Fluid Dynamics最新文献

Single-phase turbulent flow through a subchannel of a tightly spaced rod bundle with a pitch-to-diameter ratio p/d = 1.107 is investigated numerically at the Reynolds number Re = 48 400. Two low-Reynolds-number (LRN) Reynolds-averaged Navier–Stokes (RANS) models were used, namely, the (k{-} omega ~{text{SST}}) model and the ({text{BL}}{-} overline {{{upsilon }^{2}}} {text{/}}k) model, which is an elliptic-blending-based (overline {{{upsilon }^{2}}} {-} f) model implemented in the in-house EDF code₋saturne®. The numerical predictions are presented in terms of the mean velocity distribution, the turbulent kinetic energy, and variation in the wall shear stress. These predictions are then compared with experimental results given by Hooper in 1980. The results indicate that both LRN models are capable of predicting specific flow features. Furthermore, the low-Re near-wall treatment successfully reproduces the correct qualitative behavior of the wall shear stress along the rod surface, in agreement with experimental results. However, the (k{-} omega ~{text{SST}}) model demonstrated superior overall performance as compared to the ({text{BL}}{-} overline {{{upsilon }^{2}}} {text{/}}k) model, showing better accuracy in predicting the flow characteristics in this complex configuration.

Air relaxation in the afterglow of a pulsed DC discharge is simulated using the state-to-state approach and two kinetic schemes. Good agreement of the results with experiment is shown. Key factors affecting the accuracy of the simulation are identified: consideration of vibrational nonequilibrium in all molecular species, the exchange Zeldovich reaction model, and the model for vibrational energy exchanges. A reduced kinetic scheme is developed that speeds up calculations by a factor of 37 while maintaining accuracy. The proposed model is applicable in a wide range of temperatures and nonequilibrium parameters.

The behavior of solid cylinders in particulate flows is studied, focusing on their interactions with the surrounding fluid. Fluid dynamics are modeled using the power law model that accounts for the shear-thinning, Newtonian, and shear-thickening fluids. The fictitious boundary method (FBM) is employed to model the cylinder-fluid interactions within the Eulerian framework. The hydrodynamic forces exerted on the cylinder surfaces are calculated using the explicit volume integral technique. The findings indicate that the initial inter-cylinder distance and the Reynolds number significantly affect settling velocities of the two falling cylinders and their maximum separation distance. In shear-thinning fluids (left( {n = 0.9} right)), significant repulsive forces induce cylinder dispersion, especially at higher Reynolds numbers. In Newtonian fluids (n = 1), the cylinder behavior is more uniform, while in shear-thickening fluids (left( {n = 1.1} right)), the increased resistance reduces variations in the cylinder velocity. The innovation of this study lies in integrating the FBM using the power law model, enabling a comprehensive comparison of cylinder dynamics for various types of fluids. The study demonstrates how the fluid rheology influences sedimentation, cylinder separation, and settling velocities, emphasizing the significant impact of both fluid type and initial cylinder separation on the cylinder behavior. Numerical investigations using the FEATFLOW simulation tool provide high-resolution results for cylinder dynamics and sedimentation behavior under various fluid conditions.

The previously developed laboratory technique is used to perform a series of experiments to study the effect of enhanced concentration of matters dissolved in water on the condensation-induced growth and equilibrium parameters of droplets in levitating droplet clusters during their stabilization by the combined action of infrared heating and the dissolution of common salt in a water layer under the cluster. The experiments demonstrate that the equilibrium concentration of an admixture in cluster droplets can be reached even for dissolved substances that most strongly prevent the achievement of equilibrium. For the first time, experiments were conducted for mixtures of solutions of different substances. In certain cases, a non-additive effect of the solution components on droplet evaporation and stabilization was observable. The results obtained show the complosite interaction of certain aqueous solutions in small droplets, highlighting the importance of employing alternative methods and technical means in the experimental research.

The problem of determining the conditions of formation of aircraft condensation trails (contrails) during the interaction of a turbofan engine’s (turbojet) exhaust jet with the environment is considered. The gas dynamics equations (Reynolds equations) are solved numerically with regard for the nozzle shape of the turbojets under consideration. The calculations take into account the geometric and gas-dynamic parameters, including the bypass ratio, the internal and external flow characteristics, the water vapor emission, and others, that define the key processes for various types of turbojets. The obtained results are necessary for developing a criterion for the formation of stable contrails, based on the degree of water vapor supersaturation in the engine jet. Examples of contrail formation are analyzed, based on full-scale flight test data.

The interaction of the glow discharge on a flat plate with air flow in a diaphragmless shock wind tunnel is studied experimentally. As compared to the opposite directional electrodes, longer burning of discharge on the flat plate is revealed. The effect of magnetic induction on plasma luminescence and flow parameters is shown. Video frames of the discharge are given.

The results of experimental study on the explosive interaction of 3 and 10 g droplets and jets of molten bismuth with water within a range of the initial melt temperatures ({{T}_{0}}) = 600–800°C are given. High-speed video recording revealed that explosive (duration of the process less than 1 ms) breakup of liquid bismuth droplets occurs both in contact with the free surface of the coolant under the conditions of partial submersion in water, as well as within the water bulk volume. An analysis of measured pressure oscillations in the water medium showed that there is no dependence of their amplitude on the initial temperature or the droplet mass. Numerical simulations and experimental results indicate the higher values of the pressure and acoustic energy generated during underwater explosions of bismuth droplets as compared to explosions on the surface.

The microwave discharge with sparkless laser initiation is studied experimentally in the medium without external flows and under the conditions of supersonic flow at the Mach number 1.44. It is found that the properties of the active phase of the microwave discharge when applying initiation in supersonic flow differ only slightly from the case without external flows. Shadowgraph images of shock waves in front of a model cylinder in supersonic flow with the energy deposition by the discharge under consideration are obtained. Numerical simulations of the gas-dynamic processes during discharge combustion and in its wake are carried out, and the interaction of the heated gas region with the bow shock wave front is investigated. The possibility of producing a modified shock wave structure in front of the model by 100 μs with a 30–60% reduction in the stagnation pressure using an initiated microwave discharge of short duration 0.5–2 μs is demonstrated.

Experimental data on the ignition probability, the induction time, the length, and the diameter of a subcritical microwave discharge initiated by a laser pulse without laser spark are given. The studies were carried out in air at the pressures from 50 to 80 Torr, the laser pulse energies of 10–200 mJ, and the pulse supply times from –100 to 0.5 μs relative to the microwave pulse. An MI-505 magnetron, that operates at the radiation frequency of 9.6 GHz and has the pulse duration of 2.5 μs, and a focusing system were used to create microwave radiation. In the plasma zone, the microwave radiation intensity was estimated at the level of 2.0 kV/cm. An Evergreen 200 laser that creates pulses of the 10 ns half-width and the 532 nm wavelength was used to initiate the microwave discharge. The laser pulses were focused by a lens with the focal length of 250 mm. Owing to laser initiation, the threshold pressure of the microwave discharge was increased from 50 Torr (without initiation) to 80 Torr (with laser initiation). Synchronous supply of the microwave and laser pulses gives the best result, namely, discharge ignition occurs without misfires and the discharge characteristics (geometry, induction time) have a minimum spread. Premature supply of the laser pulse also makes it possible to obtain the discharge with an increased ignition threshold, but its characteristics become worse. Laser initiation also has positive effect on the supercritical microwave discharge. The results obtained can be useful in developing a plasma generator in aerodynamics and other problems with the plasma energy deposition.

The results of an analysis of the applicability of quasi-one-dimensional calculations of main gas-dynamic processes in a reflected shock tunnel for the tasks of experiment planning are given. Estimates of the test time, duration of the nozzle start-up, and parameters of supersonic gas flow at the point of mounting the model are obtained. The calculation results are compared with measurements carried out on the Large shock tunnel of the Ioffe Physical-Technical Institute. The applicability of the one-dimensional approach is shown.