Aeroacoustics

Rapid prediction of train nose entry pressure gradients

The relative merits of two rapid methods of estimating train nose entry pressure gradients are being assessed. The more accurate of the two has been developed at BU (BUHOOD40 version 6),  is analytical and allows for three-dimensional geometry. It has previously been demonstrated to give close correlation with experimental measurements. The other method is numerical and is less accurate because it is based on one-dimensional approximations, but its widespread use for predicting wave amplitudes justifies a careful appraisal of its potential for predicting pressure gradients. Both methods have the important advantage of requiring hugely less calculation time than 3D-CFD methods. It appears that the 1D method, suitably modified by a predetermined empirical adjustment, is sufficiently accurate for initial design purposes (e.g. feasibility studies), but that the 3D approach should be used for detailed design. Extensive experimental data are being used to assess the validity of the two methods.

BUHOOD40 is currently licensed to Japan Railways where it is used in the design of tunnel portals and train nose profiles. Negotiations are currently in hand for licensing in Europe.

Personnel: Professor M. S. Howe and Professor Alan Vardy (University of Dundee, UK).

Model scale tests of tunnel entrance hoods (from Howe, M. S. 2008, Journal of Sound and Vibration 314, 113 – 140).

Model scale tests of tunnel entrance hoods (from Howe, M. S. 2008, Journal of Sound and Vibration 314, 113 – 140).

High-speed trains parked in Gare de Lyon, Paris.

High-speed trains parked in Gare de Lyon, Paris.

Turbofan noise

One source of noise created by a turbofan engine comes from the interaction of the fan rotor wakes with the downstream exit guide vanes (EGV).  This fluid-structure interaction,creates both tonal and broadband noise.  The long term goal of the current research is to assess the viability of using CFD (unsteady RANS) as part of a prediction method for fan broadband noise. 

Personnel: Professor Sheryl Grace, and Douglas Sondak

Sound pressure level at the second harmonic of the blade passage frequency in the duct downstream of a turbofan rotor and exit guide vanes.

Sound pressure level at the second harmonic of the blade passage frequency in the duct downstream of a turbofan rotor and exit guide vanes.

Entropy in flow through the rotors (22 reduced to 2) and exit guide vanes (55 reduced to 5) of a turbofan at midspan.

Entropy in flow through the rotors (22 reduced to 2) and exit guide vanes (55 reduced to 5) of a turbofan at midspan.

Nonlinear flow-structure coupling of the vocal folds and the vocal tract 

Analyses are being made of the nonlinear interactions between flow in the vocal tract and glottis and sound waves produced by vibrations of the vocal folds. The mean flow through the system is produced by a nominally steady contraction of the lungs, and mechanical experiments frequently involve a ‘lung cavity’ coupled to the glottis by an experimental subglottal tube of arbitrary or ill-defined length. A simple, self-exciting single mass mathematical model of the vocal folds has been developed and is used to investigate the sound generated and the unsteady volume flux from the glottis. In experiments where the assumed absorption of sound within the sponge-like interior of the lungs is small, calculations have been performed to that show that the influence of tube length is very significant, resulting, for example, in the strong appearance of second harmonics of the acoustic field. 

Personnel: Professor M. S. Howe and Dr. Richard S. McGowan (CReSS LLC, Lexington MA).

Typical predicted acoustic pressure signature in the upper vocal tract produced by a nominally steady flow from the lungs (from Howe, M. S. and McGowan, R. S. 2009 Fluid Dynamics Research, in press; work performed under subcontract from the UCLA School of Medicine).

Typical predicted acoustic pressure signature in the upper vocal tract produced by a nominally steady flow from the lungs (from Howe, M. S. and McGowan, R. S. 2009 Fluid Dynamics Research, in press; work performed under subcontract from the UCLA School of Medicine).

Combustion chamber noise 

A specially constructed two-dimensional Rijke burner is being constructed at Twente University, and a modified Rijke tube developed at Keele University is in use at Manchester University. Both provide simplified flow and structural geometries that permit detailed numerical and analytical studies of various phenomena related to combustion noise to be made and validated. We are currently examining thermal sources of sound associated with unsteady contact between turbulent mean flow in the burner and hot boundaries, and the proper design of splitter plates (‘guide vanes’) to avoid the excitation of acoustic resonances in the burner. The following canonical problem has recently been examined at BU:

Sound generated by vortex ring impingement on a heated wall:

A vortex ring impinges normally on a heated section of a plane wall. Unsteady heat transfer starts when the vortex is within a diameter of the wall and causes a rapid increase in the emission of sound of monopole type. Experimental data (from  Arévalo et al., Physics of Fluids, 2007)  has been used to deduce that the enhancement of heat transfer by the vortex varies approximately linearly with the vortex Reynolds number, and to derive a scaling law for the acoustic pressure. The radiated pressure signature is dominated by an initial pulse of large amplitude associated with an explosive, but brief increase in the rate of heat transfer at the start of the vortex-wall interaction. 

Personnel: Professor M. S. Howe and Ms. E. Giannos Kotsari, BU; Dr. Maria A. Heckl, Keele University.

Vortex ring experiment and dependence of the maximum rate of vortex-induced heat transfer on Reynolds number (from Giannos Kotsari, E. and Howe, M. S. 2009, submitted to Journal of Sound and Vibration).

Vortex ring experiment and dependence of the maximum rate of vortex-induced heat transfer on Reynolds number (from Giannos Kotsari, E. and Howe, M. S. 2009, submitted to Journal of Sound and Vibration).

Motion and sound generated by a flag

The onset of flapping by a flag in an initial planar configuration parallel to a nominally quiescent mean stream is usually attributed to the excitation of a temporally unstable solution of the unforced, fluid coupled equation of motion. But this view takes no account of the influence of the flag pole and its wake. The Reynolds number is in practice invariably large enough to produce a disturbed vortex wake that forces the flag into motion, even at wind speeds that are theoretically subcritical according to unforced flag theory. It is only in a carefully controlled experiment involving a flag pinned to a specially streamlined support that effectively inhibits vortex shedding, that predictions of homogeneous instability theory are relevant. An analytical model has been investigated to determine the influence on the flag of forcing by the shed vorticity. Detailed predictions of the onset of flag motion are derived by linearizing the flag equation of motion and by modelling shed vorticity on both sides of the flag by simple vortex streets.

The results of this calculation are being extended to predict the sound generated by the flapping flag.

Personnel: Professor M. S. Howe and Dr. Avshelom Manela, MIT.

 

Illustrating typical predicted flag profiles driven by vortex shedding from the flag pole over a complete cycle (from Manela, A. and Howe, M. S. 2009 Journal of Fluid Mechanics, in press).

Illustrating typical predicted flag profiles driven by vortex shedding from the flag pole over a complete cycle (from Manela, A. and Howe, M. S. 2009 Journal of Fluid Mechanics, in press).

 

Self noise of a supercavitating vehicle 

The cavity of a supercavitating vehicle in water is maintained by the steady injection of gas from sources within the vehicle and cavity. An experimental and analytical investigation is being made to establish the spectrum of the acoustic ‘self noise’ produced at the nose of the vehicle. Measurements are made using a specially modified supercavity at the Penn State Applied Research Laboratory. Of particular interest is the sound generated by gas impingement on the cavity walls. In the experiment the gas enters the cavity in a radially symmetric pattern through a series of narrow jets. Experiments have been performed to measure the unsteady force applied to the gas-water interface by these jets, each one of which can then be modeled as a dipole source of sound radiating into the water.  A transfer function has been derived analytically that permits the self noise pressure at the nose of the vehicle to be expressed in terms of the pressure distribution on the cavity wall attributable to the gas jets. 

Personnel: Professor M. S. Howe and Ms Alia Foley, BU; Professor T. A. Brungart and Mr. S. D. Young, Penn State University (Research supported by the Office of Naval Research Code 333).

 

The Penn State experimental supercavitating vehicle and a schematic of the analytical model.

The Penn State experimental supercavitating vehicle and a schematic of the analytical model.