The structure of highly sheared turbulence

1995 ◽  
Vol 303 ◽  
pp. 155-167 ◽  
Author(s):  
F. A. de Souza ◽  
V. D. Nguyen ◽  
S. Tavoularis
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Turbulence Structure ◽  
Shear Rates ◽  
Energy Growth ◽  
Lower Shear ◽  
Self Similar ◽  
The Mean ◽  
Homogeneous Shear Flow ◽  
Low Shear

Uniformly sheared flows have been generated in a high-speed wind tunnel at shear rates higher than previously achieved, in an effort to approach those in the inner turbulent boundary layer. As at lower shear rates, the turbulence structure was found to attain a self-similar state with approximately constant anisotropies and exponential kinetic energy growth. The normal Reynolds stress anisotropies showed no systematic dependence upon the mean shear within the examined range; however, the shear stress anisotropy was significantly lower than the low-shear values, in conformity with boundary layer measurements and direct numerical simulations of homogeneous shear flow.

Effects of the Axisymmetric Contraction Shape on Incompressible Turbulent Flow

1976 ◽  
Vol 98 (1) ◽  
pp. 58-68 ◽  
Author(s):  
A. K. M. F. Hussain ◽  
V. Ramjee
Keyword(s):  
Boundary Layer ◽  
Turbulence Structure ◽  
Exit Plane ◽  
Core Flow ◽  
The Core ◽  
Local Contraction ◽  
Contraction Ratio ◽  
Exit Boundary ◽  
The Mean ◽  
Equal Effectiveness

The performance characteristics of four different axisymmetric contraction shapes with the same contraction ratio are experimentally investigated for incompressible flow. The pre- and postcontraction mean and turbulent velocity profiles and spectra, and the variation of the mean and turbulent velocities along the axis as a function of local contraction ratio and axial length are presented in this paper. The results show that all the nozzles are of essentially equal effectiveness as far as the core flow in the exit plane is concerned. But the mean and turbulence characteristics of the exit boundary layer, the upstream influence of the contraction, and the departure from equipartition within the nozzle vary significantly with the contraction shape. The data demonstrate the inadequacy of the Batchelor-Proudman-Ribner-Tucker theory in predicting the effect of a contraction on the turbulence structure. These data are of interest in wind tunnel and nozzle design, and in boundary layer prediction.

The reduction and elimination of a closed separation region by free-stream turbulence

2001 ◽  
Vol 446 ◽  
pp. 271-308 ◽  
Author(s):  
M. KALTER ◽  
H. H. FERNHOLZ
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Reverse Flow ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Free Stream Turbulence ◽  
The Mean

This paper is an extension of an experimental investigation by Alving & Fernholz (1996). In the present experiments the effects of free-stream turbulence were investigated on a boundary layer with an adverse pressure gradient and a closed reverse-flow region. By adding free-stream turbulence the mean reverse-flow region was shortened or completely eliminated and this was used to control the size of the separation bubble. The turbulence intensity was varied between 0.2% and 6% using upstream grids while the turbulence length scale was on the order of the boundary layer thickness. Mean and fluctuating velocities as well as spectra were measured by means of hot-wire and laser-Doppler anemometry and wall shear stress by wall pulsed-wire and wall hot-wire probes.Free-stream turbulence had a small effect on the boundary layer in the mild adverse-pressure-gradient region but in the vicinity of separation and along the reverse-flow region mean velocity profiles, skin friction and turbulence structure were strongly affected. Downstream of the mean or instantaneous reverse-flow regions highly disturbed boundary layers developed in a nominally zero pressure gradient and converged to a similar turbulence structure in all three cases at the end of the test section. This state was, however, still very different from that in a canonical boundary layer.

Meandering due to large eddies and the statistically self-similar dynamics of quasi-two-dimensional jets

2012 ◽  
Vol 692 ◽  
pp. 347-368 ◽  
Author(s):  
Julien R. Landel ◽  
C. P. Caulfield ◽  
Andrew W. Woods
Keyword(s):  
Vertical Velocity ◽  
High Speed ◽  
Turbulent Jets ◽  
Theoretical Models ◽  
Maximum Speed ◽  
Two Dimensional ◽  
The Core ◽  
Self Similar ◽  
The Mean ◽  
Large Eddies

AbstractWe investigate experimentally the structure of quasi-two-dimensional plane turbulent jets discharged vertically from a slot of width $d$ into a fluid confined between two relatively close rigid boundaries with gap $W\ensuremath{\sim} O(d)$. At large vertical distances $z\gg W$ the jet structure consists of a meandering core with large counter-rotating eddies, which develop on alternate sides of the core. Using particle image velocimetry, we observe an inverse cascade typical of quasi-two-dimensional turbulence where both the core and the eddies grow linearly with $z$ and travel at an average speed proportional to ${z}^{\ensuremath{-} 1/ 2} $. However, although the present study concerns quasi-two-dimensional confined jets, the jets are self-similar and the mean properties are consistent with both experimental results and theoretical models of the time-averaged properties of fully unconfined planar two-dimensional jets. We believe that the dynamics of the interacting core and large eddies accounts for the Gaussian profile of the mean vertical velocity as shown by the spatial statistical distribution of the core and eddy structure. The lateral excursions (caused by the propagating eddies) of this high-speed central core produce a Gaussian distribution for the time-averaged vertical velocity. In addition, we find that approximately 75 % of the total momentum flux of the jet is contained within the core. The eddies travel substantially slower (at approximately 25 % of the maximum speed of the core) at each height and their growth is primarily attributed to entrainment of ambient fluid. The frequency of occurrence of the eddies decreases in a stepwise manner due to merging, with a well-defined minimum value of the corresponding Strouhal number $\mathit{St}\geq 0. 07$.

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.

Relationship between erythrocyte aggregate size and flow rate in skeletal muscle venules

2004 ◽  
Vol 286 (1) ◽  
pp. H113-H120 ◽  
Author(s):  
Jeffrey J. Bishop ◽  
Patricia R. Nance ◽  
Aleksander S. Popel ◽  
Marcos Intaglietta ◽  
Paul C. Johnson
Keyword(s):  
Red Blood Cells ◽  
Blood Cells ◽  
High Speed ◽  
Aggregate Size ◽  
Video Camera ◽  
Low Flow ◽  
Shear Rates ◽  
Particle Length ◽  
Flow Stream ◽  
Low Shear

In previous studies we showed that intravenous infusion of Dextran 500 in the rat causes blunting of the velocity profile of red blood cells in venules at low shear rates. To determine whether this blunting is associated with the formation of red blood cell aggregates, we measured the length and width of particles in the venular flow stream at systemic hematocrits up to 20% with a high-speed video camera and a new image analysis technique. Data were obtained at various shear rates under normal (nonaggregating) conditions as well as after infusion of Dextran 500. Under normal conditions, particle length (parallel to the vessel axis) was 6.5 ± 2.7 μm and width (perpendicular to the axis) was 6.1 ± 1.7 μm, in agreement with published dimensions of individual red blood cells for this species. After Dextran 500 infusion, particle length and width increased significantly to 8.7 ± 5.1 and 10.4 ± 4.4 μm, respectively. Particle dimensions were greater in the central region of the flow stream for both normal and dextran-treated blood and increased at low flow rates with dextran-treated blood. This study provides direct confirmation of aggregate formation at low shear in venules with high-molecular-weight dextran as well as an estimate of aggregate size and range.

scholarly journals Direct numerical simulation of a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation

2017 ◽  
Vol 829 ◽  
pp. 392-419 ◽  
Author(s):  
V. Kitsios ◽  
A. Sekimoto ◽  
C. Atkinson ◽  
J. A. Sillero ◽  
G. Borrell ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Direct Numerical Simulation ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Streamwise Velocity ◽  
Far Field ◽  
Self Similar ◽  
The Mean

The statistical properties are presented for the direct numerical simulation of a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) at the verge of separation. The APG TBL has a momentum thickness-based Reynolds number range from $Re_{\unicode[STIX]{x1D6FF}_{2}}=570$ to 13 800, with a self-similar region from $Re_{\unicode[STIX]{x1D6FF}_{2}}=10\,000$ to 12 300. Within this domain the average non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}=39$, where for a unit density $\unicode[STIX]{x1D6FD}=\unicode[STIX]{x1D6FF}_{1}P_{\!e}^{\prime }/\unicode[STIX]{x1D70F}_{w}$, with $\unicode[STIX]{x1D6FF}_{1}$ the displacement thickness, $\unicode[STIX]{x1D70F}_{w}$ the mean shear stress at the wall and $P_{\!e}^{\prime }$ the far-field pressure gradient. This flow is compared with previous zero pressure gradient and mild APG TBL ($\unicode[STIX]{x1D6FD}=1$) results of similar Reynolds number. All flows are generated via the direct numerical simulation of a TBL on a flat surface with far-field boundary conditions tailored to apply the desired pressure gradient. The conditions for self-similarity, and the appropriate length and velocity scales, are derived. The mean and Reynolds stress profiles are shown to collapse when non-dimensionalised on the basis of these length and velocity scales. As the pressure gradient increases, the extent of the wake region in the mean streamwise velocity profiles increases, whilst the extent of the log-layer and viscous sublayer decreases. The Reynolds stress, production and dissipation profiles of the APG TBL cases exhibit a second outer peak, which becomes more pronounced and more spatially localised with increasing pressure gradient. This outer peak is located at the point of inflection of the mean velocity profiles, and is suggestive of the presence of a shear flow instability. The maximum streamwise velocity variance is located at a wall normal position of $\unicode[STIX]{x1D6FF}_{1}$ of spanwise wavelength of $2\unicode[STIX]{x1D6FF}_{1}$. In summary as the pressure gradient increases the flow has properties less like a zero pressure gradient TBL and more akin to a free shear layer.

Calculation of Complex Viscosities from Slow-Speed Transient Torque Measurements with the Mooney Rheometer

1982 ◽  
Vol 55 (5) ◽  
pp. 1426-1436 ◽  
Author(s):  
N. Nakajima ◽  
E. R. Harrell
Keyword(s):  
Steady State ◽  
Shear Rate ◽  
Practical Interest ◽  
Actual Process ◽  
Steady State Flow ◽  
Shear Rates ◽  
State Flow ◽  
Lower Shear ◽  
Rate Limit ◽  
Low Shear

Abstract It has been customary to assign certain time scales to given polymer processes. For example, the extrusion process is said to occur at a shear rate in the order of some hundred reciprocal seconds and injection molding at some thousand reciprocal seconds or higher. These statements are usually accompanied by instructions that the viscosity of a material is to be measured at the respective shear rate in order to characterize its processability. However, the above argument is only partially valid, and a single-point viscosity measurement is only a part of the processability evaluation. Inadequacy of the above rationale has been recognized by industry for a long time. With the rapid growth of plastics production in the 1960's, plastic processing went through a technological evolution. In the early stage of evolution of various fabrication techniques, development of suitable grades of material for the respective processes was the major effort of the plastic producers. Soon it became clear that resins which had the same viscosity at the so-called processing shear rate often behaved differently in the actual process. This led to the measurement of the steady-state flow properties at lower shear rates than the so-called processing shear rate, which was representative of the highest shear rate involved in the process. The significant observation was that the viscosity differences of resins often were magnified at the lower shear rate. Sometimes, a subtle difference in processability corresponded to a viscosity difference observable only at very low shear rates. Thus, acquisition of the steady-state flow curve from the low-shear-rate limit (i.e., the Newtonian viscosity) to the high shear rate limit (i.e., the limiting power-law region) became a subject of practical interest. The characterization of such flow curves and their relation to molecular weight distribution (MWD) became a subject of intense study for commercial plastics having a large variation in MWD.

Compressibility Effects on Turbulence Growth in High-Speed Shear Flows

1994 ◽  
Vol 47 (6S) ◽  
pp. S179-S183
Author(s):  
S. Sarkar
Keyword(s):  
Numerical Simulation ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
High Speed ◽  
Shear Flows ◽  
Length Scale ◽  
Homogeneous Shear ◽  
The Mean ◽  
Homogeneous Shear Flow ◽  
Compressibility Effects

Compressibility effects on the evolution of turbulence are obtained from direct numerical simulation of homogeneous shear flow. It is found that when the gradient Mach number - a parameter based on the mean shear rate, integral length scale and speed of sound - increases, the growth of turbulent kinetic energy is inhibited. The reduced ‘efficiency’ of production is shown to lead to the inhibited growth of turbulent kinetic energy. Implications for inhomogeneous shear flows are discussed.

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