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Capillary wicking in a thicker gel blot microfluidics paper has been investigated through a combination of an analytical framework, experiments, and numerical simulations. The primary objectives of this work are to investigate the concentration-dependent wicking process inside thicker microfluidic paper and to estimate the concentration-dependent permeability using both theoretical models and experimental data. An additional goal is to estimate the parameters for saturation-dependent flow modeling in thicker microfluidic paper. To comprehend the wicking phenomenon on thicker gel blot paper, a series of experiments employing aqueous food dye solutions at varying concentrations has been conducted. In order to calculate the temporal wicking length analytically, the Brinkman-extended Darcy equation is implemented. By modifying the permeability expression for a simple rectangular unidirectional fiber cell and pure liquid, the expression of effective permeability for the analytical framework has also been introduced. The concentrations of the food dye solutions appear to have a substantial influence on the wicking phenomenon. Effective permeability and wicking length have been found to follow a decreasing pattern at lower concentrations while both increase at higher values. Intriguingly, employing a microfluidics paper with a relatively greater thickness facilitates the visualization of the fluid front. This phenomenon is identified by the formation of an acute angle at intermediate time instants, while the fluid front angle assumes an angle nearly ∼90° during smaller and higher time instants. In order to evaluate the saturation-dependent capillary pressure and permeability, the empirical correlation of concentration-dependent Brooks and Corey parameters is additionally determined experimentally. These parameters are subsequently employed in numerical simulations to illustrate the saturation-dependent flow field using Richards' equation. Furthermore, numerical simulations based on these estimated model parameters have been conducted, and it turns out that the saturation field has an excellent agreement with the experimental results. The results of the current study can be used to design low-cost paper-based diagnostic devices for usage in healthcare and environmental applications. [ABSTRACT FROM AUTHOR]

In order to accurately predict the motion trajectory of ice shedding and ensure the safe flight of aircraft, the motion characteristics of ice shedding under two-dimensional (2D) and three-dimensional (3D) conditions are simulated and analyzed. Considering the influence of any possible shape of shedding ice and its rotation and the magnitude and direction of acceleration with time under aerodynamic force, a six degree-of-freedom analysis method is introduced in this paper. This paper proposes a theoretical model, which can be used to calculate the 3D trajectory of ice shedding with arbitrary shape. The dynamic analysis of real 3D shedding ice is carried out, and the motion behavior of shedding ice with different positions and shapes is calculated. The results show that the movement mode of the shedding ice after leaving the aircraft is translation and rotation. The shape of the low-speed region on the leeward side of the shedding ice will first increase, then decrease, and then increase with the rotation of the ice body. The influence of ice shape on ice shedding trajectory is mainly that the shedding ice continues to flip during the downstream movement of the flow field, and the projected area of the effective windward area in the lift and the drag direction changes with time. The average deviation of the shedding ice at position 5 along the spanwise is only 22.9% of that at position 1. Finally, the closer the initial position of ice shedding is to the airfoil root, the greater the probability of ice shedding hitting the aircraft fuselage. In this paper, the probability of ice shedding hitting the aircraft fuselage is 8%, which all occurred in the case with position 1 as the initial position of ice shedding. [ABSTRACT FROM AUTHOR]

The flow around a cylinder near a free surface contains many physical phenomena and mechanisms, especially when the cylinder is moving. In this paper, the two-dimensional flow of a rotating cylinder near the free surface is simulated. The dynamic behavior of the rotating cylinder and the flow characteristics such as force coefficient, vortex structure, and air entrainment under different depth ratios, Fr numbers, and rotation ratios are discussed. In such problems, the rotational motion is rarely mentioned. The two-phase model is the sharp volume of the fluid method based on the two-dimensional incompressible Navier–Stokes equation. Combined with the spatially adaptive four-octree grid, the gas–liquid interface is reconstructed with the high-density ratio (ρwater/ρair = 816). The results show that, at α ≤ 2.0, the combined effect of the free surface and the rotation causes the wake vortex to produce a positive upper and negative lower distribution, which is contrary to the single condition. For the rotating cylinder, the existence of a free surface leads to the stable vortex layer at a low rotation ratio (α = 1.0), which only occurs at the high rotation ratio (2 < α < 4.2) without free surface. For the force coefficients, the simultaneous existence of the free surface and the rotation obviously changes the value and periodicity of the coefficients, which is different from the single condition. As for the air entrainment, it can be divided into two categories within the selected parameters: vortex entrainment caused by the parallel free surface and wake jet entrainment. In the latter model, the entailed bubbles have a wider distribution in space due to the influence of shedding vortices. [ABSTRACT FROM AUTHOR]

The control of the excitation force of pump-jets is of great significance for the stealth performance of submarines. In a previous study, the authors found that the sinusoidal unevenly spaced rotor blades (SUSR) can control the excitation force of the rotor, but the suppression effect is limited by the low degree of freedom. This paper proposes random unevenly spaced rotor blades (RUSR) with higher degrees of freedom, achieving twice the suppression effect of the SUSR. A method for batch generation of RUSRs is proposed in this paper, and a large number of RUSRs are generated. The numerical simulation study was carried out by selecting the RUSRs with better suppression effects through theoretical methods, and the numerical simulation method used was improved based on the previous study. The RUSR also has an impact on the excitation force of the duct. The conclusion of this paper has guiding significance for the low excitation design of pump-jets and has the potential to promote the use of unevenly spaced rotor blades in the field of pump-jets. [ABSTRACT FROM AUTHOR]

The thermal counterflow of superfluid helium past a cylinder has been found to generate two pairs of symmetrical vortexes upstream and downstream of the cylinder in experiments. This is a unique physical phenomenon that differs from classical fluids. In this paper, we have developed a new solver on the OpenFOAM® platform based on the simplified two-fluid model proposed by Kitamura et al. and use it to simulate the thermal counterflow phenomenon. The numerical simulation results obtained by the simplified two-fluid model agree well with the experimental results. Based on this numerical simulation, an investigation has been carried out into the evolution of the four-vortex structure of the superfluid helium thermal counterflow. This paper supports the conjecture of many researchers that large scale quantum turbulence in superfluid helium can be explained using classical mechanics. Details of the theoretical basis, the solver computational procedure, the numerical simulation results, and the discussion based on the results are presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of sloshing loads acting on liquid cargo tanks based on classification rules and numerical simulations. For the design of liquid cargo tanks, sloshing load is one of the key factors that should be considered. To reduce the sloshing load and motion of the contained liquid, internal structures such as bulkheads and baffles are commonly installed in the tanks. In classification rules, methodologies to estimate the sloshing loads are proposed considering the design parameters related to these internal structure members. In this paper, we carefully investigate the limitation of classification rules through smoothed particle hydrodynamics. Our investigations focus on the impact of the design parameters on sloshing loads, the patterns of the free surface, and the characteristics of velocity distribution in the tank. Through the obtained results, it is identified that the existing classification rules lack consideration for the design parameters that affect the sloshing loads and fluid characteristics in the tank. This result provides a better understanding of the existing classification rules and their limitations. It can contribute to foundational data for the future revision of classification rules. [ABSTRACT FROM AUTHOR]

In the field of fluid mechanics, dimensionality reduction (DR) is widely used for feature extraction and information simplification of high-dimensional spatiotemporal data. It is well known that nonlinear DR techniques outperform linear methods, and this conclusion may have reached a consensus in the field of fluid mechanics. However, this conclusion is derived from an incomplete evaluation of the DR techniques. In this paper, we propose a more comprehensive evaluation system for DR methods and compare and evaluate the performance differences of three DR methods: principal component analysis (PCA), isometric mapping (isomap), and independent component analysis (ICA), when applied to cavitation flow fields. The numerical results of the cavitation flow are obtained by solving the compressible homogeneous mixture model. First, three different error metrics are used to comprehensively evaluate reconstruction errors. Isomap significantly improves the preservation of nonlinear information and retains the most information with the fewest modes. Second, Pearson correlation can be used to measure the overall structural characteristics of the data, while dynamic time warping cannot. PCA performs the best in preserving the overall data characteristics. In addition, based on the uniform sampling-based K-means clustering proposed in this paper, it becomes possible to evaluate the local structural characteristics of the data using clustering similarity. PCA still demonstrates better capability in preserving local data structures. Finally, flow patterns are used to evaluate the recognition performance of flow features. PCA focuses more on identifying the major information in the flow field, while isomap emphasizes identifying more nonlinear information. ICA can mathematically obtain more meaningful independent patterns. In conclusion, each DR algorithm has its own strengths and limitations. Improving evaluation methods to help select the most suitable DR algorithm is more meaningful. [ABSTRACT FROM AUTHOR]

Shale gas is an important unconventional natural gas resource, and it is an important target of exploration and development in recent years. Deep shale gas reservoirs are high-temperature, high-pressure, and high-stress with complex gas–water relationships, and its occurrence and flow mechanisms are still unclear, making the simulation of deep shale gas reservoirs still a challenging problem. Aiming at the key problems faced in the development of deep shale gas reservoirs, this paper establishes a flow-stress-temperature field coupled mathematical model and numerical model, studies the thermo-hydro-mechanical sequential decoupling method to solve the coupled mathematical model, and forms a multi-field coupled simulation technology which provides theoretical support for the development of deep shale gas. Based on the thermo-hydro-mechanical coupling calculation module, this paper conducts research on the influence of thermo-hydro-mechanical multi-field coupling parameters on the production and development performance, and it further verifies the practicability of our proposed model in real reservoirs. [ABSTRACT FROM AUTHOR]

This paper studies the characteristics of internal waves induced by the motion of a submerged body through a combination of experimental and numerical methods. First, by deploying an array of conductivity probes in an experiment, the temporal evolution of the internal wave at the plane of the conductivity probe array is obtained and compared with numerical simulation results based on the Navier–Stokes equations, thereby validating the accuracy of the Computational Fluid Dynamics (CFD) model. Subsequently, utilizing CFD calculations, further exploration is conducted on the spatiotemporal distribution characteristics of the internal wave, encompassing its generation and evolution, velocity field distribution, wake angle, and wave profile features. Finally, the variation of the internal wave with the Froude number (Fr) is investigated, revealing that it varies apparently with Fr, including the wave component, wave mode, wake length, wake angle, wave amplitude, and so on. It is found that when Fr is large, the internal wave converges toward the centerline behind the body, until forming a line. The wave magnitude changes with Fr in four stages, i.e., increasing with Fr from zero in the first stage, reaching a peak and decreasing thereafter in the second stage, then changing little in the third stage, and finally increasing approximately linearly in the fourth stage. [ABSTRACT FROM AUTHOR]

A V-shaped blunt leading edge(VSBLE) at the lips of three-dimensional inward-turning inlets often generates complex shock interactions and severe aerothermal loads. To date, few heat flux reduction schemes have been devised based on the generation principle of the heat flux peaks on the VSBLE. Employing a synergistic approach of theoretical analysis and numerical simulation, it is determined that the maximum pressure jump ( p max / p ∞ ) provided by the near-wall secondary shock interaction structure significantly influences the outermost heat flux peak ( q peak 1 ) and exhibits a strong correlation with the deflection angle ( θ EF ) of the upstream flow. Consequently, we design a heat flux reduction model incorporating an expansion corner (EC) to validate this finding. The simulation results demonstrated a 54.23% reduction in q peak 1 under the influence of the expansion fan generated by the EC at Mach 6, although the remaining heat flux peaks, q peak 2 and q peak 3 , experience some deterioration. After conducting a comparative analysis and modeling the causes of deterioration, we propose an improved model by designing the crotch centerline. This improved model consistently demonstrated excellent performance in reducing the maximum heat flux peak and preventing the deterioration of the other heat flux peaks at Mach 6-12. Additionally, correlations between q peak 1 and p max / p ∞ , as well as between q peak 1 and θ EF are established. This paper presents an investigation into the key factors affecting the heat flux peaks on the VSBLE and proposes a model capable of ensuring a stable reduction in the maximum heat flux of over 40% under a wide range of Mach numbers. [ABSTRACT FROM AUTHOR]

The generation of turbulent inflow conditions is a key issue in large eddy simulation (LES) or direct numerical simulation (DNS). In this paper, an improved synthetic eddy method (SEM) is proposed to generate inflow turbulence for LES and DNS. The improvements about SEM focus on the eddy radius and eddy distributions. First, the eddy radius is improved to reduce the nonphysical vortex structure on the wall caused by overestimation of the radius. Second, a sampling method using Gaussian distribution is proposed to improve the distribution of eddies, which accurately captures the randomness of turbulent structure size and is close to the actual flow field. The improved method is applied to the direct numerical simulation of the supersonic turbulent boundary layer at Mach 2.7 and the 24° compression ramp. Results indicate that the predictions yielded by the improved method are in good agreement with both DNS and experimental data. Compared to the original method, the improved method exhibits a more rapid recovery of the friction coefficient and effectively shortens the development distances. The improved SEM has enhanced the efficiency and accuracy of generating inlet turbulence, which can provide inlet turbulence boundary conditions for LES and DNS. [ABSTRACT FROM AUTHOR]

While direct numerical simulation (DNS) of multiphase flows has been the focus of many research investigations in recent years, large eddy simulation (LES) of multiphase flows remains a challenge. There is no standardized set of governing equations for multiphase LES. Different approaches and formulations have been discussed in the literature, each with its own advantages and disadvantages. In this paper, the conventional (non-weighted) filtering approach is compared with the density-weighted Favre filtering method by evaluating the subgrid scale (SGS) energy transfer for a simple test case of a shear-thinning droplet in air. The findings reveal that, unlike the Favre filtering approach, the conventional filtering method results in a notable amount of nonphysical backward scatter in the flow. Based on these results, the Favre filtering method appears preferable and is applied to the a priori analysis of shear-thinning liquid jets, where the viscosity has been modeled using the Carreau–Yasuda model. First, by explicitly filtering existing DNS data of shear-thinning jet breakup into stagnant air, the order of magnitude of different SGS terms is evaluated using the Favre filtering method. Consistent with earlier studies on Newtonian jets, the present study indicates that the diffusive term remains negligible, while the convective term plays a dominant role. Functional and structural models for the closure of the convective SGS term are assessed by means of a correlation analysis and an order of magnitude study. Existing structural models provide good results for both Newtonian and shear-thinning cases. Promising a posteriori model candidates are discussed. [ABSTRACT FROM AUTHOR]

To study the motion characteristics and cavity evolution of the cylindrical shell with different initial velocity, the numerical simulation based on the Star-CCM+ and ABAQUS collaborative simulation method is carried out in this paper. The results show that when the shell penetrates into the water at a lower speed, only a small amount of external fluid can enter into the shell, and the internal fluid shows a reciprocating motion trend. Moreover, the speed of the shell and the force at the bottom wall show a fluctuating trend. As the speed increases, the volume of fluid entering into the interior of the shell gradually increases. After the speed reaches 30 m/s, the fluid can impact the upper wall of the shell, which also causes sudden changes in velocity and displacement. When the initial velocity of the shell reaches 80 m/s, significant deformation occurs at the upper wall of the shell, and the deformation is not fully restored after the fluid leaves the upper wall. As the speed increases, the degree of deformation gradually increases, and the volume of the cavity gradually increases. [ABSTRACT FROM AUTHOR]

In the field of preparation of cerium oxide by Venturi jet pyrolysis reactor, the spray pyrolysis process can achieve energy saving and environmental protection. The atomization effect of transverse air flow on the droplet directly affects the morphology of the product, and this research can improve the high value of cerium oxide applications. In this paper, a novel Venturi jet pyrolysis reactor has been developed. The atomization stage in the preparation process was studied by combining experiment and numerical simulation. The results show that the entire atomization process can be divided into primary atomization and secondary atomization. The primary atomization process of the liquid jet consists of columnar fragmentation and surface fragmentation, which are dominated by Rayleigh–Taylor (R-T) instability and Kelvin–Helmholtz (K-H) instability respectively. In the process of atomization of liquid column, the increase in gas velocity helps to promote the atomization. The Sauter mean Diameter value at 35 and 40 m/s gas velocity is the smallest, and the atomization effect of Venturi reactor is the best. With the increase in Weissenberg number (Wi), the crushing mode of droplets in Venturi reactor gradually changes from envelope to envelope—core and further evolves into core-layer. The core-layer crushing effect is the best, and the distribution range of daughter droplets is larger. The crushing state of atomized droplets in Venturi reactor is summarized. When Wi < 12, it is vibration crushing; when 12 < Wi < 30, it is packet crushing; when 30 < Wi < 45; it is pack-core crushing; and when 45 < Wi < 75, it is stamen-layer breakage. [ABSTRACT FROM AUTHOR]

Large-span roofs can be damaged by uneven snow loads caused by snow drifting. To predict wind-induced snow loads more accurately, this paper proposes a numerical simulation method. The method considers the interaction of wind and snow, the changing characteristics of snow phase boundaries over time, and corrects for snow deposition and erosion flux. The effectiveness of the method was confirmed by simulating wind–snow flow on a three-centered cylindrical shell and comparing the results with wind tunnel measurements. The simulation considered different snowfall conditions and time-varying snow phase boundaries on a large-span shell-shaped roof. The study investigated the influence of snowfall conditions and dynamic changes in snow phase boundaries on snow cover distribution by comparing simulation results. [ABSTRACT FROM AUTHOR]

Horizontal transport of droplets is important for anti-icing, self-cleaning, and water collection technologies. In this paper, the bouncing dynamics of two off-center droplets impacting on a solid surface are explored via numerical simulations. Two kinds of surface wettability are considered. One is a superhydrophobic surface (homogeneous wettability), and the other is a superhydrophobic surface arranged with a hydrophilic strip (heterogeneous wettability). First, the effect of an eccentric angle on the horizontal transport of two droplets impacting on a homogeneous wettability surface is investigated. As the eccentric angle increases, the merged droplet after impact may experience horizontal transportation in either the positive or negative direction. When a heterogeneous wettability surface is adopted, the changes in the transport direction of the merged droplet are consistent with the location of the hydrophilic strip upon eccentric impact. By adjusting the position of the hydrophilic strip, the maximum horizontal transportation distance is 7.53 times the diameter of the initial droplet when the eccentric angle is 10°, while it is only 2.01 times the diameter of the initial droplet when the eccentric angle is 20° for a homogeneous wettability surface. It is indicated that the use of the heterogeneous wettability surface is significantly efficient for droplet control after impact. The obtained results contribute to the fundamental understanding of droplet directional migration and are valuable for related engineering applications. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel diffuse interface model of two-phase magnetohydrodynamics (MHD) based on a magnetic vector potential formulation in the three-dimensional case. This model ensures an exact divergence-free approximation of the magnetic field by introducing a magnetic vector potential A and defining the magnetic field by B = curl A. The resulting framework constitutes a highly coupled, nonlinear saddle point system consisting of the Cahn–Hilliard system and MHD potential system. To solve the model efficiently, we present two fully decoupled, first-order, linear, and unconditionally energy-stable schemes and strictly prove their well-posedness and energy stability. Finally, we present several numerical examples that demonstrate the stability and effectiveness of our schemes. [ABSTRACT FROM AUTHOR]

In recent years, the data-driven turbulence model has attracted widespread concern in fluid mechanics. The existing approaches modify or supplement the original turbulence model by machine learning based on the experimental/numerical data, in order to augment the capability of the present turbulence models. Different from the previous researches, this paper directly reconstructs a mapping function between the turbulent eddy viscosity and the mean flow variables by neural networks and completely replaces the original partial differential equation model. On the other hand, compared with the machine learning models for the low Reynolds (Re) number flows based on direct numerical simulation data, high Reynolds number flows around airfoils present the apparent scaling effects and strong anisotropy, which induce large challenges in accuracy and generalization capability for the machine learning algorithm. We mainly concentrate on the high Reynolds number turbulent flows around the airfoils and take the results calculated by the computational fluid dynamics solver with the Spallart-Allmaras (SA) model as training data to construct a high-dimensional data-driven network model based on machine learning. The radial basis function neural network and the auxiliary optimization methods are adopted, and the individual models are built separately for the flow fields of the near-wall region, wake region, and far-field region. The training data in this paper is extracted from only three subsonic flow fields of NACA0012 airfoil. The data-driven turbulence model can be applied to various airfoils and flow states, and the predicted eddy viscosity, lift/drag coefficients, and skin friction distributions are all in good agreement with the results of the original SA model. This research demonstrates the promising prospect of machine learning methods in future studies about turbulence modeling. [ABSTRACT FROM AUTHOR]

The wake behind a step cylinder [consisting of a small diameter cylinder (d) and a large diameter cylinder (D)] with diameter ratio D / d = 2 at Reynolds number ReD = 200 (the mode A* regime) is simulated by direct numerical simulations. New detailed information of the interaction between natural vortex dislocations and forced vortex dislocations is described. In the large cylinder wake, the forced and natural vortex dislocations coexist. The regular formation of forced vortex dislocation is found to be delayed under the effect of natural vortex dislocations. The occurrence of natural vortex dislocations is suppressed in the large cylinder wake close to the small cylinder. Moreover, the effect of vortex dislocations on structural loads is described. The results of this paper provide a more thorough understanding of the formation and interaction between the natural and forced vortex dislocations. [ABSTRACT FROM AUTHOR]

In recent years, interest has been growing in the stability control of cylindrical structures in a flow field using full/partial coatings of porous media. However, the partial application of porous coatings to arrays of cylinders with rough surfaces has rarely been studied. This paper numerically investigates the passive flow control of an array of rough cylinders partially coated with porous media on their leeward side in the subcritical flow regime (Re = 3900). This is achieved through two-dimensional direct numerical simulations and the Cartesian cut-cell method, which facilitate flow simulations around complex porous media and rough surfaces in an accurate and flexible manner. In contrast to macroscopic models, this study employs a quasi-microscopic model to simplify the porous structure for each cylinder, providing an accurate and intuitive means of describing the inflow details of porous media near the central cylinder. The porosities and pores per inch of the porous media are defined in terms of the alignment angles and radii. Similarly, the surface roughness of a cylinder is simulated by the attachment of small circular appendages. The accuracy and capacity of the presented numerical approach are demonstrated via a numerical case study with a single smooth cylinder. The influence of the porous media on the flows around a single rough cylinder and an array of rough cylinders is thoroughly investigated and discussed. [ABSTRACT FROM AUTHOR]

The migration and sedimentation behavior of proppant in complex fractures plays a vital role in the formation of fractures with high conductivity. Most of the existing studies on proppant sedimentation and migration are based on a single vertical fracture model that propagates in the same direction. However, due to in situ stress and reservoir heterogeneity, the propagation of fractures in reservoirs will produce multiple branch fractures that intersect with the main fractures and have a certain inclination angle. Therefore, it is necessary to study the sedimentation and migration behavior of proppants in inclined complex fractures. In this paper, the proppant particles sedimentation law in fractures is studied based on computational fluid dynamics. By changing the particle size, volume fraction, and inlet flow velocity of proppant, the migration and sedimentation law of proppant in a single fracture is simulated and verified. Then, through the establishment of a complex fracture model with multiple branches, and from the four dimensions of the vacant length of the front edge of the sand embankment, the length of the sand embankment, the highest point of the sand embankment, and the effective laying area of the proppant, the angle between the main and branch fractures in the complex fractures, the fracture angle and the inlet flow velocity, as well as the location relationship of the branch fracture setting and the migration and sedimentation of the proppant in the complex fractures are explored. [ABSTRACT FROM AUTHOR]

Fracture loss is the most common type of leakage in petroleum drilling engineering. The existing numerical simulation of the particle transport process is relatively simplified. In order to clarify the dynamic behavior characteristics of bridging particles in fractures, this paper adopts the discrete element and computational fluid dynamics coupling numerical simulation methods to establish a bidirectional coupling numerical model of bridging particles and drilling fluid to simulate the dynamic evolution process of bridging plugging particles, such as migration, bridging, and accumulation. The results show that the main factors affecting the transport pattern of plugging particles in the fracture are differential pressure, particle size, drilling fluid viscosity, drilling fluid density, fracture width, and fracture length. The drilling fluid density has a small effect on the inter-particle force in the fracture; the drilling fluid viscosity mainly affects the resistance of the particles, and the pressure difference between the inlet and outlet of the fracture mainly affects the transport speed of the particles. The particle size determines whether a plugging layer can be formed, and the best plugging effect is achieved when the particle size is 1/2 of the fracture width. [ABSTRACT FROM AUTHOR]

It is well established that a large spanwise domain size is required for accurate numerical simulations of flow past an airfoil in stall. A number of numerical experiments support this conclusion with regard to aerodynamic and turbulence statistics. However, very little has been reported concerning the effect of the span length on aeroacoustic results. In this paper, a detailed investigation is carried out into the influence of spanwise domain length on the prediction of airfoil stall noise when spanwise periodic boundary conditions are applied. This study is based on direct numerical simulations of an NACA0012 airfoil at Re∞ = 50 000 and M∞ = 0.4 at near- and full-stall conditions. There are three main findings in this paper. First, the far-field acoustics are found to be highly sensitive to the choice of spanwise domain length. In the full-stall case, a span length equal to 20% of the airfoil chord over-predicts the radiated noise by more than 10 dB at low-to-medium frequencies relative to a case with one chord length in span. Discrepancies are found to occur for acoustic wavelengths shorter than the spanwise domain size. Under near-stall conditions, the changes caused by the small spanwise domain are noticeably milder. Second, the lower noise predictions from the large span simulation at low frequencies are attributed to the spanwise breakup of large scale flow structures and reduced spanwise coherence near the trailing edge. Third, a more destructive source phase relationship is observed with a large span for medium frequencies, which was inhibited by the periodic forcing in the small span case. [ABSTRACT FROM AUTHOR]

Bidirectional chambers are well-studied in terms of the flow structure and influence of their input parameters. However, most of the available studies are based on steady-state or time-averaged research methods and do not allow to obtain data on bidirectional flow dynamics over time. The present paper reports on detailed numerical studies based on detached eddy simulations (DESs) and unsteady Reynolds-averaged Navier–Stokes methods applied for two vortex chambers with different aspect ratios. A comparison of the numerical results with the available experimental data shows that the DES method gives the most accurate results on bidirectional flow structure, turbulent fluctuations, and precessing vortex core (PVC) motion. A notable feature of the studied bidirectional flow is the central recirculation zone (CRZ) formation, which is correctly predicted by the DES method only. The presence of a CRZ in a bidirectional flow has a significant effect on turbulent velocity fluctuations and PVC behavior. It is found that CRZ formation leads to a significant decrease in radial and circumferential velocity fluctuations whereas the axial velocity fluctuations are slightly increased. Additionally, the paper reports new findings on CRZ and PVC interaction in bidirectional flows. PVC motion is almost completely nullified by the presence of a CRZ. This can prove useful in many industrial applications of bidirectional chambers, e.g., vortex thrusters and gas turbine combustors. The bidirectional swirling flow transient properties studied in this paper could assist in determining the most efficient operational modes and geometric configuration of industrial chambers as well as enabling control of turbulent fluctuations, which would allow for reliable ignition and stable combustion. [ABSTRACT FROM AUTHOR]

In this paper, we investigate two-dimensional direct numerical simulations of a transcritical shear layer. Three configurations are chosen, which are distinguished by the level of presence of two-phase phenomena. The thermodynamic model is based on a cubic equation of state. It was extended for multicomponent mixtures, and it is able to account for vapor–liquid equilibrium. The thermodynamic modeling with phase-transition is validated using experimental data from the literature. Special focus is put on the effect of the density gradient and the density changes caused by phase-transition on the development of the turbulent shear layer and the associated mixing. In addition to this, the vorticity distribution and the components of its transport equation are analyzed and compared for the different configurations. [ABSTRACT FROM AUTHOR]

The phenomenon of a hollow droplet impacting on a solid surface is widely found in various fields. The dynamic characteristics of hollow diesel droplets impacting on a solid surface are studied by combining numerical simulation and theoretical analysis. The dynamic contact angle model presented in this paper couples the advantages of existing dynamic contact angle models for simulating both the spreading and retracting stages. It also considers the continuous variation of the contact angle during the maximum spreading state. Compared to existing models, the maximum error has been reduced from 14.9% to 4.6%. The effects of impact velocity, impact angle, and volume ratio of a hollow droplet on the spreading and jetting characteristics are investigated by three-dimensional numerical simulations. It is found that air entrainment occurred in the counter-jet, and the presence of the impact angle increased the asymmetry of the counter-jet and spreading liquid film, promoting fingerlike splashing at the front liquid film. Based on energy conservation law, the theoretical prediction models of the maximum spreading coefficient of the hollow droplet impacting on the surface and the velocity of the counter-jet at the maximum spreading state are established using the multi-regional modeling method and the energy distribution principle. Compared with existing hollow droplet theoretical models, the proposed theoretical models exhibit a more concise expression, higher accuracy, and wider applicability range. [ABSTRACT FROM AUTHOR]

Thermal capillary migration is a phenomenon due to the Marangoni effect, which refers to the spontaneous motion of a liquid on a non-isothermal surface. Numerical simulation of thermal capillary migration of a droplet is extremely difficult due to the multi-physics field coupling and the violent motion of the droplet surface. In this paper, an improved smooth particle hydrodynamics (SPH) method is developed for simulating thermal capillary migration of a droplet driven by thermal gradient. In improved SPH model, an improved continuous surface force model is proposed to enhance the accuracy and stability of surface tension force calculation by introducing an improved surface tangential force. The contact angle model is utilized to model the surface wettability. The SPH method for simulating the thermal fluid flow is developed based on the continuum, momentum, and energy equations. In addition, kernel gradient correction and particle shifting technique are utilized to improve the accuracy and stability of the SPH method. The correctness and effectiveness of the improved SPH method are verified by numerical examples. Moreover, the motions of a droplet driven by thermal gradient under different conditions are investigated. Comparing with the results obtained by experiments and other resources, we can conclude that the improved SPH model is effective in modeling the thermal capillary migration of a droplet. [ABSTRACT FROM AUTHOR]

In this study, the positive and negative power-off process of a bi-directional full-flow pump is investigated by model tests and numerical simulations, and the results show that under steady conditions, the head of the bi-directional full-flow pump in the positive direction is larger than in the negative direction. The positive power-off process of the bi-directional full-flow pump is slower than the negative power-off process. The clearance backflow rate of the bi-directional full-flow pump continues to drop during the power-off process until it remains stable under the runaway condition. The clearance backflow causes the vortex at the impeller inlet and the size of the vortex gradually decreases during the power-off process. Compared to the positive power-off process, the backflow vortex ratio is greater, and therefore, the flow pattern is poorer during the negative power-off process. The entropy production rate in the negative power-off process is significantly greater than that in the positive power-off process. The pumping condition has the largest hydraulic loss in the whole power-off process. The runaway rotational speed in the positive power-off process is higher than that in the negative power-off process, and the axial force in the positive runaway condition is 1.40 times greater than in the negative runaway condition. During the power-off process, the vibration and pressure pulsation in the negative operation is significantly greater than that in the positive operation, and the acceleration of the impeller vibration from large to small is the Y (vertical) direction, Z (axial) direction, and X (horizontal) direction. The research in this paper can provide an important reference for the design and operation of the bi-directional full-flow pump. [ABSTRACT FROM AUTHOR]

This paper presents and discusses the results of the "2022 International Computational Fluid Dynamics Challenge on violent expiratory events" aimed at assessing the ability of different computational codes and turbulence models to reproduce the flow generated by a rapid prototypical exhalation and the dispersion of the aerosol cloud it produces. Given a common flow configuration, a total of 7 research teams from different countries have performed a total of 11 numerical simulations of the flow dispersion by solving the Unsteady Reynolds Averaged Navier–Stokes (URANS) or using the Large-Eddy Simulations (LES) or hybrid (URANS-LES) techniques. The results of each team have been compared with each other and assessed against a Direct Numerical Simulation (DNS) of the exact same flow. The DNS results are used as reference solution to determine the deviation of each modeling approach. The dispersion of both evaporative and non-evaporative particle clouds has been considered in 12 simulations using URANS and LES. Most of the models predict reasonably well the shape and the horizontal and vertical ranges of the buoyant thermal cloud generated by the warm exhalation into an initially quiescent colder ambient. However, the vertical turbulent mixing is generally underpredicted, especially by the URANS-based simulations, independently of the specific turbulence model used (and only to a lesser extent by LES). In comparison to DNS, both approaches are found to overpredict the horizontal range covered by the small particle cloud that tends to remain afloat within the thermal cloud well after the flow injection has ceased. [ABSTRACT FROM AUTHOR]

Resonance acoustic mixing (RAM) technology applies an external low-frequency vertical harmonic vibration to convey and mix the non-Newtonian fluid across space. However, although this method is used for various applications, its mechanism is yet not well understood. This paper investigates the flow characteristics of the power-law non-Newtonian fluids in RAM by the phase-field numerical simulation method and proposes a mixing index to study the effect of vibration acceleration amplitude, vibration frequency, and filling rate on mixing. Dynamic mode decomposition is used to decompose the vorticity field data into spatial–temporal coherent structures to explore the dynamic mechanism of mixing under the above parameters. The results show that the increase in vibration acceleration amplitude can promote mixing uniformity and rate. The decrease in vibration frequency will increase the mixing of non-Newtonian fluids, but the effect of improvement is no longer obvious when the frequency exceeds a threshold. On the one hand, increasing the vibration acceleration amplitude and decreasing the vibration frequency and filling rate appropriately can induce the 0 Hz and half of the vibration frequency component accounting for the most energy. On the other hand, it can also increase the number of modes traversed by large-scale vortices developing into small-scale vortices. Both of them can improve the mixing rate and uniformity of non-Newtonian fluids. [ABSTRACT FROM AUTHOR]

This paper performs direct numerical simulations of hypersonic boundary layer transition over a Hypersonic Transition Research Vehicle (HyTRV) model lifting body designed by the China Aerodynamic Research and Development Center. Transitions are simulated at four angles of attack: 0°, 3°, 5°, and 7°. The free-stream Mach number is 6, and the unit Reynolds number is 107 m−1. Four distinct transitional regions are identified: the shoulder cross-flow and vortex region and the shoulder vortex region on the leeward side, the windward vortex region and the windward cross-flow region on the windward side. As the angle of attack increases, the transition locations on the leeward side generally move forward and the transition ranges expand, while the transition locations generally move backward and the transition ranges decrease on the windward side. Moreover, the shoulder vortex region moves toward the centerline of the leeward side. At large angles of attack (5° and 7°), the streamwise vortex on the shoulder cross-flow and vortex region will enable the transition region to be divided into the cross-flow instability region on both sides and the streamwise vortex instability region in the middle. In addition, the streamwise vortex also leads to a significant increase in cross-flow instability in their upper region, which can generate a new streamwise vortex instability region between the two transition regions on the leeward side. Furthermore, since the decrease in the intensity and the range for the cross-flow on the windward side, the windward cross-flow region tends to become narrow and ultimately disappears. [ABSTRACT FROM AUTHOR]

Flow pattern and hydrodynamic characteristics of coarse particles in deep-sea hydraulic lifting pipes are simulated using a numerical approach developed by combining the computational fluid dynamics method with a discrete element method in the Euler–Lagrange framework. This paper examines the effects of feed concentration, two-phase flow initial mixture velocity, and particle gradation on the dynamic characteristics of particles and flow pattern in the pipe by validating the rationality of numerical simulation. The results demonstrate that particles are distributed from the pipe center to the pipe wall, while the lift force causes more particles to be distributed in the pipe center. Moreover, greater inertia makes large particles more concentrated in the center. Particle-induced turbulence modifies the lift force and causes particles to move from the pipe center to its wall. Due to the increasing trend of lift force, particles gather toward the center of the pipe at an increasing pace as the initial velocity increases. The greater the feed concentration, the more particles disperse toward the pipe wall due to the violent momentum exchange caused by the high concentration and the significantly altered lift force caused by the high turbulent kinetic energy resulting from a high concentration. From the particle gradation 1:1:1 to 1:6:1, the pressure drop decreases gradually as the reduction in small particles decreases the number of particles near the wall, and the frictional energy loss between the particles and the pipe wall decreases. [ABSTRACT FROM AUTHOR]

Cavitation erosion often occurs on the surface of many underwater applications, which can cause severe damage to materials and reduce their performance. Since the cause of erosion is the impact pressure induced by the collapse of an individual cavitation bubble near the wall, to make a better prediction and prevent the damage potential, in this paper, we carry out systematic investigations on the impact characteristics by direct numerical simulation using a vapor bubble model. The volume of fluid (VOF) method is adopted to capture the interface between the two phases. The numerical results show that pressure wave and jet are two primary inducements of the impacts on the wall. The reason for the pressure wave impacts is the pressure wave emission after the collapse of the bubble's main part. And the reason for the jet impact is the stagnation pressure in front of the jet. After a parametric study of the two impacts with respect to the initial radius, driving pressure, and stand-off distance, the predicting equations for the pressure wave impact and jet impact are proposed at γ ≥ 1.74. When γ < 1.74, the impact pattern becomes complex due to the arrival time of the two impacts and the collapse of the vapor fragments right on the wall. [ABSTRACT FROM AUTHOR]

Pressure fluctuation is an important factor affecting the stability of rotating machinery. Electric submersible pumps (ESPs) are generally arranged in a multistage series structure, and its internal unsteady flow is extremely easy to propagate and develop in the lengthy flow passage, which brings about differences in the characteristics of pressure fluctuations in each stage. In contrast to the conventional method of processing pressure fluctuation signals, we propose a parameter called "energy flow density (EFD)" of pressure pulsation by analogy with the definition of wave intensity, in order to directly quantify the intensity of pressure fluctuations. Here, we study these pressure fluctuation characteristics using a typical three-stage ESP as the research object. We apply theoretical analysis, numerical simulation, and test verification. First, in comparisons between numerical predictions of pressure fluctuation and test results, the period, amplitude, and phase of pulsation curves are highly consistent, verifying the accuracy of the numerical method employed in this paper. Next, the mechanism underlying the pressure fluctuations and the characteristics of inter-stage interference are investigated through flow field analysis. Subsequently, the results of the evaluation of the pressure fluctuations based on EFD processing are compared with those obtained in the conventional way. The results are consistent in terms of characterizing the multistage ESP pressure fluctuation characteristics, but the conventional method does not reflect subtle differences due to inter-stage propagation and coupling. However, the EFD method combines the amplitudes of all signals and provides the intensity of pressure fluctuations directly, which reflects inter-stage differences. Our results provide a theoretical basis for improving the operational stability of ESPs connected in a multistage series and have practical engineering significance. [ABSTRACT FROM AUTHOR]

With the increasing demand for underwater detection, interest in the acoustic field of range-dependent ocean waveguides is also growing. For weakly range-dependent ocean waveguides, adiabatic modes represent a compromise between accuracy and computational cost and occupy an important place in the simulation of numerical sound fields. However, either existing adiabatic-mode programs consider too few layers of media or the root-finder tends to miss roots. In addition, none of the programs can solve the acoustic field excited by a line sound source located anywhere in the plane. In this paper, we first derive an expression for the acoustic field excited by a line source by adiabatic modes and then introduce a high-precision spectral method to solve the local eigenmodes. For the lower boundary condition of the acoustic half-space, we use the eigenvalue transformation technique to transform the transcendental algebra system formed by spectral discretization into a generalized eigenvalue problem. Several representative numerical experiments are designed to verify the accuracy of the algorithm. After analyzing the parallelism, the multiprocess and multithread hybrid strategy is adopted to further accelerate the algorithm in parallel, and parallel numerical simulation is carried out on the Tianhe–2 multicore supercomputer; favorable acceleration is achieved. [ABSTRACT FROM AUTHOR]

The axial flow pump is a low-head pump widely used in farmland irrigation and water transfer engineering in plain areas. To study the effects of different guide vane rotation angles and inlet angles on the performance of axial flow pumps, based on the initial scheme of the guide vane, the efficiency of the axial flow pump is expected to be improved by adjusting the guide vane rotation angles and changing the inlet angles of the guide vane. This paper uses Computational Fluid Dynamics numerically to simulate and calculate the performance of axial flow pumps under different guide vane rotation angles and inlet angles. The results show that the influence of different guide vane rotation angles and inlet angles on the performance of the axial flow pump is reflected in the operating conditions of large-discharge conditions. Adjusting different guide vane rotation angles can significantly improve the efficiency of axial flow pumps. Moreover, the high-efficiency area of the axial flow pump is expanded nearly two times with the increase in the guide vane rotation angles under large-discharge conditions. At the same time, under the design condition, the highest efficiency of the axial flow pump is 87.69% under scheme V. However, under the linear change of the inlet angle of the guide vane, the highest efficiency of the axial flow pump can reach 87.71%. Finally, the model test verifies the reliability of the numerical simulation, and the research results are beneficial for improving the efficiency of the axial flow pump. [ABSTRACT FROM AUTHOR]

In this paper, the behaviors of single bubble in shear-thinning viscoelastic fluids were investigated using OpenFOAM. The volume-of-fluid method was used to capture the interface, and the Giesekus model was adopted for describing the rheological behaviors of shear-thinning viscoelastic fluids. The bubble cusp, negative wake, and velocity jump phenomenons in viscoelastic fluids were obtained, and the effects of wall effect, mobility factor α, and Weissenberg number (Wi) on bubble behaviors were investigated. The results showed that the viscoelastic stress is the main reason for the formation of bubble cusp, the relaxation of polymer macromolecules leads to the formation of negative wake, and the negative wake may be the main reason for the velocity jump. The open angle θ of the negative wake decreases and the distribution region in the vertical direction increases with the increasing Reynolds numbers (Re). In addition, the magnitude of the maximum negative velocity increases with Re and appears further away from the bubble cusp. Moreover, decreasing the wall effect can accelerate the formation of viscoelastic stress and make the bubble cusp appear earlier. As the mobility factor α increases, the viscosity and viscoelastic stress of the fluid near the bubble decrease; this causes the terminal velocity of the bubble to increase. As the Wi increases, both the maximum velocity and terminal velocity of the bubble increase, and the time lag occur. The viscoelastic stress τyy has a longer linear distribution at the tail of the bubble with the increase in Wi. [ABSTRACT FROM AUTHOR]

Supersonic modes, previously thought to be insignificant due to their smaller amplitude than Mack's traditional second mode, occur in hypersonic boundary layers when a disturbance travels supersonically with respect to the mean flow outside the boundary layer, causing outward-radiating acoustic waves. Very few previous studies perform Direct Numerical Simulation (DNS) of supersonic modes and instead rely on Linear Stability Theory (LST). This combined LST and DNS study investigates supersonic modes in Mach 5 flow over a blunt cold-wall cone. An LST analysis was performed in Paper I [C. P. Knisely and X. Zhong, "Sound radiation by supersonic unstable modes in hypersonic blunt cone boundary layers. I. Linear stability theory," Phys. Fluids 31, 024103 (2019)], whereas DNS is the focus of Paper II. The overall goal is to determine the mechanism of supersonic modes and the conditions under which they exist. Compared to previous pure LST studies, DNS provides the advantage of making fewer limiting assumptions and can resolve interactions between modes. The results here indicate the excitation of supersonic modes via modal interactions not resolved with LST, suggesting the inadequacy of pure LST analyses concerning supersonic modes. Unsteady DNS results verified supersonic modes in the flow with wall-to-free-stream temperature ratio T w / T ∞ = 0.2, lending credence to the modes' physical existence. However in the case of T w / T ∞ = 0.667, sound radiation was also found in DNS while LST predicted a stable supersonic mode. The mechanism for supersonic modes is attributed to a modal interaction between mode F1, mode S, and the slow acoustic spectrum. Therefore, it is necessary to perform combined LST and DNS studies of supersonic modes to reliably predict their presence and impact on transition to turbulence. [ABSTRACT FROM AUTHOR]

In recent years, vortex recognition method based on velocity gradient has rapidly become a research hotspot, accompanied by a wide range of applications. As a linear velocity field with the origin as its critical point uniquely corresponds to the velocity gradient, its streamline pattern (SP), especially the spiral streamline structure when the velocity gradient has a couple of complex eigenvalues, is studied using the right eigen representation based on the real Schur form. Compared with the left eigen representation, the right eigen representation of velocity gradient is seldom concerned. In this paper, the right eigen representation is carried out in terms of the spectral representation, and its relation with the left eigen representation is also derived. For two-dimensional case, the SP classification is listed and the typical streamlines are illustrated. After detailed investigation, it is shown that the parameters extracted from the right eigen representation would be more suitable to describe the geometrical features of the spiral streamline pattern around the critical point, and the dual directivity of spiral streamline structure is clarified. Some discussions and an illustrative example from the direct numerical simulation (DNS) data are presented. [ABSTRACT FROM AUTHOR]

In this paper, we employ direct numerical simulations for studying the dynamics of two different, but miscible polymeric drops undergoing impact-driven coalescence with one of the drops making a head-on collision on an underlying drop. We consider two separate cases: (1) a PMMA (poly-methylmethacrylate) drop impacting a PVAc (polyvinyl acetate) drop and (2) a PVAc drop impacting a PMMA drop. For each case, three separate Weber numbers (We = 0, 1, 10) are considered. The interplay of finite impact speed and the specific wetting characteristic of the underlying drop leads to fascinating fluid dynamics. For example, for the case of the PMMA drop impacting on the PVAc drop, the strong wetting behavior of the PVAc drop ensures the impact-driven formation of an accelerated disk consisting of separate layers of PMMA and PVAc. Also, there is the generation of significant capillary waves at the drop–air interface, which in turn causes significant undulations of the drop–drop interface created due to such head-on collision. On the other hand, for the case of the PVAc drop impacting on the PMMA drop, the relatively weaker wetting behavior of the PMMA drop ensures that there is no formation of an accelerated disk; rather (for large We), the PVAc drop flows on top of the PMMA drop, forms a "canopy-like" structure, and wets the solid forming a transient core–shell structure (with PMMA core and PVAc shell). Our results raise the possibility of fabricating highly complicated multi-polymeric-material components by rapid curing of the drops in their post-impact configurations. [ABSTRACT FROM AUTHOR]

The paper discusses a particular phenomenon that develops in an open channel flow when a supercritical current enters from upstream a pre-existing current. In this case, a system of two surge waves, which propagate downstream while distancing each other, develops; the two surges can be either positive or negative, as well as shock or diffuse waves, and arrange into four different configurations. A model of the double surge wave system that allows to estimate the waves characteristics such as the height and the speed of the wave fronts, is described and discussed, and a criterion to predict the wave configuration is proposed. The model is then tested against the results of lab experiments that confirmed its reliability. Some numerical simulations are also performed to describe and further discuss the double surge wave system that develops under different conditions of practical interest. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulations of propeller–hull interaction of a ship model for the dynamic condition, demonstrating the ability of the developed self-propulsion module in OpenFOAM. A dynamic motion class has been developed based on the sliding mesh method to simulate a rotating propeller with ship motions. A body-force method is also implemented to simulate propeller–hull interaction to reduce the computational cost of propeller modeling. Validation studies were carried out for the Japan Bulk Carrier (JBC) ship model. The bare-hull resistance, sinkage, and trim are verified against the experimental data. The propeller open-water hydrodynamic characteristics are then computed and validated using sliding mesh and body-force methods. Finally, the self-propulsion simulations are carried out for the JBC model in calm water with sinkage and trim at the design speed. Numerical predictions of the self-propulsion parameters and axial flow velocity distributions at different transverse locations are presented. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of eccentricity on flow characteristics and particle migration in Couette geometries. The study involves numerical simulations using the recent frame-invariant model developed by Badia et al. [J. Non-Newtonian Fluid Mech. 309, 104904 (2022)]. The study begins with a two-dimensional analysis, focusing first on the Newtonian fluid in order to thoroughly characterize the specific properties of this flow configuration. Next, the impact of eccentricity on particle migration in an isodense suspension is examined by numerical simulations based on the experiments conducted by Subia et al. [J. Fluid Mech. 373, 193–219 (1998)]. Furthermore, the study is extended to include a full three-dimensional analysis of a dense suspension flow in an eccentric Couette geometry based on resuspension experiments conducted by Saint-Michel et al. [Phys. Fluids 31, 103301 (2019)] and D'Ambrosio et al.[J. Fluid Mech. 911, A22 (2021)]. The main objective of the latter study is to investigate the influence of eccentricity on the resuspension height and on the calculation of the particle normal stress in the vertical direction through the volume fraction profile analysis. Our results show that even minimal eccentricity can lead to significant changes compared to the centered case. [ABSTRACT FROM AUTHOR]

This paper comprehensively investigates the non-axisymmetric cavity dynamics of a vertically entering sphere under the influence of nearby side-walls through experimental, numerical, and theoretical analyses. Initially, we explore the characteristics of cavity evolutions with the sidewall effect. The emergence of a twin-vortex during cavity pinch-off is observed, and detailed numerical simulations provide insights into its underlying mechanisms. Both the dimensionless distance (λ) and the Froude number Fr significantly influence the pinch-off type. A phase diagram in the λ − F r parameter space is presented, revealing the interplay between these variables. Moreover, we investigate the sidewall effect on the pinch-off time and location at low Froude numbers. The findings indicate that as λ decreases, both the pinch-off time and depth of the cavity increase. Generally, the wall effect is relatively weak when λ exceeds 4. Additionally, the pinch-off time can be described by τ = k r / g , with the constant k determined by λ. Utilizing 2D cavity theory, we estimate the pinch-off time of the water entry cavity with the sidewall effect, revealing a consistent collapse behavior with the mechanics of a two-dimensional cavity. [ABSTRACT FROM AUTHOR]

The deformation process of compound droplets moving in air flow was studied by using volume of fluid model. In this paper, the kinetic process of air-in-liquid CeO2 compound droplets in air flow is reported by numerical simulation. First, the effects of initial velocity, initial diameter, and internal and external pressure difference on droplet behavior were studied when the compound droplets finally reached the morpho-steady state. The self-compiled program is used to process and analyze the digital image of the simulated broken image. The results show that, under the action of four main forces, namely, external aerodynamic force, internal pressure, viscous force, and surface tension, the inner surface of the hollow droplets gradually begins to break and continues to expand to the outer surface to break. With the increase in the pressure difference between inside and outside of the hollow drops, the pores are severely deformed and cracked. The increase in velocity leads to the imbalance of internal and external surface forces caused by the increase in aerodynamic fluctuation on the outer surface of hollow droplets. With the decrease in the diameter of the hollow droplet, the pressure threshold of the droplet breaking decreases and it is easier to break. [ABSTRACT FROM AUTHOR]

In this paper, the three-dimensional fully compact high-order gas-kinetic scheme (HGKS) is proposed for the direct numerical simulation of compressible turbulent flows. Because of the high-order gas evolution model, the numerical fluxes as well as the point-wise conservative variables can be evaluated from the time-accurate gas distribution function at the cell interface. As a result, both the cell-averaged variables and their cell-averaged gradients can be updated inside each cell. Based on the cell averaged values and their gradients, the compact Hermite weighted essentially non-oscillatory (HWENO) scheme is developed, in which the dimension-by-dimension reconstruction is used for three-dimensional turbulences. In both normal and tangential directions, the fifth-order HWENO reconstruction is adopted. Compared with the classical WENO scheme, the stencil for the HWENO scheme only contains 33 cells for each cell. To achieve the temporal accuracy, the two-stage fourth-order temporal discretization is used. For the evaluation of point-wise variables, the simplified third-order gas-kinetic solver is used. Several classical benchmark problems are simulated, which validate the accuracy, resolution, and robustness of compact HGKS. As a comparison, the numerical results of HGKS using non-compact WENO reconstruction are also provided. Due to the compact stencil, the compact HGKS has a favorable performance for turbulence simulation in resolving the multi-scale structures. [ABSTRACT FROM AUTHOR]

Shock diffraction is a widespread phenomenon in aerospace applications, such as shock tunnel nozzle and jet exciter exit, impacting their performance significantly. This paper focuses on the transient evolution of double-sided shock diffraction in both quiescent and supersonic crossflows by unsteady numerical simulations. The characteristics of the shock wave and the vortex are revealed. In the quiescent flow, the double-sided shock diffraction exhibits remarkable symmetry. The diffracted shock retains a self-similar nature, but its intensity distribution displays non-uniform characteristics, which gradually weakens from the center to both sides. The vortices on both sides also exhibit symmetrical behavior, with their trajectory behaving in linear tendency. When the supersonic crossflow interacts with the diffracted shock, an upward-moving separation shock and an asymmetric diffracted shock are generated. The vortices remain confined beneath the boundary layer and exhibit different shapes. Moreover, due to the rapid motion of the separation shock, the relative Mach number is introduced into the free-interaction theory (FIT) to predict the shock angle of the separation shock. The F ( x ¯) values corresponding to the separation point and pressure plateau are determined to be 3.04 and 4.68, respectively. The results evaluated by modified FIT show a good agreement with the values of simulation and experiment. [ABSTRACT FROM AUTHOR]

The liquid jet formation upon the collapse of two interacting cavitation bubbles is a physics-rich complex problem, which has possible implications such as the membrane sonoporation. In this paper, we employed both experiment and numerical simulations to study the jet behavior in a bubble, which is affected by another one introduced at various time delays. We provided the regime diagram, which shows the transition of the jet direction as a function of time delay and distance between bubbles. The maximum velocity of the jet in the primary bubble was investigated. The highest velocity was achieved when the primary bubble split into two parts upon collapse. It was found that a high curvature on the rim of the collapsing primary bubble causes the lateral pinch-off, leading to the formation of a fast and thin vertical jet approaching the secondary bubble. The results would help design an effective sonoporation system. The physics depicted in this manuscript might also contribute to the deeper understanding of the jet in a bubble interacting with not only another bubble but also other boundaries. [ABSTRACT FROM AUTHOR]

This paper conducted a computational fluid dynamics study of bubbly jets (not bubble plumes due to pure gas injection) in crossflow to explore the hydrodynamics that are still unknown. A three-dimensional model was developed, calibrated, and validated by coupling the Euler–Euler two-fluid model with unsteady Reynolds-averaged Navier–Stokes approach in OpenFOAM. The results showed that the modeled gas void fraction, bubble velocity, water jet centerline trajectory, and jet expansion agree well with the experimental data. The vertical distribution of turbulent kinetic energy evolves from mono-peak to dual-peaks as the jet penetrates farther for the bubbly jet due to the interactions between bubbles and ambient water flow. Water velocity distribution was examined at cross sections of both the air- and water-phases of bubbly jets in crossflow, and counter-rotating vortex pairs can be clearly observed for both phases. Generally, the center-plane maximum concentration decreases in the crossflow direction. Compared to pure water jets, bubbly jets are stretched wider in the vertical direction due to the lift of bubbles, and thus, dilution is larger. Interestingly, the vorticity at water jet cross sections of bubbly jets evolves from two vertical "kidney-shapes" to two axisymmetric "thumb-up-shapes." Moreover, effects of ambient crossflow on bubbly jet behaviors were systematically examined. As the crossflow velocity increases, the locations of maximum concentration, maximum velocity magnitude, maximum vorticity magnitude, as well as water jet centerline, all tend to be lower for bubbly jets. [ABSTRACT FROM AUTHOR]

Swept compression ramps widely exist in supersonic/hypersonic vehicles and have become a typical standard model for studying three-dimensional (3D) shock wave/turbulent boundary layer interactions (STBLIs). In this paper, we conduct a direct numerical simulation of swept compression ramp STBLI with a 34° compression angle and a 45° sweep angle at Mach 6 using a heterogeneous parallel finite difference solver. Benefitting from the powerful computing performance of the graphics processing unit, the computational grid number exceeds 5 × 106 with the spatiotemporal evolution data of hypersonic 3D STBLI obtained. The results show that the flow of the hypersonic swept compression ramp follows the quasi-conical symmetry. A supersonic crossflow with helical motion appears in the interaction region, and its velocity increases along the spanwise direction. Fluids from the high-energy-density region pass through the bow shock at the head of the main shock and crash into the wall downstream of the reattachment, resulting in the peaks in skin friction and heat flux. The peak friction and heating increase along the spanwise direction because of the spanwise variation in the shock wave inclination. In the interaction region, the unsteadiness is dominated by the mid-frequency motion, whereas the low-frequency large-scale motion is nearly absent. Two reasons for the lack of low-frequency unsteadiness are given: (1) The separation shock is significantly weaker than the reattachment shock and main shock; and (2) because of the supersonic crossflow, the perturbations propagating at the sound speed are not self-sustaining but flow along the r-direction and toward the spanwise boundary. [ABSTRACT FROM AUTHOR]