arXiv subjects

Ricardo García-Mayoral

Publications and source records attributed to Ricardo García-Mayoral.

10 recordsLinked to original sources

arXiv2026

A systematic characterisation of canopy density based on turbulent-structure penetration

Turbulent flows over canopies of rigid elements with different geometries and Reynolds numbers (Re) are investigated to identify and characterise different canopy density regimes. In the sparse regime, turbulence penetrates relatively unhindered within the canopy, whereas in the dense regime, the penetration is limited. A common measure of canopy density is the ratio of frontal to bed area, the frontal density $λ_f$. This is effective for canopies with no preferential orientation, but we observe that it does not accurately predict the density regime for less conventional ones, so it may not encapsulate the governing physics. Instead, we propose density metrics based on the position and extent of eddies of intense Reynolds shear stress. We analyse a series of direct simulations for isotropic and anisotropic layouts, across a range of $λ_f$, height, element width-to-pitch ratio and Re. Canopies with streamwise-packed elements but large spanwise gaps allow significant turbulence penetration, and appear sparse compared to isotropic or spanwise-packed canopies with the same $λ_f$. Turbulence penetration depends on an effective spanwise gap, and increases with it, but depends also on Re. A canopy can behave as dense at low Re, but as sparser as Re increases. This suggests that turbulence penetration depends on the size of the spanwise gap relative to the typical width of the overlying eddies. Turbulence penetrates easily when the spanwise gap is larger than the eddy size, and is essentially precluded from penetrating in the opposite case. A penetration length can then be defined that is of the order of the effective spanwise gap or the eddy size, whichever is smaller. If the penetration length is small compared to the canopy height, the canopy behaves as dense; if it is comparable, as intermediate; and if it is roughly equal or larger, as sparse.

physics.flu-dyn↗

arXiv2024

Turbulent flows over porous and rough substrates

Turbulent flows over porous substrates are studied via a systematic exploration of the dependence of the flow properties on the substrate parameters, including permeability $K$, grain pitch $L$, and depth $h$. The study uses direct numerical simulations mainly for staggered-cube substrates with $L^+\approx10$ - $50$, $\sqrt{K}/L\approx0.01$ - $0.25$, and depths from $h=O(L)$ to $h\gg L$, ranging from typical impermeable rough surfaces to deep porous substrates. The results indicate that the permeability has significantly greater relevance than the grain size and microscale topology for the properties of the overlying flow, including the mean-flow slip and the shear across the interface, the drag increase relative to smooth-wall flow, and the statistics and spectra of the overlying turbulence, whereas the direct effect of grain size is only noticeable near the interface as grain-coherent flow fluctuations. The substrate depth also has a significant effect, with shallower substrates suppressing the effective transpiration at the interface. Based on the direct-simulation results, we propose an empirical `equivalent permeability' $K_{eq}^t$, that incorporates this effect and scales well the overlying turbulence for substrates with different depths, permeabilities, etc. Based on this, we propose a conceptual $h^+$-$\sqrt{K^+}$ regime diagram where, for any given substrate topology, turbulence transitions smoothly from that over impermeable rough surfaces with $h=O(L)$ to that over deep porous substrates with $h^+\gtrsim50$, with the latter limit determined by the typical lengthscale of the overlying pressure fluctuations.

physics.flu-dyn↗

arXiv2024

Resolving turbulence and drag over textured surfaces using texture-less simulations: the case of slip/no-slip textures

We study the effect of surface texture on an overlying turbulent flow for textures made of an alternating slip/no-slip pattern, a common model for superhydrophobic surfaces, but also a particularly simple form of texture. For texture sizes $L^+ \gtrsim 25$, the texture effectively imposes homogeneous slip boundary conditions on the overlying, background turbulence, but this is not its sole effect. The effective conditions only produce an origin offset on the background turbulence, which remains otherwise smooth-wall-like. For actual textures, however, for $L^+ \gtrsim 25$ the flow progressively departs from this smooth-wall-like regime, resulting in additional shear Reynolds stress and increased drag, in a non-homogeneous fashion not reproduced by the effective boundary conditions. We focus on the underlying physical mechanism of this phenomenon. We argue that it is caused by the non-linear interaction of the texture-coherent flow, directly induced by the surface topology, and the background turbulence, as it acts directly on the latter and alters it, not at the boundary where effective conditions are imposed, but within the flow itself. The interaction acts as a forcing on the governing equations of the background turbulence, and takes the form of cross-advective terms between the latter and the texture-coherent flow. We show this with simulations with the texture removed and additional, forcing terms in the Navier-Stokes equations, in addition to the effective boundary conditions. The forcing captures the effect of the non-linear interaction on the background turbulence without the need to resolve the texture. When the forcing is derived accounting for the background turbulence amplitude-modulating the texture-coherent flow, it captures the changes in the flow up to $L^+ \approx 70$--$100$, including the roughness function and also the changes in the flow statistics and structure.

physics.flu-dyn↗

arXiv2023

Examination of outer-layer similarity in wall turbulence over obstructed surfaces

Turbulent flows over canopies of rigid filaments with different densities, $λ_f$, are studied using direct simulations at Reynolds numbers $Re_τ\approx550-1000$. The canopies have heights $h^+\approx110-220$, and are an instance of obstructing substrate. We show that conventional methods used to determine the zero-plane displacement can be at odds with proper outer-layer similarity and may not be applicable for flows at moderate $Re_τ$. Instead, we determine $Δy$ and the length and velocity scales that recover outer-layer similarity by minimising the difference between the smooth-wall and canopy diagnostic function everywhere above the roughness sublayer, not just in the logarithmic layer. We also investigate if the zero-plane displacement and the friction velocity can be set independently, but find that outer-layer similarity is more consistently recovered when they are coupled. Our results suggest a modified outer-layer similarity, where the Kármán constant, $κ$, is not 0.39, but turbulence is otherwise smooth-wall-like. When the canopy is dense, the flow above the tips is essentially smooth-wall-like, with smooth-wall-like $κ\approx0.39$ and origin essentially at the tip plane. For intermediate densities, the overlying flow perceives a deeper zero-plane displacement, in agreement with previous studies, but exhibits a lower Kármán constant, $κ\approx0.34-0.36$. For sparse canopies, $κ$ tends back to its smooth-wall value, and the zero-plane-displacement height is at the canopy bed. For all canopies studied, the decrease in $κ$ never exceeds 15%, which is significantly less than that obtained in some previous works using conventional methods to assess outer-layer similarity.

physics.flu-dyn↗

arXiv2020

The smooth-wall-like behaviour of turbulence over drag-altering surfaces: a unifying virtual-origin framework

We conduct direct simulations of turbulent channels imposing different virtual origins for all three velocities using Robin, slip-like boundary conditions to study the effect of displacing the origins perceived by different flow components, a mechanism common to small-textured surfaces. We also explore this effect using opposition control. For riblets, Luchini et al. (1991) proposed that their effect could be reduced to the offset between the streamwise- and spanwise-velocity origins, the latter being the origin perceived by turbulence. Later results on superhydrophobic surfaces suggest that the apparent wall-normal-velocity origin could also play a role. Our results support that the relevant parameter is the offset between the origins perceived by the mean flow and by turbulence. The former is determined by the streamwise slip length, but the latter depends on both the wall-normal and spanwise slip lengths. The slip on the streamwise velocity fluctuations, in turn, has a negligible effect on the virtual origin for turbulence, at least in the drag-reduction regime. This suggests that the origin perceived by the quasi-streamwise vortices, which induce the cross velocities at the surface, is key in determining the virtual origin for turbulence, while that perceived by the near-wall streaks, which are associated with the streamwise velocity fluctuations, plays a secondary role. In this framework, the changes in turbulent quantities typically reported in the flow-control literature are shown to be only a result of the choice of origin and are absent when referenced to the origin experienced by turbulence. Other than this shift in origin, we show that turbulence remains essentially smooth-wall-like, and a simple expression can predict the virtual origin for turbulence. The effect on drag can be reproduced a priori by introducing the virtual origins into a smooth-wall eddy-viscosity framework.

physics.flu-dyn↗

arXiv2020

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' $τ_f$, 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 $τ_f$ at each height is similar to that over smooth-walls, even within the canopy. From this, a height-dependent scaling based on $τ_f$ 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 $τ_f$, 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 $τ_f$, 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↗

arXiv2020

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

Turbulent drag reduction by anisotropic permeable substrates $-$ analysis and direct numerical simulations

We explore the ability of anisotropic permeable substrates to reduce turbulent skin-friction, studying the influence that these substrates have on the overlying turbulence. For this, we perform DNSs of channel flows bounded by permeable substrates. The results confirm theoretical predictions, and the resulting drag curves are similar to those of riblets. For small permeabilities, the drag reduction is proportional to the difference between the streamwise and spanwise permeabilities. This linear regime breaks down for a critical value of the wall-normal permeability, beyond which the performance begins to degrade. We observe that the degradation is associated with the appearance of spanwise-coherent structures, attributed to a Kelvin-Helmholtz-like instability of the mean flow. This feature is common to a variety of obstructed flows, and linear stability analysis can be used to predict it. For large permeabilities, these structures become prevalent in the flow, outweighing the drag-reducing effect of slip and eventually leading to an increase of drag. For the substrate configurations considered, the largest drag reduction observed is $\approx 20-25\%$ at a friction Reynolds number $δ^+ = 180$.

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↗

arXiv2018

Modulation of near-wall turbulence in the transitionally rough regime

Direct numerical simulations of turbulent channels with rough walls are conducted in the transitionally rough regime. The effect that roughness produces on the overlying turbulence is studied using a modified triple decomposition of the flow. This decomposition separates the roughness-induced contribution from the background turbulence, with the latter essentially free of any texture footprint. For small roughness, the background turbulence is not significantly altered, but merely displaced closer to the roughness crests, with the change in drag being proportional to this displacement. As the roughness size increases, the background turbulence begins to be modified, notably by the increase of energy for short, wide wavelengths, which is consistent with the appearance of a shear-flow instability of the mean flow. A laminar model is presented to estimate the roughness-coherent contribution, as well as the displacement height and the velocity at the roughness crests. Based on the effects observed in the background turbulence, the roughness function is decomposed into different terms to analyse different contributions to the change in drag, laying the foundations for a predictive model.

physics.flu-dyn↗