Short duration force measurements in impulse facilities

A new multiple component stress wave force balance has been designed, calibrated and tested in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR). The balance is able to measure forces of short duration (milliseconds) on instrumented models from angles of attack from -40 to 20 degrees. Two models, a blunt cone 303mm long and a standard force reference model (HB-2) of 70mm diameter, are used to establish the accuracy of the force balance. The blunt cone tests were conducted at two different test conditions with a constant Mach number of 7.8 and total enthalpies of 3.0 and 3.5MJ/kg. At 0 degrees angle of attack and an enthalpy of 3.0MJ/kg, the measured axial coefficient was recovered to within 6% when compared to computational fluid dynamic (CFD) simulations. At –10 degrees, the axial and normal coefficients were within 6% and 9% respectively of CFD predictions while the center of pressure (based on chord length) was within 2%. Tests with the HB-2 standard force reference model were conducted at enthalpies ranging from 12 to 22 MJ/kg at an angle of attack of 0 degrees. A linear variation of the axial coefficient with the viscous similarity parameter was predicted with non-equilibrium CFD simulations assuming a laminar boundary layer. The recovered axial force coefficient remained within 6% of the CFD predictions and compared well with experimental results from other wind tunnel facilities. Reasonable comparison of pressure and heat flux measurements along longitudinal symmetry lines of the model was obtained. The accuracy of the force balance is estimated at approximately 5% for the axial component and 4% for the normal and pitching moment components