Effects of the Opening Speed of the Valve in a Diaphragmless Shock Tube for Metrological Purposes

To develop a measurement system for calibrating high-frequency dynamic pressure that is capable of acting as a primary dynamic pressure calibration standard, a new concept of the shock tube, called the diaphragmless shock tube, has recently been developed. In most such shock tubes, the diaphragm is replaced with a commercially available, ISTA KB fast-opening valve (FOV). In this article, a numerical model was built in OpenFOAM to investigate the effects of the opening mechanism of ISTA KB-40-100 FOV on the formation of the shock wave in a shock tube with an internal radius of 20 mm. Numerical simulations were performed for the driver-driven pressure ratio of 40 and the valve-opening speeds of 5, 10, and 20 m/s, which were compared with the case of instantaneous valve opening, and the results predicted by the ideal shock tube theory. The observed variables were the mass flow through the valve, as well as the pressure, the temperature, and the shock wave velocity in the formation region behind the valve. The results show that when the shock wave undergoes an initial acceleration, the shock front accelerates more at higher opening speeds of the valve. The results also show that the valve-opening mechanism generates a series of reflection waves that propagate into the driven section, giving the shock wave velocity an oscillatory character, which can affect the calibration and measurement capability of the shock tube as a primary high-frequency, time-varying, pressure calibration standard.