Monitoring of multiphase flows for superconducting accelerators and others applications

This paper is a review on implementation of measuring systems for two-phase helium, hydrogen, liquefied natural gas (LNG), and oil-formation/salty water flows. Two types of such systems are presented. The first type is based on two-phase flow-meters combining void fraction radio-frequency (RF) sensors and narrowing devices. They can be applied for superconducting accelerators cooled with two-phase helium, refueling hydrogen system for space ships and some applications in oil production industry. The second one is based on combination of a gamma-densitometer and a narrowing device. These systems can be used to monitor large two-phase LNG and oil-formation water flows. An electronics system based on a modular industrial computer is described as well. The metrological characteristics for different flow-meters are presented and the obtained results are discussed. It is also shown that the experience gained allows separationless flow-meter for three-phase oil-gas-formation water flows to be produced.     

Recent contribution in color interferometry and applications to high-speed flows

Recent improvements brought to color interferometry for analyzing high-speed flows are described through different applications. First, the optical technique based on differential interferometry using a polarized white light and one or two Wollaston prisms allows to record high-speed interferograms of the flow downstream of a circular cylinder. Then, this technique has been applied to axisymmetric flows for studying an interaction between a supersonic hot jet and a coaxial supersonic flow. Another application concerns the study of hypersonic flows using Wollaston prisms with a large birefringence angle. Finally, the analysis of gaseous mixture and the evolution of two-gases interface submitted to an acceleration is presented. Interferograms analysis is made from a modeling of interference fringes versus the optical path difference which allows to easily extract quantitative information of the gas density. In order to obtain absolute measurements of the gas density, real-time holographic interferometry has been developed using a three-color laser source and a panchromatic holographic plate. The technique generates the achromatic white fringe which makes the zero order of interference fringes easy to identify. An application is presented in a 2D subsonic wind tunnel, in which the unsteady wake flow past a cylinder is recorded at high framing rate. In this optical setup, transmission holograms are used. As a conclusion, an approach is proposed to analyze the 3D flows from real-time color holographic interferometry using reflection holograms and the problems to solve are described.     

On Jacobian matrices for flows

We present a general method for constructing numerical Jacobian matrices for flows discretized on a Poincaré surface of section. Special attention is given to Hamiltonian flows where the additional constraint of energy conservation is explicitly taken into account. We demonstrate the approach for a conservative dynamical flow and apply the technique for the general detection of periodic orbits.     

A continuum model of gas flows with localized density variations

We discuss the kinetic representation of gases and the derivation of macroscopic equations governing the thermomechanical behavior of a dilute gas viewed at the macroscopic level as a continuous medium. We introduce an approach to kinetic theory where spatial distributions of the molecules are incorporated through a mean-free-volume argument. The new kinetic equation derived contains an extra term involving the evolution of this volume, which we attribute to changes in the thermodynamic properties of the medium. Our kinetic equation leads to a macroscopic set of continuum equations in which the gradients of thermodynamic properties, in particular density gradients, impact on diffusive fluxes. New transport terms bearing both convective and diffusive natures arise and are interpreted as purely macroscopic expansion or compression. Our new model is useful for describing gas flows that display non-local-thermodynamic-equilibrium (rarefied gas flows), flows with relatively large variations of macroscopic properties, and/or highly compressible fluid flows.     

Application of multi-pulse optical imaging to measure evolution of laser-produced counter-streaming flows

A counter-streaming flow system is a test-bed to investigate the astrophysical collisionless shock(CS) formation in the laboratory. Electrostatic/electromagnetic instabilities, competitively growing in the system and exciting the CS formation, are sensitive to the flows parameters. One of the most important parameters is the velocity, determining what kind of instability contributes to the shock formation. Here we successfully measure the evolution of the counter-streaming flows within one shot using a multi-pulses imaging diagnostic technique. With the technique, the average velocity of the high-density-part(ne ≥ 8–9 × 10~(19)cm~(-3)) of the flow is directly measured to be of ～ 10~6cm/s between 7 ns and 17 ns.Meanwhile, the average velocity of the low-density-part(ne ≤ 2 × 10~(19)cm~(-3)) can be estimated as ～ 10~7cm/s. The experimental results show that a collisionless shock is formed during the low-density-part of the flow interacting with each other.     

Multifractal method for the instantaneous evaluation of the stream function in geophysical flows

Multifractal or multiaffine analysis is a promising new branch of methods in nonlinear physics for the study of turbulent flows and turbulentlike systems. In this Letter we present a new method based on the multifractal singularity extraction technique, the maximum singular stream-function method (MSSM), which provides a first order approximation to the stream function from experimental data in 2D turbulent systems. The essence of MSSM relies in relating statistical properties associated with the energy cascade in flows with geometrical properties. MSSM is a valuable tool to process sparse collections of data and to obtain instant estimates of the velocity field. We show an application of MSSM to oceanography as a way to obtain the current field from sea surface temperature satellite images; we validate the result with independent dynamical information obtained from sea level measurements.     

Void fraction prediction in two-phase flows independent of the liquid phase density changes

Gamma-ray densitometry is a frequently used non-invasive method to determine void fraction in two-phase gas liquid pipe flows. Performance of flow meters using gamma-ray attenuation depends strongly on the fluid properties. Variations of the fluid properties such as density in situations where temperature and pressure fluctuate would cause significant errors in determination of the void fraction in two-phase flows. A conventional solution overcoming such an obstacle is periodical recalibration which is a difficult task. This paper presents a method based on dual modality densitometry using Artificial Neural Network (ANN), which offers the advantage of measuring the void fraction independent of the liquid phase changes. An experimental setup was implemented to generate the required input data for training the network.ANNs were trained on the registered counts of the transmission and scattering detectors in different liquid phase densities and void fractions. Void fractions were predicted by ANNs with mean relative error of less than 0.45% in density variations range of 0.735 up to 0.98 gcm⁻³. Applying this method would improve the performance of two-phase flow meters and eliminates the necessity of periodical recalibration.     

Generalized potentiality of inhomogeneous media flows

A method for specifying a class of potential flows of inhomogeneous continuous media is developed. The general approach is based on expanding a medium material symmetry group to a special volume-preserving group, allowing us to obtain a law for the conservation of vorticity and, when there is no vorticity, to derive the unsteady Bernoulli equation. As illustrations, plane steady stationary flows of an inhomogeneous incompressible fluid and variable-entropy gas are considered. The problem of an inhomogeneous gas flow around a wedge yielding the formation of a shock wave is solved.     

Kr-PLIF for scalar imaging in supersonic flows

Experiments were performed to explore the use of two-photon planar laser-induced fluorescence (PLIF) of krypton gas for applications of scalar imaging in supersonic flows. Experiments were performed in an underexpanded jet of krypton, which exhibited a wide range of conditions, from subsonic to hypersonic. Excellent signal-to-noise ratios were obtained, showing the technique is suitable for single-shot imaging. The data were used to infer the distribution of gas density and temperature by correcting the fluorescence signal for quenching effects and using isentropic relations. The centerline variation of the density and temperature from the experiments agree very well with those predicted with an empirical correlation and a CFD simulation (FLUENT). Overall, the high signal levels and quantifiable measurements indicate that Kr-PLIF could be an effective scalar marker for use in supersonic and hypersonic flow applications.     

Gas kinetic algorithm for flows in Poiseuille-like microchannels using Boltzmann model equation

~~Gas kinetic algorithm for flows in Poiseuille-like microchannels using Boltzmann model equation1. Feynman, R., There's plenty of room at the bottom, Journal of Microelectromechanical Systems, 1992, 1: 60 -66. 2. Piekos, E. S., Breuer, K. S., Numerical modeling of micromechanical devices using the direct simulation Monte Carlo method, Transactions of the ASME, Journal of Fluids Engineering, 1996, 118: 464-469. 3. Beskok, A., Karniadakis, G. E., Trimmer, W., Rarefaction and …     

Decentralised control of material or traffic flows in networks using phase-synchronisation

We present a self-organising, decentralised control method for material flows in networks. The concept applies to networks where time sharing mechanisms between conflicting flows in nodes are required and where a coordination of these local switches on a system-wide level can improve the performance. We show that, under certain assumptions, the control of nodes can be mapped to a network of phase-oscillators.By synchronising these oscillators, the desired global coordination is achieved. We illustrate the method in the example of traffic signal control for road networks. The proposed concept is flexible, adaptive, robust and decentralised. It can be transferred to other queuing networks such as production systems. Our control approach makes use of simple synchronisation principles found in various biological systems in order to obtain collective behaviour from local interactions.     

Visualization of thermal cutting fluid flows

Outside of the fields where flow visualization is traditionally applied, there exist many processes where fluid phenomena are critical. Here, we survey flow visualization work with a focus on two thermal metal cutting processes. These two processes – plasma-arc cutting and gas assisted laser cutting – account for a large fraction of the means by which steel is cut in our world. Plasma-arc cutting utilizes an electric arc transferred between a cathode and the steel being cut to produce a high temperature gas jet that melts and removes metal. In gas assisted laser cutting, the assist jet is often high-pressure supersonic nitrogen for stainless steel, or near-atmospheric pressure, low-speed oxygen for carbon steel. Visualization of these millimeter-range diameter jets helps to understand the different roles that the assist gas has in these cutting processes, particularly with how the jets interact with the metal being cut. We describe experimental techniques for visualization of the arc jet and gas assist jet, as well as the liquid metal flows being removed from the cut and the gas flow in the torch itself. These visualizations overcome the small physical scales of the process, the bright illumination from the arc itself, and harsh high-temperature environment. The results lend perspective and understanding of the physical phenomena important to process control.     

Numerical simulations of gas-liquid two-phase flows in a micro porous structure

The lattice Boltzmann method for two-phase fluid flows is applied to the simulations of gas-liquid two-phase flows in a micro porous structure for various capillary numbers at low Reynolds numbers. The behaviors of the gas-liquid interface and the velocities of the two-phase fluid in the structure are simulated, and the permeability of gas and liquid through the structure are estimated from the calculated results. By changing the void fraction, the contact angle of the interface on walls, and the surface tension, the effect of these properties on the behaviors and the permeability of the two-phase flows in the micro porous structure is investigated. It is found that the permeability of liquid flows depends on the contact angle and it increases for hydrophobic walls. It is also seen that liquid flows are choked in pores for large void fractions and low capillary numbers.     

Kinetic theory for sheared granular flows

Rapid granular flows are far-from-equilibrium-driven dissipative systems where the interaction between the particles dissipates energy, and so a continuous supply of energy is required to agitate the particles and facilitate the rearrangement required for the flow. This is in contrast to flows of molecular fluids, which are usually close to equilibrium, where the molecules are agitated by thermal fluctuations. Sheared granular flows form a class of flows where the energy required for agitating the particles in the flowing state is provided by the mean shear. These flows have been studied using the methods of kinetic theory of gases, where the particles are treated in a manner similar to molecules in a molecular gas, and the interactions between particles are treated as instantaneous energy-dissipating binary collisions. The validity of the assumptions underlying kinetic theory, and their applicability to the idealistic case of dilute sheared granular flows are first discussed. The successes and challenges for applying kinetic theory for realistic dense sheared granular flows are then summarised.     

Composite flows

Composite flow models are an extension of the equations for a single compressible gas flows with multiple components, multiple phases, or multiple layers. Examples of such flows include the transport of oil, water, and polymers in porous reservoirs; separation of adsorbable solutes by chromatography; distillation columns; thermoclines in the ocean; multiphase flows in reactors; and separation of DNA fragments by electrophoresis. In many examples local equilibrium assumptions, such as Darcy's law or the Langmuir isotherm assumption, lead to nonlinear hyperbolic conservation laws which can be analyzed in terms of Riemann problems and elementary waves. In these cases front tracking algorithms show great promise for resolving very complicated wave interactions, in one dimension. We survey some of the recent developments in this field and present some computational examples. When local equilibrium assumptions are inappropriate, as is the case in many multiphase and multilayer flows, considerable difficulties, both theoretical and numerical, arise from the fact that the equations may be neither hyperbolic nor in conservation form. We give some examples of this and discuss the possibilities for analyzing these flows in terms of elementary and solitary waves.     

Modified relaxation time Monte Carlo method for continuum-transition gas flows

Gas flows in the continuum-transition regime often occur in micro-electro-mechanical systems. The relaxation time Monte Carlo (RTMC) method was modified by using an ellipsoid statistical model and a multiple translational temperature model in the BGK model equation to simulate continuum-transition gas flows. The modified RTMC method uses a simplified form of the generalized relaxation time, which is related to the macro velocity and the local Knudsen number. The results for Couette flow and Poiseuille flow in microchannels predicted using the modified RTMC and the DSMC are in good agreement with the modified RTMC being much faster than the DSMC for continuum-transition gas flow simulations.     

Engineering physical model of gas flows in a medium vacuum

The development of techniques for simulating gas flows in vacuum units in going from the molecular to viscous flow is hampered by the lack of adequate physical concepts of a medium vacuum. We offer an engineering physical model to simulate gas flows in vacuum units. Based on this model, a probabilistic method of simulation is worked out, and the gas flows in the molecular-viscous regime are evaluated. The paradox observed in the molecular-viscous regime is accounted for. The model is verified by experiments.     

Entropy considerations in numerical simulations of non-equilibrium rarefied flows

Non-equilibrium rarefied flows are encountered frequently in supersonic flight at high altitudes, vacuum technology and in microscale devices. Prediction of the onset of non-equilibrium is important for accurate numerical simulation of such flows. We formulate and apply the discrete version of Boltzmann’s H-theorem for analysis of non-equilibrium onset and accuracy of numerical modeling of rarefied gas flows. The numerical modeling approach is based on the deterministic solution of kinetic model equations. The numerical solution approach comprises the discrete velocity method in the velocity space and the finite volume method in the physical space with different numerical flux schemes: the first-order, the second-order minmod flux limiter and a third-order WENO schemes. The use of entropy considerations in rarefied flow simulations is illustrated for the normal shock, the Riemann and the two-dimensional shock tube problems. The entropy generation rate based on kinetic theory is shown to be a powerful indicator of the onset of non-equilibrium, accuracy of numerical solution as well as the compatibility of boundary conditions for both steady and unsteady problems.     

Subsonic non-steady-state gas flows in channels with inner cavities

A set of gasdynamic equations is given in the general form for matter with an arbitrary equation of state in the case when the entropy equation is used instead of the energy equation. In the ideal gas approximation in view of viscosity, a numerical investigation is performed of non-steady-state two-dimensional flows in a channel with a cavity. The calculation results have demonstrated that, given the flow velocity and the geometry of channel and cavity, pressure pulsations arise that are due to the departure of vortices from the cavity into the main flow. The values of the amplitude and frequency of pressure pulsations are determined. If measures are taken aimed at limiting the departure of vortices from the cavity, for example, a baffle is installed to restrict the interaction between the main flow and gas in the cavity, one can considerably increase the flow velocity in the channel, unaffected by the cavity. Such non-steady-state flows may be realized in MHD-generator channels, resonators of gas flow lasers, gas ducts for ventilation and gas transport systems, mufflers, whistles, etc.     

Chemical reaction fronts in ordered and disordered cellular flows with opposing winds

We present experiments on the motion of chemical fronts in ordered and disordered vortex flows with imposed uniform winds. Fronts in a chain of alternating vortices are found to freeze (pin to the separatrix) for a wide range of opposing winds that grows nonlinearly with the characteristic strength of the underlying vorticity. Experiments in spatially disordered flows demonstrate that freezing of fronts is common to cellular flows; furthermore, it is not dependent on boundary conditions. We therefore anticipate similar pinning in a wide range of 2D cellular flows and front-producing systems.