Correlation Measurement in a Turbulent Flow using High-Speed Computing Methods

Third-order time-correlations downstream of a grid were measured with a hot-wire anemometer using high-speed computing methods. The nonlinear response of the hot-wire to the fluctuations of velocities is taken into account as well as the effect of transverse velocities. It is found that the correlations Rt2,1(h)=u′2(t)u′(t+h)¯/(u′2)¯3/2 and Rt1,2(h)=u′(t)u′2(t+h)¯/(u′2)¯3/2 are substantially different from previous results and demonstrate that the assumption of isotropy is not adequate for these correlations downstream of a grid. The nonlinear response does not significantly affect the difference ½(Rt2,1 − Rt1,2). Since previous conclusions concerning the nature of third-order correlations were based on the measurements of such differences they masked the effects of nonlinearity on the individual correlations. Correlations of fifth-order are also presented and their relations to the third-order correlation are discussed. Although the nonlinear corrections are quite important for odd-order correlations they are negligible for correlations of even-order.