Turbulent spots in a channel: large-scale flow and self-sustainability

2013 ◽  
Vol 731 ◽  
Author(s):  
Grégoire Lemoult ◽  
Jean-Luc Aider ◽  
José Eduardo Wesfreid
Keyword(s):  
Large Scale ◽  
Large Time ◽  
Rectangular Channel ◽  
Small Scale ◽  
Turbulent Spot ◽  
Time Resolved ◽  
Image Velocimetry ◽  
Streamwise Streaks ◽  
Large Scale Flow ◽  
Scale Motion

AbstractUsing a large-time-resolved particle image velocimetry field of view, a developing turbulent spot is followed in space and time in a rectangular channel flow for more than 100 advective time units. We show that the flow can be decomposed into a large-scale motion consisting of an asymmetric quadrupole centred on the spot and a small-scale part consisting of streamwise streaks. From the temporal evolution of the energy of the streamwise and spanwise velocity perturbations, it is suggested that a self-sustaining process can occur in a turbulent spot above a given Reynolds number.

scholarly journals Time evolution of uniform momentum zones in a turbulent boundary layer

2018 ◽  
Vol 842 ◽  
pp. 554-590 ◽  
Author(s):  
A. Laskari ◽  
R. de Kat ◽  
R. J. Hearst ◽  
B. Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Small Scale ◽  
Residence Times ◽  
Time Resolved ◽  
Image Velocimetry ◽  
Measurement Plane ◽  
Very High ◽  
Flow States

Time-resolved planar particle image velocimetry was used to analyse the structuring of a turbulent boundary layer into uniform momentum zones (UMZs). The instantaneous peak-detection method employed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) and de Silva et al. (J. Fluid Mech., vol. 786, 2016, pp. 309–331) is extended to account for temporal coherence of UMZs. The resulting number of zones detected appears to follow a normal distribution at any given instant. However, the extreme cases in which the number of zones is either very high or very low, are shown to be linked with two distinct flow states. A higher than average number of zones is associated with a large-scale $Q2$ event in the log region which creates increased small-scale activity within that region. Conversely, a low number of zones corresponds to a large-scale $Q4$ event in the log region and decreased turbulent activity away from the wall. The residence times, within the measurement plane, of zones belonging to the latter scenario are shown to be on average four times larger than those of zones present during higher than average zone structuring states. For both cases, greater residence times are observed for zones of higher momentum that are generally closer to the free stream.

Effects of Squish Flow on Tangential Flow and Turbulence in a Diesel Engine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Yanzhe Sun ◽  
Kai Sun ◽  
Tianyou Wang ◽  
Yufeng Li ◽  
Zhen Lu
Keyword(s):  
Diesel Engine ◽  
Turbulent Flows ◽  
Large Scale ◽  
Radial Flow ◽  
Tangential Component ◽  
Tangential Flow ◽  
Image Velocimetry ◽  
Turbulence Production ◽  
Large Scale Flow ◽  
Main Effects

Emission and fuel consumption in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical effects of squish flow on the tangential flow and turbulence production are still far from well understood. To identify the effects of squish flow, Particle image velocimetry (PIV) experiments are performed in a motored optical diesel engine equipped with different bowls. By comparing and associating the large-scale flow and turbulent kinetic energy (k), the main effects of the squish flow are clarified. The effect of squish flow on the turbulence production in the r−θ plane lies in the axial-asymmetry of the annular distribution of radial flow and the deviation between the ensemble-averaged swirl field and rigid body swirl field. Larger squish flow could promote the swirl center to move to the cylinder axis and reduce the deformation of swirl center, which could decrease the axial-asymmetry of annular distribution of radial flow, further, that results in a lower turbulence production of the shear stress. Moreover, larger squish flow increases the radial fluctuation velocity which makes a similar contribution to k with the tangential component. The understanding of the squish flow and its correlations with tangential flow and turbulence obtained in this study is beneficial to design and optimize the in-cylinder turbulent flow.

scholarly journals Turbulent drag in a rotating frame

2016 ◽  
Vol 794 ◽  
Author(s):  
Antoine Campagne ◽  
Nathanaël Machicoane ◽  
Basile Gallet ◽  
Pierre-Philippe Cortet ◽  
Frédéric Moisy
Keyword(s):  
Large Scale ◽  
Scaling Law ◽  
Stereoscopic Particle Image Velocimetry ◽  
Turbulence Theory ◽  
Water Tank ◽  
Wave Interactions ◽  
Turbulent Drag ◽  
Image Velocimetry ◽  
Weak Turbulence Theory ◽  
Large Scale Flow

What is the turbulent drag force experienced by an object moving in a rotating fluid? This open and fundamental question can be addressed by measuring the torque needed to drive an impeller at a constant angular velocity ${\it\omega}$ in a water tank mounted on a platform rotating at a rate ${\it\Omega}$. We report a dramatic reduction in drag as ${\it\Omega}$ increases, down to values as low as 12 % of the non-rotating drag. At small Rossby number $Ro={\it\omega}/{\it\Omega}$, the decrease in the drag coefficient $K$ follows the approximate scaling law $K\sim Ro$, which is predicted in the framework of nonlinear inertial-wave interactions and weak-turbulence theory. However, stereoscopic particle image velocimetry measurements indicate that this drag reduction instead originates from a weakening of the turbulence intensity in line with the two-dimensionalization of the large-scale flow.

The Fully Developed Wake of a Circular Cylinder

1949 ◽  
Vol 2 (4) ◽  
pp. 451 ◽  
Author(s):  
AA Townsend
Keyword(s):  
Circular Cylinder ◽  
Diffusion Coefficients ◽  
Large Scale ◽  
Turbulent Transport ◽  
Turbulent Energy ◽  
Small Scale ◽  
Mean Velocity ◽  
Turbulent Fluid ◽  
Air Stream ◽  
Scale Motion

Extending previous work on turbulent diffusion in the wake of a circular-cylinder, a series of measurements have been made of the turbulent transport of mean stream momentum, turbulent energy, and heat in the wake of a cylinder of 0.169 cm. diameter, placed in an air-stream of velocity 1280 cm. sec.-1. It has been possible to extend the measurements to 960 diameters down-stream from the cylinder, and it 1s found that, at distances in excess of 600 diameters, the requirements of dynamical similarity are very nearly satisfied. To account for the observed rates of transport of turbulent energy and heat, it is necessary that only part of this transport be due to bulk convection by the slow large-scale motion of the jets of turbulent fluid emitted by the central, fully turbulent core of the wake, which had been supposed previously to perform most of the transport. The remainder of the transport is carried out by the small-scale diffusive motion of the turbulent eddies within the jets, and may be described by assigning diffusion coefficients to the turbulent fluid. It is found that the diffusion coefficients for momentum and heat are approximately equal, but that for turbulent energy is considerably smaller. On the basis of these hypotheses, it is possible to calculate $he form of the mean velocity distribution in good agreement with experiment, and to give a qualitative explanation of the apparently more rapid diffusion of heat.

Experimental Study of Large-Scale Flow Structures in an Aneurysm

2018 ◽  
Author(s):  
Paulo Yu ◽  
Vibhav Durgesh
Keyword(s):  
Large Scale ◽  
Flow Structures ◽  
Pump System ◽  
Vortical Structures ◽  
Abnormal Growth ◽  
Image Velocimetry ◽  
Large Scale Flow ◽  
The Impact ◽  
Inflow Conditions ◽  
Aneurysm Model

An aneurysm is an abnormal growth in the wall of a weakened blood vessel, and can often be fatal upon rupture. Studies have shown that aneurysm shape and hemodynamics, in conjunction with other parameters, play an important role in growth and rupture. The objective of this study was to investigate the impact of varying inflow conditions on flow structures in an aneurysm. An idealized rigid sidewall aneurysm model was prepared and the Womersley number (α) and Reynolds number (Re) values were varied from 2 to 5 and 50 to 250, respectively. A ViVitro Labs pump system was used for inflow control and Particle Image Velocimetry was used for conducting velocity measurements. The results showed that the primary vortex path varied with an increase in α, while an increase in Re was correlated to the vortex strength and formation of secondary vortical structures. The evolution and decay of vortical structures were also observed to be dependent on α and Re.

Nonlinear dynamics over rough topography: homogeneous and stratified quasi-geostrophic theory

2003 ◽  
Vol 474 ◽  
pp. 299-318 ◽  
Author(s):  
JACQUES VANNESTE
Keyword(s):  
Nonlinear Dynamics ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Small Scale ◽  
Stratified Fluids ◽  
Homogeneous Case ◽  
Bottom Boundary ◽  
One Dimensional ◽  
Averaged Equations ◽  
Large Scale Flow

The weakly nonlinear dynamics of quasi-geostrophic flows over a one-dimensional, periodic or random, small-scale topography is investigated using an asymptotic approach. Averaged (or homogenized) evolution equations which account for the flow–topography interaction are derived for both homogeneous and continuously stratified quasi-geostrophic fluids. The scaling assumptions are detailed in each case; for stratified fluids, they imply that the direct influence of the topography is confined within a thin bottom boundary layer, so that it is through a new bottom boundary condition that the topography affects the large-scale flow. For both homogeneous and stratified fluids, a single scalar function entirely encapsulates the properties of the topography that are relevant to the large-scale flow: it is the correlation function of the topographic height in the homogeneous case, and a linear transform thereof in the continuously stratified case.Some properties of the averaged equations are discussed. Explicit nonlinear solutions in the form of one-dimensional travelling waves can be found. In the homogeneous case, previously studied by Volosov, they obey a second-order differential equation; in the stratified case on which we focus they obey a nonlinear pseudodifferential equation, which reduces to the Peierls–Nabarro equation for sinusoidal topography. The known solutions to this equation provide examples of nonlinear periodic and solitary waves in continuously stratified fluid over topography.The influence of bottom topography on large-scale baroclinic instability is also examined using the averaged equations: they allow a straightforward extension of Eady's model which demonstrates the stabilizing effect of topography on baroclinic instability.

scholarly journals Gyrotactic swimmers in turbulence: shape effects and role of the large-scale flow

2018 ◽  
Vol 856 ◽  
Author(s):  
M. Borgnino ◽  
G. Boffetta ◽  
F. De Lillo ◽  
M. Cencini
Keyword(s):  
Large Scale ◽  
Stokes Equations ◽  
Navier Stokes ◽  
Small Scale ◽  
Navier Stokes Equations ◽  
Preferential Sampling ◽  
Wide Range ◽  
Shape Effects ◽  
Dilute Suspensions ◽  
Large Scale Flow

We study the dynamics and the statistics of dilute suspensions of gyrotactic swimmers, a model for many aquatic motile microorganisms. By means of extensive numerical simulations of the Navier–Stokes equations at different Reynolds numbers, we investigate preferential sampling and small-scale clustering as a function of the swimming (stability and speed) and shape parameters, considering in particular the limits of spherical and rod-like particles. While spherical swimmers preferentially sample local downwelling flow, for elongated swimmers we observe a transition from downwelling to upwelling regions at sufficiently high swimming speed. The spatial distribution of both spherical and elongated swimmers is found to be fractal at small scales in a wide range of swimming parameters. The direct comparison between the different shapes shows that spherical swimmers are more clusterized at small stability and speed numbers, while for large values of the parameters elongated cells concentrate more. The relevance of our results for phytoplankton swimming in the ocean is briefly discussed.

scholarly journals Combined measurement of velocity and temperature in liquid metal convection

2019 ◽  
Vol 876 ◽  
pp. 1108-1128 ◽  
Author(s):  
Till Zürner ◽  
Felix Schindler ◽  
Tobias Vogt ◽  
Sven Eckert ◽  
Jörg Schumacher
Keyword(s):  
Reynolds Number ◽  
Liquid Metal ◽  
Large Scale ◽  
Scale Up ◽  
Convection Cell ◽  
Small Scale ◽  
Convection Flow ◽  
Large Scale Circulation ◽  
Large Scale Flow ◽  
The Impact

Combined measurements of velocity components and temperature in a turbulent Rayleigh–Bénard convection flow at a low Prandtl number of $Pr=0.029$ and Rayleigh numbers of $10^{6}\leqslant Ra\leqslant 6\times 10^{7}$ are conducted in a series of experiments with durations of more than a thousand free-fall time units. Multiple crossing ultrasound beam lines and an array of thermocouples at mid-height allow for a detailed analysis and characterization of the complex three-dimensional dynamics of the single large-scale circulation roll in a cylindrical convection cell of unit aspect ratio which is filled with the liquid metal alloy GaInSn. We measure the internal temporal correlations of the complex large-scale flow and distinguish between short-term oscillations associated with a sloshing motion in the mid-plane as well as varying orientation angles of the velocity close to the top/bottom plates and the slow azimuthal drift of the mean orientation of the roll as a whole that proceeds on a time scale up to a hundred times slower. The coherent large-scale circulation drives a vigorous turbulence in the whole cell that is quantified by direct Reynolds number measurements at different locations in the cell. The velocity increment statistics in the bulk of the cell displays characteristic properties of intermittent small-scale fluid turbulence. We also show that the impact of the symmetry-breaking large-scale flow persists to small-scale velocity fluctuations thus preventing the establishment of fully isotropic turbulence in the cell centre. Reynolds number amplitudes depend sensitively on beam-line position in the cell such that different definitions have to be compared. The global momentum and heat transfer scalings with Rayleigh number are found to agree with those of direct numerical simulations and other laboratory experiments.

Enhanced dissipation for quasi-geostrophic motion over small-scale topography

2000 ◽  
Vol 407 ◽  
pp. 105-122 ◽  
Author(s):  
JACQUES VANNESTE
Keyword(s):  
Large Scale ◽  
Spatial Scales ◽  
Additional Term ◽  
Equation Of Motion ◽  
Small Scale ◽  
Two Dimensional ◽  
Turbulent Dissipation ◽  
Dissipative Term ◽  
History Of ◽  
Large Scale Flow

The effect of a small-scale topography on large-scale, small-amplitude oceanic motion is analysed using a two-dimensional quasi-geostrophic model that includes free-surface and β effects, Ekman friction and viscous (or turbulent) dissipation. The topography is two-dimensional and periodic; its slope is assumed to be much larger than the ratio of the ocean depth to the Earth's radius. An averaged equation of motion is derived for flows with spatial scales that are much larger than the scale of the topography and either (i) much larger than or (ii) comparable to the radius of deformation. Compared to the standard quasi-geostrophic equation, this averaged equation contains an additional dissipative term that results from the interaction between topography and dissipation. In case (i) this term simply represents an additional Ekman friction, whereas in case (ii) it is given by an integral over the history of the large-scale flow. The properties of the additional term are studied in detail. For case (i) in particular, numerical calculations are employed to analyse the dependence of the additional Ekman friction on the structure of the topography and on the strength of the original dissipation mechanisms.

Triadic scale interactions in a turbulent boundary layer

2015 ◽  
Vol 767 ◽  
Author(s):  
Subrahmanyam Duvvuri ◽  
Beverley J. McKeon
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Small Scale ◽  
Velocity Signal ◽  
Small Scales ◽  
Amplitude Modulation Coefficient ◽  
Scale Turbulence ◽  
Scale Motion ◽  
Phase Relationships

AbstractA formal relationship between the skewness and the correlation coefficient of large and small scales, termed the amplitude modulation coefficient, is established for a general statistically stationary signal and is analysed in the context of a turbulent velocity signal. Both the quantities are seen to be measures of phase in triadically consistent interactions between scales of turbulence. The naturally existing phase relationships between large and small scales in a turbulent boundary layer are then manipulated by exciting a synthetic large-scale motion in the flow using a spatially impulsive dynamic wall roughness perturbation. The synthetic scale is seen to alter the phase relationships, or the degree of modulation, in a quasi-deterministic manner by exhibiting a phase-organizing influence on the small scales. The results presented provide encouragement for the development of a practical framework for favourable manipulation of energetic small-scale turbulence through large-scale inputs in a wall-bounded turbulent flow.

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