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External Flows An internal flow is surrounded by solid boundaries that can restrict the development of its boundary layer, for example, a pipe flow. An external flow, on the other hand, are flows over bodies immersed in an unbounded fluid so that the flow boundary layer can grow freely in one direction. Examples include the flows over airfoils, ship hulls, turbine blades, etc. One of the most important concepts in understanding the external flows is the boundary layer development. For simplicity, we are going to analyze a boundary layer flow over a flat plate with no curvature and no external pressure variation. U U U U Dye streak turbulent laminar transition Boundary Layer Parameters Boundary layer thickness d: defined as the distance away from the surface where the local velocity reaches to 99% of the free-stream velocity, that is u(y=d)=0.99U. Somewhat an easy to understand but arbitrary definition. Displacement thickness d*: Since the viscous force slows down the boundary layer flow, as a result, certain amount of the mass has been displaced (ejected) by the presence of the boundary layer (to satisfy the mass conservation requirement). Imagine that we displace the uniform flow away from the solid surface by an amount d*, such that the flow rate with the uniform velocity will be the same as the flow rate being displaced by the presence of the boundary layer. d * U w (U u ) wdy , or d * (1 0 0 Amount of fluid being displaced outward equals U-u u )dy U d* Momentum Balance (on a flat plate B. L.) Example: Determine the drag force acting on a flat plate when a uniform flow past over it. Also relate the drag to the surface shear stress. (2) (1) d h tw: wall shear stresses Net Force change of linear momentum Fs V ( V dA) all surfaces V ( V dA) surface(1) V ( V dA) surface(2) d 2 U ( U )dA u dA U h u 2 dy. (Assume unit width) (1) 2 (2) 0 d d 0 0 From mass conservation: U h udy , U 2 h U udy d Fs u(u U )dy. Force Fs is the surface acting on the fluid 0 Momentum Thickness & Skin Friction The force acting on the plate is called the friction drag (D) (due to the presence of the skin friction). d D -Fs u(U u )dy 0 The drag is related to the deficit of momentum flux across the boundary layer. It can also be directly determined by the integration of the wall shear stress over the entire plate surface: D t w dA plate t w dx plate Define momentum thickness ( ) : thickness of a layer of fluid with a uniform velocity U and its momentum flux is equal to the deficit of boundary layer momentum flux. u u (1 )dy U U 0 U 2 u(U u )dy , 0 Wall Shear Stress and Momentum Thickness Therfore, the drag force can be related to the momentum thickness as D U 2 , for a unit width boundary layer and this relation is valid for laminar or turbulent flows. It is also known that D t w dx, plate dD 2 d t w U dx dx Shear stress t w can be directly related to the gradient of d the momentum thickness along the streamwise direction . dx Recall that, for laminar flow, the wall shear stress is defined as: t w (u y ) y 0 Example Assume a laminar boundary layer has a velocity profile as u(y)=U(y/d) for 0yd and u=U for y>d, as shown. Determine the shear stress and the boundary layer growth as a function of the distance x measured from the leading edge of the flat plate. u=U y u(y)=U(y/d) t w U 2 d dx For a laminar flow t w (u y ) y 0 U d from the profile. Substitute into the definition of the momentum thickness: d U y u u y y (1 )dy (1 )dy , since u U U d d d 0 0 d 6 . x Example (cont.) t w U 2 d , dx U d U 2 Separation of variables: d x 3.46 d 3.46 U x x U , 3.46 1 dd 6 dx 6 12 d d d , integrate d 2 x 12( )x2 , U U U x U x 1 , where Re x Re x d x U 3 0.289 U 2 1 tw 0.289 , tw d x Re x x Note: In general, the velocity distribution is not a straight line. A laminar flatplate boundary layer assumes a Blasius profile (chapter 9.3). The boundary layer thickness d and the wall shear stress tw behave as: U 5.0 d U x 5.0 x 0.332 U 2 , (9.13). t w , (9.14). Re x Re x Laminar Boundary Layer Development 1 d( x ) • Boundary layer growth: d x • Initial growth is fast • Growth rate dd/dx 1/x, decreasing downstream. 0.5 0 0 0.5 x 1 10 t w( x ) • Wall shear stress: tw 1/x • As the boundary layer grows, the wall shear stress decreases as the velocity gradient at the wall becomes less steep. 5 0 0 0.5 x 1 Summary of Boundary Layer2Parameters d t w U dx • Boundary Layer Thickness, d where height, y = d when u = 0.99U For a laminar flow t w (u ) y 0 y u d * U w Thickness (U du*) wdy , orSubstitute d * into (1 the definition )dy • Displacement where of the mome 0 0 • Momentum Thickness where • Wall Shear Stress tw where U d u u y y (1 )dy (1 )dy , si U U d d 0 0 d 2 d. t w U 6 , dx U d U 2 1 dd 6 dx 6 Separation of variables: 2 d d d , integra D t w dA U U • Viscous or Friction Drag D where U d 1 3.46 3.46 , where Re x tw x U x Re Cf x • Skin Friction Coefficent, Cf where 1 2 U x2 d 3.46 , d x U U U 3 0.289 U 2