Vortex particle-mesh with combined immersed boundary and mesh refinement techniques : application to bluff-body and wake-vortex flows

The present work joins a global effort in the development of efficient and accurate numerical tools for the simulation of complex fluid flows. More specifically, we focus on unsteady external flows, a fluid dynamics discipline which actually pervades applied sciences and engineering. These are the flow encountered in aircraft or car aerodynamics, wind energy, biological locomotion, etc. In the problems addressed here, we assume that the flow is incompressible and that the inertial forces dominate the viscous stresses: we are dealing with both moderate and high Reynolds number flows. Furthermore, the present work treats the flow equations in a very distinct approach. We use a vortex particle-mesh (VPM) method, which belongs to the broader class of “vortex methods”. Such methods use the vorticity-velocity formulation of the Navier-Stokes equations, rather than the velocity-pressure formulation: the vorticity field is thus the primary variable. The major advantage comes from the compactness of the vorticity field for external flows and wakes. A limited number of particles is thus required to discretize the entire flow. First, an immersed boundary technique has been developed and adapted to capture the flow past arbitrary shape bodies. The motivation is to benefit from the enhanced efficiency provided by the VPM method in terms of computational cost (as shown in previous works). The required Poisson equation is solved using an efficient grid-based solver combined with a parallel fast multipole method (which provides the required boundary conditions on each subdomain). Second, an accurate approach handling hierarchically refined meshes has been developed. We use both grid patches and particles of varying resolution. The originality of this work is the combination of the handling of multiple flow resolutions together with a full 3-D VPM Navier-Stokes solver to compute incompressible flows. This method also benefits from the versatility of the parallel fast multipole method which enables the efficient solution of the Poisson equation and straightforward domain decomposition. We have also demonstrated the potential of the mesh refinement technique on a Large-Eddy Simulation of a turbulent vortex wake rollup at very high Reynolds number.(FSA 3) -- UCL, 201

Fast-Time Modeling of Ground Effects on Wake Vortex Transport and Decay

A fast-time model for wake vortex behavior prediction in ground proximity is presented. This model takes into account the combined effects of ground proximity and wind (both crosswind and headwind components) on the wake vortex transport and decay. It aims to mimic the whole flow using a limited set of vortex particles to model both the primary two-vortex system and the ground-generated secondary vorticity. A redistribution of the vortex particles is also included in the model to limit the number of used particles and hence, guarantee a high computational efficiency. The model is integrated in the deterministic wake vortex model, which is a software that predicts, also in real-time, the temporal evolution of the wake vortices generated by a given aircraft evolving in given ambient meteorological conditions. The new ground effect model capacity is assessed using data from a measurement campaign and from large-eddy simulations. It is found that the model is able to reproduce the wake vortex behavior (both for transport and decay) for the most relevant times required for air traffic management applications

Direct numerical simulation and large-eddy simulation of wake vortices : going from laboratory conditions to flight conditions

This paper aims at presenting DNS and LES as applied to the simulation of vortex wakes: in laboratory conditions (moderate to medium Reynolds numbers) and up to real aircraft conditions (high to very high Reynolds numbers). Only incompressible °ows are considered. DNS and LES are able to capture complex 3-D physics provided one uses high quality numerical methods: methods with negligible numerical dissipation (i.e., methods that conserve energy in absence of viscosity and/or subgrid modelling) and with low dispersion errors (to properly transport complex vortical structures). Methods that can do that are: spectral methods, high order ¯nite di®erence methods, and vortex-in-cell (VIC) methods. As the problems of interest are of large spatial extent and contain vortices with small cores, it is also essential that the methods be e±ciently parallelized. As to LES of wake vortex °ows, this require subgrid scale (SGS) models that are essentially inactive during the gentle, well-resolved, phases of the °ow and within the vortex cores, and that become active only during the complex turbulent phases of the °ow. The recent multiscale models, that act solely on the high wavenumbers modes of the LES, are seen to be most appropriate. We present some illustrative examples of DNS and LES results that were obtained within our group