Computational Fluid Dynamics Method for the Analysis of the Hydrodynamic Performance in Swimming

Numerical simulations of the flow around a swimmer during the different swimming phases were carried out to understand the drag force. In mechanics of swimming, the reduction of forces, which oppose to the swimmers advancement, plays a very important part in the improvement of the performances. As a consequence, the performance improvement requires a better understanding of the structure of the fluid flow around swimmers and a good knowledge of the pressure fields and wall shear stress encountered to minimize them. This chapter will focus on computational fluid dynamics (CFD) procedures and results for this practical implication in swimming and will aim at highlighting details on numerical schemes, validations, and results showing the way CFD can be used as a powerful tool in swimming understanding

Mass Transfer

This book covers a wide variety of topics related to advancements in different stages of mass transfer modelling processes. Its purpose is to create a platform for the exchange of recent observations, experiences, and achievements. It is recommended for those in the chemical, biotechnological, pharmaceutical, and nanotechnology industries as well as for students of natural sciences, technical, environmental and employees in companies which manufacture machines for the above-mentioned industries. This work can also be a useful source for researchers and engineers dealing with mass transfer and related issues

Swimming Ability of the Enigmatic Carboniferous Fish: Tullimonstrum Gregarium

Tullimonstrum gregarium, more commonly known as the Tully Monster, is one of the strangest creatures in the fossil record. While it was traditionally considered a problematic fossil, recent studies have firmly placed the Tully Monster with the vertebrates as a relative of lamprey and hagfish. This may offer insight into the Tully Monster’s ecology, but the Tully Monster’s Swimming ability remains uncertain due to its strange body plan. This study aims to investigate the hydrodynamics of these features to gain insight into the Tully Monster’s swimming ability using computational fluid dynamics (CFD). 3D and 2D simulations of the Tully Monster revealed that the eyebar and proboscis are likely key hydrodynamic features, and that the tail fin complex could have generated pressure differentials. Pressures generated around the body also suggest the Tully Monster was a slow swimmer, and likely had a hydrodynamic tendency to descend in the water column

Geometry of unsteady fluid transport during fluid–structure interactions

We describe the application of tools from dynamical systems to define and quantify the unsteady fluid transport that occurs during fluid–structure interactions and in unsteady recirculating flows. The properties of Lagrangian coherent structures (LCS) are used to enable analysis of flows with arbitrary time-dependence, thereby extending previous analytical results for steady and time-periodic flows. The LCS kinematics are used to formulate a unique, physically motivated definition for fluid exchange surfaces and transport lobes in the flow. The methods are applied to numerical simulations of two-dimensional flow past a circular cylinder at a Reynolds number of 200; and to measurements of a freely swimming organism, the Aurelia aurita jellyfish. The former flow provides a canonical system in which to compare the present geometrical analysis with classical, Eulerian (e.g. vortex shedding) perspectives of fluid–structure interactions. The latter flow is used to deduce the physical coupling that exists between mass and momentum transport during self-propulsion. In both cases, the present methods reveal a well-defined, unsteady recirculation zone that is not apparent in the corresponding velocity or vorticity fields. Transport rates between the ambient flow and the recirculation zone are computed for both flows. Comparison of fluid transport geometry for the cylinder crossflow and the self-propelled swimmer within the context of existing theory for two-dimensional lobe dynamics enables qualitative localization of flow three-dimensionality based on the planar measurements. Benefits and limitations of the implemented methods are discussed, and some potential applications for flow control, unsteady propulsion, and biological fluid dynamics are proposed

Finite-Volume Filtering in Large-Eddy Simulations Using a Minimum-Dissipation Model

Large-eddy simulation (LES) seeks to predict the dynamics of the larger eddies in turbulent flow by applying a spatial filter to the Navier-Stokes equations and by modeling the unclosed terms resulting from the convective non-linearity. Thus the (explicit) calculation of all small-scale turbulence can be avoided. This paper is about LES-models that truncate the small scales of motion for which numerical resolution is not available by making sure that they do not get energy from the larger, resolved, eddies. To identify the resolved eddies, we apply Schumann’s filter to the (incompressible) Navier-Stokes equations, that is the turbulent velocity field is filtered as in a finite-volume method. The spatial discretization effectively act as a filter; hence we define the resolved eddies for a finite-volume discretization. The interpolation rule for approximating the convective flux through the faces of the finite volumes determines the smallest resolved length scale δ. The resolved length δ is twice as large as the grid spacing h for an usual interpolation rule. Thus, the resolved scales are defined with the help of box filter having diameter δ= 2 h. The closure model is to be chosen such that the solution of the resulting LES-equations is confined to length scales that have at least the size δ. This condition is worked out with the help of Poincarés inequality to determine the amount of dissipation that is to be generated by the closure model in order to counterbalance the nonlinear production of too small, unresolved scales. The procedure is applied to an eddy-viscosity model using a uniform mesh

Self-propelled rod-like swimmers near surfaces

Self-propelled microswimmers are biological organisms or synthetic objects that propel themselves through the surrounding fluid. Examples are sperm, various swimming bacteria such as Escherichia coli, the green alga Chlamydomonas reinhardtii and artificial bimetallic rods that catalyze chemical reactions in the surrounding hydrogen peroxide. Even though these swimmers differ in their size and driving mechanism, they can be classified as having pusher or puller polarity, which means that they are driven from the rear or the front, respectively. To study the differences in the dynamics of swimmers of different polarity, we develop a general model of rod-like swimmers and perform simulations in three dimensions, employing a particle-based mesoscopic simulation technique (multi-particle collision dynamics) for the hydrodynamic interactions. In the center of our interest are the interactions of swimmers with walls and with each other at higher densities. In the dilute case, we find that all polarities (pusher, puller and neutral) show surface adhesion, the strongest in the pusher case. For pushers, this adhesion originates from sterical alignment with the wall and hydrodynamic attraction towards the wall, making them swim closest to the wall. For pullers, we show that they swim at a slightly larger distance from the wall than pushers, and that they are inclined towards the wall by a hydrodynamic repulsion of their middle part, which also leads to strong surface adhesion. We also measure the attractive force between pusher and wall and compare it to the dipole model, which is a commonly used far-field approximation for the flow surrounding polar swimmers. Previous studies of self-propelled swimmers at high density were mostly performed in two dimensions or neglected either hydrodynamics or excluded-volume interactions. Using an efficient parallelization on GPU hardware, we are able to study the collective behavior of rods in three dimensions at various densities and driving forces, taking into account hydrodynamics and excluded-volume interactions. Our findings emphasize the importance of the polarity of swimmers: Neutrally propelled rods interact weakly via hydrodynamics, but display an isotropic-nematic phase transition at lower critical densities than passive rods. Pusher rods align parallel with each other and form medium sized motile clusters that can develop into flow defects such as jets and swirls. The clusters primarily swim close to the surfaces, where the rod concentration is highest. The surface aggregation decreases with increasing rod density. While polar order is apparent at short distances within the clusters, at longer scales the flow defects destroy the order. However, nematic order is found to be slightly positive at a system-wide scale for high-density systems, indicating that clusters can align with each other. The clusters in puller systems are radically different. At low rod densities several small non-motile hedgehog-like clusters are formed at the walls, merging into one giant, system-spanning cluster at high rod densities. These giant clusters usually include a large fraction of all rods in the system. While these are jamming clusters, they are not static but deform slowly. We conclude that the puller clusters are due to aster-like defects, which have been predicted for puller fluids, combined with excluded-volume interactions. A more specific model for sperm swimming is also being investigated. This model has been shown to display surface adhesion in the dilute solution and the capability to cluster and synchronize motion between two sperm. In multi-sperm simulations, we demonstrate the formation of small clusters by straight swimming sperm, but we find the interactions to be too weak for cluster formation among bent sperm. In order to strengthen interactions, we modify the sinusoidal beat pattern such that it displays an increasing amplitude towards the end of the tail. This indeed extends the time of two synchronized sperm swimming together, compared to the previous model

Hydrodynamics of pitching foils: flexibility and ground effects

En termes de propulsió la rigidesa flexural i l'efecte terra en una placa rectangular en piteig pur han estat investigats. Velocimetria per imatges per partícules, mesures de forces i moments amb una cèl·lula de carga de 6 eixos, mesures de velocitat i adquisició d'imatges de la cinemàtica de la placa han estat realitzades per estudiar els patrons de flux i les forces hidrodinàmiques en plaques de diferent flexibilitat. La presència de la paret va millorar la velocitat de creuer fins a un 25% i l'empenta fins a un 45% per angles escombrats de 160 i 240 graus. El mecanisme físic sota aquest efecte és discutit estudiant els camps de vorticitat produïts per l'estela de l'aleta bioinspirada en un rajiforme. Les forces hidrodinàmiques linkejades a les tècniques de visualització, van permetre calcular eficiències i camps de vorticitat promitjats en fase. Aquestes dades van revelar com l'angle escombrat de la placa juga un paper fonamental en la distribució de moment en l'estela d'una placa rígida per incrementar la propulsió. En termes de rigidesa flexural, l'òptima flexibilitat va ser determinada amb una placa semi-flexible amb una eficiència d'un 69% amb un angle d'atac de 72 graus.En términos de propulsión la rigidez flexural y el efecto suelo en una placa rectangular en puro picheo han sido investigados. Velocimetría de imágenes por partículas, medidas de fuerzas y momentos con una célula de carga de 6 ejes, medidas de velocidad y adquisiciones de imágenes de la cinemática de la placa han sido realizadas para estudiar los patrones de flujo y las fuerzas hidrodinámicas en placas con diferentes flexibilidad. La presencia de la pared mejoró la velocidad de crucero hasta en un 25% y el empuje hasta un 45% para ángulos barridos de 160 y 240 grados. El mecanismo físico bajo este efecto es discutido estudiando los campos de vorticidad producidos por la estela de la aleta bioinspirada en un rajiforme. Las fuerzas hidrodinámicas linkadas a las técnicas de visualización, permitieron calcular eficiencias y campos de vorticidad promediados en fase. Estos datos revelaron como el ángulo barrido de la placa juega un papel fundamental en la distribución de momento en la estela de un foil rígido para incrementar la propulsión. En términos de rigidez flexural la óptima flexibilidad fue determinada con la placa semi-flexible con una eficiencia de un 69% con un ángulo de ataque de 72 grados.The roles of the chordwise flexural stiffness and ground effect in a rectangular plate undergoing in pure pitching motion have been investigated. Digital Particle image velocimetry (DPIV), load measurement with a 6-axes balance, measurements of the swimming speed and image acquisition of the kinematics of the foil have been done to study the flow patterns and hydrodynamics forces around the flapping flexible plates. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45% , for swept angles of 160 and 240°. The physical mechanisms underlying of this effect are discussed by studying the vorticity dynamics in the wake of the foil. Experimental data of the hydrodynamic forces and moments allowed to obtain the efficiencies of the flapping propulsion. These load measurements were linked to the wakes of the flapping foils in order to reveal configurations with higher thrust. The momentum distribution in the wake of the foil has allowed the physical explanation for the cases with highest thrust production capacity. In terms of flexural stiffness, the optimum flexibility has been determined with the semi − flexible plate up to 69% of efficiency under a swept angle of 72 degrees for Re = O(10^4) tested in the investigation