Author(s): Janine Vogele; Robert M. Boes; Ismail Albayrak

Linked Author(s): Janine Vögele

Keywords: Two-phase flow; Dynamic pressure; Piezoresistive pressure sensors; Sensor mounting; Spectral analysis

Abstract: Accurate pressure measurements are essential for understanding hydraulic flow conditions and designing hydraulic structures adequately. However, pressure measurements are challenging in dynamic two-phase flow conditions, where air bubbles can interfere with the sensor depending on its mounting characteristics. To this end, we systematically investigated the effects of three different sensor mountings (flush, single recess, and double recess) on dynamic pressure measurements in a low-level outlet laboratory model. The study's overall goal was to find a suitable sensor mounting type to measure dynamic uplift pressures on an invert slab in a laboratory spillway model. An absolute pressure sensor with a range of 750-1350 mbar was tested with all three mountings at distances of x = 3.8 m and x = 16.6 m from the sluice gate of the low-level outlet at ten different flow conditions, with contraction Froude numbers ranging from 12.0 to 19.3. Sensor mounting had the most significant impact in the upstream region, where highly dynamic flow conditions, such as shockwaves and jet impingement on the flume bed, occurred. The flush-mounted sensor with a membrane diameter of 19 mm measured significantly higher mean and fluctuating pressures compared to both recessed sensors. While this effect was also observed in the downstream region, it was less pronounced. Both single and double recessed sensors dampened low-frequency and high-energy large-scale pressure fluctuations in all scenarios because of the smaller opening diameters of 4 and 2 mm, respectively, exposing the sensor membranes less to the highly turbulent flow. Additionally, recessing the sensors introduced resonance peaks in the power spectral density at higher frequencies. These peaks occurred at lower frequencies than expected for a Helmholtz resonator due to air bubbles being trapped within the recess structure. The deeper depth of the double-recessed sensor helped prevent air bubble entrapment, leading to improved measurement accuracy. The effects of pump and vibrational frequencies on the measurements were relevant only in the downstream section where the flow was gradually varied with less impact of shockwaves and pressure fluctuations were less compared to those at the upstream section. The present findings provide a comprehensive assessment of sensor mounting effects under different flow scenarios and measurement locations, highlighting the importance of selecting an appropriate sensor mounting to ensure accurate pressure measurements in complex, two-phase flows. Based on the results, the flush-mounting of sensors is chosen for the final spillway model, as measurements of dynamic pressure fluctuations are the primary focus.

DOI: https://doi.org/10.64697/978-90-835589-7-4_41WC-P1669-cd

Year: 2025