Fluid Dynamics最新文献

The high-speed videorecording method is used to investigate the effect of an electrostatic field (with the potential Φ = 0, 16, and 18 kV) on the flow geometry in the case of gravity-induced separation of a drop from a capillary tube. The flow videograms are analyzed and the dimensions of the characteristic structural elements, that is, the drops themselves, a bridge, and satellites, are determined. The oscillations of the linear dimensions and the mother liquid volume after drop separation are traced at (Phi = ) 0 and 18 kV. Both fundamental frequencies and their harmonics are observable in the spectra. It is found that small (12%) variations in the potential value lead to qualitative variations in the flow pattern and, in particular, to direct separation of the drop from the mother liquid without the formation of a bridge. At a constant liquid discharge in the capillary the dimensions of the separated drops decrease with increase of the voltage. The experiments show the possibility of the fine controlling of drop flows using electrostatic fields.

The disturbances generated by external turbulence in the shear layer on a flat plate suddenly set in motion are found. As the initial conditions, turbulent flow found using direct numerical simulation of the development of isotropic homogeneous turbulence is used. The solution obtained models laminar-turbulent transition in the boundary layer on a flat plate under relatively low free-stream turbulence when the transition is caused by Tollmien–Schlichting waves. The solution makes it possible to describe the process of generating various disturbances, namely, low-frequency streaky structures and instability waves and also their development in the initial stage of laminar-turbulent transition. Based on the processing of the obtained results, a simple model is proposed that relates the spectra of instability waves in the boundary layer and turbulent pulsations in free-stream flow. The dependences of the initial amplitude of instability waves and their critical amplification factors (N-factors) on the degree of flow turbulence are also obtained.

A model of oil inflow to a well in a fractured reservoir with a vast gas cap and an underlying water layer is presented in the conditions of gravity-induced segregation of fluids. Using an asymptotic analysis of the equations it was possible to simplify the description of the seepage process before and after the water and gas breakthrough into the well and at a distance from it, as well as to estimate the possibility of waterless and gasless extraction in the conditions of the stabilization of phase fractions in the total rate. It is shown that the hydrostatic equilibrium model can be used in the large-scale approximation fairly far from the well. It is noted that in most practical cases the finite-conductance effect of a fracture is negligible in the large-scale approximation, so that the model of an infinitely permeable fracture can be applied. The equations for determining gas and water fractions in the production after the breakthrough of the water and gas cones in the vicinity of the sink were derived on the flow scale. Finally, the coupling of the models presented makes it possible to describe adequately the inflow to the well before and after the breakthrough of the water and gas cones. The plausibility of the models presented is confirmed by the comparison of the calculated results with the actual data.

The drag crisis in flow past a sphere is modeled within the framework of the recently formulated scale-resolving hybrid RANS−LES approach, which includes a semi-empirical model of laminar-turbulent transition. The calculations performed in a wide Reynolds number range show that the complex model used yields a qualitatively adequate description of all aspects of the drag crisis including such fine effects, as the growth of the side force oscillation amplitude at near-critical Reynolds numbers. At the same time, the results obtained indicate that very fine computation grids should be used for obtaining qualitatively accurate predictions of the critical Reynolds number and the details of laminar-turbulent transition in near-critical flow regimes.

The generation of the suction force between a rigid cylindrical frame constricting a submerged cavitating jet and a solid surface is considered. The separation force needed to overcome the suction effect is experimentally determined in a specially developed setup. The dependences of the suction force on the frame diameter and the distance from the nozzle cavitator are obtained using numerical modeling. The volume fraction of the vapor phase inside the frame and the static pressure along the suction surface are calculated. The mechanism of the appearance of the effect and the criteria of its vanishing are explained on the basis of the data obtained.

The extent of participation of mechanisms such as the active interactions of cells with each other and with the extracellular matrix, the increased hydrostatic pressure in intercellular fluid, and enzymatic activity of cells that lead to the destruction of the extracellular matrix in the process of formation of cavities in clusters of cells formed during cluster vasculogenesis is studied. The problem of evolution of a single cluster of cells immersed in a deformable extracellular matrix is solved within the framework of a previously developed continuum multiphase model of the medium formed by two actively interacting solid phases and a fluid and the role of various cellular mechanisms discussed in the formation of hollow structures is studied. The calculations showed that the dominance of active interactions of the cell-matrix type over the intercellular interactions leads to a displacement of cells towards the outer boundary of the cluster and the creation of conditions for the formation of a cavity inside the cluster. The enzymatic activity of cells helps to free up a headroom for compaction of the cluster, due to the active intercellular interactions, and to slow down the formation of the increasing concentration profile of the cellular phase. An increase in the fluid pressure in the area occupied by cells leads to acceleration of the redistribution of concentrations of the cellular phase and matrix. The fluid pressure promotes accumulation of the cellular phase near the cluster boundary and increase in the matrix concentration in its central part. And only the joint participation of all the mechanisms considered leads to the formation of a structure in which a layer formed by the cellular phase surrounds a fluid-occupied cavity, while the matrix concentration in the cavity demonstrates the trend to its complete disappearance.

The regimes of fluid displacement from a formation developed by a system of vertical wells are studied within the framework of the cross-sectional problem of flow through a porous medium. The case of an anisotropic formation in which the effect of buoyancy of the displacing fluid is significant is considered. It is shown that in the generic case the displacement process is characterized by five similarity criteria, one of them determines the intervals of fluid flow from the well to the formation. The limiting cases of the displacement regimes in which the number of constitutive similarity parameters can be reduced are described. The influence of these parameters on the recovery factor and the sweep efficiency is studied. A diagram of displacement regimes that constrains the areas of influence of the similarity criteria and the revealed limiting cases is constructed. The results of the present study may be useful in the oil and gas industry in estimating the efficiency of various field development methods.

Numerical simulation is applied to investigate the outflow of an incompressible three-dimensional turbulent swirled wall jet. The purpose of the study is to determine the jet flow structure and to compare the characteristics of swirled and nonswirled jets. The numerical solution of the equations of motion is obtained using large eddy simulation with wall resolution (WRLES). The results of the modeling are compared with the data of a unique published study devoted to experimental investigation of swirled wall jets.

Hydrodynamic flows induced by translational oscillations of cylindrical bodies of various cross-sectional shapes are studied. The motion of fluid around oscillating bodies is described using the system of Navier–Stokes equations written in a generalized curvilinear coordinate system. Transition to a given body shape is implemented using a conformal mapping. The problem is solved using the method of successive asymptotic expansions under the assumption that the oscillation amplitudes are small. In each asymptotic approximation, the subproblems are solved numerically using the finite-difference method. Based on the results of the work, estimates of the hydrodynamic effect are obtained, the applicability of the high-frequency asymptotic approximation is estimated, and secondary stationary flows near cylinders are studied, in particular, the occurrence of directed stationary flows near an oscillating asymmetric body is considered with reference to the Joukowski airfoil.