Experimental assessment of mixing layer scaling laws in Rayleigh-Taylor instability

TL;DR

This study experimentally investigates the scaling laws of the mixing region in Rayleigh-Taylor instability within a porous medium, confirming superlinear growth predicted by prior simulations through high-resolution optical measurements.

Contribution

It provides the first experimental validation of superlinear mixing length growth in Rayleigh-Taylor instability in a porous medium, previously observed only in simulations.

Findings

Confirmed superlinear growth of mixing length during convection phase

Validated simulation predictions with experimental data

Characterized flow phases via high-resolution density measurements

Abstract

We assess experimentally the scaling laws that characterize the mixing region produced by the Rayleigh-Taylor instability in a confined porous medium. In particular, we wish to assess experimentally the existence of a superlinear scaling for the growth of the mixing region, which was observed in recent two-dimensional simulations. To this purpose, we use a Hele-Shaw cell. The flow configuration consists of a heavy fluid layer overlying a lighter fluid layer, initially separated by a horizontal, flat interface. When small perturbations of concentration and velocity fields occur at the interface, convective mixing is eventually produced: Perturbations grow and evolve into large finger-like convective structures that control the transition from the initial diffusion-dominated phase of the flow to the subsequent convection-dominated phase. As the flow evolves, diffusion acts to reduce local concentration gradients across the interface of the fingers. When the gradients become sufficiently small, the system attains a stably-stratified state and diffusion is again the dominant mixing mechanisms. We employ an optical method to obtain high-resolution measurements of the density fields and we perform experiments for values of the Rayleigh-Darcy number (i.e., the ratio between convection and diffusion) sufficiently large to exhibit all the flow phases just described, which we characterize via the mixing length, a measure of the extension of the mixing region. We are able to confirm that the growth of the mixing length during the convection-dominated phase follows the superlinear scaling predicted by previous simulations.