Includes bibliographical references (leaves 189-191). / Thesis (S.M. in Oceanographic Engineering)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2006. / (cont.) To that end, the space-time correlations of the error, turbulence, and wave processes are developed and then utilized to find the extent to which the environmental and internal processing parameters contribute to this error. It is found that the wave-induced velocities, even when filtered, introduce error variances which are of similar magnitude to that of the Reynolds stresses. / The challenge of estimating the Reynolds stress in an energetic ocean environment derives from the turbulence process overlapping in frequency, or in wavenumber, with the wave process. It was surmised that they would not overlap in the combined wavenumber-frequency spectrum, due to each process having a different dispersion relationship. The turbulence process is thought to obey a linear dispersion relationship, as the turbulent flow is advected with the mean current (Taylor's frozen turbulence approximation). However, the Acoustic Doppler Current Profiler (ADCP) looks at radial wavenumbers and frequencies, and finds overlap. Another approach is to exploit the physical differences of each process, namely that the wave induced velocities are correlated over much larger distances than the turbulence induced velocities. This method was explored for current meters by Shaw and Trowbridge. Upon adapting the method for the ADCP, it is found that the resulting Reynolds stress estimates are of the correct order of magnitude, but somewhat noisy. The work of this thesis is to uncover the source of that noise, and to quantify the performance limits of estimating the Reynolds Stress when using ADCP measurements that are contaminated with strong wave-induced velocities. / by Mark Rapo. / S.M.in Oceanographic Engineering

The present study considers the numerical simulation of the different flow characteristics involved in the conjugate heat transfer analysis of an internally cooled gas turbine blade. Conjugate simulations require full coupling of convective heat transfer in fluid regions to the heat diffusion in solid regions. Therefore, accurate prediction of heat transfer quantities on both external and internal surfaces has the uppermost importance and highly connected with the performance of the employed turbulence models. The complex flow on both surfaces of the internally cooled turbine blades is caused from the boundary layer laminar-to-turbulence transition, shock wave interaction with boundary layer, high streamline curvature and sequential flow separation. In order to discover the performances of different turbulence models on these flow types, analyses have been conducted on five different experimental studies each concerned with different flow and heat transfer characteristics. Each experimental study has been examined with four different turbulence models available in the commercial software (ANSYS FLUENT13.0) to decide most suitable RANS-based turbulence model. The Realizable k-&epsilon / model, Shear Stress Transport k-&omega / model, Reynolds Stress Model and V2-f model, which became increasingly popular during the last few years, have been used at the numerical simulations. According to conducted analyses, despite a few unreasonable predictions, in the majority of the numerical simulations, V2-f model outperforms other first-order turbulence models (Realizable k-&epsilon / and Shear Stress Transport k-&omega / ) in terms of accuracy and Reynolds Stress Model in terms of convergence.

Investigations into the turbulent flows in uniform and nonuniform open channels by previous researchers have demonstrated the requirement and importance of understanding the turbulence structures and energy losses due to irregularity in non- uniform open channels. Responding to this requirement, the turbulent flow in one special non-uniform open channel has been studied both experimentally and numerically. This non-uniform open channel was designed so that its width and bed level vary while its cross-sectional area below the water surface keeps constant. An upstream uniforzn open channel is attached to the non-uniform open channel to establish fully developed turbulent flow conditions. A downstream uniform channel is also attached for control of water depth and downstream flow condition. The experimental study consisted of measurements of turbulent velocity field with a LDV (Laser Doppler Velocimetry) and measurements of boundary shear stress (BSS) with Roving Preston tubes in the experimental channel. Turbulent velocity components in the longitudinal and vertical directions were measured with the LDV in forward scattering mode and the laser beams were focused from the channel side wall into the water. Turbulent velocity components in the longitudinal and transverse directions were measured with the LDV in back scattering mode and the laser beams are focused from above the water surface into the water. Both the forward scattering mode measurements and the back scattering mode measurements were taken at two cross sections in the upstream uniform open-channel and at twelve cross sections in the nonuniform open channel. Obtained data include mean longitudinal velocity U, transverse velocity V, vertical velocity W, turbulence intensities u^2, v^2, w^2, and Reynolds shear stresses -uv and -uw. The chief results of these measurements are: 1) There is no separation of flow in the nonuniform open channel. 2) As flow passes from wider and shallower section to narrower and deeper section, it responds as though it experiences contraction in horizontal planes and expansion in vertical planes. The reverse occurs as flow passes from narrower and deeper section to wider and shallower section; 3) The secondary currents in the nonuniform open channel are combinations of the effects of pure contraction and expansion of channel boundaries and the effects of the vortex kind secondary currents; 4) Turbulence intensities in the non-uniform open channel show similar distribution patterns to that in the uniform open-channel but their magnitudes change due to the change of channel shape; 5) Negative values of the Reynolds shear stresses, -uw, appear at the free surface and may extend to a large depth below the free surface in the nonuniform open channel. Boundary shear stresses in the experimental channel were measured with Roving Preston tubes. The use of the Roving Preston tubes was preceded with calibrations of themselves in air pipe flow and calibrations of a special pressure transducer in air and in water. Delicate measurement procedures were designed for measurements of BSS in the nonuniform open channel. The BSS were measured at one cross section in the uniform open-channel and at twelve cross sections in the nonuniform open channel. The chief results of these measurements are: 1) The irregularity of the nonuniform open channel significantly affects the distribution of the BSS but the total shear force has little change; 2) The effect of the secondary currents on the BSS is very similar to the effect of secondary currents on the ESS in uniform open channel; 3) The irregularity in the non-uniform open channel does not cause extra energy loss since there is no flow separation. The numerical study made use of a FEM (finite element method) commercial package FIDAP to simulate the turbulent flows in the experimental channel. These simulations are carried out with Speziale's eddy-viscosity anisotropic k-E model, the standard k-E model, and the RNG model. With each model, simulations were undertaken for four consecutive uniform channels of 5 m length so that fully developed turbulent flow conditions were established before entering into the simulation of flow in the non- uniform channel. In all simulations the free surfaces were fixed. Simulation results include U, V, W, k, and E. For turbulent flow in the uniform channel, only Speziale's model is capable of predicting qualitatively correct secondary currents. For turbulent flow in the non-uniform open channel, all three models gave similar simulation results. The calculated distribution patterns of U and W are in agreement with measurements except near the free surface but differences exist in magnitude. None of the three models was capable of modelling the transverse velocity V in the nonuniform open channel correctly. Further simulations are necessary with movable free surface and better boundary condition for the energy dissipation rate s in order to achieve better agreement with the experimental values, especially near the free surface.

No description available.

Couette, pipe, channel, and zero-pressure gradient (ZPG) turbulent boundary layer (TBL) flows have classically been considered as canonical wall-bounded turbulent flows since their near-wall behavior is generally considered to be universal, i.e. invariant of the flow case and the Reynolds number. Nevertheless, the idea that large-scale motions, being dominant in regions further away from the wall, might interact with and influence small-scale fluctuations close to the wall has not been disregarded. This view was mainly motivated due to the observed failure of collapse of the Reynolds normal stresses in viscous scaling. While this top-down influence has been studied extensively over the last decade, the idea of a bottom-up influence (backward energy transfer) is less examined. One exception was the recent experimental work on a Couette flow by Kawata, T. & Alfredsson, P. H. (Phys. Rev. Lett. 120, 244501, 2018). In the present work, a spectral representation of the Reynolds Stress transport equation is used to perform a scale-by-scale analysis of the terms in the equation. Two flow cases were studied: first, a Direct Numerical Simulation (DNS) of a Couette flow at a similar Reynolds number as Kawata and Alfredsson. The Reynolds number was ReT = 120, viscosity v. Second, a Large Eddy Simulation (LES) of a ZPG TBL at ReT = 730, 1270, and 2400. For both cases the classic interscale transport or turbulent kinetic energy was observed. However, also an inverse interscale transport of Reynolds shear stress was observed for both cases.

This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease. / Ph. D.

Yes / To reproduce the natural flow topography in a laboratory environment, it is crucial to recapture its bed condition in order to ensure the accurate representation. Water-worked bed represents a state-of-the-art experimentally formed bed to imitate the natural-formed channel in most rivers or natural streams. Recently, this technique has been intensively studied through experimental and computational approaches; however, its actual influence towards the near-bed flow as compared to experimentally prepared rough bed in well-packed bedform order are still yet to be investigated deeply. This experimental study systematically investigated and compared the differences in velocity distribution and three-dimensional (3D) turbulence characteristics, including turbulence intensities and Reynolds stresses, between uniform smooth bed, laboratory-prepared rough bed and water-worked bed open channel flows. The flow comparisons were concentrated at near-bed region where clear flow behaviour change can be observed. Through these comparisons, the study inspected the characteristics of water-worked bedform thoroughly, in order to inform future experimental research that tries to reproduce natural stream behaviours. / the Major State Basic Research Development Grant No. 2013CB036402 from Tsinghua University. The support from the Major State Basic Research Development Program (973 program) of China is also greatly appreciated. We also acknowledge the National Key Research and Development Project from the Ministry of Science and Technology during the Thirteenth Five-year Plan Period (Grant No. 2017YFC0403600) and the Science and Technology Projects State Grid Corporation of China (Grant No. 52283014000T).

The present work combines experimental and numerical analyses to improve current understanding of turbulence in stably stratified flows. An extensive literature review is presented on the mechanisms governing turbulence under stratified conditions, with a special focus on the Richardson number parameter, as it is often adopted as a switch to turn turbulence modelling on/off. Anisotropization of turbulence is investigated, as it is found to be an important mechanism for turbulence survival at any Richardson number, but usually overlooked in turbulence parameterizations. For this purpose, an experimental dataset previously collected over an Alpine glacier is used, with a focus on the anisotropy of the Reynolds stress tensor, as the scientific community has recently shown improvements in the description of the atmospheric surface layer by taking this aspect into account. Different sources leading stresses to deviate from the isotropic limit are explored, as well as energy exchanges across scales and between kinetic and potential reservoirs, in order to identify the main processes that should be included in turbulence parameterizations to properly represent anisotropic turbulence under stable conditions. High-resolution numerical simulations are then performed with the Weather Research and Forecasting (WRF) model to evaluate different PBL parameterizations in reproducing specific stable atmospheric conditions developing over complex terrain, and their influence on the local circulation. For this purpose, two wintertime case studies in a basin-like area of an Alpine valley are investigated. Both are fair-weather episodes with weak synoptic forcing and well-developed diurnal local circulations, differing by the thermal stratification in the basin. In particular, the influence of thermal stratification on the outbreak of a valley-exit wind coming from a tributary valley is investigated, and the influence of such type of flows on turbulence anisotropy in stably stratified conditions is discussed for future investigations.

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>

This master's thesis analyses fluid flow in an open cylindrical tank with vortex using numerical simulation. The theoretical part introduces a set of equations governing fluid flow and relations used to describe vortex motion. A general overview of terms used in computational fluid dynamics is presented. The experimental section consists of three parts. The vortex modelling is performed using ANSYS Fluent software. Data are consequently analysed within ANSYS CFD-Post software tool. Special focus is put on the vortex shape that is fitted with a curve corresponding to a probability density function of the Cauchy distribution. Results are then plotted in MATLAB software.