Experimental investigation of turbulence and turbulent thermal diffusion in strongly inhomogeneous and anisotropic forced convection

We investigate properties of turbulence and turbulent transport of non-inertial particles described in terms of turbulent thermal diffusion in strongly inhomogeneous and anisotropic convection forced by two similar turbulence generators with an oscillating membrane and a steady grid in the air flow (with the Rayleigh number about 108). Velocity field and spatial distribution of particles are measured using the particle image velocimetry system. The temperature distribution is measured in many locations using a temperature probe equipped with 12 E-thermocouples. In the forced convection, the gradients of the mean temperature field and the particle number density in the horizontal direction in the core flow are much stronger than in the vertical direction. The mean fluid velocity structure shows a transition between a single-roll pattern for isothermal turbulence to double-roll patterns with an increase in the temperature difference between the bottom and upper walls of the chamber. For larger temperature differences, the mean fluid velocity structure returns to a single-roll pattern. In the turbulent regions with large mean temperature gradients, the dominant effect of the large-scale particle clustering is turbulent thermal diffusion, resulting in the maximum of the mean particle number density being located in the regions with the minimum of the mean temperature and vice versa. Deviations from this feature are observed in the regions with strong mean fluid velocities where the mean temperature gradients are small.