Michel Bergmann

Angelo Iollo

Rajat Mittal

Influence of caudal fin rigidity on swimmer propulsion efficiency

Keywords: Caretsian grid, immersed boundaries, fish-like swimming, efficiency, tail rigidity Maillage cartésien, frontières immergées, nage de poissons, efficacité, flexibilité de la nageoire caudale

A computational model is used to examine the effect of caudal fin flexibility on the propulsive efficiency of a self-propelled swimmer. The computational model couples a penalization method based Navier-Stokes solver with a simple model of flow induced deformation and self-propelled motion at an intermediate Reynolds number of about 1000. The results indicate that a significant increase in efficiency is possible by careful choice of caudal fin rigidity. The flow-physics underlying this observation is explained through the use of a simple hydrodynamic force model.

Introduction

Investigation of swimming hydrodynamics can on the one hand provide insights into biological evolution and physiology, and on the other, lead to new bioinspired designs of underwater vehicles. Quantitative evaluation of the mechanical and fluid dynamical characteristics of swimming in experiments is challenging [START_REF] Triantafyllou | Hydrodynamics of fishlike swimming[END_REF]. In particular, precise control of the kinematics and dynamics of the experimental model is difficult; harder still is the accurate measurement of forces and power of a freely swimming model. In this regard, computational modeling and simulation is well suited for such investigation. However, three-dimensional numerical simulation of swimming modes have only become viable in the last decade [START_REF] Mittal | A versatile sharp interface immersed boundary method for incompressible flows with complex boundaries[END_REF][START_REF] Anup | A new mathematical formulation and fast algorithm for fully resolved simulation of self-propulsion[END_REF], Curet et al., 2010[START_REF] Hieber | An immersed boundary method for smoothed particle hydrodynamics of self-propelled swimmers[END_REF], Gazzola et al., 2011] due to the significant complexity and computational expense involved in such simulations.

The bending of biological propulsive structures is well known [START_REF] Mccutchen | The trout tail fin: A self-cambering hydrofoil[END_REF]. Here, we thus use numerical simulation to study this specific aspect of fish-like swimming: the influence of caudal fin flexibility on the swimming performance of the so called "carangiform" mode of swimming [START_REF] Sfatiotakis | Review of fish swimming modes for aquatic locomotion[END_REF]. A number of past numerical [Dong et al., 2010[START_REF] Ramamurti | Fluid dynamics of flapping aquatic flight in the bird wrasse: three-dimensional unsteady computations with fin deformation[END_REF] and experimental [START_REF] George V Lauder | Locomotion with flexible propulsors: I. Experimental analysis of pectoral fin swimming in sunfish[END_REF], Esposito et al., 2011] studies have examined the effect of pectoral fin deformation on labriform propulsion. However, there have been few examinations that have quantified the degree of flexibility with increase in efficiency [START_REF] Bose | Performance of chordwise flexible oscillating propulsors using a time-domain panel method[END_REF][START_REF] Prempraneerach | The effect of chordwise flexibility on the thrust and efficiency of a flapping foil[END_REF][START_REF] Vermeiden | A systematic experimental study on powering performance of flapping foil propulsors[END_REF][START_REF] Katz | Hydrodynamic propulsion by large amplitude oscillation of an airfoil with chordwise flexibility[END_REF][START_REF] Heathcote | Effect of spanwise flexibility on flapping wing propulsion[END_REF]. Esposito et al. [2011] found that there exist optimal fin flexural rigidity for maximizing thrust at fixed point, i.e., for a flapping fin that is not displacing. [START_REF] Thiria | How wing compliance drives the efficiency of self-propelled flapping flyers[END_REF] have also shown that flexible wings can lead to substantial reduction in the consumed power and to an increment of the propulsive force, also for a fixed point. [START_REF] Marais | Stabilizing effect of flexibility in the wake of a flapping foil[END_REF] observed a thrust enhancement by a factor three for a fixed flexible pitching foil compared to the rigid case. [START_REF] Vanella | Influence of flexibility on the aerodynamic performance of a hovering wing[END_REF] have used a simple two-link model to investigate the potential benefits of flexibility and highlighted the importance of considering non-linear resonances for enhancing aerodynamic performance. Finally, [START_REF] Young | Details of insect wing design and deformation enhance aerodynamic function and flight efficiency[END_REF] have examined the influence of insect wing deformation on the flight efficiency thanks to both numerical and experimental studies, highlighting the role of curvature on efficiency. Similar studies have not been undertaken for caudal-fin propulsion of a self-propelled swimmer. The vast majority of fishes and marine mammals (especially those that can swim fast) employ their caudal fin for propulsion and it is therefore expected that an analysis of this propulsion mode will yield useful insights for the design of bioinspired swimming vehicles.

It is well known that flow patterns over the body as well as in the wake significantly influence swimming performance [START_REF] Von Loebbecke | Propulsive efficiency of the underwater dolphin kick in humans[END_REF]Lauder, 2006, Zhu et al., 2002]. These patterns are in turn influenced by the geometry of the body as well as its kinematics. A number of past studies have examined these effects of body geometry as well as prescribed body and/or fin kinematics [START_REF] Mittal | A versatile sharp interface immersed boundary method for incompressible flows with complex boundaries[END_REF][START_REF] Anup | A new mathematical formulation and fast algorithm for fully resolved simulation of self-propulsion[END_REF], Curet et al., 2010[START_REF] Hieber | An immersed boundary method for smoothed particle hydrodynamics of self-propelled swimmers[END_REF][START_REF] Borazjani | Why don't mackerels swim like eels? The role of form and kinematics on the hydrodynamics of undulatory swimming[END_REF], Gazzola et al., 2011]. In the current study we focus on the efficiency improvement in a self-propelled "fish" obtained by a simple and local modification of the flexural rigidity of the caudal fin. We evaluate the swimming efficiency by employing a non-dimensional index that takes into account the total mechanical power acting on the fluid, the modeled fish velocity and the force exerted in the direction of locomotion [START_REF] Von Loebbecke | Propulsive efficiency of the underwater dolphin kick in humans[END_REF]. We concentrate on locomotion at low Reynolds numbers so that all the relevant scales of the flow are well resolved in these three-dimensional simulations.

Numerical modeling

The modeling of the flow past a deformable body and the numerical method employed ( §2.1) are basically the same as those described in [START_REF] Mittal | A versatile sharp interface immersed boundary method for incompressible flows with complex boundaries[END_REF] and [START_REF] Bergmann | Modeling and simulation of fish-like swimming[END_REF] and we only provide a brief description of the salient features.

Flow around deformable bodies

The flow is modeled by the incompressible Navier-Stokes equations (1). The kinematics of the deforming body are prescribed. The kinematics of the caudal fin can either be prescribed or computed from a unidirectional flow-induced deformation model. The trajectory of the modeled fish is computed from the integral force and torque exerted on the swimmer surface. The displacement of the swimmer across the domain is implemented via Lagrangian markers that are attached to the swimmer surface. The domain under consideration is a three-dimensional box Ω = Ω f ∪ Ω s (the "aquarium"), where Ω f is the domain filled by fluid, and Ω s is the domain defined by the swimmer. The outer boundary and the swimmer surface are denoted by ∂ Ω f and ∂ Ω s respectively. Given this, the governing incompressible Navier-Stokes equations are given by:

ρ Å ∂ u u u ∂t + ρ(u u u • ∇)u u u ã = -∇p + µ∆u u u in Ω f , (1a)
∇ ∇ ∇ • u u u = 0 in Ω f , (1b)
with initial conditions u u u(x x x, 0) = u u u 0 (x x x) in Ω f , boundary conditions u u u(x x x, t) = 0 0 0 on ∂ Ω f and u u u(x x x, t) = u u u s (x x x, t) on ∂ Ω s . Equations ( 1) are discretized in time with a second-order projection scheme [START_REF] Chorin | Numerical solution of the Navier-Stokes equations[END_REF] and spatially discretized on a fixed Cartesian mesh with grid spacing of ∆x = ∆y = ∆z = h. Since the swimmer boundary ∂ Ω s is curvilinear the Cartesian mesh does not conform to this boundary; the boundary conditions u u u(x x x, t) = u u u s (x x x, t) on ∂ Ω s are therefore imposed using a discrete forcing immersed boundary method and penalization [START_REF] Mittal | Immersed boundary methods[END_REF][START_REF] Mittal | A versatile sharp interface immersed boundary method for incompressible flows with complex boundaries[END_REF][START_REF] Bergmann | Modeling and simulation of fish-like swimming[END_REF].

Modeled fish geometry and deformation

We consider a prototype swimmer of unit length, ℓ = 1 m. At rest, the midline (backbone) of the fish-like geometry coincides with 0 ≤ x ≤ 1, y = z = 0. This backbone is then discretized by "vertebra" located at x i for i = 1, .., N. The three-dimensional fish-like shape is reconstructed using N ellipses with minor-axes y(x i ) and z(x i ). These axes lengths are found using B-splines (see figure 1). The maximum transverse dimensions are 2 y = 0.17 and 2 z = 0.24 and the caudal fin has a maximal vertical span 2 z = 0.25. These values are chosen to approximately mimic the MIT robot bluefin tuna [START_REF] Barrett | Drag reduction in fish-like locomotion[END_REF]. The final swimmer geometry is presented in figure 1.

The shape of the swimming body is obtained by deforming the body midline in the plane z = 0 as a function of time while keeping each elliptic crossection to be orthogonal to the midline during deformation. The body surface is discretized as

∂ Ω s = ∪ N s i=1 ∂ Ω i s ,
where N s denotes the number of surface sections and ∂ Ω i s defines a covering space of the surface. The velocity of each surface element, noted u u u s (x x x i , t), is then computed by tracking the Lagrangian markers corresponding to each surface element for i = 1, ..N s .

Based on previous work, the deformation of the midline is based on a traveling wave with a spacedependent amplitude [Barrett et al., 1999, Bergmann and[START_REF] Bergmann | Modeling and simulation of fish-like swimming[END_REF] as where k = 2π/λ denotes the wave number associated with a wavelength λ and ω = 2π f denotes the pulsation of the oscillations associated with frequency f . The amplitude envelope a(x) = c 0 + c 1 x + c 2 x 2 is usually defined by three parameters c 0 , c 1 and c 2 that can be adjusted. There is one constraint (the back bone length) and 4 independent parametres: c 0 , c 1 , c 2 and the abscissa of the tail extremity. Therefore we have the possibility of chosing 3 of these parameters and to determine the last thanks to the constraint. We reach a desired maximal tail excursion denoted by A, while the backbone length remains ℓ = 1 m. In what follows we will take c 0 = c 2 = 0 and c 1 = 0.1. These values correspond to a linear growth of the backward traveling wave and result in a swimming law that qualitatively agrees with those observed in Trachurus. However, these are not the values necessarely observed in a specific actual fish swimming. Without action of external forces the center-of-mass of the swimmer should not move. To respect this constraint, after deforming the swimmer as explained above, we subtract the motion of the center-of-mass from the displacement of each Lagrangian marker on the fish surface.

y mid (x) = a(x) sin (kx -ωt), (2)

Caudal fin modeling

The primary objective here is to assess the effect of caudal fin flexibility on swimming performance. To this end, the deformation can be either imposed on the whole midline length including the caudal fin (black shape shown in figure 2), or only on the portion excluding the caudal fin (which extends over the last 20% of the midline, see red or blue shapes in figure 2). For these latter cases, we model the fin by a lumped parameter elastic medium. The elastic caudal tail is composed by rigid struts joined by elastic links. Each link is subject to a couple C i that is proportional to the square of the local tangential speed V i+1 of the next junction. This couple is a crude approximation of the hydrodynamic local force exerted by the fluid on the structure. In section 4 we will show that this hypothesis is approximatively verified in our simulations.

To further simplify the model, we assume that the angles are small and the inertia and hydrodynamic forces balance for the struts following the one considered. Then, we have θi

+ β θi + kθ i = C i , i = 1, ..N L -1
where θ i is the rotation angle with respect to the previous strut, C i = -α|V i+1 |V i+1 , N L is the number of links, V i = i l i k=1 θk , l is the length of the struts. The point i = 0 corresponds to 0.8 of the midline and is the last point where the deformation is explicitly imposed by ( 2). This equation corresponds to a classical spring-damper system with a forcing term that couples the system. The anglular acceleration of a strut with respect to the previous is proportional to the sum of the couple exerted on the strut, the elastic force and the damping force. The initial condition is θ i = 0, ∀i. In the following examples we let β = 1, k = 4 • 10 3 , 0 ≤ α ≤ 16 • 10 -2 and N L = 60. These parameters were selected to obtain a realistic deformation of the tail, as in figure 2.

In summary, this crude model keeps the essential features of a visco-elastic structure, such as elastic recoil and velocity damping, mimicking the feed-back effects of actual fluid forces via the terms C i .

Force, torque and power

Let the dimensionless stress tensor T(u u u, p) = -pI I I + 1

Re (∇u u u + ∇u u u T ) and n n n the unit outward vector to ∂ Ω s , then the forces and the torques exerted by the fluid onto the bodies are:

F F F s = - ∂ Ω s T(u u u, p) n n n dx x x, M M M s = - ∂ Ω s r r r G ∧ (T(u u u, p) n n n) dx x x, (3)
with r r r G = x x xx x x G (x x x G center of mass). Since the boundary ∂ Ω s does not fit the computational mesh that will be employed, u u u and p are obtained using Lagrange interpolation. These forces and torques are used to compute the swimmer displacement. The velocity at the swimmer surface is:

u u u s = u u u + Û u u u + u u u. ( 4
)
where u u u is the imposed deformation velocity, u u u is the translation velocity and Û u u u is the rigid rotation velocity. The translation velocity can be computed from the forces F F F s by m du u u s dt = F F F s , m being the body mass. The rotation velocity is given by Û u u u s = Θ Θ Θ s × r r r G . The angular velocity Θ Θ Θ s is obtained from the torques M M M s by solving dJΘ Θ Θ s dt = M M M s , where J denoted the body inertia matrix. The force exerted by the fluid on the body surface segment ∂ Ω i s is

F F F i = (F i x , F i y , F i z ) T = - ∂ Ω i s T(u u u, p) • n n n dx x x,
where n n n the unit outward vector to ∂ Ω i s . The total work done over one stroke T is then:

W total = T N s i=1 F F F i • u u u i dt, (5)
where u u u i is the average velocity on the surface ∂ Ω i s . The useful work is defined as the part of the total work that is done by the hydrodynamic force exerted in the direction of swimming [START_REF] Von Loebbecke | Propulsive efficiency of the underwater dolphin kick in humans[END_REF]. For instance, if the swimmer velocity is positive in the x-direction, U x > 0, then the useful work is given by:

W use f ul = T N s i=1 F i x + |F i x | 2 U i x dt. ( 6
)
The propulsive efficiency can then be defined as

η = W use f ul W total . Inria (a) t = τ (b) t = τ + T 6 (c) t = τ + 2 T 6 (d) t = τ + 3 T 6 (e) t = τ + 4 T 6 (f) t = τ + 5 T 6 ›0.1 ›0.05 0 0.05 0.1 (a) (a) (b) (c) (d) (e) ( f ) t y(t) v(t)/ω τ τ + 2 π
Figure 2: Deformation of the swimmer over one swimming stroke T (corresponding to a frequency f = 2.0 Hz). The same sinusoidal swimming law is applied on the whole shape except between the black section and the tail extremity where an elastic behavior can be used. The black shape corresponds to an imposed tail deformation, the blue shape corresponds to a rigid tail and the red shape corresponds to a flexible tail.

Figure 3: Sketch of a sphere sedimentation, with an example of iso-vorticity representation.

Numerical validations

The numerical method employed has been extensively validated for several 2D test cases in [START_REF] Bergmann | Modeling and simulation of fish-like swimming[END_REF]. In what follows, 3D validations are presented. We first study the terminal velocity of a sphere falling under gravity. Then we perform a convergence study of the terminal velocity of a swimming fish with respect to spatial discretization. Finally, we show that the assumption C i = -α|V i+1 |V i+1 is reasonably verified for swimming laws similar to those investigated later in this paper.

Sedimentation of a sphere

Both experimental [START_REF] Clift | Bubbles, Drops, and Particles[END_REF] and numerical [Glowinski et al., 2001, Coquerelle andCottet, 2008] results are found in the literature for the sedimentation of a sphere under the gravity. The sketch of the configuration is given in figure 3. We chose the same non-dimensional parameters as Coquerelle and Cottet [2008], the sphere (diameter D) falls under gravity in an vertical cylinder (diameter L = 1) filled with a fluid of viscosity ν under the gravity g = -980. We model an infinite cylinder and impose homogeneous Neumann boundary conditions ( ∂ u u u ∂ n n n = 0 0 0 where n n n is the outward normal unit vector) at the top and bottom boundaries, respectively Γ T and Γ B . No slip boundary conditions, u u u = 0 0 0, are imposed on the cylinder lateral boundary, Γ L , using penalization [START_REF] Angot | A penalization method to take into account obstacles in a incompressible flow[END_REF]. Table 1 presents a comparison between terminal velocities for various diameters and viscosities. The terminal velocity, U, computed in this study is compared with experimental results U E obtained by [START_REF] Clift | Bubbles, Drops, and Particles[END_REF] and numerical results U G and U C obtained by [START_REF] Inria | A fictitious domain approach for the direct simulation of incompressible fluid flow past moving rigid bodies: Application to particulate flow[END_REF] and Coquerelle and Cottet [2008] respectively. The results reported in table 1 are computed with a spatial discretization h = 1/100. Our results are comparable to those in the literature. These are typical results for several computational set up parameters.

Swimming velocity convergence

We perform a convergence study for a unitary length swimmer with respect to the grid size. The kinematic viscosity is set to 10 -3 m 2 s -1 . The two quantities under consideration are the forward velocity V and the efficiency η ( • denotes an averaging operator acting on a given number of periods when asymptotic velocity is reached). Figure 4 shows the evolutions of the swimmer velocity norm and efficiency (averaging on the last stroke period) versus the grid size 1/h (here, we have h = ∆x = ∆y = ∆z). Both velocity and efficiency tend to converge towards limit values for fine meshes. Obviously, accuracy is improved with mesh size. However, the case h = 1/120 is a good compromise between accuracy and computation costs. Indeed, the errors between h = 1/120 and h = 1/160 are about 2% and h = 1/120 requires half CPU time than h = 1/160. We then set our spatial resolution to h = 1/120 for the following simulations. The domain under consideration is x ∈ [-2, 6], y ∈ [-2, 2] and z ∈ [-2, 2]. The finest mesh is then composed by 8 × 4 × 4 × 160 3 ≈ 5 • 10 8 mesh nodes. We considered the physical domain as an aquarium and imposed thus homogeneous boundary condition on all boundaries. In all the following simulations we used the same domain x ∈ [-2, 6], y ∈ [-2, 2] and z ∈ [-2, 2] with h = 1/120 and the mesh is composed by 8 × 4 × 4 × 120 3 ≈ 2.2 • 10 8 mesh nodes.

Force on caudal fin section

In the one-way elastic model that we have set up the force acting on the struts as F n = γ|V n |V n , where V n is the normal speed of the swimmer centerline (the backbone). We compare the actual force obtained for a self-propelling swimmer on a given section to F n . The computational set up corresponds identically to that of maximum efficiency of the next section. Three different sections are considered and the results are presented in figure 5. The top picture row shows the actual force computed on the given section vs. the model force F n on the same section. The model force is smoother but it is in substantial agreement with the actual force. The phase error is limited. The bottom picture row shows the same comparison for a model force F n computed considering the normal velocity of a section that is ∆s = 0.05 upstream. The overall deformation of the backbone will be modified and the swimming law will consequently vary. The phase error between the actual force experienced by each section considered and F n increases, confirming the hypothesis the normal force locally scales like |V n |V n . We conclude that the simplified force model is a reasonable approximation of the actual force experienced by the swimmer. With this model the fluid-structure interaction is one way and this significantly simplifies the computational set up.

Efficiency

We set the kinematic viscosity ν = 10 -3 m 2 s -1 . The swimmer is of unitary length ℓ = 1 m and hence the swimming Reynolds number is 1000V, where V the swimming velocity. Such low Reynolds numbers are chosen so as to enable high resolution of the flow. We note that a swimmer swimming at a velocity of around one body-length per time unit, Re = 10 3 can be correlated to a small swimmer of 3 cm long.

Tail rigidity

We have considered a large range of the rigidity parameter α: from 0.02 (low-flexibility tail) to 0.16 (highflexibility tail). These parameters are not directly related to the elasticity modulus of a given material, rather they are relative to the simplified elasticity model we employ. The variation of the efficiency versus the rigidity parameter α for f = 2 is shown in figure 6(a). We found that a value of α = 0.1 maximize the efficiency and that the flexible tail increases the efficiency by about 25% in comparison to the case where the deformation is prescribed for the whole body. Figure 8 shows a snapshot of the vorticity field generated by the swimmer with maximum efficiency (α = 0.1); the swimmer generates a vortex street composed of two oblique rows of vortex rings, similar to that generated by a flapping foil [START_REF] George V Lauder | Locomotion with flexible propulsors: I. Experimental analysis of pectoral fin swimming in sunfish[END_REF]. This is characteristic of the wake generated for higher values of the swimming Strouhal number et al., 2012], typically for St ≥ 0.3 [START_REF] Borazjani | Why don't mackerels swim like eels? The role of form and kinematics on the hydrodynamics of undulatory swimming[END_REF]. Here, the minimum swimming Strouhal number obtained for the swimming law considered is approximately 0.5.

St = f A V [Dai
In previous comparisons the swimmer velocity V was allowed to vary with α which introduced potentially confounding effects in the Reynolds numbers. In order to eliminate these effects, we have investigated the variation of the efficiency for a constant swimming velocity. In these cases, the frequency is kept at f = 2.0 and the tail amplitude A is regulated by a proportional feed-back controller to reach a target velocity of V = 0.4 m.s -1 , which is the minimum velocity obtained for α = 0.16. It happens that the tail excursion A is approximately constant for all αs leading to a similar swimming Strouhal number St. Figure 7(a) presents the evolution of the efficiency versus α and we find that that α ≥ 0.04 produces a higher efficiency compared to the rigid tail. In fact, for for α = 0.12 the efficiency is increased by 35%. This quite high increase for α = 0.12 can be explained by the increase of the useful work and the decrease of the total work over one stroke (see figure 7(b)).

Influence of tail curvature

In this section we investigate the influence of tail curvature on the swimming efficiency. We do that by varying the number of links in the tail. In the previous simulation we employed N L = 60 links on the tail. In what follows, we take 1 ≤ N L ≤ 5 as presented in figure 9. The case N L = 60 corresponds to α = 0.12. and the parameter α is modified for 1 ≤ N L ≤ 5 so that the trajectory of the tail is as close as possible to the one for N L = 60 with α = 0.12. Note that the shape for N L = 5 is similar to that with N L = 60. Figure 10(a) shows that efficiency is enhanced when curvature is increased (N L is increased). The evolution of the useful and total work versus the number of links N L in presented in figure 10(b). While the value of the useful work is almost constant with N L , the total work done by the swimmer over one stroke decreases with increasing N L and eventually reaches the value obtained for our test case with N L = 60. An optimal value of the flexural rigidity help to decrease the work done by the swimmer, especially the contribution due to the lateral motion. Indeed, when the swimmer velocity is constant, the total work is equal to the work done by the lateral motion.

A simple model for tail design

The largest forces and velocities on the swimmer are located on the tail. Let us consider the local contribution of the useful work W use f ul (x) relative to each section normal to the midline at abscissa x. We define by W x = 100

W use f ul (x) W use f ul
the percentage of the useful work generated between abscissa x and 1. In figure 11 we see that 75% of the total useful work is contributed by the caudal fin, i.e., the last 12% of the swimmer. Hence, in order to obtain a simple interpretation of flow physics underlying our obervations of efficiency enhancement, we now focus only on the tail segment, denoted by κ. Let e e e θ be the normal to κ as in figure 12. The relative velocity of the mid point of κ with respect to swimmer velocity is denoted Even if the values of η are slightly different from that of η obtained in figure 6(a), the overall trends are found to be similar. Indeed, the maximum efficiency is obtained for the same value of α (=0.12) in both cases.

In figure 14 we show the trajectory of κ and the corresponding model forces as a function of the tail rigidity. For rigid tails (α = 0.02) the forces are propulsive over the whole cycle but they are small in intensity and have large vertical components. For optimal rigidity (α = 0.10) the forces are propulsive over most of the cycle; their modulus is comparatively large and the vertical components are small. For high flexibility (α = 0.16) the vertical forces are small but the horizontal force is drag-generating over nearly half of the cycle. For this last case, the integral contribution over one period of the forces in the x direction becomes negative.

Conclusions

The effect of tail flexibility on swimming performance has been examined via Navier-Stokes simulations coupled with a lumped-element model of the flexible caudal fin. Our results show that allowing caudal fin deformation induced by a simple elastic model has a marked influence on the propulsive performance of the swimmer. The model shows that optimal efficiency is obtained for an intermediate flexibility of the caudal fin and that neither excessive rigidity nor compliance are conducive to efficient propulsion. We further show that a limited number of links (≈ 3) in the tail is able to generate tail curvatures sufficient to reach efficiencies obtained with a larger number of links. Our investigation is valid for low Reynolds number regimes and without taking into account possible skin effects (like polymer release) or smallscale compliant-tissue effects. Furthermore, an interpretation of these results is given based on a purely kinematic model. We obeserve that rigid caudal fins lead to excessive lateral forces that increase power consumption without generating thrust, whereas highly flexible caudal fins produces negative thrust during significant portions of the stroke. These results may lead to significant improvements in the design of underwater robots and suggest bioinspired designs for flexible fin propulsors.

This work has been supported by French National Research Agency (ANR) through COSINUS program (project CARPEINTER n. ANR-08-COSI-002). The simulations presented in this paper were carried out using the PLAFRIM experimental parallel testbed, being developed under the Inria PlaFRIM development action with support from LABRI and IMB and other entities: Conseil Régional d'Aquitaine, FeDER, Université de Bordeaux and CNRS (see https://plafrim.bordeaux.inria.fr/). Fluid Mech., 38:193-224, 2006

Inria

Figure 1 :

 1 Figure 1: Representation of the elliptical axes y(x) and z(x) defining the fish shape.

Figure 4 :

 4 Figure 4: Convergence study of the swimmer velocity V the efficiency η versus the grid size (1/h). The brackets denote an averaging operator.

Figure 5 :

 5 Figure5: Force F n on a tail section at abscissa s compared to F n = γ|V n |V n , where V n is the normal speed of the swimmer centerline (the backbone). The parameter γ is adjusted to scale F n with F n .

Inria

Figure 6 :

 6 Figure 6: Evolution of the efficiency η, the useful work W use f ul and the total work W total versus the rigidity parameter α. The dotted line corresponds to the imposed tail deformation.

Inria

Figure 7 :

 7 Figure 7: Evolution of the efficiency η, the useful work W use f ul and the total work W total versus the rigidity parameter α for V = 0.4 m.s -1 . The dotted line correspond to imposed tail deformation for V = 0.4 m.s -1

Figure 8 :Figure 9 :

 89 Figure 8: Vorticity snapshot of the wake generated by the modeled swimmer with α = 0.1 at Re = 10 3 .

Inria

Figure 10 :Figure 11 :Figure 12 :

 101112 Figure 10: Evolution of the efficiency η, the useful work W use f ul and the total work W total versus the number of links N L for V = 0.4 m.s -1 . The dotted line corresponds to imposed tail deformation for V = 0.4 m.s -1

Figure 13 :Figure 14 :

 1314 Figure 13: Evolution of the efficiency η, the useful work ‹ W use f ul and the total work ‹ W total versus the rigidity parameter α. The dotted line corresponds to the imposed tail deformation.

Table 1 :

 1 Comparison between terminal velocities for spheres falling in a fluid for various diameters and viscosities.

	Inria

  . Mattia Gazzola, Philippe Chatelain, Wim M. van Rees, and Petros Koumoutsakos. Simulations of single and multiple swimmers with non-divergence free deforming geometries. Journal of Computational Physics, 230(19):7093 -7114, 2011. ISSN 0021-9991.

	Inria

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Luc Geiller
email: luc.geiller@ac-nancy-metz.fr

Turning input into intake: autonomous reading and its impact on writing

This paper shows the importance of reading in autonomous language learning. It presents the findings of an experiment in which a group of 19-year-old French EFL learners autonomously used the press as a repository of multi-word sequences. After receiving some in-class training in active reading strategies, they adopted a top-down approach to language learning to break down a number of press articles and identify various expressions and inherent language patterns that they then memorized and wove into their own formulations during in-class essays. Data was collected by means of a computer program and analyzed by two teachers of English using a purpose-specific taxonomy of errors. The results are described and followed by a discussion of the impact of reading on self-directed language learning.

Introduction

A number of studies have shown the impact of reading on overall vocabulary learning and writing proficiency in EFL and ESL learners (e.g. [START_REF] Janopoulos | The relationship of pleasure reading and second language writing proficiency[END_REF][START_REF] Pitts | Acquiring second language vocabulary through reading: A replication of the clockwork orange study using second language acquirers[END_REF][START_REF] Lai | Effect of extensive reading on English learning in Hong Kong[END_REF][START_REF] Mason | Extensive reading in English as a foreign language[END_REF]Krashen 2004;[START_REF] Horst | Learning L2 vocabulary through extensive reading: A measurement study[END_REF]. The view and review of words and expressions in specific contexts makes it possible to infer and learn their meanings and also develop a general feel for the language. When placing special emphasis on language (not just ideas), one can arguably learn to break it down and identify (recurring) patterns. Actively engaging in this process helps understand the importance of the idiom principle and grammaticalized lexis in L2 learning. The idiom principle posits that grammar and lexis are interrelated and that '[a] language user has available to him or her a large number of semi-preconstructed phrases that constitute single choices, even though they might appear to be analysable into segments ' (Sinclair 1991: 110).

Grammaticalized lexis is the fundamental principle of the Lexical Approach (Lewis 1997) which posits that language is not divided into grammar (structure) and vocabulary (words) but consists of chunks which, when combined, produce coherent text, and that lexical phrases offer far more generative power than grammatical structures.

In a thought-provoking article, [START_REF] Ding | Text memorization and imitation: the practices of successful Chinese Learners of English[END_REF] argues that text memorization enhances noticing and rehearsal and facilitates L2 acquisition. She gives an account of the strategies used by three Chinese EFL learners to improve their English speaking skills. The learners came from the same school where they had studied English for ten hours a week in classes of about thirty students.

Although it was a daunting task that their teachers had initially forced upon them, they discovered that memorization and imitation were efficient ways to better understand language patterns and show greater command of English by using native-like sequences retrieved whole from memory.

The learners had developed a personal lexicon of prefabricated sequences and also a certain level of language awareness which enabled them to win prizes in nationwide English speaking competitions and debate tournaments. Wray, who has carried out some of the most comprehensive and unifying work on formulaic language to date, seems quite confident that aspects of the Confucian tradition of rote learning can be imported into western classrooms where memorization has been discarded as an ineffective and uncreative way to learn a second language. She argues that discouraging L2 learners from memorizing native-like multi-word sequences doesn't make much sense as it is impossible for the learners to write and speak idiomatically if they don't use them. As she explains:

The rejection of memorisation as a teaching tool is often attributed to the ill-fated investment in the audio-lingual teaching method in the 1970s, later discarded in favour of communicative teaching methods. However, many feel that the pendulum swung too far in the other direction, given that using a language effectively clearly does entail having easy access to a large amount of information stored in memory (Wray 2009: 11).

The classes préparatoires aux grandes écoles section EC, commonly called prépa EC, consist of two selective years preparing undergraduate students for competitive entry exams to France's business schools. The students' levels vary from intermediate to advanced and all have been studying English for at least five years when they start out in prépa. The program includes three hours of English teaching per week and consists in writing argumentative essays, answering reading comprehension questions, and also in translating newspaper articles and short excerpts from contemporary novels. When embarking on the prépa program, some students explain that learning grammar and vocabulary in high school has not helped them become competent users of English. In an effort to fill the gaps and prepare for their competitive exams, they turn to grammar books and word lists, convinced that eventual knowledge of rules will make it possible to combine newly learnt words. Ye t embracing the traditional view dividing language between grammar and vocabulary and trying to build English from the bottom up is arguably not the best way to proceed as it won't help them develop language-awareness skills and sound more idiomatic. If, as suggested by Wray, the pendulum has swung too far in the direction of freestyle communication in the classroom, encouraging EFL students to use native-like formulations for greater communicative power makes sense in higher education. The hypothesis is that they can be given a significant linguistic boost if encouraged to spot and memorize useful bits of prefabricated English. Initially copying and imitating native speakers can make it possible to eventually emulate them. Gaskell and Cobb (2004: 301) explain that the formal teaching of rules is not always effective in helping students with grammatical accuracy and that 'sentence-level writing errors seem immune to many of the feedback forms devised over the years'. This is something that Ellis (1994) also pointed out when he mentioned the absence of conclusive evidence that formal teaching leads to learning and argued that high levels of proficiency were not achieved through formal instruction alone but were dependent upon the capacity to initiate and manage one's own learning. This is in keeping with Little's definition of autonomy:

Essentially, autonomy is a capacity-for detachment, critical reflection, decision-making, and independent action. It presupposes, but also entails, that the learner will develop a particular kind of psychological relation to the process and content of his learning. The capacity for autonomy will be displayed both in the way the learner learns and in the way he transfers what has been learned to wider contexts (Little 1991: 4).

Programmed learning in which students make their way at their own pace through material assigned by their teacher is an efficient way to develop what Littlewood (1999: 75) calls 'reactive autonomy', 'the kind which does not create its own directions but, once a direction has been initiated, enables learners to organize their resources autonomously in order to reach their goal'.

Developing 'reactive autonomy' will eventually make it possible to achieve the ultimate goal of 'proactive autonomy' which 'affirms [the learners'] individuality and sets up directions in a world which they themselves have partially created'.

Method

Participants

The participants were 15 second-year French prépa EC students from the Lycée Fabert in Metz, France (8 male and 7 female with an average age of 19 years). They all had French L1 and had received at least five years of English instruction. The students' level varied from intermediate to advanced according to a standard placement test completed at the start of the year. Prior to starting the experiment, and since the beginning of their first year, these students had been encouraged to read the press in their own time in order to complement the work done in class and gain a sense of self-direction, a key to learning languages and to learning how to learn languages [START_REF] Holec | Learner training: meeting needs in self-directed learning[END_REF](Holec , 1981)). It is generally agreed that autonomy cannot be taught and learned but only fostered and developed [START_REF] Benson | Learner autonomy in the classroom[END_REF] and the students were thus encouraged to develop personal reading and scanning strategies and introduced to the value of imitation as a way to gain greater communicative power. The idea was to challenge the standard view dividing language into grammar and vocabulary and understand that it was possible to improve one's writing skills by memorizing native-like utterances for further use in argumentative essays. The students were thus trained to scan texts and encouraged to compile their own lists of prefabricated bits of language. The articles studied in class formed the basis for this training and the scanning activities consisted in spotting various elements (Table 1).

Table 1: Material spotted during in-class scanning activities

Collocations

The famously charismatic CEO of Apple, Steve Jobs, is a gifted public speaker.

(Source: 'Steve Jobs and 'Holy Smokes' Moments ', www.businessweek.com, 6 October 2009) 2. Fixed expressions So is India again putting the cart before the horse? (Source: 'India Food Law, The Hungry Republic ', www.bbc.co.uk, 21 December 2011)

Sentence builders

On the surface it may look as though b i g businesses, outsourcing a n d technology have brought about a socio-economic catastrophe. However winds of political change may begin to blow in favour of smaller businesses, as India's bureaucrats are starting to emerge from more diverse backgrounds.

(Source: 'Transition City Blues ', www.bbc.co.uk, 21 October 2009)

Miscellaneous

Empires always seem invincible as they are about to crumble.

Procedure

The study was conducted over a 6-week period and consisted in gauging the students' ability to spot various sequences in topic-specific reading material and to use them in related essays. The reading material assigned to the students consisted of three sets of recent newspaper articles dealing with various topics. The students' task was to scan the texts the way they had done it in class under their teacher's guidance and to memorize anything they thought they might need for further use in three essays. This was meant to document what kind of material the students select when reading English in their own time and how they assimilate the syntax and meaning of they sequences they memorize and transfer to their essays. Assignments were completed in two-week cycles, with the articles being read by the students at home in Weeks 1 and 2, and the essays written in class at the end of Week 2 without access to the preparatory texts. Forty-five argumentative essays were analyzed to throw light on the influence autonomous reading may have on L2 learning and to document how the learners went about turning the input provided by the various newspaper articles into intake.

Research instruments

Computer programs

Data from the 45 essays was collected using WCopyfind, an open-source program distributed by Lou Bloomfield of the University of Virginia which compares documents and reports similarities in their words and phrases. The program scans documents for matching words in phrases of a specified minimum length and then generates report files containing the texts and the matching phrases underlined.

Raters' evaluations

A series of 35 passages featuring the reading material occurring in the essays were submitted to one English and one Irish teacher of English who have taught in classes préparatoires. A purposespecific taxonomy of errors was designed to evaluate the students the way their examiners would most likely do it on the finals and its various tags were inserted in the essays manually by the raters (see Appendix A). There is no official grid used by examiners for the evaluation of prépa students' essays but a list of systematically stigmatized errors included in a popular student handbook by Killian and Nowak (2008: 165-167), was used and complemented from various examiners' reports offering students constructive feedback as to their performance in their competitive examinations.

Results

This section provides a qualitative analysis of selected passages from the essays (the complete list is given in Appendix A). The amount of reading material occurring in the students' essays detected by WCopyfind totals 241 words and comes in the form of 41 sequences. Two types of elements are identifiable: sentence builders and syntactically and semantically whole word sequences. Sentence builders are sequences of words providing the framework for whole sentences; they contain slots for personal expression of entire ideas:

READING MATERIAL STUDENTS' ESSAYS On the surface it may look as though big businesses, outsourcing and technology have brought about a socio-economic catastrophe.

On the surface, it may look as though France is not in favor of gay marriage.

Syntactically and semantically whole word sequences have a semantic unity and syntactic integrity and the students weave them into their own formulations:

READING MATERIAL STUDENTS' ESSAYS That would be higher than in the early 1980s when unemployment was a social challenge of paramount importance, a cause for protests and riots.

A leader has to take effectives actions to ward off any kind of plague. Then it's a social challenge of paramount importance for a leader to listen to his population.

The results show that the students have a marked preference for sentence builders: 31 sentence builders of 5 words on average occur in the essays as opposed to 10 syntactically and semantically whole word sequences of 8.6 words on average. When using the reading material, the students either operate various transfers perfectly, or make semantic and syntactic errors (only two spelling and punctuation errors were identified). In some cases they haven't fully assimilated the syntax of the sequences memorized because of the way they fail to make the necessary grammatical or lexical changes (e.g. 'Being able to gather the crowd around you *play an important role in being a leader'). In other cases, the students seem to overgeneralize because they haven't fully assimilated the meaning of these utterances. This is seen in the way they are (mis)applied and results in the raters identifying a semantic problem (e.g. 'First, the legalisation of same-sex marriage will ? increasingly become our destiny'). On the whole, 21 out of 41 formulations are successfully transferred to the essays (Table 2), which means that the students assimilate the syntax and meaning of about half the sequences they choose to memorize and insert into their own sentences. The raters identify as many syntactic as semantic errors in the twenty other formulations (Table 3).

Successful transfers

The errors identified in the essays sometimes don't affect the memorized formulations themselves but elements in the immediate co-text which bear no relation to the way the reading material might have been (mis)used. In other words, the students operate complete semantic transfers and successfully adapt the syntax of the copied sequences to weave it into their own English:

READING MATERIAL STUDENTS' ESSAYS This is evidenced by the rise of free social networking sites such as Facebook and Twitter.

1: To me, the need to allow gay marriage is evidenced by the rise of g a y p e o p l e w h o w a n t t h e i r relationship to be recognized. As hard as it is to believe, the Republican candidates for president seem to have learned very little from the moral calamities of the administration of George W. Bush 2: As hard as it is to believe, several european conservative countries G9 seem to have learned very little from current G1 d e b a te against gay marriage to protect the traditional model of the nuclear family.

Unsuccessful transfers: semantic and syntactic errors

Certain errors are symptomatic of the students' tendency to overgeneralize after reading something for the first time and memorizing it for further insertion into their own sentences (cf. passages 3, 4 and 5 below). The result strikes the raters as semantically inaccurate although the grammar is irreproachable. This shows that the students are actively engaged in a language-learning process and that breaking down the language leads to inevitable generalization in the first stages. Remarkably, students sometimes demonstrate great skill in reordering the components of original sequences (cf. passage 5).

READING MATERIAL STUDENTS' ESSAYS W h e n i t c o m e s t o y o u t h unemployment, there is precious little good news.

3: When it comes to going through V3 hard economic times, each state must be able to protect its citizens from both financial and social trouble.

S o m e i n v e s t o r s a s k e d t o b e rewarded for the extra risk and others, unwilling to start paying for credit research, just walked away.

This set off a spiral of falling bond prices, weakening banks and slowing growth.

4:

This would probably set off a spiral of a win-win situation V2 , because the G1 w a g e s w o u l d increase, enabling the state to restor S economic growth and to reduce its debt.

When he was elected president, Barack Obama made it plain that this was an event of some importance. His supporters, he said, had "put their hands on the arc of history and bent it once more toward the hope of a better day." 5: Maybe a line needs to be drawn between promising to bend the arc of history toward the hope of a better day V2 , and being realistic as well as optimistic about the recovery.

Other errors highlight what is a real catch-22 situation for L2 learners: it is impossible for them to achieve more idiomaticity without imitating native speakers, yet they are likely to sound non-native-like if they do so:

READING MATERIAL STUDENTS' ESSAYS Perhaps Obama can reach a deal with Iran, and set up a cap-and-trade system.

6: France should undergo a value shift so that the government could G8 reach a deal V1 with gay people. Perhaps Obama will lay down a path towards fixing the budget.

7: These s upporters may think gay marriage will l a y d o w n V1 a p ath towards a global agreement.

In this new world, our privacy takes a back seat to the constant drive to make information easier to obtain.

8:

It is striking to see that citizens' concerns have taken a back seat to the need to V2 save banks first.

In these examples, other raters might think that the transferred sequences used accurately (they are, after all, reused verbatim and in similar contexts). Ye t they are here dismissed as incorrect by both raters. It is not the position of the memorized sequences in relation to their immediate co-texts that is problematic to the raters (no grammatical errors are made) but the sequences themselves.

This may be due to the fact that when the raters identify other errors in the text preceding or following a copied sequence, they are conditioned into expecting other errors to occur and are thus more likely to interpret what is an 'unexpected' collocation or formulation as incorrect.

The three passages below show that syntactic modification sometimes takes precedence over rote memorization (or vice versa) in unexpected ways. In passage 9 for example, the student makes a successful semantic transfer yet fails to adapt the grammar of the original segment by using 'were' instead of 'are'. He uses exactly the same subject-verb combination as in the reading material yet makes a grammatical error when inserting it into his own sentence. In passage 10 however, the verb form is changed but a grammatical error is made as well. The use of 'will' shows that the student's knowledge of French time clauses interferes with his understanding of the way English time clauses work. In passage 11, the grammatical error is also due to interference with French grammar as the personal pronoun 'il', unlike 'he' in English, is used interchangeably with animate and inanimate nouns.

READING MATERIAL STUDENTS' ESSAYS Rajendra Pachauri, the chair of the IPCC, warned that western society must undergo a radical value shift if the worst effects of climate change were to be avoided.

9:

Eventually, if the worst effects of the crisis were G8 to be avoided, the state should staunchly try to change things for the better in the economic domain V5 . It is the central government that digs in its heels, and the louder the outcry grows, the worse the situation becomes.

10: The louder the outcry will G8 grow, the faster this agreement will G8 occur V2 .

A year later, Obama has done none of these things. "Yes he can," proclaims the cover of Newsweek, "(but he sure hasn't yet)".

11:

In this economic plight V2 , we could say that the state can, but he G4 sure hasn't yet V2 .

It is difficult to decide whether failure to use the present perfect in passage 12 below shows that active reading doesn't lead to students improving their understanding of English grammar by identifying (recurring) language patterns. Successful active reading would have arguably resulted in the learner using the present perfect as in the first clause and not the simple past. But although the tenses used in the two independent clauses don't concord, the tenses used in the second one ('did' and 'had vested') show that the student masters at least part of the English tense system. The error identified in passage 13 is questionable as the use of either the present perfect (which the rater seems to expect here) or the simple past is acceptable, depending on the point of view adopted.

Passage 14 shows that the student has successfully assimilated the syntax of the original expression and is capable of operating the necessary grammatical changes. The error identified is debatable.

READING MATERIAL

STUDENTS' ESSAYS Nothing the government has done across the last 12 months has inspired much public confidence.

12: Nothing the government has done recently has inspired much public confidence, as it did G8 not match the expectations people had vested in it. Mr Obama did little to dispel the idea that he could work miracles. At home, he seldom opens his mouth without bashing his opponents or his predecessor.

13: A good case in point is Obama who used to make people dream and d i d G8 little to dispel the idea that he could work miracles.

So far Germany is trying to have it both ways, and succeeding only in getting everyone deeper into the mire.

14: Indeed, the crisis is going to get G8 everyone deeper into the mire.

Discussion

The processes involved in assimilating prefabricated English show that the learners obviously memorize various utterances in an attempt to assimilate their meaning and syntax and don't just copy them unthinkingly. The students don't seem to use material that they don't understand and the semantic errors are ascribable to overgeneralization and subsequent misapplication of various expressions and not failure to understand them in the texts where they are initially encountered. The way in which they autonomously unpack various utterances and strive to understand their inherent patterns shows that they are actively engaged in breaking down the language and that they are committed active readers.

Hoey's lexical priming theory (2005) posits that as we acquire vocabulary it becomes loaded with the various contexts in which we repeatedly encounter it. We then expect and replicate these contexts when we read, write, hear, and speak. If the different types of information are tied to contexts and it can never be assumed that the primings that operate in one textual domain (e.g. medicine or the news) will operate in another textual domain (e.g. law or casual conversation), it is all the more difficult for learners to acquire the discourse of the various fields they write about and to gradually stop overgeneralizing. In this experiment, the students show that they are determined and perfectly able to memorize a great deal of grammaticalized lexis but that they haven't sufficiently encountered recurring elements in context to properly assimilate their meaning and syntax. Thus, it makes intuitive sense that autonomous, active reading can effectively help them activate various primings and refine their understanding of English grammar to make the necessary changes on the material that they insert into their own sentences. The software analyses show that after input has been processed, output plays a fundamental role in learning to put elements in the right order. This is consistent with Swain's hypothesis (1995) that the act of producing language (speaking or writing) constitutes, under certain circumstances, part of the process of L2 learning.

EFL students can thus become better users of English if they consciously imitate the right models and move away from a focus on single lexical items towards multi-word sequences: hence, developing personal reading and learning strategies is fundamental. As Holec explains:

The basic methodology for learner training should be that of discovery; the learner should discover, with or without the help of other learners or teachers, the knowledge and the techniques which he needs as he tries find answers to the problems with which he is faced. By proceeding largely by trial error he trains himself progressively (Holec 1980: 42).

Conclusion

If given appropriate training and repeated opportunities to use what they autonomously identify in their reading material, EFL learners can learn to substitute native-like formulations for their own non-nativelike formulations. In the context described here, the students clearly view their reading material as a language-learning tool and a repository of grammaticalized lexis (in all 45 essays, 40 copied sequences of 5.8 words on average occur). But although this experiment shows that autonomous, active reading is a way for learners to break down the language and assimilate some of its features, we now need further longitudinal studies documenting how each learner uses his or her own formulations (i.e. how inherent patterns are transferred to other contexts and used to express various ideas).

Rated essay passages

Successful transfers

Sentence builders READING MATERIAL

STUDENTS' ESSAYS Congress must ensure that the F.D.A. has adequate financing to be able to do this job. Now the state must ensure that the social cri s is w i ll be resolved by a significant help by taxing the richs V5 .

-China and the United States have been discussing human rights issues for so many years that it is baffling that American leaders remain so clueless about the Chinese government's mindset.

-Once the issue is survival, the government is in effect cornered, leaving it no choice but to resort to drastic measures.

This why it is baffling that so many governments are turning a blind eye to their suffering citizens in those G1 particular times of economic crisis. Consequently, both Wellfare State a n d B i g S o c i e t y ' s G5 myths have been debunked, leaving the population no choice but to ensure her G4 economic safetiness V1 herself G4 .

Three of the contenders for the party's nomination have now come out in favor of the torture known as waterboarding.

Only two have said it is illegal, and the rest don't seem to have the backbone to even voice an opinion on the subject.

We should have the backbone to stand up and claim "I'm a self-made-man", and to act for G3 it.

As hard as it is to believe, the Republican candidates for president seem to have learned very little from t h e m o r a l c a l a m i t i e s o f t h e administration of George W. Bush.

As hard as it is to believe, several european conservative countries G9 seem to have learned very little from current G1 debate against gay marriage to protect the traditional model of the nuclear family. A year later, [Obama] has done none of these things, and some of his supporters are starting to grow impatient.

In our open-minded societies V2 , supporters of same-sex marriage are starting to grow impatient. Many believe that big businesses could have a more influential hand in helping those who are not doing so well out of the tech boom.

There still remains G8 some sectors where G4 t he state could have an influential hand in helping citizens.

As with any new reform, adjustments will be necessary. Principals should have the option of evaluating highperforming teachers less frequently than novices or low performers.

Of course, as with any new reform, adjustments will be necessary if there is a legalisation of the G1 same-sex marriage.

The real lesson of 1989 is that nothing is ever settled.

-The real lesson of it is that nothing is ever easy in drawing the limits of V2 government action.

-The real lesson is that nothing is ever easy in political matter G2 and that there is no magic bullet to find the perfect leaders V4 .

Syntactically and semantically whole sequences

READING MATERIAL STUDENTS' ESSAYS That would be higher than in the early 1980s, when the unemployment crisis was seen as just that -a social challenge of paramount importance, a cause for protests and riots.

It's a social challenge of paramount importance for a leader to listen to his population.

One reason why so many of Mr Obama's fans are disappointed is that he promised the impossible and -such is the power of his oratory -got people to believe him.

Unsuccessful transfers: syntactic and semantic errors

Sentence builders READING MATERIAL STUDENTS' ESSAYS -Iran will keep pushing its nuclear program forward.

-We're not sure any mix of sanctions and inducements can wean Tehran of its nuclear ambitions. We are sure that a military attack would be a disaster.

Wall Street will keep pushing its crazy speculative program forward. We're not s u r e a n y m i x o f sanctions and inducements can wean crazy markets of G3 their speculative ambitions. But we are sure that a moralisation of the G1 capitalism would be a big headway V2 if the state makes rules in order to discourage the G1 bad liberalism. When sovereignty is in play, there is no longer a right or wrong side of an issue, just winning or losing.

When elimination G1 of risks is in play, it often seems like G3 there were G8 no longer a right or a wrong side of an issue, just winning or losing V3 . Tennessee has a long way to go i n improving its schools.

To conclude there is still a long way to come V1 for gay people to give a sledgehammer to break the remaining ceiling glass V5 . When it comes to youth unemployment, there is precious little good news.

When it comes to g o ing through V3 hard economic times, each state must be able to protect its citizens from both financial and social trouble. Some investors asked to be rewarded for the extra risk and others, unwilling to start paying for credit research, just walked away. This set off a spiral of falling bond prices, weakening banks and slowing growth.

This would probably set off a spiral of a win-win situation V2 , because the G1 wages would increase, enabling the state to restor S economic growth and to reduce its debt.

When it comes to youth unemployment, there is precious little good news. The number of people aged under 25 and out of work is now just below a millionaround one in five.

On the surface, it may look as though France is not in favor of gay marriage. Nonetheless, w h e n i t c o m e s t o politicians, there is precious little good new as some of them defend it V3. It is the central government that digs in its heels, and the louder the outcry grows, the worse the situation becomes.

The louder the outcry will G8 grow, the faster this agreement will G8 occur V2 .

A year later, Obama has done none of these things […]. "Yes he can," proclaims the cover of Newsweek, "(but he sure hasn't yet)."

In this economic plight V2 , we could say that the state can, but he G 4 s u r e hasn't yet V2.

Rajendra Pachauri, the chair of the Intergovernmental Panel on Climate Change (IPCC), warned that western society must undergo a radical value shift if the worst effects of climate change were to be avoided.

Eventually, if the worst effects of the crisis were G8 to be avoided, the state should staunchly try to change things for the better in the economic domain V5 .

The real lesson of 1989 is that nothing is ever settled.

The real lesson of it is that nothing is ever easy in drawing the limits of V2 government action. But how is this possible in a world where o utsiders feel free to criticize China's human rights record?

In a world where G4 outlook V1 was bleak, he gave people hope of an improvement, what G4 is a kind of idealistic vision. Perhaps Obama can reach a deal with Iran, and set up a cap-and-trade system.

France should undergo a value shift so that the government could G8 reach a deal V1 with gay people. Perhaps Obama will lay down a path towards fixing the budget.

These supporters may think gay marriage will l a y d o w n V1 a p ath towards a global agreement. In this new world, our privacy takes a back seat to the constant drive to make information easier to obtain.

It is striking to see that citizens' concerns have taken a back seat to the need to V2 save banks first.

Syntactically and semantically whole sequences READING MATERIAL STUDENTS' ESSAYS Nothing the government has done across the last 12 months has inspired much public confidence.

Nothing the government has done recently has inspired much public confidence, as it did G8 not match the expectations people had vested in it.

Mr Obama did little to dispel the idea that he could work miracles. At home, […] he seldom opens his mouth without b a s h i n g h is opponent s o r h i s predecessor.

A good case in point is Obama who used to make people dream and d i d G8 little to dispel the idea that he could work miracles.

The real lesson of 1989 is that nothing is ever settled.

N o t h i n g i s e v e r s e t t l e d so P

conservative politicians should staunchly V3 reconsider this issue. So far Germany is trying to have it both ways, and succeeding only in getting everyone deeper into the mire.

Indeed, the crisis is going to get G 8 everyone deeper into the mire.

When he was elected president, Barack Obama made it plain that this was an e v e n t o f s o m e i m p o r t a n c e . H i s supporters, he said, had "put their hands on the arc of history and bent it once more toward the hope of a better day."

Maybe a line needs to be drawn between promising to bend the arc of history toward the hope of a better day V2 , and being realistic as well as optimistic about the recovery.

(

  Source: 'How Google Stole Mighty Microsoft's Power', Time, 28 December 2009)

Table 2 : Successful vs. unsuccessful transfers Total number of memorized formulations occuring in the essays 41

 2

	Successfully transferred formulations	21
	Unsuccesfully transferred formulations	20

Table 3 : Types of errors identified in unsuccessfully transferred formulations

 3

	Total number of errors identified

Appendix A. Detailed raters' evaluations.

1. Error tags used by the raters