Improvements of the accelerometer force measurement technique

Two methods were presented to improve time resolution of direct acceleration measurement technique (accelerometer aerodynamic force balance) in extremely short test duration. The first method is a signal recovery method based on frequency domain de-convolution technique. With the method, an unsteady drag force of an 80mm diameter sphere model was measured when a planar shock wave of Ms=1.22 passed the sphere in a vertical shock tube of 300x300mm 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 method has enough accuracy and time resolution for phenomenon with a duration of a few hundred us. The other method used two accelerometers mounted on large-scale test models. In order to cancel the oscillations associated with insufficient rigidity of test models, sum-averaging of two accelerometers out-put signals was applied. With the method, drag force of a three meter length supersonic duct combustor was measured under free stream velocity 3.5km/s and dynamic pressure 120kPa in a free piston shock tunnel. This method was evaluated with thrust measurement of gaseous hydrogen injection in vacuum. A numerical simulation, which was validated with pressure measurement was also performed to evaluate the force measurement result under test flow condition. The evaluation with numerical simulation revealed that the measurement agreed with numerical value in accuracy of 5%. Since, the averaging time is 350 μs, the time resolution of the technique was probed to more than 2.88kHz.