The structure of a turbulent free shear layer in a rotating fluid

1992 ◽  
Vol 241 ◽  
pp. 469-502 ◽  
Author(s):  
A. A. Bidokhti ◽  
D. J. Tritton
Keyword(s):  
Shear Stress ◽  
Shear Layer ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Longitudinal Direction ◽  
Turbulence Energy ◽  
Spanwise Direction ◽  
Rotating Fluid ◽  
Free Shear Layer ◽  
Stress Changes

An experimental investigation has been carried out on the effects of rotation on the development and structure of turbulence in a free shear layer, oriented so that its mean vorticity is parallel or antiparallel to the system vorticity. The effective local Rossby number extended down to about 1/3. The experimental methods were hydrogen-bubble flow visualization and hot-film anemometry.In summarizing the results we refer to stabilized flow when the system vorticity has the same sign as the shear vorticity and destabilized and subsequently restabilized when it has the opposite sign (Tritton 1992). The roller eddy pattern, familiar in non-rotating flow, was observed in all stabilized flows, but was almost completely disrupted by even weak destabilization. Notable features of the quantitative results were: reorientation by Coriolis effects of the Reynolds stress tensor (inferred from the ratio of the cross-stream to longitudinal turbulence intensity and the normalized shear stress); changes in the ratio of spanwise to longitudinal intensity similar to but weaker than changes in the ratio of cross-stream to longitudinal; a gradual decrease, with increasing stabilization, of the Reynolds shear stress leading ultimately to its changing sign; an increase of the Reynolds shear stress in the destabilized range followed by rapid collapse to almost zero with restabilization. Absolute intensities did not change in line with the turbulence energy production, implying enhancement of dissipation in destabilized flow and inhibition in stabilized and restabilized. Correlation measurements indicated changes of lengthscale in the spanwise direction, and spectra indicated changes in the longitudinal direction that suggest that this enhancement and inhibition are associated with variations between fully three-dimensional and partially two-dimensional turbulence. Data for a wake in a rotating fluid (Witt & Joubert 1985) show similarities to some of the above observations and can be incorporated into the interpretation.

Effects of wavelength and amplitude of a wavy cylinder in cross-flow at low Reynolds numbers

2009 ◽  
Vol 620 ◽  
pp. 195-220 ◽  
Author(s):  
K. LAM ◽  
Y. F. LIN
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Circular Cylinder ◽  
Drag Coefficient ◽  
Shear Layer ◽  
Lift Coefficient ◽  
Three Dimensional ◽  
Spanwise Direction ◽  
Free Shear Layer ◽  
The Mean

Three-dimensional numerical simulations of laminar flow around a circular cylinder with sinusoidal variation of cross-section along the spanwise direction, named ‘wavy cylinder’, are performed. A series of wavy cylinders with different combinations of dimensionless wavelength (λ/Dm) and wave amplitude (a/Dm) are studied in detail at a Reynolds number of Re = U∞Dm/ν = 100, where U∞ is the free-stream velocity and Dm is the mean diameter of a wavy cylinder. The results of variation of mean drag coefficient and root mean square (r.m.s.) lift coefficient with dimensionless wavelength show that significant reduction of mean and fluctuating force coefficients occurs at optimal dimensionless wavelengths λ/Dm of around 2.5 and 6 respectively for the different amplitudes studied. Based on the variation of flow structures and force characteristics, the dimensionless wavelength from λ/Dm = 1 to λ/Dm = 10 is classified into three wavelength regimes corresponding to three types of wake structures. The wake structures at the near wake of different wavy cylinders are captured. For all wavy cylinders, the flow separation line varies along the spanwise direction. This leads to the development of a three-dimensional free shear layer with periodic repetition along the spanwise direction. The three-dimensional free shear layer of the wavy cylinder is larger and more stable than that of the circular cylinder, and in some cases the free shear layer even does not roll up into a mature vortex street behind the cylinder. As a result, the mean drag coefficients of some of the typical wavy cylinders are less than that of a corresponding circular cylinder with a maximum drag coefficient reduction up to 18%. The r.m.s. lift coefficients are greatly reduced to practically zero at optimal wavelengths. In the laminar flow regime (60 ≤ Re ≤ 150), the values of optimal wavelength are Reynolds number dependent.

Turbulence structure of a reattaching mixing layer

1981 ◽  
Vol 110 ◽  
pp. 171-194 ◽  
Author(s):  
C. Chandrsuda ◽  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Flow Region ◽  
Turbulent Energy ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Recirculating Flow

Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.

Large-scale modes of turbulent channel flow: transport and structure

2001 ◽  
Vol 448 ◽  
pp. 53-80 ◽  
Author(s):  
Z. LIU ◽  
R. J. ADRIAN ◽  
T. J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Shear Layer ◽  
Turbulent Kinetic Energy ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Reynolds Numbers ◽  
Upstream Region ◽  
Two Dimensional ◽  
Normal Plane

Turbulent flow in a rectangular channel is investigated to determine the scale and pattern of the eddies that contribute most to the total turbulent kinetic energy and the Reynolds shear stress. Instantaneous, two-dimensional particle image velocimeter measurements in the streamwise-wall-normal plane at Reynolds numbers Reh = 5378 and 29 935 are used to form two-point spatial correlation functions, from which the proper orthogonal modes are determined. Large-scale motions – having length scales of the order of the channel width and represented by a small set of low-order eigenmodes – contain a large fraction of the kinetic energy of the streamwise velocity component and a small fraction of the kinetic energy of the wall-normal velocities. Surprisingly, the set of large-scale modes that contains half of the total turbulent kinetic energy in the channel, also contains two-thirds to three-quarters of the total Reynolds shear stress in the outer region. Thus, it is the large-scale motions, rather than the main turbulent motions, that dominate turbulent transport in all parts of the channel except the buffer layer. Samples of the large-scale structures associated with the dominant eigenfunctions are found by projecting individual realizations onto the dominant modes. In the streamwise wall-normal plane their patterns often consist of an inclined region of second quadrant vectors separated from an upstream region of fourth quadrant vectors by a stagnation point/shear layer. The inclined Q4/shear layer/Q2 region of the largest motions extends beyond the centreline of the channel and lies under a region of fluid that rotates about the spanwise direction. This pattern is very similar to the signature of a hairpin vortex. Reynolds number similarity of the large structures is demonstrated, approximately, by comparing the two-dimensional correlation coefficients and the eigenvalues of the different modes at the two Reynolds numbers.

scholarly journals The Effect of Habitat Structure Boulder Spacing on Near-Bed Shear Stress and Turbulent Events in a Gravel Bed Channel

Water ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 1423
Author(s):  
Amir Golpira ◽  
Fengbin Huang ◽  
Abul B.M. Baki
Keyword(s):  
Shear Stress ◽  
Habitat Structure ◽  
Open Channel ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Quadrant Analysis ◽  
Bed Shear Stress ◽  
Gravel Bed ◽  
Restoration Practices ◽  
Ejections And Sweeps

This study experimentally investigated the effect of boulder spacing and boulder submergence ratio on the near-bed shear stress in a single array of boulders in a gravel bed open channel flume. An acoustic Doppler velocimeter (ADV) was used to measure the instantaneous three-dimensional velocity components. Four methods of estimating near-bed shear stress were compared. The results suggested a significant effect of boulder spacing and boulder submergence ratio on the near-bed shear stress estimations and their spatial distributions. It was found that at unsubmerged condition, the turbulent kinetic energy (TKE) and modified TKE methods can be used interchangeably to estimate the near-bed shear stress. At both submerged and unsubmerged conditions, the Reynolds method performed differently from the other point-methods. Moreover, a quadrant analysis was performed to examine the turbulent events and their contribution to the near-bed Reynolds shear stress with the effect of boulder spacing. Generally, the burst events (ejections and sweeps) were reduced in the presence of boulders. This study may improve the understanding of the effect of the boulder spacing and boulder submergence ratio on the near-bed shear stress estimations of stream restoration practices.

PIV Measurements in Square Backward-Facing Step

2007 ◽  
Vol 129 (8) ◽  
pp. 984-990 ◽  
Author(s):  
Mika Piirto ◽  
Aku Karvinen ◽  
Hannu Ahlstedt ◽  
Pentti Saarenrinne ◽  
Reijo Karvinen
Keyword(s):  
Shear Stress ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Experimental Tests ◽  
Expansion Ratio ◽  
Spanwise Direction ◽  
Duct Flow ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Stress Profiles

Measurements with both two-dimensional (2D) two-component and three-component stereo particle image velocimetry (PIV) and computation in 2D and three-dimensional (3D) using Reynolds stress turbulence model with commercial code are carried out in a square duct backward-facing step (BFS) in a turbulent water flow at three Reynolds numbers of about 12,000, 21,000, and 55,000 based on the step height h and the inlet streamwise maximum mean velocity U0. The reattachment locations measured at a distance of Δy=0.0322h from the wall are 5.3h, 5.6h, and 5.7h, respectively. The inlet flow condition is fully developed duct flow before the step change with the expansion ratio of 1.2. PIV results show that the mean velocity, root mean square (rms) velocity profiles, and Reynolds shear stress profiles in all the experimental flow cases are almost identical in the separated shear-layer region when they are nondimensionalized by U0. The sidewall effect of the square BFS flow is analyzed by comparing the experimental statistics with direct numerical simulation (DNS) and Reynolds stress model (RSM) data. For this purpose, the simulation is carried out for both 2D BFS and for square BFS having the same geometry in the 3D case as the experimental case at the lowest Reynolds number. A clear difference is observed in rms and Reynolds shear stress profiles between square BFS experimental results and DNS results in 2D channel in the spanwise direction. The spanwise rms velocity difference is about 30%, with experimental tests showing higher values than DNS, while in contrast, turbulence intensities in streamwise and vertical directions show slightly lower values than DNS. However, with the modeling, the turbulence statistical differences between 2D and 3D RSM cases are very modest. The square BFS indicates 0.5h–1.5h smaller reattachment distances than the reattachment lengths of 2D flow cases.

scholarly journals Measurement of turbulent flow in a narrow open channel

2016 ◽  
Vol 64 (3) ◽  
pp. 273-280 ◽  
Author(s):  
Sankar Sarkar
Keyword(s):  
Turbulent Flow ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Maximum Velocity ◽  
Turbulent Characteristics ◽  
Stress Changes ◽  
Rough Bed ◽  
Median Diameter ◽  
Rough Beds ◽  
Bed Material

Abstract The paper presents the experimental results of turbulent flow over hydraulically smooth and rough beds. Experiments were conducted in a rectangular flume under the aspect ratio b/h = 2 (b = width of the channel 0.5 m, and h = flow depth 0.25 m) for both the bed conditions. For the hydraulically rough bed, the roughness was created by using 3/8″ commercially available angular crushed stone chips; whereas sand of a median diameter d50 = 1.9 mm was used as the bed material for hydraulically smooth bed. The three-dimensional velocity components were captured by using a Vectrino (an acoustic Doppler velocimeter). The study focuses mainly on the turbulent characteristics within the dip that were observed towards the sidewall (corner) of the channel where the maximum velocity occurs below the free-surface. It was also observed that the nondimensional Reynolds shear stress changes its sign from positive to negative within the dip. The quadrant plots for the turbulent bursting shows that the signs of all the bursting events change within the dip. Below the dip, the probability of the occurrence of sweeps and ejections are more than that of inward and outward interactions. On the other hand, within the dip, the probability of the occurrence of the outward and inward interactions is more than that of sweeps and ejections.

Computational Analysis on Weld Formation Mechanism during Self-Reacting Friction Stir Welding

2020 ◽  
pp. 1-15
Author(s):  
Chenyu Zhao ◽  
Xun Liu
Keyword(s):  
Boundary Condition ◽  
Shear Stress ◽  
Friction Stir Welding ◽  
Shear Layer ◽  
Formation Mechanism ◽  
Fluid Model ◽  
Three Dimensional ◽  
Welding Process ◽  
Weld Formation ◽  
Friction Stir

Abstract Three-dimensional computational fluid dynamics models are developed to understand physical principles of self-reacting friction stir welding process. A novel approach of predicting the weld microstructure based on plastic strain distribution at cross-section behind the tool is proposed and verified with experimental results. Limitations and credibility of shear stress and velocity tool/workpiece boundary condition are evaluated from the perspective of the weld formation mechanism. The importance of the shear layer and its sticking/sliding transition state in weld formation mechanism is emphasized. From modeling perspective, shear stress boundary, which only represents a sliding condition, neglects the movement and effects of this shear layer. When shear layer is formed, due to the velocity discontinuity which could not be captured in fluid model, velocity boundary condition, which represents an averaging effect of sticking/sliding transition between tool and shear layer, is needed.

Turbulent Vorticity Diffusion in the Stronger Wall Jet Managed by a Flat Plate Wing in the Outer Layer

2015 ◽  
Author(s):  
Takuma Katayama ◽  
Shinsuke Mochizuki
Keyword(s):  
Shear Stress ◽  
Flat Plate ◽  
Outer Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Quantitative Relationship ◽  
Wall Jet ◽  
Mean Velocity ◽  
The Mean

The present experiment focuses on the vorticity diffusion in a stronger wall jet managed by a three-dimensional flat plate wing in the outer layer. Measurement of the fluctuating velocities and vorticity correlation has been carried out with 4-wire vorticity probe. The turbulent vorticity diffusion due to the large scale eddies in the outer layer is quantitatively examined by using the 4-wire vorticity probe. Quantitative relationship between vortex structure and Reynolds shear stress is revealed by means of directly measured experimental evidence which explains vorticity diffusion process and influence of the manipulating wing. It is expected that the three-dimensional outer layer manipulator contributes to keep convex profile of the mean velocity, namely, suppression of the turbulent diffusion and entrainment.

Development of a three-dimensional free shear layer

1998 ◽  
Vol 369 ◽  
pp. 49-89 ◽  
Author(s):  
A. J. RILEY ◽  
M. V. LOWSON
Keyword(s):  
Shear Layer ◽  
Flow Instability ◽  
Delta Wing ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Transition To Turbulence ◽  
Free Shear Layer ◽  
Velocity Data ◽  
Sweep Angle ◽  
Vortical Structures

Experiments have been undertaken to characterize the flow field over a delta wing, with an 85° sweep angle, at 12.5° incidence. Application of a laser Doppler anemometer has enabled detailed three-dimensional velocity data to be obtained within the free shear layer, revealing a system of steady co-rotating vortical structures. These sub-vortex structures are associated with low-momentum flow pockets in the separated vortex flow. The structures are found to be dependent on local Reynolds number, and undergo transition to turbulence. The structural features disappear as the sub-vortices are wrapped into the main vortex core. A local three-dimensional Kelvin–Helmholtz-type instability is suggested for the formation of these vortical structures in the free shear layer. This instability has parallels with the cross-flow instability that occurs in three-dimensional boundary layers. Velocity data at high Reynolds numbers have shown that the sub-vortical structures continue to form, consistent with flow visualization results over fighter aircraft at flight Reynolds numbers.

scholarly journals Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 2. Flow structures

2013 ◽  
Vol 734 ◽  
pp. 509-534 ◽  
Author(s):  
Mohammad Omidyeganeh ◽  
Ugo Piomelli
Keyword(s):  
Free Surface ◽  
Shear Layer ◽  
Three Dimensional ◽  
Internal Boundary ◽  
Flow Structures ◽  
Spanwise Direction ◽  
Plane Normal ◽  
Separated Shear Layer ◽  
Phase Alignment ◽  
Large Eddy

AbstractWe performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article (Omidyeganeh & Piomelli, J. Fluid Mech., vol. 721, 2013, pp. 454–483). Coherent flow structures and their statistics are discussed concentrating on two cases with the same crestline amplitudes and wavelengths but different crestline alignments: in-phase and staggered. The present paper shows that the induced large-scale mean streamwise vortices are the primary factor that alters the features of the instantaneous flow structures. Wall turbulence is insensitive to the crestline alignment; alternating high- and low-speed streaks appear in the internal boundary layer developing on the stoss side, whereas over the node plane (the plane normal to the spanwise direction at the node of the crestline), they are inclined towards the lobe plane (the plane normal to the spanwise direction at the most downstream point of the crestline) due to the mean spanwise pressure gradient. Spanwise vortices (rollers) generated by Kelvin–Helmholtz instability in the separated shear layer appear regularly over the lobe with much larger length scale than those over the saddle (the plane normal to the spanwise direction at the most upstream point of the crestline). Rollers over the lobe may extend to the saddle plane and affect the reattachment features; their shedding is more frequent than in 2D geometries. Vortices shed from the separated shear layer in the lobe plane undergo a three-dimensional instability while being advected downstream, and rise toward the free surface. They develop into a horseshoe shape (similar to the 2D case) and affect the whole channel depth, whereas those generated near the saddle are advected downstream and toward the bed. When the tip of such a horseshoe reaches the free surface, the ejection of flow at the surface causes ‘boils’ (upwelling events on the surface). Strong boil events are observed on the surface of the lobe planes of 3D dunes more frequently than in the saddle planes. They also appear more frequently than in the corresponding 2D geometry. The crestline alignment of the dune alters the dynamics of the flow structures, in that they appear in the lobe plane and are advected towards the saddle plane of the next dune, where they are dissipated. Boil events occur at a higher frequency in the staggered alignment, but with less intensity than in the in-phase alignment.

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