Reynolds Number Dependence of Mixed Structure Functions of Temperature and Longitudinal Velocity

Reynolds number dependence of mixed structure functions of longitudinal velocity u and temperature Θ is examined over a R _λ (Taylor microscale Reynolds number) range of 180-5950 for four flows. It is found that the mixed structure functions exhibit some behaviours similar to those of individual ones of the velocity and temperature. The prediction of the bivariate lognormal model for the R _λ dependence of the mixed structure functions is approached by the present measurements only at very high R _λ. At values of the longitudinal separation r close to the integral length scale as well as Taylor microscale, the velocity and temperature increments are not statistically independent. Several methods have been used to estimate the intermittency exponents μ and μ_Θ of the velocity and temperature in the lognormal model. Each method yields a different value of μ and μ_Θ, which also depend on R _λ and the type of flows. The present measurements suggest that the best estimates for μ and μ_Θ are 0.25 ± 0.05 and 0.30 ± 0.05, respectively.