Direct numerical simulation of large-eddy-break-up devices in a boundary layer

Turbulent boundary layers create aerodynamic noise inside all vehicles, and especially inside jetliners. The objective of this project is to modify the turbulence in an attached boundary layer, not to reduce skin friction, but to weaken the wall pressure fluctuations, or to shift them to less damaging frequencies and wavelengths. In order to benefit an entire airliner windshield, the effect would be sustained over about 25 boundary-layer thicknesses, δ, which far exceeds the common rule that the relaxation of the turbulence takes about 10δ. Various flow-control devices are studied by Direct Numerical Simulation, which is free of modeling and provides the full details of the pressure field. The Reynolds number is far lower than in the real flow, but this limitation of DNS is tolerable, because the focus is on the larger eddies. The multi-block implicit numerical method can represent fairly complex devices at a manageable cost. Inflow turbulence is provided by a recycling procedure derived from that of Lund, Wu and Squires, but much simpler. It occupies less than 5δ in the streamwise direction. Flow visualizations, Reynolds stresses, and spectra are shown; the baseline spectra are within the experimental scatter. Co-rotating vortex generators are tried first. They reduce the turbulence intensity away from the wall, as hoped, but actually intensify the wall pressure fluctuations and were therefore abandoned. Large-eddy-break-up devices resembling a highway bridge are tried next, and succeed in reducing the wall fluctuations, but only over about 6δ. Thus, the technology is not successful yet for a windshield, but it might be applied to other windows or other vehicles, and the simulation methodology appears to be well developed and of some interest particularly regarding inflow conditions.