Showing papers on "Flow separation published in 1997"

Turbulence Modeling Validation, Testing, and Development

Abstract: The primary objective of this work is to provide accurate numerical solutions for selected flow fields and to compare and evaluate the performance of selected turbulence models with experimental results. Four popular turbulence models have been tested and validated against experimental data often turbulent flows. The models are: (1) the two-equation k-epsilon model of Wilcox, (2) the two-equation k-epsilon model of Launder and Sharma, (3) the two-equation k-omega/k-epsilon SST model of Menter, and (4) the one-equation model of Spalart and Allmaras. The flows investigated are five free shear flows consisting of a mixing layer, a round jet, a plane jet, a plane wake, and a compressible mixing layer; and five boundary layer flows consisting of an incompressible flat plate, a Mach 5 adiabatic flat plate, a separated boundary layer, an axisymmetric shock-wave/boundary layer interaction, and an RAE 2822 transonic airfoil. The experimental data for these flows are well established and have been extensively used in model developments. The results are shown in the following four sections: Part A describes the equations of motion and boundary conditions; Part B describes the model equations, constants, parameters, boundary conditions, and numerical implementation; and Parts C and D describe the experimental data and the performance of the models in the free-shear flows and the boundary layer flows, respectively.

A new subgrid-scale orographic drag parametrization: Its formulation and testing

Abstract: A scheme for the representation of subgrid-scale orography (SSO) in numerical weather prediction and climate models is presented. the new scheme arose in part from a desire to represent nonlinear low-level mountain drag effects not currently parametrized. an important feature of the scheme is that it deals explicitly with a low-level flow which is ‘blocked’, when the effective height of the subgrid-scale orography is sufficiently high. In this new scheme, it is assumed that, for this ‘blocked’ flow, separation occurs at the mountain flanks, resulting in a form drag. This drag is parametrized on model levels which are intersected by the SSO, and provides a dynamically based replacement for envelope orography. the upper part of the low-level flow goes over the orography and generates gravity waves. At the model resolutions considered (T106 and T213) it is assumed that the length scales characteristic of the SSO are sufficiently small for the Coriolis force to be neglected. the various parameters of the scheme are adjusted using an off-line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the Pyrenees; and these estimates are validated with the PYREX data. Forecasts using T106 and T213 resolutions with this new scheme, and with mean orography, show that the forecast mountain drag consistently reproduces the drag measured during PYREX whenever the flow component normal to the ridge is large. Isentropic flow diagnostics, further, show that the new scheme has a realistic impact on the flow dynamics, reinforcing the low-level wake observed in mesoscale analyses of the flow. With this new scheme and a mean orography, the ECMWF model outperformed, in forecast skill, a version of the model which had an envelope orography and the old gravity-wave-drag scheme, while no longer suffering any disadvantages of envelope orography. the proposed low-level drag parametrization should also be relevant at model horizontal resolutions much higher than T213.

Turbulence modeling validation

Abstract: The performances of four turbulence models are evaluated against eight selected experimental flow fields. The four models are the two-equation k-e model of Launder and Sharma, the two-equation k-a> model of Wilcox, the twoequation k-03 SST model of Menter, and the one-equation eddy-viscosity model of Spalart and Allmaras. The eight turbulent flows of the validation are four fully-developed freeshear flows, an incompressibl e boundary layer, and three complex flows with flow separation. The free-shear layer flows comprise a mixing layer, a round jet, a plane jet, and a plane wake flow. The three complex flows are comprised of an adverse-pressure-gradient boundary layer, an axisymmetric shock-wave/boundary-layer interaction, and a transonic RAE 2822 airfoil flow. The experimental data for these flows is well established and has been extensively used in model developments. The numerical predictions include mean velocity profiles, spreading rates, surface pressure coefficients, skin friction, and shear-stress profiles. Most significantly, this research includes a sensitivity study on the accuracy of the solutions with respect to the effects of freestream turbulence, grid resolution, grid spacing near the wall, initial conditions, numerical methods and codes, and free stream Mach number effects on incompressible flows.

Annual Research Briefs

Abstract: This report contains the 1997 annual progress reports of the research fellows and students supported by the Center for Turbulence Research (CTR). Titles include: Invariant modeling in large-eddy simulation of turbulence; Validation of large-eddy simulation in a plain asymmetric diffuser; Progress in large-eddy simulation of trailing-edge turbulence and aeronautics; Resolution requirements in large-eddy simulations of shear flows; A general theory of discrete filtering for LES in complex geometry; On the use of discrete filters for large eddy simulation; Wall models in large eddy simulation of separated flow; Perspectives for ensemble average LES; Anisotropic grid-based formulas for subgrid-scale models; Some modeling requirements for wall models in large eddy simulation; Numerical simulation of 3D turbulent boundary layers using the V2F model; Accurate modeling of impinging jet heat transfer; Application of turbulence models to high-lift airfoils; Advances in structure-based turbulence modeling; Incorporating realistic chemistry into direct numerical simulations of turbulent non-premixed combustion; Effects of small-scale structure on turbulent mixing; Turbulent premixed combustion in the laminar flamelet and the thin reaction zone regime; Large eddy simulation of combustion instabilities in turbulent premixed burners; On the generation of vorticity at a free-surface; Active control of turbulent channel flow; A generalized framework for robust control in fluid mechanics; Combined immersed-boundary/B-spline methods for simulations of flow in complex geometries; and DNS of shock boundary-layer interaction - preliminary results for compression ramp flow.

Turbulent drag reduction using compliant surfaces

Abstract: Over the past forty years intensive investigations into the use of compliant surfaces have been undertaken, both theoretically and experimentally, in order to obtain turbulent drag reduction in boundary–layer flows. Although positive results were found in some of the studies, none of these had been successfully validated by independent researchers. In this paper the results are reported of a recent investigation carried out by the authors to verify the experimental results of Semenov in 1991 and Kulik and co–workers in 1991, who successfully demonstrated the ability of compliant surfaces to reduce the skin–friction drag and surface–flow noise in a turbulent boundary layer. A strain–gauge force balance was used in the present study to directly measure the turbulent skin–friction drag of a slender body of revolution in a water tunnel. Changes in the structure of turbulent boundary layer over a compliant surface in comparison with that over a rigid surface were also examined. The results clearly demonstrate that the turbulent skin friction is reduced for one of the two compliant coatings tested, indicating a drag reduction of up to 7 per cent within the entire speed range of the tests. The intensities of skin–friction and wall–pressure fluctuations measured immediately downstream from the compliant coating show reductions in the intensities of up to 7 and 19 per cent, respectively. The results also indicate reductions in turbulence intensity by up to 5 per cent across almost the entire boundary layer. Furthermore, an upwards shift of the logarithmic velocity profile is also evident indicating that the thickness of the viscous sublayer is increased as a result of turbulent drag reduction due to the compliant coating. It is considered that the results of the present experimental investigation convincingly demonstrate for the first time since the earlier work in Russia (by Semenov and Kulik) that a compliant surface can indeed produce turbulent drag reduction in boundary–layer flows.

Boundary Layer Development in Axial Compressors and Turbines: Part 1 of 4—Composite Picture

Abstract: Comprehensive experiments and computational analyses were conducted to understand boundary layer development on airfoil surfaces in multistage, axial-flow compressors and LP turbines. The tests were run over a broad range of Reynolds numbers and loading levels in large, low-speed research facilities which simulate the relevant aerodynamic features of modern engine components. Measurements of boundary layer characteristics were obtained by using arrays of densely packed, hot-film gauges mounted on airfoil surfaces and by making boundary layer surveys with hot wire probes. Computational predictions were made using both steady flow codes and an unsteady flow code. This is the first time that time-resolved boundary layer measurements and detailed comparisons of measured data with predictions of boundary layer codes have been reported for multistage compressor and turbine blading. Part 1 of this paper summarizes all of our experimental findings by using sketches to show how boundary layers develop on compressor and turbine blading. Parts 2 and 3 present the detailed experimental results for the compressor and turbine, respectively. Part 4 presents computational analyses and discusses comparisons with experimental data. Readers not interested in experimental detail can go directly from Part 1 to Part 4. For both compressor and turbine blading, the experimental results show large extents of laminar and transitional flow on the suction surface of embedded stages, with the boundary layer generally developing along two distinct but coupled paths. One path lies approximately under the wake trajectory while the other lies between wakes. Along both paths the boundary layer clearly goes from laminar to transitional to turbulent. The wake path and the non-wake path are coupled by a calmed region, which, being generated by turbulent spots produced in the wake path, is effective in suppressing flow separation and delaying transition in the non-wake path. The location and strength of the various regions within the paths, such as wake-induced transitional and turbulent strips, vary with Reynolds number, loading level, and turbulence intensity. On the pressure surface, transition takes place near the leading edge for the blading tested. For both surfaces, bypass transition and separated-flow transition were observed. Classical Tollmien-Schlichting transition did not play a significant role. Comparisons of embedded and first-stage results were also made to assess the relevance of applying single-stage and cascade studies to the multistage environment. Although doing well under certain conditions, the codes in general could not adequately predict the onset and extent of transition in regions affected by calming. However, assessments are made to guide designers in using current predictive schemes to compute boundary layer features and obtain reasonable loss predictions.

Turbulent drag reduction by passive mechanisms

Abstract: In many situations involving flows of high Reynolds number (where inertial forces dominate over viscous forces), such as aircraft flight and the pipeline transportation of fuels, turbulent drag is an important factor limiting performance. This has led to an extensive search for both active and passive methods for drag reduction1. Here we report the results of a series of wind-tunnel experiments that demonstrate a passive means of effectively controlling turbulence in channel flow. Our approach involves the introduction of specified patterns of protrusions on the confining walls, which interact with the coherent, energy-bearing eddy structures in the wall region, and so influence the rate at which energy is dissipated in the turbulent flow. We show that relatively small changes in the arrangement of these protrusions can alter the response of the system from one of drag decrease to increased mixing (drag enhancement).

Numerical simulations of laminar flow over a 3D backward‐facing step

Abstract: A numerical investigation of laminar flow over a three-dimensional backward-facing step is presented with comparisons with detailed experimental data, available in the literature, serving to validate the numerical results. The continuity constraint method, implemented via a finite element weak statement, was employed to solve the unsteady three-dimensional Navier–Stokes equations for incompressible laminar isothermal flow. Two-dimensional numerical simulations of this step geometry underestimate the experimentally determined extent of the primary separation region for Reynolds numbers Re greater than 400. It has been postulated that this disagreement between physical and computational experiments is due to the onset of three-dimensional flow near Re ≈ 400. This paper presents a full three-dimensional simulation of the step geometry for 100⩽ Re⩽ 800 and correctly predicts the primary reattachment lengths, thus confirming the influence of three-dimensionality. Previous numerical studies have discussed possible instability modes which could induce a sudden onset of three-dimensional flow at certain critical Reynolds numbers. The current study explores the influence of the sidewall on the development of three-dimensional flow for Re greater than 400. Of particular interest is the characterization of three-dimensional vortices in the primary separation region immediately downstream of the step. The complex interaction of a wall jet, located at the step plane near the sidewall, with the mainstream flow reveals a mechanism for the increasing penetration (with increasing Reynolds number) of three-dimensional flow structures into a region of essentially two-dimensional flow near the midplane of the channel. The character and extent of the sidewall-induced flow are investigated for 100⩽Re⩽ 800. © 1997 John Wiley & Sons, Ltd.

Properties of the mean recirculation region in the wakes of two-dimensional bluff bodies

Abstract: The properties of the time- and span-averaged mean wake recirculation region are investigated in separated flows over several different two-dimensional bluff bodies Ten different cases are considered and they divide into two groups: cylindrical geometries of circular, elliptic and square cross-sections and the normal plate A wide Reynolds number range from 250 to 140000 is considered, but in all the cases the attached portion of the boundary layer remains laminar until separation The lower Reynolds number data are from direct numerical simulations, while the data at the higher Reynolds number are obtained from large-eddy simulation and the experimental work of Cantwell & Coles (1983), Krothapalli (1996, personal communication), Leder (1991) and Lyn et al (1995) Unlike supersonic and subsonic separations with a splitter plate in the wake, in all the cases considered here there is strong interaction between the shear layers resulting in Karman vortex shedding The impact of this fundamental difference on the distribution of Reynolds stress components and pressure in relation to the mean wake recirculation region (wake bubble) is considered It is observed that in all cases the contribution from Reynolds normal stress to the force balance of the wake bubble is significant In fact, in the cylinder geometries this contribution can outweigh the net force from the shear stress, so that the net pressure force tends to push the bubble away from the body In contrast, in the case of normal plate, owing to the longer wake, the net contribution from shear stress outweighs that from the normal stress At higher Reynolds numbers, separation of the Reynolds stress components into incoherent contributions provides more insight The behaviour of the coherent contribution, arising from the dominant vortex shedding, is similar to that at lower Reynolds numbers The incoherent contribution to Reynolds stress, arising from small-scale activity, is compared with that of a canonical free shear layer Based on these observations a simple extension of the wake model (Sychev 1982; Roshko 1993a, b) is proposed

Airfoil boundary-layer development and transition with large leading-edge roughness

Abstract: An experimentalstudy of the effects of largedistributed roughness located neartheleading edgeofan airfoilhas been performed to determine the effect on boundary-layer development and transition. Boundary-layer measure- ments werecarried out on a two-dimensional NACA0012 airfoil with a 53.34-cm chord through theuse of hot-wire anemometry at Reynolds numbers of 0 .75 £ 10 6 , 1.25 £ 10 6 , and 2.25 £ 10 6 . These measurements included mean anductuating velocity, turbulence intensity, ¯ ow® eld intermittency, and associated integral parameters. The roughness used was of the type and density observed to occur during the initial glaze ice accretion process. Results have shown that the transitional boundary layer induced by large distributed roughness is markedly different from the smooth model Tollmein± Schlicting induced transition process. No fully developed turbulent boundary layers were observed to occur near the roughness location. Instead, the large distributed roughness was observed to trigger a transitional boundary layer at or very near the roughness location. This transitional boundary layer required asubstantialchordwiseextentto obtaina fully developedturbulentstate.Streamwiseturbulenceintensity levels in the roughness induced transitional region were observed to berelatively low as compared with the smooth model transitional region.

Energy injection in closed turbulent flows: Stirring through boundary layers versus inertial stirring

Abstract: The mean rates of energy injection and energy dissipation in steady regimes of turbulence are measured in two types of flow confined in closed cells. The first flow is generated by counterrotating stirrers and the second is a Couette-Taylor flow. In these two experiments the solid surfaces that set the fluid into motion are at first smooth, so that everywhere the velocity of the stirrers is locally parallel to its surface. In all such cases the mean rate of energy dissipation does not satisfy the scaling expected from Kolmogorov theory. When blades perpendicular to the motion are added to the stirring surfaces the Kolmogorov scaling is observed in all the large range of Reynolds numbers (10,Re,10) investigated. However, with either smooth or rough stirring the measurements of the pressure fluctuations exhibit no Reynolds number dependence. This demonstrates that, though the smooth stirrers are less efficient in setting the fluid into motion, their efficiency is independent of the Reynolds number so that the Kolmogorov scaling characterizes, in all cases, the dissipation in the bulk of the fluid. The difference in the global behaviors corresponds to a different balance between the role of the different regions of the flow. With smooth stirrers the dissipation in the bulk is weaker than the Reynoldsnumber-dependent dissipation in the boundary layers. With rough ~or inertial! stirrers the dissipation in the bulk dominates, hence the Kolmogorovian global behavior. @S1063-651X~97!11606-3#

Circulation around islands and ridges

Abstract: The circulation in an ocean basin containing an island is studied under nearly geostrophic, beta plane dynamics. The model is a fluid of uniform density driven by wind forcing or sources and sinks of mass at the upper boundary of the flow. The circulation is studied analytically, numerically, as well as in the laboratory through the device of the “sliced cylinder” model for the ocean circulation. Of particular interest is the estimate of the transport between the island and the oceanic basin’s boundary. The model is conceived as relevant to both the wind-driven circulation as well as the circulation of abyssal waters around deep topographic features such as mid-ocean ridge segments. Godfrey’s Island Rule for the transport is rederived in general form and the validity of the original approximation of Godfrey (1989) is examined in a variety of circumstances. In particular, the role of dissipative boundary layers and inertial effects such as vortex shedding are scrutinized to determine their role in determining the net transport around the island. Linear theory in many cases predicts a recirculation on the eastern side of the island, provided the meridional extent of the island is large enough. The existence of the recirculation, containing trapped fluid, is confirmed in both laboratory and numerical experiments and the evolution of the structure of the recirculation is examined as a function of the boundary layer Reynolds number. In both the laboratory and numerical studies, the recirculation predicted by linear theory is joined and then superseded by an inertial recirculation springing from boundary layer separation as the Reynolds number increases past a critical value. Even in the linear limit it is shown that the recirculation region, which is closed in quasigeostrophic theory, is subject to a small leak due to planetary geostrophic effects, which prediction is confirmed in the laboratory. The original island rule of Godfrey yields an estimate of the transport which is surprisingly robust and generally within 75% of the values measured in our numerical experiments. Agreement is moderately good when island western boundary layer transport is used as a basis for comparison. Several cases are discussed, however, in which the assumptions made by Godfrey are violated. One occurs when the frictional boundary layers of the island and the basin boundary overlap. We derive a threshold width for the gap for the case where the island is close to a northern or southern boundary of the basin and show how the transport is increasingly blocked as the gap is reduced. A second case occurs when the island is thin and zonally elongated so that the dissipative effects on the northern and southern boundaries of the island become important. Here the vorticity balance assumed in the simple Island Rule is fundamentally altered, and we extend the Island Rule to account for the new dissipation.

CFD calculations of S809 aerodynamic characteristics

Abstract: Steady-state, two-dimensional CFD calculations were made for the S809 laminar-flow, wind-turbine airfoil using the commercial code CFD-ACE. Comparisons of the computed pressure and aerodynamic coefficients were made with wind tunnel data from the Delft University 1.8 m x 1.25 m low-turbulence wind tunnel. This work highlights two areas in CFD that require further investigation and development in order to enable accurate numerical simulations of flow about current generation wind-turbine airfoils: transition prediction and turbulence modeling. The results show that the laminar-to-turbulent transition point must be modeled correctly to get accurate simulations for attached flow. Calculations also show that the standard turbulence model used in most commercial CFD codes, the k-{epsilon} model, is not appropriate at angles of attack with flow separation.

Similarity law for the streamwise turbulence intensity in zero-pressure-gradient turbulent boundary layers

Abstract: A similarity relationship is proposed to describe the streamwise broadband-turbulence intensity in a zero-pressure-gradient boundary layer. The formulation is applicable to the entire region of the flow beyond the viscous buffer zone and is based on the attached eddy hypothesis, the Reynolds-number-similarity hypothesis and the assumed existence of Kolmogorov eddies with a universal inertial subrange. Experimental data of the authors and those from various published works covering a large Reynolds number range are investigated in light of this formulation.

Prediction of High-Lift Flows Using Turbulent Closure Models

Abstract: The flow over two different multi-element airfoil configurations is computed using linear eddy viscosity turbulence models and a nonlinear explicit algebraic stress model. A subset of recently-measured transition locations using hot film on a McDonnell Douglas configuration is presented, and the effect of transition location on the computed solutions is explored. Deficiencies in wake profile computations are found to be attributable in large part to poor boundary layer prediction on the generating element, and not necessarily inadequate turbulence modeling in the wake. Using measured transition locations for the main element improves the prediction of its boundary layer thickness, skin friction, and wake profile shape. However, using measured transition locations on the slat still yields poor slat wake predictions. The computation of the slat flow field represents a key roadblock to successful predictions of multi-element flows. In general, the nonlinear explicit algebraic stress turbulence model gives very similar results to the linear eddy viscosity models.

Sinusoidal forcing of a turbulent separation bubble

Abstract: A turbulent separation bubble is forced by single- and double-frequency sinusoidal disturbances, with the emphasis placed on the reattachment length as a function of the forcing amplitude and frequency. The separation bubble is that formed along the side of a blunt circular cylinder with a square leading edge. In single-frequency forcing, the reattachment length attains a minimum at a particular forcing frequency, F, which scales with the frequency of shedding of vortices from the reattachment region of the separated shear layer. A flow model is presented to interpret the frequency F. Forcing of sufficiently high amplitude eliminates the recirculating region in a range of the forcing frequency. Flow visualization and a survey of the mean flow and turbulence properties demonstrate how the flow in the separated shear layer is modified by the forcing. In double-frequency forcing, the superposition of the F-component on its higher or subharmonic components is considered. A non-resonant combination of the two frequencies is also considered.

The entrance effect of laminar flow over a backward-facing step geometry

Abstract: The study investigates the entrance effect for flow over a backward-facing step by comparing predictions that set the inlet boundary at various locations upstream of the sudden expansion. Differences are most significant in the sudden expansion region. For certain cases, predictions with a high expansion number are perturbed by the entrance effect more than low-expansion-number predictions; however, the effect is localized in the sudden expansion region. Channels with low expansion numbers always experience a greater entrance effect after some distance upstream and downstream of the sudden expansion. The boundary layer growth in the inlet channel was examined using a uniform inlet velocity profile.

Effect of saltating sediment load on the determination of the mean velocity of overland flow

Abstract: Where overland flow velocity is measured using dye or salt tracing, the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor α. Flume experiments show that a varies with both Reynolds number and sediment load. For sediment-free flow over a sand-covered bed, α is less than the theoretical value of 0.67 in laminar flow and increases rapidly with Reynolds number in transitional flow and more slowly with Reynolds number in turbulent flow. For sediment-laden flow, α decreases as sediment load increases in transitional and turbulent flows. Saltating sediment extracts momentum from the flow, causing velocities near the bed to decrease and α to decrease. A multiple-regression equation is developed which can be used to predict the value of α from Reynolds number and sediment load in transitional and turbulent overland flows.

Laser Anemometer Measurements of the Flow Field in a 4:1 Pressure Ratio Centrifugal Impeller

Abstract: A laser-doppler anemometer was used to obtain flow-field velocity measurements in a 4:1 pressure ratio, 4.54 kg/s (10 lbm/s), centrifugal impeller, with splitter blades and backsweep, which was configured with a vaneless diffuser. Measured through-flow velocities are reported for ten quasi-orthogonal survey planes at locations ranging from 1% to 99% of main blade chord. Measured through-flow velocities are compared to those predicted by a 3-D viscous steady flow analysis (Dawes) code. The measurements show the development and progression through the impeller and vaneless diffuser of a through-flow velocity deficit which results from the tip clearance flow and accumulation of low momentum fluid centrifuged from the blade and hub surfaces. Flow traces from the CFD analysis show the origin of this deficit which begins to grow in the inlet region of the impeller where it is first detected near the suction surface side of the passage. It then moves toward the pressure side of the channel, due to the movement of tip clearance flow across the impeller passage, where it is cut by the splitter blade leading edge. As blade loading increases toward the rear of the channel the deficit region is driven back toward the suction surface by the cross-passage pressure gradient. There is no evidence of a large wake region that might result from flow separation and the impeller efficiency is relatively high. The flow field in this impeller is quite similar to that documented previously by NASA Lewis in a large low-speed backswept impeller.

The effect of slight viscosity on a near-critical swirling flow in a pipe

Abstract: The effect of slight viscosity on a near-critical axisymmetric incompressible swirling flow in a straight pipe is studied. We demonstrate the singular behavior of a regular-expansion solution in terms of the slight viscosity around the critical swirl. This singularity infers that large-amplitude disturbances may be induced by the small viscosity when the incoming flow to the pipe has a swirl level around the critical swirl. It also provides a theoretical understanding of Hall’s boundary layer separation analogy to the vortex breakdown phenomenon. In order to understand the nature of flows in this swirl range, we develop a small-disturbance analysis. It shows that a small but finite viscosity breaks the transcritical bifurcation of solutions of the Euler equations at the critical swirl into two branches of solutions of the Navier–Stokes equations. These branches fold at limit points near the critical swirl with a finite gap between the two branches. This means that no near-columnar equilibrium state can ex...

Prediction of the Becalmed Region for LP Turbine Profile Design

Abstract: Recent attention has focused on the so-called ``becalmed region`` that is observed inside the boundary layers of turbomachinery blading and is associated with the process of wake-induced transition. Significant reductions of profile loss have been shown for high lift LP turbine blades at low Reynolds numbers due to the effects of the becalmed region on the diffusing flow at the rear of the suction surface. In this paper the nature and the significance of the becalmed region are examined using experimental observations and computational studies. It is shown that the becalmed region may be modeled using the unsteady laminar boundary layer equations. Therefore, it is predictable independent of the transition or turbulence models employed. The effect of the becalmed region on the transition process is modeled using a spot-based intermittency transition model. An unsteady differential boundary layer code was used to simulate a deterministic experiment involving an isolated turbulent spot numerically. The predictability of the becalmed region means that the rate of entropy production can be calculated in that region. It is found to be of the order of that in a laminar boundary layer. It is for this reason and because the becalmed region may be encroached upon bymore » pursuing turbulent flows that for attached boundary layers, wake-induced transition cannot significantly reduce the profile loss. However, the becalmed region is less prone to separation than a conventional laminar boundary layer. Therefore, the becalmed region may be exploited in order to prevent boundary layer separation and the increase in loss that this entails. It is shown that it should now be possible to design efficient high lift LP turbine blades.« less

An Experimental Investigation of Transition as Applied to Low Pressure Turbine Suction Surface Flows

Abstract: Results of an experimental study of flow separation and transition in either attached boundary layers or separated shear layers over the suction surface of a simulation of a low-pressure turbine airfoil flow are presented. Detailed velocity profiles were measured with the hot-wire technique. Static pressure distributions are also presented. Flow transition is documented using measured intermittency distributions in the boundary layer and the separated shear layer. Cases for Reynolds numbers of 50,000, 100,000, 200,000 and 300,000 are reported. These Reynolds numbers are based on suction surface length and exit velocity. Three Free Stream Turbulence Intensity values, 0.5%, 2.5% and 10%, are represented. Flow separation is observed for all the low-FSTI cases. Of these, the lowest Reynolds number case was not able to complete transition of the shear layer and the separation bubble persisted over the entire blade surface. For the other low-FSTI cases, transition is observed in the shear layer over the separation bubble. This transition proceeded quickly, spreading rapidly toward the wall. Elevated FSTI drives an earlier transition than in the low-FSTI cases and the separation bubbles are smaller. For the highest Reynolds number cases with 2.5% and 10% FSTI, transition is of the attached boundary layer and no separation exists. Flow separation with shear flow transition is observed for the lower-Re cases. Models for intermittency and transition length and location from the modern literature are assessed.

Stokes flow in a slowly varying two-dimensional periodic pore

Abstract: This article presents a series solution to the velocity in a two-dimensional long sinusoidal channel. The approach is based on a stream function formulation of the Stokes problem and a series expansion in terms of the width to the length ratio, which is considered small. Results show how immobile zones may appear and even flow separation and nonturbulent eddies, even in the absence of prima facie dead-end pores. It is shown that the flow tends to concentrate in strips connecting pore throats.

Basal-flow characteristics of a non-linear flow sliding frictionless over strongly undulating bedrock

Abstract: The flow field of a medium sliding without friction over a strongly undulating surface is calculated numerically. The results are used to elucidate the basal-flow characteristics of glacier flow and they are discussed with reference to known analytical solutions. Extrusion flow is found to become increasingly pronounced as the value of n, where n is a parameter in Glen’s flow law, becomes larger. For sinusoidal bedrock undulations, a flow separation occurs if the amplitude-to-wavelength ratio exceeds a critical value of about 0.28. The main flow then sets up a secondary flow circulation within the trough, and the ice participating in this circular motion theoretically never leaves it. The sliding velocity is calculated numerically as a function of the mean basal shear stress, the amplitude-to-wavelength ratio and the flow parameter n. For moderate and high slope fluctuations, the sliding velocity is significantly different from what would be expected from results based on the small-slope approximation.

Flow Analysis in a Pump Diffuser—Part 1: LDA and PTV Measurements of the Unsteady Flow

Abstract: This paper describes experimental research aimed at improving our understanding of the complex unsteady three-dimensional flow field associated with the interaction between a pump impeller and its vaned diffuser The paper provides the results of experiments carried out using Laser Particle Tracking Velocimetry (LPTV) and Laser Doppler Anemometry (LDA), in which time-resolved details of the unsteady flow field in a vaned diffuser of a medium specific speed pump have been obtained as a function of the local position of the pump impeller blades Detailed flow field measurements have been carried out at several measurement positions in the diffuser and at a number of operating points along the pump characteristic The measurement results have been analyzed to elucidate some interesting flow features observed in this typical pump diffuser These include three-dimensional flow at the impeller outlet, flow separation in the diffuser channel, unsteady recirculation of the flow from the diffuser into the impeller, the passage of vorticity in the impeller blade wakes through the diffuser, and periodic unsteadiness and turbulence in the diffuser flow channel The relevance of these flow features to the stability of the pump characteristic is discussed

The turbulence structure of a reattaching axisymmetric compressible free shear layer

Abstract: The reattachment of a supersonic, axisymmetric shear layer downstream of a blunt-based afterbody is studied. Of primary interest are the effects of the “extra” strain rates, such as bulk compression, concave streamline curvature, and lateral streamline convergence associated with shear layer reattachment on the structure of the turbulence field. Experimental turbulence data obtained throughout the reattachment region with a two-component laser Doppler velocimeter are presented. In general, the axisymmetric compliant boundary reattachment process is shown to be different in character compared to the planar solid wall case. Most notably, significant reductions in the Reynolds stresses occur through the reattachment region due to the dominating effect of lateral streamline convergence as the flow approaches the axis. Similar to the planar solid wall case, however, a reduction in the mean turbulent transport toward the axis in the reattachment region was found, which suggests a radial containment of the large...

Tip Vortex Formation and Cavitation

Abstract: We summarize recent research on the relation between boundary layer flow, tip vortex structure for a finite span wing, and cavitation. Three hydrofoils of elliptic planform of aspect ratio 3 were constructed with different NACA cross sections. Using a sprayed oil droplet technique to visualize the boundary layer flow, each foil was found to have dramatically different flow separation characteristics on both the suction and pressure sides. Careful examination of the tip region suggests that while the initial stages of vortex roll-up from the pressure side are similar for each hydrofoil section, the vortex boundary layer interaction on the suction side differs for each section. The degree of interaction was observed to increase as the lifting efficiency decreased. Over the Reynolds number range tested, tip vortex cavitation inception has been observed to follow an almost universal scaling. Differences in this scaling law are correlated with the degree of vortex/boundary layer interaction.

Numerical prediction of laminar, transitional and turbulent flows in shrouded rotor-stator systems

Abstract: The paper deals with numerical prediction of laminar, transitional and turbulent regimes in confined flow between rotating and stationary discs. For the laminar and transitional flows, a spectral tau-Chebyshev method associated with a multistep time scheme is used. This approach allows accurate prediction of the two laminar regimes mentioned by Daily and Nece (1960) in their experimental studies. For the geometry under consideration (1/11 aspect ratio), the transition to unsteady motion occurs abruptly without any oscillatory behavior. Thus, the instabilities develop in a region localized near the external shroud, primarily along the stator side, according with experimental findings. For calculating turbulent flow regimes, one point second-order transport modeling has been implemented in a finite volume code. The superiority of advanced Reynolds stress transport models over the classical k−e model is decisive for predicting such a complex flow. This is particularly important in order to get a precise delineation of the adjacent turbulent and relaminarized regions within the cavity. This level of closure was crucial to produce numerical results in good agreement with experimental data.

Bowed stators : An example of CFD applied to improve multistage compressor efficiency

Abstract: Analysis of multistage compressor stator surface static pressure data reveals that the radial growth of suction surface corner separation prematurely separates core flow stator sections, limiting their pressure rise capability and generating endwall loss Modeling of the stator flowfield, using a three-dimensional Euler analysis, has led to the development of bowed stator shapes, which generate radial forces that reduce diffusion rates in the suction surface corners, in order to delay the onset of corner separation Experimental testing of the bowed stator concept in a three-stage research compressor has confirmed the elimination of suction surface corner separation, the resulting reduction of the endwall loss, and the increase in pressure rise capability of the stator core sections This results in more robust pressure rise characteristics and substantially improved efficiency over the entire flow range of the compressor The strong interaction effects of the bowed stator with the viscous endwall flowfield are shown to be predictable using a three-dimensional multistage Navier-Stokes analysis This permits matching of the rotors to the altered stator exit profiles, in order to avoid potential stability limiting interactions Application of bowed stators to a high bypass ratio engine eleven-stage high-pressure compressor has resulted in substantial improvement in efficiency, withmore » no stability penalty« less