Aerodynamic and aeroacoustic performance of a pitching foil with trailing edge serrations at a high Reynolds number

Abstract

Abstract The aerodynamic and aeroacoustic performance of a low-aspect-ratio ( $$\hbox {AR}=0.2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mtext>AR</mml:mtext> <mml:mo>=</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> ) pitching foil during dynamic stall are investigated numerically with focus on the effects of trailing edge serrations. A hybrid method coupling an immersed boundary method for incompressible flows with the Ffowcs Williams–Hawkings acoustic analogy is employed. Large eddy simulation and turbulent boundary layer equation wall model are also employed to capture the turbulent effects. A modified NACA0012 foil with a rectangular trailing edge flap attached to the trailing edge (baseline case) undergoing pitching motion is considered. Trailing edge serrations are applied to the trailing edge flap and their effects on the aerodynamic and aeroacoustic performance of the oscillating airfoil are considered by varying the wave amplitude ( $$2h^*= 0.05, 0.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msup> <mml:mi>h</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>0.05</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> , and 0.2) at a Reynolds number of 100,000 and a Mach number of 0.05. It is found that the reduction of the sound pressure level at the dimensionless frequency band $$St_{b}\in [1.25,4]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>S</mml:mi> <mml:msub> <mml:mi>t</mml:mi> <mml:mi>b</mml:mi> </mml:msub> <mml:mo>∈</mml:mo> <mml:mrow> <mml:mo>[</mml:mo> <mml:mn>1.25</mml:mn> <mml:mo>,</mml:mo> <mml:mn>4</mml:mn> <mml:mo>]</mml:mo> </mml:mrow> </mml:mrow> </mml:math> can be over 4 dB with the presence of the trailing edge serrations ( $$2h^*=0.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msup> <mml:mi>h</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> ), while the aerodynamic performance and its fluctuations are not significantly altered except a reduction around 10% in the negative moment coefficient and it fluctuations. This is due to the reduction of the average spanwise coherence function and the average surface pressure with respect to that of the baseline case, suggesting the reduction of the spanwise coherence and the noise source may result in the noise reduction. Analysis of the topology of the near wake coherent structure for $$2h^*=0.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msup> <mml:mi>h</mml:mi> <mml:mo>∗</mml:mo> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> reveals that the alignment of the streamwise-oriented vortex with the serration edge may reduce the surface pressure fluctuation. Graphical abstract

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