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National Advisory Committee for Aeronautics, Report - The Near Noise Field of Static Jets and Some Model Studies of Devices for Noise Reduction

Experimental studies of the pressure fluctuations near jet
exhaust streams were made during uncholced Operation of a
turbojet engine and a 1-incMMmda high-temperature model
jet and during choked operation of various sizes of model jets
with unheated air. The tests for uncholced operation indicate a
random spectrum of rather narrow band width which varies in
frequency content with axial position along the jet. Pressure
surveys from the model tests along lines parallel to the 15° jet
boundary indicate that the station of greatest pressure fluctuations
is determined by the jet velocity and the radial distance, with a
tendency of the maximum to shift downstream as either parameter
is increased. From model tests the magnitude of the fluctuations
appears to increase as about the second power of jet velocity at
points just outside the jet boundary and as increasingly higher
powers of jet velocity a distance from the boundary is increased.
A laboratory method of noise reduction with model jets was
found to produce large decreases in the magnitude of the lower—
frequency components of the spectra and thereby also to reduce
the total radiated energy.

C’hoked operation of model jets with unheated air indicates
the appearance of a discrete frequency component of very large
magnitude. Shadowgraph records of the flow show that this
condition is associated with the appearance of flow formations
suggestive of partly formed toroidal vortices in the vicinity of the
shocks. Elimination of these formations is found to eliminate
the discrete component and thereby to reduce the overall noise
level.

INTRODUCTION

It is well known that the turbojet is a. generator of intense
pressure fluctuations. In view of this fact, it is important
that the designer and operator of turbojet—powered aircraft
be able to predict the nature and severity of these fluctuations
both in the vicinity of the engine (the near field) and at large
distances from it (the far field).

The far-field aspect of the problem is of concern to a great
number of people, including airport workers as well as the
general public, and appreciable research, both theoretical and
experimental, has been done on that phase of the problem.
For example, reference 1 presents the results of an experi-
mental evaluation from model jets of the effects of various

geometric and flow parameters and compares model and full-
scale pressure fields, while a detailed survey of the pressure
field of a full-scale configuration is given in reference 2. In
reference 3 it has been shown that the problem is subject to
qualitative analytical treatment for distances that are large
relative to the radiated wavelengths.