Flow Separation and Separated Flows

AbstractFlow separation is a source of aerodynamic inefficiency, by using vortex generators flow separation can be controlled. This is of particular benefit to flows around bodies which are susceptible to separated flows, such as bodies in ground effect. Previous studies on the ability of dimples to produce vortices for flow mixing concerned heat transfer applications. Experimental measurements using Laser Doppler Anemometry (LDA) were taken in the wake of the Tyrrell026 aerofoil (Rec= 0·5 × 105) with a dimple array machined in the surface. Results for a dimple array of three rows placed forward ofx/c= 0·23 with 1·5Ddimple to dimple spacing, showed significant flow recovery in the wake. The velocity deficit ofu/Uo,min= −0·1 recovered tou/Uo,min = 0·3 with the dimple array and the size of the wake reduced by 50%; at α = 10°,h/c= 0·313. The positive effect of the dimple array on the wing reduced as the wing was brought closer to the ground.

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Numerical Investigation of Heat Transfer in Turbine Cascades With Separated Flows

Volume 3: Turbo Expo 2002, Parts A and B ◽

10.1115/gt2002-30225 ◽

2002 ◽

Cited By ~ 2

Author(s):

P. de la Calzada ◽

A. Alonso

Keyword(s):

Heat Transfer ◽

Flow Separation ◽

Turbine Blades ◽

Separated Flow ◽

Leading Edge ◽

Separated Flows ◽

Separation Flow ◽

Flow Angle ◽

Starting Point ◽

Transfer Mechanisms

Modern design of turbine blades usually requires highly loaded very thin profiles in order to save weight and cost. If local leading edge incidence is kept close to zero, then flow separation might occur on the pressure side. Although, it is known that flow separation, flow reattachment and the associated zones of re-circulation have a major impact on the heat transfer to the wall, the turbomachinery community needs an understanding of the heat transfer mechanisms in separated flows as well as models and correlations to predict it. The aim of the present investigation is a detailed study by means of an in-house CFD code, MU2 S2T, of the heat transfer mechanisms in separated flows, in particular in separation and reattachment point regions. Furthermore, an attempt is made to identify a limited number of parameters (i.e. Re, M, inlet flow angle, etc.) whose influence on the heat flux would be critical. The identification of these parameters would be the starting point to develop special correlations to estimate the heat transfer in separated flow regions.

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Evaluation of a Wall-Flow Direction Probe for Measurements in Separated Flows

Journal of Fluids Engineering ◽

10.1115/1.3241800 ◽

1982 ◽

Vol 104 (2) ◽

pp. 162-166 ◽

Cited By ~ 6

Author(s):

B. G. Shivaprasad ◽

R. L. Simpson

Keyword(s):

Flow Separation ◽

Flow Direction ◽

Separated Flows ◽

Direct Measurements ◽

Wall Flow ◽

Improved Design ◽

Downstream Flow

The upstream-downstream flow direction intermittency γpu is an important parameter that can quantitatively describe the stages of flow separation. This paper gives an improved design for a wall-flow-direction probe. Intermittency measurements made using this modified probe show agreement within experimental uncertainties with direct measurements made using a LDV, although both the unmodified and modified probe designs produce results that are consistently higher than those for the LDV.

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Numerical Investigation of Heat Transfer in Turbine Cascades With Separated Flows

Journal of Turbomachinery ◽

10.1115/1.1556014 ◽

2003 ◽

Vol 125 (2) ◽

pp. 260-266 ◽

Cited By ~ 10

Author(s):

P. de la Calzada ◽

A. Alonso

Keyword(s):

Heat Transfer ◽

Flow Separation ◽

Turbine Blades ◽

Separated Flow ◽

Leading Edge ◽

Separated Flows ◽

Separation Flow ◽

Flow Angle ◽

Starting Point ◽

Transfer Mechanisms

Modern design of turbine blades usually requires highly loaded, very thin profiles in order to save weight and cost. If local leading edge incidence is kept close to zero, then flow separation might occur on the pressure side. Although it is known that flow separation, flow reattachment, and the associated zones of recirculation have a major impact on the heat transfer to the wall, the turbomachinery community needs an understanding of the heat transfer mechanisms in separated flows as well as models and correlations to predict them. The aim of the present investigation is a detailed study by means of an in-house CFD code, MU2S2T, of the heat transfer mechanisms in separated flows, in particular in separation and reattachment point regions. Furthermore, an attempt is made to identify a limited number of parameters (i.e., Re, M, inlet flow angle, etc.) whose influence on the heat flux would be critical. The identification of these parameters would be the starting point to develop special correlations to estimate the heat transfer in separated flow regions.

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Simulation of Viscous Diffusion for Extension of the Surface Vorticity Method to Boundary Layer and Separated Flows

Journal of Mechanical Engineering Science ◽

10.1243/jmes_jour_1981_023_029_02 ◽

1981 ◽

Vol 23 (3) ◽

pp. 157-167 ◽

Cited By ~ 7

Author(s):

D. T. C. Porthouse ◽

R. I. Lewis

Keyword(s):

Boundary Layer ◽

Flow Separation ◽

Point Vortex ◽

Fluid Flows ◽

Reynolds Numbers ◽

Separated Flows ◽

Two Dimensional ◽

Drag Forces ◽

Boundary Layer Parameter ◽

Lift And Drag

A numerical method for two-dimensional incompressible viscous fluid flows is tested on the diffusion of a point vortex. It is then applied to the boundary layer to reconstruct the Blasius profile, to demonstrate flow separation, and to simulate turbulence. The significance of Thwaites' boundary layer parameter for flow separation is explained. The Kelvin-Helmholtz instability, which is responsible for two-dimensional turbulence, is represented by the motion of an array of point vortices after an initial disturbance. The formation of the Von Karman vortex street downstream of a circular cylinder is described by computer simulation, and the influence of viscous diffusion is shown. For two different cylinder Reynolds numbers the vortex shedding frequencies and oscillating lift and drag forces are evaluated.

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Transient Performance of Separated Flows: Experimental Characterization of Flow Detachment Dynamics

Volume 6: Ceramics; Controls, Diagnostics, and Instrumentation; Education; Manufacturing Materials and Metallurgy ◽

10.1115/gt2019-91020 ◽

2019 ◽

Cited By ~ 1

Author(s):

J. Saavedra ◽

G. Paniagua

Keyword(s):

Boundary Layer ◽

Flow Separation ◽

Separated Flow ◽

Flow Region ◽

Separated Flows ◽

Wall Region ◽

Flow Conditions ◽

Flow Acceleration ◽

Test Article ◽

Near Wall

Abstract The operation of compact power units at low Reynolds environments is constrained by the boundary layer detachment in the low pressure turbines stages. Flow separation is prompt by the lack of momentum on the near wall region when exposed to adverse pressure gradients. Transient flow conditions or periodic flow perturbations induced to the near wall flow may delay or prevent the flow detachment. The present investigation experimentally analyzes the behavior of separated flows based on ad-hoc wall mounted hump. The test article mimics the performance of the aft portion of the suction side of a low pressure turbine where flow separation occurs at low Reynolds and fully attached flow takes place at high Reynolds. The inception of separated flow under sudden flow release was investigated in a linear wind tunnel. The extension of the separated region and its transient development was monitored through surface pressure and temperature measurements and hotwire traverses. The inlet flow conditions to the test article were interrogated with total pressure, total temperature and hotwire traverses. A fast opening valve upstream of the settling chamber was sequentially actuated at low frequency to study the behavior of the recirculation bubble under sudden flow acceleration. Due to the sudden flow release, the near wall region overcomes the adverse pressure gradient. As the flow acceleration dilutes the boundary layer detaches and the separated flow region grows in the stream-wise direction. The comparison of the experimental results with 2D and 3D transient Computational Fluid Dynamic simulations demonstrates the ability of Unsteady Reynolds Average Navier-Stokes models to predict the dynamics of this phenomenon. However, CFD over-predicts the extension of the recirculated flow region. The integration of this research towards future control strategies will enable efficient operation of turbine-hybrid systems operating at high power.

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A nonlinear dynamic model for unsteady separated flow control and its mechanism analysis

Journal of Fluid Mechanics ◽

10.1017/jfm.2017.321 ◽

2017 ◽

Vol 826 ◽

pp. 942-974 ◽

Cited By ~ 11

Author(s):

Guoping Huang ◽

Weiyu Lu ◽

Jianfeng Zhu ◽

Xin Fu ◽

Jinchun Wang

Keyword(s):

Unsteady Flow ◽

Flow Control ◽

Nonlinear Model ◽

Flow Separation ◽

Separated Flow ◽

Approximate Model ◽

The Self ◽

Separated Flows ◽

Periodic Excitation ◽

Unsteady Separated Flow

In the analysis of the interaction between external periodic excitation and unsteady separated flow in controlling the flow separation, a new nonlinear approximate model has been established. This model is used to describe the typical chaotic and coherent characteristics of a separated flow such as small- or large-scale vortices, the injection, and the dissipation of the kinetic energy based on a simulation of a simplified cross-direction motion of free shear flows. This study presents an appropriate treatment to simulate the external periodic excitation and uses the maximum Lyapunov exponent to evaluate the degree of flow ordering in the different control states. The results of the nonlinear model are compared with experimental and numerical results, showing that the nonlinear model could be used to effectively explain the behaviours of chaotic flows and investigate the rules for controlling separated flows. In addition, as shown in the nonlinear approximate model, the self-synchronization of unsteady flow separation and periodic excitation has been analysed. Initially, the research provided an explanation of the self-synchronization mechanism, which cites that the effects of the separated flow control are independent of the phase difference between the periodic excitation and the unsteady flow. The characteristics of unsteady separated flow control have also been presented in this model, where the corresponding large eddy simulation (LES) was used for separated flows in a curved diffuser. The proper orthogonal decomposition (POD) method was used to analyse the difference between separated vortical structures with or without periodic excitation. The results showed that the model and the simulation had the same mechanism of flow control as for the separated flows. The periodic excitation transforms the original chaotic flow into a relatively ordered flow and decreases the magnitude of the chaotic unstable vortices, rather than completely eliminating the vortices, while flow mixing is reduced, inducing less energy loss.

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