Convective Vortices Research Articles

Numerical investigation of the thermocapillary migration of a water droplet in a microchannel by applying a heat source

The migration of a small droplet has been developed during the last two decades due to its applications in industry and high technology such as MEMS and NEMS devices, Lap-On-a- chip, transportation of fluids and so on. There have many studies on this topic in which the energy source as a driving force for the moving of a droplet is quite a difference like heating, magnetics, pressure, electric, laser, and so on. In this study, the numerical computation is used to investigate the transient thermocapillary migration of a water droplet in a micro-channel under the effect of heating source. For tracking the evolution of the free interface between two immiscible fluids, we employed the finite element method with the two-phase level set technique to solve the Navier-Stokes equations and continuity equation coupled with the energy equation. Both the upper wall and the bottom wall of the microchannel are set to be ambient temperature. 40mW heat source is placed at a distance of 1 mm from the initial position of a water droplet. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet, and the thermocapillary on the receding side is smaller than that on the advancing side. The temperature gradient inside the droplet increases quickly at the initial times and then decreases versus time. Therefore, the actuation velocity of the water droplet first increases significantly and then decreases continuously. Furthermore, the results also indicate that the dynamic contact angle is strongly affected by the oil flow motion and the net thermocapillary momentum inside the droplet. The advancing contact angle is always larger than the receding contact angle during the actuation process.

Generation of strong magnetic fields in a nascent neutron star accounting for the chiral magnetic effect

We propose the mean field dynamo model for the generation of strongest magnetic fields, $B\sim 10^{15}\,{\rm G}$, in a neutron star (NS) accounting for the chiral magnetic effect (CME) driven by the shock in a supernova (SN) progenitor of that NS. The temperature jump at a narrow shock front, where an initial magnetic field existing in inflowing matter rises sharply, is the source of the CME that prevails significantly the erasure of the CME due to the spin-flip through Coulomb collisions in plasma. The growth of the magnetic field just behind the shock given by the instability term $\nabla\times (\alpha {\bf B})$ in induction equation, stops after a successful SN explosion that throws out the mantle of a protoneutron star. As a result, such an explosion interrupts the transfer of strongly magnetized plasma from the shock onto NS surface and leads to the saturation of the magnetic field. Assuming the rigid protostar rotation, we employ the mean field dynamo, which is similar to the $\alpha^2$-dynamo known in the standard magnetohydrodynamics (MHD). The novelty of our model is that $\alpha^2$-dynamo is based on concepts of particle physics, applied in MHD, rather than by a mirror asymmetry of convective vortices in the rotating convection.

Application of High-Resolution IR Thermography to Study the Surface of Aqueous Solutions

The application of high-resolution IR thermography for qualitative and quantitative determination of impurity concentrations in aqueous solutions is considered. Especially efficient turned out to be comparative analysis and analysis of contaminations: a film arises on the water surface in the presence of impurities, which can be seen well on an IR imager because of the difference in the surface temperatures and structures of convective vortices. It should be especially noted that the method makes it possible to determine low concentrations of impurities of neutral molecules, a case where no changes in the solution conductivity can be recorded using standard conductometric measurements. The problems related to the film simulation and influence of low-concentration impurities on the surface and volume properties of solution are considered.

Mathematical modeling of thermal processes during "cassette" crystallization of chalcogenides

A new, relatively simple and highly efficient modification of the directional melt crystallization method in the form of a multi-cassette process has been considered. This study is based on Russian Patents and technological studies conducted at National Research and Technological University MISiS. As a result, mathematical models of a multi-cassette method have been developed for 3D radiation and conduction analysis of thermal processes in the entire volume of the heating unit and 2D analysis of convection and conduction heat exchange in a separate cassette. Parameters have been calculated on the basis of these mathematical models for clarifying the effect of heating unit component arrangement and dimensions on the formation of thermal fields in cassette units, the effect of vertical homogeneity of heat supply to the cassette unit and heating power reduction rate during crystallization on the shape of the crystallization front, as well as the effect of small asymmetry in cassette design and violation of cassette bottom cooling homogeneity on convection and asymmetrical heat transfer. Application of the conductive and radiative heat exchange model to the entire heating unit has allowed us to calculate process parameters on the basis of which we have analyzed the effect of heating unit components, their arrangement and temperature on the heat exchange conditions at the cassette unit boundaries. Application of the convective and conductive model to one growth cassette has shown that asymmetrical design and boundary thermal conditions as well as unstable vertical temperature gradient lead to the formation of convection vortices and substantial crystallization front deviation from planar shape. Calculations on the basis of the convective mass exchange model have shown that an increase in the crystallization rate by one order of magnitude greatly increases the tellurium flow into the crystal thus substantially altering the melt composition in the vicinity of the crystallization front and hence serving as a potential origin of dendrite growth. The authenticity of the calculation results has been verified in a number of tests aimed at analyzing the effect of heat and mass transport on crystallization front shape for cassette cooling rates that are typical of polycrystalline bismuth telluride growth processes.

Spatiotemporal Evolution of the Microphysical and Thermodynamic Characteristics of the 20 June 2015 PECAN MCS

Abstract This study examines microphysical and thermodynamic characteristics of the 20 June 2015 mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) experiment, specifically within the transition zone (TZ), enhanced stratiform rain region (ESR), anvil region, melting layer (ML), and the rear inflow jet (RIJ). Analyses are developed from airborne optical array probe data and multiple-Doppler wind and reflectivity syntheses using data from the airborne NOAA Tail Doppler Radar (TDR) and ground-based Weather Surveillance Radar-1988 Doppler (WSR-88D) radars. Seven spiral ascents/descents of the NOAA P-3 aircraft were executed within various regions of the 20 June MCS. Aggregation modified by sublimation was observed in each MCS region, regardless of whether the sampling was within the RIJ. Sustained sublimation and evaporation of precipitation in subsaturated layers led to a trend of downward moistening across the ESR spirals, with greater degrees of subsaturation maintained when in the vicinity of the descending RIJ. In all cases where melting was observed, the ML acted as a prominent thermodynamic boundary, with differing rates of change in temperature and relative humidity above and below the ML. Two spiral profiles coincident with the rear inflow notch provided unique observations within the TZ and were interpreted in the context of similar observations from the 29 June 2003 Bow Echo and Mesoscale Convective Vortex Experiment MCS. There, sublimation cooling and enhanced descent within the RIJ allowed ice particles to survive to temperatures as warm as +6.8°C before completely sublimating/evaporating.

Onset of Cavity Oscillation From Transverse to Longitudinal Mode in a Supersonic Flow

Abstract Experiments are carried out in supersonic flow past cavities (L/D = 1–3) to comprehend cavity oscillation from transverse to longitudinal mode. To characterize this, time-resolved schlieren images and unsteady pressure measurements are carried out. For L/D = 1, the cavity oscillation is in the transverse direction which is characterized by a single vortex along with low frequency oscillations. In the present studies, for the given length and depth, the L/D = 2 cavity shows both transverse and longitudinal mode of oscillations. This dual nature is ascertained through high-speed schlieren images and large rise in the fluctuating pressure. For L/D = 3, the cavity oscillation is in the longitudinal direction accompanied by low values of fluctuating pressure component and number of discrete modes/tones. Hence, it is believed that the transverse to longitudinal mode of cavity oscillation in the present studies occurs between L/D = 2 and 3. These cavities are driven by a thick shear layer where the vortex convection is found to be nonlinear. The comparison of dominant modes/tones with modified Rossiter and Handa relations indicates the necessity to develop a suitable model for transitional cavities. In-depth analyzes in terms of cross-correlation as well as wavelet transform divulge the basis that generates cavity modes/tones and mode switching phenomena, respectively.

Vortex elongation in outer streaming flows

We study the secondary time-averaged flow (streaming) generated by an oscillating cylinder immersed within a fluid, under high amplitude forcing so that inertial effects are significant. This streaming is decomposed into a viscous boundary layer flow where vorticity is created, and an outer flow of larger size. We operate under conditions of relatively low viscosity, so that the boundary layer is smaller than the object diameter. While for low Keulegan-Carpenter (KC) number (small enough amplitude), the size of the outer flow is typically that of the object, here we show that at large enough forcing, the outer flow stretches along the direction of the vibration by up to 8 times, while the flow still keeps its axial symmetry. We quantify the elongation through PIV measurements under an unprecedented range of frequency and amplitude, so that the streaming Reynolds number reaches values much larger than unity. The absence of significant unsteady component of vorticity outside the viscous boundary layer - and the fact that the length of elongation scales well with the streaming Reynolds number - suggest that the stretching should be due to the convection of stationary vorticity by the streaming flow itself.

Dust Devils on Titan

AbstractConditions on Saturn's moon Titan suggest that dust devils, which are convective, dust‐laden plumes, may be active. Although the exact nature of dust on Titan is unclear, previous observations confirm an active aeolian cycle, and dust devils may play an important role in Titan's aeolian cycle, possibly contributing to regional transport of dust and even production of sand grains. The Dragonfly mission to Titan will document dust devil and convective vortex activity and thereby provide a new window into these features, and our analysis shows that associated winds are likely to be modest and pose no hazard to the mission.

사각 채널 내의 볼텍스 유동에 관한 실험적 연구(2)

Mixed convective vortex flow in the 3-D horizontal rectangular channel filled with high viscous fluid is investigated for various aspect ratio experimentally. The particle image velocimetry technique with thermo-sensitive liquid crystal tracers (PIT) is used for visualizing and analysis. This method allows simultaneous measurement of velocity and temperature. Quantitative data of temperature and velocity are obtained by applying the color-image processing to a visualized image, and neural network is applied to the color-to-temperature calibration. In this study, the working fluid is silicon oil (Pr = 909), the aspect ratio (channel width to heigh) is varied from 2 to 6 at constant Reynolds number (Re = 4×10<SUP>-3</SUP>). From the present study, the temperature and velocity of mixed convective vortex flow can be visualized simultaneously, and it is found that the longitudinal roll number in the channel is increased with increasing aspect ratio, and the experimental results agree well with numerical results.

Initial results from the InSight mission on Mars

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indicate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander. Geophysical and meteorological measurements by NASA’s InSight lander on Mars reveal a planet that is seismically active and provide information about the interior, surface and atmospheric workings of Mars.

Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data

Mars’s seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earth’s microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the hammering of InSight’s Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected as of September 2019. From receiver function analysis, we infer that the uppermost 8–11 km of the crust is highly altered and/or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles. The crust beneath the InSight lander on Mars is altered or fractured to 8–11 km depth and may bear volatiles, according to an analysis of seismic noise and wave scattering recorded by InSight’s seismometer.

Simple Particle Model for Low-Density Granular Flow Interacting with Ambient Fluid

To understand the time evolutions of frontal speed and shape in a low-density granular flow, we propose a simple particle model. This model solves the equation of motion for each particle and simulates the time evolution of low-density granular flow. Spherical particles constituting a low-density granular flow slide on a slope at a steeper angle than the angle of repose. The particle motion is determined based on three forces: gravity as the driving force, repulsive force due to particle collision, and drag force due to the particle interaction through the ambient fluid. Two-dimensional numerical simulations of this model are conducted on the slope: the x–y plane parallel to the slope and the x–z plane perpendicular to the slope. In the x–y plane, particles aggregate at the moving front of the granular flow, and subsequently, flow instability occurs as a wavy pattern. This flow pattern is caused by the interparticle interaction arising from the drag force. Additionally, a vortex convection of particles is formed inside the aggregations. Simultaneously, particle aggregation is also found at the moving front of the granular flow in the x–z plane. The aggregation resembles a head–tail structure, where the frontal angle against the slope approaches 60 ∘ from a larger angle as time progresses. Comparing the numerical result by varying the particle size reveals that the qualitative dynamics of the granular flow are independent of particle size. Although the model is not realistic, our study presents a new particle-based approach that elucidates the dynamics of low-density granular flow.

Steady and unsteady numerical investigations of laminar fluid flow and heat transfer in a 180∘ bend with bypass

The present study investigates the fluid flow and heat transfer characteristics in a 180∘ bend domain with a bypass in the divider section using an in-house code based on Streamline Upwind Petrov-Galerkin Finite Element Method. The ratio of the inlet height to outlet height (IOR), location of the bypass (Lbyp) and Reynolds number (Re) are the parameters considered to assess the effects of heat transfer in a 180∘ bend. Flow in three IOR geometries i.e., IOR 1:2, 1:1, 2:1 with three Lbyp (−7, −5, −3) and Re in the range of 100–900 is investigated. IOR and Lbyp significantly affect the flow transitions from steady-state to periodic unsteady and finally to chaotic unsteady state. In this study, it is established that the IOR-bypass combination augments the heat transfer and decreases the pressure drop, which has been quantified using Thermal Performance Factor (TPF) and Nusselt number distribution (Nu). The increase in Nu and TPF for IOR 1:2 and 2:1 domains is higher than IOR 1:1 domains. A three-dimensional unsteady state investigation at Re=300 is also performed for IOR 1:2 with Lbyp=−7 to understand the convection of 3D vortices in the span-wise direction and its influence on heat transfer. From the 3D study, it is concluded that though the vortex shedding in the streamwise direction is similar to the 2D case, the spatial arrangement of 3D vortices along the span-wise direction is irregular at any given instant, which results in variable temperature and Nu distribution in the domain.

Numerical analysis of rectangular fins in a PCM for low-grade heat harnessing

Harnessing waste thermal energy from greywater (GW) in non-industrial buildings is becoming necessary to reduce energy demands, make heating/cooling technologies more efficient and to increase the share of renewables in the consumption. Harnessing this low grade energy to heat incoming cold water (CW) linked with phase change materials (PCMs) would decouple demand and supply along with integrating heat recovery with storage in a single unit, unlike technologies of the past.Radially installed rectangular copper fins around the GW and CW pipes of such heat exchanger (HE) enhance the thermal conductivity of the PCM which is the biggest obstacle in this high-impulse application, with the flow rate being high only for a short duration of time. Initially an experimental test rig is used to validate a numerical model as the basis of a sensitivity analysis to find the optimum geometric parameters of the finned HE for this application. A 40 × 90 mm fin with a 10 mm pitch provides complete phase change for both melting and freezing in the constrained time duration of 900s. Compared to a non-finned geometry this optimized fin configuration enhances the effective thermal conductivity of the PCM by a factor of 1.38 for melting and 4.75 for freezing. Although the development of buoyancy induced natural convection vortices are inhibited by fins the eventual heat transfer is enhanced due to a lower overall thermal resistance compared to a non-finned configuration. The GW to CW energy transfer efficiency is 72.4% with higher fluid flow temperature increments, compared to only 47.3% for a non-finned version.

Nonmodal nonlinear route of transition to two-dimensional turbulence

This paper provides a solution to the problem of flow transition by elucidating the interactions of nonmodal components of the spectrum of response field for zero pressure gradient boundary layer excited by a free stream convected vortex.

Acoustics-driven vortex dynamics in channel branches with round intersections: Flow mode transition and three-dimensionality

A combined experimental and large eddy simulation study was conducted to investigate acoustics-driven vortex dynamics inside channel branches with round intersections. The underlying flow mode transition and intensified flow three-dimensionality, which are closely related to the Coanda effect at round intersections, were comprehensively demonstrated. A dynamic pressure transducer array was first used to establish the relationship of the excited acoustic pressure pulsations to the channel-branch intersection radius (r) and the mainstream Reynolds number. In complementary simulations, three configurations with r/D = 0, 0.2, and 0.4 (where D is the short edge of the side-branch) were selected for demonstration. First, the simulated results were well validated in terms of acoustic pressure pulsations and phase-dependent flow fields. Subsequent analysis of the time-averaged and statistical flow characteristics revealed the existence of significantly intensified flow fluctuations inside the round channel branches having r/D = 0.2 and 0.4. Next, the proper orthogonal decomposition analysis was conducted to extract the dominant flow modes and to identify the energy transition from the streamwise vortex-shedding mode to vertical flow-oscillation mode. To this end, the influence of flow-mode transition on the phase-dependent flow fields was further investigated. The intensified branch-flow streaks resulted in a channel flow transition from synchronous convection of co-rotating vortex pairs into alternating convection of a single large-scale vortex, yielding a stronger flapping motion of the mainstream flow. Finally, the intensified flow three-dimensionality, presented by the essential spanwise Reynolds shear stresses inside the round channel branches, was found to relate to the strong turbulent mixing process caused by the flapping mainstream flow and the vertical branch flow oscillation. These findings are of great significance for industrial pipeline design and optimization.

Mathematical modeling the thermal processes during cassette crystallization of chalcogenides

An original modification of the directed crystallization method is considered as a multi-cassette process, which has comparative simplicity and high productivity. The basis of this research was domestic patents and technological research carried out at the National University of Science and Technology MISIS. As a result, mathematical models of the multi-cassette method were developed that allow both a three-dimensional radiative — conductive analysis of thermal processes in the entire volume of the hot zone and a two-dimensional analysis of convective — conductive heat transfer in a separate cassette. The parametric calculations carried out on their basis were aimed to the identifying an influence of locations and sizes of the hot zone components to a thermal field in the cassette unit; the establishing an influence of vertical heat supply equability to the cassette unit and an influence of heating power decrease during the plate crystallization, as well as to the determining an influence of small cassette design distortions and violation of cooling uniformity in its bottom part on the occurrence of convection and asymmetrical thermal field. By means of the conductive-radiative heat transfer model for the entire hot zone there were carried out parametric calculations and it was analyzed an influence of hot zone components (their locations and temperatures) on the heat exchange conditions at the cassette unit boundaries. By means of the conductive-convective model for a cassette it was determined that the boundary thermal conditions asymmetry, as well as an unstable vertical temperature gradient, result in the convective vortices and a significant deviation of the crystallization front from a flat shape. The calculations with using the convective mass transfer model showed that an increase of the crystallization rate by an order significantly increases a tellurium flux into the crystal, thereby substantially changing a melt composition near crystallization front and, thus, being a potential cause of dendritic growth. The reliability of the calculation results was checked on a number of tests, in which the influence of heat and mass transfer on the crystallization front shape was analyzed at cassette cooling rates corresponding to the growth processes of bismuth telluride polycrystals.

A stable and non-dissipative kinetic energy and entropy preserving (KEEP) scheme for non-conforming block boundaries on Cartesian grids

A stable and non-dissipative numerical scheme for Cartesian methods is proposed. The proposed scheme is based on the second-order kinetic energy and entropy preserving scheme, which the authors proposed recently, and conservation is satisfied at non-conforming boundaries where the grid refinement level is different across the computational block boundaries. In the proposed scheme, ghost cells are used at computational block boundaries for efficient parallel computation, and conservation is satisfied by assigning appropriate values to the ghost cells. Also, although the five conservative variables (i.e., mass, momentum, and total energy) are typically transferred from computational cells to corresponding ghost cells at computational block boundaries, the proposed scheme transfers two more conservative variables to satisfy conservation at non-conforming block boundaries. In a vortex convection test, the convergence rates of the L2- and L∞-error norms are examined, and the proposed scheme preserves the second-order of accuracy without inducing destructive errors at non-conforming boundaries. In an inviscid Taylor-Green vortex simulation, the proposed scheme demonstrates superior numerical stability by preserving kinetic energy and entropy. Also, the proposed scheme performs more stable computations on a non-conforming computational grid than a typical kinetic energy preserving scheme calculated on a uniform computational grid.

Experimental Study of the Convective Motions by the PIV Technique within an Evaporating Liquid Layer into the Gas Flow

We present the experimental study of convection in a horizontal liquid layer (ethanol, 3-mm deep), evaporating from a localized surface (10 × 10 mm2) into the gas flow (air). Visualization and measurements of the two-component velocity field in the liquid layer has been carried out with the Particle Image Velocimetry (PIV) technique. In our experiments we consider a novel configuration in which the gas-liquid interface is maintained in the flat position in the confined square area and the volatile liquid evaporates from the planar surface into the gas flowing along the surface. We also consider the effect of the gas velocity (0.0138–0.138 m/s) and the gas and the liquid temperature (20 °C - 50 °C) on the convective flow structure within the liquid layer. It is shown that the gas velocity and both, the gas and the liquid temperatures induce significant changes in the convective flow structure. We give the first experimental proof of the phenomenon that the motion of the gas-liquid interface goes along the counter-current direction to the gas flow as theoretically predicted. The analysis of the experimental data shows that the influence of the gas flow velocity on the Marangoni convection at the maximum temperature (50 °C) is significantly reduced owing to the growth of the diffusion resistance for the gas flow under a strong evaporation from the interface. As a result, it leads to the low surface temperature gradient, which decreases thermocapillary stresses and the circulation velocity of the first (thermocapillary) vortex within the fluid layer. Further, we observe that the disappearance of the second convective vortex circulating in the same direction with the gas flow. The governing factor, determining the structure of convective flows within the liquid is the thermocapillary effect due to the intensive evaporation provided that the cooling and the temperature distribution are uniform on the gas-liquid interface.

Tangential winds of a vortex system in a planetary surface layer

The planetary boundary layer (PBL) mediates interactions between the surface and free atmosphere. In Martian PBL, surface can force convective vortices leading to dust devils. We use the Navier–Stokes equations and the continuity equation to determine mean (with respect to time) tangential wind velocity in cylindrical co-ordinate system within the surface layer of a planetary atmosphere. We utilize Martian surface layer properties for theoretical derivation of our solution. However, our results remain valid for any planetary surface layer as long as all of our assumptions are valid. Our theoretical values of the tangential wind velocity lie well within the range of observed values. The derived equation represents the dependency of tangential velocity on both radial distances from the center of vortex, and the altitude. As we move further away from the vortex center, the effect of vortex becomes non-significant, and velocities start following the standard logarithmic profile. Due to dependency of tangential wind velocity on altitude, the tangential velocity increases as we move higher up in the vortex system. At 100 m altitude, for an order of magnitude increase in the radial distance, the mean tangential wind velocity drops by about a factor of 1.5 in magnitude.