Draining gravity current

A draining gravity current loses fluid through its substrate while spreading horizontally. For a two-dimensional viscous current of thickness $H(x,t)$ that loses fluid according to pressure-dependent Darcy drainage,when the hydrostatic pressure is $p=\rho gH$ and $k/h\propto p^{-\beta }$.

Combining the lubrication gravity-current flux with vertical imbibition under a draining liquid layer givesThe injected volume is the integral of $h+\varphi l$, not just the surface current thickness. The model neglects horizontal subsurface flow and applies only where the substrate is wetted and the surface current supplies its drainage.

For a one-sided draining gravity current with injected volume per unit width $V_0(t/\tau {)}^3$, defineThe similarity solution satisfieswhere $K=k[g^3{\tau }^3/({\nu }^3{V}_0){]}^{2/5}$. The inlet condition is $-H(0{)}^3{H}^{\prime }(0)=9$, the integral of $H+\varphi L$ is one, and both depths vanish at the advancing front. The endpoint ${\xi }_N$ gives $x_N={\xi }_{N}D{t}^2$ and depends on $K$ and $\varphi$.

If a draining gravity current is supplied with volume flux proportional to $t^{\alpha }$, a first-kind similarity solution hasprovidedThese relations follow by balancing the time derivative, horizontal lubrication theory flux, vertical drainage, and imposed inlet flux.