Overlapping Grid Technique Research Articles

Effects of particle size and reciprocating frequency of the plunger on performance and flow characteristics of a diaphragm pump

ABSTRACT The present study aims to reveal characteristics of the diaphragm pump applied in transporting the mixture of liquid and solid particles. The computational fluid dynamics (CFD) technique was used in combination with the six degree of freedom (SDOF) method to simulate the solid-liquid two-phase flow in the diaphragm pump. The zero-gap overlapping grid technique was employed to treat unsteady gap flows near the two valves. Effects of particle size and the reciprocating frequency of the plunger were investigated. The results indicate that solid particles pass smoothly through the diaphragm pump. When particle size increases from 2.0 mm to 3.0 mm, the solid volume fraction (SVF) in the pump chamber increases. When the reciprocating frequency of the plunger increases from 150 to 250 cycles per minute, the SVF increases and then decreases. When the reciprocating frequency of the plunger increases, the performance of the diaphragm pump declines, whereas the distribution of particles in the pump chamber is gradually uniformized due to the disturbance of the reversed flow from the pump outlet.

Exploring the mechanisms behind dual-peak formation in energy harvesting efficiency of semi-passive flapping airfoils with varied spring stiffness

Flapping airfoil energy harvesting has emerged as a promising paradigm inspired by birds and marine organisms. In this investigation, a semi-passive flapping airfoil device with a predetermined pitching motion was examined using a transient numerical simulation method employing an overlapping grid technique. The impact of the spring stiffness on the energy harvesting characteristics of the flapping airfoil was analyzed through the implementation of the integral momentum theorem and dynamic mode decomposition. The results demonstrated that the efficiency and power curves of the flapping airfoil exhibited a distinctive bimodal M shape in response to variations of the spring stiffness. The initial peak efficiency point, which corresponded to the system's natural frequency in proximity to the pitching frequency, was found to be heavily influenced by the presence of leading-edge vortexes, thereby influencing lift generation. By utilizing the integral momentum theorem, it was determined that the Lamb vector term, fluid acceleration term, and total pressure term predominantly contributed to the lift generation. The emergence of the second peak efficiency point was primarily linked to torque variations induced by trailing-edge vortexes.

Analysis of fluid-structure coupling during the flapping of dragonfly hind wings under different flexible distributions

Abstract To investigate the influence of flexible distribution on the aerodynamic characteristics of a dragonfly hind wing during flapping flight, this study establishes a simplified plate model of the dragonfly hind wing based on the actual size, while neglecting the wrinkled microstructure. Six different flexible distribution patterns are proposed for the model. The overlapping grid technique is employed to achieve fluid-structure coupling calculations during the flapping motion of the dragonfly hind wing, by customizing the Young’s modulus in the solid region to represent different levels of flexibility. The results show that gradually increasing the flexibility along the chordwise direction of the dragonfly wing can induce a stronger leading-edge vortex during flapping, intensifying the accumulation of vorticity near the wingtip and resulting in a higher peak lift coefficient. The flexible wing has a positive impact on the formation and development of the trailing-edge vortex, leading to an increase in the thrust coefficient peak and time-averaged thrust coefficient of the dragonfly hind wing. Consequently, all aspects of the aerodynamic characteristics are significantly improved.

Research on the multiphase flow interference and motion characteristics of vehicles during an underwater salvo

A cavitation flow can greatly impact a vehicle's attitude and stability when leaving water. This paper adopts an improved delayed detached eddy turbulence model and Schnerr–Sauer cavitation model as well as the volume-of-fluid method and an overlapping grid technique to investigate this effect. The simulation method used for the cavitation model is validated. The interference effects of a transient multiphase flow, collapse loads, and the motion instability of vehicles during an underwater salvo are studied. The results show multiple obvious pressure peaks during the process of cavity collapse, which do not overlap significantly. Instead, they are sequentially arranged from the top to the end of the bubble, and the synchronous collapse pressure peak is much stronger than the other pressure peaks. The synchronous collapse pressure has a high peak and a short pulse width, and its action position is at the bottom of the shoulder cavity. The salvo time interval is zero, the launch depth is equal to the length of the vehicle, the initial cavitation number is 0.233, and the lateral launch spacing is varied from 2 times the diameter to 5 times the diameter. When the lateral spacing is in the range of 4 times the diameter to 5 times the diameter, the effect of flow interference on the underwater travel and water exit stages disappears.

Numerical study on the pulsation characteristics of tail cavities under vertical launching conditions

This study aimed to analyze the pulsation characteristics of tail cavities at different Froude numbers under vertical launching conditions. The improved delayed detached eddy simulation (IDDES), volume of fluid (VOF) model, adaptive mesh refinement (AMR), and overlapping grid technique were adopted to model the pulsation characteristics of the tail cavity. The feasibility of the model for predicting dynamic behaviors was verified by comparing the numerical and experimental results. The results demonstrated that the volume of the tail cavity pulsated periodically. However, the concurrent expansion and contraction in the integral tail cavity resulted in a relative state of cavity volume pulsation. When the expansion rate exceeded the contraction rate, the cavity volume increased, otherwise, decreased. In addition, four flow patterns were observed in the tail cavity: smooth, irregular, hybrid, and cavity-shedding regions. An increase in the Froude number augmented the tail cavity volume and proportion of irregular regions, and reduced the proportion of hybrid regions.

Mitigation Effect of Helmholtz Resonator on the Micro-Pressure Wave Amplitude of a 600-km/h Maglev Train Tunnel

A 600-km/h maglev train can effectively close the speed gap between civil aviation and rail-based trains, thereby alleviating the conflict between the existing demand and actual capacity. However, the hazards caused by the micro-pressure wave amplitude of the tunnel that occurs when the train is running at higher speeds are also unacceptable. At this stage, mitigation measures to control the amplitude of micro-pressure waves generated by maglev trains at 600 km/h within reasonable limits are urgent to develop new mitigation measures. In this study, a three-dimensional, compressible, unsteady SST K–ω equation turbulence model, and an overlapping grid technique were used to investigate the mechanism and mitigation effect of Helmholtz resonators with different arrangement schemes on the micro-pressure wave amplitude at a tunnel exit in conjunction with a 600-km/h maglev train dynamic model test. The results of the study showed that a pressure wave forms when the train enters the tunnel and passes through the Helmholtz resonator. This in turn leads to resonance of air column at its neck, which causes pressure wave energy dissipation as the incident wave frequency is in the resonator band. This suppresses the rise of the initial compressional wave gradient, resulting in an effective reduction in the micro-pressure wave amplitude at the tunnel exit. Compared to conventional tunnels, the Helmholtz resonator scheme with a 94-cavity new tunnel resulted in a 31.87% reduction in the micro-pressure wave amplitude at 20 m from the tunnel exit but a 16.69% increase in the maximum pressure at the tunnel wall. After the Helmholtz resonators were arranged according to the 72-cavity optimization scheme, the maximum pressure at the tunnel wall decreased by 10.57% when compared with that before optimization. However, the micro-pressure wave mitigation effect at 20 m from the tunnel exit did not significantly differ from that before the optimization.

Simulation research on the outlet cavity features in the underwater launching process

This paper analyzes the evolution of cavity features and pressure characteristics during the underwater launching process. The improved delayed eddy simulation (IDDES), energy equation, and overlapping grid technique are adopted. In addition, validation of the numerical approach and grid independence is carried out. The evolution of the multiphase flow field and pressure fluctuations near the launcher outlet are studied. Results show that the gas mass escaping from the tube expands and contracts, eventually breaking off with a jet that acts on the tail of the vehicle after the vehicle leaves the tube. Four major pressure fluctuations occurred near the launcher outlet, the first two of which are generated by the alternate expansion and contraction of the high-pressure gas mass, while the subsequent fluctuations mainly originate from the intermittent escape of the remaining gas mass inside the tube, the magnitude of the pressure fluctuations decreases gradually. As the Froude number increases, the scale of the escaping high-pressure gas mass expands significantly, while the dissipation rate of the fused gas mass at the launch tube outlet also increases, the evolution mechanism of multiphase is more complex.

RANS/LES investigation on the performance of air film fusion around a vertically launched underwater vehicle

The Improved Delayed Detached Eddy Simulation (IDDES) model, energy equation, Volume of Fluid (VOF), and overlapping grid technique are adopted in this paper. Analyzing the performance of Froude numbers, and row-spacing-to-diameter ratios on the air film development process. The evolution of air film around a vertically launched underwater vehicle technology is the passive exhaust process, in which a high-pressure cavity of the limited volume is formed by crossflow shearing, environmental depressurization, and other factors. With the increase of the Froude number, the maximum diameter and length of the air film at the same feature position are increased, which is beneficial to air film fusion to a certain extent. However, research results indicate air film fusion does not occur at the large Froude numbers. In the establishment of the air film fusion model, there is a non-linear relationship between the Froude number and the pressure ratio inside and outside the cavity. As the Froude number increases, the pressure ratio becomes larger. However, the largest cross-sectional area of air film is not the largest. In the larger range of Froude numbers, the relationship between gas film fusion characteristics and Froude number is more complicated, and further research is studied. Finally, the effects of row-spacing-to-diameter ratios on the air film development process were discussed.

The transient vortex structure in the wake of an axial-symmetric projectile launched underwater

This paper provides refined wake simulations for an underwater projectile launch using an improved delayed detached eddy simulation with the energy equation, volume of fluid, and the overlapping grid technique. Additionally, the projectile wake vortex was analyzed for different Froude numbers and dimensionless transverse flow speeds. Verifications of the numerical method, grid independence, vortex identification method, and time step size are presented. Through a systematic comparison of the wake morphologies, the flow fields and vortex structures in the wakes were analyzed in detail, and the wake vortex evolution mechanisms were explored. The results show that the Kelvin–Helmholtz instability was observed, and the wake flow of the projectile launched underwater contains a complex vortical system that directly determines the wake instabilities. The resulting multiple sub-vortex structures are compact and closely arranged near the central axis without the transverse flow effect. However, compared with cases having no transverse flow, the large-scale double spiral vortex structure in the wake with a transverse flow is more difficult to fracture. In addition, the U-shaped vortex in the secondary vortex is also obviously generated in the wake during the double spiral vortex structure evolution. With an increase in the Froude number, the vortex legs are gradually apparent and, together with the shedding vortex rings in the wake, form a hairpin vortex structure. With an increase in the dimensionless transverse flow speed, the number of sub-vortex rings derived from the shedding vortex in the wake increases significantly, resulting in a more complex interaction mechanism.

Research on the effect of asymmetric bubbles on the load characteristics of projectiles during an underwater salvo

This paper analyzes the effect of bubbles on the load characteristic of projectiles launched underwater with transverse-flow under different dimensionless launch time intervals. The realizable k−ε turbulence model and energy equation, as well as the volume of fluid method and overlapping grid technique, were adopted. Additionally, validations of numerical approach and grids independence validation were carried out. Evolution of bubbles at tube outlet, time-domain characteristic and frequency domain characteristic of lateral force, vertical force-torque, and motion characteristics are studied. The results show that spilled gas from the tube outlet fuses and moves in a transverse-flow direction. The pressure pulsation, which is caused by the bubble elastic effect, leads to high-frequency pulsation laws of the load characteristic. In the initial stage of the movement, the motion laws of projectiles under different launch time intervals are similar. At ΔT≤1.2, the flow interference between two projectiles is large, which seriously affects the ballistic stability and safety.

A study on the flow interference characteristics of projectiles successively launched underwater

This paper analyzes the flow interference characteristics of projectiles successively launched underwater with 6 degrees of freedom for different dimensionless launch time intervals and different dimensionless transverse-flow speeds. The realizable k−ε turbulence model and energy equation, as well as the volume of fluid method and overlapping grid technique, are used. Additionally, a verification of the numerical method and a validation of the grid independence are presented. The pressure distribution on the inflow side and backside, the hydrodynamic parameters, the trajectory, and the pitch attitude are studied with the focus on the evolution of hairpin vortex packets in free wake flow. The results show that the hairpin vortex packet in the free wake flow is composed of a multistage hairpin vortex and a low-speed streak region with a stream scale that runs along the interior of the hairpin vortex packet. At higher transverse-flow speeds, the pressure coefficient and hydrodynamic coefficient of the second projectile increase as the launch time interval increases. Due to the effects of transverse flow, when the secondary projectile passes through the core region of the wake vortex, the pressure of uneven distribution characteristics greatly reduces and the trajectory stability increases. To maximize the efficiency and success rate of projectiles successively launched underwater, the second projectile should be launched at ΔT=2.0.

Effect of section shape, aspect ratio and thickness on the propulsive property of three-dimensional oscillating wing – a numerical study

ABSTRACT A three-dimensional numerical simulation is employed to investigate the effect of oscillating wing's section shape, aspect ratio and thickness on its propulsive property. The overlapping grid technique is employed to simulate the oscillating wing's motion at a moderate high Reynolds number of 1.0 × 104. The results show that the NACA0012 profile exhibits the best propulsive performance, compared with the other two section shape. As for the aspect ratio (AR), the mean value of the thrust force coefficient shows linear growth with the variation of AR and the optimal value of AR is suggested to be set as 3.0. Similar to the AR effect, there exists optimum value of thickness leading to the highest propulsive efficiency and the corresponding value is suggested to be set as 12% of the chord length. The vortex structures in the wake of the oscillating wing under different parameters are also presented and discussed.

Numerical analysis on the propulsive performance of oscillating wing in ground effect

The transient numerical simulation has been conducted by using of the Overlapping grid technique to investigate the ground wall effect on the propulsive performance of an oscillating wing. Both the two-dimensional and three-dimensional wing have been analyzed and the systematic analysis on the effect of the Reynolds number (Re), the average distance between the flapping foil and ground wall (h0) on the propulsive performance of oscillating wing are investigated with respect to the non-dimensional oscillating frequency (St). The simulation results demonstrate that the existence of the ground wall can produce significant influence on the fluid dynamics of oscillating wing and the ground wall effect has close connection to the magnitude of St number. In addition, the increase of Re number will facilitate the growth of both thrust force and propulsive efficiency of oscillating wing and that influence also has close connection with the St number. Finally, the existence of ground wall can also greatly improve the propulsive performance of three-dimensional oscillating wing, but the sensitivity of three-dimensional oscillating wing to the ground wall is lower than that of two-dimensional oscillating wing. The conclusions acquired here can be utilized to optimize the kinematics of the underwater vehicles swimming near ground wall.

Detached eddy simulation of the release progress of moving body separating from a cavity

Aiming at the complex unsteady flow during the release progress of moving body separating from a cavity, combined with the overlapping grid technique and Spalart Allmaras equation turbulence model, the three-dimensional unsteady flow solver is presented. On this basis, the validity of the simulation method is verified by M219 cavity example and WPFS example. Then the released moving body is conducted with SA-IDES. The research results show that upward force and pitch down moment of the moving body are affected by the fluctuating flow near the cavity and the SA – DES simulation is good in predicting high frequency flow. In conclusion, the outside shear layer and the inside fluctuating flow of the cavity has a significant impact on the aerodynamic loads and the trajectory of the moving body during the initial separation stage.

Моделирование траектории судна на волнах на базе STAR-CCM+

The reliable ship motion trajectory signal provides a basis for six-degree-of-freedom test bench to simulate ship motion in waves. The characteristics of ship motion in waves were analyzed. Aiming at the problem of complex hull shape and large displacement at sea, the dynamic overlapping grid technique was used. Based on STAR-CCM+, the free surface was solved by CFD (Computational Fluid Dynamics) numerical simulation method. The numerical model of ship motion in waves was established, and the DFBI (Dynamic Fluid Body Interaction) method was used to simulate the ship motion in the waves. The ship’s motion trajectory was obtained in the regular and irregular heading waves, which provides important support for the six-degree-of-freedom test bench wave simulation system.

Power Coefficient Analysis of Double-blade Half-rotating Impeller Tidal Turbine Operating at Yaw

The double-blade half-rotating impeller tidal turbine (DHITT) is a new type of vertical shaft tidal current turbine with lift and resistance performance. The power coefficient of the DHITT is affected by the flow direction. In order to research the power coefficient (CP) of the DHITT under different flow direction, the optimal attack flow angle of a half-impeller turbine was explored, and the fluctuation of power coefficient of the DHITT operating at yaw was analyzed based on the optimal attack flow angle. The unsteady flow of the turbine was simulated by overlapping grid technique, and the fluctuation of the turbine’s power coefficient under different flow directions was analyzed, which was verified by experiments. The results have demonstrated that the power coefficient at the optimal angle of attack is 0.53. As the yaw angle greater than 30º, the power reduction is nearly 40%, but the average efficiency loss is only 3.7% in the range of -3º to 3º.

Dynamic coupling analysis of the aerodynamic performance of a sedan passing by the bridge pylon in a crosswind

In the crosswind environment, the flow field around a bridge pylon is complex and variable so that the vehicles are prone to side slip when passing by this region. In this situation, safety and comfort are severely affected. In this study, a dynamic coupling method that uses computational fluid dynamics (CFD) coupled with multibody dynamics was adopted to investigate the aerodynamic performance and dynamic response of a sedan passing by a bridge pylon in a crosswind. The overlapping grid technique was used to achieve the movement of the sedan in the CFD simulation. The mechanism of the interaction between the flow field around the sedan and the wake vortex around the bridge pylon was demonstrated. The influence of crosswind type on the aerodynamic performance and dynamic response was further studied. The results show that the flow field around the sedan could greatly influence the behavior of the sedan, mainly in terms of changes in lateral displacement and yaw angle. Additionally, the dynamic coupling method provides a more direct and realistic way to understand the interaction between the aerodynamic loads and the dynamic responses. Of the different wind types considered (sinusoidal, linear, step), the step crosswind produces the biggest average lateral force and yaw moment resulting in the largest lateral displacement and yaw angle.

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice-podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice-podded propulsor interaction.

Overlapping Grid Technique for Numerical Simulation of a Fast-Cruising Catamaran Fitted with Active T-Foils

In marine engineering, appendages such as fin stabilizers and/or T-foils are made to rotate and to reduce the motion of ships. Research on the hydrodynamics of ships fitted with active appendages has significantly improved the design and control of such ships. However, most studies focus on fixed rather than rotating appendages, thereby ignoring the hydrodynamic unsteadiness of active appendages. To enhance the reliability and precision of the numerical simulations, we propose the use of overlapping grids for simulating advanced catamarans fitted with a pair of rotating T-foils under each bow. The fundamental purpose of the overlapping grid technique is to realize information exchange via regional overlap sharing in each subdomain of the computing domain, instead of using the method of boundary sharing, thus greatly alleviating the difficulty of generating the subdomain grid; moreover, the technique guarantees the quality of the subdomain grid. Within the main computational domain, a subdomain was allocated to accommodate the T-foil. Overlapping meshes near the interface between the two domains enable information flow during the simulation; the overlapping grids are updated at every iteration step because the subdomain rotates. The instantaneous trim and sinkage responses of the catamaran to the T-foil rotation were reproduced. From the moment the active T-foil stopped moving, there was no change in the ship’s sailing attitude, indicating that the response was in real time. By comparing with EFD data, the numerical results showed reasonable agreement, indicating the feasibility and effectiveness of the technique in simulating the hydrodynamics of ships fitted with active appendages.

Numerical study on the head-on collisions of solitary waves by a harmonic polynomial cell method

A 2D fully-nonlinear numerical wave flume is created with a piston-type wave generator based on a recently developed 2D harmonic polynomial cell method (HPC), which has been demonstrated as a highly accurate and efficient approach for modelling of water waves and their interaction with marine structures within the context of potential flow. The overlapping grid technique and a Goring’s wave making method are applied to generate solitary waves. The main focus of this paper is on the head-on collisions of pairwise solitary waves with identical or different amplitudes, including the maximum run-up and spatial phase shift during the whole colliding process. As for collision between identical solitary waves with amplitude σ =0.4, our present numerical results are in excellent agreement with experimental results both in maximum run-up and phase shifts, which indicates the accuracy of HPC method in simulating the head-on collision of solitary waves. The immediate and long-term phase shifts based on wave crest trajectories were obtained with success to explain the inconsistent results of phase shifts between experimental, fully nonlinear numerical results and the third-order approximation results. It is found from the results that the maximum run-up surpasses the sum of the initial wave amplitudes. The analysis of the phase shifts shows that the phase shifts of two solitary waves due to the head-on collision vary with the window length of data and the distance between measurement point and the collision center. The general trend is that the phase shifts measured close to the collision center are larger than those far from the collision center.