arXiv subjects

Akshath Sharma

Publications and source records attributed to Akshath Sharma.

3 recordsLinked to original sources

arXiv2019

Turbulent flows over dense filament canopies

Turbulent flows over dense canopies of rigid filaments of small size are investigated for different element heights and spacings using DNS. The flow can be decomposed into the element-coherent, dispersive flow, the Kelvin--Helmholtz-like rollers typically reported over dense canopies, and the background, incoherent turbulence. The canopies studied have spacings $s^+ = 3$--$50$, which essentially preclude the background turbulence from penetrating within. The dispersive velocity fluctuations are also mainly determined by the spacing, and are small deep within the canopy, where the footprint of the Kelvin--Helmholtz-like rollers dominates. Their typical streamwise wavelength is determined by the mixing length, which is essentially the sum of its height above and below the canopy tips. For the present dense canopies, the former remains roughly the same in wall-units, and the latter, which scales with the drag length, depends linearly on the spacing. This is the result of the drag being essentially viscous and governed by the planar layout of the canopy. In shallow canopies, the proximity of the canopy floor inhibits the formation of Kelvin--Helmholtz-like rollers, with essentially no signature for height-to-spacing ratios $h/s \approx 1$, and no further inhibition beyond $h/s \approx 6$. Very small spacings also inhibit the rollers, due to their obstruction by the canopy elements. The obstruction decreases with increasing spacing and the signature of the instability intensifies, even if for canopies sparser than those studied here the instability eventually breaks down. Simple models based on linear stability can capture some of the above effects.

physics.flu-dyn↗

arXiv2019

Scaling and dynamics of turbulence over sparse canopies

Turbulent flows within and over sparse canopies are investigated using direct numerical simulations. We focus on the effect of the canopy on the background turbulence, the part of the flow that remains once the element-induced flow is filtered out. In channel flows, the distribution of the total stress is linear with height. Over smooth walls, the total stress is only the `fluid stress' $\tau_f$, the sum of the viscous and the Reynolds shear stresses. In canopies, in turn, there is an additional contribution from the canopy drag, which can dominate within. We find that, for sparse canopies, the ratio of the viscous and the Reynolds shear stresses in $\tau_f$ at each height is similar to that over smooth-walls, even within the canopy. From this, a height-dependent scaling based on $\tau_f$ is proposed. Using this scaling, the background turbulence within the canopy shows similarities with turbulence over smooth walls. This suggests that the background turbulence scales with $\tau_f$, rather than with the conventional scaling based on the total stress. This effect is essentially captured when the canopy is substituted by a drag force that acts on the mean velocity profile alone, aiming to produce the correct $\tau_f$, without the discrete presence of the canopy elements acting directly on the fluctuations. The proposed mean-only forcing is shown to produce better estimates for the turbulent fluctuations compared to a conventional, homogeneous-drag model. The present results thus suggest that a sparse canopy acts on the background turbulence primarily through the change it induces on the mean velocity profile, which in turn sets the scale for turbulence, rather than through a direct interaction of the canopy elements with the fluctuations. The effect of the element-induced flow, however, requires the representation of the individual canopy elements.

physics.flu-dyn↗

arXiv2018

Scaling and modelling of turbulent flow over a sparse canopy

The turbulent flow within and above a sparse canopy is investigated using direct numerical simulations. The balance of Reynolds to viscous stresses within the canopy is observed to be similar to that over a smooth wall. From this, a scaling based on their local sum is proposed. Using the conventional scaling based on the total stress, the velocity fluctuations are typically reported to be reduced within the canopy compared to smooth walls. When the proposed height-dependent scaling is used, however, the fluctuations are closer to those over smooth walls. This suggests that, in a large part, the effect of the canopy can be reduced to the modification of the local scaling, rather than to the direct interaction of the canopy elements with the turbulence. Based on this, a model is proposed that consists of a drag that acts on the mean flow alone, aiming to produce the correct scaling without modifying the fluctuations directly. This model is shown to estimate the fluctuations within the canopy better than the conventional, homogeneous-drag model. Nevertheless, homogenised methods are not able to reproduce the local effects of the canopy elements. In order to capture these, another model is proposed that applies the mean-only drag on a truncated representation of the canopy geometry.

physics.flu-dyn↗