Ventilation – Having inlet airway – Including structure for mixing plural air streams together
Patent
1997-09-10
1999-12-21
Joyce, Harold
Ventilation
Having inlet airway
Including structure for mixing plural air streams together
454261, F24F 1304
Patent
active
060042044
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to an induction air handling unit, and in particular a nozzle design for an induction air handling unit.
When a gas is discharged from a duct or when it flows through a restriction in a duct, the momentum of the jet is dissipated through mixing with the downstream surroundings. A by-product of this process is the generation of noise which is radiated to the surroundings. It is well established that the sound power which is generated increases at approximately the eighth power of the jet velocity. The efficiency with which it is radiated into the far field of the surroundings depends strongly on both the rate and the scale of the mixing. For a turbulent jet of given mass flow emerging from an orifice of given cross-sectional area, the total sound power radiated decreases as the rate of mixing of the jet with the surroundings increases.
For many years it has been known that the theory of aerodynamic noise generation from turbulent shear flows advanced by Sir James Lighthill, and published in the Proceedings of The Royal Society of London, (On sound generated aerodynamically, Proc. Roy. Soc. A211, p.564, 1952--see also: Waves in fluids, Cambridge University Press, 1978) is incomplete in that it fails to yield reliable predictions of the noise generated by jets at low Mach number. The incompleteness of the Lighthill theoretical model is understandable when it is realised that it was devised before the discovery by G. L. Brown and A. Roshko (Journal of Fluid Mechanics, 64, 775-816, 1974) that the growth of the mixing layer, from which most of the noise emanates, is not continuous but is dominated by the formation of seemingly deterministic vortex-like structures of a scale which is comparable to or larger than the local thickness of the shear layer, and their non-deterministic, intermittent growth by a succession of "amalgamations" between themselves which appear to be little influenced by the turbulent shear layer which they wrap into their structures like jam into a Swiss roll. The magnitude of the disturbances in the flow is many times that which occurs in a simple turbulent shear flow and hence it is reasonable to surmise that these Brown-Roshko vortices may be generating much of the noise. Recent research by Professor N. W. M. Ko and his student Mr R. C. K. Leung at The University of Hong Kong, which has been submitted for publication in the Journal of Sound and Vibration, has advanced a new concept of the noise generation mechanism based on this surmise. The new concept derives from measurements of the processes by which successive Brown-Roshko vortex structures in the shear layer between the jet and the surroundings "pair" together causing the large scale folding, mixing and the intermittent expansion of the jet cross-section. Ko and Leung have found that if a vortex "ring" formed in the mixing layer at the edge of an axisymmetric jet is to "pair" with its predecessor, it must be accelerated rapidly by the pressure field of the leading vortex until it passes through the "eye" of that leading vortex. It is then rapidly retarded and the two vortices merge to become a single larger vortex. The process can be likened to an "extrusion" of the trailing vortex through the eye of the leading vortex. During this "extrusion" process the rates of change of the acceleration of the trailing vortex are very large. In the subject of Mechanics the rate of change of acceleration is known as the "jerk". It is expressed mathematically as the third derivative of distance with respect to time. (It will be recalled that the first derivative of distance with respect to time is the velocity and the second derivative is the acceleration. In solid mechanics it is well known that "jerk" is frequently accompanied by noise; the collision of two marbles and the tapping of a pencil on a table are typical examples). Associated with this extrusion process is a distortion of the shape of the trailing vortex from a nearly circular doughnut shape into a scalloped shape bearing similaritie
REFERENCES:
patent: 3409274 (1968-11-01), Lawton
patent: 4448111 (1984-05-01), Doherty
patent: 4665804 (1987-05-01), Miyasaka
patent: 5197920 (1993-03-01), Ganse
Luxton Russell Estcourt
Petrovic Vladimir Miodrag
Boleg Derek
Joyce Harold
Luminis PTY Ltd.
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