DNS of momentum and heat transfer over rough surfaces based on realistic combustion chamber deposit geometries

Direct Numerical Simulation (DNS) is carried out to investigate flow and heat transfer (passive scalar) over rough surfaces extracted from microscopy of combustion chamber deposits (CCD) in a direct injection test engine. The simulation set-up is a channel with one rough and one smooth wall. The roughness characteristic height, defined as maximum peak-to-valley height averaged over several subsets of the surface, in viscous units (k+) is varied from 11 to 46 in the simulations. Based on the simulation results, it is estimated that the equivalent sand roughness size of the CCD is approximately 300 µm , which is more than two times larger than the physical height of the roughness features. It is also demonstrated that the ratio of Stanton number to friction coefficient on a rough wall is smaller than that on a smooth wall at the same Reynolds number, and it decreases with an increase in k+.