FLUID In the literature: open

Turbulent enhancement of droplet collision rates at cloud Reynolds numbers

In plain words

Turbulence can make cloud droplets collide more often by clustering them and giving them different velocities. How large this effect is at the turbulence levels of real clouds, far beyond what computers can simulate, is unknown.

Precise statement

For water droplets of radii 10 to 40 micron in air turbulence with $\mathrm{Re}_{\lambda} \sim 10^4$ and $\epsilon \sim 10 \text{ to } 1000\ \mathrm{cm}^2/\mathrm{s}^3$, including gravity, hydrodynamic interactions and non-continuum lubrication, compute the collision kernel and its enhancement over gravitational collection. DNS with resolved Kolmogorov scales reach $\mathrm{Re}_{\lambda}$ of only a few hundred to about $10^3$, so the $\mathrm{Re}_{\lambda}$ dependence must be established.

What would settle it

DNS or laboratory measurements over at least a decade in $\mathrm{Re}_{\lambda}$ establishing the scaling of the kernel, extrapolated with a validated theory to $\mathrm{Re}_{\lambda} \sim 10^{4}$.

Status in the literature

Unverified note

2025 DNS compute polydisperse kernels in the bottleneck size range at $\mathrm{Re}_{\lambda}$ far below cloud values.