VirtuaSchlieren: A hybrid GPU/CPU-based schlieren simulator for ideal and non-ideal compressible-fluid flows

A schlieren post-processing tool for CFD simulations of ideal and non-ideal compressible-fluid flows is presented. The software VirtuaSchlieren simulates light propagation across a non-homogeneous medium to predict the schlieren images from an actual measurement apparatus. Trajectories of a large number of light rays-in the order of tens of millions-are reconstructed by numerical integration, from the light source to the screen plane. To this purpose, kd-tree search algorithms are implemented to retrieve the position of each ray within the CFD computational grid at each time step. The local value of the refraction index was retrieved from the interpolated value of the density at the center of each cell. The simple Gladstone-Dale and the Lorentz-Lorenz models are implemented to compute the value of the refractive index. Two search algorithms are evaluated, namely, the approximate nearest neighbor (ANN) and a simplified kd-tree search technique. The latter is implemented on both CPU and GPU architectures. The hybrid GPU/CPU implementation was successfully tested against a reference experimental schlieren image of the supersonic flow around a conical body. Numerical simulations of the supersonic expansion of non-ideal flows are also presented.