We examined and simulated the consequences of the degradation of the litter and the moss–lichen layer after fire impact, which could affect the seasonal temperature of the soil and the depth of the seasonally thawed layer (STL) in the permafrost zone. According to the analysis of satellite imagery for 2000 to 2019, the fire-disturbed area in the region of interest amounted to 20%. The main aims of the study included quantitative evaluation of the variation range of summer temperature anomalies at fire-damaged plots, summarizing the statistical norm of the STL depending on natural conditions, and numerical simulation of the response of the STL. Using Terra and Aqua/MODIS imagery, we analyzed surface temperature (in bands of λ = 10.780–11.280 and 11.770–12.270 μm) coupled with the normalized difference vegetation index (NDVI) for non-disturbed and fire-damaged sites under the same natural conditions of larch forests in Central Siberia. Heat transfer, freezing and thawing processes were numerically simulated for two extreme cases of soil conditions: dry soil and water-saturated soil. The model was also applied to soil with non-homogeneous water content. As input parameters, we used data on the properties of cryogenic soils collected in larch forests (Larix gmelinii) in the flat-mountainous taiga region of the Evenkia (Central Siberia). For post-fire plots, surface temperature anomalies observed during summer months remained significant for more than 15–20 years after fire impact, while the NDVI values were restored to the statistical norm within 7–10 years of the fire. According to the results of numerical simulation, the thickness of the STL could show a 30–50% increase compared to the statistical norm. In the first approximation, we showed the annual soil temperature dynamics at various depths in disturbed and non-disturbed plots.

We investigated changes in the temperature regime of post-fire and post-technogenic cryogenic soils of Central Siberia using remote sensing data and results of numerical simulation. We have selected the time series of satellite data for two variants of plots with disturbed vegetation and on-ground cover: natural ecosystems of post-fire plots and post-technogenic plots with reclamation as well as dumps without reclamation. Surface thermal anomalies and temperature in soil horizons were evaluated from remote data and numerical simulation and compared with summarized experimental data. We estimated the influence of soil profile disturbances on the temperature anomalies forming on the surface and in soil horizons based on the results of heat transfer modeling in the soil profile. According to remote sensing data, within 20 years, the thermal insulation properties of the vegetation cover restore in the post-fire areas, and the relative temperature anomaly reaches the level of background values. In post-technogenic plots, conditions are more “contrast” comparing to the background, and the process of the thermal regime restoration takes a longer time (>60 years). Forming “neo-technogenic ecosystems” are distinct in special thermal regimes of soils that differ from the background ones both in reclamated and in non-reclamated plots. An assumption was made of the changes in the moisture content regime as the main factor causing the long-term existence of thermal anomalies in the upper soil horizons of disturbed plots. In addition, we discussed the formation of transition zones (“ecotones”) along the periphery of the disturbed plots due to horizontal heat transfer.

A mathematical model developed to describe the interaction of a thermal convective flow with a moving body is presented. The model was implemented within the framework of the SigmaFlow computational software package, which is based on computational fluid dynamics methods. The thermal convective flow is described by the Navier–Stokes equations in the Boussinesq approximation, and the moving body model is implemented using the immersed boundary method. The article presents the results of verification of the proposed mathematical model by solving the following test problems: unsteady laminar flow around the cylinder; natural convection in the channel between two cylinders; developed convective flow in a closed rectangular area with a fixed plate. The results of a numerical study of the plate motion in a thermal convective flow in a closed volume (cuvette) with hot lower and cold upper walls are presented. The calculations showed that the moving plate has an impact on the dynamics of formation of large-scale cells, the local distribution of heat flux density on the lower wall and the integral heat flux. In particular, they revealed a local decrease in the heat flux under the plate, an increase in the number of large vortices in the cuvette and the destruction of the horizontal temperature gradient observed in the case of a fixed plate. In addition, for a fixed plate, the value of heat flux under it depends on its position, and in the case of a moving plate – on the position and direction of its movement. A qualitative comparison of calculations for two different Rayleigh numbers with the experimental data obtained at the Institute of Continuous Medium Mechanics of the Ural Branch of the Russian Academy of Sciences showed that the behavior of the plate is governed by similar regularities. Представлена математическая модель, предназначенная для описания взаимодействия свободно-конвективного потока с подвижным телом. Модель реализована в рамках расчетного программного комплекса SigmaFlow, основанного на методах вычислительной гидродинамики. Свободно-конвективное течение описывается уравнениями Навье–Стокса в приближении Буссинеска, а модель подвижного тела реализована с помощью метода погруженных границ. В статье приведены результаты верификации предложенной математической модели на следующих тестовых задачах: нестационарное ламинарное обтекание цилиндра; естественная конвекция в канале между двумя цилиндрами; развитое конвективное течение в замкнутой прямоугольной области с неподвижной пластиной. Представлены результаты численного исследования движения пластины в свободно-конвективном потоке в замкнутом объеме (кювете) с горячей нижней и холодной верхней стенками. В результате расчетов обнаружено влияние подвижной пластины на динамику формирования крупномасштабных ячеек, на локальное распределение плотности теплового потока на нижней стенке и интегральный тепловой поток. В частности, выявлено локальное уменьшение теплового потока под пластиной, увеличение числа крупных вихрей в кювете и разрушение горизонтального градиента температуры, который наблюдается в случае закрепленной пластины. Кроме этого, для закрепленной пластины величина теплового потока под ней зависит от ее положения, а в случае подвижной пластины – от положения и направления ее движения. Качественное сравнение расчетов для двух разных чисел Релея c экспериментальными данными, полученными в Институте механики сплошных сред УрО РАН, показало, что поведение пластины имеет схожие закономерности.

A comparison between the methods of solving the radiative heat-transfer equation (finite-volume and discrete-ordinate methods and diffusion approximation) is carried out. The suitability of any of these methods for various problems is considered.

The two-dimensional motions of a disc submerged at a fixed depth but floating freely in an extended layer of liquid heated by a radiation flux falling from above are studied. The motion of a body in a convective layer depends on a number of factors such as the size of the body, the geometry of the cavity and the physical properties of the fluid and the body. The main focus of this work is to reveal the role of the degree of homogeneity of the distribution of heating of the underlying surface by incident radiation. Numerical simulations allowed us not only to describe the motion of the disc, but also to show the full three-dimensional structure of the convective flow for different regimes. Verification laboratory experiments confirmed the adequacy of the numerical simulations and clarified the role of the optical properties of the floating body. The simulation results showed the sensitivity of the system to the homogeneity of the radiation flux. A change in the radiation distribution leads to a change in the structure of the large-scale circulation. In the case of homogeneous distribution, a toroidal vortex is formed with the liquid sinking in the central part, and in the case of inhomogeneous distribution with maximum heating in the centre, on the contrary, a vortex appeared with the liquid rising in the centre. Since the motion of a floating body is determined by large-scale flows, their rearrangement leads to qualitative changes in the spatial and temporal characteristics of the disc motion. The experiments with discs of different radiation transparency demonstrated that, when the transparency coefficient reaches 0.4, the wandering of the disc stops.