A Review on Generation and Mitigation of Airfoil Self-Induced Noise

Abstract

A review on passive acoustic control of airfoil self-noise by means of porous trailing edge is presented. Porous surfaces are defined using various terms such as porosity, permeability, resistivity, porosity constant, dimensionless permeability, flow control severity and tortuosity. The primary purpose of this review paper is to provide key findings regarding the sources and mitigation techniques of self-induced noise generated by airfoils. In addition, various parametric design concepts were presented, which are critically important for porous-airfoil design specifications. Most research focus on experimentation with some recent efforts on numerical simulations. Detail study on flow topology is required to fully understand the unsteady flow nature. In general, noise on the airfoil surface is linked to the vortex shedding, instabilities on the surface, as well as feedback mechanism. In addition, acoustic scattering can be minimized by reducing extent of the porous region from the trailing edge while increasing resistivity. Moreover, blowing might also be another means of reducing noise near the trailing edge. Ultimately, understanding the flow physics well provides a way to unveil the unknowns in self-induced airfoil noise generation, mitigation, and control.

Numerical Prediction of Trailing Edge Noise at Low Reynolds Number with Modified Trailing Edges of a NACA 0015 Airfoil

“…This heightened attention is attributed to the fact that noise reduction has emerged as a crucial challenge that cannot be overlooked [11]. These efforts have been deliberated carefully by acoustically tailored materials and/or altering the design configurations [15]. Many researchers have been bio-inspired by nature's silent fliers.…”

“…Consequently, as the amplified T-S wave passes the airfoil trailing edge, boundary-layer pressure fluctuations linked to turbulent are aeroacoustically scattered due to the presence of discontinuity such as trailing edge [22]. Therefore, turbulent kinetic energy is converted to a strong acoustic wave that is propagated to the far field in a dipolar or quadrupolar manner, and thus, the noise generated is often referred to as trailing-edge noise [15,[23][24][25][26][27]. This occurs at the large-physical length scale involved relative to the acoustic wavelength, for small-body limit, noise generation is limited by the large-scale vortices [25].…”

“…Therefore, trailing-edge noise is considered very important in the design of wind turbines, submarines, aircrafts and automobiles. [4,5,15,24,[29][30][31][32][33].…”

Experimental Investigation of Low Reynolds Number Flow Around a Serrated NACA 0015 Airfoil

“…The flow structures around an airfoil are entirely different for a wide range of Reynolds numbers. Flow separation and formation of laminar separation bubbles (LSB) are among the main features found in this regime [1]. The flow separates due to a strong adverse pressure gradient [2,3].…”

“…The most frequently used parameters to describe porous materials are porosity, airflow resistivity and tortuosity [36,37]. Porosity can be defined as the measure of the void space in a material whereas resistivity indicates the ability of a material to oppose the flow through its void spaces [1]. The porous material should be permeable to the normal component velocity at the source and impermeable to the average flow velocity.…”