Characterization of Communicating Turbulent Grazing Flows Through a Resolved Porous Medium

Porous media are a promising technology to reduce turbulent boundary layer trailing edge noise. However, the fact that the porous material is grazed by turbulent flow on both sides makes its characterization not trivial. This paper describes the modifications resulting from the interaction between the grazing flows through the porous medium, defined as communication. To this end, lattice-Boltzmann simulations of two communicating turbulent channel flows separated by a fully resolved porous medium are carried out. The porous medium is realized as a 75% porous triply periodic minimal surface of type Schwarz’ P. Results are compared against the case with porous medium backed by a solid wall and the smooth wall channel flow. When communication between the two channel flows is allowed, spanwise coherent structures appear that are assimilated to a shear instability at a non-dimensional frequency of Stt = 0.02. Instantaneous flow through the porous medium is observed and is driven by a time-dependent pressure differential between the channels (with a zero mean and 7.8 Pa standard deviation). This leads to a decrease in energy in turbulent scales smaller than 2.5δ and for bulk scaled frequencies greater than Stb = 0.41. These flow modifications are not observed in the non-communicating case, with the wall preventing flow through, where the topology of the fluctuating statistics is similar to the smooth wall case. Finally, the drag is found to increase by over 200% with respect to the non-communicating case and 650% with respect to a smooth turbulent channel flow. The drag increase is found to be driven by the velocity fluctuations impinging on the porous topology. The communication does not fol- low the asymptotic drag relation for the same equivalent roughness, thus entering a different drag regime.