scholarly journals Friction factor decomposition for rough-wall flows: theoretical background and application to open-channel flows

2019 ◽  
Vol 872 ◽  
pp. 626-664 ◽  
Author(s):  
V. I. Nikora ◽  
T. Stoesser ◽  
S. M. Cameron ◽  
M. Stewart ◽  
K. Papadopoulos ◽  
...  
Keyword(s):  
Friction Factor ◽  
Open Channel ◽  
Reynolds Numbers ◽  
Channel Flows ◽  
Turbulent Stress ◽  
Open Channel Flows ◽  
Navier Stokes Equation ◽  
Data Set ◽  
Secondary Currents ◽  
Wide Range

A theoretically based relationship for the Darcy–Weisbach friction factor $f$ for rough-bed open-channel flows is derived and discussed. The derivation procedure is based on the double averaging (in time and space) of the Navier–Stokes equation followed by repeated integration across the flow. The obtained relationship explicitly shows that the friction factor can be split into at least five additive components, due to: (i) viscous stress; (ii) turbulent stress; (iii) dispersive stress (which in turn can be subdivided into two parts, due to bed roughness and secondary currents); (iv) flow unsteadiness and non-uniformity; and (v) spatial heterogeneity of fluid stresses in a bed-parallel plane. These constitutive components account for the roughness geometry effect and highlight the significance of the turbulent and dispersive stresses in the near-bed region where their values are largest. To explore the potential of the proposed relationship, an extensive data set has been assembled by employing specially designed large-eddy simulations and laboratory experiments for a wide range of Reynolds numbers. Flows over self-affine rough boundaries, which are representative of natural and man-made surfaces, are considered. The data analysis focuses on the effects of roughness geometry (i.e. spectral slope in the bed elevation spectra), relative submergence of roughness elements and flow and roughness Reynolds numbers, all of which are found to be substantial. It is revealed that at sufficiently high Reynolds numbers the roughness-induced and secondary-currents-induced dispersive stresses may play significant roles in generating bed friction, complementing the dominant turbulent stress contribution.

Turbulent open-channel flows over mobile granular low-tortuosity beds: velocity distribution and friction factor

2020 ◽  
Author(s):  
Rui M L Ferreira ◽  
Rigden Y Tenzin ◽  
Ana M Ricardo
Keyword(s):  
Hydraulic Conductivity ◽  
Friction Factor ◽  
Open Channel ◽  
High Conductivity ◽  
Channel Flows ◽  
Rough Boundary ◽  
Roughness Parameters ◽  
Mean Flow ◽  
Open Channel Flows ◽  
Log Law

<p>Open channel flows over granular mobile beds are affected by the nature and intensity of hyporheic/surface mass and momentum exchanges. Near-bed surface mean flow and turbulence find an equilibrium with the flow in the hyporheic region and with the type and amount of granular material transported in equilibrium conditions. The processes involved in these adaptive process are not well known. This work addresses this knowledge gap and it is aimed at describing the effect of the hydraulic conductivity on the friction factor and on the parameters of the log-law that is thought to constitute a valid model for the turbulent flow in the overlapping region of fully developed hydraulically rough boundary layers over mobile cohesionless beds. To fulfil the objectives, experimental tests performed in high conductivity beds (mono-sized glass sphere beads) are compared with the existing database of low conductivity beds of Ferreira et al. (2012), keeping constant the range of values of porosity, Shields parameters and roughness Reynolds numbers. The hydraulic conductivity is varied by changing the tortuosity (and the dimensions of the pore paths) and not the porosity.</p><p>A new database of instantaneous velocities was acquired with Particle Image Velocimetry (PIV) and processed to gather time-averaged velocities and space-time (double-averaged) quantities, namely velocities, Reynolds stresses and form-induced stresses. The hydraulic conductivity was measured for both types of bed.</p><p>The parameters of log-law obtained from high conductivity are compared with low conductivity of existing database, for mobile and immobile bed conditions. The main finding can be summarized as follows.</p><p>i. Hydraulic conductivity does not affect the location of the zero plane of the log-law, the thickness of the region above the crests where the flow is determined by roughness.</p><p>ii. Increasing the hydraulic conductivity does not appear to decrease the value of bed roughness parameters such as the roughness heigh.</p><p>iii. Higher hydraulic conductivity is associated to a structural change: the same near-bed velocity can be achieved with lower shear stress in the inner region. A lower friction factor, (<em>u</em><sub>*</sub>/<em>U</em>)<sup>2</sup>, is thus registered.</p><p>iv. Flows over high conductivity beds appear drag-reducing even if roughness parameters do not change appreciably.</p><p> </p><p>This research was partially supported by Portuguese and European funds, within the COMPETE 2020 and PORL-FEDER programs, through project PTDC/CTA-OHR/29360/2017 RiverCure</p>

Turbulent Boundary Layers in Low Reynolds Number Shallow Open Channel Flows

1999 ◽  
Vol 121 (3) ◽  
pp. 684-689 ◽  
Author(s):  
Ram Balachandar ◽  
Shyam S. Ramachandran
Keyword(s):  
Reynolds Number ◽  
Boundary Layers ◽  
Open Channel ◽  
Reynolds Numbers ◽  
Skin Friction Coefficient ◽  
Turbulent Boundary Layers ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Mean Velocity ◽  
Low Reynolds

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.

Numerical study on secondary currents of the second kind in wide shallow open channel flows

2014 ◽  
pp. 179-187 ◽  
Author(s):  
R Suzuki ◽  
I Kimura ◽  
Y Shimizu ◽  
T Iwasaki ◽  
T Hosoda
Keyword(s):  
Open Channel ◽  
Numerical Study ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Secondary Currents

scholarly journals Flow structure in compound open-channel flows in the presence of transverse currents

2018 ◽  
Vol 40 ◽  
pp. 05024 ◽  
Author(s):  
Sébastien Proust ◽  
Vladimir Nikora
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Mixing Layer ◽  
Free Surface Flows ◽  
Channel Flows ◽  
Two Dimensional ◽  
Open Channel Flows ◽  
Secondary Currents ◽  
Flow Modification

The structure of free-surface flows is experimentally investigated in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FPs). The study focuses on the effects of transverse currents on: (i) mixing layers and quasi-two-dimensional coherent structures at the interfaces between MC and FPs; (ii) secondary currents developing across the channel; and (iii) large and very-large-scale motions that were recently observed in non-compound open channel flows. Transverse currents represent spanwise depth- and time-averaged flow from MC to FPs or vice versa. The study is based on one-point and two-point ADV measurements. Streamwise non-uniform flows are generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate the same for all scenarios. It is shown that even small transverse currents can be very effective in flow modification, as they can significantly displace the mixing layer, shear-layer turbulence, and coherent structures towards MC or FP, depending on the current direction. They can also alter the distribution and strength of the secondary currents. The interactions of quasi-two-dimensional coherent structures, very-large-scale motions, and secondary currents at different conditions are also part of this study.

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