Validation of aeroacoustic sound predictions in confined flows

  • Schoder, Stefan (TU Graz)
  • Maurerlehner, Paul (TU Graz)
  • Tieber, Johannes (TU Graz)
  • Steiner, Helfried (TU Graz)
  • Brenn, Günter (TU Graz)
  • Kaltenbacher, Manfred (TU Graz)

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In electric vehicles, noise from auxiliary aggregates can become unpleasant due to the absence of combustion engine noise. The limited space in such vehicles (packaging) often results in unfavorable duct flow geometries. Such flow conditions can cause unwanted noise, for example, in pipes of fuel cells or HVAC systems. Different aeroacoustic formulations (the Perturbed Convective Wave Equation - PCWE, Lighthill's Wave Equation - LWE, and Ribner's Dilatation Equation - RDE) are applied to simulate the flow-induced sound generation and propagation. Low Mach number turbulent pipe flows are considered, where the hybrid approach can be applied. In the first step, the unsteady turbulent flow field is solved using incompressible flow simulation (AVL FIRE), and the source terms are computed. The sources are then conservatively interpolated to the calculation grid for the acoustic propagation calculation, carried out in the last step with the Finite Element (FE) solver openCFS. The turbulent flow field and the resulting source terms are analyzed, and the mechanisms of sound generation are deduced. PCWE and RDE, based on the separation of the flow quantities into constant and fluctuating aerodynamic incompressible and acoustic parts, allow for an analysis of the flow-induced sound generation and propagation. In contrast, LWE, solved for the total pressure fluctuation, does not provide the acoustic pressure field directly. Measurements were carried out on a dedicated aeroacoustic test bench for pipe flows. The dynamic wall pressure was measured using Kulite pressure transducers or pressure field microphones depending on the flow speed. The spectra of the measured wall pressures are used to validate the simulation results. The results of the acoustic propagation simulations revealed that the PCWE and RDE are best suited for the present task. Besides reliably predicting the flow-induced sound, the numerical procedure of source term computation is straightforward for PCWE and RDE, where the relevant source term contributions solely consist of time derivatives of the pressure from the incompressible flow simulation. In contrast, the Lighthill source term involves spatial derivatives and, thus, is strongly dependent on the spatial resolution and the numerical method used for approximating these terms.