Planar potential flow on Cartesian grids

Abstract

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