Scanning stereo-PLIF method for free surface measurements in large 3D domains

Abstract

In this work, we extend a planar laser-induced fluorescence method for free surface measurements to a three-dimensional domain using a stereo-camera system, a scanning light sheet, and a modified self-calibration procedure. The stereo-camera setup enables a versatile measurement domain with self-calibration, improved accuracy, and redundancy (e.g., possibility to overcome occlusions). Fluid properties are not significantly altered by the fluorescent dye, which results in a non-intrusive measurement technique. The technique is validated by determining the free surface of a hydraulic flow over an obstacle and circular waves generated after droplet impact. Free surface waves can be accurately determined over a height of L = 100 mm in a large two-dimensional domain (y(x, z) = 120 × 62 mm 2), with sufficient accuracy to determine small amplitude variations (≈ 0.2 mm). The temporal resolution (t = 19 ms) is only limited by the available scanning equipment (f = 1 kHz rate). For other applications, this domain can be scaled as needed.

Experimental investigation of wave tip variability of impacting waves

“…A higher density ratio results in more well-developed (i.e., larger) liquid filaments. 39,40 Furthermore, the free surface at the wave crest fragments earlier for lower values of the surface tension. 38 The increase in The variability in wave impact pressure is linked to the variation in wave impact location.…”

“…On the other hand, the local flow is significantly altered by the strong gas flow over the wave crest for a gas pocket type wave impact. The local flow can be altered by the surface tension of the gas-liquid interface 38 , the gas-liquid density ratio 39,40 , the compressibility of the gas (i.e., the speed-of-sound) 18 , the possibility of phase change 5, 41,42 , and the aeration of the liquid. 9, 43,44 The scaling of the local flow is not well understood, but especially the formation of ligaments and droplets are thought to be relevant for the variability in wave impact pressures.…”

“…The shear force of the expelled gas is often postulated to result in a Kelvin-Helmholtz type instability. 40,45,53,54 Additionally, the wave tip of the plunging breaking wave is deflected by the strong gas flow prior to the impact on the wall. 35,55 Prior to impact, gas cushioning (i.e., the increase in pressure in front of the wave tip) can also result in deformation of the wave tip.…”

“…49,56,57 The wave tip deflection is shown to depend on the density ratio and the scale of the experiment. 39,40 However, accurate measurements of the wave tip deflec-143 tion have up to now not been reported.…”

“…In the present study, accurate free surface measurements The length scale of the instability depends on the wave 167 shape, density ratio, and the surface tension, which was 168 already suggested in previous work. 38,40,51,52 Addition-169 ally, the study may also provide quantitative data of the 170 wave shape, wave velocity, and wave instability for phys- This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”