Interaction between hairy surfaces and turbulence for different surface time scales

Sundin, Johan; Bagheri, Shervin

doi:10.1017/jfm.2018.935

Cited by 13 publications

(19 citation statements)

References 42 publications

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“…The understanding provided by these simulations is important when the objective is to tune the fluid–surface interactions to make the flow behave as desired, modifying the mass of the filaments or their elasticity. On the other hand, not all configurations can be treated by the pointwise approach used by Sundin & Bagheri (2019). For example, for very dense coatings additional time scales are present and play a role; in such cases the microscopic description of the fluid through the deforming filaments becomes very difficult to carry out numerically.…”

Section: Crossing the Boundary Between Mediamentioning

confidence: 99%

“…The numerical counterpart of Brücker's experiments has been recently presented by Sundin & Bagheri (2019). These authors considered a bed of elastic fibres anchored at one rigid wall and arranged in a regular square pattern, exploring systematically configurations with different ratios between f nat and f turb .…”

Section: Turbulence Above Porous and Poroelastic Layersmentioning

confidence: 99%

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Flow over natural or engineered surfaces: an adjoint homogenization perspective

Bottaro

2019

J. Fluid Mech.

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Natural and engineered surfaces are never smooth, but irregular, rough at different scales, compliant, possibly porous, liquid impregnated or superhydrophobic. The correct numerical modelling of fluid flowing through and around them is important but poses problems. For media characterized by a periodic or quasi-periodic microstructure of characteristic dimensions smaller than the relevant scales of the flow, multiscale homogenization can be used to study the effect of the surface, avoiding the numerical resolution of small details. Here, we revisit the homogenization strategy using adjoint variables to model the interaction between a fluid in motion and regularly micro-textured, permeable or impermeable walls. The approach described allows for the easy derivation of auxiliary/adjoint systems of equations which, after averaging, yield macroscopic tensorial properties, such as permeability, elasticity, slip, transpiration, etc. When the fluid in the neighbourhood of the microstructure is in the Stokes regime, classical results are recovered. Adjoint homogenization, however, permits simple extension of the analysis to the case in which the flow displays nonlinear effects. Then, the properties extracted from the auxiliary systems take the name of effective properties and do not depend only on the geometrical details of the medium, but also on the microscopic characteristics of the fluid motion. Examples are shown to demonstrate the usefulness of adjoint homogenization to extract effective tensor properties without the need for ad hoc parameters. In particular, notable results reported herein include:(i)an original formulation to describe filtration in porous media in the presence of inertial effects;(ii)the microscopic and macroscopic equations needed to characterize flows through poroelastic media;(iii)an extended Navier’s condition to be employed at the boundary between a fluid and an impermeable rough wall, with roughness elements which can be either rigid or linearly elastic;(iv)the microscopic problems needed to define the relevant parameters for a Saffman-like condition at the interface between a fluid and a porous substrate; and(v)the macroscopic equations which hold at the dividing surface between a free-fluid region and a fluid-saturated poroelastic domain.

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Section: Crossing the Boundary Between Mediamentioning

confidence: 99%

Section: Turbulence Above Porous and Poroelastic Layersmentioning

confidence: 99%

Flow over natural or engineered surfaces: an adjoint homogenization perspective

Bottaro

2019

J. Fluid Mech.

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“…[14], where the flow past a sphere with discretized porous elements on its surface is solved, or using a lattice Boltzmann method, as proposed by Ref. [15] to study the flow within a channel with a hairy surface. However, the long computational times owing to the bottleneck of the porous region discretization make these approaches inadequate for a parametric study, especially when a wide range of length scales is involved, as is the case here (with three orders of magnitude between the pappus and the filament diameter).…”

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confidence: 99%

Flow dynamics of a dandelion pappus: A linear stability approach

et al. 2019

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The study and control of flow instabilities is a key problem in aerodynamics. Aircrafts are designed not only to generate the lift force needed to balance their weight but, more importantly, to be stable and reasonably steady when in cruise conditions. Similar flow stability properties are naturally achieved by biological flying objects such as the dandelion seeds that are transported by the wind owing to a disklike structure called a pappus. The pappus creates a parachute flow configuration and is a remarkable prototype of how the wake, which would be unsteady if the pappus was completely impermeable, can be stabilized by changing the body structure so as to allow the flow to pass through. We approach the problem using the approximation of an anisotropic and nonhomogeneous rigid porous disk, combined with the linear stability analysis technique. The results show the presence of a mean porosity threshold beyond which the flow is always characterized by a separated, steady, and axisymmetric recirculating vortex ring. We compare our results with those of real dandelion pappi. The threshold is very close to the experimentally observed values of porosity, explaining why the morphology of the pappus promotes a steady wake regime.

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“…The work also summarizes the recent developments in numerical simulations of canopy flows. The simulation most relevant to this study was done by Sundin and Bagheri (Sundin and Bagheri 2019), who simulated the interaction between hairy surfaces and turbulent flows in a turbulent channel flow configuration. Their work was inspired by the experimental observation of streak stabilization in turbulent boundary flows over arrays of flexible filaments (Brücker 2011).…”

Section: Introductionmentioning

confidence: 99%

Sediment erosion in flexible canopies by vortex impact

Brücker

2020

Journal of Fluids and Structures

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A model experiment with a vortex impacting a flexible canopy filled with a thin homogeneous bed of particles is presented. Flexibility of the filaments increases the efficiency of resuspension by the amount it allows the effective canopy-height to reduce due to the reconfiguration of the flexible structures. Scaling of the results with the effective canopy height leads to a collapse of the observed resuspension in the history of several successive impacts. It is further shown that preferential pathways in the canopy play a large role in resuspension. When comparing a hexagonal arrangement to a random arrangement of the filaments at the same average porosity, one needs to double the amount of impacts to achieve the same average resuspension. Hence, it is concluded that the random path of the particles around the filaments is affected in a non-linear manner by the local resistance. Regions of locally sparse arrangements of the filaments cannot balance the trapping effect of particles within regions of dense arrangement in their travel history. Flexibility of the filaments again proves a better resuspension under such conditions.

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Interaction between hairy surfaces and turbulence for different surface time scales

Cited by 13 publications

References 42 publications

Flow over natural or engineered surfaces: an adjoint homogenization perspective

Flow over natural or engineered surfaces: an adjoint homogenization perspective

Flow dynamics of a dandelion pappus: A linear stability approach

Sediment erosion in flexible canopies by vortex impact

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