Investigation of Noise Sources in a Two-Dimensional Model Airframe Noise Problem with Wake Interaction

Abstract

Aeroacoustic predictions of a model airframe noise problem are conducted in order to assess the effects of wake and boundary layer interaction on both the flow and acoustic fields. Simulations of unsteady laminar flows, including both noise generation and its subsequent propagation to the far field, are performed for a two-dimensional configuration composed of a cylinder placed above a NACA 0012 airfoil at 5 deg. angle of incidence. The Reynolds number based on the airfoil chord is set at Rec = 5000 and the Reynolds number based on the diameter of the cylinder is Red = 200. An assessment of cylinder position and freestream Mach number effects on sound radiation is presented for M∞ = 0.1, 0.3 and 0.5. The investigation of the noise sources for airfoil and cylinder vortex shedding frequencies is also presented using a hybrid methodology which employs direct calculation for near field source computations and the Ffowcs Williams-Hawkings (FW-H) equation as the acoustic analogy formulation. In order to verify the numerical solutions, acoustic prediction results obtained by direct noise calculation (DNC) are compared to those computed by the FW-H equation and good agreement is observed. The separate contributions of the terms composing the quadrupole sources are analyzed. Even for the low Reynolds number flows investigated, Reynolds stresses are the dominant quadrupole noise sources and viscous effects as well as entropy fluctuations can be neglected. It is also found that while quadrupole sources can be neglected in noise predictions by the FW-H equation for the M∞ = 0.1 flow calculations, they should be included for the M∞ = 0.3 and 0.5 acoustic predictions at low and high frequencies. For moderate Mach number flows, at the cylinder vortex shedding frequency, dipole sources are related to both acoustic scattering and diffraction phenomena that occur along the airfoil surface and trailing edge, respectively. However, quadrupole sources are more specifically related to diffraction effects along the airfoil trailing edge. When the cylinder vortex shedding is the driving noise source mechanism, intense dipolar interference occurs for all configurations analyzed.