Laminar-turbulent Transition Model Research Articles

Linear stability of Landau jet: non-parallel effects

We perform linear stability analysis of Landau jet using quasi-parallel approximation and self-similar approach. While the former detects only sinusoidal unstable modes, the latter also captures axisymmetric disturbances in agreement with previous results. The direct comparison of dispersion curves and eigenmode shapes for m = 1 and ReD ≈ 56 indicates that low frequencies are poorly described within quasi-parallel approximation making this approach questionable for a laminar-turbulent transition model.

Hydrodynamic instabilities and nonequilibrium phase transitions

For laminar-turbulent transition model is built reconstruction of the initial stage of instability as a nonequilibrium phase transition, the mechanism of which is diffusion stratification. It is shown that the Gibbs free energy deviations from the homogeneous state (relative to the instability under consideration) is an analogue Ginzburg-Landau potentials. Numerical experiments were performed. Self-excitation of a homogeneous state by edge control condition of increasing speed. Under external influence (increase in speed at the input), there is a transition to chaos through bifurcations of period doubling, when the internal control parameter (analogue of the Reynolds number) changes, like the Feigenbaum period doubling cascade.

High-Lift Simulations of the Japan Aerospace Exploration Agency Standard Model

The Japan Aerospace Exploration Agency standard model high-lift configuration was analyzed computationally using both the OVERFLOW and LAVA codes for the third AIAA High-Lift Prediction Workshop. Simulations were performed both with and without the nacelle/pylon feature as prescribed by the workshop test cases. Without the nacelle/pylon, evidence of multiple solutions was observed when a quadratic constitutive relation was used in the turbulence modeling; however, time-accurate simulations seemed to favor one solution over the other. With the nacelle/pylon, no evidence of multiple solutions was observed. Laminar–turbulent transition modeling was also applied to the configuration, and it had an overall favorable impact on the lift predictions.

Transitional Delayed Detached-Eddy Simulation of Multielement High-Lift Airfoils

A strategy for predicting high-lift aerodynamic flows is presented that employs laminar–turbulent transition modeling, based on amplification factor transport, coupled with a hybrid Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) methodology. This modeling has been implemented in an overset, structured, finite difference computational fluid dynamics solver, and the predictive capabilities are demonstrated for the widely studied three-element MD high-lift airfoil. Lift forces, surface pressure distributions, and velocity profiles are compared for fully turbulent and transitional hybrid RANS/LES simulations. The inclusion of transition prediction has a net favorable effect when compared to an experimental reference; however, some discrepancies between measurement and prediction are not fully reconciled.

Spalart–Allmaras with Crossflow Transition Model Using Hamiltonian–Strand Approach

The current Spalart–Allmaras (SA) laminar–turbulent transition model is evaluated through the representative two- and three-dimensional test cases within the line-based Hamiltonian–Strand grid flow solver. To consider crossflow instability on transition onset, the baseline SA model has been extended by incorporating the existing crossflow model, which was originally developed to be coupled with the model. For the extension, a simple modification is only required on the baseline transition model, and the extended model has been used with both structured and unstructured/strand grid types using the present flow solver. The extended transition model is applied on the representative test cases where crossflow is dominant, such as the NLF(2)-0415 wing, ONERA M6 swept wing, inclined prolate spheroid, and a challenging sickle wing. By comparing the results with the baseline SA model, predictive results are observed through the validation of test cases. The limitations of the extended model are also found through the sickle-wing test.

Aerodynamic and aeroacoustic behavior of span-wise wavy trailing edge modified SNL 100 m blade

The aerodynamic and aeroacoustic behaviors of wavy trailing edge modified flatback wind turbine blade have been investigated through numerical simulation. Previous studies have demonstrated aerodynamic and aeroacoustic benefits of the span-wise wavy trailing edge modification on the flatback trailing edge. The previous two-dimensional airfoil test cases have been extended to a complete set of design parametric study. Aerodynamic and aeroacoustic characteristics of the parametric study have been discussed, relying on the wave size, including recently added cases. Furthermore, in the current study, the wavy trailing edge modification has been applied to the Sandia flatback blade called “SNL100-03FB” and tested in rotating blade conditions. Several variations of the wavy trailing edge design have been tested for a range of wind speeds using an overset computation domain. The numerical simulation employs the in-house developed Navier–Stokes solver, OVERTURNS, as well as Graphics Processing Unit (GPU)-accelerated solver, GPURANS3D. Both are hybrid Reynolds Averaged Navier Stokes (RANS)/Large Eddy Simulation (LES) simulation; delayed detached eddy simulation has been used with the Spalart–Allmaras turbulent model and modified [Formula: see text] laminar–turbulent transition model.

Assessment of laminar-turbulent transition models for Hypersonic Inflatable Aerodynamic Decelerator aeroshell in convection heat transfer

Hypersonic Inflatable Aerodynamic Decelerator (HIAD) has shown its great potential for future planetary explorations. However, the HIAD surface deflections could both promote boundary-layer transition early and augment heating levels sharply, which poses challenges for survivability of a Thermal Protection System (TPS). The goal of this work is to assess different transition models for the prediction of hypersonic transitional flows over scalloping deformed HIAD surface and to seek the critical factor for their capabilities to provide a reference for the application and advancement. Three representative transition models are considered: γ-Reθ, k-ω-γ and kT-kL-ω. The results show that the undulating surface causes flow separations and reattachments in valleys and strong crossflow along the leeward. k-ω-γ model can correctly predict both the shapes and locations of the transition onsets. The transition zone predicted by γ-Reθ model is much small, while kT-kL-ω is incapable of transition prediction for such a complex and irregular configuration. Moreover, this study reveals that the crossflow instability plays a dominant role in the transition. The crossflow Reynolds number Recf, whose distributions are approximately consistent with the transition zone, could be a feasible crossflow strength indicator for the transition onsets on the undulating surface. Once k-ω-γ model excludes effects of crossflow instability, it predicts incorrect transition results for the deformed surface of HIAD. Besides, a detailed analysis shows that both the crossflow instability mode and the first disturbance mode in valleys are the major contributors to the transition on the leeward surface. Near the leeward ray, the transition is mainly determined by the first disturbance mode.

Analysis of the abilities of algebraic laminar-turbulent transition models

Due to their simplicity and numerical stability, algebraic transition models seem to be a good alternative to differential transition models. The paper considers two promising algebraic transition models k-ω KD and SA BC and compares their results with the results of the most popular differential model - SST γ-Reθ. Three test cases are considered: flat plate boundary layer, NLF(1)-0414F airfoil and two NACA-0012 airfoils in tandem. Results show that although algebraic models are simpler and easier to use, their accuracy compared to the differential model leaves much to be desired.

Blunt-Wavy Combined Trailing Edge for Wind Turbine Blade Inboard Performance Improvement

Aerodynamic performance of wind turbine blade inboard is improved by modifying its trailing edge shape. A combined trailing edge modification design with blunt trailing edge and a span-wise wavy trailing edge is used. Designing blunt trailing edge at the blade inboard makes the flow become more attached than the conventional sharp trailing edge. The span-wise wavy trailing edge breaks up the Karman vortex shedding generated by the blunt trailing edge. The Sandia National Laboratory 100 meter blade, SNL100-03 was used as a baseline model. Rotating blade simulations are performed with the full-scale isolated rotor condition. Aerodynamic performance analysis and aeroacoustic performance analysis of the modified turbine blades are performed. By using the blunt-wavy combined trailing edge modification, the massive flow separation at the inboard of the baseline blade is prevented successfully. Delayed Detached Eddy Simulation (DDES) with a modified laminar-turbulent transition model (Medida - Baeder model) is used for better prediction of flow separation.

Modeling of Laminar-Turbulent Transition in Boundary Layers and Rough Turbine Blades

A local, intermittency-function-based transition model was developed for the prediction of laminar-turbulent transitional flows with freestream turbulence intensity Tu at low (Tu < 1%), moderate (1% < Tu < 3%), and high Tu > 3% levels, and roughness effects in a broad range of industrial applications such as turbine and helicopter rotor blades, and in nature. There are many mechanisms (natural or bypass) that lead to transition. Surface roughness due to harsh working conditions could have great influence on transition. Accurately predicting both the onset location and length of transition has been persistently difficult. The current model is coupled with the k–ω Reynolds-averaged Navier–Stokes (RANS) model, that can be used for general computational fluid dynamics (CFD) purpose. It was validated on the ERCOFTAC experimental zero-pressure-gradient smooth flat plate boundary layer with both low and high leading-edge freestream turbulence intensities. Skin friction profiles agree well with the experimental data. The model was then tested on ERCOFTAC experimental flat plate boundary layer with favorable/adverse pressure gradients cases, periodic wakes, and flows over Stripf's turbine blades with roughness from hydraulically smooth to fully rough. The predicted skin friction and heat transfer properties by the current model agree well with the published experimental and numerical data.

On the numerical simulations of captive, driven and freely moving cylinder

This paper presents the application of turbulence and laminar–turbulent transition models and fluid–structure capabilities to address the flow and response of captive, driven and free moving rigid cylinder for several Reynolds numbers.An investigation on the performance of the turbulence modeling with k–ω SST is presented, verifying the modeling deficiencies for this flow. The Scale Adaptive Simulations (SAS) and the Local Correlation Transition Model (LCTM or γ−Reθ), both combined with the SST, improved the agreement with experimental results for the captive cylinder flow. These studies also involve verification and validation exercises in order to quantify modeling errors of the results herein.In a second step, the use of SST with driven cylinder motions is presented, as well as with the SAS and LCTM. Finally, aiming at free-moving cylinder behavior, this work presents the study of different turbulence modeling practices for the free-moving cylinder in two degrees of freedom (DOF) with low mass ratio.The importance of turbulence effects on the moving cylinder in comparison with the fixed case is investigated. A natural conjecture is that the turbulence modeling strategy is less decisive when the cylinder is moving with driven motion and even less stringent for free motions, as the body response would filter most of the higher order turbulence effects. This issue is investigated as it would allow modeling simplifications in the application of CFD to a range of engineering problems.

The MEXICO rotor aerodynamic loads prediction: ZigZag tape effects and laminar-turbulent transition modeling in CFD

This paper aims to provide an explanation for the overprediction of aerodynamic loads by CFD compared to experiments for the MEXICO wind turbine rotor and improve the CFD prediction by considering laminar-turbulent transition modeling. Large deviations between CFD results and experimental measurements are observed in terms of sectional normal and tangential forces at the blade tip (r/R=0.82 and 0.92) of the MEXICO rotor operating in axial condition at the design tip speed ratio λ=6.7. The first part of this study identifies the effects of ZigZag tape, which is used in the experiment to trigger boundary layer transition, by analyzing the available experimental data of a single, non-rotating MEXICO rotor blade. The analysis indicates that ZigZag tape has a significant impact on sectional aerodynamic tip loads: it alters the boundary layer thickness and additionally reduces the effective airfoil camber besides the expected tripping. These additional effects most likely also occur in the rotating MEXICO experiment, reducing the sectional loads and hence lead to an overprediction by CFD. To eliminate the ZigZag tape interference, experimental data with an untripped blade is preferred to be used as validation case. In the second part of this study, a transitional flow simulation for the MEXICO rotor is performed by using RANS-based transition model k−kL−ω within OpenFOAM-2.1.1. The numerical results are compared against experimental data obtained from the untripped, new MEXICO experiments. The comparison gives that transitional simulation present a very good tip loads prediction for the untripped blade. The measured data also confirms that the ZigZag tape indeed has a significant influence on the blade tip loads in rotating conditions. The transition onset over 3D MEXICO blade is visualized and transition locations are identified. The results shown in the present study can explain the causes of the large differences between CFD and experiment observed in the MEXICO blind comparisons.

Combining the SSG/LRR-ω differential reynolds stress model with the detached eddy and laminar-turbulent transition models

The procedure of incorporating the detached eddy method and a model of laminar-turbulent transition into the SSG/LRR-ω turbulence model is presented. The approach proposed can be regarded as the generalization of the existing models intended to perform calculations with the SST turbulence model to the case of their use with the SSG/LRR-ω model. The advantage of the approach developed over the RANS turbulence models based on the Boussinesq hypothesis is demonstrated with respect to the problems of flow past an airfoil and cold jet outflow.

CFD computations of the second round of MEXICO rotor measurements

A comparison, between selected wind tunnel data from the NEW MEXICO measuring campaign and CFD computations are shown. The present work, documents that a state of the art CFD code, including a laminar turbulent transition model, can provide good agreement with experimental data. Good agreement is shown for the integral loads, radial distributions of blades forces, pressure distributions, and the velocity profiles up- and downstream of the rotor.

Effect of Wavy Trailing Edge on 100meter Flatback Wind Turbine Blade

The flatback trailing edge design for modern 100meter wind turbine blade has been developed and proposed to make wind turbine blade to be slender and lighter. On the other hand, it will increase aerodynamic drag; consequently the increased drag diminishes turbine power generation. Thus, an aerodynamic drag reducing technique should be accompanied with the flatback trailing edge in order to prevent loss of turbine power generation. In this work, a drag mitigation design, span-wise wavy trailing edge blade, has been applied to a modern 100meter blade. The span-wise trailing edge acts as a vortex generator, and breaks up the strong span-wise coherent trailing edge vortex structure at the flatback airfoil trailing edge which is a major source of large drag. Three-dimensional unsteady Computational Fluid Dynamics (CFD) simulations have been performed for real scale wind turbine blade geometries. Delayed Detached Eddy Simulation (DDES) with the modified laminar-turbulent transition model has been applied to obtain accurate flow field predictions. Graphical Processor Unit (GPU)-accelerated computation has been conducted to reduce computational costs of the real scale wind turbine blade simulations. To verify the structural reliability of the wavy modification of the blade a simple Eigen buckling analysis has been performed in the current study.

Aerothermal exploration of reaction control jet in supersonic crossflow at high altitude

Multiple reaction control jets injected normal to free-stream is used to manoeuvre aerospace vehicle at high altitudes. Detailed aerothermal analysis of a high speed aerospace vehicle with multiple lateral jets is carried out in its full trajectory covering wide range of Mach numbers and altitudes. Three dimensional RANS simulations with laminar-turbulent transition models are performed at several instances in the flight trajectory using commercial CFD solver. Numerical simulations captured all complex flow phenomena of free stream & multi-jet interaction at high altitudes and its influence on vehicle airframe temperature. Heat flux data base obtained from CFD analysis is used for transient thermal analysis of flight vehicle. High temperature local hot spots in jet wake regions and detailed thermal analysis of total vehicle provided important inputs to the system design.

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.

Numerical simulation of flow turbulization and relaminarization by active external actions

The Menter γ - Reθ laminar-turbulent transition model is generalized in order to use it with turbulence models other than SST. The agreement with the results of the initial model and the experiment for computations in conjunction with the Spalart-Allmaras turbulence model is demonstrated. The influence of a local energy supply in the boundary layer region on the region of the laminar-turbulent transition at the supersonic flow is studied numerically. It is shown that the transition can be delayed by the energy supply.

Predicting flow in aortic dissection: Comparison of computational model with PC-MRI velocity measurements

Aortic dissection is a life-threatening process in which the weakened wall develops a tear, causing separation of wall layers. The dissected layers separate the original true aortic lumen and a newly created false lumen. If untreated, the condition can be fatal. Flow rate in the false lumen is a key feature for false lumen patency, which has been regarded as one of the most important predictors of adverse early and later outcomes. Detailed flow analysis in the dissected aorta may assist vascular surgeons in making treatment decisions, but computational models to simulate flow in aortic dissections often involve several assumptions. The purpose of this study is to assess the computational models adopted in previous studies by comparison with in vivo velocity data obtained by means of phase-contrast magnetic resonance imaging (PC-MRI).Aortic dissection geometry was reconstructed from computed tomography (CT) images, while PC-MRI velocity data were used to define inflow conditions and to provide distal velocity components for comparison with the simulation results. The computational fluid dynamics (CFD) simulation incorporated a laminar–turbulent transition model, which is necessary for adequate flow simulation in aortic conditions. Velocity contours from PC-MRI and CFD in the two lumens at the distal plane were compared at four representative time points in the pulse cycle.The computational model successfully captured the complex regions of flow reversal and recirculation qualitatively, although quantitative differences exist. With a rigid wall assumption and exclusion of arch branches, the CFD model over-predicted the false lumen flow rate by 25% at peak systole. Nevertheless, an overall good agreement was achieved, confirming the physiological relevance and validity of the computational model for type B aortic dissection with a relatively stiff dissection flap.

Computational Fluid Dynamics Compatible Transition Modeling Using an Amplification Factor Transport Equation

A new laminar–turbulent transition model for low-turbulence external aerodynamic applications is presented that incorporates linear stability theory in a manner compatible with modern computational fluid dynamics solvers. The model uses a new transport equation that describes the growth of the maximum Tollmien–Schlichting instability amplitude in the presence of a boundary layer. To avoid the need for integration paths and nonlocal operations, a locally defined nondimensional pressure-gradient parameter is used that serves as an estimator of the integral boundary-layer properties. The model has been implemented into the OVERFLOW 2.2f solver. Comparisons of predictions using the new model with high-quality wind-tunnel measurements of airfoil section characteristics confirm the predictive qualities of the model, as well as its improvement over the current state of the art in computational fluid dynamics transition modeling at approximately half the computational expense.