On the Flow Past a Three-Element Wing: Mean Flow and Turbulent Statistics

Abstract

Large eddy simulations (LES) on the flow past the 30P30N three-element high-lift wing at a moderate Reynolds number \(Re_c=750,000\) and three different angles of attack \(\alpha =5\), 9 and \(23^\circ \) are conducted. The main focus is on the time-averaged statistics of the turbulent flow. The form drag noticeably increases with the angle of attack, while viscous drag remains roughly constant and contributes minimally to the total drag. This is associated with the significant pressure peaks found in the main element with increasing angles of attack and hence, the development of stronger adverse pressure gradients. At \(\alpha =23^\circ \), this leads to the development of a prominent wake downstream this element that eventually evolves into a visible recirculation region above the flap, indicating the onset of stall conditions. In the flap, strong adverse pressure gradients are observed at small angles of attack instead, i.e., \(\alpha =5\) and \(9^\circ \). This is attributed to the flap’s deflection angle with respect to the main wing, which causes a small separation of the boundary layer as the flow approaches the trailing edge. At the stall angle of attack, i.e., \(\alpha =23^\circ \), the spread of the main element wake maintains attached the flow near the flap wall, thus mitigating the pressure gradient there and preventing the flow to undergo separation. The shear layers developed on the slat and main coves are also analysed, with the slat shear layer showing more prominence. In the slat, its size and intensity noticeably decrease with the angle of attack as the stagnation point moves towards the slat cusp. Conversely, the size of the shear layer developed in the main element cavity remains approximately constant regardless of the angle of attack. At the lower angles of attack, i.e., \(\alpha =5\) and \(9^\circ \), the development of the shear layer is anticipated by the turbulent separation of the flow along the pressure side of the main wing, leading to increased levels of turbulence downstream. At the higher angle of attack, i.e., \(\alpha =23^\circ \), the shear layer is originated by the cavity separation and transition to turbulence occurs within the cavity.