Hydrodynamic Ones Research Articles

Color-gradient-based phase-field equationfor multiphase flow.

In this paper, the underlying problem with the color-gradient (CG) method in handling density-contrast fluids is explored. It is shown that the CG method is not fluid invariant. Based on nondimensionalizing the CG method, a phase-field interface-capturing model is proposed which tackles the difficulty of handling density-contrast fluids. The proposed formulation is developed for incompressible, immiscible two-fluid flows without phase-change phenomena, and a solver based on the lattice Boltzmann method is proposed. Coupled with an available robust hydrodynamic solver, a binary fluid flow package that handles fluid flows with high density and viscosity contrasts is presented. The macroscopic and lattice Boltzmann equivalents of the formulation, which make the physical interpretation of it easier, are presented. In contrast to existing color-gradient models where the interface-capturing equationsare coupled with the hydrodynamic ones and include the surface tension forces, the proposed formulation is in the same spirit as the other phase-field models such as the Cahn-Hilliard and the Allen-Cahn equationsand is solely employed to capture the interface advected due to a flow velocity. As such, similarly to other phase-field models, a so-called mobility parameter comes into play. In contrast, the mobility is not related to the density field but a constant coefficient. This leads to a formulation that avoids individual speed of sound for the different fluids. On the lattice Boltzmann solver side, two separate distribution functions are adopted to solve the formulation, and another one is employed to solve the Navier-Stokes equations, yielding a total of three equations. Two series of numerical tests are conducted to validate the accuracy and stability of the model, where we compare simulated results with available analytical and numerical solutions, and good agreement is observed. In the first set the interfacial evolution equationsare assessed, while in the second set the hydrodynamic effects are taken into account.

ЕВОЛЮЦІЯ РУХУ ДИНАМІЧНО СИМЕТРИЧНОГО ГІРОСТАТА ПІД ДІЄЮ ПОСТІЙНОГО МОМЕНТУ

Problems of the dynamics of rigid bodies containing a fluid have attracted a great deal of attention. Interest in these problems has grown in connection with the development of space technology. Similar problems also arise in the theory of aircraft and ship and in other technical questions. These problems are also of major theoretical interest. There is a necessity to study the problems of the rigid body dynamics with cavities containing a viscous fluid to calculate the motion of spacecraft about its center of mass as well as their orientation and stabilization. The problems of rigid body dynamics with cavities containing a viscous fluid are more difficalt that in the case of ideal fluid. The studies of F. L. Chernousko showed that solving the problems of the rigid body dynamics with viscous fluid in cavity can be separated into two parts: the hydrodynamic and dynamic ones, which represent a simplification of the original problem. The torques of viscous fluid in cavity forces, acting on the body, are often relatively small and can be considered as perturbations. It is natural to use the methods of small parameter to analyze the dynamics of rigid body under the action of applied torques. The method applied in this paper is the Krylov-Bogolubov asymptotic averaging method. Reference was made to the angular momentum vector of a satellite, noted during the determination of the actual orientation of the artificial earth satellite. This change was explained as a result of the presence of a the small perturbation torque, which is constant in fixed axes relative to the satellite. In the case of small torques, it is possible to use perturbation methods to obtain analytical or numerical descriptions of motion. In this paper, the motion about the center of mass of a dynamically symmetric rigid body with a cavity filled with a fluid of high viscosity and subjected to constant body-fixed torque is considered. The torque of forces acting on the rigid body by the viscous fluid in the cavity is determined by the technique developed in the works of F. L. Chernousko. We obtain the system of motion equations in the standard form. Obtained numerical solutions describe the evolution of rigid body motion under the action of small internal and external torques. Keywords: rigid body, cavity, viscous fluid, constant torque

Global high-order numerical schemes for the time evolution of the general relativistic radiation magneto-hydrodynamics equations

Modeling correctly the transport of neutrinos is crucial in some astrophysical scenarios such as core-collapse supernovae and binary neutron star mergers. In this paper, we focus on the truncated-moment formalism, considering only the first two moments (M1 scheme) within the grey approximation, which reduces Boltzmann seven-dimensional equation to a system of equations closely resembling the hydrodynamic ones. Solving the M1 scheme is still mathematically challenging, since it is necessary to model the radiation-matter interaction in regimes where the evolution equations become stiff and behave as an advection-diffusion problem. Here, we present different global, high-order time integration schemes based on Implicit-Explicit Runge-Kutta methods designed to overcome the time-step restriction caused by such behavior while allowing us to use the explicit Runge-Kutta commonly employed for the magneto-hydrodynamics and Einstein equations. Finally, we analyze their performance in several numerical tests.

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Context. Asteroseismic measurements of the internal rotation rate in evolved stars pointed to a lack of angular momentum (AM) transport in stellar evolution models. Several physical processes in addition to hydrodynamical ones were proposed as candidates for the missing mechanism. Nonetheless, no current candidate can satisfy all the constraints provided by asteroseismology. Aims. We revisit the role of a candidate process whose efficiency scales with the contrast between the rotation rate of the core and the surface. This process was proposed in previous works to be related to the azimuthal magneto-rotational instability. Methods. We computed stellar evolution models of low- and intermediate-mass stars with the parametric formulation of AM transport proposed in previous works for this instability until the end of the core-helium burning for low- and intermediate-mass stars, and compare our results to the latest asteroseismic constraints available in the post-main sequence phase. Results. Both hydrogen-shell-burning stars in the red-giant branch and core-helium-burning stars of low- and intermediate-mass in the mass range 1 M⊙ ≲ M ≲ 2.5 M⊙ can be simultaneously reproduced by this kind of parametrisation. Conclusions. Given the current constraints from asteroseismology, the core rotation rate of post-main sequence stars seems to be well explained by a process whose efficiency is regulated by the internal degree of differential rotation in radiative zones.

Causes and mechanisms of the disappearance of braided channel patterns (the example of the Białka River, Western Carpathians)

The channels of multi-threaded (braided) rivers occur commonly in areas that fulfill certain conditions such as substantial influx of bed material and gradients large enough to create significant energy of flowing water. Natural conditions favoring the formation of multithreaded channels are present in the Carpathians in Poland in Podhale – a large basin located in the piedmont area of the high-mountain Tatra massif. The area had experienced glaciation in the Pleistocene. Yet the 20th century – and especially its second half – was a period of rapid and irreversible elimination of braided channels across the region. The Białka is considered to be the last braided river in the Carpathians in Poland. Many parameters like: structure (morphologic reach sequence), degree of braiding (BI index, W/D) and also a number of hydrodynamic ones (unit stream power, critical stress, shear stress, others) were investigated in order to assess a current stage of development of the Białka river channel. The Białka river channel appears to be a complex system with a differentiated structure. Its channel system is a mosaic of different types described by the following sequence: straight-sinuous-braided. It represents an intermediate type that is somewhere between a single- and a multi-threaded channel. Future evolution of the Białka river channel appears to include further degradation and transformation into a poorer channel ecosystem.

EVOLUTION OF ROTATIONAL MOTIONS IN A RESISTIVE MEDIUM OF A NEARLY DYNAMICALLY SPHERICAL GYROSTAT SUBJECTED TO CONSTANT BODY-FIXED TORQUES

A satellite or a spacecraft in its motion about the center of mass is affected by the torques of forces of various physical nature. It is influenced by the gravitational, aerodynamic torques, the torques due to the light pressure, and the torques due to the motions of masses inside the body. These motions may have various causes, for example, the presence of fluid in the cavities in the body (for example, liquid fuel or oxidizer in the tanks of a rocket). Therefore, there is a necessity to study the problems of the dynamics of bodies with cavities containing a viscous fluid, to calculate the motion of spacecrafts about the center of mass, as well as their orientation and stabilization. The mentioned torques, acting on the body, are often relatively small and can be considered as perturbations. It is natural to use the methods of small parameter to analyze the dynamics of rigid body under the action of applied torques. The method applied in this paper is the Krylov-Bogolubov asymptotic averaging method. The studies of F. L. Chernousko showed that solving the problems of dynamics of a rigid body with a viscous fluid can be subdivided into two parts – the hydrodynamic and dynamic ones – which can greatly simplify the initial problem. We investigated the motion about its center of mass in a resistive medium of a nearly dynamically spherical rigid body with a cavity filled with a viscous fluid at small Reynolds numbers, subjected to constant body-fixed torque which is described by the system of differential equations, considering the asymptotic approximation of the moments of the viscous fluid in the cavity. The determination of the motions of forces acting on the body from side of the viscous fluid in the cavity was proposed in the works of F. L. Chernousko. We obtained the system of equations of motion in the standard form which refined in square-approximation by small parameter. The Cauchy problem for a system determined after averaging was analyzed. The evolution of the motion of a rigid body under the action of small internal and external torques of forces is described by the solutions which obtained as a result of asymptotic, analytical and numerical calculations over an infinite time interval.

Morpho-kinematics of MACS J0416.1-2403 low-mass galaxies

We use optical integral field spectroscopy from VLT/MUSE, as well as photometric observations from Hubble Space Telescope and VLT/HAWK-I, to study the morpho-kinematics of 17 low-mass (log(M/M⊙) < 9.5) MACS J0416.1-2403 cluster galaxies at R200 and five field galaxies with a redshift of z ∼ 0.4. By measuring fluxes of strong emission lines from the MUSE data, we have recovered the star formation rates, gas-phase metallicities, and spatially resolved gas kinematics, and we have also investigated the ionising mechanisms. We have analysed the structure and morphology of the galaxies from the optical and infrared photometric data, performing a multi-component decomposition into a bulge and a disk. The spatially resolved gas velocity fields of the cluster members and field galaxies were modelled using a 3D approach, which allowed us to retrieve their intrinsic gas kinematics, including the maximum rotation velocity and velocity dispersion. This enabled us to study scaling relations such as the Tully–Fisher and the stellar mass–S0.5 relation for low-mass galaxies in different environments and to search for signatures of cluster-specific processes using disturbed gas velocity fields as tracers. Most galaxies from our sample fall in the star-forming and composite region in the diagnostic diagram, which allows for the ionising sources in a galaxy to be disentangled. The cluster and field population can be classified as star-forming main-sequence galaxies, with only a sub-sample of four quenched systems. We observe significant scatter for the cluster galaxies in the mass-metallicity plane, and the lowest-mass systems deviate from the predictions of the fundamental metallicity relation, showing higher metallicities, whereas the higher-mass ones are in accordance with the model predictions. This might hint at the cutoff of pristine gas inflow and/or the removal of the hot halo gas as the mechanisms driving these offsets. Our morpho-kinematic analysis reveals a sub-sample of dwarfs with maximum velocities vmax < 50 km s−1 and vmax, gas/σgas < 1, which depart from the Tully–Fisher relation. This might indicate that their interstellar medium is affected by external environmental processes, such as ram pressure stripping. However, ∼30% of the cluster galaxies have rotation-dominated gas disks and follow the Tully–Fisher relation within 1σ. Using the S0.5 parameter, which links the dynamical support of ordered motions with that of random motions, we can differentiate between galaxies affected by gravitational processes and systems affected by hydrodynamical ones. In the stellar mass–S0.5 plane, both cluster and field galaxies follow a tight sequence, with only a sub-population of five galaxies strongly departing (> 4σ) from this relation, showing high σgas values. Both the morphology and kinematics of the outlier galaxies hint at a combination of pre-processing and cluster-specific interactions affecting their stellar and gas disks.

Gravitational torques dominate the dynamics of accreted gas at z > 2

ABSTRACT Galaxies form from the accretion of cosmological infall of gas. In the high redshift Universe, most of this gas infall is expected to be dominated by cold filamentary flows which connect deep down inside haloes, and, hence, to the vicinity of galaxies. Such cold flows are important since they dominate the mass and angular momentum acquisition that can make up rotationally-supported discs at high-redshifts. We study the angular momentum acquisition of gas into galaxies, and in particular, the torques acting on the accretion flows, using hydrodynamical cosmological simulations of high-resolution zoomed-in haloes of a few $10^{11}\, \rm M_\odot$ at z = 2. Torques can be separated into those of gravitational origin, and hydrodynamical ones driven by pressure gradients. We find that coherent gravitational torques dominate over pressure torques in the cold phase, and are hence responsible for the spin-down and realignment of this gas. Pressure torques display small-scale fluctuations of significant amplitude, but with very little coherence on the relevant galaxy or halo-scale that would otherwise allow them to effectively re-orientate the gas flows. Dark matter torques dominate gravitational torques outside the galaxy, while within the galaxy, the baryonic component dominates. The circum-galactic medium emerges as the transition region for angular momentum re-orientation of the cold component towards the central galaxy’s mid-plane.

A hydrodynamic-plastic formulation for modelling sand using a minimal set of parameters

Conventional plasticity models follow rich mathematical formulations. However, they typically lack a physical basis, so their predictive quality is strongly correlated with the number of fitting parameters used. More recent hydrodynamic principles inject physical clarity by separating scales using distinct thermal and granular temperatures. However, existing hydrodynamic models also require many parameters to reproduce a wide variety of experimental observations. This paper combines the advantages of these two approaches to construct a new physics-based hydrodynamic-plasticity model formulation, which is comprehensively predictive. An illustrative model is developed that relies on only five parameters, all of which have been previously defined and calibrated. Four parameters are easily determined from common geotechnical experiments and address diverse rate-independent phenomena, including pressure and density dependent elasticity, pressure- and density-dependent dilation behaviours, critical state relationships under continuous shearing, liquefying cyclic hystereses, and initial stress-ratio dependent extensional shear response. The one remaining parameter enables the recovery of the rate-dependent rheology of friction and density during steady flow, as well as material response during transient loading. A key feature that further distinguishes the current formulation from previous hydrodynamic ones is that here granular temperature is being related to the measurable kinetic pressure.

MODELING OF EXTRACTION PROCESS IN THE SYSTEM SOLID BODY–LIQUID

The paper reports the basic principles of modeling of extraction process through the cell walls. A solid body is assumed to be an aggregate of deformable particles, inside of which there is an extractable substance. Two stages of the process are considered: diffusion and hydrodynamic ones. The mass exchange stage inside the solid is diffusive because it has not fluidity. As far as the diffusion stage is concerned, we propose to consider the process of internal transfer as a set of pseudocatalytic reactions between the solvent molecules and the extractable substance. Based on this approach, the relationship between the diffusion coefficient and the particles structure has been derived under the condition of a cellular structure of the particles. The equations for describing the time change of the mean concentrations of a substance are given, taking into account the effect of the external mass discharge condition at interphase boundaries. It has been proved that the applied approach makes it possible to find a sufficient number of parameters for describing the different effects of extraction and gives physical meaning to interpretations of kinetic curves of extraction. As regards the hydrodynamic stage, a proper frame model of the pore space was formulated, which is deterministic and two-scale. It takes into account possible shear and volume deformations of the particles. Such two-phase medium was considered as a collection of variable intensity sources. The equations for calculating the mass transfer coefficient for the system are solid particles – liquid, taking into account the rheology of the liquid phase as a mixture of extractant and extractable substance. It has been proved that the proposed models have sufficient universality to withstand adaptation to different specific situations.

Investigation of the acoustic particle velocity using synchronized non-invasive measurements in the near and far field of a turbulent flow

The sound generation of an aeroacoustic source with dominating tonal components and the propagation into the acoustic far field is investigated based on measurements of the flow around a square rod - mounted horizontally in an open-jet closed-circuit anechoic wind-tunnel - at a Reynolds number of 6×104. Pressure time series are measured with microphones in the far field while velocity field measurements are performed simultaneously with the non-invasive particle image velocimetry (PIV) technique in the flow field. It is shown that the propagation of acoustic waves from the source region to the far field can be resolved analyzing additionally flow fields recorded in vertically shifted fields of view, if the time evolution is synchronized properly with the far-field pressure signal. Furthermore, a comparison with two analytical solutions shows that the cross-correlation function of far-field pressure and near-field velocity fluctuations, normalized by the standard deviation of the pressure signal, corresponds to the acoustic particle velocity. However, this only applies to areas where the acoustic fluctuations dominate, i.e. not in the wake or in the free shear layer between the wind tunnel flow and the quiescent air where acoustic fluctuations are superimposed by hydrodynamic ones.

BANCO DE ENSAYOS PARA LA CARACTERIZACION DE COJINETES HIDROSTATICOS

In this research work, an experimental test bench has been designed and constructed to analyze the behavior of lubricated bearings, and validate an own developed theoretical model for bearings calculations. The test bench is composed of a high precision shaft (lapped and guided by ultraprecision rolling bearings) and in the middle of the two rest points a lubricated bearing test is located. This lubricated bearing is fully monitored, with positioning probes (micrometric resolution), pressure gauges and temperature sensors. The bearing has four pads, and each pad has been monitored. The oil is pumped at constant pressure and controlled temperature, this way different operating conditions can be tested. The flow-rate of inlet fluid is measured by a flow meter. The load is applied in the radial direction, by a hydraulic actuator directly to the bearing, with a system of ropes that ensures a controlled direction of the pulling force and stability in the experiment. Axial movement is limited by rolling elements, this way the movement of the bearing is limited to the vertical plane, measuring 2D displacements in positioning probes. The bearing is manufactured with sliding polymer material, by injection process. This solution allows quick changes in the shape of the bearing’s recesses, from designs closer to hydrostatic lubrication, or hydrodynamic ones. The experimental results have been used to validate a model of bearings lubrication from internal development . This model is based on Reynolds equation solution for hydrodynamic lubrication on the bearing lands, and hydrostatic lubrication for recesses, following the current bibliography. Therefore, a test bench for hydrostatic bearing has been developed in order to vatidate a theoretical model for bearings calculations. Keywords: Hydrostatic bearings, hydrodynamic lubrication, test bench, machine tool, spindle.

ПАРАМЕТР ГАББЛА І ПОТЕНЦІАЛ КОСМОЛОГІЧНОГО СКАЛЯРНОГО ПОЛЯ

We consider a homogeneous isotropic Universe filled with cold matter (with zero pressure) and dynamic dark energy in a form of a scalar field. For known scalar field potential V(φ), the Friedmann equations are reduced to a system of the first order equation for the Hubble parameter H(z) and the second order equation for the scalar field as functions of the redshift z. On the other hand, knowledge of H(z) allows us to get the scalar field potential in a parametric form for a known cold matter content and three dimensional curvature parameter. We analyze when the accepted model mimics the dependence H(z) derived in the framework of the other models, e.g., hydrodynamic ones. Two examples of this mimicry are considered. The first one deals with the case when H2(z)~ Ωm(1+z)3+ΩΛ, but Ωm parameter overestimates the input of the cold matter (dark matter+baryons). The resulting scalar field potential is V(φ)=a+bsinh2(cφ), where the constants a,b,c depend on the Ω – parameters of the problem. In the other example we assume that some part of the dark matter has a non-zero equation of state p=wε, -1<w<1. In this case H2(z)~ Ωdm1(1+z)3(1+w)+ Ωb+Ωdm2)(1+z)3+ΩΛ. The corresponding potentials are defined for positive values of φ. For both signs of w potential V(φ) is a monotonically increasing function with typically an asymptotically exponential behavior; though for some choice of parameters we may have a singularity of V(φ)on a finite interval. Then we consider fitting of the potential for w from the interval [-0.2,0.2] for three different values of Ωdm2 by means of a simple formula Vfit(φ)=p0+p1exp(p2 φ). The dependencies pi(w) are presented and the approximation error is estimated.

Phase transition in time-reversible Navier-Stokes equations.

We present a comprehensive study of the statistical features of a three-dimensional (3D) time-reversible truncated Navier-Stokes (RNS) system, wherein the standard viscosity ν is replaced by a fluctuating thermostat that dynamically compensates for fluctuations in the total energy. We analyze the statistical features of the RNS steady states in terms of a non-negative dimensionless control parameter R_{r}, which quantifies the balance between the fluctuations of kinetic energy at the forcing length scale ℓ_{f} and the total energy E_{0}. For small R_{r}, the RNS equations are found to produce "warm" stationary statistics, e.g., characterized by the partial thermalization of the small scales. For large R_{r}, the stationary solutions have features akin to standard hydrodynamic ones: they have compact energy support in k space and are essentially insensitive to the truncation scale k_{max}. The transition between the two statistical regimes is observed to be smooth but rather sharp. Using insights from a diffusion model of turbulence (Leith model), we argue that the transition is in fact akin to a continuous second-order phase transition, where R_{r} indeed behaves as a thermodynamic control parameter, e.g., a temperature. A relevant order parameter can be suitably defined in terms of a (normalized) enstrophy, while the symmetry-breaking parameter h is identified as (one over) the truncation scale k_{max}. We find that the signatures of the phase transition close to the critical point R_{r}^{★} can essentially be deduced from a heuristic mean-field Landau free energy. This point of view allows us to reinterpret the relevant asymptotics in which the dynamical ensemble equivalence conjectured by Gallavotti [Phys. Lett. A 223, 91 (1996)PYLAAG0375-960110.1016/S0375-9601(96)00729-3] could hold true. We argue that Gallavotti's limit is precisely the joint limit R_{r}→[over >]R_{r}^{★} and h→[over >]0, with the overset symbol ">" indicating that those limits are approached from above. The limit therefore relates to the statistical features at the critical point. In this regime, our numerics indicate that the low-order statistics of the 3D RNS are indeed qualitatively similar to those observed in direct numerical simulations of the standard Navier-Stokes equations with viscosity chosen so as to match the average value of the reversible thermostat. This result suggests that Gallavotti's equivalence conjecture could indeed be of relevance to model 3D turbulent statistics, and provides a clear guideline for further numerical investigations involving higher resolutions.

The momentum budget of clustered supernova feedback in a 3D, magnetized medium

While the evolution of superbubbles driven by clustered supernovae has been studied by numerous authors, the resulting radial momentum yield is uncertain by as much as an order of magnitude depending on the computational methods and assumed properties of the surrounding interstellar medium (ISM). In this work, we study the origin of these discrepancies, and seek to determine the correct momentum budget for a homogeneous ISM. We carry out 3D hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of clustered supernova explosions, using a Lagrangian method and checking for convergence with respect to resolution. We find that the terminal momentum of a shell driven by clustered supernovae is dictated primarily by the mixing rate across the contact discontinuity between the hot and cold phases, and that this energy mixing rate is dominated by numerical diffusion even at the highest resolution we can complete, 0.03 $M_\odot$. Magnetic fields also reduce the mixing rate, so that MHD simulations produce higher momentum yields than HD ones at equal resolution. As a result, we obtain only a lower limit on the momentum yield from clustered supernovae. Combining this with our previous 1D results, which provide an upper limit because they allow almost no mixing across the contact discontinuity, we conclude that the momentum yield per supernova from clustered supernovae in a homogeneous ISM is bounded between $2\times 10^5$ and $3\times 10^6$ $M_\odot$ km s$^{-1}$. A converged value for the simple homogeneous ISM remains elusive.

Is Citizen Science a Valid Tool to Monitor the Occurrence of Jellyfish? The Spot the Jellyfish Case Study from the Maltese Islands

Gatt, M.P., Deidun, A., Galea, A., and Gauci, A., 2018. Is citizen science a valid tool to monitor the occurrence of jellyfish? The spot the jellyfish case in the Maltese Islands. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 316–320. Coconut Creek (Florida), ISSN 0749-0208.The potential of citizen science in monitoring transient biological phenomena occurring over large spatial scales, such as the blooming of jellyfish species, has received increased acknowledgment in recent years. The Spot the Jellyfish citizen science campaign was launched by the International Ocean Institute and by the University of Malta in the summer of 2010, with thousands of jellyfish spotting reports having been submitted by sea-users through the campaign website, social media site, smart phone app or even through email, since then. The 2011–2015 submitted jellyfish reports were analysed, besides a number of water quality parameters (temperature, salinity, chlorophyll a content, nutrient and phytoplankton concentration), whose values were gleaned for the marine area of interest through online satellite water quality data portals. The main aim of this exercise was to seek to identify possible relationships, through a variety of univariate and multivariate statistical techniques, between the distribution of submitted campaign jellyfish records and the same water quality parameters. Possible bias introduced in the report database through artefacts such as differences in bay frequentation were addressed through normalisation procedures. Spatial differences in the abundance of jellyfish reports submitted along different parts of the Maltese coastline emerged, with the highest number of such reports having been submitted along the north-eastern coastline of the island of Malta, and jellyfish sighting reports correlated well with the values of the selected water quality values. Despite this, our results indicate that besides the selected water quality parameters, other forces, possibly hydrodynamic ones, were contributing to the recorded spatial variability in jellyfish occurrence.

Premixed Flames Under Microgravity and Normal Gravity Conditions

Premixed conical CH4-air flames were studied experimentally and numerically under normal straight, reversed gravity conditions and microgravity. Low-gravity experiments were performed in Drop tower. Classical Bunsen-type burner was used to find out features of gravity influence on the combustion processes. Mixture equivalence ratio was varied from 0.8 to 1.3. Wide range of flow velocity allows to study both laminar and weakly turbulized flames. High-speed flame chemoluminescence video-recording was used as diagnostic. The investigations were performed at atmospheric pressure. As results normalized flame height, laminar flame speed were measured, also features of flame instabilities were shown. Low- and high-frequency flame-instabilities (oscillations) have a various nature as velocity fluctuations, preferential diffusion instability, hydrodynamic and Rayleigh-Taylor ones etc., that was explored and demonstrated.

Discrete ellipsoidal statistical BGK model and Burnett equations

To simulate non-equilibrium compressible flows, a new discrete Boltzmann model, discrete Ellipsoidal Statistical(ES)-BGK model, is proposed. Compared with the original discrete BGK model, the discrete ES-BGK has a flexible Prandtl number. For the discrete ES-BGK model in Burnett level, two kinds of discrete velocity model are introduced; the relations between non-equilibrium quantities and the viscous stress and heat flux in Burnett level are established. The model is verified via four benchmark tests. In addition, a new idea is introduced to recover the actual distribution function through the macroscopic quantities and their space derivatives. The recovery scheme works not only for discrete Boltzmann simulation but also for hydrodynamic ones, for example, based on the Navier-Stokes, the Burnett equations, etc.

Grid refinement for aeroacoustics in the lattice Boltzmann method: A directional splitting approach.

This study focuses on grid refinement techniques for the direct simulation of aeroacoustics, when using weakly compressible lattice Boltzmann models, such as the D3Q19 athermal velocity set. When it comes to direct noise computation, very small errors on the density or pressure field may have great negative consequences. Even strong acoustic density fluctuations have indeed a clearly lower amplitude than the hydrodynamic ones. This work deals with such very weak spurious fluctuations that emerge when a vortical structure crosses a refinement interface, which may contaminate the resulting aeroacoustic field. We show through an extensive literature review that, within the framework described above, this issue has never been addressed before. To tackle this problem, we develop an alternative algorithm and compare its behavior to a classical one, which fits our in-house vertex-centered data structure. Our main idea relies on a directional splitting of the continuous discrete velocity Boltzmann equation, followed by an integration over specific characteristics. This method can be seen as a specific coupling between finite difference and lattice Boltzmann, locally on the interface between the two grids. The method is assessed considering two cases: an acoustic pulse and a convected vortex. We show how very small errors on the density field arise and propagate throughout the domain when a vortical flow crosses the refinement interface. We also show that an increased free stream Mach number (but still within the weakly compressible regime) strongly deteriorates the situation, although the magnitude of the errors may remain negligible for purely aerodynamic studies. A drastically reduced level of error for the near-field spurious noise is obtained with our approach, especially for under-resolved simulations, a situation that is crucial for industrial applications. Thus, the vortex case is proved useful for aeroacoustic validations of any grid refinement algorithm.

A paradigm for modeling and computation of gas dynamics

In the continuum flow regime, the Navier-Stokes (NS) equations are usually used for the description of gas dynamics. On the other hand, the Boltzmann equation is applied for the rarefied flow. These two equations are based on distinguishable modeling scales for flow physics. Fortunately, due to the scale separation, i.e., the hydrodynamic and kinetic ones, both the Navier-Stokes equations and the Boltzmann equation are applicable in their respective domains. However, in real science and engineering applications, they may not have such a distinctive scale separation. For example, around a hypersonic flying vehicle, the flow physics at different regions may correspond to different regimes, where the local Knudsen number can be changed significantly in several orders of magnitude. With a variation of flow physics, theoretically a continuous governing equation from the kinetic Boltzmann modeling to the hydrodynamic Navier-Stokes dynamics should be used for its efficient description. However, due to the difficulties of a direct modeling of flow physics in the scale between the kinetic and hydrodynamic ones, there is basically no reliable theory or valid governing equations to cover the whole transition regime, except resolving flow physics always down to the mean free path scale, such as the direct Boltzmann solver and the Direct Simulation Monte Carlo (DSMC) method. In fact, it is an unresolved problem about the exact scale for the validity of the NS equations, especially in the small Reynolds number cases. The computational fluid dynamics (CFD) is usually based on the numerical solution of partial differential equations (PDEs), and it targets on the recovering of the exact solution of the PDEs as mesh size and time step converging to zero. This methodology can be hardly applied to solve the multiple scale problem efficiently because there is no such a complete PDE for flow physics through a continuous variation of scales. For the non-equilibrium flow study, the direct modeling methods, such as DSMC, particle in cell, and smooth particle hydrodynamics, play a dominant role to incorporate the flow physics into the algorithm construction directly. It is fully legitimate to combine the modeling and computation together without going through the process of constructing PDEs. In other words, the CFD research is not only to obtain the numerical solution of governing equations but to model flow dynamics as well. This methodology leads to the unified gas-kinetic scheme (UGKS) for flow simulation in all flow regimes. Based on UGKS, the boundary for the validation of the Navier-Stokes equations can be quantitatively evaluated. The combination of modeling and computation provides a paradigm for the description of multiscale transport process.