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Publications (10 of 41) Show all publications
Zampogna, G. A., Lacis, U., Bagheri, S. & Bottaro, A. (2019). Modeling waves in fluids flowing over and through poroelastic media. International Journal of Multiphase Flow, 110, 148-164
Open this publication in new window or tab >>Modeling waves in fluids flowing over and through poroelastic media
2019 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 110, p. 148-164Article in journal (Refereed) Published
Abstract [en]

Multiscale homogenization represents a powerful tool to treat certain fluid-structure interaction problems involving porous, elastic, fibrous media. This is shown here for the case of the interaction between a Newtonian fluid and a poroelastic, microstructured material. Microscopic problems are set up to determine effective tensorial properties (elasticity, permeability, porosity, bulk compliance of the solid skeleton) of the homogenized medium, both in the interior and at its boundary with the fluid domain, and an extensive description is provided of such properties for varying porosity. The macroscopic equations which are derived by homogenization theory employ such effective properties thus permitting the computation of velocities and displacements within the poroelastic mixture for two representative configurations of standing and travelling waves.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Homogenization, Biot-Allard equations, Poroelasticity, Interface conditions
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-240698 (URN)10.1016/j.ijmultiphaseflow.2018.09.006 (DOI)000452945400012 ()2-s2.0-85054025880 (Scopus ID)
Note

QC 20190109

Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-01-09Bibliographically approved
Rinaldi, E., Schlatter, P. & Bagheri, S. (2018). Edge state modulation by mean viscosity gradients. Journal of Fluid Mechanics, 838, 379-403
Open this publication in new window or tab >>Edge state modulation by mean viscosity gradients
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 838, p. 379-403Article in journal (Refereed) Published
Abstract [en]

Motivated by the relevance of edge state solutions as mediators of transition, we use direct numerical simulations to study the effect of spatially non-uniform viscosity on their energy and stability in minimal channel flows. What we seek is a theoretical support rooted in a fully nonlinear framework that explains the modified threshold for transition to turbulence in flows with temperature-dependent viscosity. Consistently over a range of subcritical Reynolds numbers, we find that decreasing viscosity away from the walls weakens the streamwise streaks and the vortical structures responsible for their regeneration. The entire self-sustained cycle of the edge state is maintained on a lower kinetic energy level with a smaller driving force, compared to a flow with constant viscosity. Increasing viscosity away from the walls has the opposite effect. In both cases, the effect is proportional to the strength of the viscosity gradient. The results presented highlight a local shift in the state space of the position of the edge state relative to the laminar attractor with the consequent modulation of its basin of attraction in the proximity of the edge state and of the surrounding manifold. The implication is that the threshold for transition is reduced for perturbations evolving in the neighbourhood of the edge state in the case that viscosity decreases away from the walls, and vice versa.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
nonlinear dynamical systems, nonlinear instability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-222027 (URN)10.1017/jfm.2017.921 (DOI)2-s2.0-85040834445 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180131

Available from: 2018-01-31 Created: 2018-01-31 Last updated: 2018-11-14Bibliographically approved
Sundin, J. & Bagheri, S. (2018). Interaction between hairy surfaces and turbulence for different surface time scales. Journal of Fluid Mechanics, 861, 556-584
Open this publication in new window or tab >>Interaction between hairy surfaces and turbulence for different surface time scales
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 861, p. 556-584Article in journal (Refereed) Published
Abstract [en]

Surfaces with filamentous structures are ubiquitous in nature on many different scales, ranging from forests to micrometre-sized cilia in organs. Hairy surfaces are elastic and porous, and it is not fully understood how they modify turbulence near a wall. The interaction between hairy surfaces and turbulent flows is here investigated numerically in a turbulent channel flow configuration at friction Reynolds number Re-tau approximate to 180. We show that a filamentous bed of a given geometry can modify a turbulent flow very differently depending on the resonance frequency of the surface, which is determined by the elasticity and mass of the filaments. Filaments having resonance frequencies lower than the main frequency content of the turbulent wall-shear stress conform to slowly travelling elongated streaky structures, since they are too slow to adapt to fluid forces of higher frequencies. On the other hand, a bed consisting of stiff and low-mass filaments has a high resonance frequency and shows local regions of increased permeability, which results in large entrainment and a vast increase in drag.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
flow control, flow-structure interactions, turbulent boundary layers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-241195 (URN)10.1017/jfm.2018.935 (DOI)000454407500002 ()2-s2.0-85059814760 (Scopus ID)
Note

QC 20190121

Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2019-01-28Bibliographically approved
Fabbiane, N., Bagheri, S. & Henningson, D. S. (2017). Energy efficiency and performance limitations of linear adaptive control for transition delay. Journal of Fluid Mechanics, 810, 60-81
Open this publication in new window or tab >>Energy efficiency and performance limitations of linear adaptive control for transition delay
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 810, p. 60-81Article in journal (Refereed) Published
Abstract [en]

A reactive control technique with localised actuators and sensors is used to delay the transition to turbulence in a flat-plate boundary-layer flow. Through extensive direct numerical simulations, it is shown that an adaptive technique, which computes the control law on-line, is able to significantly reduce skin-friction drag in the presence of random three-dimensional perturbation fields with linear and weakly nonlinear behaviour. An energy budget analysis is performed in order to assess the net energy saving capabilities of the linear control approach. When considering a model of the dielectric-barrier-discharge (DBD) plasma actuator, the energy spent to create appropriate actuation force inside the boundary layer is of the same order as the energy gained from reducing skin-friction drag. With a model of an ideal actuator a net energy gain of three orders of magnitude can be achieved by efficiently damping small-amplitude disturbances upstream. The energy analysis in this study thus provides an upper limit for what we can expect in terms of drag-reduction efficiency for linear control of transition as a means for drag reduction.

Place, publisher, year, edition, pages
Cambridge University Press, 2017
Keywords
boundary layer control, drag reduction, instability control
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-199471 (URN)10.1017/jfm.2016.707 (DOI)000389327700005 ()2-s2.0-84997142682 (Scopus ID)
Note

QC 20170123

Available from: 2017-01-23 Created: 2017-01-09 Last updated: 2017-11-29Bibliographically approved
Lacis, U., Taira, K. & Bagheri, S. (2016). A stable fluid-structure-interaction solver for low-density rigid bodies using the immersed boundary projection method. Journal of Computational Physics, 305, 300-318
Open this publication in new window or tab >>A stable fluid-structure-interaction solver for low-density rigid bodies using the immersed boundary projection method
2016 (English)In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 305, p. 300-318Article in journal (Refereed) Published
Abstract [en]

Dispersion of low-density rigid particles with complex geometries is ubiquitous in both natural and industrial environments. We show that while explicit methods for coupling the incompressible Navier-Stokes equations and Newton's equations of motion are often sufficient to solve for the motion of cylindrical particles with low density ratios, for more complex particles - such as a body with a protrusion - they become unstable. We present an implicit formulation of the coupling between rigid body dynamics and fluid dynamics within the framework of the immersed boundary projection method. Similarly to previous work on this method, the resulting matrix equation in the present approach is solved using a block-LU decomposition. Each step of the block-LU decomposition is modified to incorporate the rigid body dynamics. We show that our method achieves second-order accuracy in space and first-order in time (third-order for practical settings), only with a small additional computational cost to the original method. Our implicit coupling yields stable solution for density ratios as low as 10(-4). We also consider the influence of fictitious fluid located inside the rigid bodies on the accuracy and stability of our method.

Place, publisher, year, edition, pages
Academic Press, 2016
Keywords
Immersed boundary method, Fictitious fluid, Newton's equations of motion, Implicit coupling, Low density ratios, Complex particles
National Category
Physical Sciences Computer Sciences
Identifiers
urn:nbn:se:kth:diva-180478 (URN)10.1016/j.jcp.2015.10.041 (DOI)000366156600016 ()2-s2.0-84946595242 (Scopus ID)
Note

QC 20160118

Available from: 2016-01-18 Created: 2016-01-14 Last updated: 2018-01-10Bibliographically approved
Natali, D., Pralits, J. O., Mazzino, A. & Bagheri, S. (2016). Stabilizing effect of porosity on a flapping filament. Journal of Fluids and Structures, 61, 362-375
Open this publication in new window or tab >>Stabilizing effect of porosity on a flapping filament
2016 (English)In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 61, p. 362-375Article in journal (Refereed) Published
Abstract [en]

A new way of handling, simultaneously, porosity and bending resistance of a massive filament is proposed. Our strategy extends the previous methods where porosity was taken into account in the absence of bending resistance of the structure and overcomes related numerical issues. The new strategy has been exploited to investigate how porosity affects the stability of slender elastic objects exposed to a uniform stream. To understand under which conditions porosity becomes important, we propose a simple resonance mechanism between a properly defined characteristic porous time-scale and the standard characteristic hydrodynamic time-scale. The resonance condition results in a critical value for the porosity above which porosity is important for the resulting filament flapping regime, otherwise its role can be considered of little importance. Our estimation for the critical value of the porosity is in fairly good agreement with our DNS results. The computations also allow us to quantitatively establish the stabilizing role of porosity in the flapping regimes.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Flutter, Porosity, Immersed boundary, Drag reduction
National Category
Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-184555 (URN)10.1016/j.jfluidstructs.2015.11.016 (DOI)000371551800021 ()2-s2.0-84957892814 (Scopus ID)
Funder
Swedish Research Council, VR-2010-3910
Note

QC 20160406

Available from: 2016-04-06 Created: 2016-04-01 Last updated: 2017-11-30Bibliographically approved
Brosse, N., Finmo, C., Lundell, F. & Bagheri, S. (2015). Experimental study of a three-dimensional cylinder–filament system. Experiments in Fluids, 56(6), Article ID 130.
Open this publication in new window or tab >>Experimental study of a three-dimensional cylinder–filament system
2015 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 6, article id 130Article in journal (Refereed) Published
Abstract [en]

This experimental study reports on the behavior of a filament attached to the rear of a three-dimensional cylinder. The axis of the cylinder is placed normal to a uniform incoming flow, and the filament is free to move in the cylinder wake. The mean position of the filament is studied as a function of the filament length L. It is found that for long (L/D > 6.5, where D is the cylinder diameter) and short (L/D < 2) filaments, the mean position of the filament tends to align with the incoming flow, whereas for intermediate filament lengths (2 < L/D < 6.5), the filament lies down on the cylinder and tends to align with the cylinder axis. The underlying mechanism of the bifurcations is discussed and related to buckling and inverted-pendulum-like instabilities.

Keywords
Cylinder axis, Cylinder diameters, Cylinder wake, Incoming flows, Intermediate filaments, Inverted pendulum
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-170211 (URN)10.1007/s00348-015-2002-y (DOI)2-s2.0-84930659979 (Scopus ID)
Note

QC 20150629

Available from: 2015-06-29 Created: 2015-06-29 Last updated: 2017-12-04Bibliographically approved
Fabbiane, N., Simon, B., Fischer, F., Grundmann, S., Bagheri, S. & Henningson, D. S. (2015). On the role of adaptivity for robust laminar flow control. Journal of Fluid Mechanics, 767, R1-R12
Open this publication in new window or tab >>On the role of adaptivity for robust laminar flow control
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2015 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 767, p. R1-R12Article in journal (Refereed) Published
Abstract [en]

In boundary layer flows, one may reduce skin friction drag by delaying the onset of laminar-to-turbulent transition via the attenuation of small-amplitude Tollmien Schlichting (TS) waves In this work, we use numerical simulations and experiments to compare the robustness of adaptive and model-based techniques for reducing the growth of two-dimensional TS disturbances. In numerical simulations, the optimal linear quadratic Gaussian (LQG) regulator shows the best performance under the conditions it was designed for However, it is found that the performance deteriorates linearly with the drift of the Reynolds number from its nominal value. As a result, an order-of-magnitude loss of performance is observed when applying the computation-based I.QG controller in wind-tunnel experiments In contrast, it is shown that the adaptive filtered-X least-mean-squares (FXLMS) algorithm is able to maintain an essentially constant performance for significant deviations of the nominal values of the disturbance amplitude and Reynolds number.

Keywords
boundary layer control, flow control, instability control
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-154351 (URN)10.1017/jfm.2015.45 (DOI)000349688900001 ()2-s2.0-84949117226 (Scopus ID)
Funder
Swedish Research Council Formas, VR-2012-4246, VR-2010-3910
Note

Updated from Manuscript to Article. QC 20150488

Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved
Lacis, U. & Bagheri, S. (2014). A continuous description of porous and elastic media for the simulation of the flow around coated objects.
Open this publication in new window or tab >>A continuous description of porous and elastic media for the simulation of the flow around coated objects
2014 (English)Report (Other academic)
Abstract [en]

Poroelastic materials are commonly found in nature; birds are covered with all sorts of feathers, land animals are covered with different kinds of fur and fishes are covered with various fins and scales. The problem of fluid flow through such materials is very challenging both experimentally and numerically. It is impossible to experimentally measure fluid flow within the material, if the media is densely packed and have fine micro-structure. In such case, direct numerical simulations of the coupled problem with flow in the media and deformation of micro-structure are extremely costly. In order to overcome this limitation, continuum theories have been developed, where average behaviour of the poroelastic material and the fluid within is described. There have already been a significant progress towards describing poroelastic materials similar to examples found in nature; however further work to resolve issues in boundary conditions, modelling and connection between different theories is required. In the current paper, we present a summary of existing theories. Then, we select a multi-scale expansion approach, which we believe is feasible to use for description of the flow in a poroelastic material. Finally, we present preliminary results of a decoupled micro-scale problem with an expansion for two scales. We observe that the two-scale approach is problematic for poroelastic coatings of micro-structure, which is disconnected in a given plane.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-158318 (URN)
Funder
Swedish Research Council, VR-2010-3910
Note

QC 20150119

Available from: 2015-01-07 Created: 2015-01-07 Last updated: 2015-01-19Bibliographically approved
Fabbiane, N., Bagheri, S., Henningson, D. S. & Semeraro, O. (2014). Adaptive and model-based control theory applied to convectively unstable flows. Applied Mechanics Review, 66(6), 060801
Open this publication in new window or tab >>Adaptive and model-based control theory applied to convectively unstable flows
2014 (English)In: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 66, no 6, p. 060801-Article, review/survey (Refereed) Published
Abstract [en]

Research on active control for the delay of laminar-turbulent transition in boundary layers has made a significant progress in the last two decades, but the employed strategies have been many and dispersed. Using one framework, we review model-based techniques, such as linear-quadratic regulators, and model-free adaptive methods, such as least-mean square filters. The former are supported by a elegant and powerful theoretical basis, whereas the latter may provide a more practical approach in the presence of complex disturbance envi- ronments, that are difficult to model. We compare the methods with a particu- lar focus on efficiency, practicability and robustness to uncertainties. Each step is exemplified on the one-dimensional linearized Kuramoto-Sivashinsky equa- tion, that shows many similarities with the initial linear stages of the transition process of the flow over a flat plate. Also, the source code for the examples are provided. 

Keywords
Flow control, Control theory, Optimal control, Adaptive control, Boundary-layer flow, Fluid dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-154050 (URN)10.1115/1.4027483 (DOI)000346238700001 ()2-s2.0-84902832513 (Scopus ID)
Funder
Swedish Research Council, 2012- 4246
Note

QC 20141015

MATLAB scrips available at www.mech.kth.se/~nicolo/ks

Available from: 2014-10-13 Created: 2014-10-13 Last updated: 2017-12-05Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-8209-1449

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