Multiscale fluid–particle thermal interaction in isotropic turbulence

We use direct numerical simulations to investigate the interaction between the temperature field of a fluid and the temperature of small particles suspended inthe flow, employing both one- and two-way thermal coupling, in a statisticallystationary, isotropic turbulent flow. Using statistical analysis, we investigate thisvariegated interaction at the different scales of the flow. We find that the varianceof the carrier flow temperature gradients decreases as the thermal response timeof the suspended particles is increased. The probability density function (PDF) ofthe carrier flow temperature gradients scales with its variance, while the PDF ofthe rate of change of the particle temperature, whose variance is associated withthe thermal dissipation due to the particles, does not scale in such a self-similarway. The modification of the fluid temperature field due to the particles is examinedby computing the particle concentration and particle heat fluxes conditioned on themagnitude of the local fluid temperature gradient. These statistics highlight thatthe particles cluster on the fluid temperature fronts, and the important role playedby the alignments of the particle velocity and the local fluid temperature gradient.The temperature structure functions, which characterize the temperature fluctuationsacross the scales of the flow, clearly show that the fluctuations of the carrier flowtemperature increments are monotonically suppressed in the two-way coupled regimeas the particle thermal response time is increased. Thermal caustics dominate theparticle temperature increments at small scales, that is, particles that come intocontact are likely to have very large differences in their temperatures. This is causedby the non-local thermal dynamics of the particles: the scaling exponents of theinertial particle temperature structure functions in the dissipation range reveal verystrong multifractal behaviour. Further insight is provided by the flux of temperatureincrements across the scales. Altogether, these results reveal a number of non-trivialeffects, with a number of important practical consequences