Thermosuperrepellency of a hot substrate caused by vapour percolation

Drop rebound after collision with a very hot substrate is usually attributed to the Leidenfrost effect, characterized by intensive film boiling in a thin vapour gap between the liquid and substrate. Similarly, drop impact onto a cold superhydrophobic substrate leads to a complete drop rebound, despite partial wetting of the substrate. Here we study the repellent properties of hot smooth hydrophilic substrates in the nucleate boiling, non-Leidenfrost regime and discover that the thermally induced repellency is associated with vapour percolation on the substrate. The wetting structure in the presence of the percolating vapour rivulets is analogous to the Cassie-Baxter wetting mode, which is a necessary condition for the repellency in the isothermal case. The theoretical predictions for the threshold temperature for vapour percolation agree well with the experimental data for drop rebound and correspond to the minimum heat flux when spray cooling.

Viscous fluid flow between plane walls in presence of temperature gradient

В статье изучается медленный поток вязкой несжимаемой жидкости, ограниченной двумя параллельными плоскостями. Течение вызвано либо движением одной плоскости относительно другой, либо перепадом давления вдоль двух неподвижных плоскостей. Предполагается, что вязкость жидкости есть многочлен первой степени от температуры, а в рассматриваемом объеме поддерживается постоянный градиент этой величины. Это приводит к возмущению потока по сравнению с изотермическим случаем. Асимптотическими методами исследована зависимость возникающих искажений от типа потока и взаимной ориентации потока и градиента температуры. При этом перепад температуры на расстоянии, равном зазору между плоскостями, считается малым. Показано, что эти искажения могут вноситься как в скорость, так и в давление жидкости. В последнем случае при наличии в жидкости взвеси возможно возникновение дополнительной подъемной силы или силы тяги, действующей на инородные частицы. Slow flow of incompressible viscous fluid confined by two parallel planes is studied in the paper. The flow is caused either by motion of one plane with respect to another or by pressure drop along the planes. It is supposed that fluid viscosity is the first-order polynomial of temperature and that its gradient is constant in the entire domain under study. These two reasons cause the flow to be disturbed (compared with isothermal case). Dependence of these disturbances on the type of flow and on the orientation of temperature gradient with respect to the flow is investigated by asymptotic methods. Temperature drop on distances equal to the gap between planes is supposed to be small during the investigation. It is shown that both fluid velocity and pressure may be disturbed. In the last case additional lift or drag force may act on the alien particles suspended in the fluid (if it contains any).

Large-Eddy Simulation Analyses of Heated Urban Canyon Facades

Thermal convective flows are common phenomena in real urban canyons and strongly affect the mechanisms of pollutant removal from the canyon. The present contribution aims at investigating the complex interaction between inertial and thermal forces within the canyon, including the impacts on turbulent features and pollutant removal mechanisms. Large-eddy simulations reproduce infinitely long square canyons having isothermal and differently heated facades. A scalar source on the street mimics the pollutant released by traffic. The presence of heated facades triggers convective flows which generate an interaction region around the canyon-ambient interface, characterised by highly energetic turbulent fluxes and an increase of momentum and mass exchange. The presence of this region of high mixing facilitates the pollutant removal across the interface and decreases the urban canopy drag. The heating-up of upwind facade determines favourable convection that strengthens the primary internal vortex and decreases the pollutant concentration of the whole canyon by 49% compare to the isothermal case. The heating-up of the downwind facade produces adverse convection counteracting the wind-induced motion. Consequently, the primary vortex is less energetic and confined in the upper-canyon area, while a region of almost zero velocity and high pollution concentration (40% more than the isothermal case) appears at the pedestrian level. Finally, numerical analyses allow a definition of a local Richardson number based on in-canyon quantities only and a new formulation is proposed to characterise the thermo-dynamics regimes.

Vertical stellar density distribution in a non-isothermal galactic disc

ABSTRACT The vertical density distribution of stars in a galactic disc is traditionally obtained by assuming an isothermal vertical velocity dispersion of stars. Recent observations from SDSS, LAMOST, RAVE, Gaia etc. show that this dispersion increases with height from the mid-plane. Here, we study the dynamical effect of such non-isothermal dispersion on the self-consistent vertical density distribution for the thin disc stars in the Galaxy, obtained by solving together the Poisson equation and the equation of hydrostatic equilibrium. We find that in the non-isothermal case the mid-plane density is lower and the scale height is higher than the corresponding values for the isothermal distribution, due to higher vertical pressure, hence the distribution is vertically more extended. The change is $\sim \! 35 {{\ \rm per\ cent}}$ at the solar radius for a stars-alone disc for the typical observed linear gradient of +6.7 km s−1 kpc−1 and becomes even higher with increasing radii and increasing gradients explored. The distribution shows a wing at high z, in agreement with observations, and is fitted well by a double $\operatorname{sech}^{2}$, which could be mis-interpreted as the existence of a second, thicker disc, specially in external galaxies. We also consider a more realistic disc consisting of gravitationally coupled stars and gas in the field of dark matter halo. The results show the same trend but the effect of non-isothermal dispersion is reduced due to the opposite, constraining effect of the gas and halo gravity. Further, the non-isothermal dispersion lowers the theoretical estimate of the total mid-plane density i.e. Oort limit value, by 16 per cent.

Thermosuperrepellency of a hot substrate caused by vapour percolation

Drop rebound after collision with a very hot substrate is usually attributed to the Leidenfrost effect, [1-5] characterized by intensive film boiling in a thin vapour gap between the liquid and substrate. Similarly, drop impact onto a cold superhydrophobic substrate [6-8] leads to a complete drop rebound, despite partial wetting of the substrate. We have studied the repellent properties of hot smooth hydrophilic substrates in the nucleate boiling, non-Leidenfrost regime and discovered that the thermally induced repellency is associated with vapour percolation on the substrate. The wetting structure in the presence of the percolating vapour rivulets is analogous to the Cassie-Baxter wetting mode, [9] which is a necessary condition for the repellency in the isothermal case. The theoretical predictions for the threshold temperature for vapour percolation agree well with the experimental data for drop rebound and correspond to the minimum heat flux when spray cooling.

Emergence of mono-cluster flocking in the thermomechanical Cucker–Smale model under switching topologies

We study the emergent dynamics of the thermomechanical Cucker–Smale (TCS) model with switching network topologies. The TCS model is a generalized CS model with extra internal dynamical variable called “temperature” in which isothermal case exactly coincides with the CS model for flocking. In previous studies, emergent dynamics of the TCS model has been mostly restricted to some static network topologies such as complete graph, connected graph with positive in and out degrees at each node, and digraphs with spanning trees. In this paper, we consider switching network topologies with a spanning tree in a sequence of time-blocks, and present two sufficient frameworks leading to the asymptotic mono-cluster flocking in terms of initial data and system parameters. In the first framework in which the sizes of time-blocks are uniformly bounded by some positive constant, we show that temperature and velocity diameters tend to zero exponentially fast, and spatial diameter is uniformly bounded. In the second framework, we admit a situation in which the sizes of time-blocks may grow mildly by a logarithmic function. In latter framework, our temperature and velocity diameters tend to zero at least algebraically slow.

FIRE PROTECTION DISTANCES BETWEEN CONSTRUCTIONS OF DIFFERENT PURPOSE. A FIRE IN A WAREHOUSE-TYPE PREMISE. SERIES OF CALCULATIONS

Проведена серия расчетов по оценке температуры в помещении при пожаре в режиме, регулируемом вентиляцией, поскольку при этом достигается максимальное значение температуры. Рассматривается предельный случай теплообмена со стенами помещения - изотермический. Проанализирован случай пожара в зданиях типа складских и стоянок автомобилей. There is performed the series of calculations to estimate the temperature in premise during a fire in ventilation-controlled mode, because the maximum temperature achieves in this case. There is considered the extreme case of heat exchange with the walls of the room - isothermal case. The case of fire in buildings such as warehouses and car parking is examined.

The Generalized Thermodynamical Beltrami – Michell Tensorial Equations for the general Thermodynamical State of the Hooke Body: معادلات بيلترامي- ميشيل التنسورية الترموديناميكية المعممة لأجل الحالة الترموديناميكية العامة لجسم هوك

This paper concerns the mathematical linear model of the elastic, homogeneous and isotropic body, with no considerable structure and with infinitesimal elastic strains, subjected to Thermal effects, in the frame of coupled thermoelectrodynamics; discussed firstly by Hooke (in the isothermal case), and shortly called (H). In this paper, firstly we introduce the invariable tensorial traditional and Lame descriptions of the coupled dynamic, thermoelastic, homogeneous and isotropic Hooke body, which initial configuration forms a simply-connected region in the three dimensional euclidean manifold. The news of this paper consists in deriving the invariable tensorial, generalized Beltrami – Michell stress-temperature equations for the (H) thermoelastic body (in the more general case than the thermal stress state), which initial configuration forms a simply-connected region in the three dimensional euclidean manifold. Finally, we end the paper by suggesting the problem for discussing, in addition to another open problem.

General model of vertical distribution of stars in the Milky Way using complete Jeans equations

ABSTRACT The self-consistent vertical density distribution in a thin, isothermal disc is typically given by a sech2 law, as shown in the classic work by Spitzer. This is obtained assuming that the radial and vertical motions are decoupled and only the vertical term is used in the Poisson equation. We argue that in the region of low density as in the outer disc this treatment is no longer valid. We develop a general, complete model that includes both radial and vertical terms in the Poisson equation and write these in terms of the full radial and vertical Jeans equations which take account of the non-flat observed rotation curve, the random motions, and the cross term that indicates the tilted stellar velocity ellipsoid. We apply it to the Milky Way and show that these additional effects change the resulting density distribution significantly, such that the mid-plane density is higher and the disc thickness (HWHM) is lower by 30–40 per cent in the outer Galaxy. Further, the vertical distribution is no longer given as a sech2 function even for an isothermal case. These predicted differences are now within the verification limit of new, high-resolution data for example from Gaia and hence could be confirmed.