Time Accurate CFD Analysis of a Powered Resonance Tube

This CFD analysis was done as part of a project which involved both theoretical and experimental work in addition to the simulations. The objective was to develop a compact means of generating a powerful acoustic signal whose frequency could be varied at will over a relatively broad bandwidth. These devices could then be employed for active flow control applications. The method we employed was a modification of a powered resonance tube.

In a classical powered resonance tube, air is blown across a slot toward the opening of a tube-like resonator. The sound that results radiates from the slot into the flow outside. The frequency is controlled using a movable piston which varies the length of the resonator.

The innovation that was developed for this program was to add a Helmholtz resonator to the basic design. This allows a larger range of signals to be generated from a device of the same size. A schematic of both the straight tube resonator and the Helmholtz resonator are shown in the figure below. Note that the dimensions in the drawing are not actually those used in the simulations.

Schematic of a standard and a Helmholtz powered resonance tube

Below is a snapshot of pressure contours from one of the PRT simulations. This particular configuration, when scaled to lab conditions, produces a peak sound pressure level of roughly 140 db. Click on the graphic to download a short movie of the pressure field in this simulation (1.7 MB).

Pressure waves emanating from a Helmholtz powered resonance tube

Through the use of CFD analysis, we were able to visualize details of the flow in these devices which would not have been possible any other way. In addition, by varying different parameters of the simulation, we were able to explore the importance of such things as viscous effects and turbulence in the operation of PRTs.

The insights gained from the CFD analysis, combined with those from the complementary theoretical and experimental work resulted in a much deeper understanding of the physics involved than from any of the approaches by itself. I often hear CFD, experiment, and analytical methods discussed as if they were opposed to each other. This case shows how a positive synergy can be obtained between the three very different approaches to fluid mechanics analysis with each benefiting from the other.

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