Sessile super-droplets as quasi-static wall deformations in direct numerical simulation of turbulent channel flow

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Condensation is an important aspect of many flow applications due to the ubiquitous presence of humidity in the air at ambient conditions. For direct numerical simulations of such flows, simulating the gas phase as a mixture characterized by temperature and humidity coupled by the latent heat release and absorption has been shown to yield results consistent with multiphase direct numerical simulations at reduced cost. In the case of surface condensation, the deposition of condensate droplets represents an additional mechanism for flow modification. Extending the single-phase approach by tracking the mean deposition rates and consolidating the condensate mass into static super-droplets reintroduces the effects of surface droplets on the flow while retaining the computational advantages of simulating only the gas phase. Results of simulations of turbulent flow through a cooled, vertical channel with and without such droplets illustrate the additional effects captured compared to the original approach. In the immediate vicinity of a super-droplet, turbulent heat and vapor transport towards the cooled wall is enhanced. Direct impingement and deflection of the flow on the super-droplet cause a qualitative change in the distribution of the condensation rates, increasing on the surface of the super-droplets and decreasing in the surrounding regions. This modification of the near-wall transport leads to increased global cooling and drying efficiency compared to a smooth channel.