Abstract:
The present research investigates the dynamic stall behaviour of the thick NREL S814 aerofoil at the Reynolds numbers of = 0.75 × 106 and 1.1 × 105. This was achieved through 2D CFD modelling with URANs method. The aerofoil was sinusoidally pitched about the quarter chord point at a mean angle of attack, AoA, of 8° and amplitude of 5.5°. Effect of reduced frequency, = 0.073 and 0.114 was also studied. Static state studies were however conducted first to fully characterise the flow behaviour of the S814 aerofoil and for CFD model validation. Static state study revealed that a large laminar separation bubble (LSB) observed at = 1.1 × 105 relative to = 0.75 × 106 leads to sensitive and unstable flow behaviour, which strongly influences the dynamic stall behaviour. At = 0.75 × 106, delay in stall and increase in lift was found to be the result of delay in trailing edge separation and reattachment as increases. With increase in , the hysteresis behaviour is much pronounced. No vortex disturbance was found on the upper surface of the foil during the up-stroke phase as well as the down-stroke phase. An additional LSB was found on the upper surface at / = 0.35 for = 0.114 at 11.8° during the upstroke phase but not for = 0.073. At = 1.1 × 105 , simulated dynamic stall behaviour was found to differ greatly with turbulence model. This was the consequence of enlargement and bursting of the LSB observed in the static state. More detailed study such as PIV or LES is recommended to clarify the flow behaviour of dynamic stall at this Reynolds number. Comparison between simulated results at these two Reynolds numbers suggested that lift stall and maximum lift coefficient obtained near the maximum angle were delayed and increased at the lower Reynolds number. Reduction in Reynolds number tends to delay flow reattachment. An elongated and pronounced hysteresis loop is also observed when Reynolds number was reduced. The present results indicate that with lower Reynolds number, the aerofoil is subjected to the higher dynamic loading.
