Andreas Harrer

Nils Malzahn

Kay Hoeksema

Ulrich Hoppe

Learning Design Engines as Remote Control to Learning Support Environments

Keywords: Learning Design Engine, Learning Support Environment, Interoperability

Chapter 5 of the Learning Design book describes the operational model of a learning design engine based on the concept of finite automata with output alphabet. We rely on this event concept to include pre-existing learning tools in flexibe and rich learning designs.

Contribution:

We sketch an approach for the integration of complex learning environments in learning designs. Interactive learning support environments, such as argumentation or modelling tools are pre-existent and have a high potential when integrated in learning designs.

We propose an approach that aims at a clear separation of the learning design engine, the specification of the learning flow (as LD documents) and learning environments. According to its current state, the engine controls the learning environment with events (such as "start a new phase"), defined as a vocabulary for a set of environments, that are mapped to the environments' existing functionality (such as "create new workspace"). Thus the engine remotely controls the learning tools while the tools can initiate state transitions in the engine on specific events in the tool.

Learning Support Environments without Learning Design Engines and vice versa

Up to now complex learning support environments (LSE) and learning design plays are largely unrelated and co-exist, but do not co-operate. On the one hand learning support environments, such as WISE ( 2005), Co-Lab (2005) or Belvedere [START_REF] Suthers | Belvedere: Engaging students in critical discussion of science and public policy issues[END_REF]), either have a specific ("hard-wired") process model embedded or do not have an explicit learning process model at all. On the other hand environments that use learning design documents as process scaffolds or "scripts" are usually oriented towards delivery of web-content and some simple services, such as conference tools. Making the learning processes explicit in a formal specification, such as IMS/LD, offers the possibility to re-use the pedagogical rationale that is reflected within the specification. This makes it attractive for pre-existing learning support environments to utilize the formal character of IMS/LD and the availability of learning design engines (LDE), such as CopperCore (2005), to have an explicit process support within the learning environments without having to implement a process model from scratch for each individual environment. In the next section we will present our approach to achieve synergy between both lines of computer-based learning and an architecture supporting this approach.

2 Bringing it all together -flexible integration of LSE and LDE

Our starting point that leads to an integration effort of these both lines of development and research in computer-supported learning was, that we have been using the collaborative modelling and discussion support environments Cool Modes [START_REF] Pinkwart | A Plug-In Architecture for Graph Based Collaborative Modeling Systems[END_REF] and FreeStyler [START_REF] Hoppe | Integrating collaborative concept mapping tools with group memory and retrieval functions[END_REF] in a wide variety of domains (such as brainstorming, UML modelling, System Dynamics modelling, creation of argumentation graphs) and with different social forms, such as dyads, small groups and classroom groups. The scope of these tools' usage is very broad, but on the other hand setting up the scenario and having a pedagogically-based sequence of learning activities had to be done manually and without explicit process structure within the system. Thus the availability of formal descriptions of learning processes with IMS/LD and their interpretation within LD engines attracted our attention. The challenge is now to integrate a full-fledged learning support environment with a learning design engine without comprimising either of the two sides on the implementation level.

The basic idea

We propose an approach that aims at a clear separation of learning design engine, the specification and implementation of the learning flow (as LD documents) and learning environments. In this proposal we assume that the learners interact excusively with the LSE without having to know anything about being "scripted" or "scaffolded" by the LDE resp. the LD document. According to [START_REF] Vogten | An Architecture for Learning Design Engines[END_REF] learning design engines can be considered as a collection of finite state machines that react to changes of properties with state transitions by sending events of a specific output alphabet.

In the loosely-coupled connection of an engine with a learning support environment presented in figure 1, the engine controls the learning environment with output events (such as "start a new phase", event 1.), defined as a vocabulary for a set of environments, that are mapped by the environment to its existing functionality (such as "create new workspace", the configuration of the LSE through event 1.1). The learners interacting with the learning support environment create events (user action 2.), such as "phase is completed" (either directly or monitored by the LSE), that map to the input alphabet of the engine's state machines and are propagated to the LDE (message 2.1). The triggered state transition (message 2.2) causes the learning process to advance and will again trigger control messages (event 3.) to be accepted by the LSE. In that way we get the regulation cycle of figure 1 with the LDE and the LSE influencing each other's state. Using a generic vocabulary of communication primitives between the LDE and LSE has the advantage, that the LD document can be used with a variety of different LSEs without any changes to the document, given that the LSE can make use of primitives of the vocabulary. The properties, defined at IMS/LD level B, enable the dynamic regulation of the learning process within the LSE inititiated by the control messaged the LDE sends to it. The associated workspaces within our LSE are shown and hidden according to the orders the engine gives it. Depending on the state of the properties that are changed by the user's actions within the workspaces, e.g. conducting a voting, the engine's state machines are updated and lead potentially to state transitions. This is formalized in the Learning Design description as conditions, such as "if voting-result greater-than 0.8" or "if consensusachieved is true". Evaluating the conditions causes new control events that are sent to the LSE to initiate a new setup there, e.g. going from state "Decision on solution" to state "Presenting the solution" when a consensus has been reached, which means that the presentation workspace will be made visible and properly configured with the produced solution.

The practical implementation

For the practical implementation we defined an architecture that brings together LSEs and LDEs without having to make substantial changes in either of the two components: the schematic overview of the architecture can be found in figure 3 and the components introduced have the following function: • Engine Extension (CopperCore Extension): this component extends the event propagation mechanism of the learning design engine, so that on state transitions within the engine, events are sent to the LSE to remotely control the learning process according to the LD document's description. This event is sent indirectly to the LSE via the Remote Control Component

• Remote Control Component: this component is the mediator between LDE and LSE; it maps events coming from the LDE to one or more communication primitives, that build the vocabulary for remotely controlling learning support environments, such as Co-Lab or Cool Modes. These "commands" are then sent to the "remote API" of the specific LSE.

• LSE Remote API (Translator): this interface accepts communication primitives that have been defined for a variety of different LSEs and maps these primitives to the specific functionality available in the concrete LSE. For example. the communication primitive "ShowWorkspace for Decision Phase" could be mapped to calling the functionality "Make visible a workspace with title 'Decision on Solution' and add a Voting Plugin" in the Cool Modes environment (see figure 4).

The primitive that has been sent out from the Remote Control Component to the subscribers of this primitive (all LSEs that understand the primitive) is then translated to a call of the respective functionality of the LSE; thus this can be considered a remote call of the LSE functionality by the Remote Control Component.

Figure 4: The Cool Modes learning environment before (left) and after (right) transmission of the communication primitive "ShowWorkspace for VotingPhase" from CopperCore Engine. In the right part the voting plugin was added (small icon in top right corner) and an additional window appeared to conduct the voting.

If some decisions in the learning process should be taken by the teacher at runtime, the remote control component is planned to be used in combination with a GUI frontend for the teacher to control the learning process, using the same functionality as the LDE. This analogous way of using the remote control is shown in figure 3 at the bottom right.

We implemented this architecture proposal prototypically using CopperCore as LDE and Cool Modes as LSE. Both creation of events in the engine extension and the mapping to Cool Modes functionality by a "Cool Modes translator" have been realized. The Remote Control Component encapsulates the mapping from LDE events to general communication primitives and uses the Java Message Service (JMS) to publish the primitives to the translators subscribing to the Remote Control, i.e. the components that provide the remote interface to integrate Learning Support Environments into the learning design. At the moment we are in the process of generalizing and extending the vocabulary of communication primitives useful for different LSEs and the mapping to concrete LSEs, like Cool Modes and Co-Lab.

Conclusion and Outlook

The work presented here makes a first step in the integration of learning design descriptions and engines with complex learning support environments, that might be used for a variety of activities of a learning design. The basic idea is to keep the process execution and the practical activities separated, such that both engines and learning environments do not have to be modified substantially. This is achieved by definition of a vocabulary to let the LDE control remotely the LSEs that is used to regulate and configure the LSEs accoring to the state of the learning process. The central component to achieve this loose coupling is called "Remote Control Component".

In our presentation we assumed implicitly that the learners interact exclusively with the LSE without knowing anything about the process state of the learning design. When, in contrast, the learners should be made aware of the process, e.g. to give them a help in their orientation of what they have already achieved, a tool that visualizes the current state of the engine (much like the conventional function of a player) could be introduced. This process awareness tool could also have control elements for the process (such as "proceed to the next phase") to initiate events and thus state changes within the engine. Similarly a teacher could use the Remote Control Component (see figure 3 lower right) via a teacher frontend in case the teacher has to decide personally on the regulation of the learning process, e.g. if some details cannot be described in advance in the LD document.

Figure 1 :

 1 Figure 1: UML communication diagram for interaction schema between LDE and LSE To explain the basic idea we introduce the simple learning flow sketched in Figure 2 as an example. First the students explore a phenomenon (e.g. the ballistic curve of a stone) and describe what they see. Then they model the observed phenomenon within a modelling environment. Every time a student states that the model is sufficient, it is frozen and each student has to vote if he approves the model. If there is a consensus the students present their model to the teacher. If they do not agree the modelling activity is continued. This pattern is quite common for problem based learning. It can be varied, e.g. by changing the condition by which the students can finish the modelling phase. Alternatives to the

Figure 3 :

 3 Figure 3: Remote Control Architecture for interaction between LDE and LSE

Acknowledgements: We thank our student Benedikt Roth for his implementation work on integrating CopperCore and Cool Modes. Additionally we thank all the people that helped us to refine the idea of the "remote control" in discussions, such as our partners in the COSSICLE European Research Team in the Kaleidoscope Network of Excellence.


Gordon Joyes
email: gordon.joyes@nottingham.ac.uk

Bridging Cultures in Designing for Learning: An EChina~uk Project Case Study

Keywords: Professional cultures, learning design, critical incidents, learning technologists, China

This paper focuses on collaboration across professional cultures within the Sino-UK collaborative eLearning development projects at the University of Nottingham. It examines how technologists and academics developed a common understanding and effective ways of working. At the beginning of the project, the UK academics were unused to writing for online courses, and were unsure how to go about it. The technologists, on the other hand, simply wanted to be told what they needed to do, and this initially resulted in a communication breakdown. Audio and video recordings of key project meetings together with reflective journal entries and interviews with those involved in the collaboration were used to identify and explore a series of critical incidents that reveal key differences in perspective between the team members. The design approach that the team developed to bridge the differing perspectives is described as well as implications for practice.

INTRODUCTION

New learning technologies have the potential to transform the ways we learn but they rarely achieve this potential (HEFCE, 2004), often representing a poor online replica of face to face courses. The challenge is to develop creative approaches to learning design that allow for innovation and novel pedagogy so that eLearning becomes enhanced learning. In order to achieve this, participative design approaches that involve a wide range of professionals, i.e. technologists and academics need to be involved that attempt to forge a shared dialogue in relation to the new possibilities. This study explores the problematic nature of designing for enhanced learning.

The context for the study are two projects within the UK government funded eLearning International Sino-UK programme involving the University of Nottingham (UoN), UK, Beijing Foreign Studies University and Beijing Normal University, China. These online English Language Teacher (eELT) training projects sought to develop innovative eLearning in areas of language, culture & methodology to train teachers of English as a Foreign Language at secondary and tertiary levels. In the study, the focus is on the cross cultural interactions between the technologists and academics involved in the process of developing innovation as well as the Sino-UK aspects.

METHODOLOGY

Figure 1 represents the three different professional cultural contexts for designing for learning within the project. The intersections W, X, Y and Z represent possible areas of shared experience and interactions within the project that have the potential to lead to a shared understanding of what might constitute effective online learning. This study explores some of the challenges in working towards moving to the ideal represented by intersection Z by exploring the professional cultures of the techJnologists and the academics within the projectthe focus of this paper are the cultural interactions at intersection X. The study uses a critical incident approach [START_REF] Tripp | Critical incidents in teaching. Developing professional judgement[END_REF] to explore the Discourses of practice of these different professional cultures involved in the participative design of the innovative eLearning. Gee (1996:viii) describes Discourses as: 'ways of behaving, interacting, valuing, thinking, believing, speaking that are accepted as instantiations of particular roles (or types of people) by specific groups of people.'

DATA COLLECTION AND ANALYSIS

The idea that any event in professional practice can be treated as a critical incident was explored by Tripp (1993:24-25) who wrote that: 'The vast majority of critical incidents...are not at all dramatic or obvious: they are mostly straight forward accounts of very commonplace events that occur in routine professional practice which are critical in the rather different sense that they are indicative of underlying trends, motives and structures. These incidents appear to be 'typical' rather than 'critical' at first sight, but are rendered critical through analysis.'

He applied critiical incident analysis to the professional development of teachers, but the approach seems particularly useful in this study. The design approach used rapid prototyping tools such as PowerPoint and Photoshop to provide a visualization of the emerging pedagogic ideas; rapid development software, i.e. Flash was used for further development. These visualizations provided key points for critical incident analysis. The author's role as project and learning design manager meant that he needed to ensure that a common understanding in relation to pedagogy and design was emerging throughout the project and the visualizations were presented at regular meetings to share and explore the differing perspectives. The regular project design meetings were audio and video recorded and they were rich sources of data for exploring differing Discourses as revealed through differing design perspectives. These differing perspectives were explored during these meetings and then analysed as critical incidents by the author, who made entries in a reflective journal. Members of the project teams were also interviewed by a research assistant to explore their understandings of their roles and influences in the design process. The ideas that emerged were discussed further with the project teams, as the insights gained provided a means of moving the project forward.

FINDINGS The professional cultures

The following descriptions of the distinct technologist and academic professional cultures emerged from the interview data and reflections on practice within the project. A technologist subculture emerged within the project -that of the learning technologists employed to work within the project. The participative design approach used in the project meant that the project learning technologists and academics would meet regularly to discuss their ideas. The academics were unused to writing for online courses and to encourage them to move away from a text dominant approach they were encouraged to focus on developing online interactions. These interactions were to employ rich media and exploit the new technologies in order to provide an effective online student learning experience. The project learning technologist's role [START_REF] Oliver | What do Learning Technologists Do?[END_REF] was one of collaborating with technologists and other learning technologists within the University of Nottingham learning support team and with our partners in China and of exploring with the academics effective ways of realizing their ideas and of inducting them into developing online materials. The technologists took on the role of writing code, developing some interactions, designing databases, providing some graphics design support etc., to support the innovative pedagogy that arose.

The technologist culture

Members work within a central expert learning technologist team supported by other technologists. They carry out the specifications set out by academics and are accustomed to working within a linear design process in which academics specify the learning design and they then develop the online materials. They perceive the academic as having the responsibility and expertise in relation to effective pedagogy. However they are in the position of interpreting this in relation to graphic design and functionality. They traditionally have control over the technologies/learning tools that can be used within the learning design and hence have a major influence in the nature of the learning experience. However their perceptions are that they are a service culture. Notions of efficiency in relation to maximizing benefits for time spent on a project are seen as important. Developing a sense of ownership is problematic as they work on several projects at a time and have no involvement in delivery or evaluation of the learning experience and so are distanced from any learner or tutor feedback.

The academic culture

Members work as writers of content, they tend to work as individuals with particular expertise in their own subject areas. Their pedagogic approach tends to be dominated by their subject culture and this is influenced to a lesser extent by their specific national cultures. (The majority of the academics involved in the project were specialist in the Teaching of English as a Second Language and there was a shared understanding in relation to pedagogy, methodology and curriculum. However the involvement of academics who specialized in personal development provided a different pedagogic perspective.) Understanding of the needs of online learners is limited as is an understanding of the potential of eLearning. However they have a high creative potential for developing innovative online practice due to the fact that their individual subject pedagogies make specific demands of online learning. They are willing to share their approaches and are open to influence by others, i.e. other academics and the technologists within the project. However they have a limited amount of time to spend on materials development as their main focus is on research.

The project learning technologist subculture

Members work within a project based expert learning technologist team interacting with other technologists and working closely with the project academics and manager. They are perceived as bridge builders between the academics and the technologists. They need good interpersonal skills. '(The project learning designer) is a good influence in terms of team harmony, I guess, the way he works well with so many people has been influential .' -Central team technologist They understand and share the vision behind the project, have a sense of ownership even though they are on short term project based contracts. They perceive of the necessity to develop an understanding of the pedagogy involved with the learning materials so that they can offer advice on learning design so that they can help interpret this for technologists. 'I've been described as a bridge between academics and technical people. I have to convert what academics give me as raw materials into something that technical people can understand, or I take academic's stuff and put what I think is a better way in presenting it, things like putting in activities, putting in animation, then give it to technical people in terms they can understand again, because the two languages they use are entirely different.' -Project learning technologist They perceive graphic design, accessibility and effective navigation to be critical elements within the learning experience and take a lead in influencing these aspects. They provide guidance and frameworks for supporting academics in writing the materials. Their role as bridge builders and in influencing design is supported by an iterative rapid prototyping process. They have an involvement in supporting materials evaluation and so are influenced by learner and tutor feedback.

The critical incidents

Interpreting what is meant by 'not technologically possible'. An analysis of the context for learning indicated that a key motivator for establishing the 'habit' of online working would be the ways the learners experience was a personalised one. The academics felt that if the learners had a personal online space to work in; one that archived the outcomes of their online activities, their assignments and their key references, then online working would be seen as beneficial to working from text based materials or on a local computer. Initial reaction by technologists both in the UK and in China to early visualisations of these functionalities was that it was not possible to achieve this. However because the Chinese learning platforms were non commercial and bespoke database driven opened up the possibility to consider developing online personalisation in the ways the project academics wished .

Unpacking what was meant by 'not possible' within the technologists Discourse was important and also to realise why the project team considered that this ought to be possible. HEFCE wanted the projects to be involved in innovative eLearning design and this was something that clearly would be demanding for those involved, however it was clearly going to be difficult for those working within the technologists culture to buy into this. Nevertheless the project was influenced by key learning technologists who felt that academics should be asking more of technologists to solve our pedagogical challenges [START_REF] Neven | An Open and Flexible Indexation-and Query tool for Ariadne[END_REF]. 'Not possible' needed to be interpreted as 'not within our experience' and 'not a priority for us' rather than 'this cannot be achieved'. This was an important realisation. Given the funding made available by HEFCE and the availability of teams of Chinese developers, who had the appropriate programming skills, what was needed was the realisation that what was being proposed by the academics was desirable and therefore definitely possible.

The technologist culture can tend to manage 'problems' in ways that discourage innovation. The academic culture, given the external impetus to innovate, merely perceived the 'problem' as something that could be solved as pedagogically it seemed such an obvious necessity to support effective online learning. The project team, the manager, the academics and the learning technologists used this innovation and others to establish an identity for innovation within the University of Nottingham developments. The key factors driving the development were pedagogy, a sense of ownership, the external impetus to innovate and clear notions of what was possible. The outcome is the Workspace learning tool [START_REF] Joyes | When pedagogy leads technology[END_REF] which is to be provided as an open source Moodle module. This can be viewed on the eChina~UK disseminator website via link from http://www.nottingham.ac.uk/education/centres/crsflp/eChina/ .

Differing conceptions of learning

It was agreed early on in the project that the UoN would take responsibility for any interactive learning elements. These were to be developed using Macromedia Flash and the UoN project team had recruited an experienced programmer to lead these developments. However the central UoN learning team of technologists were to provide some support for these developments. They proposed a solution that would speed up the development process one that had the potential for providing reusable interactive Flash objects for use within the project and across the UoN. Early discussions revolved around the user interface that would be common to these reusable interactive Flash objects.

The following is an extract from an interview with the leader of the central team of technologists at the UoN. He was asked to describe any critical point in the project and refers to the meeting between his team and the project team to discuss the user interface. The project team had suggested top navigation as shown in figure 2 and the central team seemed to be ignoring this and had come up with a design that included navigation at the bottom of the screen. The School of Education project team could not understand why this was happening and at first assumed this was due to the power relationships that operated with and between the groups, but this was not in fact the case -the issue was due to the different pedagogic assumptions being made between the two teams. The meeting in the interview extract that is referred to is the one where there was a realisation that each team had differing perceptions of expected online learning strategies and it was these that were leading to differing design solutions.

Figure 2: The top navigation 'A meeting was held to discuss the navigation within the user interface. The project manager and the School of Education learning technologists wanted to have the back and forward arrows in a position that you'd find in a web page, i.e. at the top (this would allow users to browse the materials freely). We wanted the navigation to be at the bottom. …..Reading tradition is top left to bottom right, if you are reading constantly across the screen, your eyes would eventually finish at the bottom right hand part of the corner of the screen, so the most logical place to put a continue button to jump to the next screen is on the bottom right hand corner. So that's why we have a continue button there. They wanted to have a random search approach whereas we wanted to have logical flow approach.' -Central team technologist On the surface the difference between having navigation at the top or the bottom seems quite a trivial matter. However the pedagogic assumptions behind the two approaches are quite different and scaffold the learning experience in different ways. The learning design split the content into a series of screens, scrolling was not a feature of the design, the intention was to provide a visually interesting experience for the learner. Any extended text was to be provided in the form of pdf files that could be navigated and annotated using the sophisticated tools available within Adobe Acrobat reader. The top navigation does not expect the learner to have read all of the content before they move on as the learners are provided with the navigation tools at the top to move at will through the online pages. This allows for flexibility of learning strategy, i.e. the learner can read all of each page in turn, or they can skim through the content/activity pages to orient themselves before studying the content in more detail or they can revisit any content easily if they need to. This approach was chosen by the academics because it encourages learner autonomy. The bottom navigation approach assumes learners will need to read all that is on the page before they continue and so the navigation in the form of a continue button is provided at the end of the text. This assumes that there is only one learning strategy, i.e. move from one page to the next; the one designed in by the writer/developer. The top navigation approach prevailed, however the central technologists team still wanted to keep the continue button at the bottom right. This seemed to be redundant as there was a skip or continue button at the top.

A compromise over this was reached.

'We needed to marry the two together. The buttons at the top right hand corner, enable the users to skip ahead or move around randomly as they choose. The continue button that appears at the (bottom) right hand corner is different in term of it tells users now to move on. If there is any kind of interaction, the continue button doesn't appear until they've completed it. It works as a visual cue. It's kind of slightly different role. That's why they appear in both locations. That was a kind of agreement we came to through that collaborative discussion.' -Central team technologist In effect the continue button worked well as a visual cue for the end of an interaction and the need for the leaner to move forward. Other solutions could have involved the skip forward button to be dimmed during an interaction and to be highlighted once the interaction was completed, but the notion of there being a logical flow to the bottom right was a persistent one.

The use of text transcripts with video eLearning within China incorporates extensive use of online video. The rationale for this use is complex, but in part relates to the perceived need for the distance learner to experience a similar experience to the face to face campus one. In addition, the views of experts in China are highly regarded and the use of video helps provide authenticity and authority for online learners. The academic writers included a variety of video clips within the materials, these included academic experts, teachers discussing classroom practice and issues and an online study support mentor. The only Chinese video player interface available within the project presented the video with audio and all of the text. The project provided an opportunity to further develop this interface and the functionality of an Interactive Movie Player (IMP) began to be discussed early on in the project. From a pedagogic perspective because the online learners were second language English speakers and because some of the terminology might be new to them, it was important for them to be able to read the video transcript. In this context the audio would not provide enough information to support their learning. Hence video and the video transcript were going to be needed to be presented simultaneously.

The central technologist team raised the issue about whether video and text should be presented simultaneously as they had understood that there was a body of research evidence that indicated that this was not desirable. They were referring to the modality principle (Clarke & Mayer, 2003) the practice of presenting to-be-learned graphical information visually supported by any textual information through an auditory mode. This principle is based on cognitive research that indicates that people process pictorial information and thus video in a separate channel from auditory/ verbal information. The theory is that since the capacity of each channel is limited, graphics and supporting onscreen text compete for limited visual bandwidth as the learner cannot look at graphics and supporting text simultaneously. Conversely, when verbal information is presented in audible form it enters the cognitive system in a different channel where it can be comfortably processed simultaneously. One issue is whether video is likely to compete with a text transcript in a similar way to a graphic and its related text.

Analysis of the context for learning within the project including user needs information indicated the importance of the video clips to the learners, but that they would also need to be able to see the transcript. Recent literature about these modality effects (Ginns, 2005) reveals that much of the research was carried out with novice learners and that as learners become more expert effects on learning vary and are also dependent on subject domain and pacing of the materials.

'Clarifying the effects of element interactivity and pacing across sufficiently wide range of materials, age groups and media will allow instructional designers to make unambiguous decisions regarding the use of audio-visual or visual-only materials.' (Ginns, 2005 p 328) On closer inspection some of the 'truths' related to learning design are not well supported by experimental evidence. More importantly what needs to be reocgnised is that the pedagogy involved within these research contexts tends to be behaviourist in approach, i.e. information is presented to be learnt over a fixed period of time and learning gains over this period are measured. This is a quite distinct from the situation within this project in which it is seen as important that the learners explore the multi modal materials at their own pace and one in which actual engagement with the materials is dependant on how well they are motivated and not on payment for doing so as is the case in the research studies.

The IMP that was developed is shown in figure 3. In the IMP the learner is presented with the video/audio to play and can select to reveal the transcript underneath the video; presented as short clips shown synchronously with the audio. Full transcripts are made available as pdf files. In depth piloting of the online materials in China that made extensive use of this IMP found that the video presence of the 'author' was an important motivator for these learners [START_REF] Joyes | Researching an on online Teacher Education course[END_REF] without them they would be less likely to study the materials in depth. Thus consideration of modality effects becomes less important when the whole context for learning is considered and the technologists have to rely upon the academics to interpret this for them. Notions of 'rules' for learning design suggested by Ginns, (2005) in the quote above need to be questioned in the light of the need to consider the wider pedagogic context in which learning is to occur.

CONCLUSIONS

The three critical incidents described provide an insight into the challenges faced by cross cultural teams when designing eLearning activities. The rapid prototyping design approach allowed for early visualisation of ideas which enabled academics and technologists to engage in a dialogue around the learning design. Without this what tended to happen within the project was that the technologists would re-interpret the learning design in ways that reflected their needs and understandings of design rather than the pedagogic imperatives as perceived by the academics. This was true for both the UK and Chinese technologists. The cultures of the technologists and the academics within the project were quite different and the influences on pedagogic decisions would therefore be different. This difference in terms of skills, experiences and understandings had the potential for enabling truly innovative learning design, but in order for this to happen a means of sharing understandings and exploring these differences was necessary. It was important to critically analyse the difference in design suggested by the two cultures before any design decisions could be finally made and agreed. In effect each team's ideas had value and needed to be valued and what resulted was an informed and well justified design, see figure 2 and 3 above. The project team of learning technologists was critical in supporting this process. The fact that they identified with the project and developed an understanding of the pedagogic principles behind this enabled them to present technological solutions that were sympathetic to these pedagogic needs. They formed the much needed bridge between the academics and technologists and managed the pedo-technology interface within the project. Reflection on the process provides the following suggestions for a more effective collaborative design process:

• Discussion with all involved in the project (academics and technologists) to share the vision;

• Early discussion about the design process and the nature and expectation of the roles of the technologists and academics within this -exploration of the difference between cooperation and collaboration;

• Ongoing rapid prototyping/visualisation of pedagogic ideas;

• Analysis of critical incidents in the design process;

• At least one dedicated learning technologist who works closely with the project -the aim is to develop a sense of ownership and act as a bridge between the project academics and the other technologists involved;

• Involvement of both technologists and academics in the evaluation process and any subsequent redesign.

This research revealed the need to understand the context for design, i.e. the professional cultures involved, ways of working, infrastructure etc. in order for a successful participative design process to be utilised, one which results in innovative and effective learning design. This is the focus of further research within the eChina~UK eEducator training project at the University of Nottingham and Beijing Foreign Studies University.

In addition, this rapid prototyping design process revealed interesting pedagogic differences between the Chinese and UK cultures in relation to the nature of portfolios, assessment, the role of the tutor etc. and this will be the subject of a future paper.

Figure 1 :

 1 Figure 1: The Sino -UK contexts for designing for learning

Figure 3 :

 3 Figure 3: The Interactive Movie Player


Paul Gostelow

Ali Mahallati

Aldo Rona

Some Issues and Developments in Analytical and Experimental Work on Turbine Blade Flows

Phenomena and

Dynamics of

Areas where substantial research on planar turbine cascades is ongoing, or is still needed, are identified. Comp:ressibility effects are particularly important and are addressed in the three main sections. The modeling of the shock-boundary layer interaction is not yet reliable.

For supersonic speeds the agreement is excellent apart from the crucial region downstream of the shocks. At subsonic speeds the vortices were shed in a classical von Karman vortex street. This resulted in strong base pressure deficits causing high wake losses and ene:rgy separation in the wake. The base pressure deficit and the measurements of wake ene:rgy separation coincide and it is concluded that the two phenomena are both manifestations of von Karman vortex shedding. At Mach numbers above unity the vortex shedding was found to be one of a number of transient shedding patterns.

Rotatin g Machinery

Keywo rds

Shock-boundary ayer nteract on-base pressu re energy separat on-exot c shedd ng

INTRODUCTION

For the prediction of flows over turbine blades it is clearly essential that attention is paid to the modeling of three dimensional (3-D) flows. Arguably all turbomachinery flows are three-dimensional and unsteady. However this should not be at the expense of adequate attention to the physics of two dimensional (2-D) flows that provide the foundation and are the subject matter of th is paper. These flows have generally been treated as 2-D but are not strictly 2-D as recent publications on stationary streamwise vorticity make clear. Many details of these quasi 2-D flows are not yet fully understood. Only when these are fully represented will we acquire more confidence in blade design. An emphasis on representing the flow physics is needed, not only for code validation, but also to predict laminar boundary layers, transition to turbulence, separation, heat transfer, base pressure etc. These all affect performance and efficiency.

In this paper some obscure features of the 2-D flow over turbine blades are identified and three high speed cases are discussed in more detail. The planar cascade of blades is a model that gives valuable information and understanding for the flow through all turbomachinery blade rows.

Although the flows through a blade row are essentially 3-D and unsteady, the 2-D cascade can still give detailed information on the flow physics that is otherwise not well-predicted. Some gaps in knowledge will be identified, covering entire blade and nozzle vane surfaces, from leading edge to trailing edge and beyond. To give improved prediction capability, these gaps require improved understanding.

The focus of this work is on turbine cascades. Specifically the emphasis is on transonic and low supersonic flows. Similar behavior has been observed between tests under strong adverse pressure gradients on tri ggered spots, wake-perturbed flat plate boundary layers, and on turbine blading. The open questions in the physics of blade flows start ahead of the leading edge and continue to the trail ing edge and into the wake. Figure 1 gives the layout of a 2-D turbine nozzle passage, indicating remaining gaps in knowledge. In this paper, an attempt is made to identify areas where substantial research on planar turbine cascades is ongoing or is still needed. As an example, the question of shock-boundary layer interaction is addressed here, followed by the listing of some other phenomena.

The purpose of this investigation is to raise awareness of some features in blading aerodynamics that are not yet clearly understood. The emphasis is on the quasi two-dimensional flows covering the entire nozzle vane surfaces. Many of these features can also be found on compressor and fan blades. An attempt is made in this paper to identify some of the areas where substantial research on planar turbine cascades is ongoing or is still needed.

lt is obviously also essential that attention is paid to the modeling of three dimensional flows. lt is arguable that all turbomachinery flows are essentiall y three Outer regions of the fan blades of high by-pass engines operate with supersonic relative inlet flows and these result in a shock-boundary layer interaction in the leading edge region. This interaction, involving a thin boundary layer, mainly affects the flow rate but otherwise such interaction effects are relatively benign.

This cannot be said for turbine nozzle vanes where high loadings may call for supersonic discharge flows.

!he � odel!ng of such shock-boundary layer 1nteract1ons

IS not yet reliable. This can affect flow predictions downstream of the shock impingement and hence the loss. lt is particularly difficult when a laminar separation bubble is triggered or when the shock is oscillating under the influence of vortex shedding.

In Fig. 2 • uo ,uuu

I 0.3 / � /� � V' � 0.2 0.1 1/' V � 1 / V � V VC' � V
1-a--: :: � 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Axial chord, C,

1.6 0 M a= 0.7 (EXP) 1.4
---M a= 0.7 (CFD) 0 M a= 0.9 (EXP)

1.2

---M a= 0.9 (CFD)

0 _h, � ( VoC � 0.8 L � 'b_ . 0 oo<l � A Cl 0.6 0.4 0.2 .J V '-'\ I 1. � c 'ci-o0-q onu i i I � � if _...: :: d :/ � V A'
� !I"' -'"' � 0 .... . of the suction surface to be wrongly predicted if the representation of flows, in the interaction region and further downstream, is not correctly modeled.

Results from the subject test case are given in Figs.

3 and 4 and have been used for validating a number of codes. The most recent two-dimensional time accurate numerical simulations of the mid-span flow were performed over the speed range to Ma = 1.4; these also are given in Figs. 3 and4.

The spatial derivatives were discretized with a second-order accurate upwind Roe flux difference splitting scheme. The temporal terms were treated with the second-order implicit method. The k-w turbulence model was used for closure. Convergence was accelerated; using multi-grid techniques the a;

.0

E ::J c .c lll � iii 0 0 ....J :V .0 E ::J c .c l6 � iii 8 2 1.8 0 M a = 1.0 (EXP) 1- ---Ma= 1.0(CFD) 1.6 1.4 1.2 0 M a= 1.1 G (EXP) ---M a= 1.16 (CFD) /, \ Po "- p\ 0 �[ A€ llo 00 � f,l :;s: 0.8 0.6 I '\� L � F' / } / / 0.4 0.2 "' � :.--"' / J<l'
La--� 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Axial chord, C, mesh was refined such that the minimum value of y was less than unity. No wall functions were used and the turbulence model was integrated all the way to the wall.

2 1.8 0 M a= 1.3 (EXP) ---Ma= 1.3(CFD) � 0 I f 1.6 1.4 M a= 1.4 (EXP) ---Ma= 1.4(CFD) I � / �� I I "" 18
At discharge Mach numbers below sonic (F ig. 3) the agreement between computational prediction and experimental measurements of local Mach number is reasonable. At a Mach number of unity it is not clear whether the shock impingement is accurately predicted.

At discharge Mach numbers of 1. 16, 1 .3 and 1.4

the agreement on both surfaces is very good apart from the crucial region downstream of the shock. Shock impingement here results in a significant discrepancy between computation and experiment.

The prediction is clearly adversely affected and at a For subsonic speeds low base pressure is an essential facet of the vortex shedding process resulting in increased drag for bluff bodies and efficiency losses in turbine blades. The base pressure depends strongly dependent on Mach number. At supersonic speeds, the main causes of low base pressure are the strong spatial variations of pressure through shocks and expansions; this tends to be a relatively steady process. At subsonic speeds, shocks only beg in to play a role as the velocity reaches critical levels and, in general, the unsteady process of vortex shedding is more important.

These are therefore two distinct compressibility effects.

The main focus of this subsection is on the subsonic speed range.

Base pressure loss is an important contr ibutor to a turbine blade's total lloss. Work by Xu and Denton [4] and by Mee et al. [5] showed that the base pressure contributed a significant proportion of the total loss at

Energy Separation in Blade Wakes

Associated with the vortex shedding was the energy separation effect that was particularly strong at high subsonic speeds. On a time-averaged basis the wake center line stagnation temperature was found to be 12°C lower than for the incoming fluid. Meanwhile the stagnation temperature at the edges of the wake was 5°C higher than that of the incoming fluid. This effect had had a major adverse impact on the development

Energy Separation and Base Pressure

The finding that the distribution of the total temperature differences was correlated with the distribution of to tal pressure loss coefficients meant that the energy separation phenomenon not only caused the redistribution of the total temperature field within the wake but also affected the magnitude of the wake losses.

In Fig. 8

EXOTIC VORTEX SHEDDING AT TRANSONIC

SPEEDS

Although a link between energy separation and base drag at subsonic speeds has been demonstrated th . e shed vortices may become intermittent at supersonic speeds. The configurations are no longer solely of the conventional von Karman kind but adopt a number of different patterns. This behavior is well-known for vortex shedding at low speeds when the wake is subjected to lateral static pressure variations. In the case of the blunt trailing edge of the turbine blade at transonic speeds the pressure fluctuations do not originate on the blade surface but rather in the instability of the confluence region downstream of the trailing edge.

The most common vortex shedding mode, that predicted by von Karman, is not the only shedd . ing mode. Not all vortex shedding takes form of a classical vortex street; a wide range of 'exotic' shedding modes ex ists [12,13]. Carscallen and Gostelow [7] also discovered these anomalous patterns behind transonic turbine blading prompting an investigation of when they might occur. Two other applications. were found in the vortex-induced vibrations of bluff bodies and research on oscillating airfoils. Findings from the vortex-induced vibration work of Williamson and Roshko [12] and others clarified the shedding modes.

For some of the time vortices were shed simultaneously from the two sides of the wake, rather than alternately. This behavior can be observed in Fig. 9 for the discharge Mach number of 1.09. Other modes were observed in which vortex pairing appeared to be taking place on one side only as in Fig. 10 This has the effect of reducing the lateral distance between incipient vortices to the relatively short wake width at the downstream shock location. Nevertheless.

significant free shear layer instabilities are thought to be propagated downstream through a Ke lvin -Helmho ltz mechanism.

These phenomena, caused by vortex shedding, also play an important role in the loss-generation due to low base pressures behind blades with thick trailing edges.

They can also have a role in vortex-induced vibration of the resulting airfoil, which is effective ly a bluff body.

High speed schlieren and traverses of total pressure fluctuations have established that, although strong von There was strong interaction between the downstream shock waves and the vortex shedding process but the coupling mechanism was not understood.

Observations of how the shocks interact with shear layers suggest that the observed changes in vortex shedding from the blunt trailing edges of the transonic cascade in the Mach number range between 0.9 and 1.3 are caused by a similar self-induced oscillation mechanism. As shown in Figs. 9 and 10, shocks are generated at the confluence of the shear layers; the shocks interact the shear layers in a manner similar to the mechanism described by Mabey [14] for the biconvex airfoil.

SUMMARY

For the prediction of flows over turbine blades it is important that attention be paid to the modeling of three Figure 1 gives the layout of a turbine nozzle passage, indicating regions where there are gaps in knowledge. These cover the entire surface from leading edge to trailing edge and beyond. Many such features can also be found on compressor and fan blades. In this paper an attempt is made to identify areas where substantial research on planar turbine cascades is ongoing or is still needed. Compressibility effects are particularly important and are addressed in the three main sections of this paper.

High loading requirements in modern axial flow machines often call for transonic and supersonic flows.

The fan blades of high by-pass engines operate with supersonic inlet velocities and these result in a shock boundary layer interaction in the leading edge region. This shock interaction involves a th in boundary layer, such that the interaction effects are relatively benign.

This cannot be said for turbine nozzle vanes, where high loadings may call for supersonic discharge flows.

The modeling of the shock-boundary lay.er interaction

is not yet reliable. This can affect flow predictions downstream of the shock impingement a11d hence the loss. lt is particularly difficult when a laminar separation bubble is triggered or when the shock is oscillating under the influence of vortex shedding.

I n turbine blading the shock wave emanating from the trailing edge of one vane may impinge on the adjacent vane at around 45% of true chord (70% axial chord). There is the potential for 55% of the suction surface to be wrongly pred icted if the physical representation of flows in the interaction region and further downstream are not correctly modeled. This Computer modeling will then be able to proceed with confidence. 8

ACKNOWLEDGMENTS

Article Title 8

  dimensional and unsteady [1]. Computational grids need to be refined. Boundary layers and separation need detailed three -dimensional attention; also secondary flows, clearance, purged flow s, sweep effects, cross-flow instabilities etc. need to be incorporated. However this should not be at the expense of adequate attention to predictions of two dimensional flows that provide the foundation, and are the subject matter, of this paper. Testing on planar (rectilinear) cascades has continued in laboratory facilities around the world and is still providing valuable information. Although much of this testing is nominally two-dimensional, planar cascade tunnels are not limited to this and work on the three-dimensional aspects of the flow and on unsteady flows with wake interaction is also widespread. lt should be mentioned that the testing of annular cascades and of complete stages is also undertaken, mainly in industrial laboratories. There is, therefore, a continuum from the canonical models of flat plates and circular cylinders through rectilinear casca _ des to annular cascades and complete rota ting mach1nes. Factors govern ing the choice of model are often cost and versatility but the specific purpose of investigations may also be crucial. A pr incipal aim is to validate the prediction of computational results. These themselves may have a variety of aims, from basic physics to direct design and code validation. The codes available take on a variety of forms, from two-dimensional and time mar ? hing, through the most usual Reynolds-averaged Nav1er Stokes (RANS) codes, to Direct Numerical Simulation (DNS). In all of this a most important objective is the understanding of the basic flow physics. The ideal way of attaining this is to have a high quality planar cascade in a stable environment with good instrumentation back-up that can be worked on continuously, or at least when new staff ideas or instruments arrive. The authors are fortunate to have had such accessibility at the National Research Council of Canada (NRC) in Ottawa and most of the results presented in this paper will be drawn from that facility. Some of the issues raised by cascade tunnel testing are listed below . The question of shock ?o � nd � ry layer interactions is one on which progress IS 1mm 1nen t as new compressible models and codes are developed. -BOUNDARY LAYER INTERACTION High loading requirements in modem axial flow machines often call for transonic and supersonic flows.

Figure 2 .

 2 Figure 2. Schlleren Images of Flow at Ma = 0.7, 1.0, 1.16 and 1.3.

Figure 3 .

 3 Figure 3. Agreement between Experiment and Computation for NRC Nozzle Cascade at Four Subsonic Discharge Mach Numbers.

Figure 4 .

 4 Figure 4. Agreement between Experiment and Computation for NRC Nozzle• Cascade at Four Supersonic Discharge Mach Numbers.

Figure 5 .

 5 Figure 5. Instantaneous Computational Schlieren. View of Shock Wave Boundary Layer Interaction at Ma =1.16. discharge Mach number of 1.16 the computation under-predicts the streng1th of the expansion and of the subsequent recovery.lt may be noted that prediction of the flow regime upon shock interaction at transonic speeds is notoriously sensitive to small changes in Reynolds number.

  Figure 5 shows the numerical schlieren corresponding to the experimental result for a Mach number of 1.16. The experimental image shows that the experimental shock appears to directly impinge on the blade surface whereas the numerical schlieren indicates impingement on a separated flow region or abubble. This is consistent with the rapid expansion fan following the experimental shock that is not predicted numerically. The exper imental flow then appears more capable of recovery than the computed flow. The impingement and the flow on the downstream surface are not being correctly predicted. lt may be concluded that over the subsonic speed range (Fig .3) the flow throughout the blade passage region is correctly predicted.In this case the approaches to turbulence modeling and transition modeling are working well and existing techniques are adequate.For supersonic flows, although flows upstream of the impinging shock are modeled quite well, for the remaining 55% of true chord the observed suction surface flow is not modeled correctly. Since this is the critical region for blade profile loss generation the need for improved modeling of the shock-boundary layer interaction region is clear.The discrepancy when strong shocks are present appears to be a failure to model the shock-boundary layer interaction correctly. There is a need to investigate and improve the modeling of shock boundary layer interactions, and their influence on the downstream flow, partkularly when a laminar separation bubble is predicted or when the shock oscillates under the influence of vortex shedding.2. VORTEX SHEDDINGVortex shedding is endemic for blading having a blunt trailing edge. Trailing edges are often blunt, for blade cooling and stressing reasons, and carry a high loss

  high speeds. MacMartin and Norbury had concluded that, for bluff body flows, "calculation methods which neglect base pressure effects a re incapable of accurately calculating the flow patterns or the to tal pressure loss" [6]. Carscallen et al. [7] found that low base pressure was accompanied by the strongest amplitude of vortex shedding. The most comprehensive base pressure correlation is that of S ieverding et al . [3]. The base pressure results are compared with the Sieverding correlation by Gostelow et al. [8]. There is a significant discrepancy between the two base pressure correlations.

  of a new high-loading turbine design. lt was demonstrated that this was a manifestation of the. In energy separation the vortex cores emerge colder than the surrounding fluid and are associated with hot spots at the edge of the wake.On a time-averaged basis this results in substantial total temperature red istribution. The frequency of vortex shedding from the blades was of the order of 10 kHz.This was a requirement to measure total temperature fluctuations with a bandwidth approaching 1 00 kHz for the energy separation phenomenon to be resolved and identified. This was achieved us ing novel quartz rod-mounted thin film gages supplied by OxfordUniversity [1 0]. A Kulite pressure transducer was mounted alongside the quartz rods enabling total pressure to be measured simultaneously. lt was therefore possible, using phase averaging, to construct contours of total pressure, total temperature and entropy increase at the measurement location in the vortex wake. As an example, the total temperature contours are shown in Fig.6and the entropy contours in Fig.7. The relatively cool vortical structures on the wake center line are seen, as are the hot spots.

Figure 7 .

 7 Figure 6. Time-resolved Temperature Contours Downstream of the Blade Trailing Edge.

  Figure 8. Normalized Loss Coefficient and Total Temperature Difference as a Function of Mach Number.

  conditions schlieren observations revealed vortex shedding structures varying intermittently between the usual von Karman mode and more exotic modes. The resultant vortex shedding was also responsible for an oscillating shock that imp inged on the suction surface of the adjacent blade.The Relationship At subsonic speeds the link between energy separation and base pressure is von Karman vortex shedding.For the turbine blades the amplitude of oscillations, the energy separation and the base drag are all at their most severe at exit Mach numbers around 0.9 and, as Fig. 8 shows, fall off steeply at ex it Mach numbers above unity. A wide range of energy separation phenomena had been rev iewed by Eckert [11) who concluded that energy separati�n was primarily caused by pressure forces, acting on fluctuating curved streamlines, with only a minor contribution from viscous forces. The evidence of the results is that the driving force behind both energy separation in the wake and base drag on the blade at subsonic speeds is clearly the strong von Karman vortex shedding. Under vortex shedding conditions, the dynamic action of the vortices produces strong total temperature differences between the wake's cold core and the hot outer regions. The same process demonstrates that the dynamic action of the vortices produces strong total pressure fluctuations in the trailing edge region. This manifests itself as the low static pressure (base pressure deficit) on the wall surface. This is the principal cause of subsonic base drag. lt has therefore been demonstrated that both phenomena are the inevitable consequences of the strong von Karman vortex shedding at high subsonic speeds. The analysis indicates that the phenomena of energy separation and of base pressure deficit are inextricably linked to, and are caused by, vigorous von Karman vortex shedding. The best practical approach to allev iating energy separation and base drag is not to attempt to address these issues separate ly or directly but rather to address their cause, namely the global stability of the von Karman vortex shedding. Quite subtle changes in trailing edge geometry can minimize or even eliminate vortex shedding. The main cause of von Karman vortex shedding behind a turbine blade is the bluntness of the trailing edge.

  . Since the passage shock behavior is determined by the vortex shedding mode this anomalous behavior could have consequences for predicting the shock position, blade loading and dynamics. None of these additional transonic speed modes are explained by conventional stability theory. However Ponta and Aref [13] have performed an analysis that appears to predict the observed data well and extend them to a wider range traili ng edge shocks become oblique and the origin of the vortex street migrates from the trailing edge to the conflue nce of the two trailing edge shear layers. Previously availab le evidence had suggested that only free-stream disturbances are effective in provoking vortex-shedding instability. In the present example the visible existence and fixed location of acoustic waves precludes such a path for anti-symmetric upstream-travelling pressure waves.

Figure 9 .

 9 Figure 9. Schlieren View of Vortex Couples.

Figure 10 .

 10 Figure 10. Schlieren View of Vortex Doublets. Karman vortex shedding occurred throughout the subsonic range. Above sonic the wake vorticity distribution was intermittent with vortices being shed in one of a number of transient, yet distinct, shedding modes. These vortex-shedding modes were characterized as classical von Karman, couples, hybrid and doublets. Broadly corresponding designations from the field of vortex-induced vibration were identified in terms of the single and pairs definitions 2S, 2P, 2P* and P+S. Schlieren photographs from transonic cascades showed that the interactaon between the base flow shear layers and the shock waves are the likely mechanism causing the changes in observed vortex shedding patterns.There was strong interaction

Findings

  , from schlieren visualization, computational work and a separate hydraulic analogy experiment, have shown the shock wave I wake interaction structure at the confluence of the shear layers to be particularly dynamic and mobile. As a result an oscillatory flow is set up which causes the observed changes in vortex shedding. The base pressure recove red sig nificantly in transonic flows, where the vortex street changed its character. The anomalous vortex shedding patterns were therefore associated with a reduction in loss.

  dimensional flows. All turbomachinery flows are essentially 3-D and unsteady. Computational grids need to be refined. Boundary layers and separation need detailed three-dimensional attention; a lso secondary flows, clearance, purged flows, sweep effects, cross-flow instabilities and streamwise vortices need to be incorporated. However this should not be at the expense of adequate attention to the 2-D flow physics that provides the foundation and the subject matter of this paper. Many details of these quasi 2-D flows are not yet fully understood. Only when these are fully represented will there be more confidence in blade design. An emphasis on representing the flow physics is needed, not only for code validation, but also to predict laminar boundary layers, transition to turbulence, heat transfer, separation etc. These will all affect performance and efficiency.

  test case has been used for validating a number of codes. 2-D time-accurate numerical simulations of the mid-span flow were performed over the speed range to Ma = 1.43 and are used in Fig.2.At subsonic discharge speeds the agreement between prediction and measurements is reasonable.For supersonic discharge speeds the agreement on both surfaces is excellent apart from the crucial region downstream of the shock. The predict ion is clearly adversely affected and the computation under predicts the strength of the expansion and of the subsequent recovery.At subsonic speeds the vortices were shed in a classical von Karman vortex street. This resulted in strong base pressure deficits causing high wake losses and energy separation in the wake. The base pressure deficit and the measurements of wake energy separation are coincident and it is concluded that the two phenomena are both man ifestations of the von Karman vortex shedding. At Mach numbers above unity the von Karmc!m vortex street was found to be but one of a number of transient, yet distinct, shedding patterns. These corresponded with similar patterns observed in the field of vortex-induced vibration, The occurrence of similar changes in vortex shedding from transonic cascades suggests that the existence of an oscillating body is not a fundamental requirement. The wake will be under lateral pressure and its instab ility could be caused by an oscillating flow mechanism. Shock induced transonic flow oscillations could also change the modes of vortex shedd ing.The transonic cascade sch lieren photographs showed that the interactaon between the base flow shear layers and the shock waves, which form at Mach numbers between 0.97 and 1.2, is the likely mechanism causing the changes in observed vortex shedding patterns.In all this work an emphasis shou ld be retained on representing the flow physics. More work is needed, even to predict laminar boundary layers, heat transfer and separation. Experimental work is needed for analytical understanding and code validation.

The authors wish to acknowledge the support of the National Research Council of Canada and the University of Leicester.