Wall turbulence manipulation by large-scale streamwise vortices

2002 ◽  
Vol 473 ◽  
pp. 23-58 ◽  
Author(s):  
GAETANO IUSO ◽  
MICHELE ONORATO ◽  
PIER GIORGIO SPAZZINI ◽  
GAETANO MARIA DI CICCA
Keyword(s):  
Large Scale ◽  
Time Derivative ◽  
Vortex Generator ◽  
Turbulent Channel Flow ◽  
Wall Turbulence ◽  
Spanwise Direction ◽  
Streamwise Vortices ◽  
Turbulent Structures ◽  
Reduced Frequency ◽  
Flow Manipulation

This paper describes an experimental study of the manipulation of a fully developed turbulent channel flow through large-scale streamwise vortices originated by vortex generator jets distributed along the wall in the spanwise direction. Apart from the interest in flow management itself, an important aim of the research is to observe the response of the flow to external perturbations as a technique for investigating the structure of turbulence. Considerable mean and fluctuating skin friction reductions, locally as high as 30% and 50% respectively, were measured for an optimal forcing flow intensity. Mean and fluctuating velocity profiles are also greatly modified by the manipulating large-scale vortices; in particular, attenuation of the turbulence intensity was measured. Moreover the flow manipulation caused an increase in longitudinal coherence of the wall organized motions, accompanied by a reduced frequency of burst events, demonstrated by a reduction of the velocity time derivative PDFs and by an higher intermittency. A strong transversal periodic organization of the flow field was observed, including some typical behaviours in each of the periodic boxes originated by the interaction of the vortex pairs. Results are interpreted and discussed in terms of management of the near-wall turbulent structures and with reference to the wall turbulence regeneration mechanisms suggested in the literature.

Interactions of a stationary finite-sized particle with wall turbulence

2007 ◽  
Vol 594 ◽  
pp. 271-305 ◽  
Author(s):  
LANYING ZENG ◽  
S. BALACHANDAR ◽  
PAUL FISCHER ◽  
FADY NAJJAR
Keyword(s):  
Numerical Simulations ◽  
Finite Size ◽  
Turbulent Channel Flow ◽  
Direct Numerical Simulations ◽  
Wall Turbulence ◽  
Streamwise Vortices ◽  
Turbulent Structures ◽  
Lift Forces ◽  
Drag And Lift ◽  
Particle Force

Reliable information on forces on a finite-sized particle in a turbulent boundary layer is lacking, so workers continue to use standard drag and lift correlations developed for a laminar flow to predict drag and lift forces. Here we consider direct numerical simulations of a turbulent channel flow over an isolated particle of finite size. The size of the particle and its location within the turbulent channel are systematically varied. All relevant length and time scales of turbulence, attached boundary layers on the particle, and particle wake are faithfully resolved, and thus we consider fully resolved direct numerical simulations. The results from the direct numerical simulation are compared with corresponding predictions based on the standard drag relation with and without the inclusion of added-mass and shear-induced lift forces. The influence of turbulent structures, such as streaks, quasi-streamwise vortices and hairpin packets, on particle force is explored. The effect of vortex shedding is also observed to be important for larger particles, whoseReexceeds a threshold.

scholarly journals On the formation of streamwise vortices by plasma vortex generators

2013 ◽  
Vol 733 ◽  
pp. 370-393 ◽  
Author(s):  
Timothy N. Jukes ◽  
Kwing-So Choi
Keyword(s):  
Boundary Layer ◽  
Flow Direction ◽  
Vortex Formation ◽  
Vortex Generator ◽  
Streamwise Vortex ◽  
Spanwise Direction ◽  
Formation Mechanisms ◽  
Wall Jet ◽  
Streamwise Vortices ◽  
Barrier Discharge Plasma

AbstractThe streamwise vortices generated by dielectric-barrier-discharge plasma actuators in the laminar boundary layer were investigated using particle image velocimetry to understand the vortex-formation mechanisms. The plasma vortex generator was oriented along the primary flow direction to produce a body force in the spanwise direction. This created a spanwise-directed wall jet which interacted with the oncoming boundary layer to form a coherent streamwise vortex. It was found that the streamwise vortices were formed by the twisting and folding of the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet, which added its own vorticity to increase the circulation along the actuator length. This is similar to the delta-shaped, vane-type vortex generator, except that the circulation was enhanced by the addition of the vorticity in the plasma jet. It was also observed that the plasma vortex was formed close to the wall with an enhanced wall-ward entrainment, which created strong downwash above the actuator.

Evolution and dynamics of shear-layer structures in near-wall turbulence

1991 ◽  
Vol 224 ◽  
pp. 579-599 ◽  
Author(s):  
Arne V. Johansson ◽  
P. Henrik Alfredsson ◽  
John Kim
Keyword(s):  
Turbulent Channel Flow ◽  
Wall Turbulence ◽  
Turbulent Shear ◽  
Spanwise Direction ◽  
Wall Structures ◽  
Layer Structures ◽  
Turbulence Production ◽  
Near Wall Turbulence ◽  
Probe Measurements ◽  
Near Wall

Near-wall flow structures in turbulent shear flows are analysed, with particular emphasis on the study of their space–time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.

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.

Direct Numerical Simulation of Unstably Stratified Turbulent Channel Flow

1997 ◽  
Vol 119 (1) ◽  
pp. 53-61 ◽  
Author(s):  
O. Iida ◽  
N. Kasagi
Keyword(s):  
Channel Flow ◽  
Transport Mechanism ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Wall Turbulence ◽  
Thermal Plume ◽  
Streamwise Vortices ◽  
Mean Velocity ◽  
Thermal Plumes ◽  
Turbulent Friction

Direct numerical simulations of the fully developed horizontal channel flow under unstable density stratification were carried out to investigate interactive shear and buoyancy effects on the turbulent momentum and heat transport. As the Grashof number is increased, buoyant thermal plumes are generated. The large-scale thermal convection involving the thermal plumes diminishes the quasi-coherent streamwise vortices, which are known to play a major role in the transport mechanism of near-wall turbulence. The destruction of the streamwise vortices result in the increased bulk mean velocity and the decreased turbulent friction coefficient. The vertical fluid motion of thermal plumes drastically changes the transport mechanism of the Reynolds shear stress. The thermal plumes are spatially aligned in the streamwise direction, and the low-speed streaks and vortical structures are concentrated in the region where the thermal plume starts to rise. The Prandtl number effects on the turbulent kinetic energy are also studied when the thermal plumes emerge.

scholarly journals Collapse of transitional wall turbulence captured using a rare events algorithm

2021 ◽  
Vol 931 ◽  
Author(s):  
Joran Rolland
Keyword(s):  
Reynolds Number ◽  
Sampling Method ◽  
Rare Events ◽  
Streamwise Velocity ◽  
Wall Turbulence ◽  
Spanwise Direction ◽  
Streamwise Vortices ◽  
Large Size ◽  
Reactive Trajectories ◽  
O 10

This text presents one of the first successful applications of a rare events sampling method for the study of multistability in a turbulent flow without stochastic energy injection. The trajectories of collapse of turbulence in plane Couette flow, and their probability and rate of occurrence are systematically computed using adaptive multilevel splitting (AMS). The AMS computations are performed in a system of size $L_x\times L_z=24\times 18$ at Reynolds number $R=370$ with an acceleration by a factor ${O}(10)$ with respect to direct numerical simulations (DNS) and in a system of size $L_x\times L_z=36\times 27$ at Reynolds number $R=377$ with an acceleration by a factor ${O}(10^3)$ . The AMS results are validated by a comparison with DNS in the smaller system. Visualisations indicate that turbulence collapses because the self-sustaining process of turbulence fails locally. The streamwise vortices decay first in streamwise elongated holes, leaving streamwise invariant streamwise velocity tubes that experience viscous decay. These holes then extend in the spanwise direction. The examination of more than a thousand trajectories in the $(E_{k,x}=\int u_x^2/2\,\textrm {d}^3\boldsymbol {x},E_{k,y-z}=\int (u_y^2/2+u_z^2/2)\,\textrm {d}^3\boldsymbol {x})$ plane in the smaller system confirms the faster decay of streamwise vortices and shows concentration of trajectories. This hints at an instanton phenomenology in the large size limit. The computation of turning point states, beyond which laminarisation is certain, confirms the hole formation scenario and shows that it is more pronounced in larger systems. Finally, the examination of non-reactive trajectories indicates that both the vortices and the streaks reform concomitantly when the laminar holes close.

scholarly journals Turbulent structures in a statistically three-dimensional boundary layer

2018 ◽  
Vol 859 ◽  
pp. 543-565 ◽  
Author(s):  
Kevin Kevin ◽  
Jason Monty ◽  
Nicholas Hutchins
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Three Dimensional ◽  
Outer Region ◽  
Flow Region ◽  
Secondary Flows ◽  
Wall Turbulence ◽  
Turbulent Structures ◽  
Turbulence Structures

We investigate the behaviour of large-scale coherent structures in a spanwise-heterogeneous turbulent boundary layer, using particle image velocimetry on multiple orthogonal planes. The statistical three-dimensionality is imposed by a herringbone riblet surface, although the key results presented here will be common to many cases of wall turbulence with embedded secondary flows in the form of mean streamwise vortices. Instantaneous velocity fields in the logarithmic layer reveal elongated low-momentum streaks located over the upwash-flow region, where their spanwise spacing is forced by the $2\unicode[STIX]{x1D6FF}$ periodicity of the herringbone pattern. These streaks largely resemble the turbulence structures that occur naturally (and randomly located) in spanwise-homogeneous smooth-/rough-wall boundary layers, although here they are directly formed by the roughness pattern. In the far outer region, the large spanwise spacing permits the streaks to aggressively meander. The mean secondary flows are the time-averaged artefact of the unsteady and spanwise asymmetric large-scale roll modes that accompany these meandering streaks. Interestingly, this meandering, or instability, gives rise to a pronounced streamwise periodicity (i.e. an alternating coherent pattern) in the spatial statistics, at wavelengths of approximately 4.5$\unicode[STIX]{x1D6FF}$. Overall, the observed behaviours largely resemble the streak-instability model that has been proposed for the buffer region, only here at a much larger scale and at a forced spanwise spacing. This observation further confirms recent observations that such features may occur at an entire hierarchy of scales throughout the turbulent boundary layer.

scholarly journals Invariant solutions of minimal large-scale structures in turbulent channel flow for up to 1000

2016 ◽  
Vol 802 ◽  
Author(s):  
Yongyun Hwang ◽  
Ashley P. Willis ◽  
Carlo Cossu
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Turbulent Channel Flow ◽  
The Self ◽  
Wall Turbulence ◽  
Small Scale ◽  
Invariant Solutions ◽  
Large Scale Structures ◽  
Turbulent Statistics ◽  
Scale Structures

Understanding the origin of large-scale structures in high-Reynolds-number wall turbulence has been a central issue over a number of years. Recently, Rawat et al. (J. Fluid Mech., vol. 782, 2015, pp. 515–540) have computed invariant solutions for the large-scale structures in turbulent Couette flow at $Re_{\unicode[STIX]{x1D70F}}\simeq 128$ using an overdamped large-eddy simulation with the Smagorinsky model to account for the effect of the surrounding small-scale motions. Here, we extend this approach to Reynolds numbers an order of magnitude higher in turbulent channel flow, towards the regime where the large-scale structures in the form of very-large-scale motions (long streaky motions) and large-scale motions (short vortical structures) emerge energetically. We demonstrate that a set of invariant solutions can be computed from simulations of the self-sustaining large-scale structures in the minimal unit (domain of size $L_{x}=3.0h$ streamwise and $L_{z}=1.5h$ spanwise) with midplane reflection symmetry at least up to $Re_{\unicode[STIX]{x1D70F}}\simeq 1000$. By approximating the surrounding small scales with an artificially elevated Smagorinsky constant, a set of equilibrium states are found, labelled upper- and lower-branch according to their associated drag. It is shown that the upper-branch equilibrium state is a reasonable proxy for the spatial structure and the turbulent statistics of the self-sustaining large-scale structures.

scholarly journals Large-Scale Streamwise Vortices in Turbulent Channel Flow Induced by Active Wall Actuations

2017 ◽  
Vol 100 (3) ◽  
pp. 651-673 ◽  
Author(s):  
Weidan Ni ◽  
Lipeng Lu ◽  
Jian Fang ◽  
Charles Moulinec ◽  
Yufeng Yao
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Turbulent Channel Flow ◽  
Streamwise Vortices
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