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**Unformatted text preview: **Measurement of Boundary Layer on a Flat Plate Kay Gemba * California State University, Long Beach March 25, 2007 Abstract A basic understanding of flow characteristics over a flat plate is es- sential to a complete study of Aerodynamics. This experiment was conducted in the California State University of Long Beach, CSULB , windtunnel to gain a better understanding of the parameters and char- acteristics of fluid flow over a flat plate. Readings of the boundary layer were taken at four locations along a flat plate at an average free stream velocity U ∞ of 19 . 1 ± . 3 m s giving Reynolds numbers cor- responding to laminar through turbulent flows. The height of the boundary layer ranged from around 3 mm to 29 mm. Displacement thickness and momentum thickness values were calculated using the velocity profile. The skin-friction coefficients were determined using three separate techniques all leading to similar, yet different results. Comparing these results to a theoretical value of 0.0037, the best re- sult for C f was calculated to be 0.00372 using an equation in terms of Reynolds number for a turbulent section. 1 Objective To become familiar with a boundary layer and its parameters. 2 Background and Theory Boundary layer is a layer adjacent to a surface where viscous effects are important. Figure (1) depicts flow of a fluid over a flat plate. * [email protected] 1 Figure 1: Flow over a flat plate The fluid particles at the flat plate surface have zero velocity and they act as a retardant to reduce velocity of adjacent particles in the vertical direction. Similar actions continue by other particles until at the edge of the boundary layer where the particles’ velocity is 99% of the free stream velocity. Bound- ary layers can also be measured by more significant parameters. The main boundary layer parameters are as follows: The displacements thickness, δ * is defined as the distance by which the external streamlines are shifted due to the presence of the boundary layer: δ * = Z (1- u u ∞ ) dy (1) The momentum thickness represents the height of the free-stream flow which would be needed to make up the deficiency in momentum flux within the boundary layer due to the shear force at the surface. The momentum thick- ness for an in-compressible boundary layer is given by: θ = Z u u ∞ (1- u u ∞ ) dy (2) The skin-friction coefficient is defined as: C f = τ 1 2 ρu 2 ∞ dy (3) τ = ( ∂u ∂y ) y =0 (4) 2 The Reynolds number is a measure of the ratio of inertia forces to viscous forces. It can be used to characterize flow characteristics aver a flat plate. Values under 500,000 are classified as Laminar flow where values from 500,000 to 1,000,000 are deemed Turbulent flow. Is it important to distinguish be- tween turbulent and non turbulent flow since the boundary layer thickness varies, as Fig. (2) shows....

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