Unsteady force measurement technique in shock tubes

An aerodynamic force measurement technique with an extremely short test time was developed. A signal recovery method based on a frequency domain deconvolution technique was applied on a direct acceleration measurement with a weakly restrained test model. With the technique, an unsteady drag force of an 80 mm diameter sphere model was measured when a planar shock wave of Ms=1.22 passed the sphere in a vertical shock tube of 300x300 mm square cross section. The measured drag force was evaluated with a numerical simulation, the reliability of which was verified with pressure and optical measurements. The evaluation revealed that the present force measurement technique has enough accuracy and time resolution for phenomenon with a duration of a few hundred mus. Additionally, the unsteady force measurement of a sphere revealed that drag force had been negative for approximately 200 mus after the shock wave passed over the sphere. Numerical analysis showed that this negative drag was caused by the high pressure produced by shock waves converging at the lower part of the sphere. (C) 2004 American Institute of Physics.