Vegetation flow is commonly studied for subcritical flow motions, while in supercritical flow, the presence of vegetation often leads to enhancement of flow self‐aeration, which would in turn alter ...the flow hydraulics and water‐plant interaction. This paper presents an experimental investigation of aerated water flow cascading down a vegetated stepped chute. The 21.8° sloping chute is equipped with a 12‐m‐long uniformly stepped invert section. The 0.12‐m‐high steps have grate‐shaped rough top surface covered by 0.07‐m‐high flexible artificial plants, and three vegetation densities are tested in skimming flows, in addition to the reference cases of smooth steps and rough steps covered by bare grate only. The air‐water flow properties and free‐surface fluctuations are systematically measured from the upstream clear‐water region to the downstream fully developed aerated region. The experiment results demonstrate that, compared to a man‐made concrete stepped chute, the presence of vegetation cover on the steps reduces the overall self‐aeration and unsteady fluctuating motions of the flow, while it enhances the dissipation of the flow kinetic energy. The present finding also suggests that the modification to flow aeration and energy loss in the vegetated flow over macrostep‐shaped roughness is dependent on the flow regime which is primarily determined by the flow rate for a given slope and step‐induced macroroughness height.
Key Points
Flow aeration and flow resistance are studied in supercritical stepped chute flow with and without vegetation on the step surfaces
Vegetation on steps reduces flow self‐aeration and unsteady fluctuating motions compared to engineered concrete steps
Vegetation on steps enhances energy dissipation and air‐water mixing by producing more small‐size bubbles in water
Efficiently measuring groundwater flow in bedrock aquifers is inherently challenging due to the irregular distribution and fine scale of fractures. Recent advances in Active Distributed Temperature ...Sensing (A‐DTS) in boreholes temporarily sealed with liners have made it possible to quantify flow rates in such aquifers at many different depths using heat as a tracer, but until now only data collected under a single hydraulic condition have been published. This paper presents the first field data from multiple A‐DTS field tests conducted under different hydraulic conditions to quantify groundwater flow redistribution within a bedrock aquifer. Three separate quasi steady state A‐DTS tests were collected in a sealed borehole: (1) natural gradient condition where all boreholes were sealed with flexible and impermeable liners, (2) cross‐connected condition where a nearby borehole was open allowing vertical flow within the borehole, and (3) forced gradient condition where the nearby open borehole was pumped at a constant rate of 54 L/min. The depth‐discrete hydraulic head responses were also measured during the three tests using a string of transducers in a sealed borehole. Results provide quantifiable insights as to how the bedrock aquifer responds, including A‐DTS‐derived measurements of flow changes in fractures at multiple depths driven by changes in gradients. The results confirm that a single open borehole or long‐screened well can significantly alter the site hydraulics and demonstrate that not all large or transmissive fractures show evidence of active flow and thus, transmissivity and aperture should not be used alone to infer active flow zones.
Plain Language Summary
Measuring groundwater flow in fractured bedrock aquifers is difficult because flow is primarily controlled by small and irregularly spaced fractures. Very few tools exist to measure the natural flow through fractures in these aquifers, which is essential for understanding contaminant transport flow paths. One emerging technique, called Active Distributed Temperature Sensing (A‐DTS), uses a type of fiber optic sensor that can measure temperature at many different intervals along a fiber optic cable. This cable is lowered into a borehole, and a flexible inflatable liner is installed to prevent vertical flow within the borehole. The cable is then heated using integrated heating wires for an extended period, and the temperature response can be used to locate and estimate groundwater flow rates. This study collects field data under three different flow conditions at a site to demonstrate how the flow in a bedrock aquifer responds when it is stressed and the sensitivity of the A‐DTS technique. Results demonstrate highly variable flow with depth and that having a single open borehole on a site can strongly affect the natural flow system. A‐DTS allows efficient measurement of this variable flow with depth and provides a better understanding of these complex bedrock groundwater systems.
Key Points
A‐DTS in sealed boreholes can effectively quantify changes in fracture flow with depth in a bedrock aquifer
An open and cross‐connected borehole can significantly affect the site hydraulics
Under natural gradient conditions, transmissive fractures are not always hydraulically active
•Downstream development of initially stratified gas-liquid flow is analyzed.•The liquid film climbs up the walls gradually due to secondary flow in the gas.•The height of wall liquid film is related ...to the film roughness at the pipe bottom.•The wave-induced liquid lifting is insufficient to maintain annular film.•Liquid film wave structure is qualitatively the same as in vertical pipes.
Air-water flow in a 20 mm horizontal pipe is studied using side-view visualization with a background image and Brightness-Based Laser-Induced Fluorescence technique. The investigation is focused on the transition from stratified to annular flow patterns. Stratified flow is organized at the pipe inlet, and the dynamics of liquid lifting up the pipe walls is investigated. During the transition to annular flow, the liquid film is spread over the pipe walls in a stable and gradual manner; the spreading begins before the disturbance waves are formed. Two transition regimes are identified. At large gas and low liquid flow rates, the film is spread up the pipe walls reaching a stable height unaffected by the passing waves. At low gas and large liquid flow rates, the liquid can be lifted by the large-scale waves, but it promptly drains downwards between the waves. Secondary flow in the gas phase is considered the main mechanism of liquid lifting and the only mechanism able to create a stable annular film. The processes of formation and development of disturbance waves are qualitatively the same as previously observed in vertical pipes. Namely, the disturbance waves are formed due to the coalescence of high-frequency initial waves appearing near the inlet; the disturbance waves undergo coalescence and grow in amplitude and speed. Quantitatively, the disturbance wave formation occurs at larger distances from the inlet compared to the vertical flow, and the acceleration rate is much lower. An estimation of circumferential shear stress due to secondary flow is made based on the roughness of the liquid film surface at the bottom of the pipe. An increase in this shear stress increases the height of the liquid film on the pipe walls.
The flow characteristics of the blade unit of a tridimensional rotational flow sieve tray were investigated. First, the flow patterns are defined under different liquid arrangement methods. They are ...bilateral film flow, continuous perforated flow, and dispersion‐mixing flow in overflow distribution, film and jet flow and jet and mixed flow in spray distribution. Second, the time and frequency domain analysis of the differential pressure pulsation signal in the blade unit is carried out, the main frequencies range is 2.0−5.5 Hz. The influence of perforation and mixing intensity on the flow pattern transition is clarified. Third, the rotational flow ratio of the gas–liquid phase is measured, the gas phase rotational flow range is 0.55–0.78, and the liquid phase range is 0.15–0.42. The influence of the operating conditions on the distribution of the rotational and perforated flow is investigated. Finally, a prediction model for the rotational flow ratio is proposed.
Bacteria in porous media, such as soils, aquifers, and filters, often form surface-attached communities known as biofilms. Biofilms are affected by fluid flow through the porous medium, for example, ...for nutrient supply, and they, in turn, affect the flow. A striking example of this interplay is the strong intermittency in flow that can occur when biofilms nearly clog the porous medium. Intermittency manifests itself as the rapid opening and slow closing of individual preferential flow paths (PFPs) through the biofilm-porous medium structure, leading to continual spatiotemporal rearrangement. The drastic changes to the flow and mass transport induced by intermittency can affect the functioning and efficiency of natural and industrial systems. Yet, the mechanistic origin of intermittency remains unexplained. Here, we show that the mechanism driving PFP intermittency is the competition between microbial growth and shear stress. We combined microfluidic experiments quantifying
biofilm formation and behavior in synthetic porous media for different pore sizes and flow rates with a mathematical model accounting for flow through the biofilm and biofilm poroelasticity to reveal the underlying mechanisms. We show that the closing of PFPs is driven by microbial growth, controlled by nutrient mass flow. Opposing this, we find that the opening of PFPs is driven by flow-induced shear stress, which increases as a PFP becomes narrower due to microbial growth, causing biofilm compression and rupture. Our results demonstrate that microbial growth and its competition with shear stresses can lead to strong temporal variability in flow and transport conditions in bioclogged porous media.
The deflection flow of inlet passage seriously affects the performance of axial flow pump devices, and reduces the operation efficiency and stability of pumping station systems. In this paper, the ...influence of different deflection angles on the internal flow characteristics and outlet pulsation characteristics of the inlet passage of the vertical axial flow pump are studied. Based on the Reynolds time-averaged N-S equation of the three-dimensional incompressible fluid and the standard k-ε turbulence model, the model axial flow pump device was numerically simulated. Under optimal working conditions (Qbep = 31.04 L/s), the internal flow field of the axial flow pump was analyzed to study the change law of the axial flow pump performance under different deflection angles. Under the flow conditions of 0.6 Qbep, 1.0 Qbep and 1.2 Qbep, the pulsation characteristics of the outlet of inlet passage in axial flow pump at different deflection angles were analyzed. The result shows that with the increase of the deflection angle, the flow pattern of the inlet passage becomes turbulent, forming vortices of different sizes, the hydraulic loss of the inlet passage increases continuously, and the uniformity of the outlet flow velocity of the inlet passage increases first and then decreases. The time-domain waveform of outlet of the inlet passage at the pressure pulsation monitoring point has obvious periodicity, and the dominant frequency of the monitoring point is four times the rotation frequency, which corresponds to the number of impeller blades. It shows that the numerical calculation is in good agreement with the experimental results, which proves the reliability and validity of the numerical simulation calculation.
The industrial process involving gas liquid flows is one of the most frequently encountered phenomena in the energy sectors. However, traditional methods are practically unable to reliably identify ...flow patterns if additional independent variables/parameters are to be considered rather than gas and liquid superficial velocities. In this paper, we reported an approach to predict flow pattern along upward inclined pipes (0–90°) via deep learning neural networks, using accessible parameters as inputs, namely, superficial velocities of individual phase and inclination angles. The developed approach is equipped with deep learning neural network for flow pattern identification by experimental datasets that were reported in the literature. The predictive model was further validated by comparing its performance with well-established flow regime forecasting methods based on conventional flow regime maps. Besides, the intensity of key features in flow pattern prediction was identified by the deep learning algorithm, which is difficult to be captured by commonly used correlation approaches.
•An integrated method was developed to predict flow regimes via deep learning.•The deep learning predictive model was validated by unified models.•The model has high accuracy in predicting flow patterns of inclined flows.•Impacts of different variables on flow patterns was evaluated quantitatively.
Capability for handling entrained gas is an important design consideration for centrifugal pumps used in petroleum, chemistry, nuclear applications. An experimental evaluation on their two phase ...performance is presented for two centrifugal pumps working under air-water mixture fluid conditions. The geometries of the two pumps are designed for the same flow rate and shut off head coefficient with the same impeller rotational speed. Overal pump performance and unsteady pressure pulsation information are obtained at different rotational speeds combined with various inlet air void fractions (α0) up to pump stop condition. As seen from the test results, pump 2 is able to deliver up to 10% two-phase mixtures before pump shut-off, whereas pump 1 is limited to 8%. In order to understand the physics of this flow phenomenon, a full three-dimensional unsteady Reynolds Average Navier-Stokes (3D-URANS) calculation using the Euler–Euler inhomogeneous method are carried out to study the two phase flow characteristics of the model pump after corresponding experimental verification. The internal flow characteristics inside the impeller and volute are physically described using the obtained air distribution, velocity streamline, vortex pattern and pressure pulsation results under different flow rates and inlet void fractions. Pump performances would deteriorate during pumping two-phase mixture fluid compared with single flow conditions due to the phase separating effect. Some physical explanation about performance improvements on handing maximum acceptable inlet two phase void fractions capability of centrifugal pumps are given.
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•New quasi-two-dimensional ejector model considering flow phenomena.•Calculation of 2D flow field of mixing process of primary and secondary flows.•Consideration of effects of shock ...train and mixing layer development.•Accurate prediction of overall performance and flow parameters of ejector.•Experimental investigation of high-temperature and wet-secondary-flow conditions.
An anode gas recirculation ejector plays an important role in the performance and service life of fuel cells. This study establishes a new quasi-two-dimensional ejector model by considering the flow phenomena of the boundary layer, shock train, and mixing layer. The two-dimensional flow field of the mixing process of the primary and secondary flows is calculated using the compressible turbulent shear layer development theory. The effects of both compressible flow friction and nozzle flow separation caused by the boundary layer are considered. Furthermore, the dynamic pressure loss and mixing acceleration caused by the shock train are calculated in the model. The mixing-length theory proposed by Prandtl is used to calculate the momentum transfer between fluids, and a method for calculating the viscous dissipation of the mixing layer is presented. In addition, the effects of the flow phenomena are introduced into the governing equations through source terms. Finally, an experimental system is built to test the ejector performance under room-temperature, high-temperature, and wet-secondary-flow conditions. The performance and flow parameters obtained using the proposed model are verified through experiments and CFD simulations. The results show that the quasi-two-dimensional ejector model can accurately predict the overall performance and axial flow parameters of the ejector. The root-mean-square errors of the mass flow rates of the primary and secondary flows are 2.29% and 8.78%, respectively. The two-dimensional flow field obtained using the quasi-two-dimensional ejector model is in good agreement with the CFD simulation results. The development of the mixing layer affects the axial parameters and performance of the ejector because it influences the momentum transfer, friction loss, and viscous dissipation of the flow field. This study is of significance for accurately predicting the ejector performance as well as for optimizing the design of the ejector for anode gas recirculation fuel cell systems. This contributes to the efficient operation of fuel cells and simplification of supporting auxiliary systems, which has energy and economic benefits. The proposed ejector model can also be extended to ejectors in other fields.
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