LECTURE 14 
FLOW NOISE, THEORY AND EXPERIMENT 
E. J. Skudrzyk and G. P. Haddle 
Ordnance Research Laboratory 
The Pennsylvania State University 
University Park, Pennsylvania 
U.S.A. 
14.1. INTRODUCTION 
Flow noise is defined as the noise that is produced by the hydrodynamic flow 
around a rigid body. Thus, flow noise is produced by the turbulence in the bound- 
ary layer, by the eddies generated by the surface roughnesses, by the eddies 
shed at the tail and the fin of the moving vehicle, and by flow-excited shell and 
cavity vibrations. Flow noise is primarily due tothe centrifugal forces generated 
by the rotating eddies, to the Coriolis forces, and to various kinds of micro- 
scopic stagnation pressures caused by the eddies as they move towards or away 
from one another. All these phenomena generate great fluctuations in pressure. 
The pressure fluctuations are carried along with the flow. When they travel over 
the sensitive area of a hydrophone, they are recorded as flow noise. The flow 
noise that is recorded inside the region where it is generated is called the near- 
field flow noise. Thus, the nearfieldflow noiseis predominantly due to the steady 
and continuous transport of pressure ripples asifthey were frozen. These pres- 
sure ripples are carried along with the flow but they do not generate a pressure 
that propagates to greater distances. The nearfield flow noise is therefore not 
connected with the production of a true sound pressure nor with the production 
of sound energy. 
However, turbulence is a very unsteady phenomenon. The pressure fluctua - 
tions are not frozen in the flow; they decay because of viscosity effects and are 
regenerated by the energy fed into the fluidthrough the surface drag at the walls 
of the vehicle. They move towards and away from one another. The intermittent 
nature and the unsteadiness of the turbulence and of the nearfield pressure lead 
to the generation of a true sound that propagates all over the disturbed region 
and is radiated to greater distances. However, turbulence is a very poor sound 
radiator, and the radiation-field sound pressure is usually much smaller than 
the nearfield pressure. 
At low speed the flow noise is much weaker than the machinery noise. But 
flow noise increases greatly with the speed and eventually masks all other com- 
ponents of noise. 
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