Direct Numerical Simulation of Water Droplets in Turbulent Flow

Round 1

Reviewer 1 Report

The manuscript titled “Direct numerical simulation of water droplets in turbulent flow” by Ren et al. investigates the behaviour of falling water drops in turbulent air using state-of-the-art numerical methods. The paper is well structured and written - I enjoyed reading it. The paper draws important conclusions on why the terminal velocity of droplets decreases with increasing turbulent intensity, which I think will be interesting for the community. However, before I can recommend the paper for publication, I would like the authors to address my concerns and questions listed below.

- A first-order scheme is applied to discretise the transient term of the momentum equations. I find this very surprising, since a first-order scheme is very diffusive and artificially dissipates turbulent kinetic energy. This may be a potentially shortcoming of this study. Can you please justify this choice in detail and, if possible, provide evidence that this choice does not interfere with your conclusions.

- The authors mention on page 5 that the time-step is limited by the CFL number. I presume the CFL number is based on the flow velocity. Has the capillary time-step constraint as a result of surface tension, which would be approximately dt = 3e-5 s for the smaller drop, been considered as well and is it relevant for the conducted simulations?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Numerical simulation of water droplets in turbulence involving drop surface deformation and internal circulation is a very interesting topic and is the significance of understanding raindrop-related events.

The author has done an impressive and pioneer work by conducting direct numerical simulation in which the VOF method is applied to model the two-phase flow. Based on the simulation results, the terminal velocity, the hydrodynamics in raindrop, the external and internal flow field, and the droplet dynamics in turbulence are analyzed systematically. Thus I consider this work has referential value to understand the complicated flow dynamic around raindrop. The paper is well organized and written. Thus I recommend this work to be accepted in the present form.

Author Response

We would like to thank the reviewer for his /her very good comments.