Pulsating Flow Conditions Research Articles

Hydraulic Transport of Large Solid Particles in Inclined Pipes Under Pulsating Flow Conditions

Abstract The pressure loss due to the hydraulic transport of large solid particles should be predicted for the design of subsea mining systems. The mixture flow in a flexible jumper is expected to be unsteady during lifting operations in an actual mining system. The authors develop a one-dimensional mathematical model that predicts such pressure loss under pulsating mixture flows in a static inclined pipe assuming that the flow in the jumper is fully developed. An experiment is performed on the hydraulic transport of solid particles to obtain data for model validation. In this experiment, several kinds of solid particles are used: alumina beads, glass beads, and gravel. The experimental parameters are mixture velocity, solid concentration, pulsation period and amplitude of water velocity, and pipe inclination angle. The proposed model is validated through a comparison with experimental data. The validation confirms that the model is applicable for the prediction of the pressure loss in inclined pipes under pulsating flow conditions. Furthermore, we calculate the pressure loss due to the hydraulic transport of polymetallic sulfide ores using the proposed model. The calculation results show that the time-averaged pressure loss drastically varies with the pipe inclination angle, reaching its maximum value between the pipe inclination angles of 30 deg and 60 deg, at which the flow is inclined upward. The results also show that the amplitude of pressure loss pulsation differs little between pipe inclination angles and that the pulsation component of pressure loss should be considered in designing lifting systems.

Characterization of underwater manipulator based on the fluid-structure interaction under pulsating flow

Abstract Underwater manipulators are taking on an increasingly significant function in marine biological fishing operations. Under the action of the ocean flow field, vortex-induced vibration will be induced, which will cause fatigue damage or even fracture of the underwater manipulator. Based on the Ansys fluid-solid coupling software, this paper simulates the vortex-induced vibration response characterizations of underwater manipulators under pulsating flow conditions. The pulsation flow at the entrance is realized by changing the velocity amplitude and pulsation frequency. The influence laws of pulsating frequency and amplitude on the CL , CD, and wake vortex shedding are investigated. The results showed that the CL and CD of the underwater manipulator increased by 133.33 % and 39.18 %, respectively, with an increase in the pulsation parameter. Additionally, with the increase of pulsating frequency, the banded vortex behind the underwater manipulator is gradually obvious, and the energy in the vortex is also increasing.

Characteristics of thermo‐hydraulic flow inside corrugated channels: Comprehensive and comparative review

AbstractPrevious works that investigated the characteristics of heat transfer and fluid flow in channels with corrugated walls have been extensively reviewed in this study. In accordance with the fast increase in power consumption requirements, many researchers have investigated a new approach for cooling techniques that can enhance the cooling performance of devices without consuming more power. To improve the efficiency of energy systems, many investigators and engineers implement promising techniques such as surface optimization and additives as passive methods to augment the rates of heat transfer. Researchers investigated different corrugation profiles along with various working fluids as well as external power devices to further improve the heat exchange process of thermal systems. The aim of this article is to give a clear preview of the effects of different parameters such as wave parameters, Reynolds number, type of working fluid, and pulsating flow condition on the average and local Nusselt number, the pressure drop, the performance factors, and irreversibility. The main findings are listed in tables and depicted in figures, the matter that helps engineers and researchers to choose a suitable channel shape for their applications.

Study of Gas Film Cooling under Large Eddy Simulation: Effect of Forward and Backward Injection on Gas Turbine Cooling Efficiency and Stability

In the design of advanced gas turbines, improving thermal efficiency and power output is crucial achieve energy saving and emission reduction goals. This necessitates raising the inlet temperature, thereby highlighting the importance developing efficient cooling technologies to ensure the longevity of hot components. Gas film cooling, as a widely used traditional cooling method, is subject to the influence of fluid dynamic characteristics, especially in unstable operating environments such as pressure fluctuations in the upstream combustion chamber and interactions between stators and rotors. This study utilized large eddy simulation to analyze the effectiveness of trench film cooling under such unstable conditions, with a specific focus on the time-averaged and transient characteristics of forward and backward injection of cooling air. The study investigated the efficacy of film cooling under pulsating flow conditions, with a blowing ratio of 1.5 under steady-state conditions, utilizing cosine and square waves as pulse boundaries, and a Strouhal number of 0.254. The research findings reveal that: 1) Contrary to the results under steady-state conditions, pulsating reverse injection led to a reduction of over 15% in time-averaged cooling efficiency and a noticeable decrease in the film coverage area compared to forward injection; this phenomenon is determined by the different flow mechanisms under steady-state and pulsating states. 2) Under a certain blowing ratio, the instability of gas film cooling is induced by the temporal fluctuations of near-wall vortices, which could result in rapid temperature changes and component failures. For pulsating forward injection, additional high film cooling unsteadiness outside the slot membrane orifices was observed. In the case of backward injection, pulsations did not lead to a significant increase in instability. These research findings provide a deeper understanding of the impact of gas film cooling on the cooling efficiency and stability of gas turbines, enabling gas film cooling technology to meet the inlet temperature requirements of advanced gas turbines, thereby reducing carbon emissions and contributing to the achievement of China's dual carbon goals.

An in silico analysis of unsteady flow structures in a microaxial blood pump under a pulsating rotation speed

Background and objectiveVentricular assist devices (VADs) are generally designed to perform continuous flow. However, it has been proven that continuous flow, which is not a physiological hemodynamic state, may cause severe complications such as gastrointestinal bleeding, pulmonary hypertension, and ventricular suction. For these reasons, many pulsating blood pump control strategies have been proposed and have the potential for application in percutaneous ventricular assist devices (pVADs) or microaxial blood pumps. A few cases report extra hemolysis when introducing pulsating speed, while none involve blood pumps. This research's primary purpose is to evaluate the potential hemolysis of pVAD under pulsating flow conditions. MethodsFirst, the pulsating flow state is deduced using a heart failure model and varying speed. The heart model is established according to the pathology state collected from a clinical check. The rotation speed and boundary physical state are set to fit the heart failure model. The computational fluid dynamics (CFD) method with the hemolysis prediction model is performed. Furthermore, we used proper orthogonal decomposition (POD) analysis to reconstruct the flow field and obtain more details about shearing and transporting effects. Results(1) As a variable rotational speed was introduced, no significant gain in hemolysis accumulation appeared in pVAD. This is quite different from long-term implantable VADs. (2) Pulsation affects hemolysis mainly through pressure (or normal stress). Variable rotational speed affects hemolysis mainly through flow instability. (3) Variable rotational speed will increase the instability and influence hemolysis by transporting and shearing effects, while the transporting effect is more significant. ConclusionsThe unsteady flow state will affect the spatial distribution of hemolysis, which should be taken into account during control strategy and impeller shape design.

Fibrin Coating Contributes to the Retention of the Endothelial Layer in Pulsating Flow

The presence of a modifying coating based on extracellular matrix proteins on the inner surface of vascular prostheses is known to enhance endothelial cell adhesion and prevent detachment under pulsating flow conditions. This coating effectively reduces the risk of thrombosis and plays a critical role in determining implantation outcomes. Although proteins like collagen, fibrin, and fibrinogen are commonly used as coatings to improve cell adhesion, their relative effectiveness remains uncertain. Objective: This study aims to identify the optimal coating, based on extracellular matrix proteins, that preserves prosthesis functionality and maintains endothelial layer integrity under pulsating flow conditions. Methods: Scaffolds and vascular prostheses were fabricated using poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(ε-caprolactonone) through an electrospinning process. These structures were then modified with collagen I, fibronectin, or fibrin. Endothelial colony-forming cells (ECFCs) were seeded onto the protein-modified electrospun samples and cultured under both static and dynamic conditions. After a 3-day incubation period under static conditions, cell viability, metabolic and proliferative activity, as well as adhesive properties, were evaluated. Adhesive properties were assessed by analyzing the area occupied by the focal adhesion protein paxillin. Cell retention was determined by comparing cell density on the inner surface of 4 mm diameter vascular prostheses after a 7-day incubation period, both under pulsating flow conditions and static conditions. Results: Cell metabolic activity, viability, number, proliferation, and the area occupied by the focal adhesion protein paxillin were found to be significantly higher in samples coated with fibrin compared to those coated with collagen I and fibrinogen. The cell density (cells/cm2) of ECFCs on the inner surface of fibrin-coated prostheses showed no significant difference between dynamic and static conditions. In contrast, collagen and fibronectin coatings resulted in approximately half the cell density under pulsating flow conditions compared to static conditions. Conclusion: The fibrin coating demonstrated superior biological activity, adhesive properties, and preservation of the endothelial layer under both static and pulsating flow conditions, as compared to collagen I and fibronectin coatings. Consequently, the utilization of fibrin coating emerges as a promising option for modifying the inner surface of vascular prostheses.

Pulsating Turbulent Flows through a Square Pipe

Pulsating turbulent flows in a square pipe are studied numerically. The flow dominance regime in which the fluid flow rate remains positive in all phases of the oscillatory cycle is considered. The flows are studied at several oscillation frequencies. The results are compared with oscillating laminar flows and a steady turbulent flow in a square pipe, as well as with pulsating turbulent flows in a round pipe. The integral and fluctuating characteristics of turbulence and their dependence on the oscillation frequency are determined. In particular, it is found that at the considered Reynolds number Re = 2200 the friction coefficient in pulsating flows turns out to be lower than that in the stationary flows. The drag reduction increases with growth of the oscillation period and reaches 14.7%. A distinctive feature of turbulent flows in pipes of rectangular cross-section is the occurrence of secondary flows of Prandtl’s 2nd kind. The details of secondary flows under the pulsating flow conditions are studied at length.

Role of wall-mounted flexible flow modulator on thermo-hydraulic characteristics of pulsating channel flow

The present study focuses on numerical analysis of thermo-hydraulic characteristics in a straight isothermally heated channel with a bottom wall-mounted flexible flow modulator under pulsating flow conditions. The fluid-structure interaction problem is formulated in Arbitrary Lagrangian-Eulerian framework via finite element method for solving necessary governing equations. The pulsating characteristics of fluid flow are represented by Strouhal number (St) and amplitude of oscillation (A) and varied within the range of 0.05 ≤ St ≤ 0.40 and 0.15 ≤ A ≤ 0.50 respectively for a fixed Reynolds number. The length of the flexible modulator (Lf) has been varied within 0.4 ≤ Lf ≤ 1.0 to explore its effect on the overall performance of the system. The results have been evaluated in terms of vorticity and isotherm distributions, Nusselt number, and modulator tip displacement. Moreover, Power Spectrum Analysis aided by FFT has been conducted to interpret the oscillatory behavior of thermal frequency and modulator deflection. The study reveals that a pulsating frequency of St = 0.20 and a higher amplitude of the pulsating inflow offer the best thermal augmentation. Although the longest modulator offers higher heat transfer, an optimum length of Lf = 0.6 provides the best overall performance at minimum pressure loss.

HEAT TRANSFER ENHANCEMENT IN A CHANNEL WITH INCLINED BAFFLES UNDER PULSATING FLOW: A CFD STUDY

This study numerically investigated hydraulic and thermal performance in a channel with inclined baffles under pulsating flow conditions. The baffles were placed in a staggered arrangement. The governing equations were discretized with the finite volume method (FVM), and the pressure-velocity coupling was handled by the SIMPLE algorithm. The Strouhal number (St: 0.5,1, 2, 3, and 4), pulsation amplitude (A: 0.2, 0.5, and 0.8), and Reynolds number (200 ≤ Re ≤ 1000) were changed. The top and bottom surfaces of the channel were kept at <i>T</i><sub>ω</sub> = 350 K, and thermal improvement and friction factor were calculated for a pulsating cycle. The results were given in terms of thermal enhancement (η), relative friction factor (<i>f</i><sub>rel</sub>), and performance evaluation criteria (PEC). The flow and temperature contours were presented to determine the impacts of the pulsation frequency, the pulsation amplitude, and the Reynolds number. The results showed that the pulsation amplitude and the pulsation frequency contributed remarkably to thermal enhancement with increasing Reynolds numbers, while the heat transfer improved significantly depending on pulsation parameters together with a slight rise in friction factor. The highest thermal enhancement achieved was about 1.47 at <i>Re</i> = 1000, <i>A</i> = 0.8, and <i>St</i> = 4. The highest PEC obtained was approximately 1.12 at <i>Re</i> = 1000, <i>A</i> = 0.2, and <i>St</i> = 4.

Modeling radial turbine performance under pulsating flow by machine learning method

This work presents the development and application of a machine learning model to predict the unsteady performance of a turbocharger radial turbine subject to on-engine pulsating flow conditions. The model proposed, based on a fully connected neural network, predicts the instantaneous turbine torque and circumferentially-averaged relative inflow angle when given, as only inputs, the total pressure and temperature pulses and the time derivative of the total pressure pulse upstream of the turbine. The training data set for the model is obtained from an experimentally-validated Reynolds-averaged Navier-Stokes model and consists of various operating conditions characterized by different pulse amplitudes and frequencies. Heat transfer effects are neglected by the use of adiabatic boundary conditions while the rotational speed of the rotor is otherwise maintained fixed. Based on the results obtained, the model is shown capable of accurately predicting the turbine torque and local properties of the flow, such as the relative inflow angle, with a high degree of accuracy (coefficient of determination larger than 0.98 ). At first, the model is tested for both interpolation and extrapolation conditions. Given a training data set constituted by only three pulses characterized by different amplitudes, the model accurately predicts the turbine performance both for conditions inside and outside the range of amplitudes of the training data set. Lastly, the model is trained with a larger data set, including both variations of the pulse amplitude and frequency, in order to predict the performance of the turbine subject to a more general pulse shape. The error indicators show an improvement with respect to the extrapolation case due to the larger size of the training data set, showing also a great capability of the model to predict the unsteady performance of the turbine for a more general pulse shape. The model represents a fast and efficient approach for predicting the unsteady turbine performance as compared to more complex experimental set-ups and time-consuming 3D numerical simulations and a valid alternative to the more common 0D-1D models.

Analysis of performance and time lag of turbocharger turbines fed with gasoline and diesel burnt gases and air under pulsating flow conditions

The turbine, an enabling component of turbochargers, is a seat of highly complex unsteady flow as well as varying gas compositions between diesel and gasoline engines. Thus, it is necessary to take into account different physical models when simulating such turbines. Researchers have often considered the turbine working gas as air, which may not lead to an accurate prediction of actual turbine characteristics. In the present work, the effect of the working gas on the performance of turbocharger turbines as well as on the turbo lag was investigated through a set of numerical simulations for six wastegate opening degrees from 0% up to 100% and under a wide range of the flow pulse frequency ranging from 33.33 up to 166.66 Hz. Four working fluids were involved in this study which are the gasoline burnt gas, diesel burnt gas, air with like gasoline engine waves, and air with like diesel engine waves. The performance of the turbine and its responsiveness quantified as the time lag with burnt gases from both diesel and gasoline engines were compared to those with air. Results showed totally deviated unsteady performance characteristics compared to the turbine working with air. Results also confirmed that considering the working fluid as air may not lead to a fair representation of the actual turbine performance characteristics. The findings of this work put emphasis on the importance of the gas composition when modeling such turbines by engineers especially during the design process where a good turbocharger-engine match has to be met.

Unsteady Gas Dynamics of Radial Turbine Volutes Under Pressure Pulsations

Abstract Turbocharging is the key technique to improve engine-specific power and reduce CO2 emissions of a piston-driven engine. Repeated actions of engine pistons and valves give rise to engine pulsations resulting in intensive unsteady flow in the turbines. Understanding the pulsation effects on a turbine that is steady flow designed is crucially important for performance enhancement. The present work derives an equation that is capable of evaluating the pulsation effects on the volute-outlet flow angle. Based on the equation, a reduced-order model (1D) is proposed and applied to a single-entry mixed flow turbine. By comparing to 3D unsteady computational fluid dynamics (CFD), the ability and accuracy of the 1D model on predicting the unsteady volute-outlet flow angle are verified. The effects of the unsteady flow angle on the turbine performance are further investigated by comparing unsteady and quasi-steady flow-field characteristics given by 3D CFD. The present work unveils the volute unsteady behavior and explains the unsteady coupling mechanism between the volute and the rotor. The findings can lead to improvement of turbine design methodology under pulsating flow conditions.

Assessment of the unsteady performance of a turbocharger radial turbine under pulsating flow conditions: Parametric study and modeling

A characteristic of radial turbines for turbocharger applications subject to pulsating flow is the deviation of the turbine performance compared with corresponding continuous flow conditions, typically associated with gas-stand experiments. The performance deviations under pulsating flow generate a hysteresis loop that encloses the performance line obtained in gas-stand conditions and their intensity is demonstrated to grow with increasing pulse amplitude and frequency. Predicting the performance deviations is of great interest to improve the predictive capabilities of reduced-order models and enhance engine-turbocharger matching. In this work, the performance response of a turbocharger radial turbine is studied with respect to variations of the normalized pulse amplitude (between 0.4 and 1.6 ) and the pulse frequency (between 20 Hz and 100 Hz ). Results show that the hysteresis loop expands with increasing pulse amplitude and frequency, so that the turbine cannot be treated as a quasi-steady device. The characteristic trends of the turbine performance are also highlighted with respect to pulse amplitude and frequency variations. The expansion ratio is registered to improve by + 4.0 % with increasing pulse amplitude and decrease by - 1.3 % with increasing pulse frequency. An opposite trend is otherwise registered for the isentropic efficiency, which decreases by - 6.5 % for increasing pulse amplitude and increases by + 6.5 % for increasing pulse frequency. Finally, through a simple model, the deviations of the turbine performance from quasi-steady to pulsating flow conditions are demonstrated to depend on the time derivative of the pressure pulse and the residence time of the fluid particle rather than pulse amplitude and frequency.

Heat transfer enhancement by pulsating flow of a viscoelastic fluid in a microchannel with a rib plate

ABSTRACT In order to improve the heat transfer performance of the microchannel heat exchanger, a composite heat transfer enhancement method was proposed. Viscoelastic fluid was used as working fluid in pulsating flow condition, and rib plates were added to the microchannel to bring extra disturbance to the flow. The Oldroyd-B constitutive model of the viscoelastic fluid was used in the numerical simulation, and the flow field, temperature field, Nusselt number (Nu), and pressure drop were analyzed when the average Reynolds number (Re) is 10. Both Strouhal number (St) and amplitude are important factors affecting heat transfer, but they have an insignificant influence on pressure drop at low Reynolds number. The St = 0.125 and amplitude A = 0.8 are better parameters. The increase of Weissenberg number (Wi) will cause the vortex to split into several subsidiary vortexes during its development, which will also develop to various positions in the channel, thus further enhancing the heat transfer. When the Wi is in the range of 1 ~ 5, the performance evaluation criteria rises at a relatively fast rate, from 1 to 1.404.

New Facility for Membrane Fouling Investigations under Customizable Hydrodynamics: Validation and Preliminary Experiments with Pulsating Cross-Flow.

Flux reduction induced by fouling is arguably the most adverse phenomenon in membrane-based separation systems. In this respect, many laboratory-scale filtration studies have shown that an appropriate use of hydrodynamic perturbations can improve both performance and durability of the membrane; however, to fully understand and hence appropriately exploit such effects, it is necessary to understand the underpinning flow processes. Towards this end, in this work we propose and validate a new module-scale laboratory facility with the aim of investigating, at very well-controlled flow conditions, how hydrodynamics affects mass transport phenomena at the feed/membrane interface. The proposed facility was designed to obtain a fully developed and uniform flow inside the test section and to impose both steady and pulsating flow conditions. The walls of the facility were made transparent to grant optical accessibility to the flow. In this paper, we discuss data coming from particle image velocimetry (PIV) measurements and preliminary ultrafiltration tests both under steady and pulsating flow conditions. PIV data indicate that the proposed facility allows for excellent flow control from a purely hydrodynamic standpoint. Results from filtration tests provide promising results pointing towards pulsating flows as a viable technique to reduce fouling in membrane systems.

Numerical investigation on heat and mass transfer characteristics of ice slurry in pulsating flow

In this paper, the heat transfer enhancement of ice slurry in a horizontal pipe with constant heat flux has been investigated numerically. The model is validated with the experimental results from the literature. Then the axial distribution of the heat transfer coefficient, the bulk and wall temperature and the ice volume fraction are obtained. The factors that affect the heat transfer coefficient are analyzed. Furthermore, the heat transfer efficiency of ice slurry in pulsating flow is compared with that in steady state. It is found that the heat transfer efficiency of ice slurry can be significantly improved by pulsating flow. The maximum increment of cycle-averaged heat transfer coefficient reaches 34.32%. Moreover, the degrees of heat transfer enhancement increase with increasing pulsating frequency, relative amplitude of velocity, ice particle diameter and ice volume fraction, and decrease with increasing cycle-averaged velocity. And the heat transfer is more efficient to ice slurry with large particles and ice volume fraction under the pulsating flow conditions. The pulsating frequency is suggested to be not more than 10π Hz. Finally, the correlation for predicting cycle-averaged Nusselt number of ice slurry in pulsating flow is established, and obtaining the reasonably good prediction results.

Characterisation of a plate heat exchanger chevron type with carbon-based nanofluids under pulsed condition

In industrial-thermal applications, pulsating flow along with carbon-based nanofluids is a well adopted active method, although not in plate heat exchangers (PHEs). The performance of a PHE with carbon-based nanofluids was experimentally evaluated by superimposing pulsating flow along with steady-state flow. The results demonstrated that the use of GNP-water, hybrid GNP/MWCNT-water, and MWCNT-water nanofluids with volume fractions ranging from 0.01% to 0.1% in a steady-state flow led to improved average heat-transfer rates of 1.34, 1.27, and 1.25, respectively. Furthermore, implementation of pulsating flow enhanced the average heat-transfer rate, in comparison to that of the steady-state flow in the same nanofluids, in the range of 10.9%–28.2%, 9%–25.4%, and 7.1%–14.8%, respectively. Pulsating flow in nanofluids improved heat-transfer rate more than it did in pure water owing to the enhancement of the Brownian motion of the suspended carbon-based nanoparticles. In the considered volume fractions from 0.01% to 0.1%, the pulsating flow condition increased the pressure drop by a factor of 1.48, 1.49, and 1.62 for the MWCNT-water, hybrid GNP/MWCNT-water, and GNP-water nanofluids, respectively, in comparison to pure water. The experimental results indicated that the pulsating flow had a more profound influence on the improvement of heat-transfer rate and pressure drop in the case of GNP-based nanofluid than in the others. This could be attributed to the unique platelet shape of the GNP nanoparticles and consequently the higher Brownian motion. The improvement in the heat-transfer rate, obtained through implementation of the pulsating flow condition, outweighed the cost of increase in pressure drop in all the cases. Among the nanofluids considered, the hybrid GNP/MWCNT-water nanofluid exhibited the best overall performance of 1.2.

Computational Study of a Radial Flow Turbine Operates Under Various Pulsating Flow Shapes and Amplitudes

Abstract Radial flow turbines are extensively used in turbocharging technology due to their unique capability of handling a wide range of exhaust gas flow. The pulsating flow nature of the internal combustion engine exhaust gases causes unsteady operation of the turbine stage. This paper presents the impact of the pulsating flow of various characteristics on the performance of a radial flow turbine. A three-dimensional computational fluid dynamic model was coupled with a one-dimensional engine model to study the realistic pulsating flow. Applying square wave pulsating flow showed the highest degree of unsteadiness corresponding to 92.6% maximum mass flow accumulation due to the consecutive sudden changes of the mass flowrates over the entire pulse. Although sawtooth showed a maximum mass flow accumulation value of 88.9%, the mass flowrates entailed gradual change and resulted in the least overall mass flow accumulation over the entire pulse. These two extremes constrained the anticipated performance of the radial flow turbine that operates under realistic pulsating flow. Such constraints could develop an operating envelope to predict the performance and optimize radial flow turbines’ power extraction under pulsating flow conditions.

The supercritical question for pyroclastic dune bedforms: An overview

AbstractSedimentary structures deposited from dilute pyroclastic currents are largely dominated by the products of metre‐scale dune bedforms containing backset lamination. Since the first seminal works in the 1970s, the vast majority of the literature called these structures antidunes and chutes‐and‐pools. This consensus led to a loose terminology, either as descriptive terms for structures containing backset beds or as an interpretation of hydrodynamic conditions related to Froude‐supercritical flows. Here, the main characteristics of pyroclastic dune bedforms are summarized and classified under four categories. The literature is examined and a discussion is provided regarding a possible interpretation in terms of supercritical‐flow bedforms, including antidunes, chutes‐and‐pools and cyclic steps. The interpretation of pyroclastic dune bedforms as related to supercritical conditions is a possibility among others, yet open questions remain, and under the present state of knowledge alternative interpretations are equally valid. There is consensus that an interpretation of pyroclastic dune bedforms as related to supercritical flows implies deposition from a basal underflow supporting an internal free‐surface formed at a density interface within the overall current structure. Antidunes are likely to be limited to low‐angle, incipient bedforms. Chutes‐and‐pools may occur in the form of series of steep truncations formed by an erosive supercritical ‘chute’, with a depositional signature occurring exclusively in the subcritical ‘pool’ of a Froude jump. Rare examples of cyclic steps may be found as large‐scale undulations with superimposed metre‐scale bedforms, or for short‐scale, fully‐aggrading structures in periodic trains. Additionally, some pyroclastic bedforms may be formed by granular flows passing granular Froude jumps or as frozen granular stationary waves. The dominance of chute‐and‐pool structures over bedforms characterized by relatively more stable morphodynamics (antidunes and cyclic steps) could be the likely result of pulsating flow conditions and abrupt high‐rate deposition, impeding stable flow conditions. In a majority of cases, the growth of pyroclastic bedforms appears triggered by the influence of an inherited bed topography and by bed‐flow feedback effects. Apart from a possible interpretation as supercritical bedforms, other specific dynamics of dilute pyroclastic currents may well explain the characteristics of pyroclastic dune bedform structures. In particular, the inherent turbulence fluctuations, internal organization and highly depositional dynamics may be key to the formation of pyroclastic dune bedforms. In this respect, rather than solely focusing on the Froude dimensionless number of the formative current, theoretical considerations accounting for the Reynolds, Richardson and Rouse dimensionless parameters may be also implemented for the understanding of pyroclastic bedforms.

Pressure Loss Due to Hydraulic Transport of Large Solid Particles in Vertical Pipes Under Pulsating Flow Conditions

Abstract It is important to predict the pressure loss due to hydraulic transport of large solid particles for the design of subsea mining system. The mixture flow in the lifting pipe is expected to be unsteady in the actual mining system. The authors develop the one-dimensional mathematical model to predict the pressure loss of pulsating mixture flow in a static vertical pipe assuming that the flow in the pipe is fully developed. The experiment on hydraulic transport of solid particles was carried out to obtain the data for the investigation of the effects of flow fluctuation on pressure loss in a static vertical pipe. In the experiment, alumina beads and glass beads were used as solid particles, and the experimental parameters were mixture velocity, solid concentration, pulsating period, and pulsating amplitude. The proposed model was validated by a comparison with experimental data. Furthermore, we calculated the pressure losses due to hydraulic transports of polymetallic sulfide ores and manganese nodules using the proposed model. The calculation results showed that the fluctuating component in pulsating mixture flow should be considered for the design of lifting system and that the homogeneous mixture model could not be applied to the prediction of the pressure loss unless the mixture concentration is low and the pulsating period is short.