# CH11 - 1 External Flow Fluid Dynamics Application III...

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Unformatted text preview: 1 External Flow Fluid Dynamics Application III External Flow Chapter 11 – Drag & Lift Definition of Drag & Lift Concept of Separation & Wake 1 Drag coefficient & Influence Factors Depends on boundary factor, classified as ¡ Internal flow: fluid is entirely bounded External flow: flow is unconstrained spatially on Classification of Fluid Motions length scale relevant to body Open-channel: top surface of the fluid flow is exposed to atmosphere 2 2 Definition of Drag & Lift - I Incoming flow FREE STREAM , with uniform flow velocity U uniform flow velocity U passes over a 2D object Forces from surrounding fluid on this 2-D object are: ( a ) pressure force, F ∫ dA p ) ( r 3 ( b ) viscous force, (Pressure is gage pressure) ( ) dA w ∫ τ r Definition of Drag & Lift - II Resultant force on body Force F decomposed into ¡ Horizontal component ( ) dA dA p F w ∫ ∫ + = τ r r r ) ( L D F F F L D r r r r r + = + = ¡ Horizontal component - drag ¡ Vertical component – Lift Drag = pressure drag + viscous drag 4 ( ) dA p D w ∫ + = θ τ θ sin cos Drag is a resistance force, opposite to flow direction 3 Shear force on object surface ¡ Is caused by shear between fluid & solid surface (due to μ & velocity gradient Shear Force & Viscous Drag (due to μ & velocity gradient ) ¡ Is a function of ___________and ______of the object ¡ Also known for viscous drag , friction drag surface area length = μ du/dy 5 τ w = μ .(du/dy) Drag = pressure drag + viscous drag ( ) dA p D w ∫ + = θ τ θ sin cos Separation often occurs at sharp corners...
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CH11 - 1 External Flow Fluid Dynamics Application III...

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