Temperature fluctuations and scales in grid-generated turbulence

To study the mixing of a passive scalar in nearly isotropic turbulence, experiments have been made in isotropically mixed thermal fields with thermal mesh size M θ ( a ) equal to the momentum mesh size M , ( b ) larger than M (obtained by heating only alternate rods of the turbulence generating grid), and ( c ) smaller than M . This last condition was achieved by inserting a fine heating screen with M θ M was an increase in the relative intensity of temperature fluctuations compared with the M θ = M case, and a marginal increase in their decay rate; contrary to expectation, the ratio R of temperature to velocity integral scales in the region of approximate homogeneity did not differ from that corresponding to M θ = M . In heated screen experiments, the relative decay rate was independent of M θ / M and Δ T . For the three locations of the heating screen used in these experiments, the decay rate was also independent of the relative distance x s of the heating screen from the turbulence generating grid; however, larger x s was associated with larger relative intensity of fluctuations. To a first approximation, the ratio R approached unity according to the empirical relation R = 1 − A exp − α x θ /( UT 0 ), where x θ is downstream distance measured from the heating screen, and T 0 is a characteristic turbulence decay time scale at x 0 = 0. It was also verified that the skewness of the streamwise temperature derivative is approximately zero sufficiently downstream of the heating screen. Where the present study overlaps with previous measurements, an extensive comparison reveals several points of agreement as well as departure.