Role of Polymeric Coating on Metallic Foams to Control the Aeroacoustic Noise Reduction of Airfoils with Permeable Trailing Edges

Abstract

Studies on porous trailing edges, manufactured with open-cell Ni-Cr-Al foams with sub-millimeter pore sizes, have shown encouraging results for the mitigation of turbulent boundary-layer trailing-edge noise. However, the achieved noise mitigation is typically dependent upon the pore geometry, which is fixed after manufacturing. In this study, a step to control the aeroacoustics effect of such porous trailing edges is taken, by applying a polymeric coating onto the internal foam structure. Using this method, the internal topology of the foam is maintained, but its permeability is significantly affected. This study opens a new possibility of aeroacoustic control, since the polymeric coatings are temperature responsive, and their thickness can be controlled inside the foam. Porous metallic foams with pore sizes of 580, 800, and 1200 μm are (internally) spray-coated with an elastomeric coating. The uncoated and coated foams are characterized in terms of reduced porosity, average coating thickness and air-flow resistance. Subsequently, the coated and uncoated foams are employed to construct tapered inserts installed at the trailing edge of an NACA 0018 airfoil. The noise mitigation performances of the coated metal foams are compared to those of uncoated metal foams with either similar pore size or permeability value, and both are compared to the solid trailing edge reference case. Results show that that the permeability of the foam can be easily altered by the application of an internal coating on the metallic foams. The noise reduction characteristics of the coated foams are similar to equivalent ones with metallic materials, provided that the coating material is rigid enough not to plastically deform under flow conditions.

Noise Reduction Mechanisms of an Airfoil with Trailing Edge Serrations at Low Mach Number

“…By multiplying u i by Equation (20) and ρ , by Equation (21), taking the divergence of the sum, subtracting time derivation of the mass conservation law in Equation (20) and adding −c 2 0 ∂ 2 ρ ∂x i ∂x i with respect to the equation both sides, we finally obtain an approximation wave equation with the velocity divergence as the sound source:…”

“…For the aforementioned applications, the trailing edge noise is the most relevant noise source, especially at low Mach numbers since the turbulent fluctuations are scattered efficiently over a solid trailing edge [7]. For alleviating this dominant trailing edge noise, several passive noise-mitigation solutions such as trailing-edge brushes [8,9], sinusoidal and sawtooth serration [6,[10][11][12][13][14][15][16], slits [17,18], and porous treatments [19][20][21] have been proposed. Among these passive methods, sinusoidal and sawtooth trailing edge serrations have been of important interest for researchers [3,6].…”

Experimental investigation of the impact of porous parameters on trailing-edge noise

“…Hedayati et al [27] manufacture their trailing edge with the open-cell Ni-Cr-Al foam. This type of metal foam is characterised by sub-millimetre pore size.…”

Pneumatically-Actuated Acoustic Metamaterials Based on Helmholtz Resonators

“…Particularly, acoustic metamaterials can manipulate sound and elastic waves both spatially and spectrally in unpreceded ways [2]. Such properties include super-focusing [3], super-lensing [4], active membrane structures [5,6], cloaking [7,8], phononic plates [9], fluid cavities separated by piezoelectric boundaries [10], and tunable noise attenuation based on Helmholtz resonators [11][12][13][14][15]. The capability of metamaterials to tune their physical behavior just based on their geometrical characteristics offers a great benefit over conventional materials for application in various high-demand industries such as aerospace, automobile, and construction.…”