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 respective ly the increment of velocity and the temperature dissipation. It was s hown that these two quantities are the only parameters that govern the evolution of the pdfs for the temperature increments. With the help o f closure hypotheses on q(1) and q(2), the calculation of the asymptot ic forms of this pdf is then tractable and displays a notable deviatio n from the Gaussian behavior for large temperature increment fluctuati ons when the spatial separation r lies within the inertial and dissipa tive ranges. The objective of the present paper is to complete our the oretical approach and to investigate experimentally, from simultaneous measurements of velocity and temperature fluctuations obtained in a l aboratory wind tunnel, the evolution of the marginal and joint pdfs of the velocity and temperature increments. Attention is particularly pa id to the conditional expectations q(1) and q(2).