Temperature fluctuations and heat flux in grid-generated isotropic turbulence with streamwise and transverse mean-temperature gradients

A screen of closely spaced, parallel, thin wires was placed downstream of a grid generating nearly isotropic turbulence. The screen was normal to the flow and was heated in one of two modes: (1) periodically in time, to generate a train of transversely uniform streamwise thermal ramps, each with a uniform streamwise gradient, and (2) steadily, with transverse non-uniformity, to generate a uniform transverse thermal ramp. The simple temperature and temperature-gradient fluctuation statistical properties in both cases were found to be comparable to those encountered in earlier works with a steadily heated grid producing a uniform transverse thermal ramp. In both modes of heating the temperature fluctuations decreased initially behind the screen and then increased monotonically. The turbulent heat-transfer correlation coefficient attained an asymptotic magnitude between 0.7 and 0.8 for both modes of heating. The skewness of the temperature fluctuation derivative in the direction of the mean gradient was found to be non-zero despite the absence of mean shear.