scholarly journals Direct numerical simulation of turbulent channel flow with spanwise alternatively distributed strips control

2018 ◽  
Vol 32 (12n13) ◽  
pp. 1840004 ◽  
Author(s):  
Weidan Ni ◽  
Lipeng Lu ◽  
Jian Fang ◽  
Charles Moulinec ◽  
Yufeng Yao
Keyword(s):  
Flow Separation ◽  
Large Scale ◽  
Plane Channel ◽  
Turbulent Channel Flow ◽  
Phase Control ◽  
Vertical Shear ◽  
Small Scale ◽  
Spanwise Direction ◽  
Low Speed ◽  
Low Speed Streaks

The effect of spanwise alternatively distributed strips (SADS) control on turbulent flow in a plane channel has been studied by direct numerical simulations to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuation, and their potential in suppressing flow separation. SADS control is realized by alternatively arranging out-of-phase control (OPC) and in-phase control (IPC) wall actuations on the lower channel wall surface, in the spanwise direction. It is found that the coherent structures are suppressed or enhanced alternatively by OPC or IPC, respectively, leading to the formation of a vertical shear layer, which is responsible for the LSSVs’ presence. Large-scale low-speed region can also be observed above the OPC strips, which resemble large-scale low-speed streaks. LSSVs are found to be in a statistically-converged steady state and their cores are located between two neighboring OPC and IPC strips. Their motions contribute significantly to the momentum transport in the wall-normal and spanwise directions, demonstrating their potential ability to suppress flow separation.

Coherent structures in oscillatory boundary layers

2003 ◽  
Vol 474 ◽  
pp. 1-33 ◽  
Author(s):  
PAOLA COSTAMAGNA ◽  
GIOVANNA VITTORI ◽  
PAOLO BLONDEAUX
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Vortex Structures ◽  
Small Scale ◽  
Computational Domain ◽  
Steady Flows ◽  
Low Speed ◽  
Sequence Of Events ◽  
Low Speed Streaks ◽  
Oscillatory Boundary Layers

The dynamics of the vortex structures appearing in an oscillatory boundary layer (Stokes boundary layer), when the flow departs from the laminar regime, is investigated by means of flow visualizations and a quantitative analysis of the velocity and vorticity fields. The data are obtained by means of direct numerical simulations of the Navier–Stokes and continuity equations. The wall is flat but characterized by small imperfections. The analysis is aimed at identifying points in common and differences between wall turbulence in unsteady flows and the well-investigated turbulence structure in the steady case. As in Jimenez & Moin (1991), the goal is to isolate the basic flow unit and to study its morphology and dynamics. Therefore, the computational domain is kept as small as possible.The elementary process which maintains turbulence in oscillatory boundary layers is found to be similar to that of steady flows. Indeed, when turbulence is generated, a sequence of events similar to those observed in steady boundary layers is observed. However, these events do not occur randomly in time but with a repetition time scale which is about half the period of fluid oscillations. At the end of the accelerating phases of the cycle, low-speed streaks appear close to the wall. During the early part of the decelerating phases the strength of the low-speed streaks grows. Then the streaks twist, oscillate and eventually break, originating small-scale vortices. Far from the wall, the analysis of the vorticity field has revealed the existence of a sequence of streamwise vortices of alternating circulation pumping low-speed fluid far from the wall as suggested by Sendstad & Moin (1992) for steady flows. The vortex structures observed far from the wall disappear when too small a computational domain is used, even though turbulence is self-sustaining. The present results suggest that the streak instability mechanism is the dominant mechanism generating and maintaining turbulence; no evidence of the well-known parent vortex structures spawning offspring vortices is found. Although wall imperfections are necessary to trigger transition to turbulence, the characteristics of the coherent vortex structures, for example the spacing of the low-speed streaks, are found to be independent of wall imperfections.

scholarly journals Transfer, Segregation and Deposition of Heavy Particles in Horizontal and Vertical Turbulent Channel Flows

2003 ◽  
Author(s):  
Cristian Marchioli ◽  
Fabio Sbrizzai ◽  
Alfredo Soldati
Keyword(s):  
Coherent Structures ◽  
Particle Distribution ◽  
Outer Region ◽  
Turbulent Channel Flow ◽  
Transfer Mechanism ◽  
Wall Turbulence ◽  
Wall Region ◽  
Turbulence Structure ◽  
Low Speed ◽  
Low Speed Streaks

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. In this work, we focus on the transfer mechanism of different size particles and on the influence of gravity on particles deposition. By tracking O(105) particles in Direct Numerical Simulation (DNS) of a turbulent channel flow at Reτ = 150, we find that particles may reach the wall directly or may accumulate in the wall region, under the low-speed streaks. Even though low-speed streaks are ejection-like environments, particles are not re-entrained into the outer region. Particles segregated very near the wall by the trapping mechanisms we investigated in a previous work [1] are slowly driven to the wall. We find that gravity plays a role on particle distribution but, for small particles (τp+ < 3), the controlling transfer mechanism is related to near-wall turbulence structure.

scholarly journals A Study on the Instantaneous Turbulent Flow Field in a 90-Degree Elbow Pipe with Circular Section

2016 ◽  
Vol 2016 ◽  
pp. 1-8 ◽  
Author(s):  
Shiming Wang ◽  
Cheng Ren ◽  
Yangfei Sun ◽  
Xingtuan Yang ◽  
Jiyuan Tu
Keyword(s):  
Flow Field ◽  
Turbulent Flow ◽  
Flow Separation ◽  
Large Scale ◽  
Small Scale ◽  
Eddy Simulation ◽  
Dean Vortex ◽  
Instantaneous Flow Field ◽  
Instantaneous Pressure ◽  
Turbulent Flow Field

Based on the special application of 90-degree elbow pipe in the HTR-PM, the large eddy simulation was selected to calculate the instantaneous flow field in the 90-degree elbow pipe combining with the experimental results. The characteristics of the instantaneous turbulent flow field under the influence of flow separation and secondary flow were studied by analyzing the instantaneous pressure information at specific monitoring points and the instantaneous velocity field on the cross section of the elbow. The pattern and the intensity of the Dean vortex and the small scale eddies change over time and induce the asymmetry of the flow field. The turbulent disturbance upstream and the flow separation near the intrados couple with the vortexes of various scales. Energy is transferred from large scale eddies to small scale eddies and dissipated by the viscous stress in the end.

On the role of the outer region in the turbulent-boundary-layer bursting process

1994 ◽  
Vol 259 ◽  
pp. 345-373 ◽  
Author(s):  
ROY Y. Myose ◽  
Ron F. Blackwelder
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Outer Region ◽  
Low Speed ◽  
Moderate Reynolds Number ◽  
Eddy Structures ◽  
Low Speed Streaks ◽  
Bursting Process

The dynamics and interaction of turbulent-boundary-layer eddy structures was experimentally emulated. Counter-rotating streamwise vortices and low-speed streaks emulating turbulent-boundary-layer wall eddies were generated by a Görtler instability mechanism. Large-scale motions associated with the outer region of turbulent boundary layer were emulated with — ωzspanwise vortical eddies shed by a periodic non-sinusoidal oscillation of an airfoil. The scales of the resulting eddy structures were comparable to a moderate-Reynolds-number turbulent boundary layer. Results show that the emulated wall-eddy breakdown was triggered by streamwise acceleration associated with the outer region of turbulent boundary layer. This breakdown involved violent mixing between low-speed fluid from the wall eddy and accelerated fluid associated with the outer structure. Although wall eddies can break down autonomously, the presence of and interaction with outer-region — ωzeddies hastened their breakdown. Increasing the — ωzeddy strength resulted in further hastening of the breakdown. Conversely, + ωzeddies were found to delay wall-eddy breakdown locally, with further delays resulting from stronger + ωzeddies. This suggests that the outer region of turbulent boundary layers plays a role in the bursting process.

Implementation and Validation of a Hybrid RANS/LES Model in the Spectral Element Solver Nek5000

2014 ◽  
Author(s):  
S. Bhushan ◽  
D. K. Walters ◽  
E. Merzari ◽  
A. Obabko
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Nuclear Reactor ◽  
Plane Channel ◽  
Spectral Element ◽  
Small Scale ◽  
Test Case ◽  
Additional Contribution ◽  
Turbulent Structures ◽  
High Reynolds

A dynamic hybrid RANS/LES (DHRL) model has been implemented in the spectral-element solver Nek5000 to reduce computational expense for high Reynolds number applications. The model couples a k-ε URANS model and the dynamic Smagorinsky model for LES. The model is validated for plane channel flow at Reτ = 590 using DNS data, and compared with LES predictions. The model is then applied for the ANL-MAX case, which is a test case relevant to nuclear reactor cooling flow simulations. For the channel flow case, DHRL predictions were similar to LES on finer grids, but on coarser grids, the former predicted velocity profiles closer to DNS than the latter in the log-layer region. The improved prediction by the DHRL model was identified to be due to a 30% additional contribution of RANS stresses. For the ANL-MAX case, the URANS simulation predicts quasi-steady flow, with dominant large-scale turbulent structures, whereas LES predicts small-scale turbulent structures comparable with results in rapid mixing of cool and warm flow jets. DHRL simulations predict LES mode in the inlet jet region, and URANS mode elsewhere, as expected.

Estimating large-scale structures in wall turbulence using linear models

2018 ◽  
Vol 842 ◽  
pp. 146-162 ◽  
Author(s):  
Simon J. Illingworth ◽  
Jason P. Monty ◽  
Ivan Marusic
Keyword(s):  
Linear Model ◽  
Large Scale ◽  
Eddy Viscosity ◽  
Linear Models ◽  
Systems Approach ◽  
Wall Turbulence ◽  
Linear Estimator ◽  
Small Scale ◽  
Spanwise Direction ◽  
Time Resolved

A dynamical systems approach is used to devise a linear estimation tool for channel flow at a friction Reynolds number of $Re_{\unicode[STIX]{x1D70F}}=1000$. The estimator uses time-resolved velocity measurements at a single wall-normal location to estimate the velocity field at other wall-normal locations (the data coming from direct numerical simulations). The estimation tool builds on the work of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) by using a Navier–Stokes-based linear model and treating any nonlinear terms as unknown forcings to an otherwise linear system. In this way nonlinearities are not ignored, but instead treated as an unknown model input. It is shown that, while the linear estimator qualitatively reproduces large-scale flow features, it tends to overpredict the amplitude of velocity fluctuations – particularly for structures that are long in the streamwise direction and thin in the spanwise direction. An alternative linear model is therefore formed in which a simple eddy viscosity is used to model the influence of the small-scale turbulent fluctuations on the large scales of interest. This modification improves the estimator performance significantly. Importantly, as well as improving the performance of the estimator, the linear model with eddy viscosity is also able to predict with reasonable accuracy the range of wavenumber pairs and the range of wall-normal heights over which the estimator will perform well.

scholarly journals Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part I: Large-Scale and Storm-Scale Evolutions*

2010 ◽  
Vol 138 (12) ◽  
pp. 4285-4304 ◽  
Author(s):  
Hironori Fudeyasu ◽  
Yuqing Wang ◽  
Masaki Satoh ◽  
Tomoe Nasuno ◽  
Hiroaki Miura ◽  
...  
Keyword(s):  
Life Cycle ◽  
Tropical Cyclone ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Vertical Shear ◽  
Cloud System ◽  
Small Scale ◽  
Low Level ◽  
Multiscale Interactions ◽  
Java Sea

Abstract The Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model, successfully simulated the life cycle of Tropical Storm Isobel that formed over the Timor Sea in the austral summer of 2006. The multiscale interactions in the life cycle of the simulated storm were analyzed in this study. The large-scale aspects that affected Isobel’s life cycle are documented in this paper and the corresponding mesoscale processes are documented in a companion paper. The life cycle of Isobel was largely controlled by a Madden–Julian oscillation (MJO) event and the associated westerly wind burst (WWB). The MJO was found to have both positive and negative effects on the tropical cyclone intensity depending on the location of the storm relative to the WWB center associated with the MJO. The large-scale low-level convergence and high convective available potential energy (CAPE) downwind of the WWB center provided a favorable region to the cyclogenesis and intensification, whereas the strong large-scale stretching deformation field upwind of the WWB center may weaken the storm by exciting wavenumber-2 asymmetries in the eyewall and leading to the eyewall breakdown. Five stages are identified for the life cycle of the simulated Isobel: the initial eddy, intensifying, temporary weakening, reintensifying, and decaying stages. The initial eddy stage was featured by small-scale/mesoscale convective cyclonic vortices developed in the zonally elongated rainband organized in the preconditioned environment characterized by the WWB over the Java Sea associated with the onset of an MJO event over the East Indian Ocean. As the MJO propagated eastward and the cyclonic eddies moved southward into an environment with weak vertical shear and strong low-level cyclonic vorticity, a typical tropical cyclone structure developed over the Java Sea, namely the genesis of Isobel. Isobel experienced an eyewall breakdown and a temporary weakening when it was located upwind of the WWB center as the MJO propagated southeastward and reintensified as its eyewall reformed as a result of the axisymmetrization of an inward spiraling outer rainband that originally formed downwind of the WWB center. Finally Isobel decayed as it approached the northwest coast of Australia.

Spatial organization of large- and very-large-scale motions in a turbulent channel flow

2014 ◽  
Vol 749 ◽  
pp. 818-840 ◽  
Author(s):  
Jin Lee ◽  
Jae Hwa Lee ◽  
Jung-Il Choi ◽  
Hyung Jin Sung
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Spatial Organization ◽  
Outer Region ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Velocity Fluctuations ◽  
Spatial Features ◽  
Temporal Features ◽  
Low Speed Streaks

AbstractDirect numerical simulations were carried out to investigate the spatial features of large- and very-large-scale motions (LSMs and VLSMs) in a turbulent channel flow ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\mathit{Re}_{\tau }=930$). A streak detection method based on the streamwise velocity fluctuations was used to individually trace the cores of LSMs and VLSMs. We found that both the LSM and VLSM populations were large. Several of the wall-attached LSMs stretched toward the outer regions of the channel. The VLSMs consisted of inclined outer LSMs and near-wall streaks. The number of outer LSMs increased linearly with the streamwise length of the VLSMs. The temporal features of the low-speed streaks in the outer region revealed that growing and merging events dominated the large-scale (1–$3\delta $) structures. The VLSMs $({>}3\delta )$ were primarily created by merging events, and the statistical analysis of these events supported that the merging of large-scale upstream structures contributed to the formation of VLSMs. Because the local convection velocity is proportional to the streamwise velocity fluctuations, the streamwise-aligned structures of the positive- and negative-$u$ patches suggested a primary mechanism underlying the merging events. The alignment of the positive- and negative-$u$ structures may be an essential prerequisite for the formation of VLSMs.

Upscale Energy Transfer by the Vortical Mode and Internal Waves

2014 ◽  
Vol 44 (9) ◽  
pp. 2446-2469 ◽  
Author(s):  
Anne-Marie E. G. Brunner-Suzuki ◽  
Miles A. Sundermeyer ◽  
M.-Pascale Lelong
Keyword(s):  
Energy Transfer ◽  
Internal Wave ◽  
Internal Waves ◽  
Numerical Experiments ◽  
Large Scale ◽  
Vertical Shear ◽  
Small Scale ◽  
Driving Mechanism ◽  
Vortex Interaction ◽  
Vertical Scale

Abstract Diapycnal mixing in the ocean is sporadic yet ubiquitous, leading to patches of mixing on a variety of scales. The adjustment of such mixed patches can lead to the formation of vortices and other small-scale geostrophic motions, which are thought to enhance lateral diffusivity. If vortices are densely populated, they can interact and merge, and upscale energy transfer can occur. Vortex interaction can also be modified by internal waves, thus impacting upscale transfer. Numerical experiments were used to study the effect of a large-scale near-inertial internal wave on a field of submesoscale vortices. While one might expect a vertical shear to limit the vertical scale of merging vortices, it was found that internal wave shear did not disrupt upscale energy transfer. Rather, under certain conditions, it enhanced upscale transfer by enhancing vortex–vortex interaction. If vortices were so densely populated that they interacted even in the absence of a wave, adding a forced large-scale wave enhanced the existing upscale transfer. Results further suggest that continuous forcing by the main driving mechanism (either vortices or internal waves) is necessary to maintain such upscale transfer. These findings could help to improve understanding of the direction of energy transfer in submesoscale oceanic processes.

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