The effect of an oscillatory freestream-flow on a NACA-4412 profile at large relative amplitudes and low Reynolds-numbers

Results of an experimental investigation of the flow around a NACA-4412 profile in an oscillating freestream are presented. The experiment took place in an Eiffel-type windtunnel at a chord Reynolds-number of Re = 2 · 10⁵. Measurements of unsteady pressure distributions and boundary-layer profiles as well as flow photographs reveal that even at moderate reduced frequencies significant changes of the flow field may occur, provided that the relative amplitude of the freestream is sufficiently large. So a periodical separation and reattachment of the flow could be observed while in another case the periodical relaminarization of the boundary-layer could be found.List of symbols A relative amplitude of freestream velocity - A _I relative amplitude of first harmonic of the freestream velocity - b span of the airfoil profile - C _A lift-coefficient - C _{A st} lift-coefficient in steady freestream - C _p pressure-coefficient - d profile thickness - f frequency - H ₁₂ shape factor - k reduced frequency - l chord length - p phase-average of pressure - p ₀ total head - p static freestream pressure - p _a ambient pressure - q dynamic head - Re mean Reynolds number - Re ₂ Reynolds number - t current time - T phase time - u velocity in x-direction - u freestream-velocity - u amplitude of freestream-velocity - u _a velocity at boundary-layer edge - u _c cooling-velocity - u fluctuation of velocity in x-direction - u _rms mean square of fluctuation - û nondimensional velocity, Fig. 3 - fluctuation of velocity in y-direction - w fluctuation of velocity in z-direction - x,y,z cartesian coordinates - X _A distance of separation line from leading edge - angle of attack - nondimensional pressure gradient - boundary-layer thickness - ₁ displacement-thickness - ₂ momentum-thickness - kinematic viscosity - angular-velocity - () periodical component - (-) time-average - () stochastic component