Fluid Dynamics最新文献

Liquid fuel breakup is a critical process in the field of energy and power engineering, and understanding its mechanisms is significant to enhancing the fuel atomization efficiency. In this paper, the fuel jet breakup process and its spray characteristics are investigated numerically by using a phase-field-based lattice Boltzmann model. The spray characteristics are analyzed quantitatively from three aspects, including the spray penetration, the atomized droplet distributions, and the atomization cone angle, and a coefficient of atomization dispersion angle is proposed to describe the atomization angle and spatial dispersion of the atomized droplets. The numerical results show that the spray penetration is proportional to time before the first breakup, then it turns into the 0.6 power of time. The changes in the number of droplets, the average droplet equivalent diameter, and the droplet velocity in the jet direction as functions of time occur in accordance with the Boltzmann distribution, the logistic distribution, and the exponential associated distribution, respectively, and the bimodality is the most obvious characteristic in the probability distribution of the droplet velocity. The atomization dispersion angle tends to be steady as the fuel jet is fully developed, which is more suitable for characterizing the jet breakup process as compared to the maximum atomization angle.

The variable-geometry scramjet combustor represents a pivotal technology for wide-range and high-maneuverability aerospace vehicles. This study proposes an innovative adjustable strut/cavity configuration to achieve efficient fuel regulation. The fuel control characteristics are systematically analyzed using planar laser shadowgraphy experiments. A predictive model integrating proper orthogonal decomposition (POD) with deep multi-task learning (MTL) is developed for prediction of the full-field fuel distribution. The combustor operates under inflow conditions of the Mach number 2.0, the total temperature 300 K, and the momentum ratio 12, with geometric variations covering: the strut length (0–20%), the strut height (0–10%), the cavity length (0–30%), and the cavity depth (0–20%). The results show that (1) geometric adjustments of strut/cavity effectively modulate the global fuel distribution patterns; (2) the POD-MTL framework accurately establishes correlations between the geometric parameters and the fuel distribution, achieving the prediction accuracy with a relative error of 10%. This methodology provides theoretical foundations for real-time combustion optimization in hypersonic propulsion systems.

During high-speed flight in the atmosphere, aircraft with optical windows endure severe aerodynamic heating challenges. This study focuses on a supersonic optical dome with planar side windows. Numerical simulation methods are employed to investigate the effects of the supersonic film pressure, the film Mach number, and the type of cooling gas on the cooling performance of the film. The results indicate that the introduction of a film can alter the velocity profiles of the flow field. Increase in the the static pressure ratio and the exit Mach number of the film can extend the effective cooling length and enhance the cooling effectiveness. This improvement primarily arises from the increased film thickness above the optical window, which better isolates the window from the mainstream. Additionally, this increase suppresses the growth rate of the mixing layer generated by the interaction between the film and the mainstream, thereby extending the length of the potential-core region. However, the excessively high static pressure ratio and the Mach number can lead to waste of the cooling gas. Under the same static pressure ratio and Mach number, NH₃ exhibits a higher mass flow utilization rate and can be considered in future film cooling designs.

In firearms and ballistics, grasping the interaction between high-speed projectiles and their surroundings is crucial for optimizing trajectory, stability, and accuracy. This study explores the comparison of intricate flow dynamics involved when a bullet, traveling at the Mach number equal to 2, is fired near both a moving object or wall and a stationary wall, particularly in urban warfare scenarios. The proximity to the wall introduces an asymmetry in the pressure distribution across the bullet’s body, affecting its flow physics in diverse ways. The present analysis employs Computational Fluid Dynamics to scrutinize the flow field around a 7.82-mm bullet from an AK-47 rifle moving at supersonic speeds near the ground. Computational simulations explore phenomena at various heights from a nearby stationary or moving wall, ranging from the region of nearest influence to a ground distance five times the bullet’s diameter. The study encompasses height-to-diameter ratios h/D from 0.5 to 5, shedding light on the overall flow field, the pressure coefficient distributions, and the lift, drag, and moment coefficients. Additionally, the flow field in the wake region is examined. The results highlight the generation of a detached bow shock wave at the bullet’s tip, significantly influencing the drag coefficient experienced by the projectile. This study contrasts the effects of a stationary wall at varying proximities to the ground with those of a moving wall at equivalent distances. The comparison highlights change in flow characteristics and various parameters, providing valuable insights into high-speed projectiles in proximity to stationary walls and moving walls simultaneously (h/D) at a Mach number 2 during urban warfare scenarios. Furthermore, a study on the effect of porous wall proximity to the bullet has been conducted, and the shock absorption reduces the momentum. Understanding these phenomena is crucial for optimizing the bullet design and enhancing the effectiveness of missile ballistics in real-world applications.

The aim of this experimental study is to investigate the influence of air tab orientation on the mixing and spreading behavior of a Mach 0.8 primary jet ejected from an axisymmetric nozzle. Two air tabs have been considered in two configurations, namely, 90° and 45°, with respect to the primary jet centerline. The air tab Mach numbers, ranging from 1.1 to 1.4 in an interval of 0.1, have been considered. The jet decay along the primary jet centerline (X), along the lateral (Y), and vertical (Z) directions have been acquired. The jet half-width also has been calculated to assess the mixing/spreading enhancement induced by air tabs along the lateral and vertical directions. The percentage reduction in potential core achieved by 90° air tabs is higher than that of the 45° air tabs at all the Mach numbers studied. The 90° air tabs contract the primary jet along the vertical direction and expand it along the lateral direction. This implies a better jet mixing enhancement in the vertical direction and improves spreading along the lateral direction. On the other hand, the 45° air tabs result in rapid jet decay along both directions, in the far-field locations. The present study reveals that the 90° air tabs are best suited for applications that require better jet mixing, while the 45° air tabs are suitable for moderate jet mixing with the minimum total pressure loss.

The system of maintenance of the formation pressure is associated with the appearance of technogenic fractures near injection wells, which leads to sharp water cut of the extracted oil. Geomechanic simulators also leave out variations in the hydrodynamic fluxes due to adsorption and keeping of disperse particles in the porous medium. The quasi-one-dimensional model of the fracture development dynamics developed on the basis of mechanics of multiphase media allows one to take account of these effects. The numerical solution of these equations makes it possible to predict to a high precision the geometric parameters of a fracture at different moments of time.

The mechanisms of the bending flow of a viscous jet (Ohnesorge number greater than 0.05) flowing out of a capillary channel at a low velocity (Weber number is about unity) are experimentally investigated. The bending is due to the effect of internal forces and is not related with the interaction between the liquid and the atmosphere, which is confirmed by experiments performed in a vacuum chamber. A region of intense jet bending amounting to fifteen degrees is formed near the channel end section, at a distance of the jet diameter. Further downstream the jet is “straightened,” the angle of bending being reduced. The dependences of the greatest and overall bending angles on the jet velocity are obtained for different Ohnesorge numbers. The velocities, at which the deflection is maximum, are revealed. The deviation angle values corresponding to large velocities are determined.

An advanced physico-mathematical model of silicon vapor transfer from a melt mirror to a porous carbon article is proposed and tested in the conditions of medium vacuum in the case of vapor-phase siliconizing. The model proposed is compared both qualitatively and quantitatively with those suggested earlier. The novelty of the approach proposed lies in taking account for an additional effect in the form of possible redistribution of rarefied carrier medium, whose role is played by inert argon, as the result of displacement by silicon vapors. It is shown quantitatively to what extent the silicon vapors expel argon in the process of vapor-phase siliconizing. The dynamics of the displacement front is studied. The proposed model described by a system of partial differential equations makes it possible to calculate the mean-mass velocity of the gas mixture and the diffuse transfer of silicon vapors from the melt mirror to the specimen surface through the carrier medium.

Porous media seepage flow is the active flow control method used to reduce heat and skin friction in high-speed vehicles, but effective measurement techniques for mixing the seeping gases with the incoming boundary layer are lacking. By premixing approximately 20% acetone vapor in the cooling gas and employing the acetone planar laser-induced fluorescence (PLIF) technology, flow of the seeping gas film within the boundary layer was visualized. A correlation between the relative intensity of PLIF image grayscale and the gas film mixing rate is established. Experimental results showed that the seeping gas film layer remains initially laminar in the Mach 3 laminar boundary layer; with the lower injection rate, the film layer develops slowly and maintains a longer laminar state. As the injection rate increases, the film layer thickens significantly along streamwise direction on the porous wall. After reaching a certain thickness, instability develops, leading to intensified mixing with the incoming boundary layer downstream and the formation of large-scale mixing structures. The position of instability moves upstream with increase in the injection rates, indicating that the higher film injection rates tend to induce boundary layer instability and premature transition. When the injection rate F < 0.2%, the diffusion rate of the seeping gas film into the outer boundary layer is low, and the film maintains a high concentration at the bottom of the boundary layer. With the higher injection rates, the mixing ratio increases and diffuses outward, with a slight decrease in the normal concentration gradient of the film along the wall. For a given injection rate, the diffusion range of the seeping gas film continuously increases but does not exceed 5 mm in thickness. The study shows that the acetone PLIF technology can effectively achieve fine visualization and quantitative analysis of the mixing flow structures of seeping gases within supersonic boundary layers.

The process of hyperelastic spheres entering water under the influence of various surface geometric features is investigated, with a focus on hyperelastic spheres that have concave and convex grooves on their surfaces. The arbitrary Lagrangian–Eulerian (ALE) method is used to handle the fluid-structure interaction, considering the continuity and momentum equations of the fluid. Numerical calculations using the finite element method are employed. This incorporates a penalty function coupling algorithm and second-order accurate ALE advection techniques to address the fluid-structure coupling. The deformation, the stress distribution, and the characteristics of motion of the spheres after their entry into the water are analyzed.