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The granular Blasius problem
Published online by Cambridge University Press: 14 June 2019
Abstract
We consider the steady flow of a granular current over a uniformly sloped surface that is smooth upstream (allowing slip for $x<0$) but rough downstream (imposing a no-slip condition on
$x>0$), with a sharp transition at
$x=0$. This problem is similar to the classical Blasius problem, which considers the growth of a boundary layer over a flat plate in a Newtonian fluid that is subject to a similar step change in boundary conditions. Our discrete particle model simulations show that a comparable boundary-layer phenomenon occurs for the granular problem: the effects of basal roughness are initially localised at the base but gradually spread throughout the depth of the current. A rheological model can be used to investigate the changing internal velocity profile. The boundary layer is a region of high shear rate and therefore high inertial number
$I$; its dynamics is governed by the asymptotic behaviour of the granular rheology for high values of the inertial number. The
$\unicode[STIX]{x1D707}(I)$ rheology (Jop et al., Nature, vol. 441 (7094), 2006, pp. 727–730) asserts that
$\text{d}\unicode[STIX]{x1D707}/\text{d}I=O(1/I^{2})$ as
$I\rightarrow \infty$, but current experimental evidence is insufficient to confirm this. We show that this rheology does not admit a self-similar boundary layer, but that there exist generalisations of the
$\unicode[STIX]{x1D707}(I)$ rheology, with different dependencies of
$\unicode[STIX]{x1D707}(I)$ on
$I$, for which such self-similar solutions do exist. These solutions show good quantitative agreement with the results of our discrete particle model simulations.
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