Influence of wind tunnel freestream turbulence level on boundary-layer separation

2011 ◽  
Vol 18 (2) ◽  
pp. 203-214 ◽  
Author(s):  
I. D. Zverkov ◽  
V. V. Kozlov ◽  
A. V. Kryukov
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Boundary Layer Separation ◽  
Turbulence Level ◽  
Freestream Turbulence ◽  
Layer Separation

scholarly journals Analysis of the Effect of Vortex Generator Spacing on Boundary Layer Flow Separation Control

2019 ◽  
Vol 9 (24) ◽  
pp. 5495 ◽  
Author(s):  
Xin-kai Li ◽  
Wei Liu ◽  
Ting-jun Zhang ◽  
Pei-ming Wang ◽  
Xiao-dong Wang
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Kinetic Energy ◽  
Flow Control ◽  
Flow Separation ◽  
Lift Coefficient ◽  
Boundary Layer Separation ◽  
Wind Tunnel Experiments ◽  
Flow Separation Control ◽  
Layer Separation

During the operation of wind turbines, flow separation appears at the blade roots, which reduces the aerodynamic efficiency of the wind turbine. In order to effectively apply vortex generators (VGs) to blade flow control, the effect of the VG spacing (λ) on flow control is studied via numerical calculations and wind tunnel experiments. First, the large eddy simulation (LES) method was used to calculate the flow separation in the boundary layer of a flat plate under an adverse pressure gradient. The large-scale coherent structure of the boundary layer separation and its evolution process in the turbulent flow field were analyzed, and the effect of different VG spacings on suppressing the boundary layer separation were compared based on the distance between vortex cores, the fluid kinetic energy in the boundary layer, and the pressure loss coefficient. Then, the DU93-W-210 airfoil was taken as the research object, and wind tunnel experiments were performed to study the effect of the VG spacing on the lift–drag characteristics of the airfoil. It was found that when the VG spacing was λ/H = 5 (H represents the VG’s height), the distance between vortex cores and the vortex core radius were approximately equal, which was more beneficial for flow control. The fluid kinetic energy in the boundary layer was basically inversely proportional to the VG spacing. However, if the spacing was too small, the vortex was further away from the wall, which was not conducive to flow control. The wind tunnel experimental results demonstrated that the stall angle-of-attack (AoA) of the airfoil with the VGs increased by 10° compared to that of the airfoil without VGs. When the VG spacing was λ/H = 5, the maximum lift coefficient of the airfoil with VGs increased by 48.77% compared to that of the airfoil without VGs, the drag coefficient decreased by 83.28%, and the lift-to-drag ratio increased by 821.86%.

Effect of Unsteady Wakes on Boundary Layer Separation on a Very High Lift Low Pressure Turbine Airfoil

2010 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
High Lift ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Wake Passing ◽  
Very High

Boundary layer separation has been studied on a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes. Experiments were done under low (0.6%) and high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Wakes were produced from moving rods upstream of the cascade. Flow coefficients were varied from 0.35 to 1.4 and wake spacing was varied from 1 to 2 blade spacings, resulting in dimensionless wake passing frequencies F = fLj-te/Uave (f is the frequency, Lj-te is the length of the adverse pressure gradient region on the suction surface of the airfoils, and Uave is the average freestream velocity) ranging from 0.14 to 0.56. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer and downstream of the cascade. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. In cases without wakes, the boundary layer separated and did not reattach. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. At lower frequencies the boundary layer separated between wakes. Background freestream turbulence had some effect on separation, but its role was secondary to the wake effect.

scholarly journals Visualization of boundary layer separation and passive flow control on airfoils and bodies in wind-tunnel and in-flight experiments

2012 ◽  
Vol 25 ◽  
pp. 01078
Author(s):  
Lukas Popelka ◽  
Jana Kuklova ◽  
David Simurda ◽  
Natalie Souckova ◽  
Milan Matejka ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Flow Control ◽  
Boundary Layer Separation ◽  
Passive Flow Control ◽  
Layer Separation ◽  
Passive Flow

Effect of Unsteady Wakes on Boundary Layer Separation on a Very High Lift Low Pressure Turbine Airfoil

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
High Lift ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Wake Passing ◽  
Very High

Boundary layer separation has been studied on a very high lift, low pressure turbine airfoil in the presence of unsteady wakes. Experiments were done under low (0.6%) and high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Wakes were produced from moving rods upstream of the cascade. Flow coefficients were varied from 0.35 to 1.4 and wake spacing was varied from one to two blade spacings, resulting in dimensionless wake passing frequencies F=fLj-te/Uave (f is the frequency, Lj-te is the length of the adverse pressure gradient region on the suction surface of the airfoils, and Uave is the average freestream velocity) ranging from 0.14 to 0.56. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer and downstream of the cascade. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. In cases without wakes, the boundary layer separated and did not reattach. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. At lower frequencies the boundary layer separated between wakes. Background freestream turbulence had some effect on separation, but its role was secondary to the wake effect.

The boundary layer separation from streamlined surfaces and new ways of its prevention in diffusers

2017 ◽  
Author(s):  
Arkady Zaryankin ◽  
Andrey Rogalev ◽  
Ivan Komarov ◽  
V. Kindra ◽  
S. Osipov
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Layer Separation

scholarly journals Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

2021 ◽  
Vol 11 (6) ◽  
pp. 2593
Author(s):  
Yasir Al-Okbi ◽  
Tze Pei Chong ◽  
Oksana Stalnov
Keyword(s):  
Boundary Layer ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Shear Instability ◽  
Vortical Flow ◽  
High Angle ◽  
High Angle Of Attack ◽  
Layer Separation ◽  
Aerodynamic Performances

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.

On turbulent boundary-layer separation

1968 ◽  
Vol 32 (2) ◽  
pp. 293-304 ◽  
Author(s):  
V. A. Sandborn ◽  
C. Y. Liu
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Analytical Study ◽  
Boundary Layer Separation ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Layer Separation ◽  
Turbulent Separation ◽  
Separation Model

An experimental and analytical study of the separation of a turbulent boundary layer is reported. The turbulent boundary-layer separation model proposed by Sandborn & Kline (1961) is demonstrated to predict the experimental results. Two distinct turbulent separation regions, an intermittent and a steady separation, with correspondingly different velocity distributions are confirmed. The true zero wall shear stress turbulent separation point is determined by electronic means. The associated mean velocity profile is shown to belong to the same family of profiles as found for laminar separation. The velocity distribution at the point of reattachment of a turbulent boundary layer behind a step is also shown to belong to the laminar separation family.Prediction of the location of steady turbulent boundary-layer separation is made using the technique employed by Stratford (1959) for intermittent separation.

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