Fluid Dynamics最新文献

The multipoint illumination is used to visualize the flow pattern produced by a free falling drop in a fluid at rest. The initial stage of merging of a potassium permanganate solution drop with water and a water drop with ammonium thodanide solution as well as the spreading of a drop of aniline and crude oil in a pool of water is studied. Emphasis is placed on an analysis of the flow pattern near the cavity bottom in the impact regime, when the kinetic energy of the drop is considerably greater than its potential surface energy. An “intermediate layer” is formed under the cavity bottom upon the contact of the mixing fluids. This layer is the product of dissolution of thin fibers of the drop substance intruding into the target fluid. Poorly soluble aniline forces partially through the cavity bottom. In the experimental conditions the oil does not penetrate through the surface of the fluid in the initial stage of the flow. The values of the conventional dimensionless parameters, which are the Reynolds, Froude, Weber, Bond, and Ohnesorge numbers, are presented, together with certain additional parameters, namely, the ratios of energy components and their densities, as well as the relative densities and surface tension coefficients of the media in contact.

The aim of the study is to increase the accuracy of determining the gas-phase chemical reaction rate constants by means of numerical methods. The problems of justifying the choice of regression functions and refining their parameters in determination of the temperature dependence of the exothermic reaction rate constants are solved. The methods of solving are aimed at detecting the uncertainty components of the regression parameters associated with systematic effects, followed by their elimination or compensation. The specific features of using the Arrhenius law to approximate the temperature dependences of exothermic reactions are found from the results of numerical investigations. Despite the universal form of the law in the chemical reaction kinetics field, the utility of using the Arrhenius law as a universal model can be called into question in the regression analysis of experimental data. It is found that the unjustified choice of the regression model serves as a source of an additional uncertainty of the regression parameters, the revealed correlation dependence of the parameters results from the excessive complexity of the model. It is demonstrated that a simpler model based on a power-law function describes the temperature dependence of the exothermic reaction rate constants fairly well. Particular calculations and estimations are carried out with reference to the OH + O → O₂ + H and O₃ + H → OH + O₂ reactions over the temperature range from 150 to 500 K.

The performances of single and multiple plasma actuators on NACA 0012 airfoil at the Reynolds number of one million are investigated. A numerical method is used for flow simulation in solving the Reynolds-averaged Navier–Stokes equations together with the SST-kω turbulence model. Maxwell’s equations are also used for simulation of the electro-hydrodynamic (EHD) field using the Suzen model. The effect of the plasma actuator has been modeled at various angles of attack and actuator voltages. Initially, simulations are performed for a single actuator at different positions, and then for two actuators. Various simulations are performed for different angles of attack, voltages, and actuator positions. The results for the lift coefficient, the drag coefficient, and the aerodynamic efficiency are presented. An analysis of the results showed that in a single-actuator configuration, the installation position significantly affects the lift coefficient. In this case, the leading edge is the best point for actuator installation. Using two actuators increased the lift coefficient and decreased the drag coefficient, with the magnitude of these changes depending on the voltage and the angle of attack.

This study investigates the dynamics of single vapor bubble growth in near-saturated liquids under microgravity conditions, with particular emphasis on heat transfer mechanisms and evaporation phenomena. The research presents experimental validation of theoretical models for bubble growth kinetics across multiple pressure regimes (500–750 mbar) and thermal configurations, including systematic analysis of equivalent bubble diameter evolution, wall superheat dynamics, waiting time effects, and the influence of superheated layer characteristics on growth behavior. A generalized model has been developed based on the Labuntsov–Yagov correlation framework that incorporates time-dependent wall superheat conditions and accounts for evaporation contributions from both the contact line region and the bulk liquid-vapor interface. Comprehensive comparison between model predictions and experimental measurements demonstrates good agreement across the investigated parameter space, confirming the model’s validity and its capability to accurately capture the complex interplay between thermal boundary conditions and bubble growth dynamics under microgravity.

Numerical study of a compressible turbulent boundary layer in supersonic flow with adverse and favorable pressure gradients is carried out using the three-parameter differential RANS turbulence model. The pressure gradients are implemented by changing the Mach number of the free-stream flow along the plate. The study is carried out for a number of the free-stream Mach numbers (from 1 to 3) and two values of the temperature factor (0.5 and 1.5). Using the results of calculations of the flow and heat transfer characteristics, the dependences of the Reynolds analogy coefficient on two pressure gradient parameters are obtained.

This study presents a comprehensive evaluation and comparison on the performances of the vortex lattice method (VLM), the 3D-panel method (PM), and the computational fluid dynamics (CFD) methods for the Reynolds-averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) in predicting aerodynamic characteristics of low-aspect-ratio wings in low-Reynolds number flows. The primary aim is to investigate the capability of the models in capturing separation bubbles and complex flow behaviors with a particular focus on a wide range of high angles of attack. Accordingly, the low-aspect-ratio (AR = 1 and AR = 3) wings with a well-known NACA 4412 airfoil have been modelled and investigated at the Reynolds numbers 2.5 × 10⁴ and 7.5 × 10⁴ using the aforementioned methods to make a comparison on lift and drag prediction capabilities. The grid independence studies are conducted for both methods and ensure that the results are independent of the number of mesh or panel elements. Initially, the results of VLM and PM are compared with the RANS CFD analysis results carried out using k–k_L–ω transition turbulence model. The further analyses are performed using various turbulence models (RNG k–ε, transition SST, k–k_L–ω transition, realizable k–ε models), and the results are compared with large-eddy simulation (LES) analyses and the experimental wind tunnel data available in literature.

This work is devoted to the calculation of the distillation curve of mixed fuels including petroleum hydrocarbons and biocomponents. When calculating the distillation curves of such fuels using the standard technique, in some cases there is a significant deviation at the beginning of the graph. Therefore, in this work, activity coefficients were used to determine the saturated vapor pressure of the mixture when calculating the distillation curves. The distillation curves of the tetradecane – propanol-1 mixture were calculated taking into account the activity coefficient in the Raoult equation when determining the saturated vapor pressure of the mixture. The obtained distillation curves were compared with the experimental data also obtained by the authors of the work. Systems with different contents of the oxygen-containing component (25, 50, and 75 vol %) were considered. The UNIFAC model was used to calculate the activity coefficient. The results showed that the use of activity coefficients leads to a decrease in the discrepancy between the calculated and experimental distillation curves for mixtures with a biocomponent content of up to 50%.

Graphite ablation in subsonic jets of dissociated nitrogen, carbon dioxide and their mixture is studied using the VGU-4 high-frequency plasmatron. For additional radiative heating of the samples, a laser source is used. Numerical modeling of subsonic flows of nitrogen and carbon dioxide plasma is carried out under the experimental conditions using the Navier–Stokes equations. The heat fluxes in high-enthalpy jets are measured by probes and the plasma emission spectra in the experimental regimes are recorded. The behavior of graphite heated only by laser radiation in the nitrogen medium, heated in the nitrogen plasma and under combined heating is studied. The effect of the chemical composition of high-enthalpy flow and the heating conditions on the graphite mass loss is demonstrated.

Bulk condensation is one of the frequently encountered and exploited processes in the technologies of gas purification from impurities. The phase transition process can be conditionally divided into stages of the droplet formation and growth due to two simultaneously acting mechanisms, namely, continuing vapor condensation on the surface of formed droplets and droplet coagulation due to their collisions. Early computational estimates in regard to coagulation showed a good qualitative agreement between the calculated and experimental data, but there was a significant quantitative difference. Within the framework of the present study, a hypothesis about a possible reason of these differences is put forward: turbulent disturbances are not considered in the one-dimensional formulation. The main aim is to test the hypothesis on the need to take turbulence into account within the framework of the calculation model for expanding flow in which bulk condensation takes place. The proposed modification of the approach makes it possible to take into account the effect of turbulent disturbances on coagulation of condensation aerosol particles. This can be essential, for example, in vapor–liquid turboexpanders. The study considers the bulk condensation of heavy water vapor mixed with nitrogen, acting as a non-condensable carrier gas, in the flow part of a Laval slot nozzle with regard to coagulation and turbulence. The hypothesis on the effect of turbulence in the system of gas dynamics equations on the process of droplet (particle) coagulation of a condensing impurity in the flow is confirmed. It is found that taking turbulence into account significantly improves the numerical convergence of calculations and experiment; however, it does not provide exact agreement, which, in turn, may be caused by the adopted assumption of the Brownian coagulation approximation, as well as the use of the k–ω turbulence model. It is shown that taking turbulence into account affects the magnitude of the coagulation kernel, the maximum difference for calculations with and without turbulence being approximately 10%. Taking into account turbulence during coagulation, droplets (particles) grow to larger sizes, which in the long term makes it possible to control this process.

This paper examines the steady-state outflow of ethanol microjets in a metastable superheated state into a highly rarefied medium, accompanied by physical phenomena absent in dense gaseous environments. The formation of microjets in the form of curved, temporally and spatially unstable flows that disintegrate into droplets is reported. Flow bifurcation and explosive boiling of ethanol jets at the outlet, resulting in vapor–droplet streams, have been observed. Droplet formation on the surface of the flow source and the motion of these droplets along the surface in a direction opposite to the jet flow have also been detected, regardless of the orientation of the source in space. A mechanism explaining this reverse droplet motion is proposed.