Boundary Layer Trips for Low Reynolds Number Wind Tunnel Tests

The generation of a thick fully turbulent boundary layer is investigated in a lowspeed wind tunnel at a nominal zero pressure gradient over the Reynolds number range 0.145×106 · Rex · 0.58×106. The wind tunnel floor natural boundary layer is laminar with thickness ± between 5.76 mm and 8.13 mm. Different tripping devices are tested to trigger transition so to double the boundary layer thickness and provide a fully established turbulent velocity profile. Using a trip wire significantly increases ± but leads to an unsatisfactory velocity profiles. Using a sandpaper strip slightly increases ± but keeps the boundary layer laminar. Using a strip of sharp-edged silicon granules doubles boundary layer thickness that increases up to 20 mm and the mean velocity profiles are a good fit to the logarithmic law of the wall over the outer region of the boundary layer. The spectral decay of turbulent kinetic energy in this outer layer is exponential and close to −5/3, indicating turbulence equilibrium. This work is of practical interest to wind tunnel practitioners for generating equilibrium thick turbulent boundary layers at low Reynolds numbers.