Calculations of Wind Tunnel Circuit Losses and Speed with Acoustic Foams

The GM Aerodynamics Laboratory (GMAL) was modified in 2001 to reduce the background noise level and provide a semi-anechoic test section for wind noise testing. The walls and ceiling of the test section were lined with acoustic foam and foam-filled turning vanes were installed in the corners. Portions of the wind tunnel circuit were also treated with fiberglass material covered by perforated sheet metal panels. High skin drag due to roughness of the foam surfaces, along with high blockage due to the large turning vanes, increased the wind tunnel circuit losses so that the maximum wind speed in the test section was reduced. The present study calculates the averaged total pressure losses at three locations to evaluate the reductions in skin drag and blockage from proposed modifications to the circuit, which were intended to increase the test section wind speed without compromising noise levels. The effect of foam roughness, characterized by measurement of the boundary layer displacement thickness, was incorporated into CFD models with effective-viscosity and inner-wall-log roughness models. The mathematical correlation of the reduction of total pressure loss with the increase of test section speed is presented to justify the proposed circuit modifications. The projected reduction in the circuit total pressure loss coefficient from 0.54 to 0.44 is in good agreement with the tests.