The probability density function, and related statistics, of scalar (t
emperature) derivative fluctuations in decaying grid turbulence with a
n imposed cross-stream, passive linear temperature profile, is studied
for a turbulence Reynolds number range, Re(l), varying from 50 to 120
0, (corresponding to a Taylor Reynolds number range 30 < R(lambda) < 1
30). It is shown that the temperature derivative skewness in the direc
tion of the mean gradient, S(thetay) has a value of 1.8 +/- 0.2 (twice
the value observed in shear flows), and has no significant variation
with Reynolds number. The ratio of the temperature derivative standard
deviation along the gradient to that normal to it is approximately 1.
2 +/- 0.1 also, with no variation with Re. The kurtosis of the derivat
ives increases approximately as Re(l)0.2. The results show that the ra
re, intense temperature deviations that produce the skewed scalar deri
vative, increase in frequency, but their area fraction (of the total f
ield) becomes smaller as the Reynolds number increases. Thus, since S(
thetay) remains constant, they become sharper and more intense, occurr
ing deeper in the tails of the probability density function. Measureme
nts in a thermal mixing layer, which has a nonlinear mean temperature
profile, are also presented, and these show a similar value of S(theta
y) to the linear profile case. The experiments broadly confirm the two
-dimensional numerical simulations of Holzer and Siggia [Phys. Fluids
(in press)], as well as other recent simulations, although there are s
ome differences.