Channel Roughness Research Articles

Investigating the role of model structure and surface roughness in generating flood inundation extents using one‐ and two‐dimensional hydraulic models

Hydraulic models play an important role in determining flood inundation areas. When considering a wide array of one‐ (1D) and two‐dimensional (2D) hydraulic models, selecting an appropriate model and its calibration are crucial in an accurate prediction of flood inundation. This study compares the performance of four commonly used 1D and 2D hydraulic models, including HEC‐RAS 1D, HEC‐RAS 2D, LISFLOOD‐FP diffusive, and LISFLOOD‐FP subgrid, with respect to their model structure and their sensitivity to surface roughness characterisation. Application of these models to four study reaches with different river geometry and roughness characterisation shows that for a given set of roughness condition, the geometry, including the sinuosity, reach length and floodplain width, does not affect the performance of a 1D or 2D model. Overall, the performance of a 1D model is comparable to the 2D models used in the study, with the 2D models showing slightly better results. The performance of 2D models is affected by low channel roughness, and it improves with increasing channel roughness that enables more water to enter into the floodplain. On the contrary, the performance of 1D model is positively affected with increasing floodplain roughness. When the models are evaluated for uniform versus distributed roughness characterisation in the floodplain, the uniform surface characterisation provides the best results compared to the distributed roughness characterisation.

Numerical simulation of electroosmotic flow in rough microchannels using the lattice Poisson-Nernst-Planck methods

In this study, coupled Lattice Boltzmann method is applied to solve the dynamic model for an electroosmotic flow and investigate the effects of roughness in a 2-D flat microchannel. In the present model, the Poisson equation is solved for the electrical potential, the Nernst– Planck equation is solved for the ion concentration. In the analysis of electroosmotic flows, when the electric double layers fully overlap or the convective effects are not negligible, the Nernst-Planck equation must be used to find the ionic distribution throughout the microchannel. The effects of surface roughness height, roughness interval spacing and roughness surface potential on flow conditions are investigated for two different configurations of the roughness, when the EDL layers fully overlap through the microchannel. The results show that in both arrangements of roughness in homogeneously charged rough channels, the flow rate decreases by increasing the roughness height. A discrepancy in the mass flow rate is observed when the roughness height is about 0.15 of the channel width, which its average is higher for the asymmetric configuration and this difference grows by increasing the roughness height. In the symmetric roughness arrangement, the mass flow rate increases until the roughness interval space is almost 1.5 times the roughness width and it decreases for higher values of the roughness interval space. For the heterogeneously charged rough channel, when the roughness surface potential ψr is less than channel surface potential ψs, the net charge density increases by getting far from the roughness surface, while in the opposite situation, when ψs is more than ψr, the net charge density decreases from roughness surface to the microchannel middle center. Increasing the roughness surface potential induces stronger electric driving force on the fluid which results in larger velocities in the flow.

Understanding the effects of river regulation on Atlantic salmon fry: The importance of channel morphology

AbstractRiver regulation influences hydrological regime, channel hydraulics, and habitat quality for freshwater biota. Environmental flow assessments typically require river managers to incorporate the requirements of different freshwater species and life stages in designing discharge regimes. However, these requirements can often conflict or be poorly quantified. There is therefore increasing interest in adaptive management whereby the ecological effects of changing flow regime can be monitored and assessed. This study investigated the effects of flow regulation on Atlantic salmon fry below an impoundment. The flow regime incorporated a compensation flow with an atypical seasonal profile where higher discharges were released in the summer than autumn and a spring high flow (freshet) to facilitate downstream migration of smolts through the dam. Saturation stocking and electrofishing were combined with hydraulic and habitat models to predict the effects of discharge on habitat quality and standardized fry densities. The discharge at which the highest standardized fry densities were observed varied depending on reach morphology, particularly channel roughness. Low fry survival was consistent with predictions of consistently low habitat quality during the spring freshet. Fish locations observed during electrofishing were not always consistent with contemporary flows and may reflect the influence of antecedent flows and high site fidelity. The combination of saturation stocking, hydraulic, and habitat models provides a valuable approach for assessing the effects of regulation. It is recommended that future freshets operate at night, that uninterrupted periods of high flow are avoided during the spring, and that compensation flows reflect natural seasonal patterns.

Ensemble modeling of stochastic unsteady open-channel flow in terms of its time–space evolutionary probability distribution – Part 2: numerical application

Abstract. The characteristic form of the Saint-Venant equations is solved in a stochastic setting by using a newly proposed Fokker–Planck Equation (FPE) methodology. This methodology computes the ensemble behavior and variability of the unsteady flow in open channels by directly solving for the flow variables' time–space evolutionary probability distribution. The new methodology is tested on a stochastic unsteady open-channel flow problem, with an uncertainty arising from the channel's roughness coefficient. The computed statistical descriptions of the flow variables are compared to the results obtained through Monte Carlo (MC) simulations in order to evaluate the performance of the FPE methodology. The comparisons show that the proposed methodology can adequately predict the results of the considered stochastic flow problem, including the ensemble averages, variances, and probability density functions in time and space. Unlike the large number of simulations performed by the MC approach, only one simulation is required by the FPE methodology. Moreover, the total computational time of the FPE methodology is smaller than that of the MC approach, which could prove to be a particularly crucial advantage in systems with a large number of uncertain parameters. As such, the results obtained in this study indicate that the proposed FPE methodology is a powerful and time-efficient approach for predicting the ensemble average and variance behavior, in both space and time, for an open-channel flow process under an uncertain roughness coefficient.

Application of hybrid friction stir channeling technique to improve the cooling efficiency of electronic components

Hybrid friction stir channeling (HFSC) is a new friction stir-based method for producing internal, closed channels created simultaneously during welding of multiple metal plates. Differently from conventional friction stir channeling, that is only able to produce channels in a monolithic component, HFSC can be applied to complex structural systems involving multiple components made of similar or dissimilar materials. In this study, the channels manufactured by HFSC were open in a structural system made of AA5083, with one base plate of 5 mm thick, and one overlapping plate of 8 mm thick, used as a rib, containing the channel along conformal cooling path. The thermal performance of the HFSC channel is compared with a conventional channel, with similar shape and path, produced by milling. The channels being tested are part of an electronic device containing multiple heat sources. The HFSC channel presents 30 to 40% lower steady-state temperature and 33% higher cooling rate during the transient period than those of the milled version. Compared with the milled channel, the heat extraction capacity, per unit of mechanical pumping power, is higher for the HFSC channel. Surface roughness, microhardness, and microstructural analysis of HFSC channels are investigated to characterize the HFSC channel.

Semi-theoretical approach for energy dissipation estimation at hydraulic jumps in rough sloped channels

ABSTRACTHydraulic jumps cause significant flow energy dissipation and generally occur in association with hydraulic structures. Nevertheless, the dissipative mechanisms occurring with this hydraulic phenomenon still require investigation. There are no systematic studies analysing the hydraulic jump energy dissipation for a large range of hydraulic and geometric conditions. To the authors’ knowledge, no studies provide a complete and exhaustive analysis of the dissipative mechanism for rough sloped channels. This study proposes a novel semi-theoretical and general approach to estimate energy dissipation at hydraulic jumps in rough sloped rectangular channels, involving a large range of both hydraulic conditions and bed configurations. Two relationships were derived and their predictive capability has been tested with approximately 500 experimental data points from different studies.

LBM Investigation of Immiscible Displacement in a Channel with Regular Surface Roughness

We investigated immiscible displacement in a channel considering regular surface roughness at walls. A color-fluid LBM code is developed and validated against the Hagen–Poiseuille flow before it is employed for simulating the displacement process. The dimensionless roughness height and roughness spacing ratio are defined to characterize the surface roughness. The simulation results show that the presence of surface roughness obviously impels a finger formation in a channel. The impelling effect is more significant at larger roughness heights and medium roughness spacing ratios. The critical capillary number and viscosity ratio of a finger formation is reduced with increasing roughness height. The obvious effect of wettability on the finger development in a smooth channel is attenuated in rough channels. The attenuation magnitude increases with increasing roughness height.

Heat transfer on topographically structured surfaces for power law fluids

The three-dimensional power law fluid flow through rough microchannels has been studied numerically to determine the effects of the topographic structures on the thermal and hydrodynamic characteristics of the system. Rectangular, triangular and sinusoidal element shapes have been considered in order to investigate the effects of roughness height, width, pitch and channel separation on the pressure drop and heat transfer. Uniform wall heat flux boundary condition has been applied for all the peripheral walls.The results indicate that the global heat transfer performance can be improved or reduced by the roughness elements at the expense of pressure head when compared with the smooth channels. The maximum heat transfer performance has been obtained for triangular roughness shapes with the relative roughness height of 0.333. In most cases heat performance is smaller than unity. It is also revealed that for the sinusoidal and triangular cases by decreasing the relative roughness width, heat transfer performance is improved. Furthermore, the effects of the roughness features and the channel separation are intensified in power-law fluids. For dilatant fluids with a power-law index equal to n = 1.25, channel separation ratio reduction causes a steep rise in normalized friction factor up to 140.8 in the rectangular case. By increasing the relative roughness pitch of the cases with rectangular elements and the power-law index of n = 0.75, a reasonable heat transfer performance enhancement has been observed.

Estimating Discharge in Low‐Order Rivers With High‐Resolution Aerial Imagery

AbstractRemote sensing of river discharge promises to augment in situ gauging stations, but the majority of research in this field focuses on large rivers (>50 m wide). We present a method for estimating volumetric river discharge in low‐order (<50 m wide) rivers from remotely sensed data by coupling high‐resolution imagery with one‐dimensional hydraulic modeling at so‐called virtual gauging stations. These locations were identified as locations where the river contracted under low flows, exposing a substantial portion of the river bed. Topography of the exposed river bed was photogrammetrically extracted from high‐resolution aerial imagery while the geometry of the remaining inundated portion of the channel was approximated based on adjacent bank topography and maximum depth assumptions. Full channel bathymetry was used to create hydraulic models that encompassed virtual gauging stations. Discharge for each aerial survey was estimated with the hydraulic model by matching modeled and remotely sensed wetted widths. Based on these results, synthetic width‐discharge rating curves were produced for each virtual gauging station. In situ observations were used to determine the accuracy of wetted widths extracted from imagery (mean error 0.36 m), extracted bathymetry (mean vertical RMSE 0.23 m), and discharge (mean percent error 7% with a standard deviation of 6%). Sensitivity analyses were conducted to determine the influence of inundated channel bathymetry and roughness parameters on estimated discharge. Comparison of synthetic rating curves produced through sensitivity analyses show that reasonable ranges of parameter values result in mean percent errors in predicted discharges of 12%–27%.

Turbulence modulation and heat transfer enhancement in channels roughened by cube-covered surface

Direct numerical simulations are carried out to study turbulence modification and heat transport augmentation in rough channels with minor cubes on one wall. To account for the effect of the cubes on momentum and heat transfer in wall-bounded turbulent flows, the lattice Boltzmann method which is based on a double-distribution-function and D3Q19 model is applied to perform the numerical computation. The present study focuses on the modulations of the temperature field and heat transfer process by rough wall with different cube heights. Some typical dynamic and thermal statistics, such as the mean velocities, mean temperature, velocity fluctuations, the Reynolds stresses, and turbulent heat flux are analyzed. The similar trend of temperature and velocity roughness functions illustrates there is strong correlation between the heat transfer augmentation and a drag increase in rough channel flows. A significant heat transfer enhancement (26%, by taking into account the drag consumption) is obtained with the medium-height cubes.

Discussion of “Boundary layer development over non-uniform sand rough bed channel”, by Anurag Sharma and Bimlesh Kumar (2017)

This discussion provides highlights with regard to determination of the appropriate length for boundary layer development over non-uniform sand bed channels with and without seepage. It is demonstrated that the compound effects of Kármán’s coefficient κ and virtual origin should be accounted to quantify the behaviour of roughness length over non-uniform sand bed channels with seepage.

Effect of aspect ratio on higher order moments of velocity fluctuations in hydraulically rough open channel flow

The effect of aspect ratio on the higher order statistics of velocity fluctuations in a hydraulically rough turbulent open channel flow is investigated. In this regard, an experiment was conducted in a rough bed narrow open channel flow of aspect ratio equal to three and the instantaneous flow velocities were measured using a Nortek Vectrino+ Acoustic Doppler Velocimeter. To understand the effect of aspect ratio, the results obtained from the present study are compared with the literature data of approximately same Reynolds number and bed roughness in a wide open channel flow for turbulence intensities and higher order statistics of velocity fluctuations. Comparison of turbulence intensities between Narrow OCF and Wide OCF shows occurrence of higher streamwise and vertical turbulence intensities in the outer region of Narrow OCF. The results of third order moments of velocity fluctuations are sensitive to aspect ratio in the outer region.

Determination of manning's roughness coefficient for dhaleshwari river using hec-ras

The Dhaleshwari River is a distributary of Jamuna River and flows past Dhaka city into the Meghna estuary. The flow through the Dhaleshwari River is influenced by roughness characteristic of the channel material. Manning's n is used extensively around the world to predict the degree of roughness in channels. This paper presents a hydrodynamic HEC-RAS 1D model to estimate Manning's n for the Dhaleshwari River. The system requires data including cross-section, water level and discharge measurements. After a model is developed, it undergoes a calibration and a validation phase. Finally the calibrated and validated model result for Manning's n can be used for scenario analysis.

Large and very large scale motions in roughbed open-channel flow

Long duration PIV measurements in rough-bed (glass beads) open-channel flow (OCF) reveal that the pre-multiplied spectra of the streamwise velocity has a bimodal distribution due to the presence of large and very large scale motions (LSMs and VLSMs, respectively). The existence of VLSMs in boundary layers, pipes and closed channels has been acknowledged for some time, but strong supporting evidence for their presence in OCF has been lacking. Length scales of the large and very large scale motions in OCF exhibit different scaling properties; whereas the streamwise length of the LSM scales with the flow depth, the VLSM streamwise length does not scale purely with flow depth and may additionally depend on other scales such as the channel width, roughness height, or viscous length. Supplementary data for flows over self-affine fractal rough beds support these findings and additionally indicate that the length of VLSMs may grow along the extensive distance from the channel entrance. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed open-channel flows form independently rather than as a spatial alignment of LSMs.

Depth-averaged velocity and bed shear stress in unsteady open channel flow over rough bed

Very few studies have been carried out in the past in estimating depth-averaged velocity and bed shear stress in unsteady flow over rough beds. An experiment is thus conducted to investigate the vertical and lateral velocity profiles under unsteady flow conditions in a rough open channel for various flow depths. One hydrogram is repeatedly passed through the rectangular flume with a fixed rigid grass bed. Using micro Pitot tube and Acoustic Doppler Velocimeter (ADV), the flow patterns are investigated at both lateral and longitudinal positions over different cross-sections. For two typical flow depths, the velocities in both the rising limb and falling limb are observed. Hysteresis effect between stage-discharge (h ~ Q) rating curve between rising and falling limbs is illustrated. Lateral distribution of depth-averaged velocity and bed shear stress are plotted at three different cross sections and compared with the steady flow conditions. In falling limb of an unsteady flow case, both depth-averaged velocity and bed shear stress distribution in the central region is higher than that of steady flow case. However, in the rising limb, the bed shear stress of unsteady flow is less than that of steady flow case. Further, in an unsteady flow, the magnitude of depth-averaged velocity is found to increase towards the downstream sections. Along the downstream positions, bed shear stress values increase for lower flow depths and decrease for higher flow depth cases.

Turbulence structure of non-uniform rough open channel flow

This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.

Discharge and location dependency of calibrated main channel roughness: Case study on the River Waal

To accurately predict water levels, river models require an appropriate description of the hydraulic roughness. The bed roughness increases as river dunes grow with increasing discharge and the roughness depends on differences in channel width, bed level and bed sediment. Therefore, we hypothesize that the calibrated main channel roughness coefficient is most sensitive to the discharge and location in longitudinal direction of the river. The roughness is determined by calibrating the Manning coefficient of the main channel in a 1D hydrodynamic model. The River Waal in the Netherlands is used as a case study. Results show that the calibrated roughness is mainly sensitive to discharge. Especially the transition from bankfull to flood stage and effects of floodplain compartmentation are important features to consider in the calibration as these produce more accurate water level predictions. Moreover, the downstream boundary condition also has a large effect on the calibrated roughness values near the boundary.

Second-Law Analysis: A Powerful Tool for Analyzing Computational Fluid Dynamics (CFD) Results

Second-law analysis (SLA) is an important concept in thermodynamics, which basically assesses energy by its value in terms of its convertibility from one form to another.[...]

Evaluation of multi-mode CryoSat-2 altimetry data over the Po River against in situ data and a hydrodynamic model

Abstract Coverage of in situ observations to monitor surface waters is insufficient on the global scale, and decreasing across the globe. Satellite altimetry has become an increasingly important monitoring technology for continental surface waters. The ESA CryoSat-2 altimetry mission, launched in 2010, has two novel features. (i) The radar altimeter instrument on board of CryoSat-2 is operated in three modes; two of them reduce the altimeter footprint by using Delay-Doppler processing. (ii) CryoSat-2 is placed on a distinct orbit with a repeat cycle of 369 days, leading to a drifting ground track pattern. The drifting ground track pattern challenges many common methods of processing satellite altimetry data over rivers. This study evaluates the observation error of CryoSat-2 water level observations over the Po River, Italy, against in situ observations. The average RMSE between CryoSat-2 and in situ observations was found to be 0.38 meters. CryoSat-2 was also shown to be useful for channel roughness calibration in a hydrodynamic model of the Po River. The small across-track distance of CryoSat-2 means that observations are distributed almost continuously along the river. This allowed resolving channel roughness with higher spatial resolution than possible with in situ or virtual station altimetry data. Despite the Po River being extensively monitored, CryoSat-2 still provides added value thanks to its unique spatio-temporal sampling pattern.

Fabricating smooth PDMS microfluidic channels from low-resolution 3D printed molds using an omniphobic lubricant-infused coating

The advent of 3D printing has allowed for rapid bench-top fabrication of molds for casting polydimethylsiloxane (PDMS) chips, a widely-used polymer in prototyping microfluidic devices. While fabricating PDMS devices from 3D printed molds is fast and cost-effective, creating smooth surface topology is highly dependent on the printer's quality. To produce smooth PDMS channels from these molds, we propose a novel technique in which a lubricant is tethered to the surface of a 3D printed mold, which results in a smooth interface for casting PDMS. Fabricating the omniphobic-lubricant-infused molds (OLIMs) was accomplished by coating the mold with a fluorinated-silane to produce a high affinity for the lubricant, which tethers it to the mold. PDMS devices cast onto OLIMs produced significantly smoother topology and can be further utilized to fabricate smooth-channeled PDMS devices. Using this method, we reduced the surface roughness of PDMS microfluidic channels from 2 to 0.2 μm (10-fold decrease), as well as demonstrated proper operation of the fabricated devices with superior optical properties compared to the rough devices. Furthermore, a COMSOL simulation was performed to investigate how the distinct surface topographies compare regarding their volumetric velocity profile and the shear rate produced. Simulation results showed that, near the channel's surface, variations in flow regime and shear stress is significantly reduced for the microfluidic channels cast on OLIM compared to the ones cast on uncoated 3D printed molds. The proposed fabrication method produces high surface-quality microfluidic devices, comparable to the ones cast on photolithographically fabricated molds while eliminating its costly and time-consuming fabrication process.