The ability to predict and control mixing between two fluids is important in many engineering applications, such as between the fuel and oxidizer in combustion. Mixing two fluids can also generate undesirable noise, for example the noise associated with the exhaust of a jet engine combining with the surrounding air. One area of particular concern is the turbulent mixing that occurs in the shear layer that forms between flows of two different speeds. The mixing governs the transport of fluid properties across that layer and contributes to the noise generated within the shear layer. The mixing characteristics and sound generation change as the difference in speeds between the two flows increases. By understanding the dynamics of the shear layer, the turbulent mixing and sound generation can be better predicted and potentially modified in practical applications. Instantaneous measurements of a quantity such as temperature can provide insight into the turbulent mixing that occurs in the shear layer, as mixing governs the fluctuations in such a quantity. Laser-induced gratings (LIG) is a nonintrusive method for acquiring instantaneous temperature at specific locations within the flow field. This project aims to improve the signal strength of LIG to expand its application to a wider range of flows and to apply this new LIG technique to subsonic and supersonic jet flows so that temperature can be acquired in shear layers for variable difference in speed between the two flows. The specific objectives for this project are to 1) determine the effects of the speed difference on temperature fluctuations in the shear layers, 2) highlight the dependence of mean and fluctuating temperature on flow structure, and 3) provide insight into the thermodynamics that contribute to sound generation and mixing.
The intellectual merit of the research project encompasses an improved knowledge of the turbulent mixing that occurs in high-speed shear layers and an expanded capability of the LIG technique. The effect of compressibility, a measure of the speed difference between the flows, is a problem of practical importance that has been studied for several decades; however, measurements of the fluctuation properties crucial to understanding the problem have remained elusive. As the application of the LIG technique will be broadened to other high-speed flows, temperature can be acquired in other important flows in the future, such as planar shear layers, base flows, and jet injection flows.
The broader impacts of this study include a better understanding of the compressible, turbulent dynamics that govern mixing and sound production. The effects of compressibility on turbulence have been documented, but the underlying physical causes have yet to be fully defined. Improved understanding will be critical to applications such as supersonic fuel injection in scramjet engines, rocket propulsion, and control of noise production in commercial aircraft. Moreover, as an RUI project, the investigation will occur at a small, liberal-arts, undergraduate university. Undergraduate research assistants will be employed during the academic year and over the summers to perform the experiments and present the results, providing invaluable experience for students of many backgrounds. The experimental apparatus will be used in the fluid-mechanics laboratory in the Physics and Engineering Department, providing research and learning experience for the undergraduate engineering majors.
