A. Lepage, Y. Amosse, J.P. Brazier, M. Forte, O. Vermeersch, C. Liauzun
DOI Number: N/A
Conference number: IFASD-2019-120
The transition from laminar to turbulent boundary layer has been largely investigated in the literature on the experimental side as well as on the numerical side but the processes specifically involved in both transitional and unsteady flows (unsteady aerodynamic or aeroelasticity) have to be studied more in details. This paper describes an experimental investigation achieved through a wind tunnel test of an oscillating swept wing in low speed range. In order to force the apparition of Cross Flow instabilities, the model was installed with a 60° sweep angle under favorable pressure gradient ensuring the absence of Tollmien Schlichting instabilities. The model was equipped with hotfilm sensors to estimate the laminar or turbulent state of the boundary layer through the measurement of wall shear stress. The synchronous acquisition of dynamic motion and hotfilm signals allowed to analyse the unsteady effects on the Cross Flow induced transition. The wind tunnel test program was achieved following a parametric approach through the variation of significant parameters: the amplitude and frequency of the dynamic actuation, the flow speed velocity and the mean value of the airfoil angle of attack. The paper describes the main insights of the test campaign and the associated database and presents how the test setup provided a well-adapted mean to study dynamic motions of the transition location. In comparison to steady configuration, the transition was not located at a fixed chordwise position but covered an area with an intermediary state of the boundary layer between the laminar and turbulent states. Several tools and post processing methods were specifically tuned to calculate quantities of the transition position (time resolved signal, mean properties through a phase averaging process, intermittency ratio) in addition to classical time and frequency analyses. One of the main outcomes pointed out the mean location of the laminar – turbulent transition was weakly influenced by unsteady motion. The general trend indicated a more upstream location of few percent in chordwise in comparison to the steady case without any motion. The experimental results constitute a first step in the understanding of the interaction of Cross Flow instabilities and dynamic motion and in the assessment of ONERA modelling and numerical simulation capabilities.