Browsing by Subject "Turbulent boundary layer"
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Item An integral method for predicting unsteady turbulent boundary layers(Texas Tech University, 1983-12) Smith, Tony GlennNot availableItem Application and analysis of RANS based turbulence models for bluff body aerodynamics(Texas Tech University, 2004-12) Unhale, Sanket A.Computational Wind Engineering (CWE) is becoming more popular in the wind engineering community, for prediction of the wind loads on buildings and structures. The most important and basic requirement of CWE is successful modeling of turbulent flow in the simulated atmospheric boundary layer. Many turbulence models have been proposed and tested for a variety of flows, but they are not accurate enough to simulate the bluff body aerodynamics accurately. In the similar context, five two-equation turbulence models based on the isotropic eddy viscosity concept and Reynolds Averaged Navier Stokes (RANS) modeling approach were tested for the flow around sharp edged building models using Fluent as the solver. The numerical results were compared with the experimental findings in wind tunnel simulations and full scale measurements available in the literature. The results illustrate inaccuracy of the turbulence models to predict the pressure distribution along the building surfaces. The results were analyzed and the effects of different turbulence parameters used in the modeling and analysis were discussed. Each turbulence model was individually reviewed for the correctness of its predictions and the best model in this set was chosen. This model was further applied to different obstacle geometry in different experimental conditions to minimize the effects of unique experimental condition on analysis. One of the major problems encountered in the modeling was the type of near wall treatment used for simulation. Development of equilibrium boundary layer in the wind tunnel was used as a test condition for studying the deficiencies in different types of near wall modeling approaches. It was concluded that the standard approach of wall functions was insufficient to estimating the effects of roughness on the wall boundary on the mean flow. A new rough wall model proposed by Durbin et al [58] was implemented using the user defined functions capability of Fluent. This model was tested on a variety of flows; like flow through pipe, flow over a backward facing step, etc. This flow was then applied to the models tested in this project to analyze the effects on the rough wall model used for modeling near wall effects. Overall agreement between the computational predictions and experimental findings was acceptable.Item Composite expansions for active and inactive motions in the streamwise Reynolds stress of turbulent boundary layers(2008-05) McKee, Robert Joe, 1946-; Panton, Ronald L. (Ronald Lee), 1933-The proper scaling and prediction of the streamwise Reynolds stress in turbulent boundary layers has been a controversial issue for more than a decade as its Reynolds Number dependence can not be removed by normal scaling. One issue that may explain the unusual behavior of the streamwise Reynolds stress is that it is affected by both active and inactive motions per the Townsend hypothesis. The goal of this research is to develop a composite expansion for the streamwise Reynolds stress in turbulent boundary layers that considers active and inactive motions, explains various Reynolds Number dependencies, and agrees with available data. Data for the Reynolds shear stress and the streamwise Reynolds stress from six sources are evaluated and as appropriate plotted on inner and outer scales. A new asymptotic representation for the Reynolds shear stress, +, that meets the requirements for a proper composite expansion is developed and applied. This new Reynolds shear stress composite expansion agrees with data and allows predictions of + for any Reynolds Number. The streamwise Reynolds stress, +, can be separated into active and inactive parts and the Reynolds shear stress can be used to represent the active part. The inactive streamwise Reynolds stress, #, is separated from the complete + in part of this work. An outer correlation equation with the correct asymptotic limits for the inactive streamwise Reynolds stress is developed and shown to fit the outer part of the # data. A separate inner correlation equation for inner inactive streamwise Reynolds stress is developed and fit to data. Together these two equations form a composite expansion for the inactive streamwise Reynolds stress for flat plate boundary layers. This composite expansion for the inactive streamwise Reynolds stress can be combined with the Reynolds shear stress expansion to produce predictions for + that agree with data. Thus a composite expansion for predicting the streamwise Reynolds stress in turbulent boundary layers is developed and shown to reproduce the correct trends, to agree with the available data, and to explain the Reynolds Number dependence of the streamwise Reynolds stress.Item A direct numerical simulation of fully developed turbulent channel flow with spanwise wall oscillation(2005) Zhou, Dongmei; Ball, K. S.; Bogard, David G.Low-Reynolds-number, fully developed turbulent channel flow with wall motion has been simulated by direct numerical simulation to examine the effectiveness and the near-wall mechanics using spanwise wall oscillation to reduce friction drag. The three-dimensional unsteady Navier-Stokes (and energy) equations are solved using Fourier-Chebyshev-Tau spectral methods combined with a second-order semi-implicit time-advancement scheme. The effects of spatial resolution and computational box size on the computed turbulence and the drag reduction percentage were investigated. Finer spanwise resolution has a greater effect on achieving a better solution and the turbulent flow is well resolved for a spanwise grid spacing of Δ 3 <10 + x . It was also confirmed that the dynamics of turbulence in a natural full channel could be reproduced by a minimal channel. Parameter studies have been performed to examine the variation of drag reduction value with wall oscillation frequency, velocity amplitude, peak-to-peak amplitude, and oscillation orientation, and drag reduction data were discovered to correlate better with peak-to-peak amplitude for frequencies 01 > 0. + f in contrast to the previous finding of its correlation with peak-wall-speed. At the optimal wall oscillation conditions, net power savings of about 5% are obtained after the power input to move the wall is accounted for, even though more than 40% friction drag reduction has been achieved in the turbulence flow. Significant drag reduction is accompanied by the suppression of the turbulent bursting process and production of turbulence, and by a reduction in the intensity of streaks and streamwise vortices. A thickened viscous sublayer is indicated through the observed outward shift of statistical quantities such as velocity fluctuations and Reynolds shear stress in the moving-wall channel flow. Drag reduction by spanwise wall oscillation is mainly due to the suppression of ejection-sweep motions and the disruption in the cycle of the turbulence selfsustaining process, starting with the wall streaks that are distorted and reduced in number and extent. The intensity and the number of vortical structures are also reduced by the wall motion. The suppression of the regeneration of new streamwise vortices above the wall in turn further suppresses the ejection-sweep motions, thus leading to the reduced skin-friction levels at the wall.Item The effects of upstream mass injection by vortex generator jets on shock-induced turbulent boundary layer separation(2006) Bueno, Pablo Cesar; Clemens, Noel T.Item Impedance measurement of an orifice exposed to a turbulent boundary layer with pressure gradient(Texas Tech University, 1980-08) Chung, Chang-hwaNot availableItem On the charcterization of wakes from three-dimensional single and multiple flat plates on a ground plane(Texas Tech University, 1985-05) Barton, Gregory HaroldA flat plate is a simplified model for a heliostat, which is a large flat tracking mirror used to reflect solar energy onto a central receiving boiler in a solar collector system. This is a study of vortex shedding from 3-dimensional single flat plates on a ground plane and the modifying effect on their wakes when plates are placed upstream. The multiple plate data is characterized by the single plate wake properties based on the plate separation distance referenced to the Strouhal identity length. The new characteristic body length is simply the multiple plate separation distance which produces the same Strouhal number for single and multiple bodies. For separation distances less than the Strouhal identity length, the multiple plate characteristics are very similar to the single plate. For separation distances greater than the Strouhal identity length, a multiple plate wake scale factor is developed which transforms the streamwise wake distance from the front plate into a new streamwise coordinate.Item Particle image velocimetry study of shock-induced turbulent boundary layer separation(2003) Hou, Yongxi; Clemens, Noel T.An experimental study was conducted to investigate the characteristics of a Mach 2 shock wave / boundary layer interaction, by using particle image velocimetry (PIV). The objective was to investigate how the global flow structure is related to the shock-foot dynamics. A major component of this work was the development of a new multi-camera, multi-laser PIV system, which enables the acquisition of wide-field and time-sequenced velocity fields. The wide-field images are obtained by placing four cameras side-by-side giving an effective resolution of 4k×1k pixels. Four-image time sequences can be acquired where the time between frames is 30 to 200 µs. The PIV system was used to characterize the upstream Mach 2 boundary layer. The measured mean and RMS velocity profiles agreed well with previous measurements in compressible boundary layers and this provided important validation of the PIV system. The wide-field PIV system was used to image the entire interaction, spanning the upstream boundary layer, intermittent region, separated flow and the reattachment region on the ramp face. The separation shock wave location inferred from the PIV images agreed well with the shock-foot position inferred from the pressure data. The instantaneous vector fields reveal that boundary layer separation is not immediately induced by the shock foot, but sometimes develops substantially farther downstream. Significant reverse-flow velocities are seen in the instantaneous images, but on average no reverse-flow was observed. The global structure of the interaction was found to depend strongly on the location of the separation shock foot. Ensemble averages, conditioned upon the shock-foot position, showed that when the shock is upstream, the scale of the separated flow, the velocity fluctuations, and the domain of perturbed flow, are all substantially larger than when the shock-foot is downstream. Perhaps most importantly, the conditional upstream boundary layer profiles, conditioned on the shock position, showed that the boundary layer is thicker when the shock is upstream and vice versa. Furthermore, the conditional measurements confirmed the results of a previous study that reported a correlation between velocity fluctuations in the upstream boundary layer and shock foot motion. A preliminary study was used to test the hypothesis that acceleration fluctuations in the upstream boundary layer correlate with shock foot motion. These results showed no meaningful relationship between upstream acceleration with the shock motion, but given certain limitations of the experiment this conclusion cannot be considered definitive.Item Reducing turbulence- and transition-driven uncertainty in aerothermodynamic heating predictions for blunt-bodied reentry vehicles(2014-08) Ulerich, Rhys David; Moser, Robert deLanceyTurbulent boundary layers approximating those found on the NASA Orion Multi-Purpose Crew Vehicle (MPCV) thermal protection system during atmospheric reentry from the International Space Station have been studied by direct numerical simulation, with the ultimate goal of reducing aerothermodynamic heating prediction uncertainty. Simulations were performed using a new, well-verified, openly available Fourier/B-spline pseudospectral code called Suzerain equipped with a ``slow growth'' spatiotemporal homogenization approximation recently developed by Topalian et al. A first study aimed to reduce turbulence-driven heating prediction uncertainty by providing high-quality data suitable for calibrating Reynolds-averaged Navier--Stokes turbulence models to address the atypical boundary layer characteristics found in such reentry problems. The two data sets generated were Ma[approximate symbol] 0.9 and 1.15 homogenized boundary layers possessing Re[subscript theta, approximate symbol] 382 and 531, respectively. Edge-to-wall temperature ratios, T[subscript e]/T[subscript w], were close to 4.15 and wall blowing velocities, v[subscript w, superscript plus symbol]= v[subscript w]/u[subscript tau], were about 8 x 10-3 . The favorable pressure gradients had Pohlhausen parameters between 25 and 42. Skin frictions coefficients around 6 x10-3 and Nusselt numbers under 22 were observed. Near-wall vorticity fluctuations show qualitatively different profiles than observed by Spalart (J. Fluid Mech. 187 (1988)) or Guarini et al. (J. Fluid Mech. 414 (2000)). Small or negative displacement effects are evident. Uncertainty estimates and Favre-averaged equation budgets are provided. A second study aimed to reduce transition-driven uncertainty by determining where on the thermal protection system surface the boundary layer could sustain turbulence. Local boundary layer conditions were extracted from a laminar flow solution over the MPCV which included the bow shock, aerothermochemistry, heat shield surface curvature, and ablation. That information, as a function of leeward distance from the stagnation point, was approximated by Re[subscript theta], Ma[subscript e], [mathematical equation], v[subscript w, superscript plus sign], and T[subscript e]/T[subscript w] along with perfect gas assumptions. Homogenized turbulent boundary layers were initialized at those local conditions and evolved until either stationarity, implying the conditions could sustain turbulence, or relaminarization, implying the conditions could not. Fully turbulent fields relaminarized subject to conditions 4.134 m and 3.199 m leeward of the stagnation point. However, different initial conditions produced long-lived fluctuations at leeward position 2.299 m. Locations more than 1.389 m leeward of the stagnation point are predicted to sustain turbulence in this scenario.Item Spectral analysis of pressure fluctuations on bluff bodies placed in turbulent flows(Texas Tech University, 1998-12) Jayantha, Athukoralage SamanIn fluid dynamics, pressure fluctuations within a turbulent boundary layer play an important role in determining the noise level inside automobiles and buildings. Moreover, knowledge of pressure fluctuations in turbulent flows is desired for many engineering applications, such as the aerodynanuc noise produced by the wall pressure fluctuations due to turbulence in the boundary layer. Two examples of noise generation due to wall pressure fluctuations are noise generation on airplane surfaces and turbine blades. If wall pressure fluctuations are not controlled, undesirable structural damages can be anticipated due to pressure-induced vibrations. Prediction of complete pressure fluctuation spectra with both linear and quadratic turbulent velocity interactions is the main objective of this work. A new approach to obtain wall pressure fluctuations around bluff bodies placed in turbulent flows is described in this computational model. The spectral analysis of pressure fluctuation is performed in three steps. Three-dimensional Navier-Stokes equations are solved by a standard computational fluid dynamics (CFD) analysis to get the mean flow quantities with the help of a two-equation model, k-8 model, as the first step. In the second step, the Poisson's equation for fluctuating pressure with both linear and non-linear turbulence interacfions is solved by employing a turbulent velocity field synthesized from a stochasfic description of the three dimensional turbulent motion. Time series for pressure fluctuations at each point in space are obtamed as the result of the second step. The third step consists of the applicafion of a non-parametric estimation technique to analyze the obtained time series for fluctuating pressure in order to get the pressure fluctuation spectrum. Three different geometries (a 2-D channel, an automobile side window, and a cylinder) are considered for the present analysis. The validity of this model is confirmed by comparing the present resuts with the results from DNS channel flow data and wind tunnel experiments.Item Turbulent boundary layers over receiver arrays(2010-05) Dolder, Craig Nealon; Tinney, Charles Edmund, 1975-; Haberman, Michael R.A study of the fluctuating wall pressure and unsteady velocity field in a flat plate turbulent boundary layer flow was conducted over a moderate range of Reynolds numbers to better understand the mechanisms by which the two fields are coupled. Individual and coincident measurements of the fluctuating pressure and velocity fields were acquired using a 20 element hydrophone array and a two-component Laser Doppler Anemometer, respectively. Estimates of the velocity power spectral density (PSD) revealed two primary trends predicted by turbulence theory, k⁻¹ in the region of (ky) = 10⁰ due to anisotropy of the large scales, and k⁻⁵/³ for larger values of (ky) where structures appear more isotropic. The mean velocity profiles, having been collapsed using outer scaling variables, exhibited the presence of a slightly adverse pressure gradient with a n = 6 power law shape. As for the fluctuating wall pressure, increased Reynolds numbers produced increases in the amplitude and frequency of the characteristic signatures from which the pressure spectral densities were also found to collapse reasonably well using outer scaling variables. The results suggest the location in the flow where the mechanisms responsible for driving the fluctuating wall pressure signatures reside. Space-time correlations and frequency-wavenumber analysis reveal a convective ridge in the fluctuating wall pressure corresponding to the passage of several organized structures at 75% of the free stream velocity for all Reynolds numbers tested.