Fluid Dynamics最新文献

In modern battlefield, the requirements for long-range guidance and efficient damage of projectiles are increasing. The curved portion of projectiles is an important factor affecting the ballistic stability and aerodynamic characteristics. The optimization design of the curved portion of projectiles is of great significance for improving the guidance distance, accuracy, and the flight stability of projectiles. In this paper, a design method of the curved portion based on the exponential curve is proposed. The aerodynamic characteristics of projectiles of the curved portion based on the exponential curve and the existing typical curved portion under various Mach numbers (0.8, 1.5, and 3 Mach numbers) and various angles of attack (0, 10° and 20°) were numerically simulated by ANSYS Fluent software. The simulation results showed that when flying at various Mach numbers at zero attack angle, the projectile with logarithmic curve arc has the largest drag and drag coefficient, while the exponential curve arc has the smallest drag and drag coefficient. The drag coefficients between the two types of projectiles are 4.9, 14.5, and 13.7%, respectively, at various velocities. As compared with zero attack angle, the lift coefficient of exponential curve increases by 0.08 and 0.18 at the 10° attack angle and the 20° attack angle, and the change in the drag coefficient is only 0.008 and 0.077, which also indicates that the projectiles with the exponential curve arc have the better aerodynamic characteristics.

The present study focuses on analyzing the effects of variations in the cavity guide vane structure on the performance of a trapped vortex combustor. Steady, 2D reactive and non-reactive flow numerical simulations were carried out for a trapped vortex combustor fitted with a cavity guide vane. Three cases were introduced, namely, a) the combustor with rectangular guide vane and squared edges, b) the rectangular guide vane with rounded edges, and c) streamlined guide vane with rounded edges. Non-reactive flow cases were considered at the air velocities of 20, 30, and 40 m/s. In reactive- flow cases, the equivalence ratio is maintained with 20% excess air as 0.8. Mass entrainment into the cavity and pressure loss across the combustor are used to evaluate the effectiveness of cavity guide vane. The rectangular guide vane with rounded edges of combustor performs well in terms of the mass entrainment and the vortex structure. A peak direct mass entrainment of 3.8 kg/s was obtained for the mainstream air velocity of 40 m/s. The rectangular guide vane with rounded edges possess the superior flow and combustion characteristics at all velocities. It has been observed that the cavity guide vane structure optimization plays an important role in determining the total flow into the cavity and the number and strength of vortices formed inside the cavity.

Variations in the permeability during compression across various materials, the particle sizes, and the material combinations are investigated. The study focused on three typical materials: expanded polystyrene (EPS), polyurethane (PU), and expanded polypropylene (EPP), selected based on the particle size and the strength. The key findings are as follows: (1) The permeability of polyurethane decreases from 1121.2 Darcy (D) to 767.6 D, marking a reduction of approximately 31.5% post-compression. (2) Expanded polystyrene exhibited a significant decrease in the permeability from 547.6 to 195.2 D, a decline of 64.3%. (3) The highest permeability is observed in expanded polypropylene with a 30-fold expansion ratio demonstrating the most stable permeability. The permeability of expanded polypropylene with a 45-fold expansion ratio reduces from 695.8 to 226.4D, a decrease of roughly 67.4%. These results suggest that expanded polypropylene with a 30-fold expansion ratio and expanded polypropylene are the most effective supporting materials. Additionally, the study revealed that the particle size profoundly impacts the permeability of proppants. For both expanded polypropylene and polyurethane, the permeability and the conductivity initially increase and then decrease with enlargement of the particle sizes. Conversely, expanded polystyrene behaves differently, showing a relatively low permeability and conductivity at a 2 cm particle size. Furthermore, the permeability of material combinations with various particle sizes displays a distinct pattern as compared to single-particle-size materials. The permeability of expanded polystyrene and expanded polypropylene tend to decrease with addition of 1 cm particle size materials to other diameters. In contrast, the permeability of polyurethane increases being mixed with materials of various particle sizes. This research provides valuable insights for selecting appropriate materials in practical applications, emphasizing the nuanced considerations necessary for optimal material choice.

The blended wing body (BWB) is becoming more attractive for modern-day researchers due to its capability to be more fuel efficient, less noisy, and have the better aerodynamic performance. However, a thorough flow investigation of such bodies under varying angles of attack and sideslip conditions is necessary to further enhance the above-mentioned advantages. To investigate the effect of the angle of attack and the sideslip angles, experiments and computations are conducted for a typical BWB at the free-stream velocity of 18 m/s corresponding to the Reynolds number of 58500 based on mean aerodynamic chord. The experimental work consisted of flow visualizations using oil flow techniques and measurements of the forces using an internal 5-component strain gauge balance. Computations were also made to solve numerical simulations using the commercial software Ansys Fluent. The results show a slight variation below 5% in the pressure distribution with change in the sideslip angle. Negligible variations in the lift-to-drag ratio were observed at the lower sideslip angles, whereas slight variations below 10% were observed in the higher sideslip cases. A reasonable agreement between computational and experimental oil flows was also observed, deducing a successful application of the present technique for the investigation of the angle of attack and the sideslip angle in a BWB body.

The dielectric barrier discharge (DBD) plasma flow control technology is notable for its quick response and effective fluid control. It has attracted attention from many fields, including mechanics and aeronautics. This study employs the Suzen sophisticated volume force model to simulate complex plasma dynamics, using the Reynolds–Averaged Navier–Stokes equations by means of a density-based solver. For turbulence modeling, the k–ω SST model is adopted to capture turbulent phenomena, with the Roe method applied for discretization. The proposed numerical approach has been rigorously validated by analyzing challenging flow cases, such as flat plates and hump models. The flat plate simulation results align closely with the experimental flow velocity data, while the plasma-induced wake vortex over the hump model is significantly mitigated. Building on the RAE2822 airfoil, this investigation explores the aerodynamic behavior at various angles of attack, with and without plasma actuation. The aerodynamic response is further examined at various flight altitudes and airflow velocities following actuator activation. Findings indicate that the geometric profile influence on the airfoil’s aerodynamic properties is negligible upon actuator engagement or disengagement. As compared to an airfoil without plasma excitation, the actuated airfoil exhibits the enhanced aerodynamic traits. Notably, exciting at α = 2° optimizes outcomes by boosting the lift coefficient by 7.19% and reducing the drag coefficient by 8.45% when the free-stream Mach number is equal to 0.79. The aim of this actuation is to enhance lift and minimize drag while effectively mitigating boundary layer separation and diminishing surface vortices. Exploration of flight altitudes (H = 0.3, 2.3, and 4.3 km) revealed that the plasma actuator has a significant influence on the lift-to-drag ratios at lower altitudes, with the effects diminishing above 2.3 km. The plasma actuator is most effective in enhancing the lift-to-drag characteristics at a flight Mach number of 0.72 when flow velocities are analyzed. Thus, controlling the flight speed to maintain a constant angle of attack can significantly improve the aircraft performance. In light of these findings, incorporating the plasma flow control technology into airfoil design could be pivotal for enhancing the lift-to-drag ratio and overall flight performance of aircraft.

The article presents the derivation of the calculated relations for the model of the emissivity of the electronic vibrational bands of diatomic molecules averaged over the rotational structure. The model is based on the ab-initio expression for the integral radiation emission coefficient of the rotational line of an electron-vibrational-rotational quantum transition and the results of ab-initio calculations of the Einstein coefficients of rovibronic quantum transitions of spontaneous emission. Calculation formulas are obtained for the averaged emissivity coefficients of heteronuclear molecules. Their validity is shown for homonuclear molecules, including states of different symmetries, in the spectra of which alternating intensities of the emission of rotational lines are observed. Using the obtained relations, spectra of nonequilibrium radiation from the shock wave relaxation zone were obtained in shock wave experiments in air at a speed of 7.3 km/s and a pressure of 0.7 Torr in a low-pressure chamber. The results were compared to experimental data.

The study investigates dynamics of the bi-droplet impact and the spreading behavior in head-on and offset collisions with a focus on a parameter called the surface tension contrast κ. The aims of the study is to understand how varying surface tensions between impacting and sessile droplets affect coalescence and spreading dynamics. Using water and aqueous Sodium Dodecyl Sulfate (SDS) solutions across a range of concentrations to reduce the surface tension of the fluids as compared to water, experiments were conducted to observe the spreading lengths post-impact. High-speed photography captured the impact events, and the data were analyzed to quantify the influence of surface tension contrasts on the spreading behavior. The findings highlight the critical role of the surface tension contrast in determining the maximum spreading lengths during droplet coalescence. It is seen that, as the surface contrast increases, a decrease in maximum spreading lengths is observed. This study provides valuable insights into applications such as inkjet printing, microfabrication, and spray coating, where precise droplet deposition is crucial.

A variable-fidelity Bayesian optimization approach that leverages low-fidelity data (panel approach) to efficiently establish an initial prior for aerodynamic optimization of reusable flight vehicles is proposed. This approach demonstrates a notable advantage over traditional Bayesian optimization techniques constrained by their reliance on high-fidelity data and the associated computational expenses. A comparative analysis reveals that our approach can identify the optimized solutions that would typically require a substantial amount of data, using only a limited number of high-fidelity samples. While the traditional approach undergoes significant shifts in the search space over 50 iterations due to Bayesian optimization’s tendency to explore unknown space, our approach, employing low-fidelity data as an initial prior knowledge, achieves stability within approximately 10 iterations. Notably, with just 50 computational fluid dynamics (CFD) samples (high-fidelity data), the optimized vehicle shape demonstrates significant improvements in the lift-to-drag ratio across a broad range of the attack angles, showing a 9% enhancement at the target lift-to-drag ratio at the 10° attack angle, which is the optimization objective.

In order to improve the performance of the oscillating jet actuator, the optimization design of the actuator is carried out using numerical simulation. The aim of optimization is to improve the uniformity and range of jet sweeping; the evaluation indices of the actuator are proposed; the kriging surrogate models are established to describe the relation between the geometric parameters and evaluation indices, and the effects of the interaction between parameters on the jet are analyzed with the flow fields; the multi-objective genetic algorithm is called to complete the optimization of the actuator. The results show the followings: the interaction between the height and the length of the mixing section have a strong effect on the jet, and both of them determine the slenderness of the mixing section together; the more slender the mixing section, the easier it is for the jet to adhere to the wall of the mixing section, which leads to uneven jet sweeping and small deflection angle of the jet; the effect of the interaction between the second throat, the expansion angle and length of the expansion section on the jet is weak, which affects the degree of deflection of the jet at the second throat, the degree of the jet adhering to the wall of the expansion section and the size of the separation vortex in the expansion section, respectively; the optimized actuator increases the jet sweeping uniformity by 2% and the jet deflection angle by 5°.

A technique for the Monte Carlo simulation of the radiation in the carbon dioxide and nitrogen dissociation products in a shock wave is described: the rates of chemical reactions, as well as the excitation of electronic, vibration, and rotation levels of atoms and molecules of CO, CN, O₂, and C₂. A comparison of the results of numerical simulation and the experimental data obtained on shock tubes of the Institute of Mechanics of Moscow State University is presented.