Coherent Structures in the Similarity Region of Two-Dimensional Turbulent Jets

1984 ◽  
Vol 106 (2) ◽  
pp. 187-192 ◽  
Author(s):  
J. W. Oler ◽  
V. W. Goldschmidt
Keyword(s):  
Velocity Profile ◽  
Coherent Structures ◽  
Turbulent Jets ◽  
Jet Flows ◽  
Two Dimensional ◽  
Ordered Structure ◽  
Mean Velocity ◽  
The Mean ◽  
Plane Jet ◽  
Average Position

The strongest indication of an ordered structure in the similarity region of plane jet flows is the well documented (but controversial) apparent “flapping” behavior. Previously, the negative correlation between probes placed on opposite sides of the jet centerline has been attributed to the periodic displacement of the mean velocity profile centerline about its average position, i.e., a flapping motion. The present investigation is directed at evaluating the premise of an essentially two-dimensional von Karman vortex street as being responsible for the apparent “flapping” behavior.

Turbulent flow between a rotating and a stationary disk

2001 ◽  
Vol 426 ◽  
pp. 297-326 ◽  
Author(s):  
MAGNE LYGREN ◽  
HELGE I. ANDERSSON
Keyword(s):  
Shear Stress ◽  
Boundary Layers ◽  
Coherent Structures ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Mean Flow ◽  
Mean Velocity ◽  
Stationary Disk ◽  
Dimensional Boundary ◽  
The Mean

Turbulent flow between a rotating and a stationary disk is studied. Besides its fundamental importance as a three-dimensional prototype flow, such flow fields are frequently encountered in rotor–stator configurations in turbomachinery applications. A direct numerical simulation is therefore performed by integrating the time-dependent Navier–Stokes equations until a statistically steady state is reached and with the aim of providing both long-time statistics and an exposition of coherent structures obtained by conditional sampling. The simulated flow has local Reynolds number r2ω/v = 4 × 105 and local gap ratio s/r = 0.02, where ω is the angular velocity of the rotating disk, r the radial distance from the axis of rotation, v the kinematic viscosity of the fluid, and s the gap width.The three components of the mean velocity vector and the six independent Reynolds stresses are compared with experimental measurements in a rotor–stator flow configuration. In the numerically generated flow field, the structural parameter a1 (i.e. the ratio of the magnitude of the shear stress vector to twice the mean turbulent kinetic energy) is lower near the two disks than in two-dimensional boundary layers. This characteristic feature is typical for three-dimensional boundary layers, and so are the misalignment between the shear stress vector and the mean velocity gradient vector, although the degree of misalignment turns out to be smaller in the present flow than in unsteady three-dimensional boundary layer flow. It is also observed that the wall friction at the rotating disk is substantially higher than at the stationary disk.Coherent structures near the disks are identified by means of the λ2 vortex criterion in order to provide sufficient information to resolve a controversy regarding the roles played by sweeps and ejections in shear stress production. An ensemble average of the detected structures reveals that the coherent structures in the rotor–stator flow are similar to the ones found in two-dimensional flows. It is shown, however, that the three-dimensionality of the mean flow reduces the inter-vortical alignment and the tendency of structures of opposite sense of rotation to overlap. The coherent structures near the disks generate weaker sweeps (i.e. quadrant 4 events) than structures in conventional two-dimensional boundary layers. This reduction in the quadrant 4 contribution from the coherent structures is believed to explain the reduced efficiency of the mean flow in producing Reynolds shear stress.

Drag reduction by riblets

2011 ◽  
Vol 369 (1940) ◽  
pp. 1412-1427 ◽  
Author(s):  
Ricardo García-Mayoral ◽  
Javier Jiménez
Keyword(s):  
Velocity Profile ◽  
Drag Reduction ◽  
Cross Section ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Velocity ◽  
Plant Canopies ◽  
Stability Model ◽  
The Mean

The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A + g is a better characterization of this breakdown than the riblet spacing, with an optimum . It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y + ≈30, with typical streamwise wavelengths . They are connected with a Kelvin–Helmholtz-like instability of the mean velocity profile, also found in flows over plant canopies and other surfaces with transpiration. A simplified stability model for the ribbed surface approximately accounts for the scaling of the viscous breakdown with  A + g .

The interaction between the mean flow and coherent structures in turbulent mixing layers

1994 ◽  
Vol 260 ◽  
pp. 81-94 ◽  
Author(s):  
J. Cohen ◽  
B. Marasli ◽  
V. Levinski
Keyword(s):  
Velocity Profile ◽  
Coherent Structures ◽  
Turbulent Mixing ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Mean Flow ◽  
Mean Velocity ◽  
Turbulent Mixing Layer ◽  
Self Similar ◽  
The Mean

The nonlinear interaction between the mean flow and a coherent disturbance in a two-dimensional turbulent mixing layer is addressed. Based on considerations from stability theory, previous experimental results, in particular the modification of the mean velocity profile, the peculiar growth of the forced shear-layer thickness and the spatial growth of the disturbance amplitude, are explained. A model that assumes a quasi-parallel mean flow having a self-similar mean velocity profile is developed. The model is capable of predicting the downstream evolution of turbulent mixing layers subjected to external excitations.

Laser-Doppler anemometer measurements in drag-reducing channel flows

1975 ◽  
Vol 70 (2) ◽  
pp. 369-392 ◽  
Author(s):  
M. M. Reischman ◽  
W. G. Tiederman
Keyword(s):  
Velocity Profile ◽  
Streamwise Velocity ◽  
Laser Doppler Anemometer ◽  
Channel Flows ◽  
Two Dimensional ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Buffer Region ◽  
The Mean ◽  
Made In

The objective of this study was to make velocity measurements in drag-reducing flows which would be sufficient in scope and accuracy to test proposed models of drag-reducing flows and to yield new information about the mechanisms of drag reduction. Consequently, measurements of the mean and turbulence intensity of the streamwise velocity component were made in fully developed, turbulent, drag-reducing flow in a two-dimensional channel with a laser-Doppler anemometer. The anemometer was operated in the individual-realization mode and corrections were made to eliminate statistical biasing of the data. Two polyacrylamides and a polyethylene oxide were used to produce seven flows which had drag reductions ranging from 24 to 41 %. Measurements were also made in water to establish the standard characteristics of the flow channel.The data show that the drag-reducing mean velocity profile can be divided into three zones: a viscous sublayer, a buffer or interactive region and a logarithmic region. There is no evidence that the viscous sublayers of the drag-reducing channel flows are thicker than those in the solvent flows. In addition the normalized streamwise fluctuations are essentially the same in both the solvent and drag-reducing sublayers. The changes caused by the polymer addition occur in the buffer region. The drag-reducing buffer region is thicker and the velocity profile in the outer flow region adjusts in order to accommodate this buffer-region thickening. The measurements of the streamwise velocity fluctuations also show that the polymer additives redistribute the primary turbulent activity over a broadened buffer region. The normalized magnitude of these fluctuations is, however, considerably lower in these two-dimensional drag-reducing channel flows than in those previously reported by Rudd (1972), Logan (1972) and Kumor & Sylvester (1973). Moreover, the mean velocity profiles in the buffer region do not confirm the hypothesis of Virk, Mickley & Smith (1970) that the data will follow their proposed ‘ultimate profile’ when the drag reduction is less than that given by the maximum asymptote. The mean velocity measurements also show that the proposed methods for predicting the upward shift in the outer portion of the mean velocity profile are inconsistent and lack universality. However, these results do confirm the previous suggestions of Virk (1971), Tomita (1970) and Lumley (1973) that the buffer region is the area of importance and change in drag-reducing flows.

scholarly journals Effect of the Flame Dome Depth and Improvement of the Pressure Loss in the Dump Diffuser

1998 ◽  
Author(s):  
Shinji Honami ◽  
Wataru Tsuboi ◽  
Takaaki Shizawa
Keyword(s):  
Velocity Profile ◽  
Reynolds Stress ◽  
Flow Rate ◽  
Pressure Loss ◽  
Total Pressure ◽  
Flow Behavior ◽  
Sudden Expansion ◽  
Mean Velocity ◽  
The Mean ◽  
Stress Profiles

This paper presents the effect of flame dome depth on the total pressure performance and flow behavior in a sudden expansion region of the combustor diffuser without flow entering the dome head. The mean velocity and turbulent Reynolds stress profiles in the sudden expansion region were measured by a Laser Doppler Velocitmetry (LDV) system. The experiments show that total pressure loss is increased, when flame dome depth is increased. Installation of an inclined combuster wall in the sudden expansion region is suggested from the viewpoint of a control of the reattaching flow. The inclined combustor wall is found to be effective in improvement of the diffuser performance. Better characteristics of the flow rate distribution into the branched channels are obtained in the inclined wall configuration, even if the distorted velocity profile is provided at the diffuser inlet.

scholarly journals WKB theory for rapid distortion of inhomogeneous turbulence

1999 ◽  
Vol 390 ◽  
pp. 325-348 ◽  
Author(s):  
S. NAZARENKO ◽  
N. K.-R. KEVLAHAN ◽  
B. DUBRULLE
Keyword(s):  
Large Scale ◽  
Isotropic Turbulence ◽  
Two Dimensional ◽  
Turbulent Spot ◽  
Mean Flow ◽  
Mean Velocity ◽  
Inhomogeneous Turbulence ◽  
Large Scale Vortex ◽  
The Mean ◽  
Mean Strain

A WKB method is used to extend RDT (rapid distortion theory) to initially inhomogeneous turbulence and unsteady mean flows. The WKB equations describe turbulence wavepackets which are transported by the mean velocity and have wavenumbers which evolve due to the mean strain. The turbulence also modifies the mean flow and generates large-scale vorticity via the averaged Reynolds stress tensor. The theory is applied to Taylor's four-roller flow in order to explain the experimentally observed reduction in the mean strain. The strain reduction occurs due to the formation of a large-scale vortex quadrupole structure from the turbulent spot confined by the four rollers. Both turbulence inhomogeneity and three-dimensionality are shown to be important for this effect. If the initially isotropic turbulence is either homogeneous in space or two-dimensional, it has no effect on the large-scale strain. Furthermore, the turbulent kinetic energy is conserved in the two-dimensional case, which has important consequences for the theory of two-dimensional turbulence. The analytical and numerical results presented here are in good qualitative agreement with experiment.

scholarly journals Stochastic flow approach to model the mean velocity profile of wall-bounded flows

2019 ◽  
Vol 99 (6) ◽  
Author(s):  
Benoît Pinier ◽  
Etienne Mémin ◽  
Sylvain Laizet ◽  
Roger Lewandowski
Keyword(s):  
Velocity Profile ◽  
Stochastic Flow ◽  
Mean Velocity ◽  
The Mean ◽  
Wall Bounded Flows

A vortex-street model of the flow in the similarity region of a two-dimensional free turbulent jet

1982 ◽  
Vol 123 ◽  
pp. 523-535 ◽  
Author(s):  
J. W. Oler ◽  
V. W. Goldschmidt
Keyword(s):  
Experimental Data ◽  
Reynolds Stress ◽  
Turbulent Jet ◽  
Vortex Street ◽  
Two Dimensional ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Core ◽  
The Mean ◽  
Similarity Relationships

The mean-velocity profiles and entrainment rates in the similarity region of a two-dimensional jet are generated by a simple superposition of Rankine vortices arranged to represent a vortex street. The spacings between the vortex centres, their two-dimensional offsets from the centreline, as well as the core radii and circulation strengths, are all governed by similarity relationships and based upon experimental data.Major details of the mean flow field such as the axial and lateral mean-velocity components and the magnitude of the Reynolds stress are properly determined by the model. The sign of the Reynolds stress is, however, not properly predicted.

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