Leading-edge effects in bypass transition

2007 ◽  
Vol 572 ◽  
pp. 471-504 ◽  
Author(s):  
S. NAGARAJAN ◽  
S. K. LELE ◽  
J. H. FERZIGER
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Bypass Transition ◽  
Turbulent Spot ◽  
Eddy Simulation ◽  
Turbulence Transition ◽  
Large Eddy ◽  
Low Speed Streaks ◽  
Vortical Disturbance ◽  
Blunt Leading Edge

The effect of a blunt leading edge on bypass transition is studied by numerical simulation. A mixed direct and large-eddy simulation of a flat plate with a super-ellipse leading edge is carried out at various conditions. Onset and completion of transition is seen to move upstream with increasing bluntness. For sharper leading edges, at lower levels of turbulence, transition usually occurs through instabilities on low-speed streaks as observed by Jacobs & Durbin (2001) and Brandt et al. (2004) whereas increasing either the turbulence intensity or the leading-edge bluntness brings into play another mechanism. Free-stream vortices are amplified at the leading edge because of stretching. In the case of particularly strong vortices, this interaction induces a localized streamwise vortical disturbance in the boundary layer which then grows as it convects downstream and eventually breaks down to form a turbulent spot. These disturbances, which are localized and hence wavepacket-like, move at speeds in the range 0.55 U∞–0.65 U∞ and occur in the lower portion of the boundary layer. Simulations conducted with isolated vortices confirm such a response of the boundary layer.

Large Eddy Simulation of Bypass Transition in Vane Passage With Freestream Turbulence

2020 ◽  
Vol 142 (6) ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Bypass Transition ◽  
Freestream Turbulence ◽  
Turbulent Spot ◽  
Suction Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Inflow Turbulence

Abstract High Reynolds flow over a nozzle guide-vane with elevated inflow turbulence was simulated using wall-resolved large eddy simulation (LES). The simulations were undertaken at an exit Reynolds number of 0.5 × 106 and inflow turbulence levels of 0.7% and 7.9% and for uniform heat-flux boundary conditions corresponding to the measurements of Varty and Ames (2016, “Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels,” ASME Paper No. IMECE2016-67029). The predicted heat transfer distribution over the vane is in excellent agreement with measurements. At higher freestream turbulence, the simulations accurately capture the laminar heat transfer augmentation on the pressure surface and the transition to turbulence on the suction surface. The bypass transition on the suction surface is preceded by boundary layer streaks formed under the external forcing of freestream disturbances which breakdown to turbulence through inner-mode secondary instabilities. Underneath the locally formed turbulent spot, heat transfer coefficient spikes and generally follows the same pattern as the turbulent spot. The details of the flow and temperature fields on the suction side are characterized, and first- and second-order statistics are documented. The turbulent Prandtl number in the boundary layer is generally in the range of 0.7–1, but decays rapidly near the wall.

Large Eddy Simulation of Bypass Transition in Vane Passage With Freestream Turbulence

2019 ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Bypass Transition ◽  
Freestream Turbulence ◽  
Turbulent Spot ◽  
Suction Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Inflow Turbulence

Abstract High Reynolds flow over a nozzle guide-vane with elevated inflow turbulence was simulated using wall-resolved large eddy simulation (LES). The simulations were undertaken at an exit Reynolds number of 0.5×106 and inflow turbulence levels of 0.7% and 7.9% and for uniform heat-flux boundary conditions corresponding to the measurements of (Varty, J. W., and Ames, F. E., 2016, ASME Paper No. IMECE2016-67029). The predicted heat transfer distribution over the vane is in excellent agreement with measurements. At higher freestream turbulence, the simulations accurately capture the laminar heat transfer augmentation on the pressure surface and the transition to turbulence on the suction surface. The bypass transition on the suction surface is preceded by boundary layer streaks formed under the external forcing of freestream disturbances which breakdown to turbulence through inner mode secondary instabilities. Underneath the locally formed turbulent spot, heat transfer coefficient spikes and generally follows the same pattern as the turbulent spot. The details of the flow and temperature fields on the suction side are characterized and first and second order statistics are documented. The turbulent Prandtl number in the boundary layer is generally in the range of 0.7–1, but decays rapidly near the wall.

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

2018 ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Pressure Surface ◽  
Eddy Simulation ◽  
Large Eddy

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 scheme is added to an unstructured finite-volume solver. The performance of a number of Sub-Grid 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, whilst 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 resembled 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.

Large Eddy Simulation of the Laminar Heat Transfer Augmentation on the Pressure Side of a Turbine Vane Under Freestream Turbulence

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Leading Edge ◽  
Transition To Turbulence ◽  
Experimental Measurements ◽  
Pressure Side ◽  
Freestream Turbulence ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
Large Eddy

Vane pressure side heat transfer is studied numerically using large eddy simulation (LES) on an aft-loaded vane with a large leading edge over a range of turbulence conditions. Numerical simulations are performed in a linear cascade at exit chord Reynolds number of Re = 5.1 × 105 at low (Tu ≈ 0.7%), moderate (Tu ≈ 7.9%), and high (Tu ≈ 12.4%) freestream turbulence with varying length scales as prescribed by the experimental measurements of Varty and Ames (2016, “Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels,” ASME Paper No. IMECE2016-67029). Heat transfer predictions on the vane pressure side are in a very good agreement with the experimental measurements and the heat transfer augmentation due to the freestream turbulence is well captured. At Tu ≈ 12.4%, freestream turbulence enhances the Stanton number on the pressure surface without boundary layer transition to turbulence by a maximum of about 50% relative to the low freestream turbulence case. Higher freestream turbulence generates elongated structures and high-velocity streaks wrapped around the leading edge that contain significant energy. Amplification of the velocity streaks is observed further downstream with max rms of 0.3 near the trailing edge but no transition to turbulence or formation of turbulence spots is observed on the pressure side. The heat transfer augmentation at the higher freestream turbulence is primarily due to the initial amplification of the low-frequency velocity perturbations inside the boundary layer that persist along the entire chord of the airfoil. Stanton numbers appear to scale with the streamwise velocity fluctuations inside the boundary layer.

Interactions of Separation Bubble With Oncoming Wakes by Large-Eddy Simulation

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
S. Sarkar ◽  
Harish Babu ◽  
Jasim Sadique
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Constant Thickness ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Flow Past A Cylinder ◽  
Rotor Stator Interaction

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied using large-eddy simulation (LES). A series of airfoils of constant thickness with rounded leading edge are employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case, an infinite row of the model airfoils). This setup is a simplified representation of the rotor–stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of disturbances, becomes turbulent, and finally reattaches forming a separation bubble. In the presence of oncoming wakes, the characteristics of the separated boundary layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase-averaged results illustrate the periodic behavior of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of the boundary layer by convective wakes forming coherent vortices, which are being shed and convect downstream. Further, the transition of the separated boundary layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations, where viscous effect is substantial.

Leading Edge Contamination of a C-D Compressor Blade Using Large Eddy Simulation

2020 ◽  
Author(s):  
S. Katiyar ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Suction Surface ◽  
Local Flow ◽  
Flow Acceleration ◽  
Eddy Simulation ◽  
Large Eddy

Abstract A large-eddy simulation (LES) is employed here to predict the flow field over the suction surface of a controlled-diffusion (C-D) compressor stator blade following the experiment of Hobson et al. [1]. When compared with the experiment, LES depicts a separation bubble (SB) in the mid-chord region of the suction surface, although discrepancies exist in Cp. Further, the LES resolves the growth of boundary layer over the mid-chord and levels of turbulence intensity with an acceptable limit. What is noteworthy that LES also resolves a tiny SB near the leading-edge at the designed inflow angle of 38.3°. The objective of the present study is to assess how this leading-edge bubble influences the transition and development of boundary layer on the suction surface before the mid-chord. It appears that the separation at leading-edge suddenly enhances the perturbation levels exciting development of boundary layer downstream. The boundary layer becomes pre-transitional followed by a decay of fluctuations up to 30% of chord attributing to the local flow acceleration. Further, the boundary layer appears like laminar after being relaxed from the leading edge excitation near the mid-chord. It separates again because of the adverse pressure gradient, depicting augmentation of turbulence followed by the breakdown at about 70% of chord.

Large Eddy Simulation of a Controlled-Diffusion Cascade Blade at Varying Flow Inlet Angles

2009 ◽  
Author(s):  
W. Andrew McMullan ◽  
Gary J. Page
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Leading Edge ◽  
Fine Grid ◽  
Controlled Diffusion ◽  
Eddy Simulation ◽  
Pressure Distributions ◽  
Wide Range ◽  
Large Eddy ◽  
Flow Features

A Controlled Diffusion cascade stator blade has been studied numerically using Large Eddy Simulation (LES). The aim of the study is to assess the performance of Large Eddy Simulation in predicting flow features on a highly-loaded blade, including leading-edge separation, transition and turbulent reattachment, particularly at off-design conditions. The need for LES to be performed on high resolution grids is highlighted by preliminary simulations on a mesh typically used in Reynolds-Averaged approaches. On a fine grid, the unsteady flow features captured by time-dependent simulation yield an improvement in surface pressure distributions and boundary layer profiles, although some weaknesses are apparent in the prediction of pressure-side boundary layer properties and wake profiles. The computed loss coefficients show potential for LES to be used to obtain loss-loop data over a wide range of incidence angles.

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

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Pressure Surface ◽  
Eddy Simulation ◽  
Large Eddy

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.

Effects of incidence angle on a low-pressure turbine blade boundary layer evolution through large eddy simulation

2018 ◽  
Vol 232 (6) ◽  
pp. 722-734 ◽  
Author(s):  
Yunfei Wang ◽  
Xiuming Sui ◽  
Kai Zhang ◽  
Xiaorong Xiang ◽  
Qingjun Zhao
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Coherent Structures ◽  
Separation Bubble ◽  
Incidence Angle ◽  
Leading Edge ◽  
Pressure Side ◽  
Low Pressure Turbine ◽  
Eddy Simulation ◽  
Large Eddy

The evolution mechanism of the boundary layer and coherent structures in a low-pressure turbine blade is discussed. Five different incidence angles over the T106A blade for a Mach number Ma = 0.404 and Reynolds number Re = 0.6 × 105 (based on the axial chord and outlet velocity) are performed using large eddy simulation method. The calculation results at +7.8 incidence angle are agreed well with the experimental and direct numerical simulation data. The influence of the incidence angle on the flow field is mainly shown at the front of the suction side and pressure side. As the incidence angle changes from positive to negative, the separation bubble near the leading edge disappears and the blade loading decreases gradually. When the incidence angle reduces to −5°, separation bubble appears near the leading edge of the pressure side. At the case of incidence angle equaling −10°, the length of time-averaged separation bubble on the pressure side grows to 39% axial chord and the evolution process of the coherent structures is extremely complex. The spanwise vortexes roll up near the leading edge and gradually evolve into streamwise vortexes. High-energy fluid in the main flow was driven to near-wall zone by the rotating effect of streamwise vortexes, which increases the fluid momentum inside the boundary layer. The streamwise vortexes are stretched by the strong acceleration of the flow until they transport to the trailing edge.

Application of Turbulence Models to Bypass Transition

1997 ◽  
Vol 119 (4) ◽  
pp. 859-866 ◽  
Author(s):  
K. J. A. Westin ◽  
R. A. W. M. Henkes
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Single Point ◽  
Turbulence Models ◽  
Leading Edge ◽  
Bypass Transition ◽  
Free Stream Turbulence ◽  
Transition Prediction ◽  
Eddy Simulation ◽  
Layer Flow

The present study considers the use of low-Reynolds number, single-point closures for transition prediction at high levels of free-stream turbulence. The work is focused on two differential Reynolds Stress Transport models, which are compared with the k – ε model of Launder and Sharma. Calculations are carried out for attached boundary layer flow on a flat plate equipped with a sharp leading edge at zero-pressure gradient and for various free-stream turbulence levels. Comparisons with results from a large eddy simulation reveal significant shortcomings in the modeling of the dissipation, with a large overprediction in the pretransitional boundary layer. The present results are in some cases in conflict with other results reported in the literature, and it is shown that the discrepancies can be ascribed to the implementation of the free-stream boundary conditions.

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