21 research outputs found
Simulations of propelling and energy harvesting articulated bodies via vortex particle-mesh methods
The emergence and understanding of new design paradigms that exploit flow
induced mechanical instabilities for propulsion or energy harvesting demands
robust and accurate flow structure interaction numerical models. In this
context, we develop a novel two dimensional algorithm that combines a Vortex
Particle-Mesh (VPM) method and a Multi-Body System (MBS) solver for the
simulation of passive and actuated structures in fluids. The hydrodynamic
forces and torques are recovered through an innovative approach which crucially
complements and extends the projection and penalization approach of Coquerelle
et al. and Gazzola et al. The resulting method avoids time consuming
computation of the stresses at the wall to recover the force distribution on
the surface of complex deforming shapes. This feature distinguishes the
proposed approach from other VPM formulations. The methodology was verified
against a number of benchmark results ranging from the sedimentation of a 2D
cylinder to a passive three segmented structure in the wake of a cylinder. We
then showcase the capabilities of this method through the study of an energy
harvesting structure where the stocking process is modeled by the use of
damping elements
Planar potential flow on Cartesian grids
Potential flow has many applications, including the modelling of unsteady
flows in aerodynamics. For these models to work efficiently, it is best to
avoid Biot-Savart interactions between the potential flow elements. This work
presents a grid-based solver for potential flows in two dimensions and its use
in a vortex model for simulations of separated aerodynamic flows. The solver
follows the vortex-in-cell approach and discretizes the
streamfunction-vorticity Poisson equation on a staggered Cartesian grid. The
lattice Green's function is used to efficiently solve the discrete Poisson
equation with unbounded boundary conditions. In this work, we use several key
tools that ensure the method works on arbitrary geometries, with and without
sharp edges. The immersed boundary projection method is used to account for
bodies in the flow and the resulting body forcing Lagrange multiplier is
identified as a discrete version of the bound vortex sheet strength. Sharp
edges are treated by decomposing the body-forcing Lagrange multiplier into a
singular and smooth part. To enforce the Kutta condition, the smooth part can
then be constrained to remove the singularity introduced by the sharp edge. The
resulting constraints and Kelvin's circulation theorem each add Lagrange
multipliers to the overall saddle point system. The accuracy of the solver is
demonstrated in several problems, including a flat plate shedding singular
vortex elements. The method shows excellent agreement with a Biot-Savart method
when comparing the vortex element positions and the force
Conception et mise en oeuvre de méthodes vortex hybrides-frontiÚres immergées pour des milieux solides-fluides-poreux. Application au contrÎle passif d'écoulements.
In this work we use a hybrid vortex penalization method (HVP) to simulate incompressibleflows past bluff bodies in complex solid-fluid-porous media. In this hybrid particle approach,the advection phenomenon is modeled through a vortex method in order to benefit from thenatural description of the flow supplied by particle methods and their low numerical diffusionfeatures. A particle remeshing is performed systematically on an underlying Cartesian grid inorder to prevent distortion phenomena. On the other hand, the viscous and stretching effects aswell as the velocity calculation are discretized on the mesh through Eulerian schemes. Finally,the treatment of boundary conditions is handled with a penalization method that is well suitedfor the treatment of solid-fluid-porous media.The HVP method is applied to passive flow control. This flow control study is realized pasta 2D semi-circular cylinder and a 3D hemisphere by adding a porous layer on the surface of thebody. The presence of such porous layer modifies the characteristics of the conditions at theinterfaces and leads to a regularization of the wake and to a decrease of the aerodynamic dragof the controlled obstacle. Through parametric studies on the permeability, the thickness andthe position of the porous coating, this works aims to identify efficient control devices for flowsaround obstacles like the rear-view mirrors of a ground vehicle.Dans cette thĂšse nous mettons en oeuvre une mĂ©thode vortex hybride pĂ©nalisĂ©e (HVP) afin desimuler des Ă©coulements incompressibles autour de corps non profilĂ©s dans des milieux complexessolides-fluides-poreux. Avec cette approche particulaire hybride, le phĂ©nomĂšne de convection estmodĂ©lisĂ© Ă lâaide dâune mĂ©thode vortex afin de bĂ©nĂ©ficier du caractĂšre peu diffusif et naturel desmĂ©thodes particulaires. Un remaillage des particules est alors rĂ©alisĂ© systĂ©matiquement sur unegrille cartĂ©sienne sous-jacente afin dâĂ©viter les phĂ©nomĂšnes de distorsion. Dâautre part, les effetsdiffusifs et dâĂ©tirement ainsi que le calcul de la vitesse sont traitĂ©s sur la grille cartĂ©sienne, Ă lâaide de schĂ©mas eulĂ©riens. Le traitement des conditions de bords aux parois de lâobstacle esteffectuĂ© Ă lâaide dâune technique de pĂ©nalisation, particuliĂšrement bien adaptĂ©e au traitementde milieux solides-fluides-poreux.Cette mĂ©thode HVP est appliquĂ©e au contrĂŽle passif dâĂ©coulement. Cette Ă©tude de contrĂŽleest effectuĂ©e respectivement en 2D et en 3D autour dâun demi-cylindre et dâun hĂ©misphĂšre parlâajout dâun revĂȘtement poreux Ă la surface de lâobstacle. La prĂ©sence de cette couche poreusemodifiant la nature des conditions aux interfaces, permet de rĂ©gulariser lâĂ©coulement global etde diminuer la traĂźnĂ©e aĂ©rodynamique de lâobstacle contrĂŽlĂ©. A travers des Ă©tudes paramĂ©triquessur la permĂ©abilitĂ©, lâĂ©paisseur et la position du revĂȘtement poreux, ce travail vise Ă identifier desdispositifs de contrĂŽles efficaces pour des Ă©coulements autour dâobstacles comme des rĂ©troviseursautomobiles
C-start: optimal start of larval fish
We investigate the C-start escape response of larval fish by combining flow simulations using remeshed vortex methods with an evolutionary optimization. We test the hypothesis of the optimality of C-start of larval fish by simulations of larval-shaped, two- and three-dimensional self-propelled swimmers. We optimize for the distance travelled by the swimmer during its initial bout, bounding the shape deformation based on the larval mid-line curvature values observed experimentally. The best motions identified within these bounds are in good agreement with in vivo experiments and show that C-starts do indeed maximize escape distances. Furthermore we found that motions with curvatures beyond the ones experimentally observed for larval fish may result in even larger escape distances. We analyse the flow field and find that the effectiveness of the C-start escape relies on the ability of pronounced C-bent body configurations to trap and accelerate large volumes of fluid, which in turn correlates with large accelerations of the swimme
FluSI: A novel parallel simulation tool for flapping insect flight using a Fourier method with volume penalization
FluSI, a fully parallel open source software for pseudo-spectral simulations
of three-dimensional flapping flight in viscous flows, is presented. It is
freely available for non-commercial use under
[https://github.com/pseudospectators/FLUSI]. The computational framework runs
on high performance computers with distributed memory architectures. The
discretization of the three-dimensional incompressible Navier--Stokes equations
is based on a Fourier pseudo-spectral method with adaptive time stepping. The
complex time varying geometry of insects with rigid flapping wings is handled
with the volume penalization method. The modules characterizing the insect
geometry, the flight mechanics and the wing kinematics are described.
Validation tests for different benchmarks illustrate the efficiency and
precision of the approach. Finally, computations of a model insect in the
turbulent regime demonstrate the versatility of the software
An immersed interface method for the 2D vorticity-velocity Navier-Stokes equations with multiple bodies
We present an immersed interface method for the vorticity-velocity form of
the 2D Navier Stokes equations that directly addresses challenges posed by
multiply connected domains, nonconvex obstacles, and the calculation of force
distributions on immersed surfaces. The immersed interface method is
re-interpreted as a polynomial extrapolation of flow quantities and boundary
conditions into the obstacle, reducing its computational and implementation
complexity. In the flow, the vorticity transport equation is discretized using
a conservative finite difference scheme and explicit Runge-Kutta time
integration. The velocity reconstruction problem is transformed to a scalar
Poisson equation that is discretized with conservative finite differences, and
solved using an FFT-accelerated iterative algorithm. The use of conservative
differencing throughout leads to exact enforcement of a discrete Kelvin's
theorem, which provides the key to simulations with multiply connected domains
and outflow boundaries. The method achieves second order spatial accuracy and
third order temporal accuracy, and is validated on a variety of 2D flows in
internal and free-space domains