Mean Velocity and Entropy in Wide Channel Flows

The results of an experimental investigation of turbulent boundary layers in shallow open channel flows at low Reynolds numbers are presented. The study was aimed at extending the database toward lower values of Reynolds number. The data presented are primarily concerned with the longitudinal mean velocity, turbulent-velocity fluctuations, boundary layer shape parameter and skin friction coefficient for Reynolds numbers based on the momentum thickness (Reθ) ranging from 180 to 480. In this range, the results of the present investigation in shallow open channel flows indicate a lack of dependence of the von Karman constant κ on Reynolds number. The extent to which the mean velocity data overlaps with the log-law decreases with decreasing Reθ. The variation of the strength of the wake with Reθ is different from the trend proposed earlier by Coles.

Download Full-text

The Role of Forcing and Eddy Viscosity Variation on the Log-Layer Mismatch Observed in Wall-Modeled Large-Eddy Simulations

Journal of Fluids Engineering ◽

10.1115/1.4041562 ◽

2018 ◽

Vol 141 (5) ◽

Author(s):

Rey DeLeon ◽

Inanc Senocak

Keyword(s):

Flow Rate ◽

Eddy Viscosity ◽

Degree Of Freedom ◽

Channel Flows ◽

Momentum Balance ◽

Mean Velocity ◽

Turbulent Channel Flows ◽

The Mean ◽

Viscosity Variation

We investigate the role of eddy viscosity variation and the effect of zonal enforcement of the mass flow rate on the log-layer mismatch problem observed in turbulent channel flows. An analysis of the mean momentum balance shows that it lacks a degree-of-freedom (DOF) when eddy viscosity is large, and the mean velocity conforms to an incorrect profile. Zonal enforcement of the target flow rate introduces an additional degree-of-freedom to the mean momentum balance, similar to an external stochastic forcing term, leading to a significant reduction in the log-layer mismatch. We simulate turbulent channel flows at friction Reynolds numbers of 2000 and 5200 on coarse meshes that do not resolve the viscous sublayer. The second-order turbulence statistics agree well with the direct numerical simulation benchmark data when results are normalized by the velocity scale extracted from the filtered velocity field. Zonal enforcement of the flow rate also led to significant improvements in skin friction coefficients.

Download Full-text

Velocity distributions under floating covers

Canadian Journal of Civil Engineering ◽

10.1139/l82-008 ◽

1982 ◽

Vol 9 (1) ◽

pp. 76-83 ◽

Cited By ~ 14

Author(s):

Y. L. Lau

Keyword(s):

Turbulence Model ◽

Maximum Velocity ◽

Channel Flows ◽

Flow Depth ◽

Flow Properties ◽

Mean Velocity ◽

Bottom Boundary ◽

Standard Procedures ◽

Logarithmic Distribution ◽

The Mean

The [Formula: see text] turbulence model has been used to calculate the velocity distributions for a large number of channel flows with different top and bottom boundary roughnesses. The resulting distributions are used to review the standard procedures for stream gauging of ice-covered flows. It is found that the average of the velocities at [Formula: see text] and [Formula: see text] of the depth is indeed very nearly equal to the overall mean velocity. Examination of the velocity profiles shows that the profiles deviate from the logarithmic distribution for about 40% of the flow depth. Other flow properties, such as the location of the maximum velocity and the mean velocities in the top and bottom layers, are also examined.

Download Full-text

Annular swirling liquid layer with a hollow core

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.76 ◽

2018 ◽

Vol 841 ◽

pp. 784-824 ◽

Cited By ~ 2

Author(s):

P. M. Bardet ◽

P. F. Peterson ◽

Ö. Savaş

Keyword(s):

Free Surface ◽

Centrifugal Force ◽

Liquid Layer ◽

Inertial Confinement Fusion ◽

Air Entrainment ◽

Reynolds Numbers ◽

Channel Flows ◽

Stereoscopic Particle Image Velocimetry ◽

Hollow Core ◽

Mean Velocity

A thick turbulent annular liquid layer that swirls inside a fixed pipe is studied by means of surface observations and stereoscopic particle image velocimetry in an index-matched nozzle facility. This flow combines the complexities of swirling and free-surface turbulence. The free surface of the layer changes the boundary condition compared to filled swirling pipes and introduces the equivalent of a hollow core. The liquid layer shares similarities with turbulent open-channel flows, with the high centrifugal force across the layer having a similar effect to that of gravity in channel flows. At the surface, the restoring forces are surface tension and centrifugal acceleration. The particular nozzle under study has been envisioned for inertial confinement fusion, but the flow has relevance to systems such as compact separators or liquid rocket fuel injectors. Data are acquired at five downstream locations from the nozzle exit for four Reynolds numbers for subcritical flows. Injection flow rate and fluid kinematic viscosity are controlled independently, which allows adjusting independently the Reynolds and Taylor–Reynolds numbers. This also enables control of the centrifugal force at the free surface to test the effects of turbulence intensity on the free surface in regimes where air entrainment and droplet ejection occur. The swirl number is fixed by the design of the nozzle. From velocimetry data, mean velocity and turbulence statistics are extracted. For all the conditions tested, three flow regimes are identified: developing, developed, and transitional. The developed regime appears self-similar on the mean; the swirl creates a favourable pressure gradient that sustains the axial flow and confines the boundary layer near the wall. Large coherent vortex structures are identified in the layer. A simple model is proposed to describe the layer thickness and the velocity distribution in it. In the transitional regime, helical varicose waves generated by centrifugal instability are observed on the surface. Additionally, wall effects are visible in the bulk of the flow, and the main flow features are large overturning motions.

Download Full-text

Turbulent structure in low-concentration drag-reducing channel flows

Journal of Fluid Mechanics ◽

10.1017/s0022112088001302 ◽

1988 ◽

Vol 190 ◽

pp. 241-263 ◽

Cited By ~ 100

Author(s):

T. S. Luchik ◽

W. G. Tiederman

Keyword(s):

Leading Edge ◽

Turbulent Channel Flow ◽

Velocity Profiles ◽

Channel Flows ◽

Lower Threshold ◽

Viscous Drag ◽

Laser Doppler Velocimeter ◽

Mean Velocity ◽

Low Concentration ◽

Buffer Region

A two-component laser-Doppler velocimeter was used to measure simultaneously velocity components parallel and normal to the wall in two fully developed, wellmixed, low-concentration (1-2 p.p.m.) drag-reducing channel flows and one turbulent channel flow of water. The mean velocity profiles, root-mean-square velocity profiles and the distributions of the ūv turbulent correlation confirm that the additives modify the buffer region of the flow. The principal influence of the additives is to damp velocity fluctuations normal to the wall in the buffer region.The structural results show that the average time between bursts increased for the drag-reducing flows. When compared to a water flow at the same wall shear stress, this increase in the timescale was equal to the increase in the average streak spacing. Conditionally averaged velocity signals of y+ = 30 centred on the leading edge of a burst, as well as those centred on the trailing edge, have the same general characteristics in all three flows indicating that the basic structure of the fundamental momentum transport event is the same in these drag-reducing flows. However, it was clearly shown that the lower-threshold Reynolds-stress-producing motions were damped while the higher-threshold motions were not damped. In the buffer region of the drag-reducing flows this yields a larger mean velocity gradient with damped fluctuations normal to the wall and increased fluctuations in the streamwise direction. It is hypothesized that some strong turbulent motions are required to maintain extended polymer molecules, which produce a solution with properties that can damp lower threshold turbulence and thereby reduce viscous drag.

Download Full-text

Simulations of Channel Flows With Effects of Spanwise Rotation or Wall Injection Using a Reynolds Stress Model

Journal of Fluids Engineering ◽

10.1115/1.1343109 ◽

2000 ◽

Vol 123 (1) ◽

pp. 2-10 ◽

Cited By ~ 15

Author(s):

Bruno Chaouat

Keyword(s):

Experimental Data ◽

Reynolds Stress ◽

Reynolds Stress Model ◽

Channel Flows ◽

Fluid Injection ◽

Turbulent Regime ◽

Stress Model ◽

Mean Velocity ◽

Wall Injection ◽

Spanwise Rotation

Simulations of channel flows with effects of spanwise rotation and wall injection are performed using a Reynolds stress model. In this work, the turbulent model is extended for compressible flows and modified for rotation and permeable walls with fluid injection. Comparisons with direct numerical simulations or experimental data are discussed in detail for each simulation. For rotating channel flows, the second-order turbulence model yields an asymmetric mean velocity profile as well as turbulent stresses quite close to DNS data. Effects of spanwise rotation near the cyclonic and anticyclonic walls are well observed. For the channel flow with fluid injection through a porous wall, different flow developments from laminar to turbulent regime are reproduced. The Reynolds stress model predicts the mean velocity profiles, the transition process and the turbulent stresses in good agreement with the experimental data. Effects of turbulence in the injected fluid are also investigated.

Download Full-text

Wall shear stress determination from near-wall mean velocity data in turbulent pipe and channel flows

Experiments in Fluids ◽

10.1007/bf00189380 ◽

1996 ◽

Vol 20 (6) ◽

pp. 417-428 ◽

Cited By ~ 70

Author(s):

F. Durst ◽

H. Kikura ◽

I. Lekakis ◽

J. Jovanović ◽

Q. Ye

Keyword(s):

Shear Stress ◽

Wall Shear Stress ◽

Channel Flows ◽

Stress Determination ◽

Velocity Data ◽

Mean Velocity ◽

Wall Shear ◽

Near Wall

Download Full-text

Relative importance of mean velocity and bed shear on biofilm accumulation in open-channel flows

Water Science & Technology ◽

10.2166/wst.1995.0296 ◽

1995 ◽

Vol 32 (8) ◽

pp. 193-198 ◽

Cited By ~ 3

Author(s):

Y. L. Lau

Keyword(s):

Biofilm Formation ◽

Average Velocity ◽

Open Channel ◽

Channel Flows ◽

Shear Stresses ◽

Relative Importance ◽

Mean Velocity ◽

Biofilm Growth ◽

Roughness Elements ◽

Set Up

Experiments on biofilm growth were carried out to investigate whether bottom shear stress or average velocity is more appropriate as a parameter for investigating the effect of flow on biofilm formation in channel flows. The tests were conducted in two identical channels located side by side, using the same water supply. By having different bottom slopes or roughness elements, or both, tests were set up in which the flows in the two channels had equal velocities but different bottom shear stresses or equal bottom shear stresses but different velocities. Porcelain balls were used as bottom roughness elements and the accumulation of biofilm on the balls was monitored. Comparisons of the rates of biofilm accumulation indicate that the average velocity is the more important parameter.

Download Full-text