Unsteady flow behaviors and noise source identification of a ducted orifice using detached-eddy simulation

Abstract

In the present study, turbulent flow through a ducted orifice is numerically modeled using dynamic delayed detached-eddy simulations (dynamic delayed detached-eddy simulation) to clarify their unsteady flow behaviors and noise generation mechanisms. To this end, a total of four orifices with different thickness-to-diameter ratios (t/d = 0.5, 2, and 4) and porosities (orifice hole area to pipe area ratio, β= 20% and 31%) were chosen for comparison at a Reynolds number of 10 000. Characteristics of the unsteady turbulent flow are first examined in terms of time-mean and statistical flow quantities as well as wall pressure fluctuations. Subsequently, the coherent flow structures in the form of wavepackets are effectively evaluated through spectral proper orthogonal decomposition (SPOD) analysis. The main noise sources are identified as alternatively energetic acoustic dipoles on the orifice's leading and trailing faces, dominated by the intermittent interaction between the unsteady flow and the orifice plate, particularly at the inner edges. Comparisons of different orifice thicknesses at the same porosity (β = 31%) showed that the noise source in the thin configuration (t/d = 0.5) is alternatively dominated by the shedding and flapping behaviors of the vortical structures in the low-frequency range, while Kelvin–Helmholtz-type wavepackets result from the Kelvin–Helmholtz shear layer instability at higher frequencies. For thicker configurations (t/d = 2 and 4), reattachment of the separated shear layer occurs within the orifice throat region; the resultant SPOD modes reveal double-wavepacket structures issuing from the leading and trailing edges, with distinctly different behaviors captured at higher frequencies. Furthermore, for the thin orifice (t/d = 0.5), similar flow structures are found with different porosities (β= 20% and 31%), with intensified noise levels observed at the lower porosity.