EFDlab3_concept_dragforce - Fall 2009 Appendix C....

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Fall 2009 Appendix C. Calculation of Airfoil Drag Force Using the Momentum Integral Method Consider an experiment in which the re is drag force on an airfoil that is immersed in a steady incompressible flow. The drag force can be determined from the measurement of velocity distributions far upstream and downstream of the airfoil body (figure below). 1. Velocity far upstream is the uniform flow . 2. Velocity in the wake of the body is measured with hotwire/Pitot probe to be , which is less than due to the drag of the airfoil. 3. Objective: Find the drag force per unit length (span wise) of the airfoil. Method 1: Choose a control volume following the flow streamline. Fig. 1 Control volume following the flow streamline. Solution: Find relation between and using the mass conservation Since we choose the streamline as the control volume ( CV ), there is no mass flow across it. For the CV , is the unit normal vector and it is assumed that the CV has a unit depth. () () Thus,
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EFDlab3_concept_dragforce - Fall 2009 Appendix C....

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