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An overset grid method was used to investigate the interaction between a particle-laden flow and a circular cylinder. The overset grid method is implemented in the Pencil Code, a high-order finite-difference code for compressible flow simulation. High-order summation-by-part operators were used at the cylinder boundary, and both bi-linear Lagrangian and bi-quadratic spline interpolation was used to communicate between the background grid and the body-conformal cylindrical grid. The performance of the overset grid method was assessed to benchmark cases of steady and unsteady flows past a cylinder. For steady flow at low Reynolds number, high-order accuracy was achieved for velocity components. Results for flow in the vortex shedding regime showed good agreement to the literature. The method was also applied to particleladen flow simulations, where spherical point particles were inserted upstream of the cylinder. These inertial particles were convected towards and (possibly) past the cylinder. The simulations reproduced data from the literature at a significantly reduced cost, revealing that the previously published DNS data is less accurate than assumed for particles with very small Stokes numbers.

Two methods for solid body representation in flow simulations available in the Pencil Code are the immersed boundary method and overset grids. These methods are quite different in terms of computational cost, flexibility and numerical accuracy. We present here an investigation of the use of the different methods with the purpose of assessing their strengths and weaknesses. At present, the overset grid method in the Pencil Code can only be used for representing cylinders in the flow. For this task it surpasses the immersed boundary method in yielding highly accurate solutions at moderate computational costs. This is partly due to local grid stretching and a body-conformal grid, and partly due to the possibility of working with local time step restrictions on different grids. The immersed boundary method makes up the lack of computational efficiency with flexibility in regards to application to complex geometries, due to a recent extension of the method that allows our implementation of it to represent arbitrarily shaped objects in the flow.

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This work investigates how a turbulent free-stream (TFS) affects the transition-inwake state of flow past a cylinder. Direct numerical simulations have been performed, where the intensity of a decaying TFS varied from 3.6% to 12.2% (as it reaches the cylinder), with integral scales 0.5, 0.75 and 1.0 times the cylinder diameter for the high intensity case. Distinct effects of the TFS are observed for Reynolds numbers in the lower (Re < 190) and upper (Re ≥ 250) ranges of the transitional regime. For a flow with a laminar free-stream (LFS) the onset of the transition-in-wake is observed at a critical Reynolds number Rec ≈ 190, where the first three-dimensional effects develop in the wake (mode A instabilities). A TFS perturbs the flow, forcing the onset of the transition at a lower Reynolds number and inhibiting a sharply defined critical Reynolds number. Quasi-stable states, where the wake alternates between two-and three-dimensional vortex shedding, are observed for Re < 190. These states are closely related to intermittent vortex dislocations in the wake. In the upper Re part of the transition-in-wake regime, mode B instabilities dominate the wake of the LFS flow. A TFS stimulates the existence of mode A instabilities, resulting in mixed A-B instabilities in the wake for Re ≥ 250. This effect correlates with the turbulence intensity, and is strongest for the high intensity TFS.