VTechWorks Repository :: Browsing by Author "Glegg, Stewart A. L."

Browsing by Author "Glegg, Stewart A. L."

Now showing 1 - 16 of 16

A Comparison of Health Risk Behaviors Among College Students Enrolled in a Required Personal Health Course vs. Enrolled in an Elective Personal Health Course
Enyeart Smith, Theresa M. (Virginia Tech, 2004-04-07)
Information on the overall health risk behaviors of college students is limited and it is unknown if being enrolled in an elective or a required health course affects behavior change among the students. There are mixed reports on whether or not health education courses affect behavior change. Factors that may affect change are self-efficacy and the constructs that build the Health Belief Model (i.e. perceived susceptibility and perceived barriers). A sample of convenience was gathered for the current study using two universities in the state of Virginia. Virginia Tech students within the sample were enrolled in an elective health course (n = 375) and James Madison University students within the sample were enrolled in a required health course (n = 202). The National College Health Risk Behavior Survey (NCHRBS) and the Self-Efficacy Scale survey were used to gather information on overall health risk behaviors, health behavior changes, and self-efficacy levels of the students. To acquire health behavior change data, the NCHRBS was administered at the beginning of the Fall 2003 semester and again at the end of the semester. The results of the study indicated that, overall, the type of course a student was enrolled in and self-efficacy did not have a significant effect on health behavior change. However, possible trends were identified with alcohol use, tobacco use, and dietary behaviors, indicating that further research should be performed to analyze underlying factors, not analyzed in this study, which may be affecting health risk behaviors.
Experimental Investigation of Bio-inspired Unidirectional Canopies
Nurani Hari, Nandita; Szőke, Máté; Devenport, William J.; Glegg, Stewart A. L.; Priddin, Matthew; Ayton, Lorna J. (2022-02-08)
An analytical approach has been developed to model the rapid term contribution to the unsteady surface pressure fluctuations in wall jet turbulent boundary layer flows. The formulation is based on solving Poisson’s equation for the turbulent wall pressure by integrating the source terms (Kraichnan, 1956). The inputs for the model are obtained from 2D time-resolved Particle Image Velocimetry measurements performed in a wall jet flow. The wall normal turbulence wavenumber two-point cross-spectra is determined using an extension of the von Kármán homogeneous turbulence spectrum. The model is applied to compare and understand the baseline flow in the wall jet and to study the attenuation in surface pressure fluctuations by unidirectional canopies (Gonzales et al, 2019). Different lengthscale formulations are tested and we observe that the wall jet flow boundary layer contributes to the surface pressure fluctuations from two distinct regions. The high frequency spectrum is captured well. However, the low frequency range of the spectrum is not entirely captured. This is because we have used PIV data only up to a height of 2.3𝜹, whereas the largest turbulent lengthscales in the wall jet are on the order of 𝒚𝟏/𝟐≈𝟔𝜹. Using the flow data obtained from PIV and Pitot probe measurements, the model predicts a reduction in the surface pressure due to canopy at low frequencies.
An Experimental Investigation of Unsteady Surface Pressure on Single and Multiple Airfoils
Mish, Patrick Francis (Virginia Tech, 2003-02-06)
This dissertation presents measurements of unsteady surface pressure on airfoils encountering flow disturbances. Analysis of measurements made on an airfoil immersed in turbulence and comparisons with inviscid theory are presented with the goal of determining the effect of angle of attack on an airfoils inviscid response. Unsteady measurements made on the surface of a linear cascade immersed in periodic flow are presented and analyzed to determine the relationship between the blades inviscid response and tip leakage vortex strength. Measurements of fluctuating surface pressure were made on a NACA 0015 airfoil immersed in grid generated turbulence. The airfoil model has a 2' chord and spans the 6' Virginia Tech Stability Wind Tunnel test section. Two grids were used to investigate the effects of turbulence length scale on the surface pressure response. A large grid which produced turbulence with an integral scale 13% of the chord and a smaller grid which produced turbulence with an integral scale 1.3% of the chord. Measurements were performed at angles of attack from 0 to 20. An array of microphones mounted subsurface was used to measure the unsteady surface pressure. The goal of this measurement was to characterize the effects of angle of attack on the inviscid response. Lift spectra calculated from pressure measurements at each angle of attack revealed two distinct interaction regions; for reduced frequencies < 10 a reduction in unsteady lift of up to 7 decibels (dB) occurs while an increase occurs for reduced frequencies > 10 as the angle of attack is increased. The reduction in unsteady lift at low reduced frequencies with increasing angle of attack is a result that has never before been shown either experimentally or theoretically. The source of the reduction in lift spectral level appears to be closely related to the distortion of inflow turbulence based on analysis of surface pressure spanwise correlation length scales. Furthermore, while the distortion of the inflow appears to be critical in this experiment, this effect does not seem to be significant in larger integral scale (relative to the chord) flows based on the previous experimental work of McKeough (1976) suggesting the airfoils size relative to the inflow integral scale is critical in defining how the airfoil will respond under variation of angle of attack. A prediction scheme is developed that correctly accounts for the effects of distortion when the inflow integral scale is small relative to the airfoil chord. This scheme utilizes Rapid Distortion Theory to account for the distortion of the inflow with the distortion field modeled using a circular cylinder. Measurement of the unsteady surface pressure response of a linear cascade in periodic disturbance is presented. Unsteady pressure was measured on the suction and pressure side of two cascade blades with an array of 24 microphones (12 per blade side) mounted subsurface. The periodic disturbance was generated using a pair of vortex generators attached to a moving end wall. Measurements were made for 8 tip gaps (t/c = 0.00825, 0.0165, 0.022, 0.033, 0.045, 0.057, 0.079, 0.129) and phased averaged with respect to the vortex generator pair position. This measurement was motivated by the results presented by Ma (2003). The work of Ma (2003) suggested that tip leakage vortex shedding in the presence of a periodic disturbance is heavily influenced by the inviscid response of the cascade blade. This conclusion was arrived at by Ma's (2003) observation that as the tip gap is increased the amount of fluctuation in the tip leakage vortex circulation increases dramatically, in fact, many times the circulation in the inflow vortices. Unsteady pressure measurements reveal that the blade response involves a complex interaction of both inviscid response and viscous phenomena. However, a close relationship between unsteady tip loading and tip leakage vortex circulation is revealed suggesting the inviscid response is significant in determining the tip leakage vortex circulation. Additionally, predictions using inviscid theory agree well with measured levels of unsteady tip loading. As such, inviscid theory may be useful for predicting the tip leakage circulation and perhaps, pressure fluctuations in the tip leakage vortex.
Flow Field Analysis Around Pressure Shielding Structures
Szőke, Máté; Nurani Hari, Nandita; Devenport, William J.; Glegg, Stewart A. L.; Teschner, Tom-Robin (2021-02-08)
The flow field around a series of streamwise rods, referred to as canopies, is investigated using two-dimensional two-component time-resolved particle image velocimetry (PIV) and large eddy simulations (LES) to characterize the changes in the flow field responsible for reducing the low and high-frequency surface pressure fluctuations previously observed. It was found that an axisymmetric turbulent boundary layer (ATBL) develops over the rods, whose thickness grows at a greater rate above the rods than below. This boundary layer reaches the wall below the rods at a point where previously a saturation was found in the low-frequency noise attenuation, revealing that the ATBL is responsible for the low- frequency noise attenuation. The flow is displaced by the presence of the rods, particularly above them, which offset was primarily caused by the blockage of the ATBL. The flow below the rods exhibits the properties of a turbulent boundary layer as its profile still conforms to the logarithmic layer, but the friction velocity was found to drop. This viscous effect was found to be responsible for the high-frequency noise attenuation reported previously.
Flow Induced Noise from Turbulent Flow over Steps and Gaps
Catlett, Matthew Ryan (Virginia Tech, 2010-05-04)
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.
High Reynolds Number Turbulent Boundary Layer Flow over Small Forward Facing Steps
Awasthi, Manuj (Virginia Tech, 2012-06-15)
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.
Investigating the Aeroacoustic Properties of Kevlar Fabrics
Szőke, Máté; Devenport, William J.; Nurani Hari, Nandita; Alexander, W. Nathan; Glegg, Stewart A. L.; Li, Ang; Vallabh, Rahul; Seyam, Abdel-Fattah M. (2021-02-08)
The aeroacoustic properties of porous fabrics are investigated experimentally in an effort to find a porous fabric as an ideal interface between wind tunnel flow and quiescent conditions. Currently, the commercially available Kevlar type 120 fabric is widely used for similar applications, such as side-walls in hybrid anechoic wind tunnels or as a cover of phased microphone arrays. A total number of 8 fabrics were investigated, namely, four glass fiber fabrics, two plain weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores clogged. Two, custom-made rigs were used to quantify the transmission loss and self-noise of all eight fabrics. It was found that the pores serve as a low-resistance gateway for sound waves to pass through, hence enabling a low transmission loss. The transmission loss was found to increase with decreasing open area ratio while other fabric properties had a minor impact on transmission loss. The self-noise of the fabrics has also been evaluated and it was found that the thread density (thread per inch) is a primary factor of determining the frequency range of self-noise with the open area ratio potentially playing a secondary role in the self-noise levels. For both metrics, the mass per unit area seemed to play a minor role in the aeroacoustic performances of the fabrics. Finally, surface pressure measurements revealed that the commercially available plain Kevlar (type 120) has no quantifiable effect on the hydrodynamic pressure field passing over the fabric, sug- gesting that Kevlar behaves as a no-slip wall from the flow's perspective when no pressure difference is present on the two sides of the fabric.
Large Scale Homogeneous Turbulence and Interactions with a Flat-Plate Cascade
Larssen, Jon Vegard (Virginia Tech, 2005-01-24)
The turbulent flow through a marine propulsor was experimentally modeled using a large cascade configuration with six 33 cm chord flat plates spanning the entire height of the test section in the Virginia Tech Stability Wind Tunnel. Three-component hot-wire velocity measurements were obtained ahead, throughout and behind both an unstaggered and a 35º staggered cascade configuration with blade spacing and onset turbulence integral scales on the order of the chord. This provided a much needed data-set of much larger Taylor Reynolds number than previous related studies and allowed a thorough investigation of the blade-blocking effects of the cascade on the incident turbulent field. In order to generate the large scale turbulence needed for this study, a mechanically rotating "active" grid design was adopted and placed in the contraction of the wind tunnel at a streamwise location sufficient to cancel out the relatively large inherent low frequency anisotropy associated with this type of grid. The resulting turbulent flow is one of the largest Reynolds number (Reλ  1000) homogeneous near-isotropic turbulent flows ever created in a wind tunnel, and provided the opportunity to investigate Reynolds number effects on turbulence parameters, especially relating to inertial range dynamics. Key findings include 1) that the extent of local isotropy is solely determined by the turbulence generator and the size of the wind-tunnel that houses it; and 2) that the turbulence generator operating conditions affect the shape of the equilibrium range at fixed Taylor Reynolds number. The latter finding suggests that grid turbulence is not necessarily self-similar at a given Reynolds number independent of how it was generated. The experimental blade-blocking data was compared to linear cascade theory and showed good qualitative agreement, especially for wavenumbers above the region of influence of the wind tunnel and turbulence generator effects. As predicted, the turbulence is permanently modified by the presence of the cascade after which it remains invariant for a significant downstream distance outside the thin viscous regions. The obtained results support the claim that Rapid Distortion Theory (RDT) is capable of providing reasonable estimates of the flow behind the cascade even though the experimental conditions lie far outside the predicted region of validity.
The Noise of a Boundary Layer Flowing Over Discrete Roughness Elements
Rasnick, Matthew Byron (Virginia Tech, 2010-05-05)
This study focuses on measuring and normalizing the roughness noise of multiple roughness types across numerous layouts and flow speeds. Using the Virginia Tech Anechoic Wall Jet Facility, far field noise was recording for the flow of a turbulent wall jet boundary layer over cubes, hemispheres, and gravel, with element heights in the range of 14.3 - 55.2% of the boundary layer thickness. The sound radiated from the various layouts showed that the elements acted as independent sources when separated by three element diameters center-to-center or more. When the elements were placed shoulder to shoulder, interaction between the elements and shielding of the higher velocity flow lowered the noise per element produced. The far field roughness noise was then normalized using the theory of Glegg et al. (2007), which assumes a dipole efficiency factor. Comparisons were made between the theoretical drag spectrum model proposed by Glegg et al. (1987) and a modified version of this model made using the empirical data gathered. Overall, the theory of Glegg et al. (2007) succeeds greatly in collapsing the data into its non-dimensional drag spectra, but the original model spectrum did not fit well. The modified spectrum showed much greater fit with the data at all layouts and speeds. The collapse of the data using the theory of Glegg et al. (2007) confirms that roughness noise is dipole in nature.
Normalization of Roughness Noise on the Near-Field Wall Pressure Spectrum
Alexander, William Nathan (Virginia Tech, 2009-06-05)
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.
Prediction of Trailing Edge Noise from Two-Point Velocity Correlations
Spitz, Nicolas (Virginia Tech, 2005-05-04)
This thesis presents the implementation and validation of a new methodology developed by Glegg et al. (2004) for solving the trailing edge noise problem. This method is based on the premises that the noise produced by a surface can be computed by the integral of the cross product between the velocity and vorticity fields, of the boundary layer and shed vorticity (Howe (1978)). To extract the source terms, proper orthogonal decomposition is applied to the velocity cross spectrum to extract modes of the unsteady velocity and vorticity. The new formulation of the trailing edge noise problem by Glegg et al. (2004) is attractive because it applies to the high frequencies of interest but does not require an excessive computational effort. Also, the nature of the formulation permits the identification of the modes producing the noise and their associated velocity fluctuations as well as the regions of the boundary layer responsible for the noise production. The source terms were obtained using the direct numerical simulation of a turbulent channel flow by Moser et al. (1998). Two-point velocity and vorticity statistics of this data set were obtained by averaging 41 instantaneous fields. For comparisons purposes, experimental boundary layer data by Adrian et al. (2000) was chosen. Statistical reduction of 50 velocity fields obtained by particle image velocimetry was performed and analysis of the two-point correlation function showed features similar to the DNS data case. Also, proper orthogonal decomposition revealed identical dominant modes and eddy structures in the flow, therefore justifying considering the channel flow as an external boundary layer for noise calculations. Comparison of noise predictions with experimental data from Brooks et al. (1989) showed realistic results with the largest discrepancies, on the order of 5 dB, occurring at the lowest frequencies. The DNS results are least applicable at these frequencies, since these correspond to the longest streamwise lengthscales, which are the most affected by the periodicity conditions used in the DNS and also are the least representative of the turbulence in an external boundary layer flow. Most of the noise was shown to be produced by low-frequency streamwise velocity modes in the bottom 10% of the boundary layer and locations closest to the wall. Only 6 modes were required to obtain noise levels within 1 dB of the total noise. Finally, the method for predicting spatial velocity correlation from Reynolds stress data in wake flows, originally developed by Devenport et al. (1999, 2001) and Devenport and Glegg (2001), was adapted to boundary-layer type flows. This method, using Reynolds stresses and the prescription of a lengthscale to extrapolate the full two-point correlation, was shown to produce best results for a lengthscale prescribed as proportional to the turbulent macroscale. Noise predictions using modeled two-point statistics showed good agreement with the DNS inferred data in all but frequency magnitude, a probable consequence of the modeling of the correlation function in the streamwise direction. Other quantities associated to noise were seen to be similar to the ones obtained using the DNS.
Rotor Inflow Noise Caused by a Boundary Layer: Inflow Measurements and Noise Predictions
Morton, Michael Andrew (Virginia Tech, 2012-08-02)
A rotor immersed in a thick turbulent boundary layer produces unsteady loading on the blades which generates unwanted noise and vibration. Two point velocity fluctuations were measured in detail to determine the full four-dimensional correlation function of a boundary layer generated over a smooth wall in the Virginia Tech Stability Wind Tunnel. The correlation function reveals anisotropy in the flow dominated by a large scale correlation structure elongated in the streamwise direction and inclined relative to the wall. This correlation function was then evaluated in the blade frame of reference of an idealized 10 bladed rotor partially immersed in the flow. Blade to blade upwash coherence shows significant asymmetry which is a direct result of the anisotropy of the flow. Using a newly developed theory, the correlation function was used to predict the far-field radiated noise from the rotor at various operating and flow conditions. Predictions show the sound field is dominated by the effects of "haystacking" which is further increased with the inclusion of the presence of the wall. Directivity predictions suggest the far-field sound field acts like a monopole/dipole combination.
Sound from Rough Wall Boundary Layers
Alexander, William Nathan (Virginia Tech, 2011-09-29)
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 /Î½, 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.
A Study of Sound Generated by a Turbulent Wall Jet Flow Over Rough Surfaces
Grissom, Dustin Leonard (Virginia Tech, 2007-07-10)
The far field acoustics generated by turbulent flow over rough surfaces has been experimentally investigated in an acoustically treated wall jet facility. The facility allows direct measurement of the far field sound from small patches of surface roughness, without contamination from edge or other aerodynamic noise sources. The facility is capable of generating turbulent boundary layer flows with momentum thickness Reynolds numbers between 450 and 1160. The variation of surface conditions tested cover the range from hydrodynamically smooth surfaces through most of the transitional range, with h+ variations from 3 to 85. Single microphone narrow band acoustic spectra, measured in the far field, show sound levels as much as 15 dB above the background from 0.186 m2 roughness patches. The measurements revealed the spectral shape and level variations with flow velocity, boundary layer thickness, and roughness size; providing the first data set large enough to assess the affects of many aerodynamic properties on the acoustic spectra. Increases in the size of grit type roughness produced significant increases in acoustic levels. Patches of hydrodynamically smooth roughness generated measurable acoustic levels, confirming that acoustic scattering is at least one of the physical mechanisms responsible for roughness noise. The shapes of the measured spectra show a strong dependence on the form of the surface roughness. The acoustic spectra generated by periodic two-dimensional surfaces have a much narrower louder peak than that generated by three-dimensional grit type roughness. Measurements also show the orientation of the two-dimensional surface significantly affects the acoustic levels and directivity. The variation of sound levels with flow velocity and roughness size suggests the acoustic field is significantly affected by changes in the near wall flow due to the presence of the roughness. Current models of noise generated by rough surfaces predict the general trends seen in measurements for flows over grit and two-dimensional roughness in the range of 20
Turbulence Measurements in Trailing Vortices for B.W.I. Noise Prediction
Devenport, William J.; Glegg, Stewart A. L.; Sharma, Gautam (Virginia Polytechnic Institute and State University, 1992-06-01)
Blade wake interaction (BWI) noise is the broadband noise generated by the ingestion of turbulent tip vortices into helicopter rotors. Prediction of BWI noise requires knowledge of the turbulence structure of the tip vortex. This report describes a joint investigation to measure that structure and incorporate the results into a noise prediction scheme. Measurements were performed on the tip vortex shed by a single rectangular NACA 0012 half wing placed in a wind tunnel test section. The properties of the vortex were studied for a range of angles of attack (2.5° to 7.5°) and chord Reynolds numbers (130000 to 530000). Initially the vortex was examined for stability and probe interference effects through flow visualization. Then detailed three-component velocity, turbulence and spectral measurements were made using multiple hot wires between 20 and 30 chordlengths downstream of the wing. These measurements show the flow to consist of a small axisymmetric core surrounded by a large non-axisymmetric region in which the wing wake forms a spiral around the core. In contrast to the results of previous workers, most of whom studied vortices generated by split wing configurations, there appeared to be little axisymmetric region of merged turbulent flow. Turbulence levels in the spiral wake decay with downstream distance. They also fall as the core is approached, presumably because of the effects of the increased straining and curvature on the turbulent structures here. Turbulence spectra measured in the wake are very similar in form, regardless of conditions and exact location, and bear a strong resemblance to a von Karman spectrum. At most conditions true turbulence levels in and immediately adjacent to the core are very low. Velocity fluctuations, however, are intense as a consequence of coherent lateral motions of the core and possible wave motions and instabilities travelling along it. Velocity autospectra in the core show the lateral motions to be anisotropic at very low frequencies and isotropic and mid frequencies. A large part of these motions may well be self induced. Circulation profiles in this region show Hoffman and Joubert's semi-logarithmic region, and in one case reveal the core to be fully developed. From the point of view of BWI noise prediction the flow measurements identify three sources of velocity fluctuations; low-frequency anisotropic core motions, mid-frequency isotropic core motions, and turbulence in the surrounding spiral wake. Estimates of the noise produced by these different frequency regimes show that it is the mid-frequency isotropic motions which is the most important mechanism for noise production, but the spectral shape and the predicted directionality for this mechanism are not in agreement with the measurements of BWI noise. The original noise-prediction scheme, based on the concept of a more turbulent vortex, has been shown to be the wrong basis for the correct description of the flow, but in spite of this the results are significantly better than those presented here. This suggests that the flow measured in the wind tunnel does not have the same turbulence upwash spectrum as that encountered by the helicopter rotor. One of the features of a real helicopter rotor which was not considered is the effect of a downstream blade upon the tip vortex. This may cause vortex bursting or alter the flow structure in the rotor disc plane in other ways. Measurements to evaluate this concept are planned for the future.
Understanding Pressure Shielding by Canopies
Nurani Hari, Nandita; Szőke, Máté; Devenport, William J.; Glegg, Stewart A. L. (2021-01-01)
Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations which are arrays of streamwise rods placed parallel to the wall in order to eliminate the confounding effects of a leading edge support structure. Experiments show that such a canopy produces attenuation in three distinct frequency ranges. At low frequencies, where convective scales are much greater than the canopy height, attenuation spectra scale on the canopy height Strouhal number, but at high frequencies, a dissipation type frequency scaling appears more appropriate. There is mid-freqeuncy region which shows reduction in attenuation and is observed for all canopy structures tested. Attenuation in this region appears to scale with Strouhal number based on canopy spacing.

Browsing by Author "Glegg, Stewart A. L."

Results Per Page

Sort Options