Large eddy simulations and experiments on low-Reynolds-number laminar separation control for low-pressure turbine blades with oblong-dimpled surface

Modern high-performance low-pressure turbines (LPT) usually encounter flow problems at low Reynolds numbers, especially for high-altitude UAVs under cruise conditions. Flow separation may occur on the blade suction side, seriously degrading the turbine's aerodynamic efficiency. The present study investigates a passive laminar flow separation control method based on the oblong dimple surface structures for their more potent flow acceleration abilities. Linear cascade tests and large eddy simulations are conducted for a typical LPT blade profile, T106A, at the Reynolds numbers of 25,000, 50,000, and 100,000 based on the inlet velocity and axial chord length. Results show that applying the oblong dimples can apparently reduce the reverse flow region and improve the aerodynamic performance at low Reynolds numbers. The experimental results achieve a maximum total pressure loss reduction of 34.7% at a low Reynolds number of 25,000. The numerical simulations show the local flow acceleration after the oblong dimples and demonstrate two working modes. At low Reynolds numbers, the oblong dimples accelerate the fluid above, generating local high-speed flows, and the uneven velocity distributions in the spanwise direction feed the deformation and fragmentation of the downstream separating vortex. While at high Reynolds numbers, the oblong dimples act as vortex generators, leading to flow transition on the blade suction surface.