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

Measurements have been performed to understand the sound source mechanism in turbulent boundary layer flow over two and three-dimensional surface discontinuities whose height is smaller than the incoming boundary layer thickness. The work was performed in two different types of boundary layers: a wall-jet flow and a conventional high Reynolds boundary layer. In the wall-jet flow, measurements of far field sound from two-dimensional forward facing steps, gaps with rounded corners and swept forward facing steps with rounded corners were made. The sound from a forward facing step is shown to exhibit effects of non-compactness. Rounding the step corner results in consistent drop in sound levels but the directivity of the sound field remains unchanged. The sound from gaps is dominated by the forward step component and remains unaffected by rounding of the backward step portion. The sound from swept forward facing steps was found to approximately obey an acoustic sweep independence principle up to a sweep angle of 30 deg when the spanwise inhomogeneity in the flow is accounted for using a simple source distribution model. Sweep independence is also observed for steps with corner rounding radii up to 25% of the step height. The work performed in the high Reynolds number boundary layer included measurements on forward facing steps with rounded corners and a three-dimensional circular embossment with the same height as the forward step. The highest Reynolds number based on discontinuity height achieved in this work was approximately 93,000. The results show that rounding the forward step corner has the same qualitative effect on far field sound as in the wall-jet boundary layer. Quantitatively, for similar boundary layer edge velocity the sound is higher than in the wall-jet flow. The near field measurements show that the separation bubble downstream of the step shrinks as the step corner is rounded while the bubble upstream remains unaffected by it. The unsteady surface force in the lower half of the vertical face of the step was found to be independent of corner rounding. The force on the downstream surface shows similar character within the separation bubble for each rounding but decays faster with increasing downstream distance due to reduced bubble size. The unsteady force measurements were applied to the theory of Glegg et al. (2014) and the resultant of the unsteady forces on the vertical face and downstream surface placed at the top corner of the step is shown to qualitatively describe the far field sound. The acoustic sweep independence principle was applied to the far field sound from the circular embossment and it has been shown that the sound from the three-dimensional geometry can be predicted with reasonable accuracy using sound from a two-dimensional forward step with the same span. / Ph. D.