STATISTICS OF TEMPERATURE INCREMENTS IN FULLY-DEVELOPED TURBULENCE .2. EXPERIMENTS

A partial differential equation for the probability density function (pdf) of temperature increments in fully developed turbulence has been derived in a previous theoretical analysis [S. Vaienti, M. Ould-Rouis, F. Auselmet and P. Le Gal, Physica D 73 (1994) 99]. This equation is a function of two conditional expectations q(1) and q(2) of respectively the increment of velocity and the temperature dissipation. It was shown that these two quantities are the only parameters that govern the evolution of the pdfs for the temperature increments. With the help of closure hypotheses on q(1) and q(2), the calculation of the asymptotic forms of this pdf is then tractable and displays a notable deviation from the Gaussian behavior for large temperature increment fluctuations when the spatial separation r lies within the inertial and dissipative ranges. The objective of the present paper is to complete our theoretical approach and to investigate experimentally, from simultaneous measurements of velocity and temperature fluctuations obtained in a laboratory wind tunnel, the evolution of the marginal and joint pdfs of the velocity and temperature increments. Attention is particularly paid to the conditional expectations q(1) and q(2).