Navier slip model of drag reduction by Leidenfrost vapour layers

TL;DR

This paper investigates how Leidenfrost vapour layers reduce drag on spheres by using Navier slip models and simulations, revealing a simple parameter to characterize drag reduction across various flow conditions.

Contribution

It introduces a Navier slip model with a slip length to quantify drag reduction caused by vapour layers, validated through numerical simulations against experimental data.

Findings

Drag coefficient deviates from classical behavior at high Reynolds numbers.

Drag crisis onset depends on vapor-to-liquid viscosity ratio.

A single slip length parameter effectively characterizes drag reduction.

Abstract

Recent experiments found that a hot solid sphere that is able to sustain a stable Leidenfrost vapour layer in a liquid exhibits significant drag reduction during free fall. The variation of the drag coefficient with Reynolds number shows substantial deviation from the characteristic drag crisis behavior at high Reynolds numbers. Results obtained with liqiuds of different viscosities show that onset of the drag crisis depends on the viscosity ratio of the vapor to the liquid. The key feature of the vapour layer in facilitating tangential flow and in altering the behaviour of the boundary layer can be captured by the Navier slip model that is defined by a slip length. Here, direct numerical and large eddy simulations of flow past a sphere at moderate to high Reynolds numbers ($10^2 \leq \mbox{Re} \leq 4 \times 10^4$) are employed to quantify comparisons with experimental results in terms of variations of the drag coefficient with Reynolds number and the form of the downstream wake on the sphere. This provides a simple one parameter characterization of the drag reduction phenomenon.