Sound radiated from low Mach number turbulent boundary layer flows（Turbulent boundary layer on a smooth plate and over a small forward facing step）

Abstract

High-resolution numerical simulation has been performed to study aeroacoustic noise radiated from a turbulent boundary layer at freestream Mach number Ma = 0.3, which develops on a smooth flat plate and over a small forward-facing step. Sound waves radiated from the turbulent boundary layer on the flat plate are dominant in a very-low-frequency band and have characteristics of a linear sound source. The sound waves are observed in the far-field from the boundary propagate outside of the flow region where small hydrodynamic pressure fluctuations with a significant long-wavelength appear. The instantaneous hydrodynamic pressure fields that gradually develop or decay while moving downstream with the turbulent boundary layer are associated with the evolution of various vortical structures. The characteristics of the sound wave being a linear sound source and dominant in a very-low-frequency band are similar to those of the real high-speed train. The sound waves generated from the turbulent boundary layer over the forward-facing step with a height (SH) of SH/y+ ≅ 62 are significantly larger in a full frequency band than those radiated from the turbulent boundary layer on the flat plate. Numerical results of the turbulent boundary layer over the forward-facing step with a height of SH/y+ ≅ 30 show that the sound waves are dominant in the low-frequency band, and also step-specific sound waves are generated in the high-frequency band. Further, even if the step height is SH/y+ ≅ 7.5, the sound waves unique to the small step are generated in the high-frequency band.

Keywords

• turbulent boundary layer
• aeroacoustic noise
• low mach number
• forward facing step
• direct numerical simulation