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# boundaryla - Some Elementary Aspects of Boundary Layer...

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Some Elementary Aspects of Boundary Layer Theory R. Shankar Subramanian The failure of potential flow theory to predict drag on objects when a fluid flows past them provided the impetus for Prandtl to put forward a theory of the boundary layer adjacent to a rigid surface. Prandtl’s principal assumptions are listed below. Assumptions 1. When a fluid flows past an object at large values of the Reynolds number, the flow region can be divided into two parts. (i) Away from the surface of the object, viscous effects can be considered negligible, and potential flow can be assumed. (ii) In a thin region near the surface of the object, called the boundary layer, viscous effects cannot be neglected, and are as important as inertia. 2. The pressure variation can be calculated from the potential flow solution along the surface of the object, neglecting viscous effects altogether, and assumed to be impressed upon the boundary layer. U x laminar y turbulent Transition from laminar to turbulent flow in the boundary layer on a flat plate occurs at , where 5 Re 5 10 x ≈× Re x xU ν = . Here, is the kinematic viscosity of the fluid. 1

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The following approximate estimates can be written for the thickness of laminar and turbulent boundary layers. Laminar Boundary Layer 5 Re x x δ Turbulent Boundary Layer () 1/7 0.16 Re x x Some aspects of the drag on a body when a fluid flows past it at high Reynolds number From the four videos by Professor A. Shapiro, we learned some important ideas about the drag experienced by a body when a fluid flows past it. Professor Shapiro showed experimental observations that initially appeared to be paradoxical, and later explained them using physical aspects of boundary layer theory.
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boundaryla - Some Elementary Aspects of Boundary Layer...

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