Effect of flow acceleration and initial turbulence level on velocity fluctuations

1994 ◽  
Vol 28 (5) ◽  
pp. 624-629 ◽  
Author(s):  
V. P. Lebedev ◽  
V. V. Lemanov ◽  
S. Ya. Misyura ◽  
V. I. Terekhov
Keyword(s):  
Turbulence Level ◽  
Velocity Fluctuations ◽  
Flow Acceleration ◽  
Initial Turbulence

Experimental Study of Turbulent Flow in a Tube With Wall Suction

1974 ◽  
Vol 96 (3) ◽  
pp. 338-342 ◽  
Author(s):  
A. Brosh ◽  
Y. Winograd
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Turbulent Velocity ◽  
Turbulence Level ◽  
Mixing Length ◽  
Local Flow ◽  
Wall Suction ◽  
Velocity Fluctuations ◽  
Flow Equilibrium ◽  
Continuous Suction

The effect of wall suction on the turbulent flow of air in a porous tube has been studied. Measurements of the radial distribution of the turbulent velocity fluctuations were obtained over a range of Reynolds numbers from 104 to 2 × 105. Various suction rates were employed, for both local suction over a short length of tube and continuous suction over various lengths. The results obtained for local suction (step reduction in Reynolds number) show that approximately 40 dia are required for the turbulent velocity fluctuations to reach flow equilibrium at the lower downstream value of the Reynolds number. The results for the case of continuous suction show that after a short suction length, there is an apparent increase in the turbulence level compared with that found at the same Reynolds number with no suction. This appears to be due to the greater turbulence level which exists at the higher (presuction) Reynolds number. Longer suction lengths, above 40 dia, always result in a decrease in the turbulence level compared with turbulent flow with no suction at the same Reynolds number. The present results suggest that simple mixing-length models, incorporating local flow parameters, may be inadequate to describe the turbulent momentum transport in a tube with surface suction. Certainly, the existing mixing-length models should be re-examined in the light of this new experimental data.

scholarly journals A Bypass Transition Model for Boundary Layers

1993 ◽  
Author(s):  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Transition Model ◽  
Wall Region ◽  
Bypass Transition ◽  
Pressure Gradients ◽  
Turbulence Level ◽  
Velocity Fluctuations ◽  
Laminar Boundary Layers

Experimental data for laminar boundary layers developing below a turbulent free stream shows that the fluctuation velocities within the boundary layer increase in amplitude until some critical level is reached which initiates transition. In the near wall region, a simple model, containing a single empirical parameter which depends only on the turbulence level and length scale, is derived to predict the development of the velocity fluctuations in laminar boundary layers with favourable, zero or adverse pressure gradients. A simple bypass transition model which considers the streamline distortion in the near wall region brought about by the velocity fluctuations suggests that transition will commence when the local turbulence level reaches approximately 23%. This value is consistent with experimental findings. This critical local turbulence level is used to derive a bypass transition prediction formula which compares reasonably with start of transition experimental data for a range of pressure gradients (λθ = −0.01 to 0.01) and turbulence levels (Tu = 0.2% to 5%). Further improvement to the model is proposed through prediction of the boundary layer distortion, which occurs due to Reynolds stresses generated within the boundary layer at high free stream turbulence levels and also through inclusion of the effect of turbulent length scale as well as turbulence level.

Lagrangian Motion of Slightly Buoyant Droplets and Fluid Particles in Isotropic Turbulence

2007 ◽  
Author(s):  
Balaji Gopalan ◽  
Edwin Malkiel ◽  
Joseph Katz
Keyword(s):  
Diffusion Coefficient ◽  
Droplet Size ◽  
Isotropic Turbulence ◽  
Fluid Velocity ◽  
Sample Volume ◽  
Droplet Velocity ◽  
Fickian Diffusion ◽  
Turbulence Level ◽  
Rise Velocity ◽  
Velocity Fluctuations

The diffusion of slightly buoyant diesel oil droplets in isotropic turbulence is studied using high speed in-line digital holographic cinematography. Diesel fuel droplets with specific gravity 0.85 are injected into a 50×50×70 mm3 sample volume located at the central portion of a nearly isotropic turbulence facility. The turbulence in the sample volume is fully characterized using 2D PIV. Probability density functions of the Lagrangian droplet velocity are very close to a Gaussian distribution, which justifies the use of Taylor’s [1] model to calculate diffusion parameters. Similar to Friedman & Katz [2] data, our current results confirm that the mean rise velocity of diesel droplets becomes higher than the quiescent rise velocity at high turbulence levels. For most of the present droplet sizes and turbulence level, the rms of the horizontal droplet velocity fluctuations exceeds that of the horizontal fluid velocity fluctuations. The rms values of the vertical droplet velocity fluctuations are higher than those of the fluid only for the highest turbulence level. The droplet to fluid velocity rms ratio in both directions increases with turbulence level, but decreases with increasing droplet size. Assuming Fickian diffusion, Lagrangian auto-correlation functions of 22,000 droplet tracks are used for calculating the diffusion coefficient as functions of droplet size and turbulence level. Using all the data, we show that the diffusion coefficient scaled by quiescent rise velocity and the turbulence integral length scale is a monotonically increasing function of the turbulence level normalized by the droplet quiescent rise velocity.

Effect of the Initial Turbulence Level on an Underexpanded Supersonic Jet

AIAA Journal ◽  
2013 ◽  
Vol 51 (3) ◽  
pp. 741-745 ◽  
Author(s):  
J. Liu ◽  
K. Kailasanath ◽  
J. P. Boris ◽  
N. Heeb ◽  
D. Munday ◽  
...  
Keyword(s):  
Supersonic Jet ◽  
Turbulence Level ◽  
Underexpanded Supersonic Jet ◽  
Initial Turbulence

Condensation on Coherent Turbulent Liquid Jets: Part I—Experimental Study

1989 ◽  
Vol 111 (4) ◽  
pp. 1068-1074 ◽  
Author(s):  
S. Kim ◽  
A. F. Mills
Keyword(s):  
Surface Tension ◽  
High Speed ◽  
Reynolds Numbers ◽  
Liquid Jets ◽  
High Speed Photography ◽  
Turbulence Level ◽  
Jet Breakup ◽  
Glass Tubes ◽  
Initial Turbulence ◽  
Jet Velocities

Condensation on coherent turbulent liquid jets was investigated experimentally in order to obtain a data base for the liquid side heat transfer coefficient. Jet breakup was identified by means of high-speed photography. Nozzles were formed from smooth and roughened glass tubes to define the initial turbulence level in the jets. Jet diameters of 3–7 mm and lengths of 2–12 cm were tested at jet velocities of 1.4–12 m/s giving Reynolds numbers of 6000–40,000. Viscosity and surface tension were varied by using ethanol, and water from 277–300 K, as test liquids. The Stanton number was found to be essentially independent of jet diameter, but to decrease with length to the power of −0.57, velocity to the power of −0.20, surface tension to the power of −0.30, and viscosity to the power of −0.1.

A note on turbulence measurements with a laser velocimeter

1981 ◽  
Vol 102 ◽  
pp. 353-366 ◽  
Author(s):  
J. C. Lau ◽  
M. C. Whiffen ◽  
M. J. Fisher ◽  
D. M. Smith
Keyword(s):  
High Speed ◽  
Turbulence Level ◽  
Hot Wire ◽  
Standard Equipment ◽  
Velocity Fluctuations ◽  
Density Distributions ◽  
Significant Difference ◽  
The One ◽  
Probability Density Distributions ◽  
Skewness And Kurtosis

In recent comparative measurements using a burst-counter type laser velocimeter and a hot-wire anemometer to assess the capabilities of the velocimeter (e.g. Barnett & Giel 1976; Lau, Morris & Fisher 1979), it was found that the laser velocimeter held good promise as an instrument for turbulence research, especially in high speed, high temperature flows where a hot-wire cannot be used. The axial mean velocities obtained with the LV compared very well with hot-wire measurements. Similarly, the characteristic shapes of the spectra and probability density distributions of the velocity fluctuations were faithfully reproduced. The trends in the distributions of the various turbulence characteristics (e.g. turbulence intensity, velocity covariances, skewness and kurtosis) in a given flow field were identical to those obtained with hotwires. The one significant difference between LV and hot-wire results was the magnitudes of the turbulence level. Since the LV results were obtained with the help of the latest validation and discrimination techniques (Asher 1973), which have now become standard equipment (Durst, Melling & Whitelaw 1976), such a discrepancy was unexpected. The reason for the discrepancy is now fairly clear and a method has been suggested by Whiffen, Lau & Smith (1978) on how to eliminate the error. But the approach is lengthy and time-consuming. This paper describes a method which effectively accomplishes the same end with less effort.

A Bypass Transition Model for Boundary Layers

1994 ◽  
Vol 116 (4) ◽  
pp. 759-764 ◽  
Author(s):  
M. W. Johnson
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Transition Model ◽  
Wall Region ◽  
Bypass Transition ◽  
Pressure Gradients ◽  
Turbulence Level ◽  
Velocity Fluctuations ◽  
Laminar Boundary Layers

Experimental data for laminar boundary layers developing below a turbulent free stream show that the fluctuation velocities within the boundary layer increase in amplitude until some critical level is reached, which initiates transition. In the near-wall region, a simple model, containing a single empirical parameter, which depends only on the turbulence level and length scale, is derived to predict the development of the velocity fluctuations in laminar boundary layers with favorable, zero, or adverse pressure gradients. A simple bypass transition model, which considers the streamline distortion in the near-wall region brought about by the velocity fluctuations, suggests that transition will commence when the local turbulence level reaches approximately 23 percent. This value is consistent with experimental findings. This critical local turbulence level is used to derive a bypass transition prediction formula, which compares reasonably with start of transition experimental data for a range of pressure gradients (λ θ = −0.01 to 0.01) and turbulence levels (Tu = 0.2 to 5 percent). Further improvement to the model is proposed through prediction of the boundary layer distortion, which occurs due to Reynolds stresses generated within the boundary layer at high free-stream turbulence levels and also through inclusion of the effect of turbulent length scale as well as turbulence level.

Characteristics of Long-Period Flow Velocity Fluctuations around Tomakomai Harbor

1999 ◽  
Author(s):  
Hiroya Kinoshita ◽  
Hideki Hoshi ◽  
Youichi Atsumi ◽  
Shin-ichiro Sekiguchi ◽  
Toshihiko Yamashita
Keyword(s):  
Flow Velocity ◽  
Velocity Fluctuations ◽  
Long Period
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