Fluid Dynamics最新文献

To achieve rapid calculation of the aerodynamic forces of multi-wing butterfly-inspired ornithopters during flight, a quasi-steady aerodynamic force calculation method driven by multiple parameters and based on the blade element theory (BET) is proposed. This quasi-steady method can describe the flapping motion, the wing shape and the deformation rules of each wing, whether observed or expected, and calculate the instantaneous aerodynamic forces of the ornithopter according to the incoming flow conditions. The motion phase differences of each wing in the multi-wing ornithopter are also taken into account. Compared with the aerodynamic force measurement experiment of a 200-mm wingspan butterfly-inspired ornithopter, and considering the influence of the wing moment of inertia on the experimental measurement, the results show that the instantaneous values of the lift and thrust forces are consistent, and the maximum normalized peak error of the instantaneous lift force is 7.2%, and that of the instantaneous thrust force is 5.7%. This method can effectively describe changes in the aerodynamic force during the flight of butterfly-inspired ornithopters and provide guidance in the preliminary design stage.

The continuous advancement of space exploration has led to increasing demand for high-performance propulsion systems, which are critical for mission success, cost efficiency, and payload maximization. To address these requirements, innovative nozzle configurations are being explored to improve the propulsion efficiency and enhance the payload capabilities. Among these, the dual-bell nozzle stands out as a promising concept due to its dual-mode operation, enabling efficient performance under both sea-level and high-altitude conditions. Likewise, elliptical nozzles offer distinct advantages such as improved thrust vectoring, more uniform exhaust flow distribution, and reduced structural mass. In this study, a novel rocket nozzle configuration is introduced. This configuration represents an elliptical dual-bell nozzle, which integrates the dual-bell concept with an elliptic cross-section geometry. A numerical investigation employing computational fluid dynamics (CFD) is carried out to estimate the thermodynamic performance of the proposed design. A comparative analysis with a conventional axisymmetric dual-bell nozzle is also given. The results show that the elliptical dual-bell nozzle achieves a 28.93% increase in both the generated thrust and the thrust coefficient, thereby demonstrating the significant performance potential of the proposed design.

A numerical simulation is carried out to study the intensification of laminar heat transfer in an air flow along a stabilized hydrodynamic section in a narrow channel of height 1 and width 6, with a sparse array of oval-trench dimples (OTDs) inclined at 45°, having a depth of 0.25 and a length of 4.5, arranged at a pitch of 4 on a heated isothermal wall with the Reynolds number varying from 50 to 1500. As Re increases, a swirling flow is formed and intensified in the inclined dimple with a gradual increase in the static pressure difference between the flow’s braking zones on the windward slope of the inlet section and the negative pressure at the point of the generation of a tornado-like vortex. At high Re numbers, spiral vortices emerge from the rear of the dimple, with the flow and heat transfer gradients increasing on the leeward slope. At Re = 1500, the swirling flow’s velocity reaches 70%, the longitudinal velocity in the core of the channel flow is 2.15 times higher than the average mass velocity, and the relative heat transfer approaches 2 with relative hydraulic losses of 1.44.

The patterns of motion of marine organisms provide a new method of propulsion for underwater vehicles, in which flapping foil propulsion is one of the more representative ones. The flexible flapping foil has better propulsion efficiency. The actual flapping foil motion is active and is passively deformed by water. However, numerical simulation is realized by setting up active deformations, making it difficult to accurately reproduce the real deformations. In the study, a pressure tester was used to measure the displacement of the flapping foil to fit the real deformation of the flapping foil, the motion of the sea turtle flapping foil is simplified to the pitching motion in a two-dimensional plane, and its kinematics is modeled. Furthermore, the effects of flexibility as well as kinematic parameters on the propulsive performance of the flapping foil were investigated by means of numerical simulation. The results show that proper flexibility (R = 0.12–0.16) is beneficial to the hydrodynamic performance of flexible foils, but too much flexibility adversely affects the propulsive performance. At a frequency of 1 Hz and an amplitude of 0.075 m, the flexibility that allows the flexible foil to achieve maximum efficiency is equal to 0.13. For the Reynolds number at 35 000 for a flapping foil, the R = 0.06 foil generates more propulsive force than the R = 0.08 foil when the Strouhal number St is smaller than 0.45. Conversely, the opposite is true. The application of these research results to the design of underwater vehicles can provide new ideas for the development of underwater vehicles.

Among a variety of drag reducing techniques in high-speed vehicles, the use of aerospike is the most simple and reliable one. Unfortunately, this technique has a disadvantage that needs serious consideration. On one hand, a spike mounted on the high-speed blunt bodies reduces drag to a considerable amount. While on the other hand, the spike of a certain length tends to make the flow around the body unsteady thereby raising the possibility of structural failure or loss in maneuverability. There have been a number of studies devoting towards the flow unsteadiness arising due to the implementation of aerospike in recent past. Since the early 1950s, numerical simulation methods as well as experimental equipment have been used to conduct considerable investigation of this unsteadiness over high-speed spiked vehicles. In this study, the survey of these investigations is covered, and the recent contributions of the first author to the present field are shown. In addition to that, several areas of the science that require more research are also highlighted.

To investigate the deicing fluid concentrations across diverse global winter regions under varying ice scenarios, this study establishes a numerical model for wall-flow heat transfer of aircraft deicing fluid, validates the model accuracy via experimental verification, examines the wall-flow heat transfer characteristics under various ice types and deicing fluid concentrations, and conducts comparative analysis of the flow field and temperature field variations across distinct parametric conditions. Results demonstrate that the differential impact of distinct ice-layer materials and deicing fluid concentrations on the wall velocity field ranges between 2.4 and 5.0%. For varied ice-layer materials, both frost ice and mixed ice achieve complete melting within 240 s, with the melting duration of frost ice being one-third that of mixed ice, glaze ice attains a melting area with a radius of approximately 65 cm. Deicing fluids at various concentrations reduce melting time by from 45 to 63% as compared to hot water at identical temperatures. The 30% concentration is optimum for small-scale ice removal due to its rapid deicing performance, while the 50% solution maintains effective deicing for large aircraft wings. The 70% formulation, with higher viscosity and lower freezing point, proves suitable for extreme cold-weather operations. These findings advance precision in deicing operations across diverse ice types, demonstrating significant potential for reducing melting duration and conserving deicing fluid consumption.

This paper investigates the influence of key factors on droplet dynamics on superhydrophobic surfaces, providing theoretical guidance for their design and application. Using the fluid flow module of COMSOL Multiphysics, a two-dimensional simulation of droplet collision and coalescence–rebound was carried out on surfaces with various microstructures. The simulation results agreed well with experimental data, confirming the accuracy of the model. The study shows that as microstructural spacing increases, the solid–liquid contact area decreases, wall viscous dissipation reduces, and droplets retract and rebound more rapidly. The surface morphology, the contact angle, the droplet radius, and the initial kinetic energy strongly affect the dynamic behavior. During coalescence and bouncing, both the droplet radius and the microstructural morphology are decisive, while the higher initial velocity enhances rebound. Conversely, a smaller radii ratio between two droplets hinders detachment and may cause rebound deviation. Overall, six dominant factors were identified, namely, three related to the surface structure and three to the droplet properties. These findings establish a theoretical foundation for optimizing the design and functional application to superhydrophobic surfaces.

The numerical research work is carried out for the deflagration and detonation combustion process and pollution formation for a stoichiometric (ϕ = 1) mixture of zero carbon and a hydrocarbon fuel–air mixture in the pulse detonation combustor. Furthermore, the combustion efficiency also has been analyzed for hydrogen, kerosene and octane fuel–air mixtures inside the combustor. The SIMPLE algorithm with the finite volume discretization method is used for laminar finite rate chemistry with volumetric reaction in Ansys Fluent platform. The LES turbulence model is used to carry out calculations of the reliable and repeatable detonation wave in the pulse detonation combustor near thin boundary layer formed by the Shchelkin spiral. From the simulation, the detonation wave velocity of 2000 m/s and the reaction enthalpy of 71.4 MJ/kg are obtained for hydrogen–air combustion, which is higher as compared to those in kerosene and octane fuel–air combustion. The minimum pollutant number of 0.00000479 is obtained for hydrogen–air detonation and this magnitude is lower as compared to that for kerosene and octane–air combustion. Furthermore, the maximum combustion efficiency of 87% is obtained for hydrogen–air combustion in the detonation combustion process, which is comparatively higher than that for kerosene and octane fuel–air mixtures. Also, the combustion efficiency is more in detonation combustion for aforesaid liquid and gaseous fuel–air mixture combustion as compared to the deflagration combustion process.

The aerodynamic characteristics of amphibious aircraft in real wind and wave conditions are studied. An experimental investigation of jet lift control on airfoil in near-wave water is conducted. The novel wind-water tunnel testing methods are proposed. The study focuses on the aerodynamic performance of the basic airfoil, the lift enhancement performance of the jet, and ground/water surface effects. The results show that flap jet flow significantly increases the lift coefficient of the airfoil. When C_μ = 0.024, the lift increase is equal to approximately 48.3%. As compared to steady jet flow, unsteady pulsed jet flow has the greater lift enhancement potential, with a lift increase of about 32.5% at C_μ = 0.015. The dimensionless wave height A1 in the range from 0.653 to 0.928 results in a smoother stall characteristic compared to A1 = 0.307. The higher wave motion adversely affects the lift enhancement of the airfoil under jet control.

In this study, direct numerical simulation (DNS) is carried out to investigate flow around a wall-mounted hemisphere at a Reynolds number Re = 1000. The generation and transport characteristics of vorticity are analyzed based on the simulation results, deepening the understanding of the evolution mechanisms of vortex structures. The main flow features include near-wall recirculation vortices wrapping around the hemisphere, a large recirculation zone formed by flow separation at the apex, and hairpin vortices shedding downstream of the recirculation region along with several secondary vortical structures. From a vortex dynamics perspective, spanwise vorticity consistently dominates, contributing more than 60% to the total enstrophy. In the upstream recirculation vortices, spanwise vorticity is mainly amplified by stretching, while streamwise vorticity is generated through transfer from spanwise vorticity, manifested as spanwise stretching, spreading, and streamwise twisting of the recirculation structures. Vorticity generation occurs primarily on the windward face and upstream of the separation points due to fluid–surface interaction, followed by redistribution under the influence of surface curvature. In the near-wake evolution of arch vortices, strong transfer from wall-normal and spanwise vorticity to streamwise vorticity is identified as the key mechanism for the formation of hairpin vortices.