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Measurements were made in two-dimensional zero pressure gradient turbulent boundary layers over 5 geometries of three-dimensional densely distributed surface roughness. A 3-velocity component laser Doppler velocimeter was used to measure instantaneous velocities. These measurements permitted an independent estimate of skin friction on the surfaces using a momentum balance approach, and the validity of the von Karman constant for rough walls was tested. Five roughness fetches were evaluated: three sandpaper roughness fetches of varying grit size and two cases of uniformly distributed hemispheres of different spacing. Optical surface profilometry was used to characterize the geometry of the sandgrain surfaces. It was found that the smooth wall von Karman constant can not be assumed for densely distributed rough wall flows in order to determine the skin friction for these flows. This requires an independent measure of skin friction using more than a single boundary layer profile. Near wall flow structure measurements found that the hemispherical elements do not have high TKE or Reynolds shearing stress regions at the trailing edge of elements as had been shown for sparsely spaced cylindrical elements. This is likely due to the sharp trailing corner of the cylindrical elements, as opposed to an effect of spacing. Rather, hemispherical roughness has a periodically occurring high stress and TKE region located between two element centers in the stream-wise direction at a height of approximately 1.5 times the roughness element height. The periodic nature of the near wall flow extends to approximately 4 roughness element heights. The traditional roughness function f(λ) did not correlate well with λ or the modified Λ for the experimental data. However, it was found that the friction coefficient for the current dense roughness cases is a constant 0.004, within the experimental uncertainty. Traditional inner wall scalings, outer wall scalings, and roughness scalings were not able to collapse surface pressure fluctuation spectra for the various rough wall surfaces tested. However, the data do collapse for individual geometries based on Reynolds number. This gives rise to the ability to predict pressure fluctuation spectra at other Reynolds numbers. / Ph. D.

The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10^7. This is further complicated by the high levels of turbulence fluctuations associated with strong winds. On the other hand, numerical and wind tunnel simulations are usually carried out at smaller values of Re. Consequently, the validation of these simulations should only be based on physical phenomena derived with tools capable of their identification. In this work, two physical aspects related to extreme wind loads on low-rise structures are examined. The first includes the statistical properties and prediction of pressure peaks. The second involves the identification of linear and nonlinear relations between pressure peaks and associated velocity fluctuations. The first part of this thesis is concerned with the statistical properties of surface pressure time series and their variations under different incident flow conditions. Various statistical tools, including space-time correlation, conditional sampling, the probability plot and the probability plot correlation coefficient, are used to characterize pressure peaks measured on the top surface of a surface-mounted prism. The results show that the Gamma distribution provides generally the best statistical description for the pressure time series, and that the method of moments is sufficient for determining its parameters. Additionally, the shape parameter of the Gamma distribution can be directly related to the incident flow conditions. As for prediction of pressure peaks, the results show that the probability of non-exceedence can best be derived from the Gumbel distribution. Two approaches for peak prediction, based on analysis of the parent pressure time series and of observed peaks, are presented. The prediction based on the parent time series yields more conservative estimates of the probability of non-exceedence. The second part of this thesis is concerned with determining the linear and nonlinear relations between pressure peaks and the velocity field. Validated by analytical test signals, the wavelet-based analysis is proven to be effective and accurate in detecting intermittent linear and nonlinear relations between the pressure and velocity fluctuations. In particular, intermittent linear and nonlinear velocity pressure relations are observed over the nondimensional frequency range fH/U<0.32. These results provide the basis for flow parameters and characteristics required in the simulation of the wind loads on structures. / Ph. D.

A technique to infer circumferential total pressure distortion intensity found in serpentine inlet airflow was established using wall-pressure fluctuation measurements. This sensing technique was experimentally developed for aircraft with serpentine inlets in a symmetric, level flight condition. The turbulence carried by the secondary flow field that creates the non-uniform total pressure distribution at the compressor fan-face was discovered to be an excellent indicator of the distortion intensity. A basic understanding of the secondary flow field allowed for strategic sensor placement to provide a distortion estimate with a limited number of sensors. The microphone-based distortion estimator was validated through its strong correlation with experimentally determined circumferential total pressure distortion parameter intensities (DPCP). This non-intrusive DPCP estimation technique was then used as a DPCP observer in a distortion feedback control system. Lockheed Martin developed the flow control technique used in this control system, which consisted of jet-type vortex generators that injected secondary flow to counter the natural secondary flow inherent to the serpentine inlet. A proportional-integral-derivative (PID) based control system was designed that achieved a requested 66% reduction in DPCP (from a DPCP of 0.023 down to 0.007) in less than 1 second. This control system was also tested for its ability to maintain a DPCP level of 0.007 during a quick ramp-down and ramp-up engine throttling sequence, which served as a measure of system robustness. The control system allowed only a maximum peak DPCP of 0.009 during the engine ramp-up. The successful demonstrations of this automated distortion control system showed great potential for applying this distortion sensing scheme along with Lockheed Martin's flow control technique to military aircraft with serpentine inlets. A final objective of this research was to broaden the non-intrusive sensing capabilities in the serpentine inlet. It was desired to develop a sensing technique that could identify control efforts that optimized the overall inlet aerodynamic performance with regards to both circumferential distortion intensity DPCP and average pressure recovery PR. This research was conducted with a new serpentine inlet developed by Lockheed Martin having a lower length-to-diameter ratio and two flow control inputs. A cost function based on PR and DPCP was developed to predict the optimal flow control efforts at several Mach numbers. Two wall-mounted microphone signals were developed as non-intrusive inlet performance sensors in response to the two flow control inputs. These two microphone signals then replaced the PR and DPCP metrics in the original cost function, and the new non-intrusive-based cost function yielded extremely similar optimal control efforts. / Ph. D.

Measurements were made on three forward steps with step height to boundary layer ratio of approximately 3.8%, 15% and 60% and Reynolds number based on step height ranging from 6640 to 213,000. The measurements included mean wall pressure, single and 2 point wall pressure fluctuations, single and 2 point velocity fluctuations and, oil flow visualization. Pressure fluctuation measurements were made 5 boundary layer thicknesses upstream of step to 22 boundary layer thickness (or 600 step heights for smallest step size) downstream of the step. The results show that the steps remarkably enhance the wall pressure fluctuations that scale on the step height in the vicinity of the step and far downstream of the step. The decay of wall pressure fluctuations post reattachment is a slow process and elevated levels can be seen as far as 150 step heights downstream for the mid step size. The enhanced pressure fluctuations come from the unsteady reattachment region on top face of the step which was found to be a strong function of flow geometry and flow parameters such as Reynolds number. The 2 point pressure and velocity space-time correlations show a quasi-periodic structure which begins to develop close to the reattachment and grows in intensity and scale further downstream of reattachment and is responsible for the elevated pressure fluctuations downstream of the step. However, the velocity correlations lack in scale reflecting the fact that large scales reflected in pressure are masked by smaller scales that exist within them. / Master of Science

There have been very few studies investigating the rough wall pressure spectra under fully rough flows, which are relevant to many common engineering applications operating within this regime. This investigation uses the Virginia Tech Stability Wind Tunnel to perform experiments on a series of high Reynolds number zero pressure gradient turbulent boundary layers formed over rough walls in an effort to better understand and characterize the behavior of the rough wall pressure spectrum. The boundary layers were fully rough, and the boundary layer height remained sufficiently larger than the height of the roughness elements. Two rough surfaces were tested. One consisted of an array of 1-mm ordered hemispherical elements spaced 5.5-mm apart, and the other contained 3-mm hemispherical elements randomly spaced, but with the same element density as 1/3 of the 1-mm ordered roughness. The wall pressure spectrum and its scaling were then studied in detail, and it was found that the rough wall turbulent pressure spectrum at vehicle relevant conditions is defined by three scaling regions. One of which is a newly discovered high frequency scaling defined by viscosity, but controlled by the friction velocity adjusted to exclude the pressure drag on the roughness elements. Based on these three scaling regions an empirical model describing the wall pressure spectra for hydraulically smooth, traditionally rough, and fully rough flows was explored. Two point wall pressure fluctuations were also analyzed for each surface condition, and it was found that the roughness inhibits the convective velocities within the inner portions of the boundary layer. / Master of Science

Experimental investigations were conducted with and without the presence of the surface-mounted obstacle to quantify its effects on the far wake. The obstacle chosen for this study was a 3:2 elliptical nose NACA 0020 tail wing-body (Rood body), approximately of height equal to the boundary layer thickness at one of the measurement locations of the flow. The experiments were performed by varying the Reynolds number of the flow and manipulating the pressure gradient distributions using a NACA 0012 airfoil placed within the wind tunnel test section. The measurements were acquired utilizing a spanwise traversing boundary layer rake and a point pressure sensing microphone array. The findings reveal that the presence of the obstacle introduces disruptions in the flow, such as vortex and jet regions in the wake. However, the overall flow behavior remains consistent with that of an undisturbed turbulent boundary layer, for varying Reynolds numbers and pressure gradients. Notably, an adverse pressure gradient and lower Reynolds number both accentuate the prominence of the jet and vortex region within the wake, with the trend reversing towards the other end of the spectrum. This behavior is akin to the larger turbulent boundary layer under adverse pressure gradients and lower Reynolds numbers. Furthermore, the presence of obstacles induces an increase in the overall level of the wall pressure spectrum by approximately 2 dB, regardless of the flow condition. Additionally, it leads to a deviation in the slope of the mid-frequency range of the autospectra compared to the smooth wall case. Specifically, the mid-slope frequency of an undisturbed turbulent boundary layer is steeper than that observed in the disturbed wake flow caused by the obstacle. / Master of Science / The interaction between turbulence and aerodynamic surfaces gives rise to wall-pressure fluctuations, which in turn induce structural vibrations and acoustic noise. On surfaces turbulent flows meet, antennae, flaps, and other frequently mounted measuring devices. The flow in their wake is impacted by the coherence of a turbulent boundary layer being disrupted by these impediments mounted on aerodynamic surfaces. They also alter the nature of the pressure fluctuations that are generated on the surface of interest. The far wake of a Rood Body obstacle was studied using a point pressure sensing microphone array and a spanwise traversing boundary layer rake. Experimental measurements were taken for a range of Reynolds numbers and pressure gradient environments at the Virginia Tech Stability Wind Tunnel. Results show that the boundary layer rake measurements resolve the presence of the obstacle wake successfully, by characterizing the wake structures and confirming the presence of jet and vortex regions in the wake of the obstacle. Surface pressure measurements reveal that the presence of the obstacle causes the low-frequency content of the wall pressure to be less dominant than the no obstacle case, while the high-frequency content becomes more dominant in the presence of the obstacle. The presence of obstacles also increases the overall levels of the wall pressure spectrum by approximately 2 dB.

Surface flow noise results from fluid-surface interactions, manifesting as surface vibrations or far-field noise. Decomposing the surface pressure field reveals distinct components, with the sub-convective component being particularly critical due to its coupling with structural modes, inducing vibrations. This component, characterized by wavenumbers lower than convective wavenumbers, is significantly weaker than its convective counterpart, making it difficult to measure and model accurately. Existing studies rely on limited measurements, constrained by instrumentation and facility capabilities, leading to empirical wall pressure models with restricted accuracy and applicability. This study presents the first high-resolution measurements of sub-convective pressure fluctuations, enabling validation of wall pressure spectrum models. A novel measurement approach inspired by acoustic metamaterials was developed, employing sub-resonant cavity sensors that integrate seamlessly into existing geometries. These sensors, leveraging off-the-shelf pressure transducers, operate effectively in grazing flow environments without disturbing the flow. Their dynamic response, determined by geometry, can be optimized for specific flow conditions, offering versatility across applications. To minimize aliasing effects at low wavenumbers, an optimized sensor array with spanwise-elongated geometries was deployed linearly along the flow direction. Wind tunnel experiments across varying Reynolds numbers and pressure gradients provided crucial insights. Long statistical averages ($mathcal{O}(10^6delta/U_e)$) revealed the statistical characteristics of large-scale turbulent motions. Results showed an asymmetric convective ridge about the convective line, a sharp transition into the sub-convective domain, and sub-convective levels 30–35 dB below convective levels. Comparisons with existing models revealed discrepancies, with all models overpredicting measured levels. While the Chase model aligned over certain ranges, deviations highlight the need for improved wall pressure models. This study lays the groundwork for enhanced vibroacoustic analysis and model refinement through innovative measurement techniques. Overall, these measurements provide a refined insight into the nature of sub-convective pressure fluctuations and will aid in the development of more accurate wall pressure models, crucial for fluid-structure interaction analysis. / Doctor of Philosophy / Imagine traveling in a car or flying in a plane, tuning out conversations or music to focus on the background noise. What you'd mostly hear is a whooshing sound, a symphony of the vehicle's HVAC system, engines, and other mechanical components. But there's another significant, often overlooked, contributor to this noise: the fluid flowing around the vehicle. This phenomenon is not limited to cars and planes—it's also true for underwater vehicles. As air or water flows around a vehicle, it interacts with its surface through a thin layer called the boundary layer, whether it's the fuselage of an aircraft or the body of a car. This interaction generates fluctuating pressure forces on the surface, causing the structure to vibrate and produce noise. Unlike sticking your head out of a moving vehicle, which creates its own kind of noise, this source involves a complex interplay between the fluid flow and the structural dynamics of the vehicle. The vibrations generated from this interaction manifest as structural waves that travel much faster than the fluid itself. These waves, characterized by large spatial scales or low wavenumbers, depend on specific pressure fluctuations in the boundary layer to excite them. These particular fluctuations, called sub-convective or low-wavenumber pressure fluctuations, are much weaker—about 10,000 times less intense—than the turbulence carried by the flow. However, their overlap with the structural wave's characteristics allows for coupling, making them a crucial but elusive noise source. Measuring these weak fluctuations is incredibly challenging. Classical techniques often struggle because stronger noises, such as flow self-noise or external disturbances, can easily overwhelm the data. While some progress has been made using spatial filtering methods, these approaches often lack resolution and provide inconsistent results across studies, signaling the need for better techniques. This study introduces an innovative method inspired by acoustic metamaterials to measure these elusive pressure fluctuations with greater precision and reliability. By designing custom sensors based on multi-neck Helmholtz resonators, capable of filtering out unwanted noise, this approach offers a breakthrough in the field. The sensor design, working principle, and testing process under various flow conditions are detailed, providing insights into how flow speed impacts the fluctuations. Comparisons with existing models and measurements validate the findings, and updates to current models are proposed, paving the way for more accurate noise prediction and mitigation strategies in vehicles of all kinds.

As part of the production of certain solid-dosage pharmaceuticals, granulated ingredients are dried in a batch fluidized bed dryer. Currently, the determination of the completion of the drying process is accomplished through measurements of product or outlet air temperatures. No quantitative measurement of hydrodynamic behaviour is employed. Changes in bed hydrodynamics caused by variations in fluidization velocity may lead to increased particle attrition. In addition, excessive desiccation of the granules caused by inaccurate determination of the drying endpoint may lead to an increase in the thermal and mechanical stresses within the granules. The activity of future high-potency or peptide based drug products may be influenced by these effects. Therefore, the quantification of hydrodynamic changes may be a key factor in the tighter control of both fluidization velocity and product moisture, which are critical for maintaining product quality. <p>High-frequency measurements of pressure fluctuations in a batch fluidized bed dryer containing pharmaceutical granulate have been used to provide a global, non-intrusive indication of the hydrodynamic changes occurring throughout the drying process. A chaotic attractor comparison statistical test known as the S-statistic, has been applied to quantify these changes in drying and a related unit operation, fluidized bed granulation. The S-statistic showed a sensitivity to moisture which is not seen with frequency and amplitude analysis. In addition, the S-statistic has been shown to be useful in identifying an undesirable bed state associated with the onset of entrainment in a bed instrumented for the collection of both pressure fluctuation and entrainment data. Thus, the use of the S-statistic analysis of pressure fluctuations may be utilized as a low-cost method for determining product moisture or changes hydrodynamic state during fluidized bed drying. <p>Electrical capacitance tomography (ECT) has also been applied in this study to image the flow structure within a batch fluidized bed used for the drying of pharmaceutical granulate. This represents the first time that ECT has been applied to a bed of wet granulate material. This was accomplished through the use of a novel dynamic correction technique which accounts for the significant reduction in electrical permittivity occurring as moisture is lost during the drying process. The correction has been independently verified using x-ray tomography. <p>Investigation of the ECT images taken in the drying bed indicates centralized bubbling behaviour for approximately the first 5 minutes of drying. This behaviour is a result of the high liquid loading of the particles at high moisture. Between moisture contents of 18-wt% and 10-wt%, the tomograms show an annular pattern of bubbling behaviour with a gradual decrease in the cross-sectional area involved in bubbling behaviour. The dynamic analysis of this voidage data with the S-statistic showed that a statistically significant change occurs during this period near the walls of the vessel, while the centre exhibits less variation in dynamic behaviour. The changes identified by the S-statistic analysis of voidage fluctuations near the wall were similar to those seen in the pressure fluctuation measurements. This indicates that the source of the changes identified by both these measurement techniques is a result of the reduction in the fraction of the bed cross-section involved in bubbling behaviour. At bed moisture contents below 5-wt%, rapid divergence was seen in the S-statistic applied to both ECT and pressure fluctuation measurements. This indicates that a rapid change in dynamics occurs near the end of the drying process. This is possibly caused by the entrainment of fines at this time, or the build-up of electrostatic charge. <p>The use of the complimentary pressure fluctuation and ECT measurement techniques have identified changes occurring as a result of the reduction of moisture during the drying process. Both the localized changes in the voidage fluctuations provided by the ECT imaging and the global changes shown by the pressure fluctuation measurements indicate significant changes in the dynamic behaviour caused by the reduction of moisture during the drying process. These measurement techniques could be utilized to provide an on-line indication of changes in hydrodynamic regime. This information may be invaluable for the future optimization of the batch drying process and accurate determination of the drying endpoint.