Separation-induced Transition Research Articles

Large Eddy Simulation of Boundary Layer Transition Mechanisms in a Gas-Turbine Compressor Cascade

Large eddy simulation (LES) is used to explore the boundary layer transition mechanisms in two rectilinear compressor cascades. To reduce numerical dissipation, a novel locally adaptive smoothing (LAS) scheme is added to an unstructured finite volume solver. The performance of a number of subgrid scale (SGS) models is explored. With the first cascade, numerical results at two different freestream turbulence intensities (Ti's), 3.25% and 10%, are compared. At both Ti's, time-averaged skin-friction and pressure coefficient distributions agree well with previous direct numerical simulations (DNS). At Ti = 3.25%, separation-induced transition occurs on the suction surface, while it is bypassed on the pressure surface. The pressure surface transition is dominated by modes originating from the convection of Tollmien–Schlichting waves by Klebanoff streaks. However, they do not resemble a classical bypass transition. Instead, they display characteristics of the “overlap” and “inner” transition modes observed in the previous DNS. At Ti = 10%, classical bypass transition occurs, with Klebanoff streaks incepting turbulent spots. With the second cascade, the influence of unsteady wakes on transition is examined. Wake-amplified Klebanoff streaks were found to instigate turbulent spots, which periodically shorten the suction surface separation bubble. The celerity line corresponding to 70% of the free-stream velocity, which is associated with the convection speed of the amplified Klebanoff streaks, was found to be important.

Modelling boundary-layer transition on wings operating in ground effect at low Reynolds numbers

The transition-sensitive, three-equation k- kL- ω eddy-viscosity closure model was used for simulations of three-dimensional, single-element and multi-element wing configurations operating in close proximity to the ground. The aim of the study was to understand whether the model correctly simulated the transitional phenomena that occurred in the low Reynolds number operating conditions and whether it offered an improvement over the classical fully turbulent k-ω shear stress transport model. This was accomplished by comparing the simulation results to experiments conducted in a 2.7 m × 1.7 m closed-return, three-quarter-open-jet wind tunnel. The model was capable of capturing the presence of a laminar separation bubble on the wing and predicted sectional forces and surface-flow structures generated by the wings in wind tunnel testing to within 2.5% in downforce and 4.1% in drag for a multi-element wing. It was found, however, that the model produced insufficient turbulent kinetic energy during shear-layer reattachment, predicted turbulent trailing-edge separation prematurely in areas of large adverse pressure gradients, and was found to be very sensitive to inlet turbulence quantities. Despite these deficiencies, the model gave results that were much closer to wind-tunnel tests than those given by the fully turbulent k-ω shear stress transport model, which tended to underestimate downforce. Significant differences between the transitional and fully turbulent models in terms of pressure field, wake thickness and turbulent kinetic energy production were found and highlighted the importance of using transitional models for wings operating at low Reynolds numbers in ground effect. The k- kL- ω model has been shown to be appropriate for the simulation of separation-induced transition on a three-dimensional wing operating in ground effect at low Reynolds number.

Extending the applicability of RANS turbulence closures to the simulation of transitional flow around hydrofoils at low Reynolds number

Growing interest in marine autonomous systems such as underwater gliders and autonomous underwater vehicles (AUVs) has led to the increase of research on hydrofoil performance at low Reynolds numbers, bringing forward the issue of simulating laminar to turbulent transition using computational fluid dynamics (CFD). Most conventional turbulence closures in practical Reynolds-averaged Navier-Stokes (RANS) simulations are incapable of predicting transition with excessive turbulence generation near the leading edge, deriving from their eddy viscosity assumption. Having the potential of inherently predicting transition by solving the near wall flow accurately, three Low-Re RANS closures with anisotropy consideration are selected and evaluated for their ability to capture both separation-induced transition and natural transition. The empirical γ−Reθ transition model is also assessed in this work. Comparison with published experiments shows the capability of the four turbulence closures to capture separation-induced transition accurately. Predicting natural transition remains more challenging: two Low-Re closures exhibit the potential to capture natural transition only at medium grid finesse levels, whereas the γ−Reθ model evidences challenges in reproducing the correct flow physics. The lag elliptic blending model provides the most accurate solution owing to its consistent grid convergence, high accuracy for predicting the hydrodynamic coefficients, and ability to reproduce the correct transition physics.

Coupling of a Reynolds Stress Model with the Transition Model

A transition transport model has been coupled with the SSG/LRR- differential Reynolds stress model to form a new transition and turbulence model containing nine transport equations. The aim is to handle complex turbulent flow together with wall-bounded transitional flow for industrial applications. Based on the difference between the Menter SST turbulence model and the SSG/LRR- model, special treatments are introduced for the length-scale determining equation of the turbulence model and the transition onset function of the intermittency transport equation of the transition model. The final transitional Reynolds stress model is calibrated based on zero-pressure-gradient flat plate flow. Additionally, a study on grid influences based on flat plate flow has been conducted. As a result, the presented model is able to predict transitional flows, including Tollmien-Schlichting transition, by-pass transition, and separation-induced transition, as well as considering complex turbulent flows. The model is validated for a number of test cases, including two-dimensional airfoils and the DLR prolate spheroid. The results are compared against transitional computations using the Langtry & Menter SST model and fully turbulent computations using the standard SSG/LRR- model. The comparisons show that the nine-equation transition model has equal or higher accuracy as the SST model for different types of flows.

Effect on Torque and Thrust of the Pointed Tip Shape of a Wind Turbine Blade

This paper presents the effect of the tip shape of a wind turbine blade on aerodynamic forces, including the effects of separation, transition and stall. A National Renewable Energy Laboratory (NREL) Phase-VI wind turbine blade was used, in which the shape of the tip was modified to a pointed tip. Computational fluid dynamics (CFD) simulations were employed for the analysis and the results were compared with the original NREL blade CFD and experimental data using ANSYS CFX (Ansys Inc., Delaware, PA, USA). To predict the separation and separation-induced transition on both near wall and far away, the shear-stress-transport (SST) Gamma-Theta turbulent model was used. The stall onset of a 20° angle of attack and its effects were also analyzed and presented. The value of torque with the pointed tip blade was found to be 3%–8% higher than the original NREL blade showing the benefit of the pointed tip. Normal force coefficient is lower at the tip for the pointed tip blade, which results in lower deformation of the blade. It was found that the pointed-tip blade is more efficient in terms of generating torque than the original NREL Phase-VI blade in the dynamic stall region of 10–15 m/s wind speeds.

An algebraic intermittency model for bypass, separation-induced and wake-induced transition

An algebraic model using the intermittency concept is proposed for laminar to turbulent transition in attached and separated boundary layers. The model modifies the production terms of the k–ω RANS turbulence model by Wilcox. For attached flows, the model describes bypass transition by taking into account two main effects, which are damping of high-frequency disturbances by a laminar shear layer and breakdown of a near-wall laminar layer perturbed by streaks. For separated flow, the model describes breakdown of a laminar free shear layer. The proposed model is a modified and extended version of an earlier model by the authors for bypass transition only (Kubacki and Dick, Int. J. Heat and Fluid Flow, 58, 68–83, 2016).The primary tuning of the model has been done with flat plate T3C flows of ERCOFTAC with steady approaching flow, relevant for bypass transition and separation-induced transition. Secondary tuning has been done with two cascades with steady approaching flow at high turbulence level, one with N3-60 (Re= 6×105) steam turbine stator vanes and one with V103 (Re= 1.385×105) compressor blades. Model validation has been done for steady approaching flows of the same cascades, but including a case with low level of free-stream turbulence, and for a cascade with T106A (Re= 1.6×105) gas turbine rotor blades for high and low levels of free-stream turbulence. Further validation has been done for flow perturbed by travelling wakes through a cascade of N3-60 vanes at high and low levels of background turbulence level. The transition model produces good results for all cases, both for bypass transition in attached boundary layer state and for transition in separated boundary layer state.

Scrutinizing URANS in Shedding Flows: The Case of Cylinder in Cross-Flow in the Subcritical Regime

To unravel the widespread perception that the RANS (Reynolds-averaged Navier-Stokes) concept is unreliable in predicting the dynamics of separated flows, we assessed the performance of two RANS closure levels, the linear eddy-viscosity (LEVM) and the second-moment (Reynolds stress, RSM) approaches in a massively separated generic flow over a bluff body. Considered is the canonical, zero-turbulence, cross-flow over an infinite cylinder with reference to our LES and the available DNS and experiments at two Reynolds numbers, Re = 3.9 × 103 and 1.4 × 105, both within the sub-critical regime with laminar separation. Both models capture successfully the vortex shedding frequency, but the low frequency modulations are detected only by the RSM. At high Reynolds numbers the RSM is markedly superior to the LEVM showing very good agreement with the LES and experimental data. The RSM, accounting naturally for the stress anisotropy and phase lag between the stress and strain eigenvectors, is especially successful in reproducing the growth rate of the turbulent kinetic energy in the initial shear layer which proved to be crucial for accurate prediction of the separation-induced transition. A scrutiny of the unsteady RANS (URANS) stress terms based on the conditional phase-averaged LES data shows a remarkable similarity of the normalized coherent and stochastic (modeled) stress components for the two Reynolds numbers considered. The mixed (cross) correlations, while non-negligible at the low Re number, diminish fast relative to the stochastic ones with increasing Reynolds number and, in the whole, are not significant to undermine the URANS concept and its applicability to high Re flows of industrial relevance.

Parametric influence on convective heat transfer for an outlet guide vane

ABSTRACTImproved understanding of the impact of the operating conditions on the heat transfer and fluid flow behaviors of an outlet guide vane (OGV) is essential for accurate prediction of the lifetime of jet engines. In this article, the heat transfer characteristics of an OGV at various Reynolds numbers (Re), free stream turbulence levels, Mach number (Ma), and surface roughness are studied numerically. The Re is kept at 300,000 and 450,000, respectively, the free stream turbulence intensity ranges from 3.2% to 13%, and the turbulent length scale is varied from 1.2 to 11 mm. The Ma is selected as 0.06, 0.25, and 0.35, and the sandy grain roughness height is increased from the smooth wall level up to 160 µm. Mid-span pressure coefficient and Nu distributions are presented. Basically, the heat transfer patterns and pressure profiles are weak functions of the Re and Ma. Increasing the Re slightly moves the transition position upstream, while the Ma has no effect on the transition process. On the suction side, the transition is induced by flow separation and a bump is visible in the pressure profile. However, the turbulence intensity, turbulence length scale, and surface roughness levels have significant effects on the heat transfer and pressure distributions. On the suction side, the bump is invisible and the “separation-induced transition” is replaced by the “by pass transition”. It is also found that the transition position moves upstream as the turbulence intensity, length scale, and roughness level increase.

Boundary-layer transition prediction using a simplified correlation-based model

This paper describes a simplified transition model based on the recently developed correlation-based γ-Reθt transition model. The transport equation of transition momentum thickness Reynolds number is eliminated for simplicity, and new transition length function and critical Reynolds number correlation are proposed. The new model is implemented into an in-house computational fluid dynamics (CFD) code and validated for low and high-speed flow cases, including the zero pressure flat plate, airfoils, hypersonic flat plate and double wedge. Comparisons between the simulation results and experimental data show that the boundary-layer transition phenomena can be reasonably illustrated by the new model, which gives rise to significant improvements over the fully laminar and fully turbulent results. Moreover, the new model has comparable features of accuracy and applicability when compared with the original γ-Reθt model. In the meantime, the newly proposed model takes only one transport equation of intermittency factor and requires fewer correlations, which simplifies the original model greatly. Further studies, especially on separation-induced transition flows, are required for the improvement of the new model.

Calibration of a γ-Re θ transition model and its application in low-speed flows

The prediction of laminar-turbulent transition in boundary layer is very important for obtaining accurate aerodynamic characteristics with computational fluid dynamic (CFD) tools, because laminar-turbulent transition is directly related to complex flow phenomena in boundary layer and separated flow in space. Unfortunately, the transition effect isnt included in todays major CFD tools because of non-local calculations in transition modeling. In this paper, Menters γ - Re θ transition model is calibrated and incorporated into a Reynolds-Averaged Navier-Stokes (RANS) code - Trisonic Platform (TRIP) developed in China Aerodynamic Research and Development Center (CARDC). Based on the experimental data of flat plate from the literature, the empirical correlations involved in the transition model are modified and calibrated numerically. Numerical simulation for low-speed flow of Trapezoidal Wing (Trap Wing) is performed and compared with the corresponding experimental data. It is indicated that the γ - Re θ transition model can accurately predict the location of separation-induced transition and natural transition in the flow region with moderate pressure gradient. The transition model effectively imporves the simulation accuracy of the boundary layer and aerodynamic characteristics.

Numerical investigations of separation-induced transition on high-lift low-pressure turbine using RANS and LES methods

At low Reynolds numbers, laminar-turbulent transition occurs on the suction side of high-lift low-pressure turbine blades. The prediction of this flow is an important step in low-pressure turbine design. Thus, the laminar-turbulent transition must be modeled or resolved. The flow around the high-lift low-pressure turbine blade T106C is predicted using Reynolds-Averaged Navier-Stokes (RANS) simulations, without and with transition model, and large-eddy simulations (LES). LES are performed in order to predict the laminar separation bubble (LSB) without any laminar-turbulent transition modeling. Only two Reynolds numbers are investigated with LES, and the current study concerns also the validation of the turbulent random flow generation technique of Smirnov et al. Reynolds number and freestream turbulence effects are studied using the analysis of the unsteady behavior of the separated shear layer and the bubble. The steady flow predicted by RANS simulation with transition model and by the time-averaged LES are in good agreement with isentropic Mach number distribution at mid-span, except for the lowest Reynolds number (Re2is = 80,000). For this last case, the separation and transition points are predicted downstream of the experimental points. The spectral analysis of LES results at different locations allows determining specific frequencies of physical mechanisms. LES are able to predict LSB over the high-lift low-pressure turbine blade T106C as RANS simulation with transition model and to capture the Kelvin–Helmholtz instability which is the cause of the transition mechanism.

천이 전달 모델을 사용한 익형 유동의 예측 성능 비교

Two-dimensional prediction capability of several analysis codes, such as XFOIL, MSES, and KFLOW, is compared and analyzed based on computational results of airfoil flows. To this end the transition transport equations are coupled with the Navier-Stokes equations for the prediction of the natural transition and the separation-induced transition. Experimental data of aerodynamic coefficients are used for comparison with numerical results for the transitional flows. Numerical predictions using the transition transport model show a good agreement with experimental data. Discrepancies have been found in the prediction of the pressure drag are mainly caused by the difference in the far-field circulation correction methods.

Effect of turbulence models on predicting HAWT rotor blade performances

The effect of turbulence models on predicting the aerodynamic performance of horizontal-axis wind turbine (HAWT) rotor blades has been investigated. The flow fields around the 2-D airfoil and the 3-D blade of the NREL phase VI wind turbine rotor have been analyzed, and the results are compared between a correlation-based transition model and two other fully turbulent models. The turbulence models selected are the Spalart-Allmaras fully turbulent one-equation model, the k-ω SST fully turbulent two-equation model, and the transition model. A vertex-centered finite-volume method based on an unstructured mesh technique was used to discretize the governing Navier-Stokes equations. The inviscid fluxes were calculated by using 2nd order Roe’s FDS, and the viscous fluxes were evaluated in a central-differencing manner. For the time integration, an implicit method based on the Gauss-Seidel iteration was used. The results showed that the transition model well captures the laminar separation bubbles on the surface of the airfoil and the blade, and these separation bubbles trigger the separation-induced transition as the laminar flow separates and re-attaches as turbulent. The separation bubbles change the flow pattern on the surface of the airfoil and on the blade, and the pressure and skin-friction distributions are also changed abruptly across the laminar-turbulent transition. With properly predicted boundary-layer transition, the results of the transition model match well with the experiment. However, the results of the fully turbulent models deviate from the experiment due to the lack of the ability of capturing the boundary-layer transition. The adoption of a proper transition turbulence model is essential for the accurate prediction of the aerodynamic loads and also the rotor performance for horizontal-axis wind turbines.

Correlation-Based Transition Transport Modeling for Three-Dimensional Aerodynamic Configurations

The correlation-based transition transport model was implemented in a hybrid Reynolds-averaged Navier–Stokes solver and validated on various test cases. The model predicts two-dimensional transition phenomena such as transition due to Tollmien–Schlichting instabilities and separation-induced transition. The present work includes results for the application of the to two three-dimensional test cases, which are the inclined prolate spheroid and the ONERA M6 wing. Depending on the flow conditions, the computational results are in good agreement with the experimental data. Once the given flow conditions lead to three-dimensional transition phenomena, the transition prediction with the model is not reliable, because the model is based on the characteristics of two-dimensional boundary layers and three-dimensional transition mechanisms are not taken into account. To close this gap, the model was extended by an approach that accounts for transition due to crossflow instabilities in three-dimensional boundary layers. This approach and its implementation into the original model are introduced. First results for the flow over two different infinite swept wing configurations, which are the ONERA D wing and the NLF(2)-0415 wing, are presented.

Investigation of the laminar separation-induced transition on two low-pressure turbine rotor blades

The present study focuses on the assessment and the validation of the Langtry-Menter correlation-based transition model, recently implemented in the RANS code elsA from ONERA. The test cases are two low-pressure turbine (LPT) rotor blades, with different compressible Zweifel loading coefficients and different diffusion rates. The experimental outlet isentropic Reynolds number, based on blade chord and outlet isentropic velocity, ranges from 22,000 to 250,000 in order to investigate the complex separation-induced transition phenomenon. The turbulence intensity is the natural freestream turbulence of the facility (approximately 0.9%). The numerical test campaign provides good predictions for the low Zweifel loading coefficient case, even at really low outlet isentropic Reynolds number (22,000). However, for the high Zweifel loading coefficient case, the calculations are not able to predict the flow behaviour. The authors stress the importance of the turbulence Reynolds number effect on the prediction of separationinduced transition for strong diffusion LPT blades associated to high Zweifel loading coefficient.

Application of Non-Linear Eddy-Viscosity Model Involving A2 Stress-Invariant Transport Equation to Transitional Flows

This paper presents an effort to model the boundary layer separation-induced transition on a flat plate with a semi-circular leading edge. The modeling is performed using the three-equation non-linear eddy-viscosity model of Craft et al. (1997), where the third equation refers to the transport of the anisotropy stress invariant A2. The three-equation model is shown to generate quite realistic results, since it is able to capture the transition mechanism by predicting correctly the transition length, and by reproducing with good accuracy both the velocity and u-RMS distributions inside the separated pre-transitional, transitional, and fully turbulent boundary layer. This observation is supported also by the very accurate predictions of the skin friction coefficient and shape factor boundary layer integral parameter, especially for cases with moderate freestream turbulence intensity.

An assessment of the laminar kinetic energy concept for the prediction of high-lift, low-Reynolds number cascade flows

The laminar kinetic energy (LKE) concept has been applied to the prediction of low-Reynolds number flows, characterized by separation-induced transition, in high-lift airfoil cascades for aeronautical low-pressure turbine applications. The LKE transport equation has been coupled with the low-Reynolds number formulation of the Wilcox's k − ω turbulence model. The proposed methodology has been assessed against two high-lift cascade configurations, characterized by different loading distributions and suction-side diffusion rates, and tested over a wide range of Reynolds numbers. The aft-loaded T106C cascade is studied in both high- and low-speed conditions for several expansion ratios and inlet freestream turbulence values. The front-loaded T108 cascade is analysed in high-speed, low-freestream turbulence conditions. Numerical predictions with steady inflow conditions are compared to measurements carried out by the von Kármán Institute and the University of Cambridge. Results obtained with the proposed model show its ability to predict the evolution of the separated flow region, including bubble-bursting phenomenon and the formation of open separations, in high-lift, low-Reynolds number cascade flows.

Calculation of High-Lift Cascades in Low Pressure Turbine Conditions Using a Three-Equation Model

A three-equation model has been applied to the prediction of separation-induced transition in high-lift low-Reynolds-number cascade flows. Classical turbulence models fail to predict accurately laminar separation and turbulent reattachment, and usually overpredict the separation length, the main reason for this being the slow rise of the turbulent kinetic energy in the early stage of the separation process. The proposed approach is based on solving an additional transport equation for the so-called laminar kinetic energy, which allows the increase in the nonturbulent fluctuations in the pretransitional and transitional region to be taken into account. The model is derived from that of Lardeau et al. (2004, “Modelling Bypass Transition With Low-Reynolds-Number Non-Linear Eddy-Viscosity Closure,” Flow, Turbul. Combust., 73, pp. 49–76), which was originally formulated to predict bypass transition for attached flows, subject to a wide range of freestream turbulence intensity. A new production term is proposed, based on the mean shear and a laminar eddy-viscosity concept. After a validation of the model for a flat-plate boundary layer, subjected to an adverse pressure gradient, the T106 and T2 cascades, recently tested at the von Kármán Institute, are selected as test cases to assess the ability of the model to predict the flow around high-lift cascades in conditions representative of those in low-pressure turbines. Good agreement with experimental data, in terms of blade-load distributions, separation onset, reattachment locations, and losses, is found over a wide range of Reynolds-number values.

Separation-induced boundary layer transition: Modeling with a non-linear eddy-viscosity model coupled with the laminar kinetic energy equation

We present an effort to model the separation-induced transition on a flat plate with a semi-circular leading edge, using a cubic non-linear eddy-viscosity model combined with the laminar kinetic energy. A non-linear model, compared to a linear one, has the advantage to resolve the anisotropic behavior of the Reynolds-stresses in the near-wall region and it provides a more accurate expression for the generation of turbulence in the transport equation of the turbulence kinetic energy. Although in its original formulation the model is not able to accurately predict the separation-induced transition, the inclusion of the laminar kinetic energy increases its accuracy. The adoption of the laminar kinetic energy by the non-linear model is presented in detail, together with some additional modifications required for the adaption of the laminar kinetic energy into the basic concepts of the non-linear eddy-viscosity model. The computational results using the proposed combined model are shown together with the ones obtained using an isotropic linear eddy-viscosity model, which adopts also the laminar kinetic energy concept and in comparison with the existing experimental data.

Reynolds-Averaged Navier-Stokes Simulation of Low-Reynolds-Number Airfoil Aerodynamics

A simple procedure has been developed to improve Reynolds-averaged Navier-Stokes simulation of low-Reynolds-number airfoil aerodynamics, especially the laminar separation bubble. First, a laminar Navier-Stokes computation is performed for the selected low-Reynolds-number SD7003 airfoil case. The importance of sufficient grid resolution in the normal direction and numerical iteration has been emphasized for achieving a relatively stable, time-averaged, two-dimensional, laminar separation solution. Based on this laminar solution, the separation-induced transition is determined as the point at which the tangential velocity adjacent to the solid surface reverses its direction for the second time after the laminar separation. Then a Reynolds-averaged Navier-Stokes computation is further performed with zero production terms in the selected turbulence model before the transition point and with the complete turbulence model after the transition point. As a result, similar to large-eddy simulation and direct numerical simulation, a Reynolds-averaged Navier-Stokes approach using the Spalart-Allmaras model is able to capture a laminar separation bubble without any external transition mechanism. The robustness of the approach is validated with several other low-Reynolds-number airfoil cases.