Comparative LES and Unsteady RANS Computations for a Periodically-Perturbed Separated Flow Over a Backward-Facing Step

2005 ◽  
Vol 127 (5) ◽  
pp. 872-878 ◽  
Author(s):  
A. Dejoan ◽  
Y.-J. Jang ◽  
M. A. Leschziner
Keyword(s):  
Separation Bubble ◽  
Flow Direction ◽  
Active Flow Control ◽  
Low Frequency ◽  
Separated Flow ◽  
Backward Facing Step ◽  
Eddy Simulation ◽  
Turbulence Closures ◽  
Unsteady Rans ◽  
Perturbation Frequency

Large eddy simulation and statistical turbulence closures are used to investigate and contrast the ability of both strategies to represent the effects arising from the unsteady perturbation of a separated backward-facing-step flow caused by a slot jet injected periodically at zero net mass-flow rate into the flow at the step edge, at an angle of 45 deg relative to the flow direction. Experimental data show the effects to depend nonlinearly on the perturbation frequency, the strongest response arising at a Strouhal number of 0.2, which is the condition investigated herein. The principal response is a shortening of the separation bubble by almost 30%, a result that is highly pertinent to active flow control. As the injection frequency lies within the low-frequency range of the large scales of the turbulence spectrum, an issue of particular interest that is addressed herein is the ability of the statistical models, operating within a phase-averaged URANS framework, to reproduce the experimental observations and the response derived from the simulation.

LES and Unsteady RANS Computations for a Periodically-Perturbed Separated Flow Over a Backward-Facing Step

2004 ◽  
Author(s):  
Anne Dejoan ◽  
Yong-Jun Jang ◽  
Michael A. Leschziner
Keyword(s):  
Separation Bubble ◽  
Flow Direction ◽  
Active Flow Control ◽  
Low Frequency ◽  
Separated Flow ◽  
Backward Facing Step ◽  
Eddy Simulation ◽  
Injection Frequency ◽  
Unsteady Rans ◽  
Perturbation Frequency

Large eddy simulation and statistical turbulence closure are used to investigate and contrast the ability of both strategies to represent the effects arising from the unsteady perturbation of a separated backward-facing-step flow by means of a slot jet injected periodically at zero net mass-flow rate into the flow at the step edge at an angle of 45 degrees relative to the flow direction. Experimental data show the effects to depend non-linearly on the perturbation frequency, the strongest response arising at a Strouhal number of 0.2, which is the condition investigated herein. The principal response is a shortening of the separation bubble by almost 30%, a result that is highly pertinent to active flow control. As the injection frequency lies within the low-frequency range of the large scales of the turbulence spectrum, an issue of particular interest that is addressed is the ability of the statistical models, operating within a phase-averaged URANS framework, to reproduce the experimental observations and the response derived from the simulation.

Flow Studies of the Leading Edge Stall on a Swept-Back Wing at High Incidence

1956 ◽  
Vol 60 (541) ◽  
pp. 51-60 ◽  
Author(s):  
Joseph Black
Keyword(s):  
Boundary Layer ◽  
Liquid Film ◽  
Boundary Layer Flow ◽  
Separation Bubble ◽  
Flow Direction ◽  
Separated Flow ◽  
Leading Edge ◽  
Outer Edge ◽  
Layer Flow ◽  
Wing Tip

SummaryThe flow separation on a swept-back wing with 44 degrees leading edge sweep at 18 degrees incidence has been investigated by means of detailed pressure distribution measurements over the leading edge, boundary layer flow determination with liquid film technique, and yawmeter traverses. A wool-tuft grid was also used, and a spin detector was developed to search for regions of vorticity. These tests established that the flow separates on the leading edge; over the inboard sections it re-attaches behind a “ short” separation bubble, while outboard it only re-attaches near the trailing edge, thus forming a “ long ” separation bubble, or else fails to attach. The separated flow in what has been commonly called the tip stall does in fact take the form of a “ ram's horn “ vortex with the origin, or “ tip,” located at the junction of the two bubbles on the leading edge. The vortex lies outwards across the wing surface at approximately 20 to 25 degrees to the line-of-flight before curving aft to be shed into the wake, where it extends almost from mid semi-span to the wing tip. This vortex induces considerable changes in flow direction, both on and over the wing, and also in the wake. Thus in the wake a maximum downwash of 23 degrees is induced aft of the mid semi-span, and there is an upwash of 17 degrees at the outer edge of the vortex, almost aft of the tip. A good correlation between yawmeter results and the boundary layer flow direction indications from the liquid film technique was obtained.

scholarly journals Computational study of active flow control drag reduction device for utility vehicle

2019 ◽  
Vol 128 ◽  
pp. 09003
Author(s):  
Jęedrzej Mosiężny ◽  
Bartosz Ziegler
Keyword(s):  
Drag Reduction ◽  
Computational Study ◽  
Active Flow Control ◽  
Separated Flow ◽  
Flow Region ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation ◽  
Active Flow ◽  
Base Drag

The study presents a computational study of a drag reduction device based on an active boundary layer control for a generic truck-trailer utility road vehicle. The conceptual device is in accordance with upcoming EU regulations regarding attachable aerodynamic devices for heavy utility vehicles. Design and principles of operation of the conceptual device are presented. The device is intended to increase decrease the trailer’s base drag coefficient by manipulation of the separated flow region behind the vehicle base. Results of a steady state Reynolds averaged analysis and Delayed Detached Eddy Simulation are presented to show the discrepancies of fluid flow patterns between baseline and augmented configuration as well as between mentioned CFD approaches. Results for drag reduction for baseline truck-trailer configuration and aerodynamically augmented configuration are presented.

A Periodically Perturbed Backward-Facing Step Flow by Means of LES, DES and T-RANS: An Example of Flow Separation Control

2004 ◽  
Author(s):  
Sanjin Sˇaric´ ◽  
Suad Jakirlic´ ◽  
Cameron Tropea
Keyword(s):  
Navier Stokes ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Inlet Channel ◽  
Flow Separation Control ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Sst Model ◽  
Perturbation Frequency ◽  
Length Reduction

Turbulent flow over a backward-facing step perturbed periodically by an alternating blowing/suction through a thin slit situated at the step edge was studied computationally using the LES (Large Eddy Simulation), DES (Dettached Eddy Simulation) and T-RANS (Transient Reynolds-Averaged Navier-Stokes) techniques. The flow configuration considered (ReH = UcH/ν = 3700) has been investigated experimentally by Yoshioka et al. (2001). The periodical blowing/suction with zero mass flux is governed by a sinusoidal law: ve = 0.3Ucsin(2πfet), Uc being the centerline velocity in the inlet channel. Perturbation frequencies fe corresponding to the Strouhal numbers St = 0.08, 0.19 and 0.30 were investigated (St = feH/Uc). The experimental observation, that the perturbation frequency St = 0.19 represents the most effective case, that is the case with the minimum reattachment length, was confirmed by all computational methods applied. However, the closest agreement with experiment (the reattachment length reduction of 28.3% compared to the unperturbed case) was obtained with the LES (24.5%) and DES (35%) methods whereas the T-RANS computations show a weak sensitivity to the perturbation: 5.9% when using the Spalart-Allmaras model and 12.9% using the k–ω SST model.

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.

A Periodically Perturbed Backward-Facing Step Flow by Means of LES, DES and T-RANS: An Example of Flow Separation Control

2005 ◽  
Vol 127 (5) ◽  
pp. 879-887 ◽  
Author(s):  
S. Šarić ◽  
S. Jakirlić ◽  
C. Tropea
Keyword(s):  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Inlet Channel ◽  
Flow Separation Control ◽  
Eddy Simulation ◽  
Sst Model ◽  
Perturbation Frequency ◽  
Zero Net Mass Flux

Turbulent flow over a backward-facing step, perturbed periodically by alternative blowing∕suction through a thin slit (0.05H width) situated at the step edge, was studied computationally using (LES) large eddy simulation, (DES) detached eddy simulation, and (T-RANS) transient Reynolds-averaged Navier–Stokes techniques. The flow configuration considered (ReH=UcH∕ν=3700) has been investigated experimentally by Yoshioka et al. (12). The periodic blowing∕suction with zero net mass flux is governed by a sinusoidal law: ve=0.3Ucsin(2πfet), Uc being the centerline velocity in the inlet channel. Perturbation frequencies fe corresponding to the Strouhal numbers St=0.08, 0.19, and 0.30 were investigated (St=feH∕Uc). The experimental observation that the perturbation frequency St=0.19 represents the most effective case, that is the case with the minimum reattachment length, was confirmed by all computational methods. However, the closest agreement with experiment (the reattachment length reduction of 28.3% compared to the unperturbed case) was obtained with LES (24.5%) and DES (35%), whereas the T-RANS computations showed a weaker sensitivity to the perturbation: 5.9% when using the Spalart–Allmaras model and 12.9% using the k-ω SST model.

Structure of large-scale vortices and unsteady reverse flow in the reattaching zone of a turbulent separation bubble

1985 ◽  
Vol 154 ◽  
pp. 463-491 ◽  
Author(s):  
Masaru Kiya ◽  
Kyuro Sasaki
Keyword(s):  
Unsteady Flow ◽  
Large Scale ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Flow Direction ◽  
Longitudinal Velocity ◽  
Low Frequency ◽  
Space Time ◽  
Local Flow ◽  
Two Agents

This paper describes experiments concerning the structure of large-scale vortices and the unsteady reverse-flow properties in the reattaching zone of a nominally two-dimensional separation bubble formed at the leading edge of a blunt flat plate with right-angled corners. The experiment was performed in a wind tunnel with a constant Reynolds number 2.6 × 104 (based on the main-flow velocity and the thickness of the plate). Split-film probes, being sensitive to instantaneous reversals of flow direction, were extensively employed. An important feature of this study is a judicious use of surface-pressure fluctuations as a conditioning signal to educe the structure of the large-scale vortices.Distributions of fluctuating-velocity vectors and contour lines of high-frequency turbulent energy in a few space–time domains are presented and discussed. The most economical interpretation of these space-time distributions is that the large-scale vortices in the reattaching zone are hairpin vortices whose configuration is sketched in the text. The unsteady flow in the reattaching zone is mainly governed by two agents; the motion of the large-scale vortices and the low-frequency unsteadiness. The unsteady flow is clarified in terms of the motion (in a space–time domain) of zeros of the longitudinal velocity close to the surface of the plate; the effects of the two agents on this motion are presented separately. On the basis of these results, a mathematical model of the unsteady flow in the reattaching zone is suggested and found to yield good comparison with measured reverse-flow intermittency and frequency of local-flow reversals. It appears that the separation bubble experiences shrinkage and enlargement in connection with the low-frequency unsteadiness and that the speed of shrinkage is much greater than that of enlargement. The strength of the large-scale vortices in the reattaching zone seems to be dependent on the phase of the low-frequency unsteadiness.

Comparison of Flying-Hot-Wire and Stationary-Hot-Wire Measurements of Flow Over a Backward-Facing Step

1999 ◽  
Vol 121 (2) ◽  
pp. 441-445 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Hot Wire Anemometry ◽  
Blunt Leading Edge

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

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