Threshold Flow Velocity Research Articles

Incipient Motion of Single Shells under Currents in Flume Experiments

Understanding the motion thresholds of shells is important, as shell motion allows the analysis of beach profiles, prevents excessive erosion of the coastline, and helps to resource the use of discarded shells, providing new ideas for the protection of beaches. In this study, the orientational motions and motion thresholds of two types of typical molluscan shells, bivalve and gastropod shells, were investigated by means of flume experiments. The final orientations with the statistically highest number of occurrences during the orientational motions of each shell were used as the initial orientations for the respective threshold flow velocity measurements. The critical Shields parameter and the incipient mean velocity of the flow were used to represent the critical threshold of the motion. The critical Shields parameters for bivalve shells in the convex upward position were overall higher on average than those for gastropod shells. The experimental data showed that the incipient mean flow velocities of bivalve shells in the convex upward position were about 1.4–2.8 times larger than those in the convex downward position. The incipient mean velocity data were regressed to obtain the motion threshold equations applicable to bivalve shells in the convex upward and convex downward positions as well as gastropod shells under different final orientations.

Study on the incipient motion of deposited particles caused by hydraulic erosion in mining-induced fracture: Optimizing the process parameters of HET

Hydraulic erosion technology (HET) is a method of injecting a large amount of water into a blocked fracture to enhance its permeability. However, the incipient motion characteristics of the deposited particles in a fracture during hydraulic erosion are still unclear. This study conducted particle incipient motion experiments under the hydraulic conditions in a fracture, and investigated the particle incipient motion behavior and the variation in the characteristics of the threshold flow velocity with a change in the particle diameter. In addition, a new formula was developed to predict the threshold flow velocity of the deposited particle in the fracture, and the effects of fluid density, viscosity, and viscous sub-layer thickness on the threshold flow velocity were discussed. Furthermore, a method for optimizing the process parameters of HET was proposed. Results revealed that hydraulic erosion causes three particle motion states and four particle motion behavior in fractures. In addition, with an increase in the particle diameter, the threshold flow velocity first decreased and then increased. Further, increasing the fluid density and reducing the fluid viscosity were observed as two effective means for reducing the threshold flow velocity in fractures. Furthermore, an increasing fluid density was more sensitive to a reduction in the threshold velocity in fractures. These findings are expected to guide the optimization of the process parameters of HET.

Impact of solid and hollow bluff bodies on the performance and dynamics of flag-based energy harvester

This study investigates the energy harvesting potential of the wake of different bluff bodies using a piezoelectric membrane. Both hollow and solid C-shaped cylinders with similar dimensions in normal and inverted orientations are placed upstream of the membrane as a kinetic source of energy. A series of water channel tests are performed by varying stream-wise gap (x*) 0.5 ≤ x* ≤ 4.0 between the cylinder and membrane and flow velocity (V) 0.12 ≤ V ≤ 0.26 m/s. The instability region is expanded remarkably by using different cross-sections. Different flapping modes are observed, including biased, intermittent, and continuous. It is shown that the time-mean wake fluctuates with changes in the cross-section of the bluff body and has a substantial influence on the energy harvesting efficiency and dynamical behaviors of the membrane in terms of peak-to-peak oscillation amplitude and flapping frequency. A threshold flow velocity of 0.2 m/s for continuous energy harvesting is defined for all bluff bodies. The highest electrical energy is obtained at a flow velocity of 0.26 m/s and 2.5 diameters downstream from the hollow C-shaped bluff body when placed in an inverted orientation. A comparison of all the configurations of the bluff bodies indicates that the hollow C-shaped bluff body in inverted orientation produces the strongest wake, which results in a 36 % increase in flapping amplitude and a 35 % improvement in harvested power for the piezo-membrane based energy harvester compared to the baseline case (a solid C-shape).

Influence of shielding gas flow consistency on parts quality consistency during large-scale laser powder bed fusion

Large-scale laser powder bed fusion (large-scale LPBF) has been widely applied in aerospace, automobile, military and other fields to obtain light-weighted, highly customized parts. However, as the building area grows, it becomes more difficult to produce consistent parts at different locations on the building area. One of the biggest challenges it to maintain high-speed shielding gas flow with uniform flow direction across the enlarged powder bed area. In this study, two types of gas flow inlets were designed: inlet without grid (no restriction on the gas inflow) and inlet with grid (aim at limiting the degrees of freedom on z direction) to study the factors related to shielding gas flow influencing parts quality consistency in large-scale LPBF. The threshold flow velocity distribution on the base plate and quality consistency of built parts (density and tensile properties) under two inlet conditions were compared. According to the experimental results and the analysis of high-speed imaging and CFD simulation, it can be concluded that building position and consistency of flow characteristics on the powder bed influence the consistency of parts quality, which is crucial during large-scale LPBF.

Receiver operating characteristic curve analysis of arteriovenous dialysis access flow using ultrasound.

Despite numerous studies investigating the use of ultrasound (US) in assessing arteriovenous fistulas (AVF), there are no universally agreed threshold flow velocities in diagnosing significantly abnormal flow that are useful in predicting thrombotic flow-related dysfunction. This study evaluates a predictive model using receiver operating characteristic curve (ROC) analyses to establish threshold velocities. Five hundred and eleven US scans were analysed. ROC curves were used to determine the optimal threshold time average mean velocity (TAMV), peak systolic velocity (PSV) and end diastolic velocity (EDV) of the brachial artery supplying the AVF in determining the need for intervention or thrombosis within 3months of the scans. Estimated flow volume (FV) ROC was used as an evaluative comparison. There were 356 negative and 155 positive scan results in relation to the need for intervention or thrombosis. Empirical flow velocity parameters of TAMV, EDV and PSV were analysed using ROC curves, yielding an area under the curve (AUC) of 0.95, 0.92 and 0.86, respectively. FV ROC analysis yields a comparative AUC of 0.90. A TAMV cut-off at 48.6cm/s yielded the highest AUC. Subgroup analysis yielded an optimal TAMV cut-off of 45 cm/s for forearm and 49 cm/s for arm AVF. The EDV was also highly predictive of outcomes. PSV has the lowest accuracy. The TAMV of inflow brachial artery to AVF is highly predictive of outcomes of thrombotic flow-related dysfunction. Our study confirms TAMV cut-offs of 45 cm/s for forearm and 49 cm/s for arm AVF. These results require prospective validation.

Role of the Jet Angle, Particle Size, and Particle Concentration in the Degradation Behavior of Carbon Steel under Slow Slurry Erosion-Corrosion Conditions

Erosion-corrosion behavior of piping systems is a critical issue for their durability. This work concerns the erosion-corrosion behavior of carbon steel as a function of abradant characteristics as particle size and concentration. Degradation tests were performed in a jet erosion-corrosion cell with a maximum flow rate of 4.8 m/s, and jet angles comprised 30° and 90°. Abradant particles consisted of angular alumina powder with a mean diameter of 181, 219, and 359 µm. A critical threshold flow velocity of about 2.5 m/s was determined when experiments were performed with particles with diameters of 181 µm and jet angles of 45°. Even if erosion did not occur, the degradation rate increased compared with the stagnant condition because of dissolved dioxygen supply. A maximum of erosion-corrosion of 4 mg × cm−2 × h−1 was determined for the jet angle of 45°, irrespective of the particle sizes. The increase of abradant concentration led to a higher degradation rate regardless of the jet angle. However, the degradation rates tended to limit values of 7 mg × cm−2 × h−1 at 45° and 5 mg × cm−2 × h−1 at 90°. Above a critical concentration, a slowdown of the degradation was measured, suggesting that particle behavior in dense fluid acts on material degradation. This critical concentration can be understood from the interactions of the particles in concentrated media that modify trajectories in the flow and at the metal surface, reducing their kinetic energy consequently.

Comparison of Gravity Independence Criteria for Two-Phase Flow

The effect of gravity on two-phase flow phenomena depends on the relative magnitude of the gravity force to inertia, viscous, and surface tension forces. Two-phase flow research has investigated heat transfer coefficients, critical heat flux, and flow regimes at different gravity levels and inclination angles, and several researchers proposed threshold flow velocities above which two-phase flow would be independent of the gravity force. This is a promising approach for a safe design of two-phase flow systems employed in microgravity or to build orientation-independent two-phase flow systems operating under the influence of gravity. This paper summarizes the derivation of several proposed approaches found in the open literature and compares their results via a standardized reference velocity. It is found that groups of criteria show similar trends and magnitudes of the reference velocity in parametric studies, despite a generally wide spread of velocities looking at all criteria. A validation of the two-phase flow criteria for two-phase systems is suggested as important future research.

Hydrological management strategies for the control of algal blooms in regulated lowland rivers

AbstractReservoirs of lowland floodplain rivers with eutrophic backgrounds cause variations in the hydrological and hydraulic conditions of estuaries and low‐dam reservoir areas, which can promote planktonic algae to proliferate and algal bloom outbreaks. Understanding the ecological effects of variations in hydrological and hydraulic processes in lowland rivers is important for algal bloom control. In this study, the middle and lower reaches of the Han River, China, a typical regulated lowland river with a eutrophic background, are selected. Based on the effect of hydrological and hydraulic variability on algal blooms, a hydrological management strategy for river algal bloom control is proposed. The results showed that (a) differences in river morphology and background nutrient levels cause significant differences in the critical threshold flow velocities for algal bloom outbreaks between natural river and low‐dam reservoir sections; there is no uniform threshold flow velocity for algal bloom control. (b) There are significant differences in the river hydrological/hydraulic conditions between years with and without algal blooms. The average river flow, water level and velocity in years with algal blooms are significantly lower than those in years without algal blooms. (c) For different river sections where algal blooms occur and to meet the threshold flow velocities, the joint operation of cascade reservoirs and diversion projects is an effective method to prevent and control algal blooms in regulated lowland rivers. This study is expected to deepen our understanding of the ecological significance of special hydrological processes and guide algal bloom management in regulated lowland rivers.

CFD study on the effect of various geometrical parameters of honeycomb type orifices on pressure drop and cavitation characteristics

Flow zoning devices in nuclear power plants are used to achieve desired pressure drop and flow rates. Cavitation in these devices may cause erosion, flow chocking, noise, vibration, resulting in severe damage. Optimization of flow conditions, geometrical parameters, and prediction of the hydrodynamics of the flow field are necessary to avoid cavitation in these devices. In the present study, Computational Fluid Dynamics (CFD) simulations have been carried out to model flow through honeycomb shaped orifices. The effect of various geometrical parameters such as flow area, the spacing between plates, number of plates, orientation between plates (inline or offset) on pressure drop, and cavitation characteristics has been studied. The multiphase mixture model (water-liquid and water vapor), in conjunction with the Schnerr and Sauer cavitation model, is used to model the flow through orifice plates. Axial distribution profiles and contours of vapor fraction and pressure coefficient have been plotted to analyze the cavitation characteristics for different plate configurations and to get a better understanding of the flow field. Lowest pressure values are observed at the orifice throat and near plate exit. Threshold flow velocities above which flow is subjected to cavitation have been identified for different geometrical configurations. An increase in the number of plates reduces the probability of cavitation. Reduction in the flow area of the orifice plate increases the cavitation potential and pressure drop. However, an increase in plate spacing enhances the pressure recovery and reduces non-uniformities in the flow field.

The role of hydrodynamic impact force in subaerial boulder transport by tsunami—Experimental evidence and revision of boulder transport equation

The hydrodynamic impact contributes to the initiation of subaerial boulder transport by tsunamis and tsunami-like flows. Here, we consider two approaches to modelling this impact force—hydrostatic and hydrodynamic—and assess the suitability of each in boulder transport models. The model proposed, solved as an ordinary differential equation, eliminates the need to assume a conservative value of flow acceleration. Introducing the hydrodynamic impact force reduces, by almost 45%, the recognised overestimation of threshold flow velocity in simple hydrodynamic models for boulder transport. By contrast, including the hydrostatic impact force reduces the overestimation by less than 10%. We found the new model consistent with the results of simple flume experiments. In these experiments, three rectangular limestone blocks of 27–70 cm3 in subaerial pre-transport setting condition were subjected to tsunami-like flow by sudden opening of a sluice gate.

Review Article: Advances in modeling of bed particle entrainment sheared by turbulent flow

Bed particle entrainment by turbulent wall-shear flow is a key topic of interest in hydrodynamics because it plays a major role to govern the planetary morphodynamics. In this paper, the state-of-the-art review of the essential mechanisms governing the bed particle entrainment by turbulent wall-shear flow and their mathematical modeling is presented. The paper starts with the appraisal of the earlier multifaceted ideas in modeling the particle entrainment highlighting the rolling, sliding, and lifting modes of entrainment. Then, various modeling approaches of bed particle entrainment, such as deterministic, stochastic, and spatiotemporal approaches, are critically analyzed. The modeling criteria of particle entrainment are distinguished for hydraulically smooth, transitional, and rough flow regimes. In this context, the responses of particle size, particle exposure, and packing condition to the near-bed turbulent flow that shears the particles to entrain are discussed. From the modern experimental outcomes, the conceptual mechanism of particle entrainment from the viewpoint of near-bed turbulent coherent structures is delineated. As the latest advancement of the subject, the paper sheds light on the origin of the primitive empirical formulations of bed particle entrainment deriving the scaling laws of threshold flow velocity of bed particle motion from the perspective of the phenomenological theory of turbulence. Besides, a model framework that provides a new look on the bed particle entrainment phenomenon stemming from the stochastic-cum-spatiotemporal approach is introduced. Finally, the future scope of research is articulated with open questions.

Nonlinear dynamics of cantilevered pipes conveying fluid: Towards a further understanding of the effect of loose constraints

The nonlinear dynamics of a fluid-conveying cantilevered pipe with loose constraints placed somewhere along its length is investigated. The main objective of this study is to determine the effects of several geometrical and physical parameters of the loose constraints on the characteristics and behavior of pipes conveying fluid. Based on the full nonlinear equation of motion, the dynamical behavior of the pipe system is investigated. Phase portraits and bifurcation diagrams are constructed for a selected set of system parameters. Typical results are firstly compared to numerical ones reported previously and excellent agreement is obtained. Then, the threshold flow velocities for several key bifurcations including pitchfork, period doubling, chaos, and sticking behaviors are predicted, showing that in many cases, the gap size, stiffness, and asymmetry of the loose constraints have remarkable effects on the nonlinear responses of the cantilevered pipe conveying fluid. For a pipe system with small/large constraint gap sizes, small constraint stiffness, or large constraint offset, some of the complex dynamical behaviors including chaos and period-doubling bifurcations would disappear, at least in the flow velocity range of interest.

Prediction of Bridge Pier Scour Using Genetic Programming

This paper presents the use of genetic programming (GP) as a tool to predict pier scour depths based on clear-water conditions of laboratory measurements by past researchers. Four main dimensionless parameters-pier width, approaching flow depth, threshold flow velocity, and channel open-ratio-are considered for predicting the scour depth. The performance of the GP equation is verified by comparing the results with those obtained by empirical equations. It is found that the scour depth at bridge piers can be efficiently predicted using the GP model. The advantage of the GP model is confirmed by comparing the GP results of scour depths with the large-scale model studies and field data.

Effect of geometric parameters of the bistable fluidic amplifier in the liquid-jet hammer on its threshold flow velocity

The effect of the geometric parameters of the bistable fluidic amplifier in a liquid-jet hammer on its threshold flow velocity was investigated computationally. The flow fields of 33 fluidic amplifiers with different dimensions, each attached to an identical actuating mechanism, were studied to obtain their threshold flow velocities, using Computational Fluid Dynamics (CFD), with the technique of dynamic mesh and a user-defined function. The computational results indicate that each of the geometric parameters has a significant effect on the threshold flow velocity. Five representative computational results were validated by laboratory measurement. The agreement between the simulations and measurements is considered to be quite acceptable. Physical analysis of typical simulation results is also presented.

Performance studies on the desiccant packed bed with varying particle size distribution along the bed

The transient heat and mass transfer in a desiccant packed bed containing varying particle diameter distribution along the axial direction has been investigated using the pseudo gas controlled approach that considers the heat conduction in the bed. The numerical results of the present model and the experimental data from literature show good agreement with a maximum root of mean square of errors of 3% and 2% for exit air temperature and humidity ratio, respectively. The improvement in the total mass adsorbed and/or reduction in pressure drop has been investigated for various cases of packed bed namely, uniform particle diameter, linear, parabolic and cubic ascending and descending distributions. It has been found that there is a 25.7% reduction in pressure drop with negligible reduction in the total mass adsorbed for a desiccant bed with cubic type particle size distribution when compared to the bed with uniform particle diameter of 1.0 mm. A threshold flow velocity exists below which the total mass adsorbed is independent of particle diameter distribution type.

BINARY FISH PASSAGE MODELS FOR UNIFORM AND NONUNIFORM FLOWS

ABSTRACTBinary fish passage models are considered by many fisheries' managers to be the best available practice for culvert inventory assessments and for fishway design. Misunderstandings between different passage‐modelling approaches often arise, however, because of the absence of a formal comparison between the different approaches that include detailed derivations with consistent terminology and example applications. In this paper, one‐dimensional binary fish passage models were reviewed, and derivations from basic principles were provided for clarification. For uniform flow, a simple exhaustion‐threshold (ET) model equation was derived that predicts the flow velocity threshold in a fishway that causes exhaustion at a given maximum distance of ascent if a fish swims at the optimal ground speed. Velocities at or above the threshold predict failure to pass (exclusion). Velocities below the threshold predict passage. The ET model was therefore intuitive and easily applied to predict passage or exclusion. It was also shown to be consistent with the ascent‐distance (AD) model that shows that fish must adopt an optimal ground speed to maximize AD. The limitation of passage models to uniform flow was addressed by deriving a general model framework for passage that accounts for nonuniform flow conditions more commonly found in the field, including backwater and drawdown water‐surface profiles. Comparison of these models with experimental observations of volitional passage for top‐performing western mosquitofish Gambusia affinis indicates reasonable prediction of binary outcomes (passage or exclusion) if the flow velocity is not near the threshold flow velocity. More research is needed on fish behaviour, passage strategies under nonuniform flow regimes and stochastic methods that account for individual differences in swimming performance. Future experiments should track and measure ground speeds of ascending fish to test passage strategies and to improve model predictions. Stochastic models, such as Monte‐Carlo techniques, that account for different passage performance among individuals and allow prediction of the percentage of fish passing are needed to improve fish passage prediction. Published in 2011 by John Wiley & Sons, Ltd.

A local scour prediction method for pile caps in complex piers

The outcomes of an experimental study on local scour at a pile cap are presented. Experiments were conducted with the approaching flow having an undisturbed flow depth and a threshold flow velocity. The main variables investigated were pile cap dimensions and location relative to the streambed. According to the rate of change in scour depth, the scour at a pile cap for different cap levels was divided into four cases. Equations for a correction factor for these four cases are derived. The correction factor Kc has the effect of reducing the scour depth from a corresponding full-depth pier of the same width as the pile cap. A new methodology is presented to estimate local scour depth at a pile cap as a component of a complex pier. The proposed method was evaluated with the results from this experimental study and historical measurements. The proposed method, which corresponds closely to the observations, can be used to predict local scour at a pile cap as a component of a complex pier in the superposition method. It is also applicable to the prediction of local scour due to a caisson being sunk onto a mobile bed in a current.

Determination of the hydrophysical parameters of soil in an erosion model

The rates of the soil loss were studied in a wide range of flow velocities from 0.41 to 1.64 m/s. The results confirmed the main implications of the hydrophysical model: (1) the rate of the soil loss in the region of flow velocities exceeding the threshold value by 1.2–1.5 times and more is proportional to the cubic flow velocity; (2) the detachment of particles in the pre-threshold region is probable in nature, and the relationship between the soil loss and flow velocity is S-shaped. On this basis, a procedure was developed for the experimental determination of the soil erosion parameters: the erodibility, the threshold flow velocity, and the coefficient characterizing the variance of the particles’ resistance to detachment.

Unity of mechanisms of water and wind erosion of soils

An equation for the threshold velocity of the water or wind flow at which erosion of a homogeneous model soil begins was derived on the basis of accepted and explicitly formulated suppositions and limitations and tested using experimental data. The validity of the proposed mechanisms and equations describing the lifting force of the soil-eroding water or wind flow and the interaggregate cohesion in a model homogeneous incoherent soil was confirmed. The limit for the decrease in the threshold flow velocity with decreasing size of the soil particles (aggregates) was theoretically substantiated. The first unified equation of the threshold velocity of the water or wind flow for a homogeneous model soil in dimensionless variables was derived and experimentally justified.

Flow-induced clustering and alignment of vesicles and red blood cells in microcapillaries

The recent development of microfluidic devices allows the investigation and manipulation of individual liquid microdroplets, capsules, and cells. The collective behavior of several red blood cells (RBCs) or microcapsules in narrow capillaries determines their flow-induced morphology, arrangement, and effective viscosity. Of fundamental interest here is the relation between the flow behavior and the elasticity and deformability of these objects, their long-range hydrodynamic interactions in microchannels, and thermal membrane undulations. We study these mechanisms in an in silico model, which combines a particle-based mesoscale simulation technique for the fluid hydrodynamics with a triangulated-membrane model. The 2 essential control parameters are the volume fraction of RBCs (the tube hematocrit, H(T)), and the flow velocity. Our simulations show that already at very low H(T), the deformability of RBCs implies a flow-induced cluster formation above a threshold flow velocity. At higher H(T) values, we predict 3 distinct phases: one consisting of disordered biconcave-disk-shaped RBCs, another with parachute-shaped RBCs aligned in a single file, and a third with slipper-shaped RBCs arranged as 2 parallel interdigitated rows. The deformation-mediated clustering and the arrangements of RBCs and microcapsules are relevant for many potential applications in physics, biology, and medicine, such as blood diagnosis and cell sorting in microfluidic devices.