Water flooding technology is an important measure to enhance oil recovery in oilfields. Understanding the pore-scale flow mechanism in the water flooding process is of great significance for the optimization of water flooding development schemes. Viscous action and capillarity are crucial factors in the determination of the oil recovery rate of water flooding. In this paper, a direct numerical simulation (DNS) method based on a Navier–Stokes equation and a volume of fluid (VOF) method is employed to investigate the dynamic behavior of the oil–water flow in the pore structure of a low-permeability sandstone reservoir in depth, and the influencing mechanism of viscous action and capillarity on the oil–water flow is explored. The results show that the inhomogeneity variation of viscous action resulted from the viscosity difference of oil and water, and the complex pore-scale oil–water two-phase flow dynamic behaviors exhibited by capillarity play a decisive role in determining the spatial sweep region and the final oil recovery rate. The larger the viscosity ratio is, the stronger the dynamic inhomogeneity will be as the displacement process proceeds, and the greater the difference in distribution of the volumetric flow rate in different channels, which will lead to the formation of a growing viscous fingering phenomenon, thus lowering the oil recovery rate. Under the same viscosity ratio, the absolute viscosity of the oil and water will also have an essential impact on the oil recovery rate by adjusting the relative importance between viscous action and capillarity. Capillarity is the direct cause of the rapid change of the flow velocity, the flow path diversion, and the formation of residual oil in the pore space. Furthermore, influenced by the wettability of the channel and the pore structure’s characteristics, the pore-scale behaviors of capillary force—including the capillary barrier induced by the abrupt change of pore channel positions, the inhibiting effect of capillary imbibition on the flow of parallel channels, and the blockage effect induced by the newly formed oil–water interface—play a vital role in determining the pore-scale oil–water flow dynamics, and influence the final oil recovery rate of the water flooding.

Water flooding technology is widely used to improve oil recovery efficiency in oilfields. The capillary barrier effect induced by the complex pore structures in the reservoir rocks is a crucial reason for the trapping of a great deal of residual oil in oil reservoirs after water flooding. However, the formation condition along with the effect on the recovery rate of the capillary barrier under different wettability conditions should be investigated further. To bridge the gap between the microscopic mechanism of the capillary barrier effect and the macroscopic mechanism of oil displacement efficiency, a simple conceptual capillary model is constructed to obtain the formation conditions of the capillary barrier using the analysis method, and its influence on macroscopic oil displacement efficiency in the porous media model with an opening angle of 45° is systematically investigated in this study using direct numerical simulations (DNS) coupled with the volume of fluid method. The results showed that the capillary barrier effect plays a significant role in the formation of the residual oil in the reservoir rock and the contact angle and the opening angle are the primary factors for the formation of the capillary barrier. The capillary force is the driving force when the oil–water interface advances in the throat channel under water-wet conditions, while the capillary force hinders the movement of oil–water movement when the liquid flows out of the throat channel and when θ + β > 90^o. Furthermore, the highest oil displacement efficiency is achieved at the intermediate capillary number and in the case that the minimum conditions of occurrence of the capillary barrier phenomenon are satisfied. This is of great significance for controlling the optimized contact angle to further enhance the oil recovery rate of current oil reservoirs using waterflooding technology.

Trees have a significant impact on the airflow and pollutant diffusion in the street canyon and are directly related to the comfort and health of residents. In this paper, OpenFOAM is used for simulating the airflow and pollutant diffusion in the street canyon at different height–width ratios and tree layouts. Different from the drag source model in the previous numerical simulation, this study focuses on the characterization of the blocking effect of tree branches on airflow by using more precise and real tree models. It is found that the airflow is blocked by the tree branches in the canopy, resulting in slower airflow and varying velocity direction; the air flows in the pore area between trees more easily, and the vortex centers are different in cases where the street canyon shape and tree layout are different. Low-velocity airflow distributes around and between two tree canopies, especially under the influence of two trees with different spacing. At the height of the pedestrian, the tree branches change the vortex structure of airflow, and thereby high pollutant concentration distribution on both sides of the bottom of the leeward side of the street canyon changes constantly. In the street canyon, the small change in tree spacing has a very limited influence on the pollutant concentration. The street canyon has the lowest average pollutant concentration at the largest y-axis direction spacing between two trees.

Tree is an essential factor affecting airflow and pollutant diffusion in the urban street canyon. The wind environment in the urban street canyon will be effectively improved by expounding the mechanism and implementing greening measures. Moreover, it will help decrease the pollutant concentration around the street canyon. This paper reviews the airflow and pollutant diffusion numerical simulation in the street canyon under the tree influence. Firstly, the numerical mathematical model used for pollutant diffusion and airflow in urban street canyons under the influence of trees is summarized. The representation of trees’ numerical mathematical model in the simulation domain is mainly proposed. Secondly, the wind environment and pollutant distribution factors influencing urban street canyons are elaborated and analyzed, including tree characteristics, layout, street canyon shape, and thermal. Furthermore, current research progress and deficiencies are discussed. Finally, the future research direction of wind environment and pollutant distribution simulation in urban streets under the influence of trees is pointed out.

Immiscible displacement in porous media is a crucial microscale flow phenomenon in many fields, necessitating an understanding of the flow mechanisms under dynamic injection velocity and mixing wettability to predict and affect this flow accurately. Initially, a dynamic injection velocity method and a computational domain model considering non-dominant/dominant wetting angles were proposed. Then, microscale flow phenomena were modeled in a pore throat structure and doublet geometry under mixed wetting conditions. Finally, the influence of dynamic injection velocity and mixed wettability on microscale flow were investigated using numerical simulations. The results indicate that when stepwise and piecewise linear changes in injection velocity are observed, unlike continuous injection, two preferential displacement pathways are predominantly formed in the porous media. As the difference between the maximum and minimum injection velocity increases, the recovery efficiency initially decreases and then increases. Recovery efficiency is higher under piecewise linear injection velocity changes. The non-dominant wetting angle determines the distribution and flow of oil-water two-phase systems in porous media. With a dominant controlling wetting angle of 45°, as the non-dominant wetting angle increases, the flow phenomenon changes from one preferential pathway in the back region (30°, 45°) to two preferential pathways (60°, 90°, 120°) and then to one preferential pathway in the middle porous media (150°). As the degree of the non-dominant wetting angle increases, the recovery efficiency first increases and then decreases, with a maximum and minimum difference of 13.6%.

Understanding the two-phase displacement behaviors of oil and water in porous media under different reservoir development modes for enhanced oil recovery is essential. In this paper, the influence of development measures, such as increasing the injection rate, changing the inlet/outlet position, increasing the water viscosity, and reducing the surface tension coefficient, on oil–water dynamic behaviors was studied using a numerical simulation based on the study of the formation of a high-water-cut channel by water flooding at different injection rates. The results show that blockage and restart occur during displacement in the pore–throat channel and during staggered displacement in different pore channels. With an increase in the injection rate, the recovery increases first and then decreases. All the different development measures can increase the swept area and recovery factor. The recovery factor increases significantly by reducing the surface tension coefficient at medium/high injection rates (≥0.01 m/s) and by increasing the viscosity of the water at low injection rates (<0.01 m/s). The numerical simulation study preliminarily revealed the influence of different development measures on displacement behaviors in the pore model. It thus provides theoretical support for understanding the law of oil and water movement in reservoirs.