Inhomogeneous Velocity Field Research Articles

Wind noise due to flow induced around a windscreen in large-scale turbulence

Turbulent wind noise on microphones is a ubiquitous problem for sound measurement and recording outdoors. Windscreens, such as open-cell foam spheres, are effective at reducing wind noise. The unsteady pressure sources induced on the windscreen surface must contribute less noise on average than the stagnation pressure on a bare microphone, or windscreens would not be effective. To quantify this noise reduction mechanism, a model is presented for the unsteady pressure within an impermeable windscreen of arbitrary shape immersed in an initially isotropic turbulent flow. Only turbulent scales larger than the windscreen are considered, which contribute the most intense low-frequency band of the noise spectrum in most practical situations. The inhomogeneous velocity field around the windscreen is modeled in terms of an induced irrotational blocking field that meets the kinematic boundary condition. The interior pressure is expressed as a convolution of the surface pressure field with a filter function in wavevector space. The wind noise wavevector spectrum is then shown to be only a function of the inflow isotropic velocity spectrum and the windscreen shape. Solutions are obtained for the pressure frequency spectrum within a spherical windscreen for a von Kármán turbulence model and compared with prior results in literature.

Solving the Hydrodynamical System of Equations of Inhomogeneous Fluid Flows with Thermal Diffusion: A Review

The present review analyzes classes of exact solutions for the convection and thermal diffusion equations in the Boussinesq approximation. The exact integration of the Oberbeck–Boussinesq equations for convection and thermal diffusion is more difficult than for the Navier–Stokes equations. It has been shown that the exact integration of the thermal diffusion equations is carried out in the Lin–Sidorov–Aristov class. This class of exact solutions is a generalization of the Ostroumov–Birikh family of exact solutions. The use of the class of exact solutions by Lin–Sidorov–Aristov makes it possible to take into account not only the inhomogeneity of the pressure field, the temperature field and the concentration field, but also the inhomogeneous velocity field. The present review shows that there is a class of exact solutions for describing the flows of incompressible fluids, taking into account the Soret and Dufour cross effects. Accurate solutions are important for modeling and simulating natural, technical and technological processes. They make it possible to find new physical mechanisms of momentum transfer for the design of new types of equipment.

SIMULATION OF TWO-COMPONENT POWDER MOLDING MELTING UNDER VACUUM SINTERING CONDITIONS

Vacuum sintering of metal powders under controlled heating conditions belongs to the traditional powder technologies employed to obtain dense composite materials. Despite the long history of studies on sintering processes, modeling in this area remains relevant since it provides deeper understanding of the associated physical phenomena. In the present work, we studied a two-component powder pressing system melting model based on the multiphase filtration theory that takes into account the differences in the melting temperatures of the components. The appearance of the liquid phase was modeled by introducing a melting temperature interval. An algorithm for the numerical realization of the model was developed. Employing a titanium-aluminum as an example, we demonstrate that even a temperature field with weak inhomogeneity can cause an inhomogeneous velocity field and redistribution of the fusible component.

Гидравлическое сопротивление системы управления движением инспекционного снаряда с клапаном обтекаемой формы в газопроводе низкого давления

The main problem while developing inspection shells for low-pressure gas pipelines is mainte-nance of their uniform motion under the action of the pumped gas. In the present work, we propose a solution to this problem based on changing the cross-section of the central flow channel of the smart pig by moving an axially symmetric valve along the axis of the channel. Two types of valves with a drop-shaped and a cone-shaped front part were considered. The choice of the valve shape and size can be made by calculating the forces acting on the smart pig during gas flow. The prob-lem was solved by the methods of computational fluid dynamics under the assumption of axial flow symmetry. The gas was considered to be compressible. The calculations showed that the total force acting on the valve and walls of the flow channel does not depend on its shape and, depending on the valve position, can change by more than two times. The optimum length of a drop-shaped valve is nearly half that of a conical valve. Although the drop-shaped valve produces a more inhomoge-neous velocity field than the conical valve, the former is more acceptable for regulating gas flow through the central channel of the inspection smart pig.

Anomalous diffusion in a circular comb with external velocity field

Transport of particles confined in a branched circular medium under influence external force is a common phenomenon in nature and science. In this paper, the diffusion of particles, under an external homogeneous and inhomogeneous velocity field, in a circular comb has been studied and the impact of the geometry on the process has been discussed. The process is a circular motion under external velocity for fixed radius r=R and is interspersed with a radial motion inward and outward of the circle. The diffusion-advection equation that describes this process has been derived and solved numerically. The results show an enhance in the diffusion process in the circular direction according to the external force and the radius of the circle, where a transition between sub and normal diffusion in the case of homogeneous velocity, and a transition between sub and fast diffusion in the case of in-homogeneous velocity. However, due to the finite region of the structure, the process tends to saturate the structure with time. On the other hand, the speed of enhancement in the diffusion process and its saturation decreases as the radius of the circle increases. This model can be served in the analysis of the restricted diffusion of spin-bearing particles in circular layers in in-homogeneous magnetic fields.

Construction of Inhomogeneous Velocity Fields Using Expansions in Terms of Eigenfunctions of the Laplace Operator

In this paper we present a method for constructing inhomogeneous velocity fields of an incompressible fluid using expansions in terms of eigenfunctions of the Laplace operator whose weight coefficients are determined from the problem of minimizing the integral of the squared divergence. A number of examples of constructing the velocity fields of plane-parallel and axisymmetric flows are considered. It is shown that the problem of minimizing the integral value of divergence is incorrect and requires regularization. In particular, we apply Tikhonov’s regularization method. The method proposed in this paper can be used to generate different initial conditions in investigating the nonuniqueness of the solution to the Navier – Stokes equations.

Analysis of Microplastic Particle Transmission

The progressive increase in the mass of microplastics in the ecosystem obliges us to urgently define measures to reduce its adverse effects, which primarily requires an understanding of the genesis of its presence and the dynamics of expansion through the biosphere. This paper aims to contribute to the understanding of the dynamics of microplastic particle motion, especially in the context of deposition rate with respect to microplastic material density, microplastic particle size and especially with respect to microplastic particle shape (which significantly affects shape resistance forces). For this purpose, an overview of existing works in the field of modeling the motion of microplastics is given, and a numerical model for modeling the transport of microplastic particles in an inhomogeneous fluid velocity field for laminar flow is formed. The proposed model is thus based on a system of two nonlinear ordinary differential equations.

Data centre floor tiles influence on server fan power consumption

This study presents the flow of cooling air in a single rack data centre test bench with 40 servers with cold/hot aisle containment and a raised floor with a floor tile. The inhomogeneous pressure and velocity field in the cold aisle and the effect on the local server fan power were assessed. Computational fluid dynamic simulations as well as measurements were performed. Due to the cold/hot aisle containment, the floor jet caused a low pressure region for the lowest rack positions 1–10. In the simulation this led to a predicted increase in server fan power consumption of up to 6% for the lower rack positions. In the experiments, an increase of the server fan power for lower rack positions was likewise noticeable but was blurred due to the variance between individual servers. The influence of the vertical pressure distribution on the total server fan power consumption was minor (below 2%) if leakage is neglected. Six partially covered floor tile configurations were investigated. Partially blocking the floor tile to redistribute the flow showed no beneficial effect in homogenising the vertical pressure distribution because of the resulting increase in floor jet velocity. The simulations over-predicted the vertical pressure inhomogeneity in the contained cold aisle because leakage was not taken into account.

Vibroequilibria in microgravity: Comparison of experiments and theory.

Experiments on vibrated fluids confined in cylindrical and cuboidal containers were performed under the reduced gravity conditions of a parabolic flight. The results constitute a systematic quantitative investigation of the vibroequilibria effect, which refers to the reorientation of vibrated fluids in response to the inhomogeneous oscillatory velocity field and the accompanying dynamic pressure. This effect is amplified in microgravity where the restoring force of gravity is small or absent. Here the vibrations are transmitted via a pair of piezoelectric ceramics and a cantilever beam, excited in a resonant mode. The first and second resonances exhibit different types of motion and lead to different types of vibroequilibria surfaces, one with a dip or crater in the interior and the other flattened compared to the unforced reference experiment. The general tendency for interfaces to orient more perpendicular, on average, to the vibrational axis is confirmed. In the case of water in a cuboidal container, a quantitative comparison is made with vibroequilibria theory and with direct simulations of the Navier-Stokes equations. The good agreement confirms the predictions of vibroequilibria theory and suggests the capacity of this phenomenon to manipulate and position fluids in space environments through the choice of frequency and resonant mode.

The effect of an unsteady flow incident on an array of circular cylinders

In this paper we investigate the effect of an inhomogeneous and unsteady velocity field incident on an array of rigid circular cylinders arranged within a circular perimeter (diameter $D_{G}$) of varying solid fraction $\unicode[STIX]{x1D719}$, where the unsteady flow is generated by placing a cylinder (diameter $D_{G}$) upwind of the array. Unsteady two-dimensional viscous simulations at a moderate Reynolds number ($Re=2100$) and also, as a means of extrapolating to a flow with a very high Reynolds number, inviscid rapid distortion theory (RDT) calculations were carried out. These novel RDT calculations required the circulation around each cylinder to be zero which was enforced using an iterative method. The two main differences which were highlighted was that the RDT calculations indicated that the tangential velocity component is amplified, both, at the front and sides of the array. For the unsteady viscous simulations this result did not occur as the two-dimensional vortices (of similar size to the array) are deflected away from the boundary and do not penetrate into the boundary layer. Secondly, the amplification is greater for the RDT calculations as for the unsteady finite Reynolds number calculations. For the two highest solid fraction arrays, the mean flow field has two recirculation regions in the near wake of the array, with closed streamlines that penetrate into the array which will have important implications for scalar transport. The increased bleed through the array at the lower solid fraction results in this recirculation region being displaced further downstream. The effect of inviscid blocking and viscous drag on the upstream streamwise velocity and strain field is investigated as it directly influences the ability of the large coherent structures to penetrate into the array and the subsequent forces exerted on the cylinders in the array. The average total force on the array was found to increase monotonically with increasing solid fraction. For high solid fraction $\unicode[STIX]{x1D719}$, although the fluctuating forces on the individual cylinders is lower than for low $\unicode[STIX]{x1D719}$, these forces are more correlated due to the proximity of the cylinders. The result is that for mid to high solid fraction arrays the fluctuating force on the array is insensitive to $\unicode[STIX]{x1D719}$. For low $\unicode[STIX]{x1D719}$, where the interaction of the cylinders is weak, the force statistics on the individual cylinders can be accurately estimated from the local slip velocity that occurs if the cylinders were removed.

High-precision sound source localization method based on successive approximation in an inhomogeneous sound velocity field

In an inhomogeneous sound velocity field, the conventional time of flight (ToF) method cannot derive an exact distance directly. Therefore, a high-precision sound source localization method is required to address this difficult problem. In this study, we propose a novel high-accuracy successive approximation sound source localization method (SALM) in the inhomogeneous sound velocity field. The ToFs from a sound source to multiple receivers are used to constitute a real time vector. And the ToF from the assumed sound source to each receiver is considered a virtual time vector. It can be calculated according to the distribution of the sound velocity field, regardless of where the sound source is in the field. Then, modified vector and iterative formula can be constructed given the minimum difference between the roles of real and virtual time vectors. By using successive approximation method, the assumed sound source position approaches the actual position systematically until satisfies the localization accuracy requirements. Numerical results indicate that the SALM can considerably improve the localization accuracy. For example, in the 100 m × 100 m horizontal region, the ranging error of the SALM can be lower than 1 mm with more than 20 iterations when the velocity of sound is distributed unevenly within ±5 m/s. Moreover, increasing the number of receivers ensures a significant improvement in the approximate speed. And the localization accuracy can also be improved infinitely by increasing the number of iterations. Finally, the SALM is also applicable to sound source localization in a 3D space.

Temperature and Velocity of the Gas–Vapor Mixture in the Trace of Several Evaporating Water Droplets

The optical techniques (particle image velocimetry (PIV), laser-induced phosphorescence (LIP), planar laser-induced fluorescence (PLIF)) are used to study unsteady and inhomogeneous temperature and velocity fields of a gas–vapor mixture forming in the immediate vicinity of rapidly evaporating water droplets. Experiments involve various arrangements of several (two, three, and five) water droplets in a heated air flow. We establish the dependencies of the temperature and velocity of a gas–vapor mixture in the trace of each droplet on the heating time, velocity and temperature of the air flow, initial dimensions, and droplet arrangement scheme. Distinctive features of the synergistic effect of a droplet group on their temperature and aerodynamic traces are identified. Longitudinal and transversal dimensions of the aerodynamic and thermal traces of evaporating droplets are established. The length of the temperature trace of one droplet equals 10–12 of its radii, and the width of the temperature and aerodynamic trace of a droplet is no larger than its diameter.

Mechanisms of surface formation at treatment with compression plasma flows

The mechanisms of the formation of the surface relief at treatment with intense plasma flows are considered in this paper. The main attention is paid to the formation of a wave-like relief of the surface. On the basis of the presented theoretical model for the formation of wave-like surface relief, it is shown that the inhomogeneous velocity field formed in the melt during its radial motion over the target surface leads to an increase in the elastic energy of the melt. It can be accompanied by the melt decomposition into separate fragments. If the elastic energy is sufficient to form a new surface the fragmentation of the melt takes place. The decomposition of the melt will not be observed if the time during which the elastic energy necessary for decay is stored is less than the characteristic hydrodynamic time. It is shown that the thinner ring of the melt is broken down into smaller fragments with the same difference in the velocities of the outer part of the melt and the inner one. As the velocity difference increases, the fragments become more elongated, which agrees well with the experimental data. Thus, the phenomenon observed in the experiment of the formation of the wave-like relief of the surface was explained.

Temperature and velocity fields of the gas-vapor flow near evaporating water droplets

Experimental studies of unsteady temperature and velocity fields of the gas-vapor flow in the immediate vicinity of evaporating water droplets were performed in the interests of developing high-temperature (over 300°С) gas-vapor-droplet applications. The formation time of virtually homogeneous (temperature variations of under 2–3°С) temperature fields of evaporating water droplets was established using Particle Image Velocimetry, Laser Induced Phosphorescence, Planar Laser Induced Fluorescence. We observed highly inhomogeneous and unsteady temperature and velocity fields of the gas-vapor mixture and determined the lateral and transversal dimensions of the aerodynamic and thermal traces of evaporating water droplets. The degree of impact of several parameters on the latter was evaluated: initial temperature (20–500 °C) and velocity of the incoming flow (0.5–5 m/s), as well as the initial droplet size (1–2 mm). The role of evaporation and convective heat exchange between a droplet and gases in forming of thermal and aerodynamic trace was analyzed. The heat flow density was found to vary nonlinearly with time (in the immediate vicinity of an evaporating droplet), and the impact of droplet trace temperature and velocity on the heat flow density was evaluated. Both advantages and limitations of the used techniques are established in terms of reliable plotting of unsteady temperature and velocity fields of gas-vapor mixture in the trace of evaporating water droplets, considering the heating lag of the latter.

Assessing a new velocity field in Greece towards a new semi-kinematic datum

Greece has always been an interesting study area for the geodesists due to its intense geodynamic behaviour. Especially, the exploitation of a modern geodetic velocity field using GNSS becomes a crucial requirement for geodetic purposes. The main aim of the present study is the assessment of a modern geodetic velocity model. This procedure is based on the unification of various reference frames, implemented from individual extended GNSS campaigns in different epochs with the effective involvement of the Hellenic Military Geographic Service. The implementation of geodetic velocities in the inhomogeneous Greek velocity field improves significantly the reliability of transformations, more than 60% with respect to the horizontal component, applying a vice versa time shift to a common epoch. We also emphasise the necessity to adopt a velocity model in order to process regional GNSS networks and therefore to establish a modern semi-kinematic reference frame in Greece.

An optimal geodetic dynamic reference frame realization for Greece: Methodology and application

On this study the theoretical and practical approach of an optimal geodetic referenceframe realization is described in detail. The Hellenic area due to its strong geodynamic behaviorcreates a rather inhomogeneous velocity field. This issue plays a critical role in stable and longtermgeodetic datum. The methodology of the Minimization of the Kinetic Energy Criterion(MKEC) is defined and analyzed on a permanent GNSS network consisting of 151 stations. Thesites velocities values were estimated from seven years continuously data. The applicationformula shows that the kinematic energy is minimized more than 60 percent in two differentapproaches for the International Terrestrial Reference Frame and the European TerrestrialReference System 1998 (ETRS89). In addition, the realization of the ETRS89 approach in Greeceis not offering any significant advantage. However, we proposed a new strategy, which minimizesthe total kinetic energy and it is found effective in local areas with strong geodynamic activity likethe Greece case.

Migration of a granule in a rapidly oscillating inhomogeneous velocity field of a viscous fluid

A closed system of equations for calculating the drift of an inertial heavy granule in an inhomogeneous velocity field of the carrier fluid is derived by the Krylov-Bogolyubov method of averaging over fast oscillations of velocity. Exact analytical solutions are presented and compared with the results of the numerical solution of the equations of granule motion. The influence of the granule inertia, acceleration of mass forces, and frequency of oscillations on the granule dynamics in an inhomogeneous oscillating velocity field of the fluid is studied.

Effective dispersion in conditioned transmissivity fields

We analyze the impact of conditioning to measurements of hydraulic transmissivity on the transport of a conservative solute. The effects of conditioning on solute transport are widely discussed in the literature, but most of the published works focuses on the reduction of the uncertainty in the prediction of the plume dispersion. In this study both ensemble and effective plume moments are considered for an instantaneous release of a solute through a linear source normal to the mean flow direction, by taking into account different sizes of the source. The analysis, involving a steady and spatially inhomogeneous velocity field, is developed by using the stochastic finite element method. Results show that conditioning reduces the ensemble moment in comparison with the unconditioned case, whereas the effective dispersion may increase because of the contribution of the spatial moments related to the lack of stationarity in the flow field. As the number of conditioning points increases, this effect increases and it is significant in both the longitudinal and transverse directions. Furthermore, we conclude that the moment derived from data collected in the field can be assessed by the conditioned second-order spatial moment only with a dense grid of measured data, and it is manifest for larger initial lengths of the plume. Nevertheless, it seems very likely that the actual dispersion of the plume may be underestimated in practical applications.

Effective orthorhombic anisotropic models for wavefield extrapolation

Wavefield extrapolation in orthorhombic anisotropic media incorporates complicated but realistic models, to reproduce wave propagation phenomena in the Earth's subsurface. Compared with the representations used for simpler symmetries, such as transversely isotropic or isotropic, orthorhombic models require an extended and more elaborated formulation that also involves more expensive computational processes. The acoustic assumption yields more efficient description of the orthorhombic wave equation that also provides a simplified representation for the orthorhombic dispersion relation. However, such representation is hampered by the sixth-order nature of the acoustic wave equation, as it also encompasses the contribution of shear waves. To reduce the computational cost of wavefield extrapolation in such media, I generate effective isotropic inhomogeneous models that are capable of reproducing the first-arrival kinematic aspects of the orthorhombic wavefield. First, in order to compute traveltimes in vertical orthorhombic media, I develop a stable, efficient and accurate algorithm based on the fast marching method. The derived orthorhombic acoustic dispersion relation, unlike the isotropic or transversely isotropic one, is represented by a sixth order polynomial equation that includes the fastest solution corresponding to outgoing P-waves in acoustic media. The effective velocity models are then computed by evaluating the traveltime gradients of the orthorhombic traveltime solution, which is done by explicitly solving the isotropic eikonal equation for the corresponding inhomogeneous isotropic velocity field. The inverted effective velocity fields are source dependent and produce equivalent first-arrival kinematic descriptions of wave propagation in orthorhombic media. I extrapolate wavefields in these isotropic effective velocity models using the more efficient isotropic operator, and the results compare well, especially kinematically, with those obtained from the more expensive anisotropic extrapolator.%%%%Wavefield extrapolation in orthorhombic anisotropic media incorporates complicated but realistic models, to reproduce wave propagation phenomena in the Earth's subsurface. Compared with the representations used for simpler symmetries, such as transversely isotropic or isotropic, orthorhombic models require an extended and more elaborated formulation that also involves more expensive computational processes. The acoustic assumption yields more efficient description of the orthorhombic wave equation that also provides a simplified representation for the orthorhombic dispersion relation. However, such representation is hampered by the sixth-order nature of the acoustic wave equation, as it also encompasses the contribution of shear waves. To reduce the computational cost of wavefield extrapolation in such media, I generate effective isotropic inhomogeneous models that are capable of reproducing the first-arrival kinematic aspects of the orthorhombic wavefield. First, in order to compute traveltimes in vertical…

Shallow-water gaseohydrothermal plume studies after massive eruption at Panarea, Aeolian Islands, Italy

Marine water dynamics in the near field of a massive gas eruption near Panarea (Aeolian Islands volcanic arc, SE Tyrrhenian Sea) is described. ADCP current-meters were deployed during the paroxysmal phase in 2002 and 2003 a few meters from the degassing vent, recording day-long time series. Datasets were sorted to remove errors and select good quality ensembles over the entire water column. Standard deviation of error velocity was considered a proxy for inhomogeneous velocity fields over beams. Time series intervals had been selected when the basic ADCP assumptions were fulfilled and random errors minimized. Backscatter data were also processed to identify bubbles in the water column with the aim of locating bubble-free ensembles. Reliable time series are selected combining these data. Two possible scenarios have been described: firstly, a highly dynamic situation with visible surface diverging rings of waves, entrainment on the lower part of the gas column, detrainment in the upper part and a stagnation line (SL) at mid depth where currents were close to zero and most of the gas bubbles spread laterally; secondly, a less dynamic situation with water entraining into the gas plume at all depths and no surface rings of diverging waves. Reasons for these different dynamics may be ascribed to changes in gas fluxes (one order of magnitude higher in 2002). Description of SL is important to quantify its position in the water column and timing for entrainment–detrainment, and it can be measured by ADCP and calculated from models.