Effect of Leading Edge Blowing for Aerofoil Subjected to Laminar and Turbulent Inflows

Abstract

The aim of this thesis is the investigation on the aerodynamic performance, flow pattern and aeroacoustics of a NACA65(12)-10 aerofoil with different straight blowing as an active flow-control device and serrated leading edges as a passive flow-control device leading edges at high and low turbulent flow. Aerofoil subjected to leading edge blowing can be regarded as one of the effective flow control approaches that can harvest multiple benefits. In this thesis, small orifices are implemented at the leading edge of an aerofoil to facilitate injection of mass flow against the incoming flow with low and elevated freestream turbulence intensities. The aeroacoustics investigation reveals that the largest level of reduction in the turbulence–leading edge interaction noise is associated with a larger concentration of orifices per unit span before reaching a critical velocity, whilst a lower concentration of spanwise orifices is more desirable after the critical velocity. There also exists an optimal blow rate to tackle this particular noise source. It is envisaged that leading edge blowing, an active flow control approach, could produce the same mechanisms as those produced by a serrated leading edge to enhance the aerodynamic and aeroacoustic performances of aerofoil. It is interesting to note that the effective margin of the leading edge blowing volume flow rate is so narrow that small variation in Q  from the most optimised value would produce a large difference in the noise performance. The concept of the leading edge blowing is to minimise the interaction of an incoming turbulent flow with the leading edge of the aerofoil. Through blowing, the leading edge jet continuously opposes, and possibly dissipates the incoming turbulent eddies by either displacing the leading edge stagnation point of the aerofoil, or creating a “buffer zone” over the region around the aerofoil leading edge. It seems that one, or possibly both of these mechanisms could be very sensitive to the blowing volume flow rate, which is related to the exit jet velocity. Although not shown here, PWL as high as 9 dB can be achieved by one of the leading edge blowing configurations. NACA 65(12)-10 was chosen as the baseline aerofoil in this experimental study. Four types of serrated leading edge in different combinations of serration wavelength  and amplitude A, as well as straight leading edge with different blow rates Q  and spanwise air hole spacing  , were investigated. Based on the results so far, there exists an explicit relationship between the A and Q , as well as  and   for the aerodynamic lift and drag coefficients across a wide range of angle of attack, it is improved the stall angle and larger lift coefficient. However, at the poststall regime, the most effective configuration switches back to the one with larger concentration of orifices per unit span. Additionally, active and passive flow control techniques for serratedblowing (Hybrid) leading edge designs with superior aerodynamic or aeroacoustic performance with the influencing a number of serratedblowing leading edge devices were implemented. For high turbulent intensity and the serration-blowing leading edge is clearly beneficial, with a significant reduction in the turbulent broadband noise, up to 4 dB in some cases with the influencing factors blowing rate (Q՛=0.5-4.5 liter/min) and the sound power level reduction of the noise up to 3.7 dB for low turbulent intensity. The sound reduction involves the elimination of tonal effects. Thus, a noise reduction jump from 12 dB in elevated turbulent instances to 33 dB in the case of low Tu compares with basline case. Through noise and velocity measurements close to the leading and trailing edges of an aerofoil, the reduction of the noise is found to be primarily caused by the serration geometry. The new serrated-blowing (hybrid) leading edges have the potential to enhance serration technology's industrial worthiness in obtaining low noise radiation.