11-bl - Lecture 11 Boundary Layers and Separation Applied...

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1 Lecture 11 – Boundary Layers and Separation Applied Computational Fluid Dynamics Instructor: André Bakker http://www.bakker.org © André Bakker (2002-2006) © Fluent Inc. (2002)
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2 Overview Drag. The boundary-layer concept. Laminar boundary-layers. Turbulent boundary-layers. Flow separation.
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3 The surrounding fluid exerts pressure forces and viscous forces on an object. The components of the resultant force acting on the object immersed in the fluid are the drag force and the lift force. The drag force acts in the direction of the motion of the fluid relative to the object. The lift force acts normal to the flow direction. Both are influenced by the size and shape of the object and the Reynolds number of the flow. p < 0 U p > 0 U τ w The drag force
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4 Drag prediction The drag force is due to the pressure and shear forces acting on the surface of the object. The tangential shear stresses acting on the object produce friction drag (or viscous drag). Friction drag is dominant in flow past a flat plate and is given by the surface shear stress times the area: Pressure or form drag results from variations in the the normal pressure around the object: In order to predict the drag on an object correctly, we need to correctly predict the pressure field and the surface shear stress. This, in turn, requires correct treatment and prediction of boundary layers and flow separation. We will discuss both in this lecture. w viscous d A F τ . , = = A n pressure d da p F ,
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5 Viscous boundary layer An originally laminar flow is affected by the presence of the walls. Flow over flat plate is visualized by introducing bubbles that follow the local fluid velocity. Most of the flow is unaffected by the presence of the plate. However, in the region closest to the wall, the velocity decreases to zero. The flow away from the walls can be treated as inviscid, and can sometimes be approximated as potential flow. The region near the wall where the viscous forces are of the same order as the inertial forces is termed the boundary layer. The distance over which the viscous forces have an effect is termed the boundary layer thickness. The thickness is a function of the ratio between the inertial forces and the viscous forces, i.e. the Reynolds number. As Re increases, the thickness decreases.
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Effect of viscosity The layers closer to the wall start moving right away due to the no-slip boundary condition. The layers farther away from the wall start moving later. The distance from the wall that is affected by the motion is also called the viscous diffusion length. This distance increases as time goes on. The experiment shown on the left is performed with a higher viscosity
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11-bl - Lecture 11 Boundary Layers and Separation Applied...

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