Local Flow Properties Research Articles

Partial surface texturing: A mechanism for local flow reconditioning in lubricated contacts

The texture hydrodynamics occurring in optimized steady-state lubricated contacts is theoretically investigated recurring to the recent Bruggeman texture hydrodynamics model. In particular, an attempt is made to unravel the intimate relationship between the texture-induced local flow properties and the macroscopic characteristics of the contact such as friction and supported load. The existence of different mechanisms of fluid flow harvesting and channeling is discussed, highlighting their strict relationship to the particular texture physical and topological properties. Whilst their single-species optimization does not produce particularly interesting performances in comparison to classical geometries, their optimized synergistic interaction, instead, provides a remarkable load generation and friction reduction almost independently from the macroscopic geometrical characteristics of the contact. The provided discussion can be easily extended to other contact geometries, such as for journal bearings, wet clutches and dynamic sealings, as well as to bio-tribology and soft contact applications.

Aerodynamic force by Lamb vector integrals in compressible flow

A new exact expression of the aerodynamic force acting on a body in steady high Reynolds number (laminar and turbulent) compressible flow is proposed. The aerodynamic force is obtained by integration of the Lamb vector field given by the cross product of vorticity times velocity. The result is obtained extending a theory developed for the incompressible case. A decomposition in lift and drag contribution is obtained in the two-dimensional case. The theory links the force generation to local flow properties, in particular to the Lamb vector field and to the kinetic energy. The theoretical results are confirmed analyzing numerical solutions obtained by a standard Reynolds Averaged Navier-Stokes solver. Results are discussed for the case of a two-dimensional airfoil in subsonic, transonic, and supersonic free stream conditions.

Experimental Investigation of Two-Phase Electrolysis and Multiphysics Modelling

During two-phase electrolysis processes, for example for hydrogen production, there are bubbles which are created at electrodes. This implies a great vertical motion source in the normal earth gravity field and then a quite important natural two-phase convection. All other fields are then affected. Heat, mass and electricity transfers are modified due to both bubbles screening (at surface and in volume) and to bubbles transport promotion.Many numerical modeling for two-phase processes such as kerosene pulverization in engines or coal combustion sciences have shown the difficulties of these multi-physics processes. Both particles and reactor scales must be considered according with a strong coupling modeling. In these processes the particles injection is “in the flow”. In boiling or electrolysis processes, a new difficulty is added: particles birth or injection is strongly coupled to the local flow properties and leads to a complex boundary condition at surfaces. Electrical and electrochemical properties and processes are disturbed. This disturbance can lead to the modification of the local current density and to anode effects for example.The both experimental and numerical investigation is here presented. Experiments are performed in laboratory and in zero gravity (CNES grant and parabolic flights opportunities).

On the physical mechanisms governing self-excited pressure surge in Francis turbines

The required operating range for hydraulic machines is continually extended in an effort to integrate renewable energy sources with unsteady power outputs into the existing electrical grid. The off-design operation however brings forth unfavorable flow patterns in the machine, causing dynamic problems involving cavitation, which may represent a limiting factor to the energy production. In Francis turbines it is observed that the self-excited oscillation of a vortex rope in the draft tube cone prevents the delivery of maximum power when required. This phenomenon is referred to as full load pressure surge and has been the object of extensive research during the past decades. Several contributions deepened its understanding through measurement and simulation of the local flow properties and the global stability parameters. The draft tube pressure level and the runner outlet swirl are identified as key variables in the modelling of the vortex rope dynamics. Recently, a cyclic appearance of blade cavitation has been observed at overload conditions in a multiphase numerical simulation coupling the runner and the draft tube. From the analysis of the simulation it becomes obvious that the cyclic appearance of blade cavitation has a direct effect on the runner outlet swirl, thus introducing an additional interaction mechanism that is not accounted for in formerly published models. For the presented work, the results of this numerical study are confirmed experimentally on a reduced scale model of a Francis turbine. Several wall pressure measurements in the draft tube cone are performed, together with high speed visualizations of the vortex rope and the blade cavitation. The flow swirl is calculated based on Laser Doppler Velocimetry measurements. A possible mechanism explaining the coupling between the self-excited pressure and vortex rope oscillation and the cyclic appearance of the blade cavitation is proposed. Furthermore, the streamwise propagation speed of the flow swirl in the draft tube is calculated. The results offer important insights in the physics of high load pressure surge and contribute to the further development of numerical draft tube flow and stability models.

Teflon probing for the flow characterization of arc-heated wind tunnel facilities

The experimental flow characterization of the arc-heated wind tunnel of the University of Texas at Arlington is investigated in this work using ablative Teflon probes in combination with total pressure measurements. A parallel analytical work, focused on the dimensional analysis of the ablation process, has been conducted with the purpose of improving existing semi-empirical correlations for the heat blockage due to the mass injection inside the boundary layer. A control volume analysis at the receding surface of the specimens is used to calculate the wall heat transfer for a non-ablating probe by including the blockage effect. The new correlations, obtained for the convective blockage, show an improvement of the correlation coefficient of 110 % with respect to those available in literature, once a new blowing parameter containing the stagnation pressure is introduced. A correlation developed by NASA during the Round-Robin program, which relates the Teflon mass loss rate to the total pressure and cold-wall heat flux measured experimentally, is also used to predict the wall heat transfer referred to the ablation temperature of Teflon. For both approaches, a simplified stagnation point convective heat transfer equation allows the average stagnation enthalpy to be calculated. Several locations downstream of the nozzle exit have been surveyed, and selected points of the facility’s performance map have been used for the experimental campaign. The results show that both approaches provide similar results in terms of stagnation heat flux and enthalpy prediction with uncertainties comparable to those provided by standard intrusive heat flux probes (δ q max < 25 %). The analysis of the Teflon’s ablated surface does not reveal significant flow non-uniformities, and a 1.14 heat flux enhancement factor due to the shock–shock interaction is detectable at x = 3.5 in. from the nozzle exit plane. The results show the use of ablative probes for the flow characterization of arc plasma facilities to be promising for the dual purpose of calculating the local flow properties (i.e., heat flux and enthalpy) as well as verifying the uniformity of the flow by inspecting the footprint of the plume on the exposed surfaces.

Experimental investigation on behavior of multiple vertical buoyant jets discharged into a stagnant ambient

The behavior of merging buoyant jets discharged from uni-directional diffuser has been examined considering the interaction of individual axi-symmetric jets. Some series of experiments have been performed to define the range of merging transition based on the variations of local flow properties. The properties for the large eddies on the boundaries; passing frequency, vortex spacing, and Strouhal number, are also measured to investigate the evolution of individual jets to a merged flow. The results of this study show that the local flow properties change from those of axi-symmetric to merged flow over a merging transition where b/ps = 0.3 to 1.2. The flapping motion is observed as the individual jets get merged with neighboring ones. After the merging transition (b/ps = 0.3 to 1.2), the flows show a typical behavior of plane flows with a constant value of Strouhal number.

The influence of vegetation on turbulence and bed load transport

AbstractVegetation is ubiquitous in river channels and floodplains and alters mean flow conditions and turbulence. However, the effects of vegetation patches on near‐bed turbulence, bed load transport rates, and sedimentation are not well understood. To elucidate the influence of emergent vegetation on local and patch‐averaged bed load transport, we conducted a set of experiments in which we varied the mean flow velocity (U), total boundary shear stress (τ), or vegetation density between runs. We measured 2D velocity fields using Particle Imaging Velocimetry and bed load fluxes using high‐speed video. Simulated rigid vegetation caused bed load fluxes to vary spatially by an order of magnitude, causing distinct scour zones adjacent to, and depositional bed forms between stems. These local patterns of sedimentation could impact recruitment and survival of other plants. Large bed load fluxes were collocated with high near‐bed turbulence intensities that were three to four times larger than spatially averaged values. Higher vegetation densities increased the importance of inward and outward interactions, particularly downstream of vegetation. At the patch scale, greater stem densities caused either an increase or decrease in run‐averaged bed load fluxes, depending on whether U or τ was held constant between runs. This implies that sedimentation in vegetation patches is not only a function of bed grain size, sediment supply, and vegetation density and species, but whether vegetation significantly impacts mean and local flow properties, which could depend on vegetation location. Commonly used bed load transport equations did not accurately predict average sediment fluxes in our experiments unless they accounted for the spatial variability in the near‐bed Reynolds stress.

Flows of rarefied gaseous mixtures in networks of long channels

A method is presented to calculate the steady flows of rarefied gaseous mixtures in networks of long channels. The approach is based on the kinetic level. First, the McCormack linearized kinetic model is solved to obtain the local flow properties in the channels in a wide range of gaseous rarefaction and mole fraction. Second, the global flow properties including the flow rates and the distribution of the pressure and the mole fraction are deduced. An integral equation is introduced in order to determine the flow rates as functions of the differences of the partial pressures between the two ends of each channel. The conservation of mass at the nodes of the network results into a system of linear algebraic equations. The overall mathematical problem is solved iteratively. Pressure driven flows of He/Xe and He/Ar through an example network of circular tubes are calculated at intermediate values of the gaseous rarefaction. The results of the flow rates and the pressures and the mole fractions at the nodes in the whole system and the representative distributions of the pressure and the mole fraction along the channels are presented and commented on.

The dam-break problem for concentrated suspensions of neutrally buoyant particles

AbstractThis paper addresses the dam-break problem for particle suspensions, that is, the flow of a finite volume of suspension released suddenly down an inclined flume. We were concerned with concentrated suspensions made up of neutrally buoyant non-colloidal particles within a Newtonian fluid. Experiments were conducted over wide ranges of slope, concentration and mass. The major contributions of our experimental study are the simultaneous measurement of local flow properties far from the sidewalls (velocity profile and, with lower accuracy, particle concentration) and macroscopic features (front position, flow depth profile). To that end, the refractive index of the fluid was adapted to closely match that of the particles, enabling data acquisition up to particle volume fractions of 60 %. Particle migration resulted in the blunting of the velocity profile, in contrast to the parabolic profile observed in homogeneous Newtonian fluids. The experimental results were compared with predictions from lubrication theory and particle migration theory. For solids fractions as large as 45 %, the flow behaviour did not differ much from that of a homogeneous Newtonian fluid. More specifically, we observed that the velocity profiles were closely approximated by a parabolic form and there was little evidence of particle migration throughout the depth. For particle concentrations in the 52–56 % range, the flow depth and front position were fairly well predicted by lubrication theory, but taking a closer look at the velocity profiles revealed that particle migration had noticeable effects on the shape of the velocity profile (blunting), but had little impact on its strength, which explained why lubrication theory performed well. Particle migration theories (such as the shear-induced diffusion model) successfully captured the slow evolution of the velocity profiles. For particle concentrations in excess of 56 %, the macroscopic flow features were grossly predicted by lubrication theory (to within 20 % for the flow depth, 50 % for the front position). The flows seemed to reach a steady state, i.e. the shape of the velocity profile showed little time dependence.

Large probe arrays for measuring mean and time dependent local oil volume fraction and local oil velocity component distributions in inclined oil-in-water flows

Arrays of dual-sensor and four-sensor needle conductance probes have been used to measure the mean and time dependent local properties of upward inclined, bubbly oil-in-water flows (also known as dispersed oil-in-water flows) in a 153mm diameter pipe. The flow properties that were measured were (i) the local in-situ oil volume fraction α; (ii) the local oil velocity uo in the axial direction of the pipe (the Z direction); and (iii) the local oil velocity uY in the direction from the lower side of the inclined pipe to its upper side (the Y direction). Oil velocities in the X direction (orthogonal to the Y and Z directions) were found to be negligible. For all of the flow conditions investigated it was found that the mean value of α varied from a maximum value at the upper side of the inclined pipe to a minimum value at the lower side, and that the rate of decrease of this mean value of α with distance in the −Y direction became greater as the pipe inclination angle θ from the vertical was increased. It was also found that the mean value of uo was greatest at the upper side of the inclined pipe and decreased towards the lower side of the inclined pipe, the rate of decrease with distance in the −Y direction again becoming greater as θ was increased. For θ=45o, a water volumetric flow rate Qw=16.38m3h−1, an oil volumetric flow rate Qo=6.0m3h−1 and using a sampling period T=0.05s over a total time interval of 60s, it was found that at the upper side of the inclined pipe the standard deviation in uo was 31.6% of the mean value of uo. Furthermore for T=0.05s, θ=30o, Qw=16.38m3h−1 and Qo=6.0m3h−1 it was found that the standard deviation in the cross-pipe oil velocity component uY was approximately equal to the standard deviation in the axial velocity component uo. These large temporal variations in the local flow properties have been attributed to the presence of large scale Kelvin–Helmholtz waves which intermittently appear in the flow. It is believed that the techniques outlined in this paper for measuring the standard deviation of local flow properties as a function of the sampling period T will be of considerable value in validating mathematical models of time dependent oil–water flows. It should be noted that the principal focus of this paper is on the measurement techniques that were used and the methods of data analysis rather than the presentation of exhaustive experimental results at numerous different flow conditions.

Laser-Induced Spark Ignition of Premixed Confined Swirled Flames

Optimization of the ignition location is of crucial importance for many combustion systems and requires advanced knowledge of the ignition process for both fundamental and applied configurations. In this article, a premixed swirl burner was designed to experimentally study the impact of the spark location on successful ignition and to detail the scenario from ignition to flame stabilization. Two swirl numbers were investigated to evaluate their impact on the ignition process. Particular attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic particle image velocimetry) as well as on ignition conditions. Ignition probability maps were obtained for a constant level of deposited energy. Contrary to previous studies, no correlation between local turbulent kinetic energy and ignition probability was observed, and a deeper analysis of the temporal evolution of the flame kernel within the combustion chamber is required. Coupling fast flame visualization with the corresponding pressure signal demonstrated that the efficiency of the ignition location was not only controlled by the local flow properties, but also by the early flame kernel development, linked by its typical trajectories within the combustion chamber.

Turbulent flows in highly elastic wormlike micelles

This work reports on an experimental study of elastic turbulence in a semi-dilute wormlike micelle system made of a highly elastic betaine surfactant solution. The temporal evolution of both rheological quantities and local flow properties is monitored by combining global rheology, optical visualization, and ultrasonic velocimetry. Even at the smallest applied shear rates or shear stresses, we find that the micellar sample develops large Weissenberg (Wi) numbers, leading the flow to undergo a transition to elastic turbulence. Three-dimensional flows are indeed observed all along the flow curve, which therefore cannot be interpreted in the framework of classical shear banding. Strong fluctuations are also recorded in the rheological quantities, in the reflected light intensity, and in velocity profiles. We show that the power spectral densities (PSDs) of these fluctuations display power law behaviours with exponents ranging from −1 to −3 depending on the applied shear stress or shear rate. The exponents inferred from local velocity measurements are found to be spatially dependent, pointing to inhomogeneous turbulence. The nature of the instability and of the transition to elastic turbulence is further discussed in light of recent experimental and theoretical works on wormlike micelles and polymers.

On the existence of a simple yield stress fluid behavior

Materials such as foams, concentrated emulsions, dense suspensions or colloidal gels, are yield stress fluids. Their steady flow behavior, characterized by standard rheometric techniques, is usually modeled by a Herschel–Bulkley law. The emergence of techniques that allow the measurement of their local flow properties (velocity and volume fraction fields) has led to observe new complex behaviors. It was shown that many of these materials exhibit shear banding in a homogeneous shear stress field, which cannot be accounted for by the standard steady-state constitutive laws of simple yield stress fluids. In some cases, it was also observed that the velocity fields under various conditions cannot be modeled with a single constitutive law and that nonlocal models are needed to describe the flows. Doubt may then be cast on any macroscopic characterization of such systems, and one may wonder if any material behaves in some conditions as a Herschel–Bulkley material. In this paper, we address the question of the existence of a simple yield stress fluid behavior. We first review experimental results from the literature and we point out the main factors (physical properties, experimental procedure) at the origin of flow inhomogeneities and nonlocal effects. It leads us to propose a well-defined procedure to ensure that steady-state bulk properties of the materials are studied. We use this procedure to investigate yield stress fluid flows with MRI techniques. We focus on nonthixotropic dense suspensions of soft particles (foams, concentrated emulsions, Carbopol gels). We show that, as long as they are studied in a wide (as compared to the size of the material mesoscopic elements) gap geometry, these materials behave as ‘simple yield stress fluids’: they are homogeneous, they do not exhibit steady-state shear banding, and their steady flow behavior in simple shear can be modeled by a local continuous monotonic constitutive equation which accounts for flows in various conditions and matches the macroscopic response.

Multiphase flow dynamics of pyroclastic density currents during the May 18, 1980 lateral blast of Mount St. Helens

The dynamics of the May 18, 1980 lateral blast at Mount St. Helens, Washington (USA), were studied by means of a three‐dimensional multiphase flow model. Numerical simulations describe the blast flow as a high‐velocity pyroclastic density current generated by a rapid expansion (burst phase, lasting less than 20 s) of a pressurized polydisperse mixture of gas and particles and its subsequent gravitational collapse and propagation over a rugged topography. Model results show good agreement with the observed large‐scale behavior of the blast and, in particular, reproduce reasonably well the front advancement velocity and the extent of the inundated area. Detailed analysis of modeled transient and local flow properties supports the view of a blast flow led by a high‐speed front (with velocities between 100 and 170 m/s), with a turbulent head relatively depleted in fine particles, and a trailing, sedimenting body. In valleys and topographic lows, pyroclasts accumulate progressively at the base of the current body after the passage of the head, forming a dense basal flow depleted in fines (less than 5 wt.%) with total particle volume fraction exceeding 10−1 in most of the sampled locations. Blocking and diversion of this basal flow by topographic ridges provides the mechanism for progressive current unloading. On ridges, sedimentation occurs in the flow body just behind the current head, but the sedimenting, basal flow is progressively more dilute and enriched in fine particles (up to 40 wt.% in most of the sampled locations). In the regions of intense sedimentation, topographic blocking triggers the elutriation of fine particles through the rise of convective instabilities. Although the model formulation and the numerical vertical accuracy do not allow the direct simulation of the actual deposit compaction, present results provide a consistent, quantitative model able to interpret the observed stratigraphic sequence.

Modeling the thermochemical degradation of biomass inside a fast pyrolysis fluidized bed reactor

AbstractA fast pyrolysis process in a bubbling fluidized bed has been modeled, thoroughly reproduced and scrutinized with the help of a combined Eulerian/Lagrangian simulation method. The 3‐D model is compared to experimental results from a 100 g/h bubbling fluidized bed pyrolyzer including such variables as particle composition at the outlet and gas/vapor/water yields as a function of fluidization conditions, biomass moisture concentrations, and bed temperatures. Multiprocessor simulations on a high‐end computer have been carried out to enable the tracking of each of the 0.8 million individual discrete sand and biomass particles, making it possible to look at accurate and detailed multiscale information (i.e., any desired particle property, trajectory, particle interaction) over the entire particle life time. The overall thermochemical degradation process of biomass is influenced by local flow and particle properties and, therefore, accurate and detailed modeling reveals unprecedented insight into such complex processes. It has been found, that the superficial fluidization velocity is important while the particle moisture content is less significant for the final bio‐oil yield. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3030–3042, 2012

Experimental and Numerical Characterization of Flow-Induced Vibration of Multispan U-tubes

This paper describes a test program that was developed to measure the dynamic response of a bundle of steam generator U-tubes with anti-vibration bar (AVB) supports, subjected to Freon two-phase cross-flow. The tube bundle geometry is similar to the geometry used in preliminary designs for future CANDU steam generators. This test program is one of the initiatives that Atomic Energy of Canada Limited (AECL) is undertaking to demonstrate that the tube support design for future CANDU steam generators meets the stringent requirements associated with a 60-year lifetime. In particular, the tests will address issues related to in- and out-of-plane fluidelastic instability and random turbulent excitation of a U-tube bundle with AVB supports. Therefore, the measurements include tube vibration amplitudes and frequencies, work-rate due to impacting and sliding motion of the tubes against their supports, bulk process conditions and local two-phase flow properties. Details of the test rig setup and the measurement techniques are described in the paper. Moreover, a numerical prediction of the U-tube vibration response to flow was performed with AECL’s pipo-fe code. A summary of the numerical results is presented.

Dynamically adaptive grid based discontinuous Galerkin shallow water model

A Godunov-type numerical model, which is based on the local planar Runge–Kutta discontinuous Galerkin (RKDG2) solutions to the two dimensional (2D) shallow water equations (SWEs) on a dynamically adaptive quadrilateral grid system, is developed in this work for shallow water wave simulations, with particular application to flood inundation modeling. To be consistent with the dynamic grid adaptation, the well-balanced RKDG2 framework is reformulated to facilitate realistic flood modeling. Grid adaptation and redistribution of flow data are automated based on simple measures of local flow properties. One analytical and two diagnostic test cases are used to validate the performance of the dynamically adaptive RKDG2 model against an alternative RKDG2 code based on uniform quadrilateral meshes. The adaptive model is then assessed by further applying it to reproduce a laboratory-scale tsunami benchmark case and the historical Malpasset dam-break event. Numerical evidence indicates that the new algorithm is able to resolve the moving wave features adequately at much less computational cost than the refined uniform grid-based counterpart.

Experimental and Numerical Investigations of Two-Phase Electrolysis Processes - Electrical Energy Conversion in Hydrogen Production

During two-phase electrolysis processes, for example for hydrogen production, there are bubbles which are created at electrodes. This implies a great vertical motion source in the normal earth gravity field and then a quite important natural two-phase convection. All other fields are then affected. Heat, mass and electricity transfers are modified due to both bubbles screening (at surface and in volume) and to bubbles transport promotion. Many numerical modeling for two-phase processes such as kerosene pulverization in engines or coal combustion sciences have shown the difficulties of these multi-physics processes. Both particles and reactor scales must be considered according with a strong coupling modeling. In these processes the particles injection is “in the flow”. In boiling or electrolysis processes, a new difficulty is added: particles birth or injection is strongly coupled to the local flow properties and leads to a complex boundary condition at surfaces. Electrical and electrochemical properties and processes are disturbed. This disturbance can lead to the modification of the local current density and to anode effects for example. There is few works concerning the local modelling of electrochemic al processes during a two-phase electrolysis process. There are also few local experimental measurements in term of chemical composition, temperature or current density which will allow the numerical calculations validation. The present work shows the started numerical modeling strategy and the first results, both experimental and numerical obtained.

Design study of a regenerative pump using one-dimensional and three-dimensional numerical techniques

Regenerative pumps are low cost, compact and able to deliver high heads at low flowrates. Furthermore with stable performance characteristics they can operate with very small NPSH. The complexity of the flowfield is a serious challenge for any kind of mathematical modelling. This paper compares an analytical and numerical technique of resolving the performance for a new regenerative pump design. The performance characteristics computed by a CFD approach and a new one-dimensional model are compared and matched to experimental test results. The approaches of both modelling techniques are assessed as potential design tools. The approaches are shown to not only successfully resolve the complex flowfield within the pump; the CFD is also capable of resolving local flow properties to conduct further refinements. The flow field is represented by the CFD as it has never been before. A new design process is suggested. The new regenerative pump design is considered with a comparable duty centrifugal pump, proving that for many high head low flowrate applications the regenerative pump is a better choice.

Couette grain flow experiments: The effects of the coefficient of restitution, global solid fraction, and materials

Granular flows are complex flows of solid granular material which are being studied in several industries. However, it has been a challenge to understand them because of their non-linear and multiphase behavior. The present experimental work investigates granular flows undergoing shear, by specifically studying the interaction between rough surfaces and granular flows when the global solid fraction and the material comprising the rough shearing surface are varied. A two-dimensional annular shear cell, with a stationary outer ring and inner driving wheel, and digital particle tracking velocimetry (DPTV) technique were used to obtain local granular flow properties such as velocity, local solid fraction, granular temperature, and slip. A customized particle drop test apparatus was built to experimentally determine the coefficient of restitution (COR) between the granular and surface materials using high-speed photography. Results showed that wheel surface materials that produce higher COR values exhibit higher velocity and granular temperature values near the wheel, and lower slip velocities. The local solid fraction appears inversely related to the COR values. The global solid fraction seemed to correspond with velocity and granular temperature, while displaying an inverse relationship to slip. Results also showed an initial decrease in the kinetic energy of the flow as the global solid fraction increased, due to the formation of a distinct contact region. This was followed by a rise in kinetic energy as the global solid fraction continued to increase, based on the increase of particles present in the kinetic region of the flow.