Laminar buffet and flow control
Résumé
An experimental investigation of the transonic flow past the laminar OALT25 airfoil has been conducted to analyze the impact of laminar flow upon the shock wave dynamics and the existence of a laminar buffet like phenomenon. Tests have been carried out at freestream Mach numbers varying in the range 0.7 to 0.8, angle of attack from 0.5 to 4 degrees and with two tripping configurations at the upper surface of the wing. The (airfoil) chord based Reynolds number is about 3 millions. Results obtained from pressure taps and sensors measurements, as well as Schlieren visualizations of the flow reveal the presence of a laminar buffet phenomenon in sharp contrast with the turbulent phenomenon, as it features a (freestream and chord) based normalized frequency of about unity while turbulent buffet occurs for a frequency close to 0.07 (Jacquin et al., AIAA Journal, vol. 47, number 9, 2009). A low frequency mode, at a frequency about 0.05 is also present in the laminar situation, notably lower than the high frequency component. The latter exhibits strong oscillations of the shock foot and vertical wavelike deformations of the shock wave and the former moves the shock back and forth over a small portion of chord, quite similarly to the turbulent phenomenon. The mean flow past the laminar wing is characterized by a laminar separation bubble under the shock foot which likely contributes much to the novel dynamics revealed by the present experiments. Two control strategies of the unsteady shock wave are implemented, one consisting of three-dimensional bumps and one consisting of steady jets blowing transversely to the freestream. It is found that bumps provide a significant reduction of the buffet intensity in the laminar situation. The jets are able to completely remove the flow unsteadiness in both laminar and turbulent conditions.
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