Large-scale motions and inner/outer layer interactions in turbulent Couette–Poiseuille flows

2011 ◽  
Vol 680 ◽  
pp. 534-563 ◽  
Author(s):  
SERGIO PIROZZOLI ◽  
MATTEO BERNARDINI ◽  
PAOLO ORLANDI
Keyword(s):  
Large Scale ◽  
Flow Mode ◽  
Modulation Effect ◽  
Wide Range ◽  
Flow Features ◽  
The Mean ◽  
Flow Configurations ◽  
Nonlinear Modulation ◽  
High Reynolds ◽  
Poiseuille Flows

We investigate the organization of the momentum-carrying eddies in turbulent Couette–Poiseuille flows. The study relies on a direct numerical simulation (DNS) database covering a wide range of flow configurations from pure Couette to pure Poiseuille flows, at Reτ ≈ 250 (based on the flow properties at the stationary wall). The study highlights the occurrence of streaky patterns of alternating high and low momentum throughout the channel for all flow configurations, except near zeros of the mean shear, where streaks are suppressed. The mean streak spacing shows a relatively universal distribution in the core of the channel, where it ranges from 50 to 100 local viscous units. The validity of the local viscous scaling in collapsing flow features at different wall distances is confirmed by the analysis of the spanwise velocity spectra, which also highlights (in the case of Couette-like flows) the onset of a secondary low-wavenumber flow mode, superposed on the high-wavenumber flow mode that is responsible for the inner-layer dynamics. The effect of the former mode on the latter is studied by means of the two-point amplitude modulation coefficient, which brings to light a nonlinear modulation phenomenon. Physical mechanisms to explain the modulation effect are proposed, based on the interpretation of the conditional average events. Note that, although similar mechanisms have been previously observed in high-Reynolds-number turbulent boundary layers and channels, the modulation effect is here rather associated with the intrinsic large-scale dynamics of Couette-like flows, and takes place at DNS-accessible Reynolds numbers. We thus believe that the study of Couette-like flows may give an alternative avenue for probing inner/outer interaction effects than canonical channel flows.

Turbulent Shear Flow Over Surface Mounted Obstacles

1990 ◽  
Vol 112 (4) ◽  
pp. 376-385 ◽  
Author(s):  
W. H. Schofield ◽  
E. Logan
Keyword(s):  
Shear Flow ◽  
Shear Layers ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Mean Flow ◽  
Wide Range ◽  
Flow Features ◽  
Model Geometry ◽  
The Mean ◽  
High Reynolds

The mean flow field surrounding obstacles attached to a wall under a turbulent boundary layer is analyzed. The analysis concentrates on how major features of the flow are influenced by model geometry and the incident shear flow. Experimental data are analyzed in terms of nondimensionalized variables chosen on the basis that their effect on major flow features can be simply appreciated. The data are restricted to high Reynolds number shear layers thicker than the attached obstacle. The work shows that data from a wide range of flows can be collapsed if appropriate nondimensional scales are used.

Optimal energy growth in stably stratified turbulent Couette flow

2021 ◽  
Author(s):  
Grigory Zasko ◽  
Andrey Glazunov ◽  
Evgeny Mortikov ◽  
Yuri Nechepurenko ◽  
Pavel Perezhogin
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Couette Flow ◽  
Large Scale ◽  
Thin Layers ◽  
Plane Couette Flow ◽  
Turbulent Layer ◽  
Wide Range ◽  
The Mean ◽  
Optimal Disturbances

<p>In this report, we will try to explain the emergence of large-scale organized structures in stably stratified turbulent flows using optimal disturbances of the mean turbulent flow. These structures have been recently obtained in numerical simulations of turbulent stably stratified flows [1] (Ekman layer, LES) and [2] (plane Couette flow, DNS and LES) and indirectly confirmed by field measurements in the stable boundary layer of the atmosphere [1, 2]. In instantaneous temperature fields they manifest themselves as irregular inclined thin layers with large gradients (fronts), spaced from each other by distances comparable to the height of the entire turbulent layer, and separated by regions with weak stratification.</p><p>Optimal disturbances of a stably stratified turbulent plane Couette flow are investigated in a wide range of Reynolds and Richardson numbers. These disturbances were computed based on a simplified linearized system of equations in which turbulent Reynolds stresses and heat fluxes were approximated by isotropic viscosity and diffusion with coefficients obtained from DNS results. It was shown [3] that the spatial scales and configurations of the inclined structures extracted from DNS data coincide with the ones obtained from optimal disturbances of the mean turbulent flow.</p><p>Critical value of the stability parameter is found starting from which the optimal disturbances resemble inclined structures. The physical mechanisms that determine the evolution, energetics and spatial configuration of these optimal disturbances are discussed. The effects due to the presence of stable stratification are highlighted.</p><p>Numerical experiments with optimal disturbances were supported by the RSF (grant No. 17-71-20149). Direct numerical simulation of stratified turbulent Couette flow was supported by the RFBR (grant No. 20-05-00776).</p><p>References:</p><p>[1] P.P. Sullivan, J.C. Weil, E.G. Patton, H.J. Jonker, D.V. Mironov. Turbulent winds and temperature fronts in large-eddy simulations of the stable atmospheric boundary layer // J. Atmos. Sci., 2016, V. 73, P. 1815-1840.</p><p>[2] A.V. Glazunov, E.V. Mortikov, K.V. Barskov, E.V. Kadantsev, S.S. Zilitinkevich. Layered structure of stably stratified turbulent shear flows // Izv. Atmos. Ocean. Phys., 2019, V. 55, P. 312–323.</p><p>[3] G.V. Zasko, A.V. Glazunov, E.V. Mortikov, Yu.M. Nechepurenko. Large-scale structures in stratified turbulent Couette flow and optimal disturbances // Russ. J. Num. Anal. Math. Model., 2010, V. 35, P. 35–53.</p>

Entry Length Requirements for Two- and Three-Dimensional Laminar Couette–Poiseuille Flows

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
Bayode E. Owolabi ◽  
David J. C. Dennis ◽  
Robert J. Poole
Keyword(s):  
Three Dimensional ◽  
Reynolds Numbers ◽  
Channel Height ◽  
Development Length ◽  
Square Duct ◽  
Entry Length ◽  
Wide Range ◽  
High Reynolds ◽  
Pressure Driven Flow ◽  
Poiseuille Flows

In this study, we examine the development length requirements for laminar Couette–Poiseuille flows in a two-dimensional (2D) channel as well as in the three-dimensional (3D) case of flow through a square duct, using a combination of numerical and experimental approaches. The parameter space investigated covers wall to bulk velocity ratios, r, spanning from 0 (purely pressure-driven flow) to 2 (purely wall driven-flow; 4 in the case of a square duct) and a wide range of Reynolds numbers (Re). The results indicate an increase in the development length (L) with r. Consistent with the findings of Durst et al. (2005, “The Development Lengths of Laminar Pipe and Channel Flows,” ASME J. Fluids Eng., 127(6), pp. 1154–1160), L was observed to be of the order of the channel height in the limit as Re→0, irrespective of the condition at the inlet. This, however, changes at high Reynolds numbers, with L increasing linearly with Re. In all the cases considered, a uniform velocity profile at the inlet was found to result in longer entry lengths than in a flow developing from a parabolic inlet profile. We show that this inlet effect becomes less important as the limit of purely wall-driven flow is approached. Finally, we develop correlations for predicting L in these flows and, for the first time, also present laser Doppler velocimetry (LDV) measurements of the developing as well as fully-developed velocity profiles, and observe good agreement between experiment, analytical solution, and numerical simulation results in the 3D case.

scholarly journals Hierarchical Porous Polyamide 6 by Solution Foaming: Synthesis, Characterization and Properties

Polymers ◽  
2018 ◽  
Vol 10 (12) ◽  
pp. 1310 ◽  
Author(s):  
Liang Wang ◽  
Yu-Ke Wu ◽  
Fang-Fang Ai ◽  
Jie Fan ◽  
Zhao-Peng Xia ◽  
...  
Keyword(s):  
Large Scale ◽  
Polyamide 6 ◽  
Solution Concentration ◽  
The Novel ◽  
Foaming Agent ◽  
Hierarchical Porous ◽  
Wide Range ◽  
The Mean ◽  
Novel Strategy

Porous polym er materials have received great interest in both academic and industrial fields due to their wide range of applications. In this work, a porous polyamide 6 (PA6) material was prepared by a facile solution foaming strategy. In this approach, a sodium carbonate (SC) aqueous solution acted as the foaming agent that reacted with formic acid (FA), generating CO2 and causing phase separation of polyamide (PA). The influence of the PA/FA solution concentration and Na2CO3 concentration on the microstructures and physical properties of prepared PA foams were investigated, respectively. PA foams showed a hierarchical porous structure along the foaming direction. The mean pore dimension ranged from hundreds of nanometers to several microns. Low amounts of sodium salt generated from a neutralization reaction played an important role of heterogeneous nucleation, which increased the crystalline degree of PA foams. The porous PA materials exhibited low thermal conductivity, high crystallinity and good mechanical properties. The novel strategy in this work could produce PA foams on a large scale for potential engineering applications.

Optical Validation of Ejector Flow Characteristics Predicted by Computational Analysis

2012 ◽  
Author(s):  
Adrienne B. Little ◽  
Yann Bartosiewicz ◽  
Srinivas Garimella
Keyword(s):  
Large Scale ◽  
Waste Heat ◽  
Turbulence Models ◽  
Ideal Gas ◽  
Computational Simulations ◽  
Design Data ◽  
Local Flow ◽  
Wide Range ◽  
Flow Features ◽  
Flow Phenomena

Passive, heat actuated devices can offer simple and energy-efficient options for a variety of end uses. An ejector pump is one such device that provides reasonable pressure head with no electrical input or moving parts. Useful for a wide range of applications from nuclear reactor cooling to vapor compression in waste-heat-driven heat pumping and work recovery systems, the flow phenomena inside an ejector must be understood to achieve improvements in component design and efficiency. In an effort to obtain insights into the flow phenomena inside an ejector, and to evaluate the effectiveness of commonly used computational tools in predicting these conditions, this study presents a set of shadowgraph images of flow inside a large-scale air ejector, and compares them to computational simulations of the same flow. On-design and off-design conditions are considered where the suction flow is choked and not choked, respectively. The computational simulations used for comparison apply k-ε RNG and k-ω SST turbulence models available in ANSYS FLUENT to 2D, locally-refined rectangular meshes for ideal gas air flow. Experimental and computational results show that on-design ejector operation is predicted with reasonable accuracy, but accuracy with the same models is not adequate at off-design conditions. This is attributed to an inability of turbulence models to predict shock/expansion interaction with the motive jet boundary, as well as the strength and position of flow features. Exploration of local flow features shows that the k-ω SST model predicts the location of flow features, as well as global inlet mass flow rates, with greater accuracy. It is concluded that to provide a rigorous validation of turbulence models for the application of modeling ejector flow, it is necessary to rely on off-design data where more complex phenomena occur, such as flow separation, strong boundary layer/shock interaction, and unsteady flow. Such validation will help refine turbulence models for future ejector design purposes, and allow for more efficient ejector operation.

Using URANS to Predict Turbulent Flow Features in a Channel with Lateral Slot

2013 ◽  
Vol 394 ◽  
pp. 101-107 ◽  
Author(s):  
Tiago de Melo ◽  
Jhon Goulart ◽  
Sandi Souza
Keyword(s):  
Large Scale ◽  
Rectangular Channel ◽  
Lateral Wall ◽  
Navier Stokes ◽  
Large Scale Structures ◽  
Commercial Code ◽  
Flow Features ◽  
The Mean ◽  
Gap Width ◽  
Reynolds Average

Employing a commercial code an unsteady Reynolds Average Navier-Stokes (URANS) with Spalart-Allmaras as turbulence model numerical calculations were performed in order to predict the mean and velocity fluctuations fields in a rectangular channel with a lateral slot. The slot is attached to a lateral wall channel, being characterized by its deepness p and the gap width d. Simulations were performed keeping constant the slot deepness p and the length L while the gap width d was increased from 2 up to 6 mm. Three test sections involving p/d ratios12.50, 6.25 and 4.17were studied. Main results revealed that turbulence production increases with gap dimension decreasing. Large scale structures appearance were also the target of this paper. The study showed gap width plays an important role on this issue. As the gap width was increased large scales structures could be observed farther from channels entrance. Moreover, a kind of viscous effect in the gap was observed. As gap become very tight the frequency of coherent motions is reduced.

scholarly journals Simultaneous skin friction and velocity measurements in high Reynolds number pipe and boundary layer flows

2019 ◽  
Vol 871 ◽  
pp. 377-400 ◽  
Author(s):  
R. Baidya ◽  
W. J. Baars ◽  
S. Zimmerman ◽  
M. Samie ◽  
R. J. Hearst ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Large Scale ◽  
Streamwise Velocity ◽  
Momentum Equation ◽  
The Self ◽  
Self Similarity ◽  
Self Similar ◽  
The Mean ◽  
High Reynolds

Streamwise velocity and wall-shear stress are acquired simultaneously with a hot-wire and an array of azimuthal/spanwise-spaced skin friction sensors in large-scale pipe and boundary layer flow facilities at high Reynolds numbers. These allow for a correlation analysis on a per-scale basis between the velocity and reference skin friction signals to reveal which velocity-based turbulent motions are stochastically coherent with turbulent skin friction. In the logarithmic region, the wall-attached structures in both the pipe and boundary layers show evidence of self-similarity, and the range of scales over which the self-similarity is observed decreases with an increasing azimuthal/spanwise offset between the velocity and the reference skin friction signals. The present empirical observations support the existence of a self-similar range of wall-attached turbulence, which in turn are used to extend the model of Baarset al.(J. Fluid Mech., vol. 823, p. R2) to include the azimuthal/spanwise trends. Furthermore, the region where the self-similarity is observed correspond with the wall height where the mean momentum equation formally admits a self-similar invariant form, and simultaneously where the mean and variance profiles of the streamwise velocity exhibit logarithmic dependence. The experimental observations suggest that the self-similar wall-attached structures follow an aspect ratio of$7:1:1$in the streamwise, spanwise and wall-normal directions, respectively.

Turbulent flow of a fluid with anisotropic viscosity

2016 ◽  
Vol 792 ◽  
pp. 252-273 ◽  
Author(s):  
Tim Grünberg ◽  
Thomas Rösgen
Keyword(s):  
Turbulent Flow ◽  
Large Scale ◽  
Tracer Particle ◽  
Mean Flow ◽  
Flow Topology ◽  
Velocity Statistics ◽  
The Mean ◽  
High Reynolds ◽  
Ultrasound Velocimetry ◽  
Anisotropic Viscosity

We ask if and how the large-scale structure of a turbulent flow depends on anisotropies introduced at the smallest scales. We generate such anisotropy on the viscous scale in a paramagnetic colloid whose rheology is modified by an external, uniform magnetic field. We report measurements in a high Reynolds number turbulence experiment ($R_{{\it\lambda}}=120$). Ultrasound velocimetry provides records of tracer particle velocity. Distinct changes in the velocity statistics can be observed from the dissipative scales up to the mean flow topology.

scholarly journals Development and Calibration of a New Dripper-Based Rainfall Simulator for Large-Scale Sediment Wash-Off Studies

Water ◽  
2020 ◽  
Vol 12 (1) ◽  
pp. 152
Author(s):  
Juan Naves ◽  
Jose Anta ◽  
Joaquín Suárez ◽  
Jerónimo Puertas
Keyword(s):  
Physical Model ◽  
Large Scale ◽  
Terminal Velocity ◽  
Final Solution ◽  
Rainfall Simulator ◽  
Natural Rainfall ◽  
Raindrop Size Distribution ◽  
Wide Range ◽  
The Mean ◽  
Raindrop Size

Rainfall simulators are useful tools for controlling the main variables that govern natural rainfall. In this study, a new drop-forming rainfall simulator, which consists of pressure-compensating dripper grids above a horizontal mesh that breaks and distributes raindrops, was developed to be applied in wash-off experiments in a large-scale physical model of 36 m2. The mesh typology and size, and its distance to drippers, were established through a calibration where rain uniformity and distributions of raindrop sizes and velocities were compared with local natural rainfall. Finally, the rain properties of the final solution were measured for the three rain intensities that the rainfall simulator is able to generate (30, 50 and 80 mm/h), obtaining almost uniform rainfalls with uniformity coefficients of 81%, 89% and 91%, respectively. This, together with the very suitable raindrop size distribution obtained, and the raindrop velocities of around 87.5% of the terminal velocity for the mean raindrop diameter, makes the proposed solution optimal for wash-off studies, where rain properties are key in the detachment of particles. In addition, the flexibility seen in controlling rain characteristics increases the value of the proposed design in that it is adaptable to a wide range of studies.

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