Fluids – Lecture 3 Notes - MIT

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Fluids Lecture 3 Notes1. 2-D Aerodynamic Forces and Moments2. Center of Pressure3. Nondimensional CoefficientsReading: Anderson Forces and MomentsSurface force distributionThe fluid flowing about a body exerts a local force/area (or stress)~fon each point of thebody. Its normal and tangential components are the pressurepand the shear stress .( magnitude greatly exaggerated)Vff RMRLDNA resultant force, and moment about ref. pointalternative components of resultant forceplocal pressure and shear stress componentsforce/area distribution on airfoil rdsrThe figure above greatly exaggerates the magnitude of the stress component just to makeit visible. In typical aerodynamic situations, the pressurep(or even the relative pressurep p ) is typically greater than by at least two orders of magnitude, and so~fis verynearly perpendicular to the surface. But the small often significantly contributes to drag,so it cannot be neglected stress distribution~fintegrated over the surface produces a resultant force~R, and alsoa momentMabout some chosen moment-reference point.

For 2-D bodies such as airfoils, the appropriate reference area/span is simply the chord c, and the reference length is the chord as well. The local coefficients are then defined as follows. Local Lift coefficient: cℓ ≡ L′ q ∞ c Local Drag coefficient: cd ≡ …

  Airfoils