Numerical Simulation on the airfoil self-noise at low mach number flows

Airfoil self-noise is a common phenomenon for many engineering applications. Aim to study the underlying mechanism of airfoil self-noise at low Mach number and moderate Reynolds number flow, a numerical investigation is presented on noise generation by flow past symmetric NACA airfoils. Based on a high-order accurate numerical method, both the near-field hydrodynamics and the far-field acoustics are computed simultaneously by performing Direct Numerical Simulation (DNS). To improve computational efficiency, all the simulations are performed by using a multi-size mesh and multi-time step strategy. The present DNS solver is first validated by the simulation of flow over NACA0012 airfoil at zero incidence. The Reynolds number under consideration varies from 2×10⁵ to 5×10⁵. The mean pressure coefficient distribution on airfoil surface is found to show a good agreement with experimental result. By fitting the data, the formula describing the relation between the Strouhal number of computed tones and the Reynolds number is in the form of the Paterson formula. Based on the validated DNS solver, the NACA0018 airfoil is simulated at Reynolds number of1.6×10⁵. The mean flow properties agree well with the experimental measurements. The characteristics of aerodynamic noise are investigated at various angles of attack. The obtained results show that inclining the airfoil could enlarge turbulent intensity and produce larger scale of vortices. Noise sources are identified in the region of flow separation and reattachment and also in the wake flow. The sound radiation is mainly towards the upper and lower directions of the airfoil surface. At higher angle of attack, the tonal noise tends to disappear and the noise spectrum displays broad-band features.