ALBERT

Notes: Conclusions The presented principle of a sting balance applying semiconductor strain gauges bears a number of advantages in comparison with classical strain gauge balances. Simultaneous, highly time resolved measurement of all six aerodynamic force components is possible. The manufacturing is comparatively easy, a milling machine is sufficient to achieve reasonable accuracy. The balance needs only little space, which makes it suitable for small and especially for short models. The disadvantages of semiconductor strain gauges like zero drift and temperature effects do not play a significant role during the test time of five milliseconds. However, they have to be taken into account during the long lasting calibration process. The attachment of the model to the cross is critical with regard to preloadings and hysteresis. Therefore a connecting part is used between balance and model, which should not be removed once the balance is calibrated. The model should be attached to the balance so that the centre of mass is located at the origin of the coordinate system shown in Fig. 2. Then, the moment of inertia with respect to the balance fixed coordinate system is minimal, whereas the natural frequencies of the rotating degrees of freedom are maximized. If the flow over a model is studied the coordinate system only has to be rotated to obtain the aerodynamic forces. Furthermore, gauging and wiring of the strain gauges is uncomplicated, all locations of the gauges are easy to reach. As the construction is compact and of high stiffness it may withstand high loads. The existing balance has a design load of 1,000 N. Strain gauges with a gauge factor of k=130 were used. With an amplification factor of 10,000 even forces below 1 N can be observed, using an appropriate amplifier (e.g. “2311 Signal Conditioning Amplifier”, Vishay Measurements Group). The accuracy of the measurement mainly depends on the efforts invested in the calibration and data processing. With a suitable algorithm as proposed by Schnabl (1987), an accuracy in the order of one percent should be possible. So far the balance has not been applied for measurements on a model, but a number of tests is being made to investigate the natural frequencies and short time response. The results of these tests will lead to the final geometry of the balance and its attachment to the model. It is hoped to have the next prototype manufactured, gauged and calibrated by the end of this year. First results of the investigation of a model with the balance might be presented in 1990.

Notes: Abstract A simple nanosecond spark source (Nanospark) has been constructed from several small ceramic capacitors, one thyratron and one coaxial cable. During operation, a 500 pF capacitor in the spark head is pulsecharged in about 70 ns with automatic preionization from a high-voltage pulse generator switched by a thyratron. This charge is available for discharge when the gas breakdown starts. The preionization is useful to start the gas breakdown easily and to decrease the pulse to pulse jitter. This Nanospark produces 12–15 ns (FWHM) light pulses with a time jitter of less than 60 ns when operated at 8–11 kV and at air of atmospheric pressure. The initiation of the spark discharge is highly reliable. The spark has adequately exposed polaroid film (ASA 3000).

Notes: Abstract An experimental study was made of the stability of converging cylindrical shock waves. The experiments were conducted on annular shock tubes equipped with a double exposure holographic interferometer in the Stoßwellenlabor, RWTH Aachen, and in the Institute of High Speed Mechanics, Tohoku University, Sendai, for shock Mach numbers of 1.1 to 2.1 in air. By comparing these two different shock tube experiments, it is found that in the former facility the mode-three instability is predominant at the center of convergence, whereas the mode-four instability appears in the latter setup. The instabilities are denoted in this way because the shock and the flow field behind it reveal a remarkable triangular and quadrangular symmetry, respectively. It is concluded that the converging cylindrical shock wave is always unstable and sensitive to the structure of the annular shock tube. Usefulness of holographic interferometry to this kind of shock wave research is also demonstrated.