Evolution Of Flow Field Research Articles

Study on the characteristics of the transient flow field under different underwater environments

The underwater muzzle transient flow field is an unsteady, multiphase complex flow field interacting with projectiles and containing various shock wave structures. The turbulent mixing of gunpowder gas and water has a significant impact on the development of the muzzle gas flow field. Moreover, the muzzle gas flow field disturbs the motion of the projectile, thereby affecting shooting accuracy. As part of this research, an unsteady multiphase flow model of the underwater muzzle transient flow field is established by combining the theories of multiphase flow and turbulent mixing. The volume of fluid model is employed to trace the two-phase interface, while the gas–liquid turbulent mixing is described by the standard k–ε turbulence model. Furthermore, the cavitation model is used to describe the cavitation phenomenon caused by the motion of the projectile. The established numerical model is validated by comparing underwater launching experimental results. Accordingly, the muzzle flow field of a 30 mm underwater gun under different water depth conditions is numerically calculated. The results demonstrate that, as the water depth increased, the gunpowder gas is exposed to relatively high water pressure during the expansion process, resulting in a continuous decrease in the core area of the gas, and the Mach disk is also increasingly closer to the muzzle. At different water depths, the diameter of the Mach disk conforms to the binomial law with time, while the displacement of the Mach disk from the muzzle increases exponentially with time.

Experimental and numerical investigation on the premixed methane/air flame propagation in duct with obstacle gradients

This work experimentally and numerically investigates the effect of the obstacle gradient on the characteristics of the methane/air explosion in obstructed ducts. The obstacle gradient is expressed through various blockage ratios, and three different obstacle gradients are investigated, i.e., C357, C555, and C753. Noted that C357 means that the blockage ratios of three obstacles arranged sequentially in the duct are 0.3, 0.5 and 0.7 respectively, and so do C555 and C753. A two-dimensional (2D) model is adopted and the Scale-Adaptive Simulation method with the thickened flame model is considered. Experimental results show that the obstacle gradient significantly affects the flame evolution structure, flame propagation speed and overpressure. The obstacle gradient is accountable for the “tulip flames” appearing downstream of the obstacle. For the case with a fixed methane volume fraction, the average flame front speed is fixed. However, with the increasing obstacle gradient, the maximum flame front speed increases until it achieves the maximum at C357, and so does the maximum overpressure. The numerical simulation can predict the flame evolution behaviour precisely. It is evident that the generation of different flame shapes appearing is derived from the flow field evolution of unburned gas downstream of the flame front.

Research on similarity of water entry load for scaled-down underwater vehicle based on different model test environments

The incompatibility of similarity quotients and the lack of test simulation capability can be effectively addressed by overcoming “similarity constraint” using diverse model test environments (varying parameters such as atmospheric pressure and gravitational acceleration). In order to examine the scaling effects in different model test environments, this paper presents a numerical study of the prediction accuracy of a flat-head scaled-down vehicle for full-scale prototype water entry impacts in the normal compression, reduced compression and hypergravity model test environments using the experimentally validated FLUENT overlapping mesh technique. The study compared and analyzed the pressure load and flow field evolution under different initial conditions. The findings revealed that the over-simulation of the entry cushion effect of the scaled-down model and dynamic pressure in the normal compression environment led to inaccurate prediction of the full-scale prototype. The prediction results improved in the reduced compression environment than in the normal compression environment. However, the cavitation effect was over-simulated and the dynamic pressure was weakened. In contrast, the hypergravity environment enables the simulation of solid and fluid inertia to the same extent, satisfying the scaled-down model for full prediction of the prototype water entry cavity evolution and impact loads.

Nonlinear reduced-order modeling for three-dimensional turbulent flow by large-scale machine learning

A large-scale machine learning-based nonlinear reduced-order modeling method was developed for a three-dimensional turbulent flow field (Re=1000) using a neural-network with unsupervised learning. First, a mode decomposition method was applied to three-dimensional flow field data using a convolutional-autoencoder-like neural network. Then, a reduced-order model (ROM) was constructed using long short-term memory neural networks (LSTMs). Consequently, it was demonstrated that the time evolution of the turbulent three-dimensional flow field can be simulated at a significantly lower cost (approximately three orders of magnitude) without a major loss in accuracy. However, neural-network-based mode decomposition for the three-dimensional flow field requires a huge computational cost in terms of calculation and memory usage. Therefore, a distributed machine-learning method was implemented using a hybrid parallelism scheme tailored to the network structure. Thus, it was possible to decompose 1.7 million cells of the three-dimensional flow field data into 64 modes and reproduce those with sufficient accuracy. In this study, a uniform flow around a circular cylinder model was used as a test case. To validate the method, the reduction performance of the proposed mode decomposition method was compared with the proper orthogonal decomposition (POD) method. Furthermore, the target flow field was reproduced using ROM, and the reconstruction accuracy was evaluated in terms of various criteria compared with the accuracy based on POD in conjunction with Galerkin projection method.

Evolution of flow field around high-speed trains meeting at the tunnel entrance under strong wind-rain environments

Tropical storms pose a great threat to the traffic safety of tunnel-embankment transition sections in coastal areas, especially when two trains are meeting. A series of 3D computational fluid dynamics numerical simulations of wind-rain-tunnel-embankment-train are established using the Eulerian multiphase and the Shear-Stress Transport k-ω models. The numerical model's reliability is verified by wind tunnel tests using rainfall simulation technology. The differences in the train's aerodynamic performance before and during the meeting under different wind-rain conditions are analyzed. The rain phase's impact mechanism on the flow field is revealed. Results show that: Before the meeting, the rain phase will worsen the train's aerodynamic performance. When the wind speed and rainfall intensity is 20 m/s and 400 mm/h, the head train's average lift force (C‾y), yawing and pitching moments (C‾mz) increase by 6.25%, 9.68% and 10.31%, respectively. The rain phase increases the wind-rain load amplitude during the meeting, and the head train's pitching moment increases by 10.7%. The moment is more worthy of attention than that of force under a rainy day, and the change rate of C‾mz is 4.9 times of that of C‾y. The amplification effect of rain on wind-rain loads may endanger the driving safety of trains at the tunnel entrance.

The investigation of a likely scenario for natural tornado genesis and evolution from an initial instability profile

A likely mechanism for the little-understood tornado genesis is proposed and its numerical implementation is presented. The Burgers-Rott vortex with its axis in the vertical direction is introduced as an instability mechanism, and the flow field then evolves under the influence of the atmospheric pressure, temperature and density variations with altitude. Buoyancy effects are implemented using the Boussinesq model. Results are presented and discussed for a set of conditions including mesh type and size, different turbulence models, and a few different boundary conditions. Post-processed results of the transient simulations including animations contain a wealth of information to help analyze tornado behavior. Velocity contours, pressure contours, vorticity contours, streamlines, and iso-surfaces show the evolution of a complex flow field possessing many characteristics of a tornado. At longer times from the start, the flow field becomes more asymmetric with the vortex core becoming more twisted, and the eye of the vortex drifting away from the axis of the computational domain. The single initial vortex then transitions into multiple vortices of varying size and orientation. These high Reynolds number (ReΓ ∼109) simulation results show flow fields that resemble highly unsteady, turbulent flows with large regions of flow separation, and large eddy size range.

Physics-informed neural networks (PINNs) for 4D hemodynamics prediction: An investigation of optimal framework based on vascular morphology

Hemodynamic parameters are of great significance in the clinical diagnosis and treatment of cardiovascular diseases. However, noninvasive, real-time and accurate acquisition of hemodynamics remains a challenge for current invasive detection and simulation algorithms. Here, we integrate computational fluid dynamics with our customized analysis framework based on a multi-attribute point cloud dataset and physics-informed neural networks (PINNs)-aided deep learning modules. This combination is implemented by our workflow that generates flow field datasets within two types of patient personalized models - aorta with fine coronary branches and abdominal aorta. Deep learning modules with or without an antecedent hierarchical structure model the flow field development and complete the mapping from spatial and temporal dimensions to 4D hemodynamics. 88,000 cases on 4 randomized partitions in 16 controlled trials reveal the hemodynamic landscape of spatio-temporal anisotropy within two types of personalized models, which demonstrates the effectiveness of PINN in predicting the space-time behavior of flow fields and gives the optimal deep learning framework for different blood vessels in terms of balancing the training cost and accuracy dimensions. The proposed framework shows intentional performance in computational cost, accuracy and visualization compared to currently prevalent methods, and has the potential for generalization to model flow fields and corresponding clinical metrics within vessels at different locations. We expect our framework to push the 4D hemodynamic predictions to the real-time level, and in statistically significant fashion, applicable to morphologically variable vessels.

Large eddy simulation on trenched film cooling with forward and backward injections under pulsed conditions

Considering the strong turbulence effect and inevitable environmental pulsation in a practical film cooling process, this work exhibits both time-averaged and transient performances of trenched film cooling with forward and backward cooling air injections by a series of large eddy simulations. The blowing ratio in the steady case is 1.5, while cosine and square waves are selected as pulsed boundaries with a Strouhal number of 0.254. The spatial-temporal evolution of flow and temperature fields is revealed, from which two important conclusions are obtained: 1) opposite to the results obtained under steady conditions, the pulsed backward injection decreases the time-averaged cooling effectiveness by over 15%, and the film covered area also shrinks remarkably compared to forward injections. This phenomenon is determined by the different flow mechanisms between the steady and pulsed conditions. 2) Under a steady blowing ratio, the film cooling unsteadiness is caused by the temporal fluctuation of the near-wall vortex, which may induce a fast change in temperature and component failure. For the pulsed forward injection, additional high film cooling unsteadiness appears in the lateral sides of the film hole inside the trench. For the backward injection, the pulsation does not result in an obvious increase in unsteadiness.

Study on Effect of Piston Bowl Depth on Intake Air Flow Behaviour Using Ansys Fluent Cold Flow

Gas flow behaviour is an extremely complicated question on internal combustion engine and plays a key role in air-fuel mixing performance. A proper mixture process significantly improves the combustion performance and thus enhances the engine efficiency. This study estimates the effect of different piston bowl depths on the flow field development inside the combustion chamber of a CNG engine using Ansys Fluent. The cold flow model was utilized without considering the combustion behaviour. The three-dimensional model was developed based on a single cylinder CNG converted engine. Four different bowl heights were applied while the compression ratio was maintained as the designed value of 10. The tumble and swirl as well as the turbulent flow pattern were investigated to consider the gas flow behaviour. The simulation results indicate that the tumble and swirl ratio increase with increasing of piston bowl depth. The case of bowl height 17 mm shows better flow behaviour and higher velocity for a good mixing performance as well as turbulence dissipation.

Forebody Articulation at High Angles of Incidence

An experimental investigation was carried out to understand the effect of forebody articulation on the aerodynamic characteristics and the flowfield development over a slender body at high angles of incidence. The experiments were carried out at the Florida A&M University and Florida State University low-speed wind tunnel over a range of forebody deflection (, , and ) and flow conditions. The results show that a positive forebody deflection increases the normal-force coefficient, whereas a negative forebody deflection decreases the normal-force coefficient at high angles of incidence. Pitching moment characteristics showed that all forebody deflections tested produce a pitching moment despite minimal normal force at low angles of incidence. Lateral aerodynamic characteristics showed that the side-force onset angle does not solely depend on the semi-apex angle of the forebody (as predicted for axisymmetric slender bodies), and the role of the forebody-to-aftbody transition geometry in initiating vortex asymmetry is equally important. The particle image velocimetry results show that a positive forebody deflection increases the size, strength, and circulation of the vortices developing on the body, leading to an increase in aerodynamic forces and moments. The flowfield development for a negative forebody deflection is dominated by secondary shear-layer vortices originating from the forebody to aftbody junction.

Multi-physical Modeling and Adjusting for Ultrasonic Assisted Soft Abrasive Flow Processing

The polishing efficiency of the soft abrasive flow (SAF) method is low, which is not in line with the concept of carbon emission reduction in industrial production. To address the above issue, a two-phase fluid multi-physics modeling method for ultrasonic-assisted SAF processing is proposed. The acoustics-fluid coupling mechanic model based on the realizable k-ε model and Helmholtz equation is built to analyze the cavitation effect. The results show that the proposed modeling and solution method oriented to ultrasonic-assisted SAF processing have better revealed the flow field evolution mechanism. The turbulence kinetic energy at different ultrasonic frequencies and amplitudes is studied. Simulation results show that the ultrasonic vibration can induce a cavitation effect in the constrained flow channel and promote the turbulence intensity and uniformity of the abrasive flow. A set of comparative polishing experiments with or without ultrasonic vibration are conducted to explore the performance of the proposed method. It can be found that the ultrasonic-assisted SAF method can improve the machining efficiency and uniformity, to achieve the purpose of carbon emission reduction. The relevant result can offer a helpful reference for the SAF method.

Study on the effect of inflow direction on the hydrodynamic characteristics of underwater manipulators

When operating in water, an underwater manipulator is subjected to different directions of flows. Thus, its hydrodynamic performance and flow-field evolution vary. Herein, the hydrodynamic coefficients and flow field characteristics of an underwater manipulator with different incoming flow directions in a subcritical range are studied. The effects of the different inflow directions on the hydrodynamic coefficients and the flow field evolution of the sectional shape, spacing, and arrangement order are numerically investigated. Results indicate that the hydrodynamic coefficients of the upstream circular section decrease at γ = 60° as the angle of the inflow direction increases. The root-mean-square value of the lift coefficient of the downstream elliptical section increases by a factor 1.5 with an increase in the inflow angle. When the elliptical section is upstream, the working attitude of the underwater manipulator is relatively stable. The research conclusions of this work will provide help for the precise positioning and fishing of underwater manipulators in the ocean.

The effect of gravity on self-similarity of Worthington jet after water entry of a two-dimensional wedge

The effect of gravity on the self-similarity of jet shape at late stage of Worthington jet development is investigated by experiment in the study. In addition, the particle image velocimetry (PIV) method is introduced to analyze the development of flow field. There is a linear scaling regarding the axial velocity of the jet and the scaling coefficient increases with the Froude number.

Research on time series prediction of the flow field in supersonic combustor based on deep learning

The detection of supersonic combustion flow field provides valuable reference information for the efficient combustion of hypersonic vehicles. From the perspectives of scramjet operation during flight as well as experiment where the measured information is limited, flow field prediction using deep learning technology is a promising method that can provide future flow field evolution in scramjet combustors. This study proposes a convolutional neural network model of multi-temporal pressure information fusion to predict combustion flow field based on wall pressure. In addition, it analyzes the effect of various wall pressures on the flow field prediction. An experimental dataset of supersonic ground-pulse wind tunnel tests with different injection pressures is built through the combustion chamber wall under self-ignition conditions of a hydrogen-fueled scramjet. The results showed that the predicted flow field of the combustor agrees with the ground test, average correlation coefficient reached more than 90% and when the wall pressure number is 32, the average peak signal to noise ratio reaches about 23. and the main wave structure and turbulent boundary layer region are restored even if there is a shock boundary layer interaction.

Transmissive Mode Laser Micro-Ablation Performance of Ammonium Dinitramide-Based Liquid Propellant for Laser Micro-Thruster.

The transmissive mode laser micro-ablation performance of near-infrared (NIR) dye-optimized ammonium dinitramide (ADN)-based liquid propellant was investigated in laser plasma propulsion using a pulse YAG laser with 5 ns pulse width and 1064 nm wavelength. Miniature fiber optic near-infrared spectrometer, differential scanning calorimeter (DSC) and high-speed camera were used to study laser energy deposition, thermal analysis of ADN-based liquid propellants and the flow field evolution process, respectively. Experimental results indicate that two important factors, laser energy deposition efficiency and heat release from energetic liquid propellants, obviously affect the ablation performance. The results showed that the best ablation effect of 0.4 mL ADN solution dissolved in 0.6 mL dye solution (40%-AAD) liquid propellant was obtained with the ADN liquid propellant content increasing in the combustion chamber. Furthermore, adding 2% ammonium perchlorate (AP) solid powder gave rise to variations in the ablation volume and energetic properties of propellants, which enhanced the propellant enthalpy variable and burn rate. Based on the AP optimized laser ablation, the optimal single-pulse impulse (I)~9.8 μN·s, specific impulse (Isp)~234.9 s, impulse coupling coefficient (Cm)~62.43 dyne/W and energy factor (η)~71.2% were obtained in 200 µm scale combustion chamber. This work would enable further improvements in the small volume and high integration of liquid propellant laser micro-thruster.

Flow structure and parameter evaluation of conical convergent–divergent nozzle supersonic jet flows

Supersonic gas jets in conical convergent–divergent nozzles are studied numerically using the OpenFOAM rhoCentralFoam solver. The spatiotemporal evolution of the jet flow field is analyzed. The influence of the operating conditions on the flow field is studied parametrically, including the nozzle pressure ratio (NPR), area ratio, and throat position. The behaviors and mechanisms of the double-diamond structure, throat wave, and exit wave are interpreted in detail. The results show that a conical convergent–divergent nozzle always generates shock waves. The throat shock reaches its maximum length during the initial stage, then becomes slightly shorter before becoming stationary, dominated by the exit velocity. Furthermore, it is shown that the jet flow changes from overexpansion to underexpansion with increasing NPR. With an increasing area ratio, the trend is the opposite. The throat position affects the jet divergence angle at the nozzle exit, consequently causing a variation in the core radius of the jet. It is further shown that the double-diamond structure does not always appear. The throat shock angle, exit wave angle, and shear layer width directly affect the shape of the double-diamond structure. The favorable pressure gradient of the nozzle ultimately dominates the changes in the length of the throat wave and exit wave.

Study on the influence of helicopter rotor flow on projectile motion

In this paper, the muzzle flow field generated during helicopter gun firing is numerically studied. Based on Navier-Stokes equation and Standard k − ε turbulence model, the projectile motion is realized by using overlapping mesh technology. The development of the muzzle flow field formed by complex wave systems such as Mach disk and shock wave under the influence of helicopter rotor flow at different speeds, as well as the force and motion state of the projectile in the flow field are analysed. The results show that the degree of asymmetry of the flow field, the force and offset of the projectile are positively correlated with the rotor flow velocity. The research will help to improve the understanding of the muzzle flow field of helicopter gun and provide a new idea for improving the firing accuracy of helicopter gun..

Shock wave dynamics and venting overpressure hazards induced by indoor premixed hydrogen/air explosion

To further investigate the venting shock wave dynamics and overpressure hazards induced by indoor hydrogen/air explosion, an explosion-venting model of 6 m × 3 m × 2.5 m with a vent was established by computational fluid dynamics (CFD) technology. The results show that the combustion rate structure of venting flame composed of three peaks completely reflects the evolution of the venting flow field, and the significance and arrival time of the peak R3 reflect the severity and hysteresis of the external explosion; The propagation of the external explosion wave in two directions, away from and near the vent, resulting in the evolution of the traditional two-wave and three-area structure into a three-wave and four-area structure. The new wave structure contains the reverse cancellation of the external explosion wave and explosion-venting wave, and the co-direction superposition with the precursor wave, in which the wave-wave superposition forms a new significant peak Ps-max; The distribution characteristics of venting overpressure hazard under the external explosion and wave-wave superposition become unpredictable. The severe damage zone to building will evolve repeatedly on the explosion-venting path, and the dead zone may appear far away from the vent. The research results provide a scientific basis for the prevention and mitigation of explosion disasters in hydrogen-related industrial plants.

Cluster Partition Operation Study of Air-Cooled Fan Groups in a Natural Wind Disturbance

This study discusses the influence of natural wind on the air flow of air-cooled condensers (ACCs) and then proposes a partition speed-regulation strategy for a fan group with enhanced generalized capability, which is of great practical significance for optimizing energy-saving operations. The stochastic time-varying features of natural wind are characterized by sine–Gaussian, Weibull, and composed winds. In a natural wind disturbance, using the Sugon Supercomputing Center, the transient numerical simulation of the dynamic evolution of the ACC flow field was found: the dynamic system of air flow is a typical time-varying nonlinear process. Cluster analysis was used to extract the nonlinear features of air flow, divide the fan group into four subregions with generalization capability, and implement a partitioned speed operation. It was found that giving priority to increasing the fan speed in the headwind partition can suppress the natural wind disturbance and improve the overall air flow, thus reducing the fan speed in the leeward partition, which reduces the overall air flow loss. The dynamic characteristics of the fan group obtained from the simulation and the proposed fan partition method can guide the optimized energy-saving operation of ACCs.

Numerical Investigation of the Stress on a Cylinder Exerted by a Stratified Current Flowing on Uneven Ground

In this study, a three-dimensional internal wave (IW)—cylinder—terrain coupled numerical model is established. Based on the large-eddy simulation (LES) method, the IW mechanical characteristics of the cylinder and the flow field evolution around the cylinder over different types of terrains are explored. The similarities and differences in the mechanical characteristics of the cylinders in the environments with and without terrains are compared. The research results show that, when the IWs propagate over terrain, the waveform structures are prone to continuous changes. The intense reverse alternating flow of the upper and the lower water, bounded by the pycnocline, results in huge IWs forces differences between the case without terrains and the cases with terrains. In the case without terrains, the maximum horizontal resultant force on the cylinder is positive, while the resultant forces are negative in the cases with terrain. Compared with the case without terrain, the shallow-water effect caused by the combined action of the terrain and the IWs enhances the flow field strength, making the lower parts of the cylinder suffer larger horizontal forces in the opposite direction to the IW direction. Moreover, the additional vortices produced by the interaction between the IWs and the terrain causes a more complex flow field around the cylinder and the greater forces on the cylinder.