Evaluation of free-stream turbulence intensity in the Mars Wind Tunnel under low Reynolds number conditions

It is known that the aerodynamic characteristics of low-Reynolds number airfoils be are susceptible to the free-stream turbulence in a wind tunnel. To control the turbulence intensity of the Mars Wind Tunnel (MWT) at Tohoku University, we built three types of turbulence grids and evaluated their effects on the flow fields at low pressure conditions. A hot-wire anemometer was used to measure velocity fluctuations in the test section for total pressures of 5, 11 and 22 kPa and the free-stream turbulence intensity was evaluated. The frequency analysis showed that the flow conditions were classified into four types depending on total pressure and flow velocity. The flow with no grid was almost laminar and the corresponding turbulence intensity was ranged from 0.5 up to 1.5 %. The effect of the turbulence grid was dependent on the grid type, particularly on a wire diameter. Using relatively thick wires, the turbulence intensity could be raised up to 3.0 %. The flow behind the grids was mostly turbulent except for some transitional conditions. These results indicated that, by using a properly-designed turbulence grid, we could produce relatively large free-stream fluctuations even at low Reynolds number condition.