External Flow Field Research Articles

Analytical solution of supersonic axisymmetric flow around a sharp convex corner

Supersonic axisymmetric expansion flow is a typical and fundamental issue in gas dynamics. It plays a vital role in the high-speed external and internal flow fields regarding the contour design and performance evaluation of supersonic/hypersonic vehicles and their propulsion systems. The supersonic two-dimensional (2D) planar expansion flow is dominated by the well-known Prandtl–Meyer (P–M) theory. However, no similar explicit relation exists for the supersonic axisymmetric expansion flow, and only the computational fluid dynamics results could be employed at present. Therefore, this work focuses on developing the analytical solution of supersonic axisymmetric flow around a sharp convex corner on the basis of the generic gasdynamic functions in a newly established coordinate system for addressing the aforementioned issue. Theoretical derivations and numerical results prove that the flow deflection angle and Mach number in supersonic axisymmetric flow around a sharp convex corner obey the identical law to the 2D planar situation, that is, the P–M theory, while the local axisymmetric expansion fan is not the simple wave flow despite the conical flow. Meanwhile, the method of characteristics is employed to further explicate the intrinsic connection and difference between the 2D and axisymmetric sharp convex corner flow. The equivalence of sharp corner and curved surface flows with the identical deflection angle is discussed, and three limitations of the proposed analytical solution are clarified.

Design and optimization of dry powder jet cleaning nozzles for airport navigation lights

The research of this work is based on the nozzle design and optimization of dry powder (NaHCO3) jet cleaning system for airport navigation aid lamps. In this cleaning system, compressed air drives dry powder (NaHCO3) abrasive, and a gas-solid mixed high-speed jet is sprayed onto the surface of the luminaire through the nozzle to scour and clean the dirt attached to the surface of the luminaire. Based on the computational fluid dynamics and flow field theory, three types of nozzle structures, namely tapered nozzle, Laval nozzle, and flat nozzle, were studied; the DPM (Deformable part model) model in Fluent was used to analyze the internal and external flow fields of the nozzle mixed with the gas-solid two-phase flow to obtain the velocity field, pressure field and nozzle wear rate of the mixed gas, and the flat nozzle was selected and the nozzle parameters were optimized. The optimized level combination of flat type nozzle structure is obtained with a flat width of 4mm, flat height of 24.5mm, and flat length of 55mm.

Influence of the Projectile Rotation on the Supersonic Fluidic Element

The effects of projectile rotation on the internal and external flow fields of the supersonic fluidic element are numerically studied using sliding grid technique and the RNG k-ε turbulence model. The effects of rotating speed on internal and external flow fields, switching time and output characteristics are studied. The results show that: for the external flow field, there is no obvious change in the flow field structure at low angular velocity; when the angular velocity increases to 20 r/s, the flow field structure becomes obviously asymmetric due to the Coriolis force; the flow field far away from the surface of the projectile body (more than 0.3 m) is much more affected than the flow field near the surface of the projectile body. The influence of projectile rotation on the internal flow field is much weaker than on the external flow field, and the change of internal flow field is not obvious when the rotational speed is less than 20 r/s. The switching time decreases with the increase in angular velocity, and within normal range of the angular velocity, the deviation of switching time from that without rotation is within 5%. The change of thrust distribution is not obvious when the rotational speed is less than 20 r/s. However, when the rotational speed reaches 50 r/s, the thrust of the middle part of the right nozzle increases by about 20 N.

Backward Monte Carlo method for simulating spectral radiation characteristics of boost-gliding vehicle

The boost-gliding hypersonic vehicle possesses unique features of high Mach number, high stealth, and high maneuverability, becoming a new air defense threat. The detection of its spectral radiation characteristics is one of the early warning ways. The study of spectral radiation characteristics is essentially a process of solving the Radiative Transfer Equation (RTE). Backward Monte Carlo Method (BMCM) is the most accurate method to solve the RTE compared with other methods, such as the Finite Volume Method and Line-Of-Sight method. In this study, numerical simulations of spectral radiation characteristics of an HTV2-shaped aircraft, at three trajectory points of 70 km, 50 km, and 30 km, are conducted using the BMCM. The incoming flow around the aircraft is calculated by a two-temperature OpenFOAM solver. In particular, a long-range detection model for the BMCM is developed to improve simulation efficiency. The absorption coefficients are calculated by the Line-By-Line method. Eventually, a code termed Plume Work Station-Radiation is developed to obtain spectral radiation intensities of an HTV2-shaped aircraft in the range of 1−15μm. Simulation results indicate that the high-temperature aircraft surface makes a vital contribution to the total spectral radiation characteristics, while the external flow field only has a finite contribution at some sensitive bands. The spectral radiation intensity at 50 km is the highest among the three trajectory points. An intersection point appears between the spectral radiation intensities of 70 km and 30 km due to the localized high-temperature surface at 70 km. The wavelength of the intersection point varies with the detector direction, which is attributed to the variation of the projected area of the high-temperature surface with the detector direction.

Influence of vent size and vent burst pressure on vented ethylene-air explosion: Experimental and numerical study

This paper describes the numerical simulation of the vented explosion of ethylene-air mixtures. The influence of concentration, vent size, and vent burst pressure on the vented ethylene-air explosion is investigated by comparing simulation results and experiments. The different phases of external flame propagation are concluded by comparing the experimental photos and the simulation results. The external explosion process is captured by the high-speed camera and simulation. It is found that the unburned mixture expanded to the external atmosphere, which was ignited by the flame. The flow velocity, velocity vector, and turbulence kinetic energy are applied to analyze the mechanism of external flame propagation. The velocity vector is deflected to both sides of the flame front and the reverse flow velocity is captured in the external flow field. The change of ethylene concentration and the venting flow velocity is used to explain the relationship between the pressure rise, chemical reaction rate, and venting flow. Results show that the internal pressure for AV= 0.18 m2 is significantly greater than the internal pressure for AV= 0.55 m2 in this case. The smaller vent size results in higher flow velocity and turbulence kinetic energy inside of the vessel. The vent burst pressure growth promotes the acceleration of the flame front inside of the vessel. The variation curve of ethylene concentration and venting flow velocity has a similar law with different vent burst pressures. The experimental and simulated results can provide technical and theoretical support for the design of gas explosion prevention in the field of process safety and risk engineering.

Surface strengthening in confined spaces: A novel deflecting abrasive waterjet peening for improving the surface integrity of nickel-based superalloys GH4169

Abrasive waterjet peening (AWJP) has become an essential surface treatment technique to improve the surface integrity and mechanical properties of metallic parts due to its high controllability and small thermal influence. However, since existing AWJP processes are based on straight nozzles, they are difficult for complex geometrical profiles and confined spaces treatment, such as dovetail slots. In the present work, a novel deflecting AWJP (DAWJP) process with a designed deflecting nozzle was proposed and verified by experiments on the nickel-based superalloy GH4169. First, the DAWJP's deflecting nozzle was designed based on the AWJP's straight nozzle and deflecting pin with deflection angle. Second, the internal and external flow field distribution and abrasive movement state of deflecting nozzles with different deflection angles (R = 0, 1, 1.5, 2 mm) were studied by using the FLUENT CFD software. Finally, the surface integrity of the GH4169 specimen treated by the proposed DAWJP with an optimized deflecting nozzle was investigated, including microstructure, height difference, surface roughness and microscopic morphology, surface and subsurface microhardness and compressive residual stress (CRS). The results of this study highlighted that the deflecting nozzle with a deflection angle of 1.5 mm could obtain relatively optimized jet flow velocities of the water and abrasive mixed phases. The GH4169 specimen treated by the DAWJP with different processing parameters using the optimized deflection angle formed a plastic deformation depth of 22–51 μm, a height difference between the as-received and DAWJP treated surface of 4–48 μm, and the surface roughness Ra and Sa of 0.199–0.503 μm and 0.535–0.953 μm. The surface microhardness and CRS of the DAWJP treated specimen were approximately 522–573 HV and 773–991 MPa, which is 7.6 %–18.1 % and 209 %–296 % times higher than the as-received specimen, respectively. The work-hardened layer and CRS depth of the near surface of the specimen induced by the DAWJP treatment were approximately 120–180 μm and 112–203 μm. This work presents a novel DAWJP method and investigates its effect on the surface integrity of the GH4169 material, which is expected to have applications in improving the surface integrity and mechanical properties of metallic parts with confined spaces.

Research on a New Type of Catamaran with Shock-Absorbing and Energy-Saving Suspension Structure

At present, the suspended structure catamaran is composed of the main ship on the water and the side bodies symmetrically distributed on the hull. Such a structure avoids the direct contact of the main ship with the water surface, and has good stability. In this paper, a shock absorption system for ships is designed. It is undertaken by the double wishbone independent suspension system, which includes the upper wishbone, the lower wishbone, the bracket, the shock absorber and other components. The double wishbone suspension system is used in the shock absorption of this project, which can well cope with the huge impact brought by the waves. The 3D model of the hull is modeled, and the external flow field simulation of the catamaran is carried out by using starccm+ software, and the design optimization of the tail structure and thruster is realized.

Fluid-Thermal Interaction Simulation of a Hypersonic Aircraft Optical Dome

Hypersonic aircraft design is an enabling technology. However, many problems are encountered, including the design of the hood. The aircraft optical dome can become heated due to aerodynamic effects. Since the optical dome of a hypersonic aircraft should satisfy optical imaging requirements, a conventional ablative coating cannot be adopted. The aerodynamic heating characteristics during the whole flight must be studied. In this study, a numerical simulation method for the aerodynamic heat of hypersonic aircraft under long-term variable working conditions is proposed. In addition, the numerical simulation of the external flow field and structure coupling of the aerodynamic heat problem is performed. The dynamic parameters of temperature and pressure are obtained, and the thermal protection basis of the internal equipment is obtained. Numerical results indicate that the average temperature and maximum temperature of the optical dome for inner and outer walls exhibit an “M” shape with time, with two high-temperature cusps and one low-temperature cusp. The time of average temperature coincides with that of maximum wall temperature. During the flight, the wall pressure changes with time, exhibiting the characteristics of higher temperature at both ends of the flight and lower temperature in the middle. The structural temperature of the hypersonic aircraft is higher than that of the external flow behind the shock wave after 310 s. Therefore, this study provides a reliable reference for the preliminary design and parameter research of optical domes of hypersonic aircraft.

Study on Interior Aerodynamic Noise Characteristics of the High-Speed Maglev Train in the Low Vacuum Tube

As the next-generation high-speed transportation system, the low vacuum tube high-speed maglev system combines the tube with a certain degree of vacuum with the high-speed maglev train, which can realize high-speed operation under low aerodynamic resistance and noise mode. In order to study the interior aerodynamic noise characteristics of the high-speed maglev train in the low vacuum tube, a computational model of the external flow field of the high-speed maglev train in a low vacuum tube was established, and the computational model of the interior aerodynamic noise of the high-speed maglev train was established using the statistical energy analysis method; then the interior aerodynamic noise characteristics of the high-speed maglev train in the low vacuum tube were studied. The research results show that in the low vacuum tube, the distribution of the interior aerodynamic noise of the high-speed maglev train shows the characteristics of large head car and tail car and small middle car, and the aerodynamic noise on the top of the car is smaller than that on the floor. With the increase in frequency, the sound pressure level of the interior aerodynamic noise of the high-speed maglev train has the tendency of increasing first and then decreasing, and the main energy of the interior aerodynamic noise is distributed in the range of 200–1000 Hz. From the perspective of the total sound pressure level of the interior aerodynamic noise, the interior aerodynamic noise of the tail car is the greatest, followed by the head car, and the interior aerodynamic noise of the middle car is the smallest. As the direction of the travel of the maglev train will change, the optimization design of the interior aerodynamic noise of the head and tail cars should be emphasized.

Aspect ratio and combined installation in fluidic oscillator-crossflow interactions

This paper describes a computational study of fluidic oscillator-crossflow interactions. Fluidic oscillators are high potential active flow control devices, which offer promise for cooling and flow separation. Previous experimental studies have investigated the internal/external flow fields of fluidic oscillators. This improved the understanding of three-dimensional flow fields; however, this has been limited to single aspect ratio nozzles in single variable installations. Research has not analyzed the effects of varying both oscillator parameters and installation conditions. Therefore, the present aim is to enhance flow field understanding by examining three aspect ratio (AR) nozzles in default perpendicular condition and in a “skewed incline” configuration (β = 60°, α = 75°). An unsteady Reynolds averaged Navier Stokes model was validated with prior experimental data and then used to evaluate time-averaged and time-accurate flow fields at three different blowing ratios. Strouhal number analysis uncovers the effectiveness of parametric changes. Internal flow fields are presented, identifying key differences and similarities. External flow field analysis in regard to spreading angles 55°–65° observes greater wall-normal penetration for lower AR oscillators with improved spanwise penetration for larger AR's. The combined installation produces stronger primary vortices, improving penetration along the wall, while inclination keeps vortices close to the wall. Oscillator effectiveness is dependent on the application.

Visualization of the external flow field during a vented explosion for hydrogen-air mixtures: Effects of hydrogen concentrations and vent areas

To study the effects of hydrogen concentrations and vent areas on the vented hydrogen-air explosion, series of experiments were performed in a cylindrical tube under the conditions of 1 atm and 10 ℃. The pressure oscillation frequency is proportional to the hydrogen concentration, while the Helmholtz oscillation disappeared gradually with the increase of Kv. The position inside the container which was farther away from the vent cover would experience the higher dynamic loading. The volume concentration of the hydrogen at the external field (cExt) was always lower than the initial hydrogen concentration, that increased with the increase of the initial concentration, while it was zero at some concentrations with the increase of Kv. The external flame velocity of rich fuel was always larger than that of lean fuel. The deflagration to detonation (DDT) in the outer field was easier to achieve for the concentration near to 50 vol% and for lower Kv. The secondary explosion wave (PExt) increased firstly and then decreased with the increase of hydrogen concentration, which decreased with the increase of Kv in general. The maximum surface temperature rose with the increase of concentration and dropped with the increase of Kv in general. The high-temperature field was closer to the vent for lower concentration and higher Kv.

Study on bottom thermal environment of launch vehicle during high altitude flight

Aiming at the problem of the thermal environment at the bottom of the launch vehicle in the high-altitude flight stage, taking the launch vehicle with core stage four-way machine as the model, the coupled convection / radiation heat transfer problem at the high-altitude of the launch vehicle is deeply studied through the numerical simulation method, the gas jet flow model of the launch vehicle in the high-altitude flight stage is established, and the flow field information of the rocket under different working conditions and the heat flow at the bottom of the rocket are obtained. The numerical results are compared with the experimental data to verify the effectiveness of the selected model and calculation method. The research shows that the external flow field of the rocket body will have a great impact on the thermal environment at the bottom. With the continuous increase of flight altitude, the convective heat flow at the bottom of the rocket body first rises and then decreases. In the high-altitude stage, the total heat flow at the bottom of the rocket body is dominated by radiant heat flow, the convective heat flow outside the main nozzle is dominant, and the radiant heat flow outside the traveling jet nozzle is dominant.

A Review on Hydrodynamic Performance and Design of Pump-Jet: Advances, Challenges and Prospects

A pump-jet, which is generally and widely adopted on underwater vehicles for applications from deep sea exploration to mine clearing, consists of a rotor, stator, and duct, with the properties of high critical speed, high propulsion efficiency, great anti-cavitation performance, and low radiated noise. The complex interaction of the flow field between the various components and the high degree of coupling with the appendage result in the requirements of in-depth research on the hydrodynamic performance and flow field for application and design. Due to the initial application on the military field and complicated structure, there is scant literature in the evaluation of pump-jet performance and optimal design. This paper, in a comprehensive and specialized way, summarizes the pump-jet hydrodynamic performance, noise performance, and flow field characteristics involving cavitation erosion and vortices properties of tip-clearance, the interaction between the rotor and the stator and the wake field, as well as the optimal design of the pump-jet. The merits and applications range of numerical and experimental methods are overviewed as well as the design method. It also concludes the main challenges faced in practical applications and proposes a vision for future research. It was found that the compact structure and complex internal and external flow field make the pump-jet significantly different, also leading to higher performance. As the focus of cavitation research, vortices interact with the complex structure of the pump-jet, leading to instabilities of the flow field, such as vibration, radiated noise, and cavitation erosion. The effective approaches are adopted to reduce radiated pump-jet with minimal influence on the hydrodynamic performance, such as eliminating the tip clearance and installing the sawtooth duct. Advanced optimal technology can achieve high performance, cavitation performance, and acoustic performance, possessing good prospects. Further developments in investigation and the application of pump-jets in the multidisciplinary integration of fluid dynamics, acoustics, materials, chemistry, and bionics should be the main focus in future research.

Vented hydrogen-air explosions at elevated static activation pressures

The flame behaviors and pressure characteristics of vented hydrogen-air explosions at elevated static activation pressures (Pstat) were investigated using a 20 L spherical vessel. For vented flame, it was found that the maximum flame lengths and widths raised with the increase in venting diameters (Dv) and equivalent ratios (Φ) by dominating the concentration of vented unburned gas, while they were insensitive to the change of Pstat. Based on the variation law, non-dimensional formulas for predicting maximum flame length and width were established and the relative error was within 50 %. For internal pressure, the pressure curves were characterized with notably oscillations and an empirical formula coupled with NFPA 68 was proposed with a relative error within 25 %. For external pressure, the mechanism of the external explosion was revealed with the assistance of BOS images. The intensity of external explosion mainly depended on the external flow field intensity and the concentration of the unburned gas. The peaks of external explosion pressure and the pressure recession indices reached their maximum at Dv = 125 mm due to the conflicting effect of the increased Dv.

Numerical analysis of deformation and failure mechanism of metal truss structure of spacecraft under re-entry aerothermodynamic environment

A coupled aerodynamic-thermoelastic analysis framework is presented to simulate the deformation and failure mechanism of metal truss structure for spacecraft under re-entry aerothermodynamic environment. The three-dimensional dynamic thermo-mechanical coupling finite element algorithm is developed by computable modeling of thermoelasticity and heat conduction equations to solve the thermo-mechanical coupling response of spacecraft structure. The aerodynamic boundary conditions such as temperature, heat flux and pressure distribution are computed and provided by the gas-kinetic unified algorithm (GKUA) based on the Boltzmann model equation. The linear interpolation method is designed and employed to couple the boundary conditions between the external aerodynamic flow field and the solid structure interface. The present thermo-mechanical coupling algorithm is verified on one-dimensional and two-dimensional transient heat flow problems with analytical solutions, and good consistency is achieved. The reliability of the GKUA for solving the boundary conditions of the external flow field and the dynamic thermo-mechanical coupling algorithm for the thermal response behavior of the internal structure material is verified by the consistency calculation of the internal and external temperature distribution. Then, the aerodynamic-thermoelastic analysis framework is further applied to study the deformation behavior of structural thermo-mechanical responses and failure mechanism of a hollow sphere under re-entry aerothermodynamic environment with emphasis placed on the influence of flight speeds, rarefied effects, algorithm schemes and damping effects. The numerical results obtained by the presented algorithm including the internal temperature, thermal stress and deformation distribution law are compared and analyzed. Finally, the deformation and failure mechanism of spacecraft structure during re-entry process are solved and analyzed by the present algorithm framework. The displacement, temperature and stress distribution contours of the structure are presented to make a preliminary understanding and evaluation on the process of spacecraft deformation and failure disintegration. It is indicated that the present coupled aerodynamic-thermoelastic analysis framework can provide a reliable, applicable and efficient platform for the thermo-mechanical coupling responses and failure mechanism of re-entry spacecraft in the end of life.

Effects of injection angle and injector mode variation on the fluidization performance of a powder-fueled ramjet

Fluidization is a major problem in the supply system of a powder-fueled ramjet. The performance of fluidization mainly depends on the injection way of fluidizing gas. This paper has investigated the effects of variation of the injection angles and injector modes on the performance of fluidization, using the Euler-Lagrange approach for gas-particle flow. The injection angle varies among 30°, 45°, 60° and 90°, the injector mode includes single injector and dual injectors. We conclude that due to higher mixing quality and particle mass flow rate, the system performs better for both single-injector and dual-injector modes at an angle of 45°. Based on the analysis in the fluidization domain and external flow field, we observe that the dual-injector mode performs significantly better than the single-injector mode.

Experimental investigation of internal and external flow fields of jetting nozzles with different structures

Jetting has been widely used in the petroleum fields, the nozzle has been investigated for a long time and the structure has been optimized for different applications as the key part of jetting. However, most of these researches are focused on the outer flow with simulations, the real flow characteristics are not been revealed clearly. A visual experimental platform was built with Particle Image Velocimetry (PIV) and a high-pressure pump. 40 nozzles of different profiles were machined and measured. The internal and outer flow fields were captured by PIV under low velocity, and the pressure drop and flow displacement were measured under high-pressure conditions in this paper. The jetting performance such as discharge coefficient, flow core length, jet angle, and velocity distributions were analyzed. The internal flow characteristics including the flow field near the nozzle wall were captured by PIV. The converging angle and the throat section length play an important role in the jetting performance and are suggested to be set to 15° and 3 times the nozzle diameter when designed. The throat length has little effect on the flow field of the streamlined nozzle compared to the cone-straight nozzle, especially the concave curve nozzle. The vortex generated at the corner and the entrance of the throat section can indicate the boundary layer separation and transition, which agrees with our previous simulation work. All work will provide theoretical support for the design of the nozzle on-site application.

Analysis of Flow Field Aero-Optical Effects on the Imaging by Near-Earth Space All-Time Short-Wave Infrared Star Sensors

In order to study the performance of the all-time short-wave infrared (SWIR) star sensor mounted on a high-speed platform in near-earth space, a model of aero-optical effects is established to quantitatively analyze the influence of the flow field on its imaging. Firstly, the LES model is chosen to calculate the external flow field characteristics. Then, the degradation results of the transmission effect are analyzed based on the refractive index field, and the imaging degradation model of the star sensor under aero-thermal conditions is constructed. The optical transmission effect simulation shows that the star point offset increases with the rise of flight speed. Compared with the visible band, when the velocity reaches 8Ma, the star point offset in the J, H, and Ks bands declines by 46.6%, 62.1%, and 71.1%, respectively. The research on the thermal radiation effect presents that the stargazing ability of star sensors is barely affected with a speed of no more than 4Ma. As the speed increases to 8Ma, the detectable limiting magnitudes of the J, H, and Ks bands are 3.943, 2.674, and 1.313. H band and the middle region are suggested to maximize the performance of SWIR star sensors. The research quantitatively analyzes the influence of flow field aero-optical effects on the image quality, which provides theoretical support for the system design, parameter selection, and installation position of SWIR star sensors applied in near-earth space.

Optimization on the jet characteristics of the main nozzle of hot-blowing snow removal device

The exhaust devices used by snow removal vehicles are mainly based on hot-blowing snow removal. Due to the inherent characteristics of the turbojet engine such as low flow rate and high exhaust temperature, the modified hot-blowing snow removal device has problems of low snow removal efficiency and the possibility of ablation and damage to the road surface. In order to solve the problem, transforming the turbofan engine with a high flow rate and a low exhaust temperature into a hot-blowing snow removal device is an important improvement to achieve efficient and safe snow removal operations. In order to convert a medium bypass ratio turbofan engine into a hot-blowing snow removal device, the computational fluid dynamics (CFD) software CFX was used to analyze the internal and external flow field characteristics of the main nozzle of an exhaust device under the 60 % rated condition of the engine. The effective area sizes of the jet velocity and temperature predicted by the external flow field were used to optimize the layout of the main nozzle.

Hydrodynamic and Flow Field Characteristics of Water Jet Propulsion under Mooring Conditions

The acceleration characteristics of a water jet-propelled ship during startup are related to its performance under mooring conditions. Water jet propulsion cavitation during startup increases the vibration and noise of the whole ship. Therefore, accurately predicting and analyzing the performance, hydrodynamics and flow field characteristics of water jet-propelled ships under mooring conditions can help elucidate the startup characteristics of the ships and optimize their acceleration strategies. In this study, the hydrodynamic and flow field characteristics of water jet propulsion and water jet propulsion ships under mooring conditions were studied using three-dimensional numerical modeling. First, the hydrodynamic performance of the water jet propeller was analyzed, and the relevant flow field law was derived. Then, the hydrodynamic performance, internal and external flow field characteristics, pulsation pressure and flow rate at the nozzle, and pulsation pressure at the monitoring points around the impeller of the water jet propulsion ship model were analyzed under mooring conditions. We obtained the open-water law for the water jet propeller and the hydrodynamic force and flow field law for a two-pump water jet propulsion ship. The ship model developed in this study provides a good theoretical foundation for further research on water jet propulsion.