Resistance in steep open channels due to randomly distributed macroroughness elements at large froude numbers
01 Dec 2017-Journal of Hydrologic Engineering (American Society of Civil Engineers (ASCE))-Vol. 22, Iss: 12, pp 04017052
TL;DR: In this article, energy loss in a steep open channel due to randomly spaced spherically shaped macroroughness elements such as boulders was investigated using a three-dimensional fluid dynamics solver.
Abstract: Energy loss in a steep open channel due to randomly spaced spherically shaped macroroughness elements such as boulders was investigated using a three-dimensional fluid dynamics solver. Firs...
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TL;DR: In this paper, three-dimensional numerical simulations were performed for different flow rates and various geometrical parameters of step-pools in steep open channels to gain insight into the occurrence of energy loss and its dependence on the flow structure.
Abstract: Three-dimensional numerical simulations were performed for different flow rates and various geometrical parameters of step-pools in steep open channels to gain insight into the occurrence of energy loss and its dependence on the flow structure. For a given channel with step-pools, energy loss varied only marginally with increasing flow rate in the nappe and transition flow regimes, while it increased in the skimming regime. Energy loss is positively correlated with the size of the recirculation zone, velocity in the recirculation zone and the vorticity. For the same flow rate, energy loss increased by 31.6% when the horizontal face inclination increased from 2° to 10°, while it decreased by 58.6% when the vertical face inclination increased from 40° to 70°. In a channel with several step-pools, cumulative energy loss is linearly related to the number of step-pools, for nappe and transition flows. However, it is a nonlinear function for skimming flows.
9 citations
TL;DR: In this paper, a series of tests of T-shape roughness elements, fixed height, arranged in three different configurations, differ in the number of lines of roughness element.
Abstract: The hydraulic behavior of the flow can be changed by using large-scale geometric roughness elements in open channels. This change can help in controlling erosions and sedimentations along the mainstream of the channel. Roughness elements can be large stone or concrete blocks placed at the channel's bed to impose more resistance in the bed. The geometry of the roughness elements, numbers used, and configuration are parameters that can affect the flow's hydraulic characteristics. In this paper, velocity distribution along the flume was theoretically investigated using a series of tests of T-shape roughness elements, fixed height, arranged in three different configurations, differ in the number of lines of roughness element. These elements were used to find the best configuration of roughness elements that can be applied to change the flow's hydraulic characteristics. ANSYS Parametric Design Language, APDL, and Computational Fluid Dynamics, CFD, was used to simulate the flow in an open channel with roughness elements. CFD can be used to study the hydrodynamics of open channels under different conditions with inclusive details rather than relying on the costly field and time-consuming. Runs were implemented with different conditions, the discharge, and water depth in upstream and downstream of the flume. T-shape roughness elements with height equal to 3cm placed in three different configurations, two lines, four lines, and fully rough configurations were tested. The results show that the effect of roughness elements increasing with increasing the number of lines of roughness elements. Cases of four lines and fully rough configurations have almost the same hydraulic performance by having the same results of the velocity decrease percentage, which is decreased by approximately about 66% and 61% of the control case's velocity in the zone near the roughness elements consequently. But the difference is that four lines configuration is affected in a part of the test section. This behavior increases the velocity values by about 11% in the other side and by about 10% near the free surface in the case of four lines configuration and increased by about 32% above the roughness elements in a fully rough configuration.
2 citations
TL;DR: In this paper , a hybrid model based on Structure from Motion (SfM) and computational fluid dynamics (CFD) was proposed for step-pool morphology analysis in mountain streams.
Abstract: . Step-pool systems are common bedforms in mountain streams and have been utilized in river restoration projects around the world. Step-pool units exhibit highly non-uniform hydraulic characteristics which have been reported to closely interact with the morphological evolution and stability of step-pool features. However, detailed information of the three- 10 dimensional hydraulics for step-pool morphology has been scarce due to the difficulty of measurement. To fill in this knowledge gap, we established a hybrid model based on the technologies of Structure from Motion (SfM) and computational fluid dynamics (CFD). The model used 3D reconstructions of bed surfaces with an artificial step-pool unit built by natural stones at six flow rates as inputs for CFD simulations. The hybrid model succeeded in providing high-resolution visualization of 3D flow structures for the step-pool unit. The results illustrate the segmentation of flow regimes below the step, i.e., the 15 integral jump at the water surface, streaky wake vortexes near the bed, and high-speed jets in between. The highly non-uniform distribution of turbulence energy in the pool has been revealed and two energy dissipaters with comparable capacity are found to co-exist in the pool. Pool scour development under flow increase leads to the expansion of the jump and wake vortexes but this increase stops for the jump at high flows close to the critical condition for step-pool failure. The micro-bedforms as grain clusters developed on the negative slope affect the local hydraulics significantly but this influence is suppressed at pool bottom. 20 The drag forces on the step stones increase with discharge before the highest flow is used while the lift force has a larger magnitude and wider varying range. Our results highlight the feasibility and great potential of the hybrid model approach combining physical and numerical modeling in investigating the complex flow characteristics of step-pool morphology. u (2) u the instantaneous velocity fluctuation in three directions. 215 The Q-criterion (Hunt et al., 1988; Flow science, 2016) was used to calculate visualize coherent flow the step-pool unit and the Q criterion was calculated by Eq. 3. We turbulent dissipation at the downstream area of x0 and reaches the maximum earlier than the TKE in the The turbulent dissipation rate on the negative slope remains at a low level, even lower than near the step toe (Fig. to Fourth, the maximum value of flow kinetic energy, TKE and turbulent dissipation in the pool increases during a from 5.0 to 43.6 L/s, but decreases when the flow further increases to 49.9 L/s further occurrence
2 citations
TL;DR: In this paper , the effects of the spatial distribution of roughness elements on water surface levels and head loss were analyzed in a 15 m long, 0.9 m wide flume with a slope of 5% under large Froude numbers.
Abstract: Accurate estimation of head loss introduced via randomly placed roughness elements found in natural or constructed streams (e.g., fish passages) is essential in order to estimate flow variables in mountain streams, understand formation of niches for aquatic life, and model flow structure. Owing to the complexity of the involved processes and the often missing detailed data regarding the roughness elements, the head loss in such streams is mostly approximated using empirical models. In our study, we utilize flume experiments to analyze the effects of the spatial distribution of roughness elements on water surface levels and head loss and, moreover, use the produced data to test three empirical models estimating head loss. The experiments were performed in a 15 m long, 0.9 m wide flume with a slope of 5% under large Froude numbers (2.5–2.8). Flow velocities and water levels were measured with different flow rates at 58 points within a 3.96 m test section of the flume. We could show that different randomly arranged patterns of roughness elements significantly affected head loss (differences up to 33.6%), whereas water jumps occurred when flow depths were in the same size range as the roughness elements. The roughness element position and its size influenced water surface profiles. None of the three tested empirical models were able to well reproduce the differences in head loss due to the different patterns of roughness elements, with overestimated head loss from 12 to 94.7%, R2 from 41 to 73%, NSE from −21.1 to 0.09, and RRMSE from 18.4 to 93%. This generally indicates that these empirical models are conditionally suitable to consider head loss effects of random patterns of roughness elements.
1 citations
TL;DR: In this article , a combined approach based on the technologies of Structure from Motion (SfM) and computational fluid dynamics (CFD) is proposed to visualize the 3D flow structures for the step-pool unit.
Abstract: Abstract. Step-pool systems are common bedforms in mountain streams and have been utilized in river restoration projects around the world. Step-pool units
exhibit highly nonuniform hydraulic characteristics which have been reported to closely interact with the morphological evolution and stability of
step-pool features. However, detailed information on the three-dimensional hydraulics for step-pool morphology has been scarce due to the difficulty
of measurement. To fill in this knowledge gap, we established a combined approach based on the technologies of structure from motion (SfM) and
computational fluid dynamics (CFD). 3D reconstructions of bed surfaces with an artificial step-pool unit built from natural stones at six flow rates
were imported to CFD simulations. The combined approach succeeded in visualizing the high-resolution 3D flow structures for the step-pool unit. The
results illustrate the segmentation of flow velocity downstream of the step, i.e., the integral recirculation cell at the water surface, streamwise
vortices formed at the step toe, and high-speed flow in between. The highly nonuniform distribution of turbulence energy in the pool has been
revealed, and two energy dissipaters of comparable magnitude are found to co-exist in the pool. Pool scour development during flow increase leads to
the expansion of recirculation cells in the pool, but this expansion stops for the cell near the water surface when flow approaches the critical
value for step-pool failure. The micro-bedforms (grain clusters) developed on the negative slope affect the local hydraulics significantly, but this
influence is suppressed at the pool bottom. The drag forces on the step stones increase with discharge (before the highest flow value is
reached). In comparison, the lift force consistently has a larger magnitude and a more widely varying range. Our results highlight the feasibility and great
potential of the approach combining physical and numerical modeling in investigating the complex flow characteristics of step-pool morphology.
References
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TL;DR: In this paper, the assessment of the Darcy-Weisbach flow resistance coefficients f at five steep mountain rivers located in the northern mountain range surrounding the city of Cochabamba in Bolivia is presented.
Abstract: The present paper illustrates the assessment of the Darcy-Weisbach flow resistance coefficients f at five steep mountain rivers located in the northern mountain range surrounding the city of Cochabamba in Bolivia. Extensive field works were carried out. Several empirical formulas were tested. Results show that some of the aforementioned formulas provide good results in the assessment of f, being the model of Ugarte & Madrid-Aris (1994) the most accurate one. Plots showing direct logarithmic relationships between f and the riverbed slope So were obtained for each studied river, as well as an overall relationship of f vs. So.
7 citations
01 Jan 2014
TL;DR: In this article, a simulation of free and submerged hydraulic jumps in a rectangular channel downstream a sluice gate is performed using an Acoustic Doppler Velocimeter (ADV) and a Particle Image Velocimer (PIV).
Abstract: Hydraulic jump is one of the most extended and effective mechanism for hydraulic energy dissipation. Usually, hydraulic jump characteristics have been studied through physical models. Nowadays, computational fluid dynamics (CFD) are an important tool that can help to analyze and to understand complex phenomena that involve high turbulence and air entrainment cases. Free and submerged hydraulic jumps are studied in a rectangular channel downstream a sluice gate. Velocity measurements are carried out by using an Acoustic Doppler Velocimeter (ADV) and a Particle Image Velocimeter (PIV). The CFD models boundary conditions are based on laboratory measurements. Air-water two-phase flows are considered in the simulations. The closure problem is solved by using a two-equations turbulence model. Water depths, hydraulic jumps lengths and velocity profiles are compared with laboratory measurements.
6 citations
Dissertation•
01 Jan 2013
2 citations
TL;DR: In this article, the authors present advice for young professional engineers from both a professional and personal viewpoint from both the perspective of the engineer and the engineer's own experience in the field.
Abstract: Presents advice for young professional engineers from both a professional and personal viewpoint.
1 citations