© 1949 by Oxford University Press
THE EFFECT OF WIRE GAUZE ON SMALL DISTURBANCES IN A UNIFORM STREAM
( Trinity College Cambridge )
The paper sets out to give a comprehensive account of the effect of woven wire gauze on small disturbances to a uniform stream. The relevant aerodynamic characteristics of gauze are the pressure drop and deviation in direction which it imposes on a uniform stream incident at an arbitrary angle to the plane of the gauze. Measurements by Dryden and Schubauer suggest that these two characteristics may be uniquely related for all gauzes and Reynolds numbers, and enable the deviation to be determined when the pressure drop is known.
The effect of placing a gauze at right angles to a uniform stream, on which is superimposed a small steady disturbance to the longitudinal velocity, is determined in terms of the gauze characteristics. The limitation to a small disturbance linearizes the problem and the effect of gauze is to reduce the intensity of the disturbance while leaving its (arbitrary) shape unchanged. Previous attempts to solve this problem have been made; it is shown that they all make special assumptions about the side-force exerted by the gauze and that Collar's analysis is incorrect on another account. The theoretical reduction in the disturbance is consistent with the few published measurements. There is one gauze—with a pressure-drop coefficient of about 2·8·which entirely removes all small steady longitudinal disturbances.
The effect of gauze on a small arbitrary unsteady disturbance—i.e. on turbuleace— is also determined, on the assumption that the gauze wires produce no wake turbulence. The intensities of fluctuations parallel and normal to the plane of the gauze are found to be reduced by different amounts and if time turbulence is isotropic far upstream from the gauze it is axially symmetrical downstream from the gauze. The reduction ill turbulent energy depends on the three-dimensional energy spectrum of the oncoming turbulence and is thus not unique, even when the oncoming turbulence is isotropie Some estimates of the reduction in intensity obtained from an approximation to the spectrum function in isotropie turbulence are presented. They are found to be close to the reduction factors appropriate to a simplified type of turbulence in which the longitudinal velocity does not vary in the direction of the stream. it. is possible to deduce the effect of gauze on other characteristics of the turbulence; one immediate result is that the longitudinal integral scale is reduced to zero by the gauze which removes steady longitudinal disturbances.