Local Flow Reversal Research Articles

Experimental study on the flat loop heat pipe with minichannel wick under low heat flux

Stable operating under low heat flux condition is crucial for loop heat pipe. To investigate the performance of the flat loop heat pipe under low heat flux, a visual structure of evaporator with longitudinal replenishment characterized by minichannel wick is proposed and fabricated. The minichannel wick increases the permeable area and reduces flow resistance to enhance the heat transfer capacity. However, under low heat flux condition, excessive liquid permeation into the vapor line and the local reverse vapor flow may deplete the liquid storage in the compensation chamber, resulting in a wick temperature increase of 4.7 °C under extreme condition. Lowering the heat sink temperature does not necessarily lead to a lower stable wick temperature in specific cases but rather poses operational challenges and further raises the stable wick temperature by 9 °C. By visualizing the flow structure, it is found that arduous vapor–liquid prestart distribution leads to prolonged circulation establishment time and higher wick temperature, resulting in a higher stable wick temperature. As the heat flux increases, the effect of prestart distribution on the stable wick temperature weakens as the redistribution capability improves.

Numerical Investigation of Flow Evolution in Centrifugal Compressors During Surge

Abstract Surge can lead to violent flow fluctuations in the compression system and damage to the blade structures. In this article, a fully three-dimensional numerical model of the centrifugal compressor surge is developed, and the accurate transient flow evolutions in different components during the surge are studied in detail. The results show that in the surge initiation, the pressure distortion caused by the asymmetric geometry of the volute at the diffuser outlet transfers along blade passages to the impeller inlet, which induces two types of stall cells with different rotating speeds and sizes developing independently in two isolated circumferential positions at the impeller inlet. With the surge development, the two types of stall cells come into contact and are mixed, which causes the asymmetric local reverse flow near the casing of the impeller leading edge. Subsequently, the reverse flow extends to the full annuls at the impeller inlet, and the compressor pressure ratio falls abruptly. At the same time, several expansion waves arise in the impeller and travel downstream along the volute and the outlet pipe. As reflected by the nozzle, these expansion waves travel back upstream into the impeller. The findings of this research have great implications for the asymmetric flow control methods, which develop novel asymmetric geometries to counteract the influence of the volute and extend the compressor stable operation range.

Numerical Study on Single Turn Pulsating Heat Pipe with Additional Branch Configuration

The simple wickless structure and strong heat transfer potential of pulsating heat pipes (PHP) have prompted widespread interest in transferring heat from electronic devices. This work proposes a novel single-turn PHP with convergent and divergent shaped additional branches (ABs) (C-PHP and D-PHP) in the evaporator section, which could increase latent heat transfer in the evaporator by minimizing the saturation temperature and improving the unidirectional flow and thermal performance of PHP. Numerical simulations were performed using computational fluid dynamics (CFD) techniques to understand the thermo-hydrodynamics and properties of the proposed novel PHPs. This study provides the outcomes of novel PHPs and a comparison with the performance of traditional PHP (T-PHP) without AB and straight AB PHP (S-PHP). The results demonstrated that the startup time of novel C-PHP and D-PHP were improved by 32.6% and 22.4%, respectively, compared to the T-PHP, “stopovers” and “local flow reversals” are minimized by the use of AB. The average flow velocity of novel C-PHP and D-PHP was improved by 16% when compared to T-PHP and 11.4% compared to S-PHP, the directional component in C-PHP is substantially stronger than S-PHP and D-PHP.

Experimental investigation of the crossflow of water in a 7-pin wire-wrapped rod bundle

In this study, the crossflow of water in a 7-pin wire-wrapped rod bundle was investigated experimentally using particle image velocimetry and the technique of matching index of refraction. Three Reynolds numbers, 3000, 6000, and 9000, were used to examine the effect of the Reynolds number on crossflow. A large-dimension rod bundle was utilized to provide sufficient space for high-resolution measurements. The cross-sectional flow distribution in the wire-wrapped rod bundle was obtained. Both global circulation flow in the bundle outer region and local reverse flow in the bundle interior were identified. In general, crossflow distribution is complex and is characterized by vortices of various scales. Previously, a large-scale vortex has been observed near the rod, which is related to the wall-fluid interaction. However, small vortexes are likely to appear on the lead side of the wire, which is attributed to the blockage effect of the wire. The distributions of the dimensionless velocity and turbulent kinetic energy in three typical subchannels were studied. The features of streamwise flow were also examined briefly. The flow blockage and redistribution caused by the wrapped wires were observed. The experimental data can be applied to validate CFD codes, turbulence models, and subchannel crossflow models.

Abstract Number ‐ 63: A swine model to simulate the mechanism of aspiration thrombectomy.

Introduction Aspiration thrombectomy is an effective technique to recanalize large vessel occlusions (LVO) in stroke patients and has become one of the major techniques. Advancements in technology have enabled the use of catheters with larger lumens and higher aspiration forces; however, the successful reperfusion rate of the first pass remains at 57–63%. This poses a question as to why this rate is still suboptimal when conducting aspiration thrombectomy. We established a swine model for LVO, which provides concurrent fluoroscopic and transmural visualization of real‐time vessel responses during aspiration thrombectomy, to investigate the mechanism of aspiration thrombectomy. Methods Under general anesthesia, a common carotid artery (CCA) and a superficial cervical artery (SCA) of Yorkshire swine (n = 3) were surgically exposed, and an LVO was reproduced by injecting a radiopaque clot analog via a guiding catheter. Each target artery was treated using aspiration catheters with different inner diameters (0.058, 0.068, and 0.088 inches). The CCA group (n = 7) represented large diameter vessels (4‐5mm) simulating a human ICA, and the SCA group (n = 6), with bifurcations, represented small diameter vessels (2‐3mm) simulating a human MCA. To make a consistent clot location in the CCA group, mild, non‐flow‐limiting stenosis was created using a 4‐0 Prolene suture. Fluoroscopy and a high‐resolution digital microscope camera were used simultaneously to monitor angiographic and transmural vessel behavior during the procedure. Average vessel diameter, local blood pressures proximal and distal to the occlusion site, the presence or absence of vessel collapse / reverse flow during the procedures, and pre‐and post‐angiographic findings were evaluated. Results Both fluoroscopic and transmural visualization of mechanical thrombectomy was achieved in all vessels allowing the observation of real‐time arterial wall and clot response during the procedure. The mean blood pressures distal and proximal to the occlusion site showed no significant differences between the two groups. With remote aspiration, all vessels in the SCA group (Mean Diameter: 2.34 ± 0.49mm) showed immediate vessel collapse, and regardless of the lumen size of the catheter, they all failed to recanalize the vessels. Effective clot ingestion was achieved only when direct contact aspiration was applied (5 of 6 vessels). None of the vessels in the CCA group (Mean Diameter: 5.16 ± 0.54 mm) showed vessel collapse during remote aspiration. Six out of 7 vessels (85.7%) treated with remote aspiration showed local reverse flow followed by complete aspiration when the tip of the aspiration catheter was placed 5–20mm away from the clot. One CCA vessel required direct contact aspiration to achieve clot ingestion. Conclusions This swine model to analyze vessel behavior during aspiration thrombectomy may help us understand the mechanism of aspiration thrombectomy. The model may contribute to improving the techniques and the development of new aspiration devices.

CFD Simulation of Centrifugal Pump with Different Impeller Blade Trailing Edges

The centrifugal pump is one of the most widely used types of power machinery in the field of ship and ocean engineering, and the shape of the impeller blade trailing edge has an important influence on their performance. To reveal the mechanism of the effect of different trailing edges on external performance, the internal flow of 16 types of impeller blade trailing edges of a centrifugal pump, consisting of Bezier trailing edges, rounding on the pressure side, cutting on the suction side, and the original trailing edge is studied by numerical simulation. The reverse flow, shaft power, and energy loss distribution in the impeller and diffuser along the streamwise direction are analyzed by calculating them on each micro control body sliced from the fluid domain. The entropy production theory and Ω-vortex identification method are used to display the magnitude and location of energy loss and the vortex structure. Finally, a static structural analysis of the impeller with different trailing edges is performed. The results show that different impeller trailing edges can clearly affect the efficiency of the pump, i.e., the thinner the trailing edge, the higher the efficiency, with the thickest model reducing efficiency by 5.71% and the thinnest model increasing efficiency by 0.59% compared to the original one. Changing the shape of the impeller trailing edge has a great influence on the reverse flow, shaft power, and energy loss near the impeller trailing edge and diffuser inlet but has little influence on the leading part of the impeller. The distribution of local entropy production rate, energy loss, and reverse flow along the streamwise direction shows similar rules, with a local maximum near the leading edge of the impeller due to the impact effect, and a global maximum near the impeller trailing edge resulting from strong flow separation and high vortex strength due to the jet-wake flow. Thinning the impeller trailing edge and smoothing its connection with the blade can reduce the vortex strength and entropy production near the impeller trailing edge and diffuser inlet, improve the flow pattern, and reduce energy loss, thus improving the pump efficiency. In all models, the maximum equivalent stress is less than 6.5 MPa and the maximum total deformation is less than 0.065mm. The results are helpful for a deeper understanding of the complex flow mechanism of the centrifugal pump with different blade trailing edges.

Coupled electrohydrodynamic transport in rough fractures: a generalized lubrication theory

Fractures provide pathways for fluids and solutes through crystalline rocks and low permeability materials, thus playing a key role in many subsurface processes and applications. In small aperture fractures, solute transport is strongly impacted by the coupling of electrical double layers at mineral–fluid interfaces to bulk ion transport. Yet, most models of flow and transport in fractures ignore these effects. Solving such coupled electrohydrodynamics in realistic three-dimensional (3-D) fracture geometries poses computational challenges which have so far limited our understanding of those electro-osmotic effects’ impact. Starting from the Poisson–Nernst–Planck–Navier–Stokes (PNPNS) equations and using a combination of rescaling, asymptotic analysis and the Leibniz rule, we derive a set of nonlinearly coupled conservation equations for the local fluxes of fluid mass, solute mass and electrical charges. Their solution yields the fluid pressure, solute concentration and electrical potential fields. The model is validated by comparing its predictions to the solutions of the PNPNS equations in 3-D rough fractures. Application of the model to realistic rough fracture geometries evidences several phenomena hitherto not reported in the literature, including: (i) a dependence of the permeability and electrical conductivity on the fracture walls’ charge density, (ii) local (sometimes global) flow reversal, and (iii) spatial heterogeneities in the concentration field without any imposed concentration gradient. This new theoretical framework will allow systematically addressing large statistics of fracture geometry realizations of given stochastic parameters, to infer the impact of the geometry and various hydrodynamic and electrical parameters on the coupled transport of fluid and ions in rough fractures.

多目的最適化による遠心ポンプの低流量域における右上がり不安定特性の改善

Centrifugal pumps, which are widely used in industry for transporting liquids in plants and in water purification systems, and it is necessary to improve the instability characteristics at extremely low flow rate conditions. The negative slope of the characteristic curve is the evidence of instability of the flow, and it must be based on the local reverse flow zone and pre-whirl at impeller inlet. It is possible to predict the negative slope of the characteristics by unsteady numerical analysis at low flow rate, however, optimization design search requires many numerical analysis of individuals, it is not realistic solution to use the unsteady numerical analysis for the optimization.

Mechanism of low frequency high amplitude pressure fluctuation in a pump-turbine during the load rejection process

ABSTRACT Owing to the complexity of the load rejection process, the accompanied complex low frequency pressure fluctuations and their sources have not been determined. Herein, the load rejection transient process of a pump-turbine was simulated with a three-dimensional (3-D) large eddy simulation method and a dynamic mesh technology. The simulation results were validated against the experimental data. Through the joint time-frequency analysis of simulated pressure, the complex low frequency pressure fluctuation components were captured, which are generally lower than the rated rotational frequency of the runner. A comprehensive formation mechanism of these complex low frequency pressure fluctuation components was attributed to three transient hydraulic phenomena and their interactions: the water hammer in the pump-turbine, local reverse flow near the runner inlet, and 3-D blocking water ring in the vaneless space. These findings facilitate the elucidation and elimination of complex low frequency high amplitude pressure fluctuations during the load rejection transient process.

Genesis of Antibiotic Resistance LIX: Fluid Resuscitation Induced Aberrant Intracranial Pressure (“Monroe‐Kellie” Doctrine (MKD) Contort Tethered Leucocytes / Platelets Crest “Critical Closing Pressure” (CCP)

NCT01663701 noted that the median volume of iv fluid administered within 6 hours of presenting to ER in the sepsis intervention group was 3.5 L. The mean volume administered during the similar time interval in prior sepsis resuscitation trials were 5.0 L (EGDT); 5.1 L (ProCESS); and 4.5 L (ARISE). Criticisms of NCT01663701 such as a. the suggested vs administered median volume of fluid administered during the initial phase of resuscitation in the sepsis protocol group (3.5L) may have been higher than the 30 mL/kg recommended by SSC guidelines; b.”Each 1‐hour delay in antibiotic administration was associated with an absolute increase in mortality in the range of 5% to 10%”; and c. the median volume of iv fluid administered between ER presentation and 6 hours for BMI 18.5 kg/m2 ~ 70 mL/kg, here we present a plausible pathophysiological sequel as a contributing factor for mortality rate. Within the six hrs. of fluid resuscitation, a surge of fluid bolus mounted an adverse pressure gradient on the sphincters in metarterioles deregulating the pulsatile blood flow into the capillary bed. Based on the “Whole Brain Fluid and Osmolyte Network Model or” or “The whole brain network” approach a mechanistic model inclusive of major cranial compartments simulated the effect of osmotic therapy predicted that critical intracranial pressure (ICP) increased to >30 mmHg, with insufficient cerebral perfusion, and ventriculomegaly. The ventricular enlargement, in turn, reduces extracellular milieu for fluid exchange lead to a chronic increase in ventricular size shifts attributed to osmolyte administered. It is our hypothesis that after fluid resuscitation blood flows against an adverse pressure gradient in the capillary bed. The fluid particles such as activated and/or aggregating Cellular Aggregates near to the endothelial layer due to their low kinetic energy, “Cellular Aggregates” flows in the opposite direction. Such Cellular Aggregates cannot surmount the adverse pressure, lead to the formation of eddies. Such eddies in the arterial end are likely to cause local flow reversal zone and which retard the velocity concurrently the pressure of the blood flow. Under the low velocity and pressure gradient, the cellular eddies start separating from the endothelium referred to as boundary layer separation causing the blood flow to decelerate against an adverse pressure gradient. Then in the upstream part which is the venous end of the capillary bed, where the cellular eddies achieve boundary layer separation, causing blood pressure to drop from that of the arterial end creating the eddies to drag. At this stage, the pressure distribution in the capillary bed is at the stagnation point, where the velocity becomes zero at which cellular eddies form DIC. Sustained fluid bolus with vasopressor likely to form a non‐pulsatile flow pattern forming turbulent eddies in the venous end and disseminate the coagulated eddies leading to an aberration in the “MKD”. Dysregulated ICP, in turn, alter the ARGR leading to hypoperfusion subsequently coma.Support or Funding InformationSupported by pd funds and in part CME activities of Subburaj Kannan MD PhD for www.aaets.org

A study in fractional diffusion: Fractured rocks produced through horizontal wells with multiple, hydraulic fractures

The spatiotemporal evolution of transients in fractured rocks often displays unusual characteristics and is traced to multifaceted origins such as micro-discontinuity in rock properties, rock fragmentation, long-range connectivity and complex flow paths. A physics-based model that incorporates transient propagation wherein the mean square displacement of the diffusion front follows a nonlinear scaling with time is proposed. This model is based on fractional diffusion. The motivation for fractional flux laws follows from the existence of long-range connectivity that results in the mean square displacement of fronts moving faster than predicted by classical models; correspondingly, obstructions and discontinuities, local flow reversals, intercalations, etc. produce the opposite effect with fronts moving at a slower rate than classical predictions. The interplay of these two competing behaviors is quantified. We simulate transient production in a porous rock at the Theis scale as a result of production through a horizontal well consisting of multiple hydraulic fractures. Asymptotic solutions are derived and computations verified. The practical potential of this model is described through an example and the movement of fronts under the constraints of this model is demonstrated through the new expressions developed in this work. We demonstrate that this model offers a potential avenue to explain other behaviors noted in the literature. Though this work is developed in the context of applications to the earth sciences (production of hydrocarbons, extraction of geothermal resources, sequestration of radioactive waste and other fluids, groundwater flow), a minimal change in the Nomenclature permits application to other contexts. Ideas proposed here are particularly useful in the context of superdiffusion in bounded systems which until now, in many ways, has been considered to be an open problem.

Three-Dimensional Computational Fluid Dynamics Prediction of Turbocharger Centrifugal Compression System Instabilities

The present work combines experimental measurements and unsteady, three-dimensional computational fluid dynamics predictions to gain further insight into the complex flow-field within an automotive turbocharger centrifugal compressor. Flow separation from the suction surface of the main impeller blades first occurs in the mid-flow range, resulting in local flow reversal near the periphery, with the severity increasing with decreasing flow rate. This flow reversal improves leading-edge incidence over the remainder of the annulus, due to (a) reduction of cross-sectional area of forward flow, which increases the axial velocity, and (b) prewhirl in the direction of impeller rotation, as a portion of the tangential velocity of the reversed flow is maintained when it mixes with the core flow and transitions to the forward direction. As the compressor operating point enters the region where the slope of the constant speed compressor characteristic (pressure ratio versus mass flow rate) becomes positive, rotating stall cells appear near the shroud side diffuser wall. The angular propagation speed of the diffuser rotating stall cells is approximately 20% of the shaft speed, generating pressure fluctuations near 20% and 50% of the shaft frequency, which were also experimentally observed. For the present compressor and rotational speed, flow losses associated with diffuser rotating stall are likely the key contributor to increasing the slope of the constant speed compressor performance curve to a positive value, promoting the conditions required for surge instabilities. The present mild surge predictions agree well with the measurements, reproducing the amplitude and period of compressor outlet pressure fluctuations.

Wall embedded electrodes to modify electroosmotic flow in silica nanoslits.

Electroosmotic flow in a silica slit channel with nonuniform surface charge density is investigated. In nanoconfinement, the electrical double layer occupies a non-negligible fraction of the system. Therefore, modifying the charge density on specific locations on the channel wall surface allows effective manipulation of the electroosmotic flow rates. In the present study, extensive (160 ns) nonequilibrium molecular dynamics simulations are conducted to investigate the ability of controlling the electroosmotic flow control in a nanoslit by patterning the surface potential. The mechanism to modify the surface charge consists of a set of charged electrodes embedded within one of the channel walls. The presence of the embedded electrodes results in the redistribution of ions in the electrolyte solution and in the alteration of the electroosmotic flow throughout the nanochannel. Indeed, the results reveal significant changes in the electroosmotic driving force and velocity profiles including local flow reversal. This study provides physical insight into the direct manipulation of the electrokinetic flow in nanoslits.

Electrokinetic transport in silica nanochannels with asymmetric surface charge

The characteristic high surface area to volume ratio causes ionic transport in nanofluidic devices to be governed by surface properties like surface charge. In recent years, experimental demonstrations have shown gate electrodes embedded in the nanochannel wall to systematically alter the surface potential to manipulate ionic transport in nanochannels. Using state-of-the-art non-equilibrium molecular dynamics simulations, electrokinetic transport of an aqueous KCl solution confined in a ~7 nm deep silica nanochannel is reported under the influence of asymmetric surface charge to investigate the effect of engineered surface charge on the resulting velocity and ion number density profiles within these nanochannels. Significant changes in the velocity magnitude and profile were observed in the silica nanochannels with two charged surface patches compared to a homogenously charged silica channel. The surface charged patches cause regions of local flow reversal which were indicated by an observed negative velocity. The simulations provide a theoretical foundation to systematically manipulate electrokinetic flow inside nanochannels by using engineered surface charge at the nanochannel walls.

Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid–structure interaction approach

Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid–structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our hemodynamic shearing device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization.

Noise at the mid to high flow range of a turbocharger compressor

The acoustic and performance characteristics of an automotive centrifugal compressor are studied on a steady-flow turbocharger test bench, with the intention of identifying operating regions associated with flow noise within the compressor and connected ducting. Near choke, discrete tones including rotor-order frequency and its harmonics (bladepass) are observed. As the flow rate is reduced, the current compressor exhibited a broadband elevation of noise in the 4-12 kHz band, which was evident both in the upstream compressor duct and external sound pressure level (SPL) measurement locations. At all rotational speeds studied here, the total SPL in this frequency range demonstrates a strong dependence on the incidence angle of the incoming flow with the blades at the inducer of the impeller. When the incidence angle is further increased (mass flow rate is decreased) beyond a critical value, the temperature near the inducer tips increases sharply, suggesting local flow reversal, and the total SPL in the 4-12 kHz range suddenly reduces.

Extreme solitary waves on falling liquid films

AbstractDirect numerical simulation (DNS) of liquid film flow is used to compute fully developed solitary waves and to compare their characteristics with the predictions of low-dimensional models. Emphasis is placed on the regime of high inertia, where available models provide widely differing results. It is found that the parametric dependence of wave properties on inertia is highly non-trivial, and is satisfactorily approximated only by the four-equation model of Ruyer-Quil & Manneville (Eur. Phys. J. B, vol. 15, 2000, pp. 357–369). Detailed comparison of the asymptotic shapes of upstream and downstream tails is performed, and inherent limitations of all long-wave models are revealed. Local flow reversal in front of the main hump, which has been previously discussed in the literature, is shown to occur for an inertia range bounded from below and from above, and the boundaries are interpreted in terms of the capillary origin of the phenomenon. Computational results are reported for the entire range of Froude numbers, providing benchmark data for all wall inclinations.

Numerical investigation of buoyant effect on flow and heat transfer of Lithium–Lead Eutectic in DFLL–TBM

Thermal–hydraulic behaviors of Lithium–Lead Eutectic (PbLi) in Chinese ITER Dual Functional Lithium–LeadTest Blanket Module (DFLL–TBM) were investigated numerically by using CFD code ANSYS CFX. The influences of buoyant effect on flow and heat transfer of PbLi were analyzed and the validity of Boussinesq approximation for liquid PbLi with non-uniform heat generation was discussed. Comparisons between two scenarios with and without buoyancy were made and the results shown that the flow field was significantly modified compared with the case of non-buoyancy effect. The local reverse flow phenomena and recirculation flows in the front duct near the first wall (FW) were found. Furthermore, a fractional outflow was also observed at inlet under the condition of inlet mass flow rate 2.165 kg/s. The results indicated that the buoyancy-induced mixed convection occurred in front duct can enhance the heat transfer process and made the temperature field of PbLi more uniform that can improve the operational safety of DFLL–TBM. The present work is expected to be helpful for the designs of DFLL–TBM.

Experimental study of turbulence, sedimentation, and coignimbrite mass partitioning in dilute pyroclastic density currents

Laboratory density currents comprising warm talc powder turbulently suspended in air simulate many aspects of dilute pyroclastic density currents (PDCs) and demonstrate links between bulk current behavior, sedimentation, and turbulent structures. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers match those of natural PDCs as does the ratio of thermal to kinetic energy density. The experimental currents have lower bulk Reynolds numbers than natural PDCs, but the experiments are fully turbulent. Consequently, the experiments are dynamically similar to the dilute portions of some natural currents. In general, currents traverse the floor of the experimental tank, sedimenting particles and turbulently entraining, heating, and thermally expanding air until all particle sediments or the currents become buoyant and lift off to form coignimbrite plumes. When plumes form, currents often undergo local flow reversals. Current runout distance and liftoff position decrease with increasing densimetric Richardson number and thermal energy density. As those parameters increase, total sedimentation decreases such that >50% of initial current mass commonly fractionates into the plumes, in agreement with some observations of recent volcanic eruptions. Sedimentation profiles are best described by an entraining sedimentation model rather than the exponential fit resulting from non-entraining box models. Time series analysis shows that sedimentation is not a constant rate process in the experiments, but rather occurs as series of sedimentation–erosion couplets that propagate across the tank floor tracking current motion and behavior. During buoyant liftoff, sedimentation beneath the rising plumes often becomes less organized. Auto-correlation analysis of time series of particle concentration is used to characterize the turbulent structures of the currents and indicates that currents quickly partition into a slow-moving upper portion and faster, more concentrated, lower portion. Air entrainment occurs within the upper region. Turbulent structures within the lower region track sedimentation–erosion waves and indicate that eddies control deposition. Importantly, both eddies and sedimentation waves track reversals in flow direction that occur following buoyant liftoff. Further, these results suggest that individual laminations within PDC deposits may record passage of single eddies, thus the duration of individual PDCs may be estimated as the product of the number of laminations and the current's turbulent timescale.

Vortical structures and heat transfer in a round impinging jet

In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.