Turbulent jet impingement: surface roughness effects on wall-heat transfer

Direct numerical simulation is used to investigate heat transfer and flow characteristics of turbulent jets impinging on smooth and rough plates. The jet issues from a fully-developed turbulent pipe flow at $Re = 5300$ (based on pipe diameter $D$ and mean bulk velocity $U_b$) and impinges normally onto a plate located $2D$ away from the nozzle section. Surface roughness is resolved in the simulations using a body-fitted computational mesh within the framework of a spectral element flow solver. Although surface roughness is typically associated with a global increase of wall-heat transfer with respect to a comparable smooth wall configuration, the present investigation shows that, for the considered configuration, such a conclusion is not necessarily true locally. At the investigated Reynolds number and for the considered surface roughness, the main effect of surface roughness on the wall-heat transfer results in greater fluctuations in the near impingement region. Nevertheless, the mean wall-heat transfer does not vary significantly along the impingement plate compared to the smooth wall case, except in the range of radial distances $D < r < 2D$.