Analysis of Laminar Boundary-Layer Separation in Retarded Flow over Bodies of Revolution

2021 ◽  
Author(s):  
Ahmer Mehmood ◽  
Babar Hussain Shah ◽  
Muhammad Usman ◽  
Iqrar Raza
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Flow Separation ◽  
Boundary Layer Separation ◽  
The Body ◽  
Bodies Of Revolution ◽  
Layer Separation ◽  
Transverse Curvature ◽  
Favorable Pressure Gradient ◽  
Curvature Parameter

Laminar boundary-layer separation phenomenon is one of the interesting and important aspects of boundary-layer flows. It occurs in various physical situations because of decreasing wall shear stress. Retarded flow velocities are one of the reasons to happen this event. Flow separation can be prevented or delayed by utilizing bodies of revolution as surface transverse curvature produces the effects of the nature of favorable pressure-gradient which in turn increases wall shear stress that keeps the flow attached to the surface. Bodies of revolution whose body contour follows power-law form also play a vital role to delay flow separation. Bodies of revolution of varying cross-sections and involving surface transverse curvature (TVC) are utilized to examine their effects on flow separation. Particularly, a convex transverse curvature has been considered due to its effects of the nature of favorable pressure-gradient which causes to delay the flow separation. A retarded flow velocity of Görtler’s type is considered in this study to investigate flow separation process. A detailed analysis is provided to understand the flow separation by calculating separation points under various assumptions. It has been observed that the body contours exponent n and the convex transverse curvature parameter k play an assistive role in the delaying of boundary-layer separation even under the influence of strong retardation. Results are presented through various Tables and graphs in order to highlight the role of the power-law exponent of external velocity m, the convex transverse curvature parameter k, and the body contours exponent n on separation points.

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.

Detecting boundary-layer separation point with a micro shear stress sensor array

2007 ◽  
Vol 139 (1-2) ◽  
pp. 31-35 ◽  
Author(s):  
Kui Liu ◽  
Weizheng Yuan ◽  
Jinjun Deng ◽  
Binghe Ma ◽  
Chengyu Jiang
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Sensor Array ◽  
Boundary Layer Separation ◽  
Separation Point ◽  
Stress Sensor ◽  
Layer Separation ◽  
Shear Stress Sensor

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%.

scholarly journals On unsteady boundary-layer separation in supersonic flow. Part 1. Upstream moving separation point

2011 ◽  
Vol 678 ◽  
pp. 124-155 ◽  
Author(s):  
A. I. RUBAN ◽  
D. ARAKI ◽  
R. YAPALPARVI ◽  
J. S. B. GAJJAR
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Supersonic Flow ◽  
Body Surface ◽  
Boundary Layer Separation ◽  
The Body ◽  
Separation Point ◽  
Coordinate Frame ◽  
Characteristic Time Scale ◽  
Layer Separation

This study is concerned with the boundary-layer separation from a rigid body surface in unsteady two-dimensional laminar supersonic flow. The separation is assumed to be provoked by a shock wave impinging upon the boundary layer at a point that moves with speed Vsh along the body surface. The strength of the shock and its speed Vsh are allowed to vary with time t, but not too fast, namely, we assume that the characteristic time scale t ≪ Re−1/2/Vw2. Here Re denotes the Reynolds number, and Vw = −Vsh is wall velocity referred to the gas velocity V∞ in the free stream. We show that under this assumption the flow in the region of interaction between the shock and boundary layer may be treated as quasi-steady if it is considered in the coordinate frame moving with the shock. We start with the flow regime when Vw = O(Re−1/8). In this case, the interaction between the shock and boundary layer is described by classical triple-deck theory. The main modification to the usual triple-deck formulation is that in the moving frame the body surface is no longer stationary; it moves with the speed Vw = −Vsh. The corresponding solutions of the triple-deck equations have been constructed numerically. For this purpose, we use a numerical technique based on finite differencing along the streamwise direction and Chebyshev collocation in the direction normal to the body surface. In the second part of the paper, we assume that 1 ≫ Vw ≫ O(Re−1/8), and concentrate our attention on the self-induced separation of the boundary layer. Assuming, as before, that the Reynolds number, Re, is large, the method of matched asymptotic expansions is used to construct the corresponding solutions of the Navier–Stokes equations in a vicinity of the separation point.

scholarly journals Electrical Swath Ships with Underwater Hulls Preventing the Boundary Layer Separation

2020 ◽  
Vol 8 (9) ◽  
pp. 652
Author(s):  
Igor Nesteruk ◽  
Srecko Krile ◽  
Zarko Koboevic
Keyword(s):  
Boundary Layer ◽  
Weight Ratio ◽  
Boundary Layer Separation ◽  
Water Area ◽  
The Body ◽  
Layer Separation ◽  
Small Water ◽  
High Reynolds ◽  
Body Shapes ◽  
Neutrally Buoyant

The body shapes of aquatic animals can ensure a laminar flow without boundary layer separation at rather high Reynolds numbers. The commercial efficiencies (drag-to-weight ratio) of similar hulls were estimated. The examples of neutrally buoyant vehicles of high commercial efficiency were proposed. It was shown that such hulls can be effectively used both in water and air. In particular, their application for SWATH (Small Water Area Twin Hulls) vehicles is discussed. In particular, the seakeeping characteristics of such ships can be improved due to the use of underwater hulls. In addition, the special shaping of these hulls allows the reducing of total drag, as well as the energetic needs and pollution. The presented estimations show that a weight-to-drag ratio of 165 can be achieved for a yacht with such specially shaped underwater hulls. Thus, a yacht with improved underwater hulls can use electrical engines only, and solar cells to charge the batteries.

Flow separation on a β-plane

1980 ◽  
Vol 99 (2) ◽  
pp. 399-409 ◽  
Author(s):  
Lee-Or Merkine
Keyword(s):  
Boundary Layer ◽  
Flow Separation ◽  
Layer Structure ◽  
Boundary Layer Separation ◽  
Rotating Flows ◽  
Boundary Layer Structure ◽  
Layer Separation

Boundary-layer structure of prograde and retrograde rotating flows past a cylinder on a β-plane is investigated. It is found that β inhibits boundary-layer separation for prograde flows but it exerts no influence on the boundary-layer structure for retrograde flows. The results agree with the few available experimental observations.

Micro-Actuated Delta Wings Induced Control of Boundary Layer Separation

2006 ◽  
Author(s):  
Augusto Lori ◽  
Savaas Xanthos ◽  
Mahmoud Ardebili ◽  
Yiannis Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separated Flow ◽  
Adverse Pressure Gradient ◽  
Boundary Layer Separation ◽  
Wall Region ◽  
Delta Wings ◽  
Layer Separation ◽  
Favorable Pressure Gradient ◽  
Dimensional Boundary

Control of boundary layer separation has been investigated employing an array of micro-actuated delta winglets. The flow with the array is simulated computationally in an initially two-dimensional boundary layer, which is subjected to a Favorable Pressure Gradient (FPG) that accelerated the flow substantially, followed by an Adverse Pressure Gradient (APG) where the flow decelerated, the two successive distortions cause a flow separation in the boundary layer developing on the opposite wall of the wind tunnel. The simulations capture vortices formed by the impulsive motion of the delta wings. The vortices are part of recirculating zone in the wake of the actuators, which as they advect downstream, bring high momentum fluid into the near wall region of a separated flow. Preliminary results indicate micro-actuated delta wings array affect boundary layer separation favorably.

Vehicles Drag Reduction With Control of Critical Reynolds Number

2013 ◽  
Vol 135 (10) ◽  
Author(s):  
E. L. Amromin
Keyword(s):  
Boundary Layer ◽  
Active Flow Control ◽  
Substantial Reduction ◽  
Side Surface ◽  
Adverse Pressure Gradient ◽  
Middle Surface ◽  
Boundary Layer Separation ◽  
The Body ◽  
Whole Body ◽  
Layer Separation

Various vehicles have been designed as short blunt bodies. Drag coefficients of these bodies are high because adverse pressure gradients cause boundary layer separation from their surfaces, but a reduction of the size of separation zone allows for a substantial reduction of the body drag. It can be done via displacement of their boundary layer separation far downstream. In this study, such displacement was achieved with a combination of passive and active flow control. First, the whole body side surface includes two constant pressure surfaces of selected lengths and the surface of a high adverse pressure gradient in the middle of them. Second, the boundary layer suction maintained on this middle surface prevents separation there. The concept feasibility is manifested for very short axisymmetric bodies (of length to width ratios from 1.02 till 1.25). For moderate Reynolds numbers (from 3,000,000 to 10,000,000) and at the optimum suction intensity, the total drag coefficient of the designed bodies is about tenfold lower than the drag of spheroids of the same slenderness. The 3D design problem is also considered.

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
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