Fluorescence imaging study of free and impinging supersonic jets: Jet structure and turbulent transition

A series of experiments into the behavior of underexpanded jet flows has been conducted at NASA Langley Research Center. This work was conducted in support of the Return to Flight effort following the loss of the Columbia. The tests involved simulating flow through a hypothetical breach in the leading edge of the Space Shuttle Orbiter along its reentry trajectory, with the goal of generating a data set with which other researchers can test and validate computational modeling tools. Two nozzles supplied with high-pressure gas were used to generate axisymmetric underexpanded jets exhausting into a low-pressure chamber. These nozzles had exit Mach numbers of 1 and 2.6. Reynolds numbers based on nozzle exit conditions ranged from about 200 to 35,000, and nozzle exit-to-ambient jet pressure ratios ranged from about 1 to 37. Both free and impinging jets were studied, with impingement distances ranging from 10 to 40 nozzle diameters, and impingement angles of 45??, 60??, and 90??. For the majority of cases, the jet fluid was a mixture of 99.5% nitrogen seeded with 0.5% nitric oxide (NO).;Planar laser-induced fluorescence (PLIF) of NO was used to non-intrusively visualize the flow with a temporal resolution on the order of lets. PLIF images were used to identify and measure the location and size of flow structures. PLIF images were further used to identify unsteady jet behavior in order to quantify the conditions governing the transition to turbulent flow. This dissertation will explain the motivation behind the work, provide details of the laser system and test hardware components, discuss the theoretical aspects of laser-induced fluorescence, give an overview of the spectroscopy of nitric oxide, and summarize the governing fluid mechanical concepts. It will present measurements of the size and location of flow structures, describe the basic mechanisms and origins of unsteady behavior in these flows, and discuss the dependence of such behavior on particular flow structures. Finally, correlations describing the relationship between flow conditions and the degree of flow unsteadiness at a given location along the jet axis will be presented

Experimental Investigation of Supersonic Jets Using Optical Diagnostics

The complexity of many fluid flows and phenomena is a well-known characteristic driven primarily by turbulence, which has been a focal point of study for decades. Most engineering applications in fluids will encounter turbulence, and hence the need to understand how turbulence might influence the problem at hand is omnipresent. In many turbulent flows, there are large-scale coherent structures which directly influence macro-scale processes of engineering relevance, such as noise production. Over decades of study, it has been demonstrated that similar structures are often observed across many flowfields, despite differences in characteristic parameters, and this has led to the pursuit of simplified models through the use of these dominant, shared structures. Large-scale, coherent structures are of particular importance in turbulent jets, as they represent efficient sources of sound. Noise reduction of subsonic and supersonic fluid jets represents a large interest in the study of acoustic production in jets, and much of it is viewed in the context of controlling these large-scale structures. Supersonic jets in particular may emit an intense sound known as jet screech as a consequence of these structures. This noise source easily has the potential to be damaging to both structures and humans in close proximity, and is a particular target of noise reduction efforts. Turbulent flowfields from two supersonic, underexpanded, screeching jets are analyzed by means of three non-intrusive, high-speed, optical diagnostics. The first technique is high-speed schlieren. The second technique is pulse-burst particle image velocimetry (PB-PIV). The third technique is known as focused laser differential interferometry (FLDI). Extensive spectral, statistical, and modal decomposition analyses are used in this work to identify, extract, and characterize the most energetic features and coherent structures associated with jet screech. The large field of view of the image-based datasets is fully taken advantage of by creating spatial maps of spectral and statistical quantities, which highlight regions of increased fluctuations or activity. These are shown to agree with, or demonstrate additional features that could not be reproduced by the modal analyses. Modal analyses are used to evaluate the structure of the most energetic components in the flow of both screeching jets

First results of the french national project "Drive": experimental data for the evaluation of hydrogen risks onboard vehicles, the validation of numerical tools and the edition of guidelines

Hydrogen has been used in many industrial and commercial applications with quite an exemplary safety record. Should it become more extensively used in the transport industry, it is thought that the existing safety procedures will provide only limited guidance for hydrogen­powered vehicles. However, automotive makers will have to ensure that this new technology is as safe as the conventional one and it is therefore important to understand in the early stage of development hydrogen behaviour on­board the vehicle in order to identify the acceptable risks and to control any hazardous risk. Since only a small number of these vehicles are in operation today, data available on safety aspects are quite limited

The Effect of Pulsed Injection on Shear Layer Dynamics in a Scramjet Combustion Chamber

One of the greatest problems that scramjet research faces is fuel air mixing. The residence time for a scramjet engine, or the time it takes for a volume of air to completely pass through the engine, is on the order of 0.1 ms. In that extremely short period of time fuel must be injected and fully mirco-mixed at stoichiometric ratios with the combustion chamber airflow. The fuel-air mixture must then be combusted and expanded through the nozzle to produce thrust. The goal of this research is to develop a new more efficient method of fuel air mixing within a scramjet combustion chamber. A possible way to speed up the mixing process of parallel injection without incurring the total pressure losses that would occur in normal injection is to inject the fuel from the rear side of a backward facing step. Backward facing steps in supersonic flow produce a Prandtl-Meyer expansion fan followed by a shear layer. The instabilities in this shear layer have dominant resonant frequencies. It is believed that if fuel is injected in pulses that impinge on the shear layer at these dominant resonant frequencies that the shear layer will resonate. When the shear layer resonates the vortices that form in the shear layer will grow in magnitude, thus mixing the injected fuel with the air. To test this hypothesis a new test section was designed and built that features a one inch step under which an injector can be housed. This new test section was installed in the supersonic facility at the University of Kansas. Two injectors were also designed that each feature a face plate, one with eight injection ports arranged in a ring and one with 5 injection ports. Between the face plate and a back plate there is a cavity that houses a rotating valve that is powered by a pneumatic motor. Five valves were built: one with 8 teeth, one with 16 teeth, one with 5 teeth that are the same size as the gaps between the teeth, one with 5 teeth where the teeth are 50% larger than the gaps, and one with 5 teeth where the teeth are 50% smaller than the gaps. The 8 tooth valve and 16 tooth valve where used with the 8 port injector face plate. The 5 tooth valves were used with the 5 port injector face plate. As the valve rotates the teeth block and unblock the injection ports injecting carbon dioxide gas into the test section. The 8 port injector was tested over a range of frequencies from 1.6 kHz to 10.0 kHz. The 5 port injector was tested for each valve over a range of frequencies from 1.0 kHz to 4.0 kHz. Static pressure data was taken along the upper and lower walls of the test section by means of an array of pressure sensors. The pressure data from the test section was compared to results generated using a three dimensional CFD simulation of the test section. Overall the pressure data on the lower wall agreed reasonably well with the CFD simulation. The vorticity and turbulence contours generated by the STAR-CCM+ simulation suggest that as a pulse is injected into the test section from the step it causes the shear layer to curve outward near the point of injection. After the pulse the shear layer returns to the state it was in before injection. The shear layer showed no resonance behavior as a result of pulsed injection. A spectral analysis was performed on the wall static pressure data. The results of this analysis showed no indication of resonance behavior of the shear layer in the wind tunnel tests