scholarly journals Effects of different mean velocity ratios on dynamics characteristics of a coaxial jet

2008 ◽  
Vol 12 (2) ◽  
pp. 49-58 ◽  
Author(s):  
Ali Mergheni ◽  
Toufik Boushaki ◽  
Jean-Charles Sautet ◽  
Gille Godard ◽  
Ticha Ben ◽  
...  
Keyword(s):  
Shear Stress ◽  
Velocity Ratio ◽  
Diameter Ratio ◽  
Outer Diameter ◽  
Mean Velocity ◽  
Potential Core ◽  
Zero Crossing ◽  
Core Length ◽  
The Mean ◽  
Outer Potential

The flow field of a coaxial jet configuration having inner and outer diameter ratio Di /Do = 0.33 is studied for four values of the velocity ratios and m = Ui /Uo = 5.17, 1.13, 0.77, and 0.54. The profiles of the mean axial velocity, of the axial turbulence intensities, and of the shear stress are described for the initial and fully zones. The obtained results show the inner potential core length of the coaxial jet strongly depends on the velocity ratio while the outer potential core for jets having velocity ratios greater than unity seems to be insensitive to the velocity ratio. As expected, the inner jet core length is seen to decrease with decreasing velocity ratio; jets with velocity less than unity develop faster than those with m greater than unity and the Reynolds stress show a zero-crossing in the near-region. .

The Effect of the Velocity Ratio on the Diffusion Inhibition of Coaxial Jet

2019 ◽  
Author(s):  
Fujio Akagi ◽  
Takumi Etou ◽  
Yuito Fukuda ◽  
Ryoya Yoshioka ◽  
Youichi Ando ◽  
...  
Keyword(s):  
Shear Layer ◽  
Energy Transport ◽  
Velocity Ratio ◽  
Streamwise Velocity ◽  
Inhibition Mechanism ◽  
Outer Diameter ◽  
Potential Core ◽  
Round Jet ◽  
Eddy Simulation ◽  
Core Length

Abstract The mixing characteristics of a coaxial round jet having inner to outer diameter ratio di/do = 0.67 was investigated to establish the method for mass and energy transport with inhibiting its diffusion. In the present study, the effect of the velocity ratio on the streamwise length of the potential core of the central round jet, the streamwise velocity distribution, and the evolution of vortcies of a coaxial jet was evaluated by using Large Eddy Simulation and experimental flow visualization to clarify diffusion inhibition mechanism of a central jet. Three velocity ratios γ = 0.25, 0.5, and 0.75 were conducted under a single condition of the central jet Reynolds number of 2000. The results show that the potential core length of a central jet become the longest on condition of the velocity ratio of 0.5, and the length was 15 times of the inner nozzle diameter. This value is equivalent to 3.8 times in the case of a general round jet. In other velocity ratio, it was also confirmed that the potential core length of a central jet becomes short by enhancement of vortices generation in the shear layer of annular and central jet. It is concluded that the velocity gradient in the shear layer is the important parameter in determining the diffusion of a central jet.

Turbulent Mixing in the Developing Region of Coaxial Jets

1973 ◽  
Vol 95 (3) ◽  
pp. 467-473 ◽  
Author(s):  
D. Dura˜o ◽  
J. H. Whitelaw
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Velocity Ratio ◽  
Jet Flows ◽  
Normal Stresses ◽  
Downstream Location ◽  
Mean Velocity ◽  
Coaxial Jets ◽  
Developing Region ◽  
The Mean

Measurements of mean velocity, the three normal stresses and Reynolds shear stress are reported in the developing region of coaxial jet flows. The measurements were obtained with three velocity ratios, i.e., values of the ratio of maximum initial pipe velocity to maximum initial annulus velocity of 0, 0.23, and 0.62 and at downstream distances up to 17 outer diameters. The results show that coaxial jets tend to reach a self-preserving state much more rapidly than axisymmetric single jets; that the mean velocity, normal stresses, and Reynolds shear stress attain this state at a similar downstream location; and that, for the particular geometry investigated, a velocity ratio of around 0.15 corresponds to the slowest rate of development. Relationships between mean velocity gradient, Reynolds shear stress, and turbulent kinetic energy are examined to assess their ability to characterize the present flow: the results indicate the need to take account of the normal stresses in any proposed mathematical model.

scholarly journals Effect of Mean Velocity-to-Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 883
Author(s):  
Nargess Moghaddassi ◽  
Seyed Habib Musavi-Jahromi ◽  
Mohammad Vaghefi ◽  
Amir Khosrojerdi
Keyword(s):  
Experimental Investigation ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Scour Depth ◽  
Incipient Motion ◽  
Mean Velocity ◽  
Straight Path ◽  
Meandering Channel ◽  
Bed Topography ◽  
The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

scholarly journals Global energy fluxes in turbulent channels with flow control

2018 ◽  
Vol 857 ◽  
pp. 345-373 ◽  
Author(s):  
Davide Gatti ◽  
Andrea Cimarelli ◽  
Yosuke Hasegawa ◽  
Bettina Frohnapfel ◽  
Maurizio Quadrio
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Energy Dissipation ◽  
Velocity Field ◽  
Flow Control ◽  
Reynolds Shear Stress ◽  
Power Input ◽  
Energy Fluxes ◽  
Mean Velocity ◽  
The Mean

This paper addresses the integral energy fluxes in natural and controlled turbulent channel flows, where active skin-friction drag reduction techniques allow a more efficient use of the available power. We study whether the increased efficiency shows any general trend in how energy is dissipated by the mean velocity field (mean dissipation) and by the fluctuating velocity field (turbulent dissipation). Direct numerical simulations (DNS) of different control strategies are performed at constant power input (CPI), so that at statistical equilibrium, each flow (either uncontrolled or controlled by different means) has the same power input, hence the same global energy flux and, by definition, the same total energy dissipation rate. The simulations reveal that changes in mean and turbulent energy dissipation rates can be of either sign in a successfully controlled flow. A quantitative description of these changes is made possible by a new decomposition of the total dissipation, stemming from an extended Reynolds decomposition, where the mean velocity is split into a laminar component and a deviation from it. Thanks to the analytical expressions of the laminar quantities, exact relationships are derived that link the achieved flow rate increase and all energy fluxes in the flow system with two wall-normal integrals of the Reynolds shear stress and the Reynolds number. The dependence of the energy fluxes on the Reynolds number is elucidated with a simple model in which the control-dependent changes of the Reynolds shear stress are accounted for via a modification of the mean velocity profile. The physical meaning of the energy fluxes stemming from the new decomposition unveils their inter-relations and connection to flow control, so that a clear target for flow control can be identified.

Fully-Developed Laminar Flow in Sinusoidal Grooves

2001 ◽  
Vol 123 (3) ◽  
pp. 656-661 ◽  
Author(s):  
Scott K. Thomas ◽  
Richard C. Lykins ◽  
Kirk L. Yerkes
Keyword(s):  
Contact Angle ◽  
Shear Stress ◽  
Volumetric Flow Rate ◽  
Numerical Data ◽  
Mean Velocity ◽  
Constant Property ◽  
Fully Developed Flow ◽  
Vapor Interface ◽  
The Mean ◽  
Poiseuille Number

The flow of a constant property fluid through a sinusoidal groove has been analyzed. A numerical solution of the conservation of mass and momentum equations for fully developed flow is presented. The mean velocity, volumetric flow rate, and Poiseuille number are presented as functions of the groove geometry, meniscus contact angle, and shear stress at the liquid-vapor interface. In addition, a semi-analytical solution for the normalized mean velocity in terms of the normalized shear stress at the meniscus is shown to agree with the numerical data quite well.

Establishment of the Wake Behind a Disk

1964 ◽  
Vol 86 (4) ◽  
pp. 869-880 ◽  
Author(s):  
Thomas Carmody
Keyword(s):  
Shear Stress ◽  
Ambient Pressure ◽  
Continuous Distribution ◽  
Flow Characteristics ◽  
Graphical Form ◽  
Stress Distributions ◽  
Mean Velocity ◽  
Pressure Distributions ◽  
Energy Relationships ◽  
The Mean

An air-tunnel study of the establishment of the wake behind a disk at a Reynolds number of approximately 7 × 104 was undertaken. On the basis of the measured data, such a wake is fully established, that is, similarity profiles of the flow characteristics are formed, within 15 diameters of the disk, and approximately 95 percent of the transfer of energy from the mean motion to the turbulence motion takes place within 3 diameters of the disk, in the region of the mean standing eddy. The measured mean ambient-pressure and mean total-pressure distributions, mean velocity distributions, turbulence-intensity and shear-stress distributions, and the mean streamline pattern are presented in graphical form, as are the quantitative balances of the integrated momentum and mean-energy relationships. A stream function consisting of a continuous distribution of doublets is introduced to extend the radial limit of understanding of the flow characteristics to a very large if not infinite radius. Considerable attention is given to the problem of obtaining and interpreting turbulence shear-stress data immediately downstream from the point of flow separation. The applicability of a local diffusion coefficient or virtual viscosity of the Boussinesq or Prandtl type for relating the turbulence shear stress to the radial gradient of mean axial velocity is discussed. The Bernoulli sum and the energy changes along individual streamlines investigated in an associated study are incorporated herein to obtain a quantitative estimate of the local errors involved in the turbulence-shear-stress measurements.

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.

Combined effects of flow curvature and rotation on uniformly sheared turbulence

2009 ◽  
Vol 628 ◽  
pp. 371-394 ◽  
Author(s):  
D. C. ROACH ◽  
A. G. L. HOLLOWAY
Keyword(s):  
Shear Stress ◽  
Wind Tunnel ◽  
Model Development ◽  
Three Dimensional ◽  
Simple Shear Flow ◽  
Test Case ◽  
Mean Velocity ◽  
Tunnel Section ◽  
Flow Curvature ◽  
The Mean

This paper describes an experiment in which a uniformly sheared turbulence was subjected to simultaneous streamwise flow curvature and rotation about the streamwise axis. The distortion of the turbulence is complex but well defined and may serve as a test case for turbulence model development. The uniformly sheared turbulence was developed in a straight wind tunnel and then passed into a curved tunnel section. At the start of the curved section the plane of the mean shear was normal to the plane of curvature so as to create a three-dimensional or ‘out of plane’ curvature configuration. On entering the curved tunnel, the flow developed a streamwise mean vorticity that rotated the mean shear about the tunnel centreline through approximately 70°, so that the shear was nearly in the plane of curvature and oriented so as to have a stabilizing effect on the turbulence. Hot wire measurements of the mean velocity, mean vorticity, mean rate of strain and Reynolds stress anisotropy development along the wind tunnel centreline are reported. The observed effect of the mean shear rotation on the turbulence was to diminish the shear stress in the plane normal to the plane of curvature while generating non-zero values of the shear stress in the plane of curvature. A rotating frame was identified for which the measured mean velocity field took the form of a simple shear flow. The turbulence anisotropy was transformed to this frame to estimate the effects of frame rotation on the structure of sheared turbulence.

scholarly journals High-resolution velocity measurement in the inner part of turbulent boundary layers over super-hydrophobic surfaces

2016 ◽  
Vol 801 ◽  
pp. 670-703 ◽  
Author(s):  
Hangjian Ling ◽  
Siddarth Srinivasan ◽  
Kevin Golovin ◽  
Gareth H. McKinley ◽  
Anish Tuteja ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Velocity Profile ◽  
Drag Reduction ◽  
Slip Velocity ◽  
Reynolds Shear Stress ◽  
Turbulent Boundary Layers ◽  
Mean Velocity ◽  
Roughness Elements ◽  
The Mean

Digital holographic microscopy is used for characterizing the profiles of mean velocity, viscous and Reynolds shear stresses, as well as turbulence level in the inner part of turbulent boundary layers over several super-hydrophobic surfaces (SHSs) with varying roughness/texture characteristics. The friction Reynolds numbers vary from 693 to 4496, and the normalized root mean square values of roughness $(k_{rms}^{+})$ vary from 0.43 to 3.28. The wall shear stress is estimated from the sum of the viscous and Reynolds shear stress at the top of roughness elements and the slip velocity is obtained from the mean profile at the same elevation. For flow over SHSs with $k_{rms}^{+}<1$, drag reduction and an upward shift of the mean velocity profile occur, along with a mild increase in turbulence in the inner part of the boundary layer. As the roughness increases above $k_{rms}^{+}\sim 1$, the flow over the SHSs transitions from drag reduction, where the viscous stress dominates, to drag increase where the Reynolds shear stress becomes the primary contributor. For the present maximum value of $k_{rms}^{+}=3.28$, the inner region exhibits the characteristics of a rough wall boundary layer, including elevated wall friction and turbulence as well as a downward shift in the mean velocity profile. Increasing the pressure in the test facility to a level that compresses the air layer on the SHSs and exposes the protruding roughness elements reduces the extent of drag reduction. Aligning the roughness elements in the streamwise direction increases the drag reduction. For SHSs where the roughness effect is not dominant ($k_{rms}^{+}<1$), the present measurements confirm previous theoretical predictions of the relationships between drag reduction and slip velocity, allowing for both spanwise and streamwise slip contributions.

scholarly journals Experimental Study of Mean Flow Characteristics in the Near Field of Wedge-Shaped Swirling Jets

2020 ◽  
Vol 5 (10) ◽  
pp. 1199-1203
Author(s):  
Md. Mosharrof Hossain ◽  
Muhammed Hasnain Kabir Nayeem ◽  
Dr. Md Abu Taher Ali
Keyword(s):  
Flow Field ◽  
Near Field ◽  
Flow Characteristics ◽  
Velocity Profiles ◽  
Mean Flow ◽  
Mean Velocity ◽  
Potential Core ◽  
Swirling Jets ◽  
The Mean ◽  
Sinusoidal Flow

In this investigation experiment was carried out in 80 mm diameter swirling pipe jet, where swirl was generated by attaching wedge-shaped helixes in the pipe. All measurements were taken at Re 5.3e4. In the plain pipe jet the potential core was found to exist up to x/D=5 but in the swirling jet there was no existence of potential core. The mean velocity profiles were found to be influenced by the presence of wedge-shaped helixes in the pipe. The velocity profiles indicated the presence of sinusoidal flow field in the radial direction existed only in the near field of the jet. This flow field died out after x/D=3 and the existence of jet flow diminished after x/D=5.

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