Large eddy simulation of confined turbulent flows for aeroacoustics with application to phonation

Jung Soo Suh, Purdue University

Abstract

Large eddy simulations (LES) of low-speed, wall-bounded turbulent flows were conducted by numerically integrating the compressible Navier-Stokes equations in a generalized curvilinear coordinate system. An efficient third-order additive semi-implicit Runge-Kutta method for time advancement and a sixth-order accurate, compact finite-difference scheme for spatial discretization were used. The convective terms in the wall-normal direction were treated implicitly to remove the time-step limitation associated with the use of fine meshes in the near-wall region for high Reynolds number viscous flows. The dynamic Smagorinsky subgrid-scale eddy viscosity model was used to close the filtered equations. Generalized characteristic-based nonreflecting boundary conditions were used together with the exit zone approach. Characteristic-based interface conditions that accurately respect acoustic wave propagation were used for communication between the adjacent computational blocks. The accuracy and efficiency of the numerical scheme was assessed by simple acoustic model problems and by comparing LES predictions for fully-developed turbulent channel flow and turbulent separated flow in an asymmetric diffuser to previous direct numerical simulation (DNS) and experimental data, respectively. LES predictions for both flows are in reasonable agreement with the DNS and experimental mean velocity and turbulence statistics. The findings suggest that the numerical approach employed here offers comparable accuracy to similar recent studies at approximately one-third of the computational cost and may provide both an accurate and efficient way to conduct computational aeroacoustics (CAA) studies for low Mach number, confined turbulent flows. Finally, CAA studies of flow through the human vocal tract were conducted. Rigid models of both converging and diverging glottal passages, each featuring a 20 degree included angle and a minimal glottal width of 0.04 cm with transglottal pressure pf 15 cm H2O, were studied. Asymmetry of the flow due to the Coanda effect and transition to turbulence were observed. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was applied to decomposer the near-field acoustic sources into its monopole, dipole, and quadrupole contributions on far-field sound. The results showed that dipole sources due to the unsteady forces exerted on the duct wall are dominant.

Degree

Ph.D.

Advisors

Frankel, Purdue University.

Subject Area

Mechanical engineering

COinS