An Experimental Verification of the Isotropy of Turbulence Produced by a Grid

T E WORK of Dryden, Simmons and others in adapting the hot wire anemometer to the measurement of up-and-down stream turbulent velocity fluctuat ions is now well known. More recently Simmons, in an as yet unpublished report, has applied the inclined wire direction meter to the measurement of the turbulent velocities normal to the mean velocity. The method is a versatile one; and, in the present instance, is used to evaluate correlation coefficients Rv = ViV2/ vvl v connecting simultaneous values of the transverse velocities a t two fixed points in the turbulent region behind a grid. In the experimental arrangement shown in Fig. 1, a, by c and d represent heated plat inum wires each about 0.15 cm. long and 0.00025 cm. diameter. These are supported in pairs on light wire prongs (Fig. l a ) , and are placed in the turbulent stream of air with their apex pointing upstream. With the currents and i2 adjusted by means of R and Rz so t ha t the wires are heated to about 2 0 0 ° C , the resistance R2 and R^ and, if necessary, the orientations of the wire holders are adjusted until the Wheatstone bridges stay in mean balance (as registered by the galvanometers G) independently of small changes in the wind velocity U. The turbulent weaving of the air in the plane XO Y, say, of the wires a and b cools them differentially, and so alters their resistances differently. The resulting unbalance e of the Wheatstone bridge records, therefore, the stream's directional deviations v/U from the mean direction OX. The resistance-capacity coupled amplifier A i supplies to one coil of an electrodynamometer D a current proportional to instantaneous values of e. * On connection of the other coil of the electrodynamometer, the instrument deflects an amount