Experimental Study of Subcavity in Supersonic Cavity Flow

An experimental and numerical study is conducted on a rectangular open cavity with a length to depth ratio of 2 at Mach number 1.71 by placing a subcavity at different locations. The subcavity at the front wall has already been established as a passive control device experimentally. In addition, it has been observed that it can act as a passive resonator. However, in the current study, it is found that the location of the subcavity and its dimensions play a crucial role in determining the types of oscillations existing inside the cavity. Cavity models with a subcavity length to main cavity length of 0.2 (l/L = 0.20) were investigated by placing the subcavity at the front wall, aft wall, and simultaneously at both front and aft walls. High speed schlieren visualization revealed the presence of different shock features associated with the cavity flow field. Statistical techniques such as fast Fourier transform, spectrogram, coherence, and correlation are employed to analyze the unsteady pressure data. Numerical computations are carried out to validate the experimental results and also to explore the flow physics. The front wall subcavity acts as a passive control device with a maximum reduction of 34.1 dB in the sound pressure level for the most dominant tone, and there is also a notable reduction in the overall sound pressure level by 11.7 dB. In the case of front wall subcavity, the acoustic wave gets inclined as it interacts with the subcavity, thereby displacing the shear layer to form a dome-shaped structure. The aft wall subcavity acts as a passive resonator with distinct fluid-resonant oscillations and the respective modal frequencies differ widely from those predicted using Rossiter’s expression. The shear layer interacts with a recirculation region formed inside the subcavity at the aft wall, thereby mitigating the effect due to direct impingement of the shear layer on the aft wall. The subcavity at both walls acts as a passive suppression device with a reduction of 34.9 dB in the sound pressure level for the most dominant mode and also with a reduction of 14.5 dB in the overall sound pressure level.

Oscillatory characteristics of cavities in supersonic flow

The behavior of oscillation modes in a supersonic flow of a Mach number of 1.58 over a cavity of and was studied using wind-tunnel experiments. Through instantaneous shadowgraph visualization, the leading-edge shock and trailing-edge shock were both found to be at antiphase to one another for the first mode. Simultaneous unsteady pressure measurements were carried out at the front and aft walls of the cavity. From a wavelet analysis, the dominant mode was found to be the second mode, which prevailed for a major duration of the measurement time. Mode switching was observed between the first and second modes of oscillation. The mean velocity of the vortical disturbance convecting downstream and the acoustic disturbance travelling upstream inside the cavity was determined from a cross-correlation analysis. An examination using wavelet coherence revealed the coherence and phase difference existing between the pressure signal at the front and aft walls. The phase difference estimated from the measurements indicated the strong influence of cavity acoustic resonance on the cavity oscillations. However, the phase difference values showed a slight deviation between the acoustic resonance and cavity modes owing to the standoff distance between the trailing-edge shock and the aft wall.

Experiments on flow past cavity and the application of modal decomposition techniques

On shock train interaction with cavity oscillations in a confined supersonic flow

Numerical simulations and experiments were carried out to investigate the unsteady flow in a two-dimensional rectangular open cavity with emphasis on the effects of length-to-depth ratio on the shear-layer development and cavity drag. The cavity was exposed to a supersonic flow of Mach 2. The results of the simulations were compared and analyzed with experimental data. Detailed predictions of flow features and frequencies showed a good agreement with the experiments. Particularly, the results indicated how the cavity flowfield changes when the length-to-depth ratio of the cavity is changed. Below a certain value of length-to-depth ratio, self-sustained periodic oscillations are strong and coherent, and the cavity drag is low. Beyond this value, the oscillations are weak, frequencies are mostly broadband, and the drag is high. This transition was found to occur for a cavity of length-to-depth ratio of 5. The investigation has also explored features of cavity shear-layer development, hitherto unexplained, and these are elucidated.

High performance miniaturized electronic devices require enhanced, compact and reliable thermal management system. As an efficient compact space cooling technique, flow boiling in microchannels has recently gained wide acceptance. However, weak buoyancy effects and microgravity in avionics and numerous space systems operations hinder the performance of flow boiling microchannel thermal management system due to poor bubble departure capacity and unfavorable development of flow regimes. Here we report the flow boiling silicon nanowires (SiNWs) microchannels which can favorably regulate two-phase flow regimes by enhancing explosive boiling, minimizing bubble departure diameter, and smoothing flow regime transition. Extensive experimental investigations along with high speed visualizations are performed. The experiments are performed with the dielectric fluid HFE-7100 in a forced convection loop for wide range of heat and mass fluxes. High speed flow visualizations have been employed at up to 70 k frames per second (fps) to understand the boiling mechanism in terms of bubble dynamics, flow patterns, and flow regime developments for SiNWs microchannels. These studies show that SiNWs reduce intermittent flow regimes (slug/churn), improve rewetting and maintain thin liquid film at wall. Therefore, flow boiling in SiNW microchannels is promising to thermal management owing to its high heat transfer rate with low pressure drop and negligible microgravity sensitivity.

Identification of inducer cavitation instabilities using high-speed visualization

Experimental Investigation of Flow Boiling in Straight and Diverging Rectangular Miniature Channels with High-Speed Flow Visualization

High-speed flow visualisation has played an important role in the investigations conducted at the Stoßwellenlabor of the RWTH Aachen University for many decades. In addition to applying the techniques of high-speed imaging, this laboratory has been actively developing new or enhanced visualisation techniques and approaches such as various schlieren methods or time-resolved Mach-Zehnder interferometry. The investigated high-speed flows are inherently highly transient, with flow Mach numbers ranging from about M = 0.7 to M = 8. The availability of modern high-speed cameras has allowed us to expand the investigations into problems where reduced reproducibility had so far limited the amount of information that could be extracted from a limited number of flow visualisation records. Following a brief historical overview, some examples of recent studies are given, which represent the breadth of applications in which high-speed imaging has been an essential diagnostic tool to uncover the physics of high-speed flows. Applications include the stability of hypersonic corner flows, the establishment of shock wave systems in transonic airfoil flow, and the complexities of the interactions of shock waves with obstacles of various shapes.

In Chap. 2, the flow boiling heat transfer and pressure characteristics of three different expanding microgap heat sinks have been reported. This present chapter aims to understand the parametric effects including heat flux, mass flux, and channel dimension on flow boiling instabilities in expanding microgap visually and experimentally. High-speed flow visualizations from the top of the microgap heat sink are conducted simultaneously along with the experiments. The different flow patterns are presented and discussed to illustrate the bubble characteristics in the flow boiling in expanding microgap heat sinks. The measured flow boiling instability data are presented in this section for three different microgap configurations: straight microgap of depth 200 μm and two expanding microgaps having the same inlet depth 200 μm with gradually increasing exit depth 300 μm and 460 μm. Results are presented for wide range of heat and mass fluxes. Results show that two-phase expanding microgap heat sink has novel potential to mitigate the flow instabilities and flow reversal issues as the generated vapor has room to expand downstream and also due to reducing shear force along the direction of expansion of the test section. Key details of the experimental setup, procedure, test section, and experimental conditions are described in Chap. 2.

An experimental investigation of the flow boiling of dielectric refrigerant R245fa is conducted in microgaps with enhancement features. A silicon microgap of height 200 μm populated with pin fin arrays of diameter 150 μm with spacing 200 μm in both horizontal and vertical directions is examined. For five different test conditions and in a wide range of mass flux from 781 to 5210 kg/m2s, and inlet temperatures in the range of 13–18 °C, average single-phase and two-phase heat transfer coefficients, pressure drop, and exit vapor quality are reported. Three major flow patterns are observed in the pin finned area using high-speed flow visualization at frame rate of 2229 fps: foggy, bubbly, and slug flow. Based on the experimental data, a flow regime map is constructed.

From measurements of hydrodynamics to computation of species transport in falling films

This paper presents preliminary results of an experimental study on the occurrence and development of the upper part-load instability in a reduced-scale Francis turbine. The study includes draft tube pressure measurements, high-speed flow visualization, and particle image velocimetry. Our results reveal that for an operating point within the range of the upper part-load instability (70 to 85 % of the nominal discharge), the vortex rope has a circular cross section in non-cavitating conditions, which is preserved even after the appearance of cavitation within the vortex core. It is only below a certain cavitation number that the vortex cross section turns into an ellipse, which is associated with an abrupt increase in the pressure fluctuations with a distinct peak in the frequency domain. A further decrease in the cavitation number results in a constant decrease in the activated frequency while the amplitude of these oscillations experience a rise followed by a quick drop. Phase-averaged velocity fields show that the occurrence and development of cavitation within the vortex rope result in a more diffused distribution of the angular momentum. The instantaneous velocity fields, on the other hand, reveal that the elliptical vortex has various states with either diffused or concentrated velocity distributions, which makes the use of the averaged velocity field for this point less relevant.

The formation of a plasma discharge on the surface of aerodynamic bodies can be helpful to manipulate flow conditions. Here, plasma is used to mitigate the massive flow separation observed for an airfoil model at a high incidence angle. High-speed flow visualizations are provided to demonstrate the plasma/gas interaction for flow in initially quiescent air and in the presence of a 20 m/s cross-stream velocity. The results reveal that flow reattachment results from a significant modification of the vortex street developing within the free separated shear layer.