Direct Numerical Simulation and Large Eddy Simulation of Laminar Separation Bubbles at Moderate Reynolds Numbers

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
Francois Cadieux ◽  
Julian A. Domaradzki ◽  
Taraneh Sayadi ◽  
Sanjeeb Bose
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Transition To Turbulence ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Large Eddy

Flows over airfoils and blades in rotating machinery for unmanned and microaerial vehicles, wind turbines, and propellers consist of different flow regimes. A laminar boundary layer near the leading edge is often followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. The separated turbulent flow then reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. Typical Reynolds-averaged Navier–Stokes (RANS) turbulence modeling methods were shown to be inadequate for such laminar separation bubble flows (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Direct numerical simulation (DNS) is the most reliable but is also the most computationally expensive alternative. This work assesses the capability of large eddy simulations (LES) to reduce the resolution requirements for such flows. Flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble is considered. Benchmark DNS data for this configuration are generated with the resolution of 59 × 106 mesh points; also used is a different DNS database with 15 × 106 points (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Results confirm that accurate LES are possible using O(1%) of the DNS resolution.

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.

The Pervasive Effect of the Calmed Region

2005 ◽  
Author(s):  
R. L. Thomas ◽  
J. P. Gostelow
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Laminar Separation Bubble ◽  
Free Shear Layer ◽  
Hot Wire ◽  
Strong Suppression ◽  
Velocity Fluctuations ◽  
Laminar Separation

Experiments have been conducted relating to the interaction of imposed freestream wakes upon a flat plate laminar separation bubble under an adverse pressure gradient. Controlled wakes, representative of those seen in turbomachinery environments, were used to investigate unsteadiness effects upon a separating boundary layer that undergoes natural transition in the free shear layer under steady conditions. Hot-wire anemometry using a single hot-wire has shown leading edge boundary layer disturbances induced under each passing wake, which grow steadily via by-pass and natural transition methods into turbulent strips that convect with the flow. These disturbances are of such strength that the separated region is resisted and effectively swept away by the passing turbulence, momentarily giving rise to a wholly attached laminar boundary layer. Controlling the chord-wise proximity of neighboring wakes allowed for the investigation of the effect and extent of the calmed region behind each induced turbulent strip. Measurements have shown that a strong suppression of velocity fluctuations is seen related to the proximity of the turbulent strips. Turbulence level reductions of up to 40% have been demonstrated as wake spacing is reduced. Even for those cases where systematic wakes are sufficiently close together to prevent the development of a visible calmed region, very strong calming influences are seen in the wake induced turbulent domain that would have normally been occupied by the calmed flow.

Computational Study of Flow Characteristics of Thick and Thin Airfoil With Implicit Large-Eddy Simulation at Low Reynolds Number

2011 ◽  
Author(s):  
Ryoji Kojima ◽  
Taku Nonomura ◽  
Akira Oyama ◽  
Kozo Fujii
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Eddy Simulation ◽  
Naca0012 Airfoil ◽  
Large Eddy ◽  
Implicit Large Eddy Simulation

The flow fields around NACA0012 and NACA0002 at Reynolds number of 23,000, and their aerodynamic characteristics are analyzed. Computations are conducted with implicit large-eddy simulation solver and Reynolds-averaged-Navier-Stokes solver. Around this Reynolds number, the flow over an airfoil separates, transits and reattaches, resulting in generation of a laminar separation bubble at angle of attack in the range of certain degrees. Over a NACA0012 airfoil a separation point moves toward its leading edge with increasing angle of attack, and a separated flow may transit to create a short bubble. On the other hand, over a NACA0002 airfoil a separation point is kept at its leading edge, and a separated flow may transit to create a long bubble. Moreover, there appears nonlinearity in lift curve for NACA0012 airfoil, but does not appear in that for NACA0002 in spite of existence of a laminar separation bubble.

scholarly journals Direct numerical simulation of ‘short’ laminar separation bubbles with turbulent reattachment

2000 ◽  
Vol 403 ◽  
pp. 223-250 ◽  
Author(s):  
M. ALAM ◽  
N. D. SANDHAM
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Laminar Boundary Layer ◽  
Stokes Equations ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Wall Turbulence ◽  
Mean Flow ◽  
Laminar Separation

Direct numerical simulation of the incompressible Navier-Stokes equations is used to study flows where laminar boundary-layer separation is followed by turbulent reattachment forming a closed region known as a laminar separation bubble. In the simulations a laminar boundary layer is forced to separate by the action of a suction profile applied as the upper boundary condition. The separated shear layer undergoes transition via oblique modes and Λ-vortex-induced breakdown and reattaches as turbulent flow, slowly recovering to an equilibrium turbulent boundary layer. Compared with classical experiments the computed bubbles may be classified as ‘short’, as the external potential flow is only affected in the immediate vicinity of the bubble. Near reattachment budgets of turbulence kinetic energy are dominated by turbulence events away from the wall. Characteristics of near-wall turbulence only develop several bubble lengths downstream of reattachment. Comparisons are made with two-dimensional simulations which fail to capture many of the detailed features of the full three-dimensional simulations. Stability characteristics of mean flow profiles are computed in the separated flow region for a family of velocity profiles generated using simulation data. Absolute instability is shown to require reverse flows of the order of 15–20%. The three-dimensional bubbles with turbulent reattachment have maximum reverse flows of less than 8% and it is concluded that for these bubbles the basic instability is convective in nature.

Response of a Laminar Separation Bubble to an Impinging Wake

2003 ◽  
Author(s):  
J. P. Gostelow ◽  
R. L. Thomas
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Side Wall ◽  
Leading Edge ◽  
Good Representation ◽  
Laminar Separation Bubble ◽  
Rotating Disc ◽  
Laminar Separation ◽  
Section Data ◽  
Layer Velocity

Laminar separation and transition phenomena were investigated experimentally in the wake-disturbed flow over a 2.4 m long flat plate. A controlled diffusion pressure distribution, representative of that on a compressor blade, was imposed but with sufficiently strong loading to cause laminar separation. Boundary layer velocity traverses were performed at several longitudinal stations. Wakes were generated upstream by a single rod, parallel to the leading edge, attached to a rotating disc mounted flush in the side-wall of the working section. Data are presented in the form of velocity traces, and contours of velocity and turbulent intermittency. The results highlight the interaction between the incoming wake and the natural boundary layer, which features a long and thin laminar separation bubble; they demonstrate that wind tunnel experiments provide a good representation of boundary layer behavior under wake disturbances on turbomachinery blading. The calmed region behind the disturbance is a feature that is even stronger behind a wake interaction than behind a triggered turbulent spot. Intermittency values for the undisturbed flow in the separation bubble reattachment region are well-represented by Narasimha’s universal intermittency distribution, lending support to the use of intermittency-based predictive routines in calculations of blade boundary layers.

Response of a Laminar Separation Bubble to an Impinging Wake

2005 ◽  
Vol 127 (1) ◽  
pp. 35-42 ◽  
Author(s):  
J. P. Gostelow ◽  
R. L. Thomas
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Rotating Disk ◽  
Good Representation ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Section Data ◽  
Controlled Diffusion ◽  
Layer Velocity

Laminar separation and transition phenomena were investigated experimentally in the wake-disturbed flow over a 2.4 m long flat plate. A controlled diffusion pressure distribution, representative of that on a compressor blade, was imposed but with sufficiently strong loading to cause laminar separation. Boundary layer velocity traverses were performed at several longitudinal stations. Wakes were generated upstream by a single rod, parallel to the leading edge, attached to a rotating disk mounted flush in the sidewall of the working section. Data are presented in the form of velocity traces and contours of velocity and turbulent intermittency. The results highlight the interaction between the incoming wake and the natural boundary layer, which features a long and thin laminar separation bubble; they demonstrate that wind tunnel experiments provide a good representation of boundary layer behavior under wake disturbances on turbomachinery blading. The calmed region behind the disturbance is a feature that is even stronger behind a wake interaction than behind a triggered turbulent spot. Intermittency values for the undisturbed flow in the separation bubble reattachment region are well represented by Narasimha’s universal intermittency distribution, lending support to the use of intermittency-based predictive routines in calculations of blade boundary layers.

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