Mikael A. Langthjem

Numerical Simulation of the Hole-Tone Feedback Cycle Based on the Discrete Vortex Method and the Acoustic Analogy

An axisymmetric numerical simulation approach to the holetone feedback problem is developed. It is based on the discrete vortex method and an ‘acoustic analogy’ representation of flow noise sources. The shear layer of the jet is represented by ‘free’ discrete vortex rings, and the jet nozzle and the end plate by bound vortex rings. A vortex ring is released from the nozzle at each time step in the simulation. The newly released vortex rings are disturbed by acoustic feedback. The simulated frequencies f follow the criterion L/uc + L/c0 = n/f where L is the gap length, uc is the shear layer convection velocity, c0 is the speed of sound, and n is a mode number (n = 1/2, 1, 3/2, ...). This is in agreement with experimental observations. The numerical model also display mode shifts (jumps in the chosen value of n), as seen in experiments.© 2003 ASME

THE JET HOLE-TONE OSCILLATION CYCLE SUBJECTED TO ACOUSTIC EXCITATION : A NUMERICAL STUDY BASED ON AN AXISYMMETRIC VORTEX METHOD(Sound and Nature)

An axisymmetric numerical simulation approach to the hole-tone feedback problem is presented. It is based on the discrete vortex method and an ‘acoustic analogy’ representation of ∞ow noise sources. The shear layer of the jet is represented by ‘free’ discrete vortex rings, and the jet nozzle and end plate by bound vortex rings. This model is capable of predicting the sound-generating unsteady jet ∞ow with good accuracy. In the acoustic model, a monopole source term alone is capable of predicting the correct frequency spectrum, and the correct sound pressure level of the dominating frequency component (the hole tone) in the acoustic near fleld. The last part of the paper presents several case studies on suppression of the hole tone by forced acoustic excitation of the shear layer near the nozzle exit.