On the Dynamics of Overshooting Convection in Spherical Shells: Effect of Density Stratification and Rotation

Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics, as this process may lead to mixing of chemical species and contribute to the transport of angular momentum and magnetic fields. We present a series of fully nonlinear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell that are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars that however have not been studied systematically together via global simulations. We demonstrate that the overshoot lengthscale is not simply a monotonic function of the density stratification in the convective region, but instead it depends on the ratio of the density stratifications in the two zones. Additionally, we find that the overshoot lengthscale decreases with decreasing Rossby number Ro and scales as Ro0.23 while it also depends on latitude with higher Rossby cases leading to a weaker latitudinal variation. We examine the mean flows arising due to rotation and find that they extend beyond the base of the convection zone into the stable region. Our findings may provide a better understanding of the dynamical interaction between stellar convective and radiative regions, and motivate future studies particularly related to the solar tachocline and the implications of its overlapping with the overshoot region.

Dissolved oxygen stratification in a small lake depending on water temperature and density and wind impact

The dissolved oxygen (DO) is one of the most important parameters in lakes ecosystem. Variability of DO in lakes is associated with the absorption of oxygen due to the decomposition of organic matter and chemical reactions and the release of oxygen as a result of photosynthesis. The DO concentration also depends on seasonal changes in water temperature and mixing regime. The aim of this work is to assess the influence of seasonal thermal and density stratification on the DO stratification in a small mesotrophic lake and to develop a regression DO model, with water temperature and density and characteristics of wind regime as independent variables. Long-term measurements of water temperature and DO in small Karelian Lake Vendyurskoe in 2007-2020 were used. At the stage of spring-summer heating, three periods are considered when the water column was in the state of homothermy (May 15-June 15), weak stratification (July 15-August 15), and strong stratification (July 15-August 15). The wind load (number of days with wind speed more than 3 m/s for each period) was analysed based on the weather station Petrozavodsk data. As a result of multiple regression analysis, taking into account the wind load, dependences of DO stratification on water temperature stratification (R2 = 0.51) and water density stratification (R2 = 0.61) are found. Obtained regression DO models can be used for solving various environmental tasks.

Adjustment of drilling slurry density during transportation by Platform Supply Vessels

Annotation – The issues of maintaining the drilling fluid density during its transportation from shore to the oil drilling platform by the Platform Supply Vessels were considered. The research was carried out on the vessel of 5650 deadweight tons, which made 2 ... 2.5 day voyages from port to drilling platform with possible waiting for direct approach to the platform for 1 ... 1.5 days. The drilling fluid was transported in four tanks of equal-size, arranged in pairs on each side of the vessel. It was found that during this period of time there is a latent stratification of drilling fluid along the depth of cargo tank, causing stratification of its density. The density of the drilling fluid was measured at depths corresponding to 10, 50 and 90 % of the total tank depth. The drilling fluid density stratification is defined as the relative density change in the upper (at 10 % depth) and lower (at 90 % depth) parts of the cargo tank. The hydrometer was used to measure density, allowing measurements in the range of 650 ... 1630 kg/m3 with an accuracy of 1 kg/m3 while controlling the temperature. Density measurements were taken at 6-hour intervals. It has been experimentally proved that during the 48-hour transportation of the drilling fluid with the density of 1295 kg/m3, the density value at the indicated depths is 1163 and 1524 kg/m3 respectively, while the density stratification reaches 31 %. It is offered to prevent the density stratification by creating a forced X-shaped circulation of the drilling fluid between cargo tanks which stand side by side. It is offered to regulate and maintain automatically the drilling fluid density in the range of 2 ... 7 % by using programmable controllers. When the density stratification reaches 7 %, the controller switches on the circulation pumps and circulates the drilling fluid between tanks which stand side by side. This increases the drilling fluid uniformity and helps to reduce the density stratification along the depth of the cargo tank. The process of additional circulation of the drilling fluid is rationally ensured until the density stratification reaches a value of 2 %. Fulfilment of the above-mentioned conditions will ensure that the drilling fluid maintains its operational properties and the energy consumption associated with the additional operation of the circulation pumps is minimised.

Core Eigenmodes and their Impact on the Earth’s Rotation

Changes in the Earth’s rotation are deeply connected to fluid dynamical processes in the outer core. This connection can be explored by studying the associated Earth eigenmodes with periods ranging from nearly diurnal to multi-decadal. It is essential to understand how the rotational and fluid core eigenmodes mutually interact, as well as their dependence on a host of diverse factors, such as magnetic effects, density stratification, fluid instabilities or turbulence. It is feasible to build detailed models including many of these features, and doing so will in turn allow us to extract more (indirect) information about the Earth’s interior. In this article, we present a review of some of the current models, the numerical techniques, their advantages and limitations and the challenges on the road ahead.

Development of a method for maintaining the performance of drilling fluids during transportation by Platform Supply Vessel

The object of research is the process of transportation of drilling fluid used for lubrication and cooling of drilling equipment of offshore oil production platforms. The subject of the study is the stratification of the density of the drilling fluid along the height of the cargo tank in which it is transported. The technology of transportation of drilling fluid on the Platform Supply Vessel is considered. A problematic point in ensuring this process is that during the transportation of drilling fluids, due to the action of gravitational forces on organic and inorganic compounds in their volume, there is a latent change in their dispersion over the volume of the fluid. This leads to the stratification and stratification of the fluid density along the height, as well as to the formation of sediments at the bottom of the cargo tanks, in which the drilling fluid is transported. The study is aimed at developing a technology that maintains a constant value of the density of the drilling fluid along the depth of the tank in which it is transported. The studies were carried out in the vessel system for transporting drilling fluid of a specialized marine vessel of the Platform Supply Vessel type with a deadweight of 5850 tons. It is experimentally established that for a transportation time of 6–36 hours, the density stratification of the drilling fluid is 3.04–32.04 %. As a method that ensures the minimum stratification of the density of the drilling fluid during its transportation, it is proposed to use an additional X-shaped circulation of the drilling fluid in the volume of adjacent cargo tanks. Studies have confirmed that the density stratification over a time period of 6–36 hours decreases to a range of 2.30–9.01 %. The complex use of additional X-shaped circulation and simultaneous air supply to the bottom of the cargo tank provides a density stratification value of 0.73–2.93 %. The proposed technology was tested on a specialized seagoing vessel of the Platform Supply Vessel type with a deadweight of 5850 tons and can be used on offshore vessels that ensure the operation of offshore oil production platforms.