Rough Flow Conditions Research Articles

A Study on the Friction Factor and Reynolds Number Relationship for Flow in Smooth and Rough Channels

The shear velocity and friction coefficient for representing the resistance of flow are key factors to determine the flow characteristics of the open-channel flow. Various studies have been conducted in the open-channel flow, but many controversies remain over the form of equation and estimation methods. This is because the equations developed based on theory have not fully interpreted the friction characteristics in an open-channel flow. In this paper, a friction coefficient equation is proposed by using the entropy concept. The proposed equation is determined under the rectangular, the trapezoid, the parabolic round-bottomed triangle, and the parabolic-bottomed triangle open-channel flow conditions. To evaluate the proposed equation, the estimated results are compared with measured data in both the smooth and rough flow conditions. The evaluation results showed that R (correlation coefficient) is found to be above 0.96 in most cases, and the discrepancy ratio analysis results are very close to zero. The advantage of the developed equation is that the energy slope terms are not included, because the determination of the exact value is the most difficult in the open-channel flow. The developed equation uses only the mean velocity and entropy M to estimate the friction loss coefficient, which can be used for maximizing the design efficiency.

Revisiting rough-wall turbulent boundary layers over sand-grain roughness

Abstract

Sediment Bed-Load Transport: A Standardized Notation

Morphodynamic processes on Earth are a result of sediment displacements by the flow of water or the action of wind. An essential part of sediment transport takes place with permanent or intermittent contact with the bed. In the past, numerous approaches for bed-load transport rates have been developed, based on various fundamental ideas. For the user, the question arises which transport function to choose and why just that one. Different transport approaches can be compared based on measured transport rates. However, this method has the disadvantage that any measured data contains inaccuracies that correlate in different ways with the transport functions under comparison. Unequal conditions also exist if the factors of transport functions under test are fitted to parts of the test data set during the development of the function, but others are not. Therefore, a structural formula comparison is made by transferring altogether 13 transport functions into a standardized notation. Although these formulas were developed from different perspectives and with different approaches, it is shown that these approaches lead to essentially the same basic formula for the main variables. These are shear stress and critical shear stress. However, despite the basic structure of these 13 formulas being the same, their coefficients vary significantly. The reason for that variation and the possible effect on the bandwidth of results is identified and discussed. A further result is the finding that not only shear stress affects bed-load transport rates as is expressed by many transport formulas. Transport rates are also significantly affected by the internal friction of the moving sediment as well as by the friction fluid-bed. In the case of not fully rough flow conditions, also viscous effects and thus the Reynolds number becomes of importance.

A novel approach for the prediction of the incipient motion of sediments under smooth, transitional and rough flow conditions using Geno-Fuzzy Inference System model

In this present study, a novel approach is introduced for accurate prediction of the incipient motion of uniform grain particles in sand and gravel-bedded open channels under unidirectional flow by improving Sugeno Fuzzy Inference System (Sugeno FIS). The Adaptive Neural Fuzzy Inference System (ANFIS) tool is based on Sugeno FIS.Consequent part of the ANFIS tool is limited to either as a constant or a linear function. This means that not only a non-linear function is available for the consequent part of the model but also it cannot be represented by the constant and linear functions,at the same time. ANFIS tool optimizes antecedent parameters (fuzzy sets) and consequent parameters (constant or linear functions) by utilizing neural and least square methods, respectively. In this study, a novel hybrid model named as Geno-Fuzzy Inference System (GENOFIS) is introduced by integrating improved Sugeno FIS and Genetic Algorithms (GAs) that refer to where, the antecedent (fuzzy sets) and consequent (constant, linear and non-linear functions) parameters are optimized using Genetic Algorithms (GAs) tool. A quantitative comparison is implemented between the ANFIS and GENOFIS models using root mean square error (RMSE) and the Nash-Sutcliffe coefficient of efficiency (CE) as evaluation criteria by using three different types of incipient motion of sediment measurements that exist in the literature as reference, visual and development of competence functions. The results of this present study demonstrated that the novel GENOFIS model provided more accurate prediction results in comparisons with the ANFIS model results for three different types of incipient motion of sediment.

Particle transport mechanics and induced seismic noise in steep flume experiments with accelerometer‐embedded tracers

AbstractRecent advances in fluvial seismology have provided solid observational and theoretical evidence that near‐river seismic ground motion may be used to monitor and quantify coarse sediment transport. However, inversions of sediment transport rates from seismic observations have not been fully tested against independent measurements, and thus have unknown but potentially large uncertainties. In the present study, we provide the first robust test of existing theory by conducting dedicated sediment transport experiments in a flume laboratory under fully turbulent and rough flow conditions. We monitor grain‐scale physics with the use of ‘smart rocks’ that consist of accelerometers embedded into manufactured rocks, and we quantitatively link bedload mechanics and seismic observations under various prescribed flow and sediment transport conditions. From our grain‐scale observations, we find that bedload grain hop times are widely distributed, with impacts being on average much more frequent than predicted by existing saltation models. Impact velocities are observed to be a linear function of average downstream cobble velocities, and both velocities show a bed‐slope dependency that is not represented in existing saltation models. Incorporating these effects in an improved bedload‐induced seismic noise model allows sediment flux to be inverted from seismic noise within a factor of two uncertainty. This result holds over nearly two orders of magnitude of prescribed sediment fluxes with different sediment sizes and channel‐bed slopes, and particle–particle collisions observed at the highest investigated rates are found to have negligible effect on the generated seismic power. These results support the applicability of the seismic‐inversion framework to mountain rivers, although further experiments remain to be conducted at sediment transport near transport capacity. © 2018 John Wiley & Sons, Ltd.

Bed roughness effects on boundary‐layer turbulence and consequences for odor‐tracking behavior of blue crabs (Callinectes sapidus)

We employed laser‐induced fluorescence (LIF) measurements of odorant plume structure and behavioral observations to examine how turbulence affects the three‐dimensional structure of odorant plumes and subsequently mediates olfactory search efficiency and success in our model organism, the blue crab (Callinectes sapidus). The turbulent characteristics of two laboratory flumes (one for behavioral quantification, one for signal‐structure quantification) were systematically varied by changing the bed substrate roughness to create smooth, transitional, and fully rough flow conditions. Generally, increasing bed roughness caused greater mixing, decreased the time‐averaged odorant concentration and concentration variance, and increased the plume width and homogeneity. Foraging success and speed of blue crabs attempting to locate the odorant source both declined consistently with increasing bed roughness. The variation in signal structure at the height of the antennules among bed‐roughness treatments explains the observed behavior differences in crab foraging speed. In contrast, steering (path linearity) appeared to be unaffected by bed‐roughness‐induced turbulence. The transverse correlation function for odorant concentration at sensors separated across the width of the plume was examined among bed‐roughness treatments, and, ultimately, the correlation function was found to be related to the spatial position of tracking blue crabs. The spatial arrangement of blue crab chemosensors combined with the three‐dimensional structure of odor plumes account for the differential effects of turbulence on the speed and success of crab‐tracking behavior.

On Detection of a Wave Age Dependency for the Sea Surface Roughness

The wave age dependency of the nondimensional sea surface roughness (also called the Charnock parameter) is investigated with data from the new field measurement program at Rodsand in the Danish Baltic Sea. An increasing Charnock parameter with inverse wave age is found, which can be described by a power-law relation. Friction velocity is a common quantity in both the Charnock parameter and wave age. Thus self-correlation effects are unavoidable in the relation between them. The significance of self-correlation is investigated by employing an artificial “dataset” with randomized wave parameters. It is found that self-correlation severely influences the relation. For the Rodsand dataset the difference between real and randomized “data” was found to be within the measurement uncertainty. By using a small subset of the data it was found that the importance of self-correlation increases for a narrower range of wave age values. This supports the conclusion of Johnson et al. that because of the scatter and self-correlation problems the coefficients of the power-law relation can only be obtained from the analysis of an aggregated dataset with a wide wave age range combining measurements from several sites. The dependency between wave age and sea roughness has been discussed extensively in the literature with different and sometimes conflicting results. A wide range of coefficients has been found for the power-law relation between the Charnock parameter and wave age for different datasets. It is shown that self-correlation contributes to such differences, since it depends on the range of wave age values present in the datasets. Also, data are often selected for rough flow conditions with the Reynolds roughness number. It is shown that for datasets with large scatter this can lead to misleading results with regard to the relationship between wave age and Charnock parameter. Two different methods to overcome this problem are presented.

On the wave age dependence of wind stress over pure wind seas

Data from five recent field campaigns are selected for pure wind sea, deep water, and fully rough flow conditions. The combined data set includes a wide range of wave ages, with high variability in both friction velocity and wave phase speed. These data, which are expected to follow Monin‐Obukhov similarity scaling, are used to investigate the influence of wave age on wind stress. The relationship between the dimensionless roughness and inverse wave age is found to be zo/σ = 13.4 (u*/cp)3.4, where zo is the surface roughness length, σ is the standard deviation of the surface elevation, u* is the friction velocity, and cp is the wave phase speed at the peak of the spectrum. This relationship, which represents a significant dependence of roughness on wave age, was obtained using a procedure that minimizes the effects of spurious correlation in u*. It is also shown to be consistent with the wave age relationship derived using an alternate form of the dimensionless roughness, namely, the Charnock parameter zog/u*2, where g is the gravitational constant.

Laboratory measurements of mass transfer of carbon dioxide and water vapour for smooth and rough flow conditions

Laboratory measurements of mass transfer of carbon dioxide and water vapour for smooth and rough flow conditions

Laboratory measurements of mass transfer of carbon dioxide and water vapour for smooth and rough flow conditions

Mass transfer rates of carbon dioxide (CO2) and water (H2O) have been measured in a 32-m wind-wave tunnel, with a beach in the middle providing 2 sections of 16 m fetch. The wind speed (referred to 10 m) range explored was 1 to 24 m/s, covering a full range of aerodynamic conditions from smooth to fully rough. A pronounced minimum in the Dalton number for both CO2 and H2O is revealed. The minimum Dalton number, at wind speeds between 2 and 3 m/s, indicates a characteristic common point in the transfer mechanism for both gas and aqueous phase controlled constituents. This point corresponds to the occurrence of the first wind generated wavelets. At higher winds, the transfer rates of CO2 increase more rapidly than those of H2O, reflecting the increased importance of wave-generated turbulence for the ventilation of water phase limited compounds at the air-water interface.

The effect of temperature factor on the transition from partially to fully rough flow conditions

Experimental results are presented for the effect of the temperature factor (T w/T f) on the transition from partially to fully rough flow conditions in gas (air) flow in annular channels with the inner tube being rough. An empirical relation is suggested for finding the optimum, as regards heat transfer augmentation, transition Reynolds numbers with the allowance for the temperature factor effect.

Surface drag coefficient dependence on the aerodynamic roughness of the sSea

Analysis of logarithmic wind profiles measured to a height of 10 m on the south shore of Long Island indicates three distinct classes of aerodynamic roughness of the sea surface. On the basis of the roughness Reynolds number, defined as u/sub */z/sub 0//v, where u/sub */ is the surface friction velocity, z/sub 0/ the roughness length, and v the kinematic viscosity, the sea surface is aerodynamically smooth for u/sub */z/sub 0//v less than or equal to 0.15. There is a moderately rough transition regime for 0.15 < u/sub */z/sub 0//v < 4, the surface becoming fully rough for the roughness Reynolds numbers exceeding 4. The surface drag coefficient C/sub D/sub 6// obtained from the mean wind speed at a height of 6 m is found to vary from 0.75 x 10/sup -3/ for an aerodynamically smooth sea surface to 1.9 x 10/sup -3/ for a fully rough surface. Moderately rough flow conditions yielded a C/sub D/sub 6// of 1.15 x 10/sup -3/. Wind profile measurements from two other offshore or coastal sites showed good agreement. Mean wind speeds ranged from 3.5 to 10 m s/sup -1/. The drag coefficient did not vary with mean wind speed but was weakly dependentmore » on the atmospheric stability. Charnock's constant ..cap alpha.. in z/sub 0/ = au/sub *//sup 2//g, where g is the gravitational acceleration, was found to have a mean value of 0.016 for moderately rough conditions and 0.072 for fully rough conditions.« less

The Roughness Length for Water Vapor Sensible Heat, and Other Scalars

Abstract An expression is presented for the roughness length of water vapor (or sensible heat, etc.) that is the surface value intercept of the straight line resulting from a semi-logarithmic plot of the mean specific humidity (or potential temperature, etc.) profile in the dynamic sublayer. The derivation is based on the standard assumption of continuity in the mean profile at the interface between the interfacial transfer sublayer and the fully turbulent surface sublayer. It is found that the resulting formulation yields similar results for a number of empirical and theoretical equations for the interfacial transfer that are available in the literature. The roughness length of any scalar admixture depends not only on the nature of the surface but also on the intensity of the surface shear stress and on the molecular diffusivity and the viscosity. In meteorological applications under rough flow conditions this roughness length may be considerably smaller than the aerodynamic roughness length Z0, whereas ...