The immersed boundary method proposed by Pinelli et al. (2010) has been implemented as a new object oriented library within the open source CFD solver OpenFOAM for incompressible bluff body fluid flows. The method encompasses the presence of fixed and moving solid obstacles in a computational mesh, without conforming to their boundaries. Standard Cartesian meshes are employed(uniform or stretched), which allows to use efficient and accurate flow solvers. The immersed obstacles are defined using a body force added on the conservation equations, and evaluated on Lagrangian markers that can move over the Eulerian mesh to capture the motion or the deformation of the body.
The integration of the method in the finite-volume formalism and the succesfull integration of the method into the PISO algorithm will be detailed and a careful verification will be provided using a manufactured solution. The efficiency and the accuracy of the algorithm has been studied on various 2D and 3D simulations of flows around fixed and moving cylinder , including careful comparisons with available numerical and experimental results of the literature. Analysis of the computational cost, numerical behavior and accuracy of the numerical method show that the global properties of the OpenFOAM solver are not alterated. A quasi-linear scalability with the number of processors (up to 96) is obtained, with a slope slightly lower than the ideal scalability a feature that has been reported already in existing OpenFOAM studies.
Work has been validated at Reynolds numbers in the range Re=30-500 and is in good agreement with reference data reported in the literature. Work is already in progress to extend the algorithm to the simulation of fluid structure interaction with induced oscillation and turbulent flows around bluff bodies for which preliminary results are in good agreement with reference data reported in the literature.
Summary Aerodynamic characteristics of various geometries are predicted using a finite element formulation coupled with several numerical techniques to ensure stability and accuracy of the method. First, an edge‐based error estimator and anisotropic mesh adaptation are used to detect automatically all flow features under the constraint of a fixed number of elements, thus controlling the computational cost. A variational multiscale‐stabilized finite element method is used to solve the incompressible Navier‐Stokes equations. Finally, the Spalart‐Allmaras turbulence model is solved using the streamline upwind Petrov‐Galerkin method. This paper is meant to show that the combination of anisotropic unsteady mesh adaptation with stabilized finite element methods provides an adequate framework for solving turbulent flows at high Reynolds numbers. The proposed method was validated on several test cases by confrontation with literature of both numerical and experimental results, in terms of accuracy on the prediction of the drag and lift coefficients as well as their evolution in time for unsteady cases.
The Ahmed body is an academic test-case meant to reproduce a wide range of the flow features encountered in automotive aerodynamics. The present work relies on high resolution Automated Stereo-PIV to clarify the structure and dynamics of the longitudinal vortices at high Reynolds number. The vortex core undergoes a sharp decay of streamwise velocity in the near wake which suggests possible occurrence of vortex breakdown. Confirmation comes from the sign switch of the azimuthal vorticity in the longitudinal plane intersecting the vortex core. The location of the sign switch seems strongly correlated to the modification of streamwise velocity.
