A novel wavefront measuring camera for quantitative measurement of density in high-speed gas flows

Abstract

This paper presents the development of a novel wavefront measuring camera capable of detecting both the amplitude and phase of the captured light wave simultaneously. The main objective of the present work is to develop a simple "aim and shoot" camera system for quantitative estimation of density variations in high-speed gas flow fields. The interrogating beam which is a plane wave used here gets distorted by flow induced change in refractive index gradients. Wavefront distortion is quantitatively measured by inspecting the projected pattern through the embedded mask of a modified CMOS image sensor, which samples the incoming wavefront space continuously. Post-processing of the captured images through Fourier- and windowed Fourier transform schemes reveals the change in phase and amplitude of the captured wave. The captured phase of the wavefront is used in an iterative tomography scheme to estimate the density distribution of the flow field. The utility of the developed camera is demonstrated in the quantitative visualization of the high-speed flow fields around test objects subjected to hypersonic flows at Mach numbers 8.89 and 5.82 in hypersonic shock tunnel facility (HST2) and also to visualize the flow field generated at the exit of a convergent-divergent nozzle (Mach number 2.9). It is observed that the recovered quantitative density values from the experiments match well with the results obtained through computational fluid dynamic simulations demonstrating the proficiency of the proposed wavefront measuring camera for high-speed flow diagnostics.

Time-resolved quantitative visualization of complex flow field emitted from an open ended shock tube using a wavefront measuring camera

“…The schematic of the experimental setup is shown in Fig.2. For this quantitative flow visualization, the wavefront measuring camera [36,37] is used to capture the flow field images with spatial resolution of 16μm/pixel and 14-7-15, 20-09-17, 09-02-18 exposure time of 0.8 s  . The camera unit is triggered by the signal derived from one of the pressure transducers of the driven section with the help of a control unit (see Fig.2), which controls the timing of triggering signals used in different repeated experiments.…”

“…It has 31mm inner diameter for both driver and driven sections, of lengths 400mm and 600mm respectively. Quantitative visualization of the flow field is accomplished by using the wavefront measuring camera [36,37] that can measure the phase of the interrogating beam, although its current hardware doesn't support high frame rate recording matching the need to image the dynamic flow field at the exit of the ST. Therefore, images of the flow field is captured in a series of identical experiments (one image at a time) by triggering the camera unit at different time instances covering the first 300μs time of flow evolution after the shock discharge from the ST.…”

“…In this paper, quantitative visualization of shock induced flow field discharged from the open end of a shock tube is presented, in which the discharged flow field is investigated through our recently developed wavefront measuring camera [36,37]. For the set of experiments, a miniaturized, table-top shock tube, also known as Reddy tube [34,35] is used, which shares the similar working principle as by the Stalker tube [33].…”

“…During experiments, mentioned ST is driven manually to increase the pressure of the driver side by using a piston arrangement until it breaks open the primary diaphragm creating planer shock wave, which undergoes diffraction on release through the exit of the ST to the ambient. The diffracted shock at different instant of time is investigated quantitatively by our quantitative wavefront measuring camera set-up [36,37] by capturing conjugative images of the flow field with a spatial resolution of 16μm/pixel and temporal resolution of 50 s  for the first 300 s  of flow evolution. Captured images are processed to recover density distribution of the flow field, which are further analyzed and compared with the one obtained through computational fluid dynamic (CFD) simulation.…”

“…The quantitative flow field measurements described in this manuscript is based on the home built quantitative wavefront measuring camera [36,37].The camera is built around a modified CMOS image sensor embedded with a semi-transparent mask positioned facing the sensor in close proximity, which provides the ability to detect the direction of the incident light wavefront. Under geometrical optics, the casted shadow of the mask on the sensor plane is the eikonal approximation of the captured light wavefront through the mask pattern and thus any variation of the captured wavefront would displace the casted pattern from its original position when illuminated by a plane wave.…”