$358 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

5084 We, 09:15-09:30 (P29) Influence of wing kinematics on performance in hovering insect f l ight

F.M. Bos 1 , D. Lentink 2, B.W. van Oudheusden 1 , H. Bijl 1 . 1Aerospace Engineering, Delft University of Technology, Delft, The Netherlands, 2Experimental Zoology, Wageningen University, Wageningen, The Netherlands

We present Navier-Stokes simulations of flow around a hovering insect wing. Our interest is twofold: (1) to thoroughly assess the accuracy of our numerical model for these challenging type of applications, and (2) to further unravel the unsteady aerodynamics of flapping insect flight. As numerous studies have appeared on the hovering fruit fly, Drosophila Melanogaster, this was selected as a test case. However, all studies, both experimental and computational, use simplified wing kinematic models, ranging from pure harmonic motion [1] to the symmetrised and less detailed version of the actual flapping motion used in the Robofly experiments [2] at CalTech. As aerodynamic performance strongly depends on vortex dynamics, and as vortex dynamics is tightly related to the wing kinematics, these simplifications can have a strong effect on performance. Since realistic wing kinematics of a hovering fruit fly were available to us, we decided to focus on the influence of different wing kinematic models on the performance. For this, three simplified models are compared with the actual hovering fruit fly wing kinematics (which to our knowledge, has not been simulated yet). To solve the flow for these different kinematic models a finite volume code is used which solves the time-dependent Navier-Stokes equations. To implement the wing motion the mesh moves according to the ALE [3] implementation. The O-type structured mesh is body conformal and fixed to the wing in order to preserve mesh quality. Both translational and rotational cylinder motions were simulated in order to validate our method. For the comparison of the influence of wing kinematic models on performance four different wing kinematic models were chosen. Two of these, pure harmonic motion and the Robofly experimental kinematics have appeared in literature. The third model was chosen to investigate the effect of symmetry, and the last represents the actual fruit fly kinematics. It was found that the actual fruit fly wing kinematics result in forces that differ significantly from those resulting from the simplified wing kinematic models. Different characteristic features, like the angle of attack and the deviation of the stroke plane, are causing respectively higher performance and more balanced forces.

References [1] Z.J. Wang, M.B. Birch, M.H. Dickinson. Unsteady forces and flows in low

Reynolds number hovering flight: Two-dimensional computations vs. robotic wing experiments. The Journal of Experimental Biology 2004; 207: 461-474.

[2] M.H. Dickinson, F. Lehmann, S.P. Sane. Wing rotation and the aerodynamic basis of insect flight. Science 1999; 284: 1954-1960.

[3] J.H. Ferziger, M. Peric. Computational Methods for Fluid Dynamics, 3rd edn. 2002, Springer, Berlin.

6806 We, 09:30-09:45 (P29) Propulsive efficiency of 2D f lexible f lapping foi ls

O. Boiron 1, J. Polansky 1, J. Pokorn~ 2. IlRPHE UMR 6594, Equipe de Biom#canique, Marseille, France, 2 University of West Bohemia, Pilsen, Czech Republic

The carangiform and thunniform types of swimming use for a propulsion further confined tail part of the body. These methods are known as the most efficient mode of swimming evolved in the aquatic environment. Thrust is produced mainly by the circulation induced by the periodic shedding of large vortex (known as the reverse Karman street) at the tail of the fin and is under the influence of several non-dimensional parameters like Strouhal number, heave to chord ratio, maximum angle of attack, etc. Several studies [1,2] are devoted to the determination of the optimal set of parameters for rigid foils and showed as one of the major result that, like a number of fish and marine cetaceans species, the optimal Strouhal number fall in a relatively small range, namely 0 .25<St<0.4. In this numerical study we focus our attention in the performances of 2D flexible flapping foils where the foils are modelized as beams according to the Euler-Bernouilli theory. The influence of flexural rigidity on global efficiency is specially explored and comparisons are made between inviscid and viscous fluids flow. Inviscid flows around flexible foils are solved using unsteady panel method, viscous laminar flows are solved using ALE formulation on unstructured meshes. These methods are applied in order to assess the performances of flexible foils for finswimming application with prescribed motion issue from experi- ments. Finswimming is a new sport activity where leg's propulsive action is fundamental in order to improve the performance.

References [1] GS Triantaphyllou. Optimal thrust development in oscillating foils with application

to fish propulsion.

Oral Presentations

[2] FS Hover et al. Effect of angle of attack profiles in flapping foil propulsion. J. Fluid Struct. 2004; 19: 37-47.

17.5. Terrestrial Locomotion

6144 We, 11:00-11:15 (P32) Is there a proximo-distal gradient of l imb muscle function? A.A. Biewener 1 , E. Yoo 1 , M.P. McGuigan 2 . 1CFS, Harvard University, Bedford, MA, USA, 2Sport and Exercise Science, University of Bath, England

Changes in mechanical work must occur when animals change speed or move over different grades. However, when animals and humans move at steady speeds on the level, certain muscles do little net work, which may increase their force economy and favor elastic energy savings. We hypothesize that Iong-fibered proximal muscles are recruited to modulate work production, whereas short-fibered distal muscles, often with long tendons, exhibit less of a contribution to limb work. To test this hypothesis, we examined the in vivo fascicle strains measured via sonomicrometry in two proximal muscles (biceps femoris, BF, and vastus lateralis, VL) and three distal muscles (medial and lateral gastrocnemius, MG & LG, and superficial digital flexor, SDF) of the goat hindlimb. In vivo muscle strains were examined while goats (N =5) walked and trotted on a treadmill during level, incline (+15 °, 27% grade) and decline (-15 °) conditions. During trotting BF stance phase strains were: (Level) -28.7±4.1%; (Incline) -39.3±6.4%; (Decline) -14.2±5.8%; whereas VL strains were: 2.6±0.2%, -4.1±4.6% & 7.6±6.2% across grades. LG-MG strains averaged: -11.1±3.5%, -22.9±5.5% & -7.2±2.8%, and SDF strains were: -5.4±2.4%, -8.6±3.5% & -4.1±2.3% for level, incline and decline conditions. Patterns of muscle work determined for the LG, MG & SDF from in vivo tendon force recordings matched the strain patterns of these muscles, with changes in work by the LG-MG muscles being greater than the SDF across grades. These results support the role of the proximal BF as contributing most to work modulation, compared with the distal muscles. However, the VL also exhibited low net strains similar to the SDF, suggesting its role to transmit force and stabilize the knee rather than to contribute significant work. All five muscles shifted strain and work (LG, MG & SDF) patterns consistent with grade changes: increasing shortening on 150 grade and reducing shortening or shifting to lengthening on -150 grade, compared with level (NIH AR047679).

6155 We, 11 : 15-11:30 (P32) Linking muscle-tendon dynamics to limb control and running stability M.A. Daley, A.A. Biewener. Concord Field Station, Harvard University, Cambridge MA, USA

We use an integrative experimental approach to study the link between in vivo muscle control, limb function and body mechanics during running in rough versus uniform terrain. This leads to novel insights into how humans and animals control their limbs to move effectively over level ground as well as maintain stability and prevent injury in rough terrain. Running guinea fowl repeatedly encountered obstacle perturbations (5 and 7cm in height) on a treadmill. Analysis of in vivo muscle function (force, length and EMG) during obstacle negotiation reveals that two distal limb muscles, the gastrocnemius and digital flexors, play key roles in the link between limb posture and stabilizing dynamics. These short-fibered, pinnate muscles have long tendons acting across multiple distal limb joints. We hypothesized that their architecture amplifies the link between limb posture, proprioceptive feedback and muscle force-length dynamics. The gastrocnemius exhibits an inverse relationship between relative limb length and muscle fiber length at the time of ground contact (p =0.004), such that on obstacle steps this muscle can be 30% longer at the onset of force than during level running. This also correlates with higher peak EMG intensity, mean muscle force and net work during stance. As a result, the gastrocnemius increases force production by 80% and work output by more than 100% during 7 cm obstacle steps. The digital flexor also exhibits a significant inverse relationship between relative limb length and muscle fiber length (p<0.001). However, this muscle exhibits dynamic changes in recruitment, force, strain and work in response to changes in limb posture. Consequently, this muscle exhibits a damping behavior that depends on how the foot interacts with the substrate. Thus, although these muscles contribute relatively little work for incline running or jumping, they contribute substantial work for rapid obstacle negotiation. Supported by NIH AR047679.

6173 We, 11:30-11:45 (P32) Biomechanics of muscle synergies underlying postural control L.H. Ting. Laboratory for Neuroengineering, W.H. Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, GA USA

Muscle synergies that co-activate multiple muscles across the limb have been identified as a possible neural mechanism that simplifies the functional