The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Sedat F. Tardu - One of the best experts on this subject based on the ideXlab platform.
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Stochastic Synchronization of the Wall Turbulence
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2010Co-Authors: Sedat F. TarduAbstract:Instantaneous amplitude and phase concept emerging from analytical signal formulation is applied to the wavelet coefficients of streamwise velocity fluctuations in the buffer layer of a near Wall turbulent flow. Experiments and direct numerical simulations show both the existence of long periods of inert zones wherein the local phase is constant. These regions are separated by random phase jumps. These behaviours are reminiscent of phase synchronization phenomena observed in stochastic chaotic systems. The lengths of the constant phase inert (laminar) zones reveal a type-I intermittency behaviour. The observed phenomena are related to the footprint of coherent structures convecting in the low buffer layer that synchronizes the Wall Turbulence.
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Enhancement of Suboptimal Controllability in Wall Turbulence
IUTAM Symposium on Flow Control and MEMS, 2008Co-Authors: Olivier Doche, Sedat F. Tardu, Vincent KubickiAbstract:The Wall Turbulence is forced to a predictable state through localized time-periodical blowing. It is shown through experiments and direct numerical simulations that the temporal waveform of the localized blowing plays a crucial role in the response of turbulent Wall drag. Imposed unsteadiness increases the capacity of the controllability significantly. Results on the optimum periodical temporal waveform of localized blowing are also discussed. We use the characteristics of the cyclostationnarity to achieve this particular goal.
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Active control effectiveness and synchronization of Wall Turbulence under localized imposed unsteadiness
Physics of Fluids, 2007Co-Authors: Sedat F. Tardu, Olivier DocheAbstract:The effect of a spatially localized time-periodic perturbation on the efficiency of suboptimal control of the Wall Turbulence is analyzed. It is shown that the imposed unsteadiness with a frequency in the median production range doubles the turbulent drag reduction under suboptimal control strategy. It is further observed that the spatially averaged turbulent Wall shear is synchronized in time with the imposed perturbation waveform. This is related to the synchronization of the unstable periodic orbits present in the near-Wall Turbulence in connection with the regeneration cycle of Turbulence producing coherent structures.
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Response of the near Wall Turbulence to localized unsteady blowing periodical and dissymetric in time
2004Co-Authors: Sedat F. Tardu, Olivier DocheAbstract:The active control of the near Wall Turbulence by suboptimal or optimal strategies is plausible, but it requires unfeasibly dense distribution of the sensors and actuators at the Wall. One possible way to remedy to this situation is to go through dual control methods. The latter consist of exciting the near Wall Turbulence locally to render it more predictable and controllable. Such a strategy has been used by Tardu (1998, 2001) who investigated the reaction of the near Wall Turbulence to a space localized oscillating blowing sinusoidal in time with small severity.
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Response of the near Wall Turbulence to localized unsteady blowing periodical and dissymetric in time
2004Co-Authors: Sedat F. Tardu, Olivier DocheAbstract:The active control of the near Wall Turbulence by suboptimal or optimal strategies is plausible, but it requires unfeasibly dense distribution of the sensors and actuators at the Wall. One possible way to remedy to this situation is to go through dual control methods. The latter consist of exciting the near Wall Turbulence locally to render it more predictable and controllable. Such a strategy has been used by Tardu (1998, 2001) who investigated the reaction of the near Wall Turbulence to a space localized oscillating blowing sinusoidal in time with small severity. He showed that the flow is partly relaminarized during the oscillation cycle until roughly 50 Wall units downstream of the slot. This is due to the creation of a positive spanwise vorticity layer that dilutes the negative vorticity existing near the Wall. The relaminarized phase is unstable. The diffusion of the induced vorticity layer is constrained into the low buffer layer with limited effect of turbulent mixing, when the frequency of the oscillating blowing is large. Consequently the vorticity concentrates, becomes compact and rolls-up into a coherent spanwise vortex at approximately the beginning of the low buffer layer. It increases the Wall shear stress as it is convected downstream, resulting in an important drag penalty compared with steady blowing with the same mean severity.
Ivan Marusic - One of the best experts on this subject based on the ideXlab platform.
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attached eddy model of Wall Turbulence
Annual Review of Fluid Mechanics, 2019Co-Authors: Ivan Marusic, J P MontyAbstract:Modeling Wall Turbulence remains a major challenge, as a sufficient physical understanding of these flows is still lacking. In an effort to move toward a physics-based model, A.A. Townsend introduc...
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scaling of the streamwise Turbulence intensity in the context of inner outer interactions in Wall Turbulence
Physical Review Fluids, 2017Co-Authors: Ivan Marusic, Woutijn J Baars, N HutchinsAbstract:The mechanisms that underlie the interactions between the inner and outer regions of Wall-bounded Turbulence are considered, and their effect on the near-Wall Turbulence intensity is discussed.
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Progress in Wall Turbulence 2 - Progress in Wall Turbulence 2
ERCOFTAC Series, 2016Co-Authors: Michel Stanislas, Javier Jimenez, Ivan MarusicAbstract:This is the proceedings of the ERCOFTAC Workshop on Progress in Wall Turbulence: Understanding and Modelling, that was held in Lille, France from June 18 to 20, 2014. The workshop brought together world specialists of near Wall Turbulence and stimulated exchanges between them around up-to-date theories, experiments, simulations and numerical models. This book contains a coherent collection of recent results on near Wall Turbulence including theory, new experiments, DNS, and modeling with RANS, LES.The fact that both physical understanding and modeling by different approaches are addressed by the best specialists in a single workshop is original
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on the logarithmic region in Wall Turbulence
Journal of Fluid Mechanics, 2013Co-Authors: Ivan Marusic, J P Monty, Marcus Hultmark, Alexander SmitsAbstract:Considerable discussion over the past few years has been devoted to the question of whether the logarithmic region in Wall Turbulence is indeed universal. Here, we analyse recent experimental data in the Reynolds number range of nominally for boundary layers, pipe flow and the atmospheric surface layer, and show that, within experimental uncertainty, the data support the existence of a universal logarithmic region. The results support the theory of Townsend (The Structure of Turbulent Shear Flow, Vol. 2, 1976) where, in the interior part of the inertial region, both the mean velocities and streamwise Turbulence intensities follow logarithmic functions of distance from the Wall.
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On the logarithmic region in Wall Turbulence
Journal of Fluid Mechanics, 2013Co-Authors: Ivan Marusic, J P Monty, Marcus Hultmark, Alexander SmitsAbstract:Considerable discussion over the past few years has been devoted to the question of whether the logarithmic region in Wall Turbulence is indeed universal. Here, we analyse recent experimental data in the Reynolds number range of nominally $2\times 1{0}^{4} \lt {\mathit{Re}}_{\tau } \lt 6\times 1{0}^{5} $ for boundary layers, pipe flow and the atmospheric surface layer, and show that, within experimental uncertainty, the data support the existence of a universal logarithmic region. The results support the theory of Townsend ( The Structure of Turbulent Shear Flow , Vol. 2, 1976) where, in the interior part of the inertial region, both the mean velocities and streamwise Turbulence intensities follow logarithmic functions of distance from the Wall.
Beverley Mckeon - One of the best experts on this subject based on the ideXlab platform.
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phase relations in a forced turbulent boundary layer implications for modelling of high reynolds number Wall Turbulence
Philosophical Transactions of the Royal Society A, 2017Co-Authors: Subrahmanyam Duvvuri, Beverley MckeonAbstract:Phase relations between specific scales in a turbulent boundary layer are studied here by highlighting the associated nonlinear scale interactions in the flow. This is achieved through an experimental technique that allows for targeted forcing of the flow through the use of a dynamic Wall perturbation. Two distinct large-scale modes with well-defined spatial and temporal wavenumbers were simultaneously forced in the boundary layer, and the resulting nonlinear response from their direct interactions was isolated from the Turbulence signal for the study. This approach advances the traditional studies of large- and small-scale interactions in Wall Turbulence by focusing on the direct interactions between scales with triadic wavenumber consistency. The results are discussed in the context of modelling high Reynolds number Wall Turbulence.
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Nonlinear interactions isolated through scale synthesis in experimental Wall Turbulence
Physical Review Fluids, 2016Co-Authors: Subrahmanyam Duvvuri, Beverley MckeonAbstract:An experimental investigation of nonlinear scale interactions in a forced turbulent boundary layer is presented here. A dynamic Wall perturbation mechanism was used to externally force two distinct large-scale synthetic modes with well-defined spatial and temporal wave numbers in a fully turbulent flow. The focus is on characterizing the nonlinear flow response at triadically consistent wave numbers that arises from the direct interactions of the two synthetic modes. These experimental results isolate triadic scale interactions in Wall Turbulence in a unique fashion, and provide the ability to explore the dynamics of scale coupling in a systematic and detailed manner. The ideas advanced here are intended to contribute towards modeling efforts of high-Reynolds-number Wall Turbulence.
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Wall Turbulence with Designer Properties: Identification, Characterization and Manipulation of Energy Pathways
2016Co-Authors: Beverley Mckeon, Joel A. Tropp, Mark Sheplak, David B. GoldsteinAbstract:Abstract : The research performed under this BRI award targeted the identification, characterization and manipulation of energy pathways in Wall Turbulence. The objectives were pursued separately and collaboratively by the California Institute of Technology (Caltech), the University of Florida (UF) and the University of Texas at Austin (UT) teams through a joint numerical, experimental and modeling effort. Our approach applied the team's state-of-the-art capabilities in experiments (Caltech), simulation (UT), MEMS sensor development (UF) and linear algebra techniques (Caltech) to the resolvent formulation of the Navier-Stokes equations for Wall Turbulence proposed by McKeon and Sharma (2010). This formulation explicitly identifies the pathways for extraction of energy from the mean flow and transfer between wavenumbers, and thus can be used to identify forcing distributions required to manipulate the spectrum. Energetic pathways between modes have been identified, highlighted and characterized by experimentally exciting two modes and providing the first-known observation of a single triadic interaction.
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Experimental manipulation of Wall Turbulence: a systems approach
Physics of Fluids, 2013Co-Authors: Beverley Mckeon, Ati S. Sharma, Ian JacobiAbstract:We review recent progress, based on the approach introduced by McKeon and Sharma [J. Fluid Mech. 658, 336–382 (2010)10.1017/S002211201000176X], in understanding and controlling Wall Turbulence. The origins of this analysis partly lie in nonlinear robust control theory, but a differentiating feature is the connection with, and prediction of, state-of-the-art understanding of velocity statistics and coherent structures observed in real, high Reynolds number flows. A key component of this line of work is an experimental demonstration of the excitation of velocity response modes predicted by the theory using non-ideal, but practical, actuation at the Wall. Limitations of the approach and promising directions for future development are outlined.
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Predicting structural and statistical features of Wall Turbulence
arXiv: Fluid Dynamics, 2010Co-Authors: Beverley Mckeon, Ati S. Sharma, Ian JacobiAbstract:The majority of practical flows, particularly those flows in applications of importance to transport, distribution and climate, are turbulent and as a result experience complex three-dimensional motion with increased drag compared with the smoother, laminar condition. In this study, we describe the development of a simple model that predicts important structural and scaling features of Wall Turbulence. We show that a simple linear superposition of modes derived from a forcing-response analysis of the Navier-Stokes equations can be used to reconcile certain key statistical and structural descriptions of Wall Turbulence. The computationally cheap approach explains and predicts vortical structures and velocity statistics of turbulent flows that have previously been identified only in experiments or by direct numerical simulation. In particular, we propose an economical explanation for the meandering appearance of very large scale motions observed in turbulent pipe flow, and likewise demonstrate that hairpin vortices are predicted by the model. This new capability has clear implications for modeling, simulation and control of a ubiquitous class of Wall flows.
Javier Jimenez - One of the best experts on this subject based on the ideXlab platform.
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Progress in Wall Turbulence 2 - Progress in Wall Turbulence 2
ERCOFTAC Series, 2016Co-Authors: Michel Stanislas, Javier Jimenez, Ivan MarusicAbstract:This is the proceedings of the ERCOFTAC Workshop on Progress in Wall Turbulence: Understanding and Modelling, that was held in Lille, France from June 18 to 20, 2014. The workshop brought together world specialists of near Wall Turbulence and stimulated exchanges between them around up-to-date theories, experiments, simulations and numerical models. This book contains a coherent collection of recent results on near Wall Turbulence including theory, new experiments, DNS, and modeling with RANS, LES.The fact that both physical understanding and modeling by different approaches are addressed by the best specialists in a single workshop is original
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Near-Wall Turbulence
Physics of Fluids, 2013Co-Authors: Javier JimenezAbstract:The current state of knowledge about the structure of Wall-bounded turbulent flows is reviewed, with emphasis on the layers near the Wall in which shear is dominant, and particularly on the logarithmic layer. It is shown that the shear interacts with scales whose size is larger than about one third of their distance to the Wall, but that smaller ones, and in particular the vorticity, decouple from the shear and become roughly isotropic away from the Wall. In the buffer and viscous layers, the dominant structures carrying turbulent energy are the streamwise velocity streaks, and the vortices organize both the dissipation and the momentum transfer. Farther from the Wall, the velocity remains organized in streaks, although much larger ones than in the buffer layer, but the vortices lose their role regarding the Reynolds stresses. That function is taken over by Wall-attached turbulent eddies with sizes and lifetimes proportional to their heights. Two kinds of eddies have been studied in some detail: vortex clusters, and ejections and sweeps. Both can be classified into a detached background, and a geometrically self-similar Wall-attached family. The latter is responsible for most of the momentum transfer, and is organized into composite structures that can be used as models for the attached-eddy hierarchy hypothesized by Townsend [“Equilibrium layers and Wall Turbulence,” J. Fluid Mech.11, 97–120 (1961)]. The detached component seems to be common to many turbulent flows, and is roughly isotropic. Using a variety of techniques, including direct tracking of the structures, it is shown that an important characteristic of Wall-bounded Turbulence is temporally intermittent bursting, which is present at all distances from the Wall, and in other shear flows. Its properties and time scales are reviewed, and it is shown that bursting is an important part of the production of turbulent energy from the mean shear. It is also shown that a linearized model captures many of its characteristics.
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Progress in Wall Turbulence: Understanding and Modeling - Progress in Wall Turbulence: Understanding and Modeling
ERCOFTAC Series, 2011Co-Authors: Michel Stanislas, Javier Jimenez, Ivan MarusicAbstract:This is the proceedings of the ERCOFTAC Workshop on Progress in Wall Turbulence: Understanding and Modelling,that was held in Lille, France from June 18 to 20, 2014. The workshop brought together world specialists of near Wall Turbulence and stimulated exchanges between them around up-to-date theories, experiments, simulations and numerical models. This book contains a coherent collection of recent results on near Wall Turbulence including theory, new experiments, DNS, and modeling with RANS, LES.The fact that both physical understanding and modeling by different approaches are addressed by the best specialists in a single workshop is original
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progress in Wall Turbulence 2 understanding and modelling
2011Co-Authors: Javier Jimenez, Ivan Marusic, Michel StanislasAbstract:This is the proceedings of the ERCOFTAC Workshop on Progress in Wall Turbulence: Understanding and Modelling,that was held in Lille, France from June 18 to 20, 2014. The workshop brought together world specialists of near Wall Turbulence and stimulated exchanges between them around up-to-date theories, experiments, simulations and numerical models. This book contains a coherent collection of recent results on near Wall Turbulence including theory, new experiments, DNS, and modeling with RANS, LES.The fact that both physical understanding and modeling by different approaches are addressed by the best specialists in a single workshop is original
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Wall Turbulence without Walls
Springer Proceedings in Physics, 2009Co-Authors: Yoshinori Mizuno, Javier JimenezAbstract:The dynamics of the inner layer of Wall Turbulence are now fairly well understood[1]. The next target are the overlying outer layers, including the logarithmic layer. There is increasing evidence that those layers, which account for most of the flow thickness, are relatively independent of the near-Wall region, mainly from rough- and active-Wall experiments and simulations[2, 3], but it is unclear whether a physical Wall is required for their canonical behaviour. On the other hand, this is an interesting physical question, and sets a baseline requirement for LES, because the buffer layer often has to be modelled differently from the rest of the flow.
Hideaki Mouri - One of the best experts on this subject based on the ideXlab platform.
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Unlikely existence of kx−1 spectral law in Wall Turbulence: An observation of the atmospheric surface layer
Physics of Fluids, 2019Co-Authors: Hideaki Mouri, Takeshi Morinaga, Shigenori HaginoyaAbstract:For Wall Turbulence, a range of streamwise wavenumbers kx has been predicted such that the spectral density of streamwise velocity fluctuations is proportional to kx−1. The existence or nonexistence of this kx−1 law is examined here. We observe the atmospheric surface layer over several months, select suitable data, and use them to synthesize the energy spectrum that would represent Wall Turbulence at a very high Reynolds number. The result is not consistent with the kx−1 law. It is, rather, consistent with a recent correction to the prediction of a model of energy-containing eddies that are attached to the Wall. The reason for these findings is discussed mathematically.For Wall Turbulence, a range of streamwise wavenumbers kx has been predicted such that the spectral density of streamwise velocity fluctuations is proportional to kx−1. The existence or nonexistence of this kx−1 law is examined here. We observe the atmospheric surface layer over several months, select suitable data, and use them to synthesize the energy spectrum that would represent Wall Turbulence at a very high Reynolds number. The result is not consistent with the kx−1 law. It is, rather, consistent with a recent correction to the prediction of a model of energy-containing eddies that are attached to the Wall. The reason for these findings is discussed mathematically.
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Logarithmic scaling for fluctuations of a scalar concentration in Wall Turbulence.
Physical review. E, 2017Co-Authors: Hideaki Mouri, Takeshi Morinaga, Toshimasa Yagi, Kazuyasu MoriAbstract:Within Wall Turbulence, there is a sublayer where the mean velocity and the variance of velocity fluctuations vary logarithmically with the height from the Wall. This logarithmic scaling is also known for the mean concentration of a passive scalar. By using heat as such a scalar in a laboratory experiment of a turbulent boundary layer, the existence of the logarithmic scaling is shown here for the variance of fluctuations of the scalar concentration. It is reproduced by a model of energy-containing eddies that are attached to the Wall.
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Mathematical model for logarithmic scaling of velocity fluctuations in Wall Turbulence.
Physical Review E, 2015Co-Authors: Hideaki MouriAbstract:For Wall Turbulence, moments of velocity fluctuations are known to be logarithmic functions of the height from the Wall. This logarithmic scaling is due to the existence of a characteristic velocity and to the nonexistence of any characteristic height in the range of the scaling. By using mathematics of random variables, we obtain its necessary and sufficient conditions. They are compared with characteristics of a phenomenological model of eddies attached to the Wall and also with those of the logarithmic scaling of the mean velocity.
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Mathematical model for logarithmic scaling of velocity fluctuations in Wall Turbulence.
Physical review. E Statistical nonlinear and soft matter physics, 2015Co-Authors: Hideaki MouriAbstract:For Wall Turbulence, moments of velocity fluctuations are known to be logarithmic functions of the height from the Wall. This logarithmic scaling is due to the existence of a characteristic velocity and to the nonexistence of any characteristic height in the range of the scaling. By using the mathematics of random variables, we obtain its necessary and sufficient conditions. They are compared with characteristics of a phenomenological model of eddies attached to the Wall and also with those of the logarithmic scaling of the mean velocity.