CFDa.5.external - Fluids Review TRN-1998-004 External Flows...

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© Fluent Inc. 12/05/10 E1 Fluids Review TRN-1998-004 External Flows
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© Fluent Inc. 12/05/10 E2 Fluids Review TRN-1998-004 Overvie w Characteristics of flow past an object Concepts of lift and drag Drag Friction Drag (due to shear stress) Pressure Drag (or form drag) Lift surface pressure distribution e.g. airfoils and buildings circulation
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© Fluent Inc. 12/05/10 E3 Fluids Review TRN-1998-004 External flow past an object An object immersed in a moving fluid (or moving object in a stationary fluid) experiences a resultant force due to the interaction between the object and the surrounding fluid. U U
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© Fluent Inc. 12/05/10 E4 Fluids Review TRN-1998-004 Lift and Drag - Basic Concepts The surrounding fluid exerts pressure forces and viscous forces on the 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 D acts in the direction of the motion of the fluid relative to the object. The lift force L acts normal to the flow direction p < 0 U p > 0 U τ w Lift Drag 2 2 2 1 . . 2 1 . . U A C D U A C L D L ρ = ρ =
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© Fluent Inc. 12/05/10 E5 Fluids Review TRN-1998-004 Drag (1) The drag force is due to the pressure and shear forces acting on the surface of the object. Drag is influenced by the shape of the object and the Reynolds number of the flow. The tangential shear stresses acting on the object produce friction drag (or viscous drag ). Friction drag is dominant in flow past a flat plate. Pressure or form drag results from the normal stress and becomes more important in flows past bluff bodies.
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© Fluent Inc. 12/05/10 E6 Fluids Review TRN-1998-004 Drag (2) Analytical solutions of the distribution of pressure and shear stress are rare, in particular for complex shapes which typically exhibit flow separation.
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