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Abstract:The interaction of steady hot streaks from an annular ring of combustion chambers with turbine rotating blades can potentially generate tonal noise. The relevance of this source mechanism in aeroengine noise is controversially discussed in the literature. In the present paper, the streak-turbine interaction is investigated using the computational fluid dynamics (CFD) method called harmonic balance (HB)-a truncated non-linear frequency domain approach with a high potential of reducing the computational effort. The investigated high-pressure turbine is composed of a single stator-rotor stage. The first part of the present paper compares the results obtained with the HB method to those obtained with the more established time-accurate unsteady Reynolds-averaged Navier-Stokes (URANS) approach and investigates their sensitivity with respect to the computational mesh density. Thereby, no streaks are simulated, and only the turbine alone tones are considered. Convincing results are obtained on aerodynamics and acoustics. The second part of the paper deals with a parametric study on the acoustical impact of steady hot streaks. The streaks are prescribed at the inlet of the stage using a boundary condition as an attempt to simulate a ring of combustor nozzles. No vorticity is coupled into the computational domain, as the objective is to measure the effect of temperature inhomogeneity only. Overall, the turbine appears to be slightly quieter with hot streaks. The streak-to-stator-vane ratio of 1-to-2 explains the generation of new acoustic modes with distinct azimuthal orders. The acoustic power amplitude of those additional modes scales roughly with the square of the temperature difference, the fourth-power of the diameter, and the square of the entropy difference. This last result agrees well with the 1D theory of Marble and Candel. The acoustic contribution of the unsteady force on the rotor blades due to the overspeed measured in the wakes of the streaks-resulting from the flow acceleration through the stator-remains an open question.

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The application of ultrasonic cavitation in chemical processing is a widespread method to accelerate dissolution processes. In recycling technology, often bulk material is introduced into a reactor forming a packed bed of granular particles. Thereby, the cavitation activity at the material surfaces is of interest. In this numerical study, the packed bed is simplified by sphere packings of different arrangement and porosity. The ultrasound propagation model considers the effect of cavitation bubbles by a linearized damping ansatz. A strong sound scattering by the spheres is found for packings with low porosities, while the sphere arrangement is less important.

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