A common challenge in electrochemical machining (ECM) with insulating masks is the concentration of electric field at mask edges, which leads to localized over-etching and limits machining accuracy. This paper proposes a generic solution: the use of an insoluble auxiliary anode. This component, consisting of a metal substrate coated with an insoluble conductive layer, is installed in the non-machining area at the same potential as the workpiece. It functions by redistributing the local electric field, thereby suppressing the edge concentration effect without undergoing dissolution itself. The efficacy of this method is rigorously validated through its application in counter-rotating electrochemical machining (CRECM), a precision variant of ECM used for generating convex structures on difficult-to-machine titanium alloys. Coupled multiphysics simulation confirms that the auxiliary anode reduces the peak current density at the insulating edge by 37.5

Counter-rotating electrochemical machining (CRECM) is an effective method for the large-scale removal of difficult-to-machine materials from revolving parts, offering high machining efficiency, no residual stress, and zero tool wear. However, as the height of the CRECM-processed convex structure increases, stray corrosion caused by submerged machining becomes increasingly severe, reducing the accuracy of high-convex features (height > 10 mm). To address this limitation, this study proposes a novel electrolyte flow strategy: CRECM with internal flushing (IF-CRECM). In this approach, the electrolyte flows from the electrode through outlet slits into the inter-electrode gap (IEG), and the degree of electrolyte diffusion is controlled by varying the number of slits, thereby suppressing stray corrosion. A multiphysics model coupling gas-liquid two-phase flow with electric field distribution is developed to investigate the corrosion suppression mechanism of IF-CRECM. Detailed analyses of the gas-liquid distribution and stray current behavior in submerged CRECM and IF-CRECM confirm that the electrolyte is effectively confined within the IEG, resulting in a substantial reduction in the stray corrosion of convex structures during counter-rotation. Experimental validation further demonstrates that IF-CRECM achieves a 93.96

This research leverages resolved large eddy simulation to comprehensively explore the profound influence of yaw angle on the wake dynamics of tandem square cylinders. A rigorous grid sensitivity analysis substantiates the fidelity of the computational mesh, thereby guaranteeing the reliability and accuracy of the simulations. The findings reveal that the yaw angle exerts a significant and intricate impact on the aerodynamic coefficients of the cylinders, highlighting a nuanced interplay between drag and lift forces. Specifically, the upstream cylinder exhibits elevated mean drag coefficients, a trend that diminishes progressively with increasing yaw angles. In contrast, the mean lift coefficient of the downstream cylinder increases with rising yaw angle, while that of the upstream cylinder initially decreases until a critical threshold is reached, beyond which it subsequently ascends. Through meticulous analyses of the flow field and modal characteristics, including proper orthogonal decomposition, the study elucidates that an escalation in yaw angle induces wake deflection, thereby mitigating wake-induced interactions on the downstream cylinder and amplifying the yaw effect. These insights, garnered through a combination of computational and analytical methodologies, provide a deeper understanding of the fluid dynamics governing the wake dynamics of tandem square cylinders under yawed conditions.

In natural environments, fish almost always swim in groups. Investigating the coupled mechanism of biomimetic fish exhibiting autonomous swimming capabilities advances our understanding of fish schooling phenomena and simultaneously aids in refining the structural and formation configurations of underwater robotic vehicles. This work innovatively develops an algorithm based on the Direct-Forcing Immersed Boundary Method (DF-IBM) and implements it in an efficient, modular software program written in C++. The program accelerates the calculation process by using a multigrid method. Validation against a benchmark case of flow around a cylinder, with comparison to data from the existing literature, verifies the program’s precision with discrepancies of less than 3.6%. Based on this algorithm, the paper analyzes the incompressible viscous flow during the movement of parallel-aligned biomimetic fish. It uncovers the interaction between the fish’s motion and the surrounding flow field and also reveals the hydrodynamic mechanisms of the group motion of the parallel-aligned biomimetic fish. The flow field under varying spacing and phases between the parallel-aligned biomimetic fish proves that the interaction between the flow fields induced by the two fish bodies becomes increasingly significant when decreasing the lateral spacing from 1.4L to 0.6L. Notably, an initial lateral convergence of the fish bodies is observed, followed by a sideways swimming pattern at a particular pitch angle, accompanied by a decrement in their forward swimming velocity as they approach each other. Additionally, this study compares flow field alterations in parallel-aligned biomimetic fish with identical lateral spacing but opposing flapping phases. The findings indicate that, irrespective of the phase, the fish exhibit an initial convergence followed by a sideways motion at a specific pitch angle. However, due to disparities in the tail’s flow field, a larger pitch angle is generated when the fish swim in unison. All the findings above will provide a solid theoretical foundation for the design and optimization of underwater robotic vehicles.

The counter-rotating electrochemical machining(CRECM) shows unique potential in the machining of thin-walled rotating parts with complex convex structures. CREM realizes the shaping of complex convex structures through the relative rotation of the cathode and anode.The complex motion pattern and electric field distribution make it difficult to apply the existing cathode design methods to CRECM. To solve this problem, the matrix equations of cathode motion based on the kinematics and the electric field simulation model are established. The motion trajectories and edge contours at different angles are analyzed. The rotational overlap theory of motion trajectories under the windows at different angles is proved. Besides, the relationship between electric field distribution and the convex structure forming under different angle windows is studied, and the fundamental reason for deviations occurs when the convex profile is rotated to coincide is revealed. Therefore, a prediction model of the sidewall dissolution is established to correct this deviation, thereby deriving a high-precision design formula for the cathode windows of the high convex structures. By designing a cathode with oval-like windows to curry out CRECM experiments, the array-arranged(30 × 5) circular high convex structure with a maximum roundness error of 0.065 mm is successfully fabricated.

This work develops an Updated Lagrangian Smoothed Particle Hydrodynamics (ULSPH) framework to simulate high-speed icebreaking by a hemispherically capped cylinder (HCC). Using a self-programmed C++ code with Drucker–Prager damage criteria, this work systematically analyzes how impact velocity (100–200 m/s), ice thickness (10–40 cm), and impact angle (60–90°) govern structural loads and ice failure modes. The head of the HCC is always the stress concentration area, and the peak value of the impact force increases non-linearly with increasing the initial velocity from 100 m/s to 200 m/s. The increase in ice layer thickness from 10 cm to 40 cm raises the peak value of the impact force by 18.1%. The ice layer deformation shows three-stage characteristics: collision depression, penetration perforation, and through-spray. When the impact angle α is non-vertical, the strain of the ice layer is asymmetrically distributed, and the component of the peak impact force along the y direction increases significantly with the decrease in the impact angle, reaching 129.3 kN at α = 60°. Results reveal velocity-driven nonlinear force amplification, asymmetric strain distribution at oblique angles, and critical stress concentration at the HCC head, providing design insights for polar equipment.

Thin-walled annular parts are widely used in the aerospace industry. There are some irregular island structures with non-periodic distribution on the inner surfaces of annular parts, which poses a significant challenge to manufacture such annular parts using conventional mechanical methods. Electrochemical machining (ECM) has exhibited good prospects for manufacturing thin-walled annular parts, with the advantages of no tool wear, no processing deformation and a high material removal rate. However, there is little research related to fabricate convex structures on the inner surfaces of annular parts, although ECM is vital to the machining of thin-walled annular parts. In this study, a flexible cathode tool was used to machine irregular convex structures on the inner surfaces of annular parts via co-rotating ECM. The complex motion of the cathode tool and conductive slider was simulated using matrix equations, and the digital simulation of the material removal process was realized based on matrix equations and Faraday's law. Based on the mathematical model, a design method for a flexible cathode tool was developed, and three main parameters were determined: the position of the window, radius of the cathode, and width of the window. Comparative experiments were performed using the flexible and conventional cathode tools. The results showed that the selective removal of convex structures was achieved by using the flexible cathode tool. The insulated sheet needed to be attached to the position where the convex structure was retained. An aero-engine casing with many irregular island structures on the inner surface was successfully machined. The height of the fabricated island structure is 6 mm. The sidewall thickness of the aero-engine casing is 1.2 mm. Therefore, co-rotating ECM with a flexible cathode tool offers excellent machining ability for irregular island structures on the inner surfaces of annular parts.

During the water entry process of a trans-domain morphing aircraft, significant impact forces are generated when the aircraft hits the water surface, which will potentially cause the deformation of the cabin structure and might damage the structure or onboard devices. Thus, it is necessary to investigate the water entry process of the cabin structure. This paper analyses changes in fluid loads and the corresponding structural responses during the water entry process. Firstly, the numerical model is established for the water entry process and the modeling method is validated by comparing the results to the experimental data. An empirical formula is developed to correlate the impact loads with the water entry velocities. Then, fluid–structure interaction analysis of the water entry process is performed using a two-way coupling approach. The relationship between structural deformation and the water entry process is then investigated. The results are compared with those without considering the structural deformation. The empirical formula is then modified to reflect the effects of the deformation. The results show that structural deformation will disperse the impact load, which represents different responses compared to the rigid cabin structure.

Electrochemical machining (ECM) presents numerous applications in the production of aero-engine components. The effects of current density on surface quality during ECM of René 88DT in NaCl-ethylene glycol and NaNO3-aqueous solutions was investigated here. The results indicated that the René 88DT surface has lower sensitivity to current density and more homogeneous electrochemical dissolution in NaCl-glycol solution than in NaNO3-aqueous solution, which results in the suppression of stray corrosion and consistently high surface quality regardless of current density. These strengths are attributed that C2H5O2- and Cl- ions replace OH– ions and combine with alloy ions to form solubles and complexes, which prevent the formation of obstructive passive film, oxide layer, and insoluble electrolytic products. The electrochemical dissolution model in two solutions was established. Furthermore, the high tensile strength and precision machinability of René 88DT in NaCl-glycol solution were also demonstrated.

An adaptive particle refinement (APR) algorithm has been developed for the smoothed particle hydrodynamics (SPH) method to augment the resolution of the region of interest to achieve high accuracy and simultaneously reduce the cost of computational resources. It is widely applied in the field of fluid-controlling problems involving large interface deformations, such as the two-phase flow and fluid–structure interaction because this algorithm can capture the interface with high accuracy. Nonetheless, existing APR algorithms widely encounter computational dispersion issues at the interface of regions of different particle resolutions. Moreover, traditional shifting algorithms applied in the APR processes also have difficulties in dealing with particles with different smooth lengths. In this work, an algorithm for fast particle generation was first developed based on the accelerated ray method, which accelerates the discretization of the flow field into particles. Then, a dynamic refinement/coarsening algorithm based on the APR algorithm is proposed to solve the computational dispersion problem that occurs at the refinement/coarsening interfaces. In addition, the shifting algorithm was improved in this work to ensure the particles are always well distributed during numerical calculations and, thus, can efficiently facilitate the adaptive particle refinement/coarsening processes. Comparative analysis indicates that the robust algorithms developed for the SPH method in this work can lead to more precise and reasonable flow fields compared with the conventional SPH adaptive methods.

The water entry of a torpedo is a complex nonlinear problem, involving transient impact, free surface deformation, droplet splashing, and fluid–structure coupling, which poses severe challenges to traditional mesh methods. The meshless smoothed particle hydrodynamics (SPH) method shows unique advantages in capturing the complex features of the water entry of the torpedo at different entry angles. However, it still suffers from some inherent shortcomings, such as low surface discretization accuracy, poor discretization flexibility, and low calculation efficiency. In this study, an improved adaptive SPH algorithm is proposed to investigate the water entry of the torpedo accurately and efficiently. This method integrates meshless point generation and adaptive techniques simultaneously. The numerical results demonstrate that when the torpedo vertically enters the water at different velocities, the induced impact loads acting on the head of the torpedo fluctuate significantly with two peak values in the initial stage and thereafter stabilize in a later stage. The impact load acting on the torpedo, the entry depth of the torpedo, the splash height of the droplets, and the size of the cavity generated around the torpedo increase with the increment in the entry velocity. When the torpedo enters the water at different entry angles under the same initial entry velocity, both the vertical and the horizontal movements of the torpedo are observed, which results in more complex variations in parameters along the x- and y-axes. The findings and the corresponding numerical method in this study can provide a certain basis for the future designs of the entry trajectory and the structural bearing capacity of torpedoes.

The electrochemical dissolution behavior of cobalt-based superalloy Haynes 188 was observed by an in-situ observation device. The results showed that at low current density, a dense oxide film was formed on the carbide surface to protect its dissolution. But at high current density, carbide dissolved and produced CO bubbles until falling off. The dissolution of carbide at high current density resulted in the reduction of the high ten-point mean roughness (Rz) and the improvement of surface quality. A dissolution model under different current densities was established to reveal the mechanism of influence of carbide on surface quality under different current densities.

Here, we propose an information preservation (IP) optimization approach for supersonic flow simulations. Based on the conversion of variables integrated in a half-space and analogical analysis, we obtained a relationship between the integrated variables, used in the governing equations of the preserved information, and the macroscopic variables used in Navier–Stokes equations. Computationally, the flux for the correlation terms is reconstructed using the Advection Upstream Splitting Method (AUSM) scheme, giving the IP method windward characteristics. Additionally, the pressure boundary treatment is modified to improve the stability and accuracy of the IP method in the simulation of rarefied supersonic flows driven by high pressures. The particle number threshold method is proposed to solve the “carbuncle point” problem, which is caused by insufficient local collisions. The IP optimization method combines the advantages of the AUSM scheme and low statistical scatter method. Thus, it can be used to investigate the real-time variations in a rarefied supersonic flow through simulations of the supersonic microscale jet flow and hypersonic macroscale nozzle flow. The results indicate that the real-time statistical accuracy of the IP optimization method is significantly higher than that of the direct-simulation Monte Carlo method, and the correctness and accuracy of this method are satisfactory as well. Therefore, the proposed method can unveil the mechanisms underlying the variations and unsteadiness in rarefied supersonic flows on different scales.

A new numerical simulation method called the Meshless Direct Simulation Monte Carlo (DSMC) Method is presented in this article for solving a rarefied flow field problem. It uses a discrete points group instead of using a conventional mesh to discretize the computational domain. The meshless theory is used to solve the governing equations of the rarefied flow field. Numerical challenges related to mesh distortion and low accuracy encountered in conventional mesh technology are eliminated. The meshless technology used in the DSMC method constructs the molecular cloud structure and the virtual volume to establish the methodology of molecular fast search, molecular collision pair number calculation, and molecular cloud sampling. The numerical results of a two-dimensional flow around a cylinder, a nozzle flow, a three-dimensional flow around a sphere, and a Mars probe re-entry flow are presented to demonstrate the feasibility, accuracy, and robustness of the Meshless DSMC method. This study provides the basis for meshless technology development in the field of rarefied flow.

The migration movement of fish in regular formation is a common phenomenon in nature. Exploring the principle of hydrodynamics is not only conducive to further understanding the internal mechanism of the fish swarm, but also conducive to optimizing the structural design and formation design of underwater vehicles. The immersed boundary method used in this paper is a non-fitting Cartesian mesh method, which can better deal with the problems of complex geometric shapes and moving boundaries without generating fitting meshes as frequently as the traditional mesh generation method. In this study, a solving program is developed based on the immersion boundary direct force algorithm, which uses multi-grids and non- uniform grids to accelerate the solution of the flow field. The program is verified by a typical example of cylindrical flow, and the corresponding accuracy of the program is demonstrated through comparing the achieved results with that reported in the existing literature. Based on this verified program, the movements of the juxtaposed bionic fishes in incompressible viscous flow are analyzed, the interactions between the movements of fishes and the surrounding flow field are captured, and, importantly, the hydrodynamic mechanism of juxtaposed bionic fishes swimming in a shoal of fish is revealed. Different characteristics of flow fields induced by the juxtaposed bionic fishes’ movements with the same swing frequency but in different phases, namely in-phase and anti- phase, are observed and analyzed. The result shows that the anti- phase scenario is more beneficial to enhancing the overall propulsion performance of the shoal of fish.

Counter-rotating electrochemical machining (CRECM) is an innovative ECM method that can be used to manufacture revolving parts, especially thin-walled casings with complex convex structures. As titanium alloys are easy to passivate and difficult to machine, active NaCl solution is often used to obtain a good surface quality from ECM. However, the top surfaces of convex structures will be subjected to severe over cutting using NaCl solution, resulting in poor machining accuracy. Therefore, an insulation coating is employed to protect the non -processed area by shielding stray currents in this paper. A simulation model is established to investigate the evolution of convex structures with an insulation coating. According to the simulated convex structures, the top of the convex structure suffers from serious stray corrosion without the protection of insulation coating, and the sidewall inclination angle is 25.42 degrees. However, the insulation coating enables convex structures without over cutting, achieving a sidewall inclination angle of only approximately 1.1 degrees. Under the protection of insulation coating, the width and height decrease linearly with time, and the radius of the root fillet increases towards an approximately constant value. The experimental results indicate that stray currents can be shielded completely with an insulation coating, and no electrochemical dissolution occurs on the top of the convex structure. The deviation between simulation and experimental results does not exceed 10 %, demonstrating reliability of the proposed model. Cylindrical and conical parts with a grid-like convex structure are successfully produced with a surface roughness value of Ra1.8 mu m. This fully verifies the effectiveness of an insulation coating and demon-strates the excellent processing capability of CRECM.

This study experimentally and numerically investigates the performance of a circular cylinder with a spiral grooved surface in terms of reducing wind drag. Its application in the overhead high-power conductor plays a vital role, especially in typhoon conditions. Wind tunnel tests have shown that at the critical Reynolds number (Re), the coefficients of wind drag decrease to a greater extent in a spiral grooved cylinder than in a smooth circular one. Moreover, a cylinder with a shallow groove and a small number of spirals could reduce the coefficient of drag in typhoon conditions. To gain an insight into the underlying fluid mechanism, a large-eddy simulation of turbulent flow from a critical to a super-critical Re has been carried out to approximate the flow separation and turbulent eddies over the spiral grooved cylinder. The results of the wind tunnel test have been used as a benchmark for the numerical results. The flow characteristics have been established about the near-wall flow separation and far wake flow, the pressure coefficient, the skin-friction coefficient, drag coefficient, and Q -criterion field.

There are serious aerodynamic heating and high resistance phenomena for near space supersonic aircraft. Application of opposing jet flow is an effective technology for drag and heat reduction. This paper uses DSMC method to simulate the external opposite flow of aircraft moving in the supersonic flow in rarefied environment of near space. It analyzes the advantages and disadvantages of drag and heat reduction of opposing jet in the different conditoins by adjusting speeds of the jet in order to obtain an optimized scheme. The numerical results show that the peak values of aircraft surface pressure, surface heat flux and friction are reduced by 76.6%, 93.5% and 50.0% respectively. Application of opposing jet in rarefied environment can effectively reduce the drag and aerodynamic heating for supersonic aircraft. The present study provides an effective reference for the aircraft design on the drag and heat reduction.

Fish-like robots have been widely used in intelligent surveillance and investigation because of their high swimming efficiency and low traveling noise. Numerical simulations are usually selected to simulate the movement modes and hydrodynamic characteristics of fish-like robots during design and manufacture. However, the body-fitted grid method traditionally utilized in numerical simulations often has difficulty dealing with moving solid boundaries. In this work, the immersed boundary method, superior in handling the moving boundary conditions, is employed to simulate the movement of a fish-like robot swimming in high Reynolds number flows in combination with the RANS turbulence model. The numerical method is first validated using a fluid flowing over a square block, and the corresponding results are in good agreement with the ones reported in reference. Then, the swing of the fish-like robot under three different Reynolds numbers is studied. The lift coefficient and the drag coefficient of the fish-like robot decrease with increasing the Reynolds number. This paper provides remarkable support for future designs and applications of fish-like robots.

The Information Preservation (DSMC-IP) method is improved by using Advection Upstream Splitting Method (AUSM) splitting scheme, to solve the problems that the Direct Simulation of Monte Carlo (DSMC) method is effected by a large statistical dissipation, and that he traditional IP method cannot simulate the strong shock wave accurately. The fluxes of correlation terms in governing equations are reconstructed by local Mach numbers as the standard, so that the calculation is more accurate in accordance with the flow characteristics on both sides of the shock wave, thus forming a new DSMC-IP method with high statistical accuracy and hypersonic flow simulation ability. This method is utilized to simulate supersonic flow around the cylinder and hypersonic flow around the nozzle with divergent angle, the results of the AUSM splitting DSMC-IP method is basically consistent with the results of DSMC method, however the former has lower statistical scatter, and the flow characteristics are clearer. The surface characteristic coefficients differences between the results and the experiment or reference values are less than 5.5%. It is proved that the AUSM splitting DSMC-IP method is accurate and effective in the hypersonic flow simulation.