An experimental investigation of an incompressible, turbulent junction vortex formed at the base of a streamlined cylinder with a circular leading edge placed normal to a flat surface is documented. The flow environment is characterized by a body Reynolds number of 183,000, based on the diameter of the model. The investigation centered around measurements of mean velocity vectors at over 1,700 locations on the plane of symmetry. In addition, extensive turbulence measurements in regions of interest on the plane of symmetry were also made. All of the measurements are performed with a two-color, two-component, frequency shifted laser Doppler velocimeter. The mean flow field shows the presence of a single vortex on the plane of symmetry and a singular separation point upstream of the vortex. From the measured data, the locations of the vortex center and the singular separation point were determined. The mean velocity field correlates well with previously published surface pressure data and surface flow visualization results. Extensive comparisons were made with earlier Conrad and five-hole pressure probe measurements and hot film measurements. Comparisons with the Conrad probe results were made in a two-dimensional turbulent boundary layer, and with the five-hole probe results for the plane of symmetry junction vortex flow at six stations in the upstream region and four stations inside the separated region. Hot film results were compared at two stations in the upstream region. Excellent agreement was found in the two-dimensional turbulent boundary layer. On the plane of symmetry junction vortex flow, agreement among the three instruments was good, especially for data taken in the upstream region and away from the floor. Turbulence data were also obtained with the laser Doppler velocimeter in conjunction with a minicomputer. Results indicate the turbulence level inside the separated region is significantly higher than in the upstream region. Highest levels of turbulence were found near the singular separation point and near the center of the vortex. Doppler spectra variations in these regions were also shown. In addition, Doppler histograms obtained by a PDP-11 minicomputer were compared to Doppler spectra obtained using an FFT. Excellent agreement was found between the Doppler spectra and the histograms obtained from the two different instruments. / Ph. D.

Many flow cases in fluid dynamics face undesirable flow separation due to ad-verse pressure gradients on wall boundaries. This occurs, for example, due togeometrical reasons as in a highly curved turbine-inlet duct or on flow-controlsurfaces such as wing trailing-edge flaps within a certain angle-of-attack range.Here, flow-control devices are often used in order to enhance the flow and delayor even totally eliminate flow separation. Flow control can e.g. be achieved byusing passive or active vortex generators (VGs) for momentum mixing in theboundary layer of such flows. This thesis focusses on such passive and activeVGs and their modelling for computational fluid dynamics investigations. First, a statistical VG model approach for passive vane vortex genera-tors (VVGs), developed at the Royal Institute of Technology Stockholm andthe Swedish Defence Research Agency, was evaluated and further improvedby means of experimental data and three-dimensional fully-resolved computa-tions. This statistical VVG model approach models those statistical vortexstresses that are generated at the VG by the detaching streamwise vortices.This is established by means of the Lamb-Oseen vortex model and the Prandtllifting-line theory for the determination of the vortex strength. Moreover, thisansatz adds the additional vortex stresses to the turbulence of a Reynolds-stresstransport model. Therefore, it removes the need to build fully-resolved three-dimensional geometries of VVGs in a computational fluid dynamics mesh. Usu-ally, the generation of these fully-resolved geometries is rather costly in termsof preprocessing and computations. By applying VVG models, the costs arereduced to that of computations without VVGs. The original and an improvedcalibrated passive VVG model show sensitivity for parameter variations suchas the modelled VVG geometry and the VVG model location on a flat plate inzero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil withits high-lift system extracted. It could be shown that the passive VG modelqualitatively and partly quantitatively describes correct trends and tendenciesfor these different applications. In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow atTechnische Universitä̈t Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. Dependencies of the generated vortices and their characteristics on the VGJsetup parameters could be identified and quantified. These dependencies wereused as a basis for the development of a new statistical VGJ model. This modeluses the ansatz of the passive VVG model in terms of the vortex model, theadditional vortex-stress tensor, and its summation to the Reynolds stress ten-sor. Yet, it does not use the Prandtl lifting-line theory for the determinationof the circulation but an ansatz for the balance of the momentum impact thatthe VGJ has on the mean flow. This model is currently under developmentand first results have been evaluated against experimental and fully-resolvedcomputational results of a flat plate without pressure gradient. / <p>QC 20120511</p>

The object of the present work is to experimentally study the case of a turbulent boundary layer subjected to an Adverse Pressure Gradient (APG) with separation and reattachment. The effect of unsteadiness on turbulent boundary layer separation by means two different methods were explored viz. the effect of local forcing by acoustic waves and effect of wakes on separation bubble. The experiments were conducted in a low speed open circuit blower type wind tunnel. The turbulent separation bubble was created on the test plate by a contoured ceiling which created the adverse pressure gradient. The velocities were measured using single element hot wire and X-wire. Limited studies on quasi shear stress were also conducted using surface mounted hot film probes. Static pressure was measured using a projection manometer. Boundary layer is tripped near the leading edge of the flat plate to ensure a turbulent boundary layer. Surface pressure distribution and flow visualization were conducted as part of diagnostics. In the case of laminar separation bubble, lot of investigations have been done on the effect of unsteady wake and the most important conclusion was that the wake induces `bypass' transition to turbulence and since the turbulent boundary layer is more resistant to separation, it remains attached. In the case of turbulent separation bubble, laminar-turbulent transition is not relevant and if the bubble is suppressed, it should be by some other mechanism. This is what we seek to unravel in this study. A closer look at the mean velocity profiles reveal the occurrence of inflection point before separation as in the case of laminar separation bubble and the peak values of turbulence intensities correspond to the location of point of inflection. Turbulent separation correlations proposed by various investigators were compared with the present results and are found to be in good agreement. Surface flow visualization pictures are used to get qualitative information. The wall forcing on the separation bubble was done using a speaker which blows a small amount of air when the diaphragm moves up and sucks in when the diaphragm moves down. The blowing effect seems to be more effective in suppressing the separation compared to suction. The interaction with wake is studied using an unsteady bar which is moving up and down. The inflection point in the mean velocity distribution seems to move closer to the wall with the impingement o the wake. Also the turbulence intensities have increased and seem to move closer to the wall. The displacement and momentum thickness have increased and the shape factor has decreased which indicates suppression of the bubble. The quasi shear stress in the separated region also increased which indicates suppression of separation. While the oncoming unsteady wake might be a parcel of fluid with defect velocity when seen in isolation, in comparison to the velocity defect in the separation bubble, it is a region of velocity excess. As a result, one can expect the impingement of the unsteady wake on the TSB to transport momentum thereby contributing to separation reduction. But the mechanism of separation is different from laminar separation bubble affected by wakes. The suppression in the case of turbulent separation bubble is partly due to the entrainment of turbulence and partly due to the kinematic impact of the wake on the bubble.

A deterministic approach based on a modal expansion and modal receptance method has been developed to evaluate the airborne sound insulation of aircraft panels with stringer and ring frame attachments. Furthermore, this method was extended to predict the noise radiation of stiffening panel subjected to TBL excitation. This approach integrates with the fast and accurate methods in evaluating the modal excitation terms and modal radiation efficiency. Based on these advantages, the effects of the curvature, overpressure, stringers, ring frames, hydrodynamic coincidence, composite structures and structural dissipation on the acoustical properties of a typical aircraft panel are able to be investigated efficiently. Theoretic predictions were compared with laboratory measurements conducted on both model structures and aircraft panels. It was found that a small curvature may result in significant deterioration of the sound transmission loss at frequencies of interest. Unlike a flat uniform panel, the theoretical prediction for curved panels from the infinite model can not provide good agreement with the measurement close to and well below the ring frequency. However, in this frequency range, the finite model has been proved to be applicable For the large curved airplane panels studied here, it was found that the ring frames have little influence on sound transmission loss in the frequency range of interest. However the stringers may have considerable influence on sound transmission loss. The stringer improves this for a curved panel around the ring frequency, but it may result in a potential deterioration of the sound transmission loss above the ring frequency. In this study it is evident that the sound transmission loss of the composite skin attached with composite stringers is lower than that of the metallic panel attached with metallic stringers. At frequencies higher than the corresponding ring frequency of the curved panel, both experiment and theoretical prediction reveal that the overpressure at the concave side tends to reduce the sound transmission loss at the rate of about 0.5dB /10000 Pa. While at lower frequencies, say well below the ring frequency, the overpressure may increase or reduce sound transmission loss of a finite panel, depending on the shift of the resonant frequencies resulting from the overpressure. For TBL excitation, numerical investigation reveals that the panel with the ring frames behaves more like a sub-panel between two frames. Below 500Hz, the ring frames slightly enhance the sound radiation while dramatically increasing it around 1.3kHz. The TBL forcing field excites the same vibration lever for the panel with and without ring frame attachments, but the modes excited for the panel with ring frames radiate more sound. Unlike the ring frames, the stringers increase sound radiation below 1kHz. Above 1kHz, the sub-panels between two bays respond independently and the stringer effects is therefore not obvious. / <p>QC 20100908</p>

The present understanding of the structure and dynamics of turbulent boundary layers on aerodynamically smooth walls has been clarified over the last decade or so. However, the dynamics of turbulent boundary layers over rough surfaces is much less well known. Nevertheless, there are many industrial and environmental flow applications that require understanding of the mean velocity and turbulence in the immediate vicinity of the roughness elements.</p> <p>This thesis reports the effects of surface roughness on the flow characteristics in a turbulent boundary layer. Both experimental and numerical investigations are used in the present study. For the experimental study, comprehensive data sets are obtained for two-dimensional zero pressure-gradient turbulent boundary layers on a smooth surface and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. Three different probes, namely, Pitot probe, single hot-wire, and cross hot-film, were used to measure the velocity fields in the turbulent boundary layer. A Pitot probe was used to measure the streamwise mean velocity, while the single hot-wire and cross hot-film probes were used to measure the fluctuating velocity components across the boundary layer. The flow Reynolds number based on momentum thickness, , ranged from 3730 to 13,550. The data reported include mean velocity, streamwise and wall-normal turbulence intensities, Reynolds shear stress, triple correlations, as well as skewness and flatness factors. Different scaling parameters were used to interpret and assess both the smooth- and rough-wall data at different Reynolds numbers, for approximately the same freestream velocity. The appropriateness of the logarithmic law and power law proposed by George and Castillo (1997) to describe the mean velocity in the overlap region was also investigated. The present results were interpreted within the context of the Townsends wall similarity hypothesis. </p> <p>Based on the mean velocity data, a novel correlation that relates the skin friction to the ratio of the displacement and boundary layer thicknesses, which is valid for both smooth- and rough-wall flows, was proposed. In addition, it was also found that the application of a mixed outer scale caused the velocity profile in the outer region to collapse onto the same curve, irrespective of Reynolds numbers and roughness conditions. The present results showed that there is a common region within the overlap region of the mean velocity profile where both the log law and power law are indistinguishable, irrespective of the surface conditions. For the power law formulation, functional relationships between the roughness shift, and the power law coefficient and exponent were developed for the transitionally rough flows. The present results also suggested that the effect of surface roughness on the turbulence field depends to some degree on the specific characteristics of the roughness elements and also the component of the Reynolds stress tensor being considered. </p> <p>In the case of the numerical study, a new wall function formulation based on a power law was proposed for smooth and fully rough wall turbulent pipe flow. The new formulation correctly predicted the friction factors for smooth and fully rough wall turbulent pipe flow. The existing two-layer model realistically predicted the velocity shift on a log-law plot for the fully rough turbulent boundary layer. The two-layer model results also showed the effect of roughness is to enhance the level of turbulence kinetic energy and Reynolds shear stress compared to that on a smooth wall. This enhanced level extends into the outer region of the flow, which appears to be consistent with present and recent experimental results for the boundary layer.

Infrared transition location estimates can be fast and useful measurements in wind tunnel and flight tests. Because turbulent boundary layers have a much higher rate of convective heat transfer than laminar boundary layers, a difference in surface temperature can be observed between turbulent and laminar regions of an airfoil at a different temperature than the free stream air temperature. Various implementations of this technique are examined in a wind tunnel. These include using a heat lamp as an external source and circulating fluid inside of the airfoil. Furthermore, ABS plastic and aluminum airfoils are tested with and without coatings such as black paint and surface wraps. The results show that thermal conduction within the model and surface reflections are the driving issues in designing an IR system for detecting transition. Aluminum has a high thermal diffusivity so is a poor choice for this method. However, its performance can be improved using an insulating layer. Internal fluid circulation was far more successful than the heat lamp because it eliminates the reflected IR due to the heat lamp. However, using smooth surface wraps can mitigate reflection issues caused by the heat lamps by reducing the scatter within the reflection, producing an IR image with fewer contaminating reflections.

The present understanding of the structure and dynamics of turbulent boundary layers on aerodynamically smooth walls has been clarified over the last decade or so. However, the dynamics of turbulent boundary layers over rough surfaces is much less well known. Nevertheless, there are many industrial and environmental flow applications that require understanding of the mean velocity and turbulence in the immediate vicinity of the roughness elements.</p> <p>This thesis reports the effects of surface roughness on the flow characteristics in a turbulent boundary layer. Both experimental and numerical investigations are used in the present study. For the experimental study, comprehensive data sets are obtained for two-dimensional zero pressure-gradient turbulent boundary layers on a smooth surface and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. Three different probes, namely, Pitot probe, single hot-wire, and cross hot-film, were used to measure the velocity fields in the turbulent boundary layer. A Pitot probe was used to measure the streamwise mean velocity, while the single hot-wire and cross hot-film probes were used to measure the fluctuating velocity components across the boundary layer. The flow Reynolds number based on momentum thickness, , ranged from 3730 to 13,550. The data reported include mean velocity, streamwise and wall-normal turbulence intensities, Reynolds shear stress, triple correlations, as well as skewness and flatness factors. Different scaling parameters were used to interpret and assess both the smooth- and rough-wall data at different Reynolds numbers, for approximately the same freestream velocity. The appropriateness of the logarithmic law and power law proposed by George and Castillo (1997) to describe the mean velocity in the overlap region was also investigated. The present results were interpreted within the context of the Townsends wall similarity hypothesis. </p> <p>Based on the mean velocity data, a novel correlation that relates the skin friction to the ratio of the displacement and boundary layer thicknesses, which is valid for both smooth- and rough-wall flows, was proposed. In addition, it was also found that the application of a mixed outer scale caused the velocity profile in the outer region to collapse onto the same curve, irrespective of Reynolds numbers and roughness conditions. The present results showed that there is a common region within the overlap region of the mean velocity profile where both the log law and power law are indistinguishable, irrespective of the surface conditions. For the power law formulation, functional relationships between the roughness shift, and the power law coefficient and exponent were developed for the transitionally rough flows. The present results also suggested that the effect of surface roughness on the turbulence field depends to some degree on the specific characteristics of the roughness elements and also the component of the Reynolds stress tensor being considered. </p> <p>In the case of the numerical study, a new wall function formulation based on a power law was proposed for smooth and fully rough wall turbulent pipe flow. The new formulation correctly predicted the friction factors for smooth and fully rough wall turbulent pipe flow. The existing two-layer model realistically predicted the velocity shift on a log-law plot for the fully rough turbulent boundary layer. The two-layer model results also showed the effect of roughness is to enhance the level of turbulence kinetic energy and Reynolds shear stress compared to that on a smooth wall. This enhanced level extends into the outer region of the flow, which appears to be consistent with present and recent experimental results for the boundary layer.

The objective is to contribute to several issues regarding the traditional view of the local structure of passive scalar fields: (1) probability density function (PDF) of the scalar concentration and scalar gradient, (2) the scalar power spectrum, (3) the structure functions, and (4) correlation functions and multi-point correlators. In addition, the research provides a geometric description of two-dimensional transects of the passive scalar iso-surfaces using the tools of fractal geometry. The local structure is analyzed as a function of large-scale anisotropy, intermittency factor, Reynolds number, and initial condition of the scalar injection. Experiments were performed in the bed boundary layer produced by a uniform depth open channel flow of water in a tilting flume for Re_lamda = 63, 94, and 120. A small nozzle iso-kinetically delivers a passive scalar of high Schmidt number ( Sc = 1000) at mid-depth to generate the turbulent scalar field. Three nozzle diameters are used to study the effects of the injection length scale. High-resolution planar laser induced fluorescence (PLIF) technique is used to measure the scalar field. The local structure far from isotropic and is influenced even at the smallest scales by large-scale anisotropy, initial injection length scale and the Reynolds number of the flow. The PDF of the scalar fluctuations is non-Gaussian and dependent on large-scale anisotropy. The PDF of scalar gradients show the influence of large-scale anisotropy on the structure at the smallest scales. The spectrum of the scalar field deviates from the in the inertial convection regime and is dependent on large-scale anisotropy, external intermittency, and low Reynolds number. There is no evidence of Batchelors k^-1 scaling law. The scaling exponents of the even-ordered structure functions appear to be inversely correlated with the kurtosis of the scalar fluctuations. The fractal geometry of the two dimensional transects of passive scalar iso-surfaces is scale dependent. The fractal dimension is 1.0 at the smallest length scale and increases in a universal manner in the viscous-convective regime. The coverage length underestimate reflects this universal behavior with practical significance. The lacunarity function shows that the instantaneous scalar field is most in-homogenous around the Kolmogorov scale.

abstract: Rotorcraft operation in austere environments can result in difficult operating conditions, particularly in the vicinity of sandy areas. The uplift of sediment by rotorcraft downwash, a phenomenon known as brownout, hinders pilot visual cues and may result in a potentially dangerous situation. Brownout is a complex multiphase flow problem that is not unique and depends on both the characteristics of the rotorcraft and the sediment. The lack of fundamental understanding constrains models and limits development of technologies that could mitigate the adverse effects of brownout. This provides the over-arching motivation of the current work focusing on models of particle-laden sediment beds. The particular focus of the current investigations is numerical modeling of near-surface fluid-particle interactions in turbulent boundary layers with and without coherent vortices superimposed on the background flow, that model rotorcraft downwash. The simulations are performed with two groups of particles having different densities both of which display strong vortex-particle interaction close to the source location. The simulations include cases with inter-particle collisions and gravitational settling. Particle effects on the fluid are ignored. The numerical simulations are performed using an Euler- Lagrange method in which a fractional-step approach is used for the fluid and with the particulate phase advanced using Discrete Particle Simulation. The objectives are to gain insight into the fluid-particle dynamics that influence transport near the bed by analyzing the competing effects of the vortices, inter-particle collisions, and gravity. Following the introduction of coherent vortices into the domain, the structures convect downstream, dissipate, and then recover to an equilibrium state with the boundary layer. The particle phase displays an analogous return to an equilibrium state as the vortices dissipate and the boundary layer recovers, though this recovery is slower than for the fluid and is sensitive to the particle response time. The effects of inter-particle collisions are relatively strong and apparent throughout the flow, being most effective in the boundary layer. Gravitational settling increases the particle concentration near the wall and consequently increase inter-particle collisions. / Dissertation/Thesis / M.S. Aerospace Engineering 2011

Flush-mounted cavity hot-wire probes have been around since two decades, but have typically not been applied as often compared to the traditional wall hot-wires mounted several wire diameters above the surface. While the latter suffer from heat conduction from the hot wire to the substrate in particular when used in air flows, the former is belived to significantly enhance the frequency response of the sensor. The recent work using a cavity hotwire by Gubian et al. (2019) came to the surprising conclusion that the magnitute of the fluctuating wall-shear stress τ+w,rms reaches an asymptotic value of 0.44 beyond the friction Reynolds number Re τ ∼ 600. In an effort to explain this result, which is at odds with the majority of the literature, the present work combines direct numerical simulations (DNS) of a turbulent channel flow with a cavity modelled using the immersed boundary method, as well as an experimental replication of the study of Gubian et al. in a turbulent boundary layer to explain how the contradicting results could have been obtained. It is shown that the measurements of the mentioned study can be replicated qualitatively as a result of measurement problems. We will present why cavity hot-wire probes should neither be used for quantitative nor qualitative measurements of wall-bounded flows, and that several experimental short-comings can interact to sometimes falsely yield seemingly correct results.