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Turbulent flow over a rough surface produces sound that radiates outside the near wall region. This noise source is often at a lower level than the noise created by edges and bluff body flows, but for applications with large surface area to perimeter ratios at low Mach number, this noise source can have considerable levels. In the first part of this dissertation, a detailed study is made of the ability of the Glegg & Devenport (2009) scattering theory to predict roughness noise. To this end, comparisons are made with measurements from cuboidal and hemispherical roughness with roughness Reynolds numbers, hu_Ï /ν, ranging from 24 to 197 and roughness height to boundary layer thickness ratios of 5 to 18. Their theory is shown to work very accurately to predict the noise from surfaces with large roughness Reynolds numbers, but for cases with highly inhomogeneous wall pressure fields, differences grow between estimation and measurement. For these surfaces, the absolute levels were underpredicted but the spectral shape of the measurement was correctly determined indicating that the relationship of the radiated noise with the wavenumber wall pressure spectrum and roughness geometry appears to remain relatively unchanged. In the second part of this dissertation, delay and sum beamforming and least-squares analyses were used to examine roughness noise recorded by a 36-sensor linear microphone array. These methods were employed to estimate the variation of source strengths through short fetches of large hemispherical and cuboidal element roughness. The analyses show that the lead rows of the fetches produced the greatest streamwise and spanwise noise radiation. The least-squares analysis confirmed the presence of streamwise and spanwise aligned dipoles emanating from each roughness element as suggested by the LES of Yang & Wang (2011). The least-squares calculated source strengths show that the streamwise aligned dipole is always stronger than that of the spanwise dipole, but the relative magnitude of the difference varies with frequency. / Ph. D.

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Recirculating turbulent flow within a cavity with an inlet wall jet was examined. In steady water flow velocity profiles were constructed with measurements taken with a Laser Doppler Anemometer system mounted on a traversing mechanism. Two test cases were examined Re jet = 1167 and Re jet = 3231 as well as developing wall jet profiles. The results are presented with mean velocity plots and turbulent kinetic energy contours. Comparisons are then made with results obtained using a finite difference computational scheme based on the k - e turbulence model. Good agreement was obtained between the computer code predictions and the experimental data. / Thesis / Master of Engineering (ME)

The aerodynamic flow and fluctuating surface pressure of a plane, turbulent, two-dimensional wall jet flow into still air over smooth and rough surfaces has been investigated in a recently constructed wall jet wind tunnel testing facility. The facility has been shown to produce a wall jet flow with Reynolds numbers based on the momentum thickness, Re<SUB>&delta</SUB> = &deltaU<SUB>m</SUB>/&nu, of between 395 and 1100 and nozzle exit Reynolds numbers, Re<SUB>j</SUB> = U<SUB>m</SUB>b/&nu, of between 16000 and 45000. The wall jet flow properties (&delta, &delta<SUP>*</SUP>, &theta, y<SUB>1/2</SUB>, U<SUB>m</SUB>, u<SUP>*</SUP>, etc.) were measured and characterized over a wide range of initial flow conditions and measurement locations relative to the wall jet source. These flow properties were measured for flow over a smooth flow surface and for flow over roughness patches of finite extent. The patches used in the current study varied in length from 305 mm to 914 mm (between 24 and 72 times the nozzle height, b) and were placed so that the leading edge of the patch was fixed at 1257 mm (x/b = 99) downstream of the wall jet source. These roughness patches were of a random sand grain roughness type and the roughness grain size was varied throughout this experiment. The tests covered roughness Reynolds numbers (k<SUP>+</SUP>) ranging from less than 2 to over 158 (covering the entire range of rough wall flow regimes from hydrodynamically smooth to fully rough). For the wall jet flows over 305 mm long patches of roughness, the displacement and momentum thicknesses were found to vary noticeably with the roughness grain size, but the maximum velocity, mixing layer length scale, y<SUB>/2</SUB>, and the boundary layer thickness were not seen to vary in a consistent, determinable way. Velocity spectra taken at a range of initial flow conditions and at several distinct heights above the flow surface showed a limited scaling dependency on the skin friction velocity near the flow surface. The spectral density of the surface pressure of the wall jet flow, which is not believed to have been previously investigated for smooth or rough surfaces, showed distinct differences with that seen in a conventional boundary layer flow, especially at low frequencies. This difference is believed to be due to the presence of a mixing layer in the wall jet flow. Both the spectral shape and level were heavily affected by the variation in roughness grain size. This effect was most notable in overlap region of the spectrum. Attempts to scale the wall jet surface pressure spectra using outer and inner variables were successful for the smooth wall flows. The scaling of the rough wall jet flow surface pressure proved to be much more difficult, and conventional scaling techniques used for ordinary turbulent boundary layer surface pressure spectra were not able to account for the changes in roughness present during the current study. An empirical scaling scheme was proposed, but was only marginally effective at scaling the rough wall surface pressure. / Ph. D.

Roughness noise can be a significant contributor of sound in low Mach number, high Reynolds number flows. Only a small amount of experimental research has been conducted to analyze roughness noise because of its often low energy levels that are hard to isolate even in a laboratory setting. This study details efforts to scale the roughness noise while independently varying roughness size and edge velocity. Measurements were taken in the Virginia Tech Anechoic Wall Jet Facility for stochastic rough surfaces varying from hydrodynamically smooth to fully rough as well as deterministic rough surfaces including 1mm and 3mm hemispheres and a 2D wavy wall. Inner and outer variable normalizations were applied to recorded far field data in an attempt to find specific driving variables of the roughness noise. Also, a newly formulated derivation that attempts to scale the far field sound from a single point wall pressure measurement was used to collapse the far field noise. From the results, the inner and outer variable scalings were unable to collapse the noise generated by all velocities and roughness sizes. The changing spectral shapes of noise generated by rough surfaces with significantly varying wavenumber spectra make it impossible to scale the produced noise using the proposed inner and outer variable scalings. They use only one a single scaling value for the entire frequency range of each spectrum. The analyzed wall pressure normalization, which is inherently frequency dependent, produces a tight collapse within the uncertainty of the measurements for all rough surfaces studied except the larger hemispherical roughness which had individual elements that dominated the surrounding region of the wall pressure microphone. This indicates that the roughness generated noise is directly proportional to the wall pressure spectrum. The collapsed data displayed a slope of Ï ^2, the expected dipole efficiency factor. This is the clearest confirmation to date that the roughness noise source is of a dipole nature. / Master of Science

The flow of jets in confining enclosures has significant application in many engineering processes. In particular, two jet flows have been studied; the impingement of axisymmetric jets in a confined space and a turbulent inlet wall jet in a confining enclosure. The impingement of axisymmetric jets in a cavity has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. When the flow field was examined under various geometrical and fluid parameters several flow regions were found, depending on the geometrical and fluid parameters. Initially, a steady flow field existed for all arrangements for Red < ~90 but subsequent increments in the fluid velocity caused an oscillating flow field to emerge. The onset of the oscillations and the upper limit of finite oscillations were found to be a function of the nozzle diameter to chamber dimension ratio. Although steady numerical simulations predicted the steady flow field well, steady simulations of the oscillating flow field over-predicted the peak axial velocities. The oscillating flow field is considered to be a class of self-sustaining oscillations where instabilities in the jet shear layer are amplified because of feed back from pressure disturbances in the impingement region. The turbulent wall jet in a cavity has been studied using flow visualization, laser Doppler anemometry (LDA), particle streak velocimetry (PSV) and numerical simulations. Instantaneous PSV measurements agreed well with time averaged LDA measurements. Two dimensional simulations using an algebraic stress turbulence model (ASM) were in better agreement with the experimental data than two and three dimensional simulations using a k - ε turbulence model in the wall jet region. A wall jet growth rate was found to be 54% higher than a wall jet in stagnant surroundings due to the enclosure boundaries. / Thesis / Doctor of Philosophy (PhD)

The existence of small surface discontinuities on a flow surface generate significant pressure fluctuations which can manifest as radiated far field sound and affect the fluctuating near wall pressure field exerted on the flow surface. A significant amount of research has been performed on various step and gap flows; however few have dealt with step heights that are small relative to the incoming boundary layer. Fewer still have been concerned with measuring the effect on the fluctuating wall pressure field or the radiated far field sound from these small surface discontinuities. This study presents the work aimed at scaling the radiated sound from small forward and backward steps, detailing the surface pressure field as a result of these steps, and detailing the far field sound radiated from gap configurations of similar dimension. These measurements were performed in the Virginia Tech Anechoic Wall Jet facility for step heights that ranged from approximately 10% to 100% of the incoming boundary layer height. The results show the influence of step height and boundary layer velocity on the far field sound from forward and backward steps. Very little directivity is seen for either source and the larger step heights considered in this study are shown to not be acoustically compact. A new mixed scaling normalization is proposed for the far field spectra from both types of step, which is shown to reliably collapse the data. Backward steps are shown to be much weaker producers of far field sound than a similarly sized forward step. The implications of this behavior are discussed with respect to the far field sound measured from various gap flows. The fluctuating wall pressure field was measured upstream and downstream of both step configurations. The data shows a slow recovery of the wall pressure field with lasting disturbances up to 100 step heights downstream of the step feature. / Master of Science