Investigation of droplet-mediated sensible and latent heat fluxes in a turbulent air flow over a waved water surface by direct numerical simulation

The objective of the present study is to investigate sensible and latent heat transfer mediated by evaporating saline droplets in a turbulent air flow over a waved water surface by performing direct numerical simulation. Equations of the air-flow velocity, temperature and humidity are solved simultaneously with the two-way-coupled equations of individual droplets coordinates and velocities, temperatures and masses. Two different cases of air and water surface temperatures,T_a = 27 ⁰C, T_s = 28 ⁰C,  and T_a = -10 ⁰C, T_s = 0 ⁰C, are considered and conditionally termed as "tropical cyclone" (TC) and "polar low"  (PL) conditions, respectively. Droplets-mediated sensible and latent heat fluxes, Q_S and Q_L, are integrated along individual droplets Lagrangian trajectories and evaluated as distributions over droplet diameter at injection, d, and also obtained as Eulerian, ensemble-averaged fields. The results show that under TC-conditions, the sensible heat flux from droplets to air is negative whereas the latent heat flux is positive, and thus droplets cool and moisturize the carrier air. On the other hand, under PL-conditions, Q_S and Q_L  are both positive, and Q_L – contribution is significantly reduced as compared to Q_S - contribution. Thus in this case, droplets warm up the air. In both cases, the droplet-mediated enthalpy flux, Q_S + Q_L , is positive, vanishes for sufficiently small droplets (with diameters d ≤ 150 μm) and further increases with d. The results also show that the net fluxes are reduced with increasing wave slope.

This work is supported by the Ministry of Education and Science of the Russian Federation (Task No. 0030-2019-0020). Numerical algorithms were developed under the support of RFBR (Nos. 18-05-60299, 18-55-50005, 18-05-00265, 20-05-00322). Postprocessing was performed under the support of the Russian Science Foundation (No. 19-17-00209).