On the effect of mean flow profile, wavelength and array length on focal-resolution of a quadrupole source using aeroacoustic time-reversal

This paper analyses the effect of the mean flow field, source wavelength and array length on the focal-resolution characteristics of an idealised lateral quadrupole source obtained using aeroacoustic Time-Reversal (TR). The TR simulations were implemented by numerically solving the 2-D Linearised Euler Equations (LEE) and enforcing the time-reversed acoustic pressure (stored during forward simulation) at the nodes of two Line Arrays (LAs) located outside the flow and on opposite sides. The TR simulation in a stationary medium and in a uniform mean flow indicates that interaction between the converging and diverging waves in the vicinity of quadrupole focal spots perpendicular to the LAs leads to the reinforcement of acoustic power at these regions. This results in formation of side-lobes of a relative magnitude larger than the quadrupole focal spots in the source maps, thereby making it difficult to characterise a quadrupole. During TR simulation of a quadrupole in a fully-developed non-uniform shear flow, refraction of the back-propagated waves through the shear-layer forces the acoustic fluxes to be focused almost completely at the focal spots, thereby significantly improving the quadrupole focal-resolution characteristics.