46 results on '"Turbulence simulations"'
Search Results
2. Virus transmission by aerosol transport during short conversations.
- Author
-
Singhal, Rohit, Ravichandran, S., Govindarajan, Rama, and Diwan, Sourabh S.
- Subjects
AEROSOLS ,SARS-CoV-2 ,INFECTIOUS disease transmission ,EPIDEMIOLOGY ,PROBABILITY theory - Abstract
The SARS-CoV-2 is transmitted not only through coughing, but also through breathing, speaking or singing. We perform direct numerical simulations of the turbulent transport of potentially infectious aerosols in short conversations, involving repetitive phrases separated by quiescent intervals. We estimate that buoyancy effects due to droplet evaporation are small, and neglect them. A two-way conversation is shown to significantly reduce the aerosol exposure compared with a relative monologue by one person and relative silence of the other. This is because of the 'cancelling' effect produced by the two interacting speech jets. Unequal conversation is shown to significantly increase the infection risk to the person who talks less. Interestingly, a small height difference is worse for infection spread, due to reduced interference between the speech jets, than two faces at the same level. For small axial separation, speech jets show large oscillations and reach the other person intermittently. We suggest a range of lateral separations between two people to minimize transmission risk. A realistic estimate of the infection probability is provided by including exposure through the eyes and mouth, in addition to the more common method of using inhaled virions alone. We expect that our results will provide useful inputs to epidemiological models and to disease management. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
3. Experimental evidence of edge intrinsic momentum source driven by kinetic ion loss and edge radial electric fields in tokamaks
- Author
-
Watkins, J. [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)]
- Published
- 2016
- Full Text
- View/download PDF
4. Mean flows and blob velocities in scrape-off layer (SOLT) simulations of an L-mode discharge on Alcator C-Mod
- Author
-
Zweben, S. [Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)] (ORCID:0000000217380586)
- Published
- 2016
- Full Text
- View/download PDF
5. Integrated fusion simulation with self-consistent core-pedestal coupling
- Author
-
Holland, C. [Univ. of California, San Diego, CA (United States)]
- Published
- 2016
- Full Text
- View/download PDF
6. The role of neutral gas in validated global edge turbulence simulations.
- Author
-
Zholobenko, W., Stegmeir, A., Griener, M., Conway, G.D., Body, T., Coster, D., and Jenko, F.
- Subjects
- *
TURBULENCE , *IONIZING radiation , *FUSION reactors , *PLASMA density , *ION temperature , *TURBULENT shear flow , *PLASMA turbulence - Abstract
To make predictions for and design fusion reactors, a multitude of physical processes must be considered. In the edge and scrape-off layer (SOL), turbulent fluctuations intertwine with the plasma background, which is largely determined by neutral gas, and magnetic geometry plays an important role. A diffusive neutrals model has now been implemented in the global Braginskii edge turbulence code GRILLIX. The code is based on the flux-coordinate independent (FCI) approach, which allows efficient turbulence simulations in diverted equilibria. We validate simulations across the ASDEX Upgrade edge and SOL against measurements in discharge # 36190, and find much better agreement thanks to the neutrals. Disentangling the effects of the neutral gas, we find that it affects the plasma in several ways. Firstly, the ionization of neutrals modifies the radial profiles of plasma density and temperature, leading to a transition of the turbulence drive from the general ballooning type to the ion temperature gradient type. Secondly, strong poloidal asymmetries can be induced due to divertor recycling, depending on the ionization pattern. As ballooned perpendicular plasma transport is stronger at the low-field side, neutrals penetrate deeper into the plasma at the high-field side, leading to significant ionization and radiation there. With increasing divertor neutrals density, the targets cool down while plasma density increases, more strongly at the high-field side. Much of the dynamics take place directly around the X -point and along the separatrix, which can be resolved by the FCI approach. Potential remains in extending the model and the code, but our results build confidence that predictive capability is within reach for major design questions for fusion reactors, such as the near SOL fall-off length. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
7. Modeling the effect of lithium-induced pedestal profiles on scrape-off-layer turbulence and the heat flux width
- Author
-
Zweben, S. [Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)]
- Published
- 2015
- Full Text
- View/download PDF
8. Modifying mixing and instability growth through the adjustment of initial conditions in a high-energy-density counter-propagating shear experiment on OMEGA
- Author
-
Kline, J. [Los Alamos National Lab. (LANL), Los Alamos, NM (United States)]
- Published
- 2015
- Full Text
- View/download PDF
9. Impact of inward turbulence spreading on energy loss of edge-localized modes
- Author
-
Kim, S. [NFRI, Daejeon (South Korea). WCI Center for Fusion Theory]
- Published
- 2015
- Full Text
- View/download PDF
10. Implementation of a generalized actuator disk wind turbine model into the weather research and forecasting model for large-eddy simulation applications
- Author
-
Lundquist, J. [Univ. of Colorado, Boulder, CO (United States); National Renewable Energy Lab., Golden, CO (United States)]
- Published
- 2014
- Full Text
- View/download PDF
11. Isolation, decomposition, and mechanisms of the aerodynamic nonlinearity and flow field phenomenology of structure-motion-induced dynamics in fluid-structure interactions
- Abstract
This study focuses on the aerodynamic nonlinearity and flow field phenomenology of structure-motion-induced dynamics in fluid-structure interactions (FSI), which is essential for response prediction. Through dynamic-meshing large-eddy simulations with near-wall resolution, the nonlinear aerodynamic damping in the still wind has been isolated by forced vibration, and its phenomenological characteristics and physical mechanisms have been analyzed. The results show that nonlinear aerodynamic damping can account for up to 30% of the total damping, which cannot be ignored in response prediction. The study also reveals that the three-dimensional vorticity dynamics vary nonlinearly with structure motion, leading to the hysteresis effect between aerodynamic forces and displacement. Furthermore, in-depth phenomenological analysis discloses eight types of coherent flow field substructures, including the Stick, Phone, Bowknot, Crutch, Droplet, Bat, Horn, and Flag substructures, which are solely induced by structural motion. Insights into these substructures' formation, evolvement, dissipation, and superposable magnitude have been disclosed. This research offers a new perspective on understanding the physical nature of aerodynamic damping in FSI, serving as a reference for various FSI applications, including bridges, high-building design, and other related fields.
- Published
- 2023
12. Effects of time-filtering the Navier-Stokes equations
- Author
-
Oberle, Daniel, Pruett, C. David, and Jenny, Patrick
- Subjects
Doppler effect ,Linear filters ,Fourier analysis ,Frequency spectrum ,Computational fluid dynamics ,Turbulence theory and modelling ,Navier Stokes equations ,Turbulence simulations ,Turbulent flows - Abstract
The underlying premise of temporal large eddy simulation (TLES) is that the attenuation of high-frequency content also attenuates the high-wavenumber content. Yet, to date, the effect in wavenumber space of removing high-frequency oscillations by time-domain filtering is not well understood. In this work, we numerically investigate the relationship between the frequency and wavenumber with particular attention to the role of the temporal residual-stress in TLES. Moreover, since under-resolved simulations that use high-order, non-dissipative numerical methods require some measure of artificial dissipation for stabilization, we also discuss the regularization term with practical relevance to under-resolved applications of TLES. Specifically, we analyze the effects of Eulerian time-domain filtering with a causal exponential filter on homogeneous isotropic turbulence. The data are generated by direct numerical simulation of the Navier-Stokes equations, which are driven to maintain an average Reynolds number ( Reλ ) of 200. A priori, Fourier transformations of the velocity fields were performed in order to compute the unfiltered and filtered energy and dissipation spectra in both wavenumber space and wavenumber-frequency space. Furthermore, the amount of unresolved dissipation of an insufficiently resolved simulation was approximated in an attempt to estimate the required additional artificial dissipation. The results indicate that the numerically motivated stabilization term can be reduced due to temporal filtering. Moreover, it has been shown that a sharp cutoff in the frequency domain does not translate into a sharp cutoff in the wavenumber space. Thus, a hybrid model that combines temporal filtering for the residual-stress and spatial filtering for stabilization might be advantageous., Physics of Fluids, 35 (6), ISSN:1070-6631, ISSN:1089-7666, ISSN:0031-9171
- Published
- 2023
13. Numerically stable formulations of convective terms for turbulent compressible flows.
- Author
-
Coppola, G., Capuano, F., Pirozzoli, S., and de Luca, L.
- Subjects
- *
COMPRESSIBLE flow , *TURBULENT flow , *NAVIER-Stokes equations , *VISCOSITY , *CONVECTIVE flow , *ENERGY conservation - Abstract
Abstract A systematic analysis of the discrete conservation properties of non-dissipative, central-difference approximations of the compressible Navier–Stokes equations is reported. A generalized splitting of the nonlinear convective terms is considered, and energy-preserving formulations are fully characterized by deriving a two-parameter family of split forms. Previously developed formulations reported in literature are shown to be particular members of this family; novel splittings are introduced and discussed as well. Furthermore, the conservation properties yielded by different choices for the energy equation (i.e. total and internal energy, entropy) are analyzed thoroughly. It is shown that additional preserved quantities can be obtained through a suitable adaptive selection of the split form within the derived family. Local conservation of primary invariants, which is a fundamental property to build high-fidelity shock-capturing methods, is also discussed in the paper. Numerical tests performed for the Taylor–Green Vortex at zero viscosity fully confirm the theoretical findings, and show that a careful choice of both the splitting and the energy formulation can provide remarkably robust and accurate results. Highlights • Energy-preserving split forms in compressible flow equations are studied. • A novel two-parameter family of energy-preserving splittings is derived. • A dynamic splitting procedure with optimal conservation properties is proposed. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
14. Virus transmission by aerosol transport during short conversations
- Abstract
The SARS-CoV-2 is transmitted not only through coughing, but also through breathing, speaking or singing. We perform direct numerical simulations of the turbulent transport of potentially infectious aerosols in short conversations, involving repetitive phrases separated by quiescent intervals. We estimate that buoyancy effects due to droplet evaporation are small, and neglect them. A two-way conversation is shown to significantly reduce the aerosol exposure compared with a relative monologue by one person and relative silence of the other. This is because of the 'cancelling' effect produced by the two interacting speech jets. Unequal conversation is shown to significantly increase the infection risk to the person who talks less. Interestingly, a small height difference is worse for infection spread, due to reduced interference between the speech jets, than two faces at the same level. For small axial separation, speech jets show large oscillations and reach the other person intermittently. We suggest a range of lateral separations between two people to minimize transmission risk. A realistic estimate of the infection probability is provided by including exposure through the eyes and mouth, in addition to the more common method of using inhaled virions alone. We expect that our results will provide useful inputs to epidemiological models and to disease management., QC 20230213
- Published
- 2022
- Full Text
- View/download PDF
15. Explicit Runge–Kutta schemes for incompressible flow with improved energy-conservation properties.
- Author
-
Capuano, F., Coppola, G., Rández, L., and de Luca, L.
- Subjects
- *
RUNGE-Kutta formulas , *NAVIER-Stokes equations , *INCOMPRESSIBLE flow , *EDDIES , *ENERGY conservation , *CONVECTIVE flow - Abstract
The application of pseudo-symplectic Runge–Kutta methods to the incompressible Navier–Stokes equations is discussed in this work. In contrast to fully energy-conserving, implicit methods, these are explicit schemes of order p that preserve kinetic energy to order q , with q > p . Use of explicit methods with improved energy-conservation properties is appealing for convection-dominated problems, especially in case of direct and large-eddy simulation of turbulent flows. A number of pseudo-symplectic methods are constructed for application to the incompressible Navier–Stokes equations and compared in terms of accuracy and efficiency by means of numerical simulations. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
16. Quality Measures of Mixing in Turbulent Flow and Effects of Molecular Diffusivity
- Author
-
Quoc Nguyen and Dimitrios V. Papavassiliou
- Subjects
turbulent transport ,turbulent mixing ,Lagrangian modeling ,turbulence simulations ,Thermodynamics ,QC310.15-319 ,Descriptive and experimental mechanics ,QC120-168.85 - Abstract
Results from numerical simulations of the mixing of two puffs of scalars released in a turbulent flow channel are used to introduce a measure of mixing quality, and to investigate the effectiveness of turbulent mixing as a function of the location of the puff release and the molecular diffusivity of the puffs. The puffs are released from instantaneous line sources in the flow field with Schmidt numbers that range from 0.7 to 2400. The line sources are located at different distances from the channel wall, starting from the wall itself, the viscous wall layer, the logarithmic layer, and the channel center. The mixing effectiveness is quantified by following the trajectories of individual particles with a Lagrangian approach and carefully counting the number of particles from both puffs that arrive at different locations in the flow field as a function of time. A new measure, the mixing quality index Ø, is defined as the product of the normalized fraction of particles from the two puffs at a flow location. The mixing quality index can take values from 0, corresponding to no mixing, to 0.25, corresponding to full mixing. The mixing quality in the flow is found to depend on the Schmidt number of the puffs when the two puffs are released in the viscous wall region, while the Schmidt number is not important for the mixing of puffs released outside the logarithmic region.
- Published
- 2018
- Full Text
- View/download PDF
17. Statistically steady states of forced isotropic turbulence in thermal equilibrium and non-equilibrium.
- Author
-
Donzis, Diego A. and Maqui, Agustin F.
- Subjects
TURBULENT flow ,VIBRATION (Mechanics) ,THERMAL equilibrium ,DEGREES of freedom ,MACH number ,REYNOLDS number - Abstract
We investigate statistically steady states of turbulent flows when molecular degrees of freedom, in particular vibration, are taken into account. Unlike laminar flows initially in thermal non-equilibrium which asymptotically relax towards thermal equilibrium, turbulent flows present persistent departures from thermal equilibrium. This is due to fluctuations in temperature and other thermodynamic variables, which are known to increase with turbulent Mach number. Analytical results are compared to direct numerical simulations at a range of Reynolds and Mach numbers as well as molecular parameters such as relaxation times. Turbulent fluctuations are also shown to disrupt the distribution of energy between translational-rotational-vibrational modes even if thermal equilibrium is attained instantaneously relative to turbulence time scales, an effect that increases with characteristic relaxation times. Because of the nonlinear relation between temperature and vibrational energy in equilibrium, the fluctuation of the latter could be strongly positively skewed with long tails in its probability density function. This effect is stronger in flows with strong temperature fluctuations and when vibrational modes are partially excited. Because of the finite-time relaxation of vibration, departures from equilibrium result in very strong transfers of energy from the translational-rotational mode to the vibrational mode. A simple spectral model can explain the stronger departures from thermal equilibrium observed at the small scales. The spectral behaviour of the instantaneous vibrational energy can be described by classical phenomenology for passive scalars. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
18. Numerical treatment of the energy equation in compressible flows simulations.
- Author
-
De Michele, C. and Coppola, G.
- Subjects
- *
FLOW simulations , *SPEED of sound , *EQUATIONS , *NAVIER-Stokes equations , *EULER equations , *COMPRESSIBLE flow , *ENTHALPY - Abstract
We analyze the conservation properties of various discretizations of the system of compressible Euler equations for shock-free flows, with special focus on the treatment of the energy equation and on the induced discrete equations for other thermodynamic quantities. The analysis is conducted both theoretically and numerically and considers two important factors characterizing the various formulations, namely the choice of the energy equation and the splitting used in the discretization of the convective terms. The energy equations analyzed are total and internal energy, total enthalpy, pressure, speed of sound and entropy. In all the cases examined the discretization of the convective terms is made with locally conservative and kinetic-energy preserving schemes. Some important relations between the various formulations are highlighted and the performances of the various schemes are assessed by considering two widely used test cases. Together with some popular formulations from the literature, also new and potentially useful ones are analyzed. • Analysis of the conservation properties of the discretizations of the compressible Euler equations. • Study of induced discrete evolution of derived thermodynamic quantities. • Assessment of the robustness with respect to energy variable and split form choice. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
19. The effect of electrostatic charges on particle-laden duct flows
- Author
-
Claus Bissinger, Holger Grosshans, Mathieu Calero, Miltiadis Papalexandris, and UCL - SST/IMMC/TFL - Thermodynamics and fluid mechanics
- Subjects
Prandtl number ,FOS: Physical sciences ,02 engineering and technology ,particle laden flows ,01 natural sciences ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,symbols.namesake ,turbulence simulations ,Particle dynamics ,0103 physical sciences ,Streamlines, streaklines, and pathlines ,Duct (flow) ,particle/fluid flow ,Physics ,Mechanical Engineering ,Fluid Dynamics (physics.flu-dyn) ,Reynolds number ,Mechanics ,Physics - Fluid Dynamics ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Aerodynamic force ,Mechanics of Materials ,symbols ,Carrier fluid ,0210 nano-technology ,electrohydrodynamics - Abstract
We report on direct numerical simulations of the effect of electrostatic charges on particle-laden duct flows. The corresponding electrostatic forces are known to affect particle dynamics at small scales and the associated turbophoretic drift. Our simulations, however, predicted that electrostatic forces also dominate the vortical motion of the particles, induced by the secondary flows of Prandtl's second kind of the carrier fluid. Herein we treated flows at two frictional Reynolds numbers ($Re_\mathrm{\tau}=$ 300 and~600), two particle-to-gas density ratios ($\rho_\mathrm{p}/\rho=$ 1000 and 7500), and three Coulombic-to-gravitational force ratios ($F_\mathrm{el}/F_\mathrm{g}=$ 0, 0.004, and 0.026). In flows with a high density ratio at $Re_\mathrm{\tau}=$ 600 and $F_\mathrm{el}/F_\mathrm{g}=$ 0.004, the particles tend to accumulate at the walls. On the other hand, at a lower density ratio, respectively a higher $F_\mathrm{el}/F_\mathrm{g}$ of 0.026, the charged particles still follow the secondary flow structures that are developed in the duct. However, even in this case, the electrostatic forces counteract the particles' inward flux from the wall and, as a result, their vortical motion in these secondary structures is significantly attenuated. This change in the flow pattern results in an increase of the particle number density at the bisectors of the walls by a factor of five compared to the corresponding flow with uncharged particles. Finally, at $Re_\mathrm{\tau}=$ 300, $\rho_\mathrm{p}/\rho=$ 1000, and $F_\mathrm{el}/F_\mathrm{g}=$ 0.026 the electrostatic forces dominate over the aerodynamic forces and gravity and, consequently the particles no longer follow the streamlines of the carrier gas.
- Published
- 2020
20. Analysis of pedestal plasma transport
- Author
-
Stacey, W [Georgia Institute of Technology]
- Published
- 2010
- Full Text
- View/download PDF
21. Imposing virtual origins on the velocity components in direct numerical simulations
- Author
-
Garazi Gómez-de-Segura, Ricardo García-Mayoral, and Apollo - University of Cambridge Repository
- Subjects
Fluid Flow and Transfer Processes ,Offset (computer science) ,Turbulence ,Mechanical Engineering ,Boundary plane ,Turbulent boundary layers ,02 engineering and technology ,Mechanics ,Surface finish ,Turbulence simulations ,Condensed Matter Physics ,01 natural sciences ,Robin boundary condition ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,Flow control ,020303 mechanical engineering & transports ,0203 mechanical engineering ,Drag ,0103 physical sciences ,Mean flow ,Boundary value problem ,Geology - Abstract
The relative wall-normal displacement of the origin perceived by different components of near-wall turbulence is known to produce a change in drag. This effect is produced for instance by drag-reducing surfaces of small texture-size like riblets and superhydrophobic surfaces. To facilitate the research on how these displacements alter near-wall turbulence, this paper studies different strategies to model such displacement effect through manipulated boundary conditions. Previous research has considered the effect of offsetting the virtual origins perceived by the tangential components of the velocity from the reference, boundary plane, where the wall-normal velocity was set to zero. These virtual origins are typically characterised by slip-length coefficients in Robin, slip-like boundary conditions. In this paper, we extend this idea and explore several techniques to define and implement virtual origins for all three velocity components on direct numerical simulations (DNSs) of channel flows, with special emphasis on the wall-normal velocity. The aim of this work is to provide a suitable foundation to extend the existing understanding on how these virtual origins affect the near-wall turbulence, and ultimately aid in the formulation of simplified models that capture the effect of complex surfaces on the overlying flow and on drag, without the need to resolve fully the turbulence and the surface texture. From the techniques tested, Robin boundary conditions for all three velocities are found to be the most satisfactory method to impose virtual origins, relating the velocity components to their respective wall-normal gradients linearly. Our results suggest that the effect of virtual origins on the flow, and hence the change in drag that they produce, can be reduced to an offset between the virtual origin perceived by the mean flow and that perceived by the overlying turbulence, and that turbulence remains otherwise smooth-wall-like, as proposed by Luchini (1996). The origin for turbulence, however, would not be set by the spanwise virtual origin alone, but by a combination of the spanwise and wall-normal origins. These observations suggest the need for an extension of Luchini’s virtual-origin theory to predict the change in drag, accounting for the wall-normal transpiration when its effect is not negligible.
- Published
- 2020
- Full Text
- View/download PDF
22. An efficient particle tracking algorithm for large-scale parallel pseudo-spectral simulations of turbulence.
- Author
-
Lalescu, Cristian C., Bramas, Bérenger, Rampp, Markus, and Wilczek, Michael
- Subjects
- *
TRACKING algorithms , *PARALLEL algorithms , *TURBULENCE , *TURBULENT flow , *FLOW simulations - Abstract
Particle tracking in large-scale numerical simulations of turbulent flows presents one of the major bottlenecks in parallel performance and scaling efficiency. Here, we describe a particle tracking algorithm for large-scale parallel pseudo-spectral simulations of turbulence which scales well up to billions of tracer particles on modern high-performance computing architectures. We summarize the standard parallel methods used to solve the fluid equations in our hybrid MPI/OpenMP implementation. As the main focus, we describe the implementation of the particle tracking algorithm and document its computational performance. To address the extensive inter-process communication required by particle tracking, we introduce a task-based approach to overlap point-to-point communications with computations, thereby enabling improved resource utilization. We characterize the computational cost as a function of the number of particles tracked and compare it with the flow field computation, showing that the cost of particle tracking is very small for typical applications. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
23. Parametric forcing approach to rough-wall turbulent channel flow.
- Author
-
Busse, A. and Sandham, N. D.
- Subjects
CHANNEL flow ,FLUID dynamics ,BOUNDARY layer (Aerodynamics) ,AERODYNAMICS ,TURBULENCE - Abstract
The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier–Stokes equations. This force term contains two parameters, related to the density and the height of the roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the equivalent sand grain roughness. The effects of the roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions, it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity for both the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the roughness layer. In extremely rough cases, spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian, Andersson & Manhart (Intl J. Heat Fluid Flow, vol. 25, 2004, pp. 373–383) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglected. [ABSTRACT FROM PUBLISHER]
- Published
- 2012
- Full Text
- View/download PDF
24. Numerically stable formulations of convective terms for turbulent compressible flows
- Abstract
A systematic analysis of the discrete conservation properties of non-dissipative, central-difference approximations of the compressible Navier–Stokes equations is reported. A generalized splitting of the nonlinear convective terms is considered, and energy-preserving formulations are fully characterized by deriving a two-parameter family of split forms. Previously developed formulations reported in literature are shown to be particular members of this family; novel splittings are introduced and discussed as well. Furthermore, the conservation properties yielded by different choices for the energy equation (i.e. total and internal energy, entropy) are analyzed thoroughly. It is shown that additional preserved quantities can be obtained through a suitable adaptive selection of the split form within the derived family. Local conservation of primary invariants, which is a fundamental property to build high-fidelity shock-capturing methods, is also discussed in the paper. Numerical tests performed for the Taylor–Green Vortex at zero viscosity fully confirm the theoretical findings, and show that a careful choice of both the splitting and the energy formulation can provide remarkably robust and accurate results., Peer Reviewed, Postprint (author's final draft)
- Published
- 2019
25. Lagrangian block hydrodynamics for environmental fluid mechanics simulations.
- Author
-
Tan, Lai-wai and Chu, Vincent H.
- Abstract
The Lagrangian block hydrodynamics is formulated based on the block advection of fluid. By enforcing the mass and momentum conservations on the Lagrangian mesh, the numerical oscillation problem encountered in the classical Eulerian computational methods is circumvented. A large number of the previously computationally difficult problems in environmental fluid mechanics are successfully simulated using the method. Examples of these simulations are described in this paper. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
26. Magnetic helicity fluxes in an α2 dynamo embedded in a halo.
- Author
-
Hubbard, Alexander and Brandenburg, Axel
- Subjects
- *
TURBULENCE , *DYNAMICS , *MAGNETIC fields , *STRAINS & stresses (Mechanics) , *BUFFER states (International relations) , *COMPUTER simulation - Abstract
We present the results of simulations of forced turbulence in a slab where the mean kinetic helicity has a maximum near the mid-plane, generating gradients of magnetic helicity of both large and small-scale fields. We also study systems that have poorly conducting buffer zones away from the midplane in order to assess the effects of boundaries. The dynamical α quenching phenomenology requires that the magnetic helicity in the small-scale fields approaches a nearly static, gauge independent state. To stress-test this steady state condition we choose a system with a uniform sign of kinetic helicity, so that the total magnetic helicity can reach a steady state value only through fluxes through the boundary, which are themselves suppressed by the velocity boundary conditions. Even with such a set up, the small-scale magnetic helicity is found to reach a steady state. In agreement with the earlier work, the magnetic helicity fluxes of small-scale fields are found to be turbulently diffusive. By comparing results with and without halos, we show that artificial constraints on magnetic helicity at the boundary do not have a significant impact on the evolution of the magnetic helicity, except that 'softer' (halo) boundary conditions give a lower energy of the saturated mean magnetic field. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
27. Asymptotic-Preserving methods and multiscale models for plasma physics
- Author
-
Pierre Degond, Fabrice Deluzet, The Royal Society, Department of Mathematics [Imperial College London], Imperial College London, Institut de Mathématiques de Toulouse UMR5219 (IMT), Institut National des Sciences Appliquées - Toulouse (INSA Toulouse), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Université Toulouse 1 Capitole (UT1), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Université Toulouse - Jean Jaurès (UT2J)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS), Université Toulouse Capitole (UT Capitole), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut National des Sciences Appliquées - Toulouse (INSA Toulouse), Institut National des Sciences Appliquées (INSA)-Université de Toulouse (UT)-Institut National des Sciences Appliquées (INSA)-Université Toulouse - Jean Jaurès (UT2J), Université de Toulouse (UT)-Université Toulouse III - Paul Sabatier (UT3), and Université de Toulouse (UT)-Centre National de la Recherche Scientifique (CNRS)
- Subjects
Anisotropic elliptic equations ,Technology ,Physics and Astronomy (miscellaneous) ,NAVIER-STOKES EQUATIONS ,RUNGE-KUTTA SCHEMES ,010103 numerical & computational mathematics ,01 natural sciences ,09 Engineering ,Plasma ,[MATH.MATH-MP]Mathematics [math]/Mathematical Physics [math-ph] ,Statistical physics ,Limit (mathematics) ,Diffusion (business) ,Anisotropy ,Navier–Stokes equations ,Mathematical Physics ,ComputingMilieux_MISCELLANEOUS ,Physics ,Numerical Analysis ,02 Physical Sciences ,Applied Mathematics ,Mathematical Physics (math-ph) ,PARTICLE SIMULATION ,Asymptotic-Preserving method ,Computer Science Applications ,Magnetic field ,Physics, Mathematical ,010101 applied mathematics ,Computational Mathematics ,Classical mechanics ,ANISOTROPIC ELLIPTIC PROBLEMS ,Modeling and Simulation ,Physical Sciences ,symbols ,MICRO-MACRO DECOMPOSITION ,Computer Science, Interdisciplinary Applications ,QUASI-NEUTRAL LIMIT ,Quasi-neutrality ,Singular perturbation ,FOS: Physical sciences ,Singular limit ,TOKAMAK TURBULENCE ,symbols.namesake ,NUMERICAL TRANSPORT PROBLEMS ,Physics::Plasma Physics ,0101 mathematics ,Debye length ,01 Mathematical Sciences ,Science & Technology ,KINETIC-EQUATIONS ,Physics - Plasma Physics ,Plasma Physics (physics.plasm-ph) ,Computer Science ,TURBULENCE SIMULATIONS ,Drift limit - Abstract
The purpose of the present paper is to provide an overview of As ymptotic- Preserving methods for multiscale plasma simulations by ad dressing three sin- gular perturbation problems. First, the quasi-neutral lim it of fluid and kinetic models is investigated in the framework of non magnetized as well as magne- tized plasmas. Second, the drift limit for fluid description s of thermal plasmas under large magnetic fields is addressed. Finally efficient nu merical resolutions of anisotropic elliptic or diffusion equations arising in ma gnetized plasma simu- lation are reviewed.
- Published
- 2017
28. INTERSTELLAR TURBULENCE I: Observations and Processes.
- Author
-
Elmegreen, Bruce G. and Scalo, John
- Subjects
- *
TURBULENCE , *FLUID dynamics , *AERODYNAMIC noise , *ASTROPHYSICS , *ASTRONOMY - Abstract
Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structure functions, the direction and locality of energy transfer and cascade, velocity probability distributions, and turbulent pressure. We emphasize expected differences between incompressible and compressible turbulence. Theories of magnetic turbulence on scales smaller than the collision mean free path are included, as are theories of magneto-hydrodynamic turbulence and their various proposals for power spectra. Numerical simulations of interstellar turbulence are reviewed. Models have reproduced the basic features of the observed scaling relations, predicted fast decay rates for supersonic MHD turbulence, and derived probability distribution functions for density. Thermal instabilities and thermal phases have a new interpretation in a supersonically turbulent medium. Large-scale models with various combinations of self-gravity, magnetic fields, supernovae, and star formation are beginning to resemble the observed interstellar medium in morphology and... [ABSTRACT FROM AUTHOR]
- Published
- 2004
- Full Text
- View/download PDF
29. The statistical distributions of one-dimensional “turbulence”
- Author
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Peyrard, Michel
- Subjects
- *
TURBULENCE , *HARMONIC oscillators , *HARMONIC motion , *DISTRIBUTION (Probability theory) - Abstract
We study a one-dimensional discrete analog of the von Kármán flow widely investigated in turbulence, made of a lattice of anharmonic oscillators excited by both ends in the presence of a dissipative term proportional to the second-order finite difference of the velocities, similar to the viscous term in a fluid. The dynamics of the model shows striking similarities with an actual turbulent flow, both at local and global scales. Calculations of the probability distribution function of velocity increments, extensively studied in turbulence, with a very large number of points in order to determine accurately the statistics of rare events, allow us to provide a meaningful comparison of different theoretical expressions of the PDFs. [Copyright &y& Elsevier]
- Published
- 2004
- Full Text
- View/download PDF
30. A direct numerical simulation study of the influence of flame-generated vorticity on reaction-zone-surface area in weakly turbulent premixed combustion
- Author
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Shinnosuke Nishiki, Tatsuya Hasegawa, Andrei Lipatnikov, Vladimir Sabelnikov, Chalmers University of Technology [Göteborg], DMPE, ONERA, Université Paris Saclay (COmUE) [Palaiseau], ONERA-Université Paris Saclay (COmUE), Central Aerohydrodynamic Institute (TsAGI), Kagoshima University, Nagoya University, DMPE, ONERA, Université Paris Saclay [Palaiseau], and ONERA-Université Paris-Saclay
- Subjects
Baroclinity ,SURFACE DYNAMICS ,Computational Mechanics ,Direct numerical simulation ,Combustion ,Enstrophy ,01 natural sciences ,010305 fluids & plasmas ,FLOW DYNAMICS ,Physics::Fluid Dynamics ,COMBUSTION ,[SPI]Engineering Sciences [physics] ,NAVIER STOKES EQUATIONS ,0103 physical sciences ,TURBULENT FLOWS ,Stratified flow ,Physics::Chemical Physics ,Fluid Flow and Transfer Processes ,Physics ,[PHYS]Physics [physics] ,010304 chemical physics ,Turbulence ,Mechanical Engineering ,Mechanics ,Vorticity ,VORTEX DYNAMICS ,Condensed Matter Physics ,Vortex ,COMPUTATIONAL FLUID DYNAMICS ,FLAMES ,13. Climate action ,Mechanics of Materials ,THERMAL EFFECTS ,TURBULENCE SIMULATIONS - Abstract
International audience; Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., σ = 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.
- Published
- 2019
31. On the modelling of wavepacket scattering noise with coherence effects
- Author
-
Peter Jordan, André V. G. Cavalieri, Filipe D. da Silva, Instituto de Plasmas e Fusão Nuclear [Lisboa] (IPFN), Instituto Superior Técnico, Universidade Técnica de Lisboa (IST), Acoustique, Aérodynamique, Turbulence (2AT ), Département Fluides, Thermique et Combustion (FTC), Institut Pprime (PPRIME), Université de Poitiers-Centre National de la Recherche Scientifique (CNRS)-ENSMA-Université de Poitiers-Centre National de la Recherche Scientifique (CNRS)-ENSMA-Institut Pprime (PPRIME), Université de Poitiers-Centre National de la Recherche Scientifique (CNRS)-ENSMA-Université de Poitiers-Centre National de la Recherche Scientifique (CNRS)-ENSMA, Université de Poitiers-Centre National de la Recherche Scientifique (CNRS)-ENSMA, ENSMA-Centre National de la Recherche Scientifique (CNRS)-Université de Poitiers-ENSMA-Centre National de la Recherche Scientifique (CNRS)-Université de Poitiers-Institut Pprime (PPRIME), ENSMA-Centre National de la Recherche Scientifique (CNRS)-Université de Poitiers-ENSMA-Centre National de la Recherche Scientifique (CNRS)-Université de Poitiers, and ENSMA-Centre National de la Recherche Scientifique (CNRS)-Université de Poitiers
- Subjects
Signal processing ,Acoustics and Ultrasonics ,Wave packet ,Astrophysics::High Energy Astrophysical Phenomena ,Entropy ,02 engineering and technology ,Turbulence simulations ,01 natural sciences ,Source field ,Directivity ,Acoustic analogies ,010305 fluids & plasmas ,symbols.namesake ,0203 mechanical engineering ,Arts and Humanities (miscellaneous) ,0103 physical sciences ,Trailing edge ,Acoustical properties ,Fluid mechanics ,Physics ,Scattering ,Coordinate system ,[SPI.FLUID]Engineering Sciences [physics]/Reactive fluid environment ,Computer simulation ,Acoustic field ,Computational physics ,Noise ,020303 mechanical engineering & transports ,Mach number ,symbols ,High Energy Physics::Experiment ,Acoustic noise ,Large eddy simulation - Abstract
International audience; An investigation of a wavepacket model for free-jet and jet-surface interaction noise was conducted. The source term for the axisymmetric mode was extracted from a Mach 0.9 jet large eddy simulation and employed to adjust the parameters of a simple source model. Streamwise coherence decay, in particular, was considered. The source model was propagated with both the free-field and tailored Green's function for a semi-infinite flat plate positioned at a distance of r/D = 1 from the jet axis. Significant deviations were observed in the prediction of the low-angle directivity of the isolated jet as well as in the reproduction of the characteristics of the source field. However, the effects of trailing edge noise were well reproduced. The installed jet case, at the region dominated by trailing-edge scattering, showed very little sensitivity to the coherence decay, a crucial feature in the isolated jet case. In this sense, the modelling of the installed-jet case proved to be much simpler.
- Published
- 2019
32. Implicit Large-Eddy Simulation of a Wingtip Vortex
- Author
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Julien F. A. Hoessler, David Moxey, Mark J. Taylor, Spencer J. Sherwin, Jean-Eloi W. Lombard, Sridar Dhandapani, Engineering & Physical Science Research Council (EPSRC), McLaren Racing Limited, and Royal Academy Of Engineering
- Subjects
Technology ,STABILIZATION ,Engineering ,TIP VORTEX ,4-VORTEX AIRCRAFT WAKE ,Direct numerical simulation ,Aerospace Engineering ,01 natural sciences ,0901 Aerospace Engineering ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,symbols.namesake ,0103 physical sciences ,Aerospace & Aeronautics ,0101 mathematics ,Aerospace engineering ,Engineering, Aerospace ,SVV-LES ,Science & Technology ,business.industry ,Reynolds number ,Mechanics ,Solver ,ELEMENT METHODS ,NACA airfoil ,Vortex ,010101 applied mathematics ,SPECTRAL VANISHING VISCOSITY ,Boundary layer ,symbols ,Wingtip vortices ,UNSTEADY-FLOW ,business ,Reynolds-averaged Navier–Stokes equations ,TRAILING VORTICES ,NEAR-FIELD ,TURBULENCE SIMULATIONS ,0913 Mechanical Engineering - Abstract
In this article, recent developments in numerical methods for performing a large-eddy simulation of the formation and evolution of a wingtip vortex are presented. The development of these vortices in the near wake, in combination with the large Reynolds numbers present in these cases, makes these types of test cases particularly challenging to investigate numerically. First, an overview is given of the spectral vanishing viscosity/implicit large-eddy simulation solver that is used to perform the simulations, and techniques are highlighted that have been adopted to solve various numerical issues that arise when studying such cases. To demonstrate the method’s viability, results are presented from numerical simulations of flow over a NACA 0012 profile wingtip at Rec=1.2×106 and they are compared against experimental data, which is to date the highest Reynolds number achieved for a large-eddy simulation that has been correlated with experiments for this test case. The model in this paper correlates favorably...
- Published
- 2016
33. Numerically stable formulations of convective terms for turbulent compressible flows
- Author
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L. de Luca, Gennaro Coppola, Sergio Pirozzoli, Francesco Capuano, Coppola, G., Capuano, F., Pirozzoli, Sergio, de Luca, Luigi, Universitat Politècnica de Catalunya. Departament de Mecànica de Fluids, and Universitat Politècnica de Catalunya. GReCEF- Grup de Recerca en Ciència i Enginyeria de Fluids
- Subjects
Convection ,Physics and Astronomy (miscellaneous) ,Compressible Navier–Stokes equations ,FOS: Physical sciences ,010103 numerical & computational mathematics ,Computational fluid dynamics ,Turbulence simulations ,Energy conservation ,01 natural sciences ,Applied mathematics ,0101 mathematics ,Entropy (energy dispersal) ,Compressible Navier–Stokes equation ,Mathematics ,Numerical Analysis ,Internal energy ,Turbulence ,Applied Mathematics ,Fluid Dynamics (physics.flu-dyn) ,Physics - Fluid Dynamics ,Dinàmica de fluids computacional ,Computational Physics (physics.comp-ph) ,Computer Science Applications ,Vortex ,010101 applied mathematics ,Computational Mathematics ,Nonlinear system ,Modeling and Simulation ,Compressibility ,compressible ,Navier–Stokes ,equations ,energy ,conservation ,turbulence ,simulations ,Physics - Computational Physics ,Enginyeria mecànica::Mecànica de fluids [Àrees temàtiques de la UPC] - Abstract
A systematic analysis of the discrete conservation properties of non-dissipative, central-difference approximations of the compressible Navier–Stokes equations is reported. A generalized splitting of the nonlinear convective terms is considered, and energy-preserving formulations are fully characterized by deriving a two-parameter family of split forms. Previously developed formulations reported in literature are shown to be particular members of this family; novel splittings are introduced and discussed as well. Furthermore, the conservation properties yielded by different choices for the energy equation (i.e. total and internal energy, entropy) are analyzed thoroughly. It is shown that additional preserved quantities can be obtained through a suitable adaptive selection of the split form within the derived family. Local conservation of primary invariants, which is a fundamental property to build high-fidelity shock-capturing methods, is also discussed in the paper. Numerical tests performed for the Taylor–Green Vortex at zero viscosity fully confirm the theoretical findings, and show that a careful choice of both the splitting and the energy formulation can provide remarkably robust and accurate results.
- Published
- 2018
- Full Text
- View/download PDF
34. Imposing virtual origins on the velocity components in direct numerical simulations.
- Author
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Gómez-de-Segura, Garazi and García-Mayoral, Ricardo
- Subjects
- *
DRAG reduction , *VELOCITY , *COMPUTER simulation , *TURBULENT boundary layer , *SUPERHYDROPHOBIC surfaces , *SURFACE texture , *CHANNEL flow - Abstract
• Methods to impose virtual origins on the velocities in turbulent direct simulations are studied. • Robin conditions for all three components are identified as a suitable method to impose such origins. • The origin for near-wall turbulence is controlled by the spanwise and wall-normal virtual origins. • The origin for the mean velocity profile, in turn, is set by the streamwise origin. The relative wall-normal displacement of the origin perceived by different components of near-wall turbulence is known to produce a change in drag. This effect is produced for instance by drag-reducing surfaces of small texture-size like riblets and superhydrophobic surfaces. To facilitate the research on how these displacements alter near-wall turbulence, this paper studies different strategies to model such displacement effect through manipulated boundary conditions. Previous research has considered the effect of offsetting the virtual origins perceived by the tangential components of the velocity from the reference, boundary plane, where the wall-normal velocity was set to zero. These virtual origins are typically characterised by slip-length coefficients in Robin, slip-like boundary conditions. In this paper, we extend this idea and explore several techniques to define and implement virtual origins for all three velocity components on direct numerical simulations (DNSs) of channel flows, with special emphasis on the wall-normal velocity. The aim of this work is to provide a suitable foundation to extend the existing understanding on how these virtual origins affect the near-wall turbulence, and ultimately aid in the formulation of simplified models that capture the effect of complex surfaces on the overlying flow and on drag, without the need to resolve fully the turbulence and the surface texture. From the techniques tested, Robin boundary conditions for all three velocities are found to be the most satisfactory method to impose virtual origins, relating the velocity components to their respective wall-normal gradients linearly. Our results suggest that the effect of virtual origins on the flow, and hence the change in drag that they produce, can be reduced to an offset between the virtual origin perceived by the mean flow and that perceived by the overlying turbulence, and that turbulence remains otherwise smooth-wall-like, as proposed by Luchini (1996). The origin for turbulence, however, would not be set by the spanwise virtual origin alone, but by a combination of the spanwise and wall-normal origins. These observations suggest the need for an extension of Luchini's virtual-origin theory to predict the change in drag, accounting for the wall-normal transpiration when its effect is not negligible. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
35. Boundary-consistent B-spline filtering schemes and application to high-fidelity simulations of turbulence.
- Author
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Bay, Yong Yi, Bodony, Daniel J., and Freund, Jonathan B.
- Subjects
- *
BURGERS' equation , *TURBULENCE , *MATHEMATICAL ability , *CHANNEL flow , *FINITE differences - Abstract
• B-spline-based filters are provably dissipative for periodic and bounded domains. • A dynamic procedure outperforms a priori selection of filtering parameters. • An efficient implementation algorithm makes cost comparable to compact (implicit) finite difference filters. A filtering operation, based on B-spline discretizations, is introduced to target weakly growing mesh-scale oscillations that can arise in high-fidelity turbulence simulations. This is a spectral regularization that can be described using the singular values of a banded matrix operator, with the filtering strength set by a scalar- or vector-valued penalty parameter. The penalty parameter can be specified though it can also be advantageously selected to minimize the generalized cross validation (GCV) measure of distance between the pre- and post-filtered solutions. Efficient algorithms are developed to compute both the scalar and vector penalty parameters. The B-spline filter has a sharper localization to high-wavenumber than compact or explicit filters of the same stencil width and is demonstrated for solutions of the Burgers' equation, decaying Burgers' turbulence, and compressible Navier–Stokes turbulent channel flow. These simulations confirm the scheme's numerical stability and ability to narrowly target the high wavenumber components of numerical solutions. An advantage over finite-difference filters is that these B-spline filters are stable on bounded domains and even preserve formal order of accuracy. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
36. Explicit Runge–Kutta schemes for incompressible flow with improved energy-conservation properties
- Abstract
The application of pseudo-symplectic Runge–Kutta methods to the incompressible Navier–Stokes equations is discussed in this work. In contrast to fully energy-conserving, implicit methods, these are explicit schemes of order p that preserve kinetic energy to order q, with q>p. Use of explicit methods with improved energy-conservation properties is appealing for convection-dominated problems, especially in case of direct and large-eddy simulation of turbulent flows. A number of pseudo-symplectic methods are constructed for application to the incompressible Navier–Stokes equations and compared in terms of accuracy and efficiency by means of numerical simulations., Peer Reviewed, Postprint (author's final draft)
- Published
- 2017
37. A new subgrid characteristic length for turbulence simulations on anisotropic grids
- Abstract
Direct numerical simulations of the incompressible Navier-Stokes equations are not feasible yet for most practical turbulent flows. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grained simulations. In this regard, eddy-viscosity models for Large-Eddy Simulation (LES) are probably the most popular example thereof. This type of models requires the calculation of a subgrid characteristic length which is usually associated with the local grid size. For isotropic grids, this is equal to the mesh step. However, for anisotropic or unstructured grids, such as the pancake-like meshes that are often used to resolve near-wall turbulence or shear layers, a consensus on defining the subgrid characteristic length has not been reached yet despite the fact that it can strongly affect the performance of LES models. In this context, a new definition of the subgrid characteristic length is presented in this work. This flow-dependent length scale is based on the turbulent, or subgrid stress, tensor and its representations on different grids. The simplicity and mathematical properties suggest that it can be a robust definition that minimizes the effects of mesh anisotropies on simulation results. The performance of the proposed subgrid characteristic length is successfully tested for decaying isotropic turbulence and a turbulent channel flow using artificially refined grids. Finally, a simple extension of the method for unstructured meshes is proposed and tested for a turbulent flow around a square cylinder. Comparisons with existing subgrid characteristic length scales show that the proposed definition is much more robust with respect to mesh anisotropies and has a great potential to be used in complex geometries where highly skewed (unstructured) meshes are present., Peer Reviewed, Postprint (author's final draft)
- Published
- 2017
38. Explicit Runge-Kutta schemes for incompressible flow with improved energy-conservation properties
- Author
-
Gennaro Coppola, Luis Rández, Francesco Capuano, L. de Luca, Capuano, Francesco, Coppola, Gennaro, L., Randez, DE LUCA, Luigi, Universitat Politècnica de Catalunya. Departament de Mecànica de Fluids, and Universitat Politècnica de Catalunya. GReCEF- Grup de Recerca en Ciència i Enginyeria de Fluids
- Subjects
Work (thermodynamics) ,pseudo-symplecticity ,Física::Física de fluids [Àrees temàtiques de la UPC] ,Physics and Astronomy (miscellaneous) ,Mathematics::Analysis of PDEs ,010103 numerical & computational mathematics ,Turbulence simulations ,Energy conservation ,Kinetic energy ,01 natural sciences ,energy-conservation ,incompressible Navier-Stokes equation ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,turbulence simulations ,Incompressible flow ,0103 physical sciences ,Runge-Kutta method ,0101 mathematics ,Mathematics ,Numerical Analysis ,Pseudo-symplecticity ,Turbulence ,Applied Mathematics ,Mathematical analysis ,Runge–Kutta method ,Computer Science Applications ,Computational Mathematics ,Runge–Kutta methods ,Pressure-correction method ,Modeling and Simulation ,Compressibility ,Incompressible Navier–Stokes equations - Abstract
The application of pseudo-symplectic Runge–Kutta methods to the incompressible Navier–Stokes equations is discussed in this work. In contrast to fully energy-conserving, implicit methods, these are explicit schemes of order p that preserve kinetic energy to order q, with q>p. Use of explicit methods with improved energy-conservation properties is appealing for convection-dominated problems, especially in case of direct and large-eddy simulation of turbulent flows. A number of pseudo-symplectic methods are constructed for application to the incompressible Navier–Stokes equations and compared in terms of accuracy and efficiency by means of numerical simulations.
- Published
- 2017
39. A new subgrid characteristic length for turbulence simulations on anisotropic grids
- Author
-
Roel Verstappen, Assensi Oliva, F. X. Trias, Maurits H. Silvis, Andrey Gorobets, Universitat Politècnica de Catalunya. Departament de Màquines i Motors Tèrmics, Universitat Politècnica de Catalunya. CTTC - Centre Tecnològic de la Transferència de Calor, and Computational and Numerical Mathematics
- Subjects
Length scale ,Characteristic length ,Física::Física de fluids [Àrees temàtiques de la UPC] ,Computational Mechanics ,FOS: Physical sciences ,Computational fluid dynamics ,Turbulence simulations ,01 natural sciences ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,Fluid dynamics ,0103 physical sciences ,Turbulent flows ,Vortex dynamics ,Statistical physics ,0101 mathematics ,Navier–Stokes equations ,Turbulència ,Fluid Flow and Transfer Processes ,Physics ,business.industry ,Turbulence ,Viscosity ,Mechanical Engineering ,Turbulence modeling ,Fluid Dynamics (physics.flu-dyn) ,Physics - Fluid Dynamics ,Condensed Matter Physics ,Navier Stokes equations ,010101 applied mathematics ,Mechanics of Materials ,Mesh generation ,Dinàmica de fluids ,business ,Large eddy simulation - Abstract
Direct numerical simulations of the incompressible Navier-Stokes equations are not feasible yet for most practical turbulent flows. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grained simulations. In this regard, eddy-viscosity models for Large-Eddy Simulation (LES) are probably the most popular example thereof. This type of models requires the calculation of a subgrid characteristic length which is usually associated with the local grid size. For isotropic grids this is equal to the mesh step. However, for anisotropic or unstructured grids, such as the pancake-like meshes that are often used to resolve near-wall turbulence or shear layers, a consensus on defining the subgrid characteristic length has not been reached yet despite the fact that it can strongly affect the performance of LES models. In this context, a new definition of the subgrid characteristic length is presented in this work. This flow-dependent length scale is based on the turbulent, or subgrid stress, tensor and its representations on different grids. The simplicity and mathematical properties suggest that it can be a robust definition that minimizes the effects of mesh anisotropies on simulation results. The performance of the proposed subgrid characteristic length is successfully tested for decaying isotropic turbulence and a turbulent channel flow using artificially refined grids. Finally, a simple extension of the method for unstructured meshes is proposed and tested for a turbulent flow around a square cylinder. Comparisons with existing subgrid characteristic length scales show that the proposed definition is much more robust with respect to mesh anisotropies and has a great potential to be used in complex geometries where highly skewed (unstructured) meshes are present., Comment: 27 pages, 13 figures, 1 table
- Published
- 2017
- Full Text
- View/download PDF
40. Paths of energy in turbulent channel flows
- Author
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Andrea Cimarelli, E. De Angelis, Carlo Massimo Casciola, A. Cimarelli, E. De Angeli, and C.M. Casciola
- Subjects
Physics ,Turbulence ,K-epsilon turbulence model ,Mechanical Engineering ,Mechanics ,Condensed Matter Physics ,Space (mathematics) ,Physics::Fluid Dynamics ,TURBULENT BOUNDARY LAYER ,Flow (mathematics) ,Mechanics of Materials ,Energy cascade ,Turbulence kinetic energy ,turbulence simulation ,turbulent boundary layers ,turbulent flows ,TURBULENT FLOWS ,Scaling ,TURBULENCE SIMULATIONS ,Communication channel - Abstract
The paper describes the energy fluxes simultaneously occurring in the space of scales and in the physical space of wall-turbulent flows. The unexpected behaviour of the energy fluxes consists of spiral-like paths in the combined physical/scale space where the controversial reverse energy cascade plays a central role. Two dynamical processes are identified as driving mechanisms for the fluxes, one in the near-wall region and a second one further away from the wall. The former, stronger, one is related to the dynamics involved in the near-wall turbulence regeneration cycle. The second suggests an outer self-sustaining mechanism which is asymptotically expected to take place in the eventual log layer and could explain the debated mixed inner/outer scaling of the near-wall statistics. The observed behaviour may have strong repercussions on both theoretical and modelling approaches to wall turbulence, as anticipated by a simple equation which is shown able to capture most of the rich dynamics of the shear-dominated region of the flow.
- Published
- 2013
41. Parallel filtering in global gyrokinetic simulations
- Author
-
Yasuhiro Idomura, Stephan Brunner, T. Vernay, Laurent Villard, Alberto Bottino, Sébastien Jolliet, Paolo Angelino, Ben F. McMillan, and Trach-Minh Tran
- Subjects
Tokamak Turbulence ,Equations ,Physics and Astronomy (miscellaneous) ,Turbulence Simulations ,Gyrokinetics ,Flows ,Transport ,Matrix (mathematics) ,symbols.namesake ,Driven ,Wavenumber ,Statistical physics ,Ion ,Pic Code ,Physics ,Numerical Analysis ,Applied Mathematics ,Spectrum (functional analysis) ,Mathematical analysis ,Eulerian path ,Solver ,Computer Science Applications ,Computational Mathematics ,Nonlinear system ,Fourier transform ,Plasmas ,Modeling and Simulation ,Field solver ,Itg ,symbols - Abstract
In this work, a Fourier solver [B.F. McMillan, S. Jolliet, A. Bottino, P. Angelino, T.M. Tran, L Villard, Comp. Phys. Commun. 181 (2010) 7151 is implemented in the global Eulerian gyrokinetic code GT5D [Y. Idomura, H. Urano, N. Aiba, S. Tokuda, Nucl. Fusion 49 (2009) 0650291 and in the global Particle-In-Cell code ORB5 [S. Jolliet, A. Bottino, P. Angelino, It Hatzky. T.M. Iran, B.F. McMillan, O. Sauter, K. Appert, Y. Idomura, L Villard, Comp. Phys. Commun. 177 (2007) 4091 in order to reduce the memory of the matrix associated with the field equation. This scheme is verified with linear and nonlinear simulations of turbulence. It is demonstrated that the straight-field-line angle is the coordinate that optimizes the Fourier solver, that both linear and nonlinear turbulent states are unaffected by the parallel filtering, and that the k(parallel to) spectrum is independent of plasma size at fixed normalized poloidal wave number. (C) 2011 Elsevier Inc. All rights reserved.
- Published
- 2012
42. A mimetic finite difference discretization for the incompressible Navier–Stokes equations
- Author
-
Luca Bonaventura and Antonella Abba
- Subjects
business.industry ,Applied Mathematics ,Mechanical Engineering ,Mathematical analysis ,mimetic discretizations ,Computational Mechanics ,Finite difference ,Finite difference method ,computational fluid dynamics ,Computational fluid dynamics ,Computer Science Applications ,Physics::Fluid Dynamics ,turbulence simulations ,Mechanics of Materials ,Incompressible flow ,Pressure-correction method ,Hagen–Poiseuille flow from the Navier–Stokes equations ,Boundary value problem ,Navier-Stokes equations ,incompressible flow ,business ,Navier–Stokes equations ,Mathematics - Abstract
The results of a mimetic finite difference discretization of the three-dimensional, incompressible Navier-Stokes equations are compared with more traditional finite difference schemes. The proposed method handles both momentum advection and diffusion in a vorticity-preserving manner and allows for simple treatment of rigid wall boundary conditions. The results obtained in various tests demonstrate the advantages of the proposed method.
- Published
- 2008
43. Physical and numerical investigation of cavitating flows around a pitching hydrofoil
- Author
-
Yin Lu Young, Antoine Ducoin, Biao Huang, University of Michigan [Ann Arbor], University of Michigan System, Beijing Institute of Technology (BIT), Institut de Recherche de l'Ecole Navale (IRENAV), Université de Bordeaux (UB)-Institut Polytechnique de Bordeaux-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Arts et Métiers Sciences et Technologies, HESAM Université (HESAM)-HESAM Université (HESAM), Laboratoire de recherche en Hydrodynamique, Énergétique et Environnement Atmosphérique (LHEEA), and École Centrale de Nantes (ECN)-Centre National de la Recherche Scientifique (CNRS)
- Subjects
Flow (psychology) ,Computational Mechanics ,02 engineering and technology ,Turbulence simulations ,Bubble dynamics ,01 natural sciences ,010305 fluids & plasmas ,Physics::Fluid Dynamics ,symbols.namesake ,0203 mechanical engineering ,0103 physical sciences ,Turbulent flows ,[PHYS.MECA.MEFL]Physics [physics]/Mechanics [physics]/Fluid mechanics [physics.class-ph] ,Rotating flows ,Navier–Stokes equations ,Fluid Flow and Transfer Processes ,Physics ,Cavitation ,Turbulence ,Mechanical Engineering ,Multiphase flow ,Reynolds number ,Laminar flow ,Mechanics ,Condensed Matter Physics ,Vortex ,020303 mechanical engineering & transports ,Classical mechanics ,Mechanics of Materials ,symbols - Abstract
International audience; The objective of this paper is to investigate cavitating flows around a pitching hydrofoil via combined physical and numerical studies. The aims are to (1) improve the understanding of the interplay between unsteady cavitating flow, hydrofoil motion, and hydrodynamic performance, (2) quantify the influence of pitching rate on subcavitating and cavitating responses, and (3) quantify the influence of cavitation on the hydrodynamic load coefficients and surrounding flow structures. Results are presented for a NACA66 hydrofoil undergoing controlled, slow (α̇ =6∘/s) and fast (α̇ =63∘/s) pitching motions from α = 0° to α = 15° and back to α = 0° for both subcavitating and cavitating conditions at a moderate Reynolds number of Re = 750 000. The experimental studies were conducted in a cavitation tunnel at the French Naval Academy, France. The numerical simulations are performed by solving the incompressible, multiphase Unsteady Reynolds-Averaged Navier-Stokes Equations via the commercial code CFX using a transport equation-based cavitation model; a modified k-ω SST turbulence model is used to account for the effect of local compressibility on the turbulent eddy viscosity. The results showed that increases in the pitching rate suppressed laminar to turbulent transition, delayed stall, and significantly modified post-stall behavior. Cavitation inception at the leading edge modified the pressure distribution, which in turn significantly changed the interaction between leading edge and trailing edge vortices, and hence the magnitude as well as the frequency of the load fluctuations. For a fixed cavitation number, increases in pitching rate lead to increase in cavitation volume, which in turn changed the cavity shedding frequencies and significantly modified the hydrodynamic loads. Inversely, the leading edge cavitation observed for the low pitching velocity case tends to stabilize the stall because of the decrease of the pressure gradient due to the formation of the cavity. The results showed strong correlation between the cavity and vorticity structures, which suggest that the inception, growth, collapse and shedding of sheet/cloud cavities are important mechanisms for vorticity production and modification.
- Published
- 2013
44. A Key to Improved Ion Core Confinement in the JET Tokamak : Ion Stiffness Mitigation due to Combined Plasma Rotation and Low Magnetic Shear
- Abstract
New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation., QC 20111017
- Published
- 2011
- Full Text
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45. Experimental Study of the Ion Critical-Gradient Length and Stiffness Level and the Impact of Rotation in the JET Tokamak
- Abstract
Experiments were carried out in the JET tokamak to determine the critical ion temperature inverse gradient length (R/L-Ti = R vertical bar del T-i vertical bar/T-i) for the onset of ion temperature gradient modes and the stiffness of Ti profiles with respect to deviations from the critical value. Threshold and stiffness have been compared with linear and nonlinear predictions of the gyrokinetic code GS2. Plasmas with higher values of toroidal rotation show a significant increase in R/L-Ti, which is found to be mainly due to a decrease of the stiffness level. This finding has implications on the extrapolation to future machines of present day results on the role of rotation on confinement., QC 20100525
- Published
- 2009
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46. Closed 1st-Order And 2nd-Order Moment Equations For Stochastic Nonlinear Problems with Applications To Model Hydrodynamic And Vlasov-Plasma Turbulence
- Abstract
Working along the lines of a procedure outlined by Keller, a technique is developed for deriving closed first_ and second_order moment equations for a general class of stochastic nonlinear equations by performing a renormalization at the level of the second moment. The work of Weinstock, as reformulated recently by Balescu and Misguich, is extended in order to obtain two equivalent representations for the second moment using an exact, nonperturbative, statistical approach. These general results, when specialized to the weak_coupling limit, lead to a complete set of closed equations for the first two moments within the framework of an approximation corresponding to Kraichnan's direct_interaction approximation. Additional restrictions result in a self_consistent set of equations for the first two moments in the stochastic quasilinear approximation. Finally, the technique is illustrated by considering its application to two specific physical problems: (1) modelhydrodynamicturbulence and (2) Vlasov_plasma turbulence in the presence of an external stochastic electric field.
- Published
- 1976
- Full Text
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