Sliding Mesh Technology Research Articles

Unbalanced force analysis on valve core clamping in hydraulic spool valves

Hydraulic systems are widely used in nuclear power plant equipments. The reliability and stability of hydraulic systems can be seriously impacted by the valve core clamping resulting from unbalanced forces. This paper investigates the unbalanced force characteristics of hydraulic spool valve cores to identify the mechanisms behind such failures. Numerical models are established for hydraulic spool valve cores using sliding mesh technology with varying control face structures and space angles between the inlet and outlet, which are validated by experiments. The turbulent flow is modeled using the RNG k-ε model. Simulation results reveal that larger openings weaken the unbalanced force distribution while the larger inlet flow rates exacerbate the unbalanced force distribution, and control face structures significantly impact force distribution by altering the throttling effect of the throttle orifice. Furthermore, the impact of the space angles between the inlet and outlet is little compared with other parameters. These findings provide a reference for designing and optimizing hydraulic spool valves, contributing to higher reliability and stability of hydraulic systems.

Numerical investigation on unsteady characteristics of ducted fans in ground effect

Ducted fans are widely used in various applications of Unmanned Aerial Vehicles (UAVs) due to the high efficiency, low noise and high safety. The unsteady characteristics of ducted fans flying near the ground are significant, which may bring stability problems. In this paper, the sliding mesh technology is applied and the Unsteady Reynolds Averaged Navier-Stokes (URANS) method is adopted to evaluate the influence of ground on the aerodynamic performance of ducted fans. The time-averaged results show that the ground leads to the decrease of duct thrust, the increase of rotor thrust and the decrease of total thrust. The transient results show that there exist small-scale stall cells with circumferential movements in ground effect. The stall cells start to appear at the blade root when the height is 0.8 rotor radius distance, and arise at both the blade root and tip when the height drops to 0.2. It is found that the unsteady cells rotate between blade passages with an approximate relative speed of 30%-80% of the fan speed, and lead to thrust fluctuations up to 37% of the total thrust. The results are essential to the flight control design of the ducted fan flying vehicle, to ensure its stability in ground effect.

Numerical Study of Fuselage Impact on Acoustic Characteristics of a Helicopter Rotor

The paper presents the results of the simulation of unsteady turbulent flow generated by helicopter main rotor in the presence of the fuselage with an emphasis on the analysis of the influence of the fuselage on the rotor-induced flow and the rotor-generated acoustic field. The Reynolds-averaged Navier–Stokes equations with the Spalart–Allmaras turbulence model are used to simulate the Caradonna–Tung rotor and ROBIN fuselage interaction in hovering flight. The governing equations are discretized using the vertex-centered control volume method on mixedelement unstructured meshes with the sliding mesh technology to treat the rotor. The acoustic field generated by the rotor+fuselage interaction is comparatively analyzed against the case of an isolated rotor. It is found that the presence of fuselage significantly changes the rotor-generated acoustics. In particular, the presence of the fuselage noticeably distorts the directivity of acoustic radiation and increases the overall sound pressure level under the fuselage up to 20 dB, emphasizing the importance of the influence of fuselage on the helicopter acoustics.

Computational Investigation of Blade–Vortex Interaction of Coaxial Rotors for eVTOL Vehicles

In the design of electric vertical takeoff and landing (eVTOL) vehicles, coaxial rotors have garnered significant attention due to their superior space usage and aerodynamic efficiency compared to standard rotors. However, it is challenging to study the flow field near the rotors due to the blade–vortex interface (BVI) and vortex–vortex contact between two rotors. Using sliding mesh technology and Reynolds-averaged Navier–Stokes (RANS) solvers, a numerical method was established to simulate the flow field of a coaxial rotor in hover, which was verified by experiments. Using this method, this paper analyzes the relationship between position and intensity of the tip vortex of the upper rotor, the axial velocity of induced flow and the load distribution on the blades at the azimuth when the BVI phenomenon occurs with a difference in rotational speed and rotor spacing. The results indicate that, when the BVI phenomenon appears, the blade-tip vortex of the top rotor rapidly dissipates, and the load distribution of the lower blade changes due to the induced flow of the vortex. When the rotational speed increases, the spanwise thrust coefficient of each rotor changes slightly. The vortex–vortex interaction becomes stronger, which leads to vortex pairing. When the distance between the rotors decreases, the BVI phenomenon occurs at an earlier azimuth and the location of the BVI moves towards the tip of the lower blade. The vortex–vortex interaction is also enhanced, which leads to vortex pairing and vortex merging.

Unsteady non-equilibrium condensation flow of 3-D wet steam stage of steam turbine with roughness using sliding mesh method

This research develops the non-equilibrium condensation model with sliding mesh technology to solve the unsteady condensing flow inside a 3D wet steam stage of steam turbine with transient rotor-stator interaction. The maximum fluctuation of time-dependent condensation parameters is predicted. The condensation loss and entropy generation considering the off-design operation and rough blades are also evaluated quantitatively. The results showed that the secondary expansion and condensation occur near the rotor trailing edge. At design operation, the time-dependent subcooling fluctuates from −9.81 K to 8.06 K at the maximum fluctuation location. The frozen rotor method over-predicts the expansion and condensation characteristics in the steam turbine stage. Moreover, the maximum relative fluctuation of time-dependent wetness is 37.14% when it changes from 0.022 to 0.048. At off-design operation, the p-T diagram is applied to compare the expansion and condensation processes. The inlet subcooling increases by 40 K, resulting in an increase of 110.34% in outlet wetness. The phase of condensation loss with high off-design inlet subcooling is ahead of that with low off-design inlet subcooling. The fluctuation of time-dependent condensation loss with off-design inlet subcooling is about 102.28 kW. In addition, the back pressure ratio changes from 0.55 to 0.10, resulting in an increase of 190.91% in outlet wetness. The fluctuation of time-dependent condensation loss with off-design back pressure ratio can reach 112.3 kW. Besides, the maximum time-averaged entropy generation and exergy destruction due to the increase of surface roughness can reach 9.37 kJ kg−1 K−1 and 5.71 kW.

Numerical study on transient aerodynamic behaviors in a subway tunnel caused by a metro train running between adjacent platforms

Aerodynamic effects, including slipstream, piston wind, drag, and pressure wave, in the tunnel caused by a metro train running between two adjacent platforms were simulated using a metro train model with six cars, an RNG ᴋ–ε turbulence model, and the sliding mesh technology, and they were verified through some full-scale tests and scale model experiments. Theoretical analysis and computational fluid dynamics (CFD) were used to investigate the formation and spatial–temporal distribution characteristics of the wind environment in a tunnel. The effect of uniform speed on the transient aerodynamic behaviors in the tunnel was analyzed. The results show that there are differences in aerodynamic behaviors in a subway tunnel when a train is in the acceleration and deceleration stages. Piston wind is a kind of high-quality wind resource, and it is better to arrange the wind turbines close to the destination platform. Some results can provide theoretical support for the recovery of wind energy resources and the layout of fans in the tunnel. Further, they can increase the development and utilization of urban underground space.

Transient Behavior of a Prototype Centrifugal Pump with Abrupt Decreasing Flow Rate

Centrifugal pump often operates at different working flow rates to meet engineering requirement. To better reveal the transient behavior of centrifugal pump in the process of decreasing flow rate, the finite volume method (FVM), RNG k-ε turbulence model, sliding mesh technology, and user-defined functions (UDF) were employed to simulate the three-dimensional unsteady viscous incompressible flow in a low-specific-speed centrifugal pump during the abrupt valve-off period. The results show that the differences are very obvious between transient and quasi-steady calculations. The velocity is maximum on the wall of hub and shroud, while the velocity is minimum and uniform distribution at middle positions. The transient flow field lags behind the quasi-steady flow field, which may be related to the reasons; namely, kinetic energy cannot convert pressure energy in time.

Fluid dynamic analysis of liquefied natural gas flow through a cryogenic ball valve in liquefied natural gas receiving stations

Cryogenic ball valves are one of the most important fluid control devices in liquefied natural gas (LNG) receiving stations. The dynamic flow characteristics of cryogenic ball valves directly affect the stability of a pipeline system. In this paper, the sliding mesh technology was adopted to simulate the dynamic flow characteristics inside the cryogenic ball valve. Firstly, the energy loss inside the cryogenic ball valve during the opening and closing process was analyzed. It is found that the energy loss is larger when the spool degree is less than 30°, while the fluid resistance is very small at the rest of the degree. Then, the dynamic flow characteristics of the cryogenic ball valve at different spool rotational speeds were investigated. It is found that the velocity value and the flow turbulence are gradually increased when the spool rotational speed increases from 1.25 deg/s to 10 deg/s. Finally, dynamic flow characteristics during the valve opening process with fluctuation of inlet velocity were analyzed. It is found that the distribution of velocity and pressure is approximately consistent, while the value of velocity and pressure is increased with the increase of inlet velocity. It is helpful for the design of cryogenic ball valves and the improvement of the whole LNG pipeline system.

Aerodynamic performance and dynamic behaviors of a train passing through an elongated hillock region beside a windbreak under crosswinds and corresponding flow mitigation measures

Based on unsteady simulations and decomposed sliding mesh technology, train aerodynamic and dynamic behaviors near an elongated hillock beside a windbreak under a crosswind were analyzed. First, the sudden deterioration of the aerodynamic performance was explained. Then, a mitigation measure was proposed to reduce the impact of the elongated hillock on the train. Finally, the dynamic responses for the original elongated hillock and mitigation measure were compared. The results showed that two symmetric vortices along the railway were generated by the elongated hillock, and in the position between these two vortices, a strong wind speed occurred, which resulted in a sudden impact on the train, and the aerodynamic impacts were the largest on the head car. A certain distance from the elongated hillock to the windbreak could make a mitigation effect. For the original elongated hillock, there was a severe yawing phenomenon of more than 83 mm/3° from the centerline of the car body for the head car. For the mitigation measure, the safety index and vibration displacement of the head car and tail car decreased effectively.

Numerical investigations of flow characteristics of a pumpjet propulsor in oblique inflow

In this study, a detailed analysis of hydrodynamic performance of a pumpjet propulsor (PJP) in different oblique inflow angles (β=0⁰, 5⁰, 10⁰, 15⁰, 20⁰, 25⁰, 30⁰, 40⁰) and different rotational speeds (n=3000rpm, 3600rpm, 4200rpm) is investigated numerically. The SST k-ω turbulence model and sliding mesh technology based on the Reynolds Averaged Navier-Stokes (RANS) method are employed. The structured grids are adopted to the whole computational domain. The grid independent inspection is carried out and verified. A ducted propeller (the Ka4-70 propeller in 19A duct) is selected to validate the numerical method by comparison of its results with experimental data. Based on the numerical method, it can be found that the propulsor efficiency decreased with the increasing of different β. The propulsor efficiency changes tinily in little oblique angle and decreases significantly when the oblique angle is larger than 20⁰. The thrust coefficient and torque coefficient of PJP are obtained. The increase of β has a negative impact on the balance performance of PJP. The pressure distributions of pressure and suction side of rotor blade with different β are analyzed. Owing to the influence of β, the pressure of the upper left region of rotor blade drops significantly. The enlargement of the local load on pressure side and the appearance and expansion of the low-pressure region on suction side occour at the bottom right region of rotor blade. The radial distributions of the axial velocity (Vz) and the tangential velocity (Vt) with different Span and β along the axial direction are investigated. The increase of β resultes in a severe oscillatory behavior of the flow velocity, which easily induces the occurrence of cavitation phenomenon and mechanical vibration. The velocity contours of the axial cross section with different β are presented. Due to the change of β, the flows are blocked by the flow-guide and duct, there are obvious high-velocity and low-velocity regions of the flows.

Numerical analysis of the effects of stern flaps on ship resistance and propulsion performance

When a stern flap is installed on a high-speed surface ship, its role is often to reduce the ship resistance. However, the performance of a stern flap determined based on direct conversion of ship model resistance test results often has a large error. In this study, the ship model and stern flap coupling resistance tests and self-propulsion tests with integrated coupling of a ship-propeller-rudder-stern flap were performed based on the Reynolds-averaged Navier–Stokes method. The hull and propeller motions were based on overlapping grid and sliding mesh technology. The results show that while a stern flap reduces the ship resistance, it also affects the ship model flow field to a certain extent, consequently affecting the propulsion performance of the ship. The improvement in ship propulsion performance is one of the main factors contributing to the energy savings achieved by installing a stern flap, with a contribution between 30% and 50%. The integrated ship-propeller-rudder-stern flap calculation method is significantly improved compared with traditional ship model resistance tests as a means of determining the stern flap performances of full-scale ships.

Numerical analysis of unsteady hydrodynamic performance of pump-jet propulsor in oblique flow

In this study, the SST k−ω turbulence model and the sliding mesh technology based on RANS method have been adopted to simulate the exciting force and hydrodynamic of a pump-jet propulsor in different oblique inflow angle (0°, 10°, 20°, 30°) and different advance ratio (J = 0.95, J = 1.18, J = 1.58).The fully structured grid and full channel model have been adopted to improved computational accuracy. The classical skewed marine propeller E779A with different advance ratio was carried out to verify the accuracy of the numerical simulation method. The grid independence was verified. The time-domain data of pump-jet propulsor exciting force including bearing force and fluctuating pressure in different working conditions was monitored, and then which was converted to frequency domain data by fast Fourier transform (FFT). The variation laws of bearing force and fluctuating pressure in different advance ratio and different oblique flow angle has been presented. The influence of the peak of pulsation pressure in different oblique flow angle and different advance ratio has been presented. The results show that the exciting force increases with the increase of the advance ratio, the closer which is to the rotor domain and the closer to the blades tip, the greater the variation of the pulsating pressure. At the same time, the exciting force decrease with the oblique flow angle increases. And the vertical and transverse forces will change more obviously, which is the main cause of the exciting force. In addition, the pressure distribution and the velocity distribution of rotor blades tip in different oblique flow angles has been investigated.

Study of the unsteady aerodynamic performance of an inter-city train passing through a station in a tunnel

Based on sliding mesh technology, this paper numerically investigates the aerodynamic performance of an 8-car inter-city train passing through a underground station at full speed. The effects of the gap between the car and the platform screen doors on the aerodynamic performance of the train passing through underground stations are analyzed. The curve of pressure variations at survey points on the train and the tunnel surface obtained from numerical simulation agree with those obtained from the moving model test. The asymmetrical internal structure of the station and the abrupt change in cross-section are the main factors contributing to the poor running stability of the inter-city train, with car 1 and car 8 showing the worst running stability. When the gap between the car and the platform screen doors increases from 130 mm to 425 mm, the maximum side force drops by 52.72% when car 1 passes the platform screen doors, and a linear relationship between the maximum side force and the gap between the car and the platform screen doors is observed.

Effect of train length on fluctuating aerodynamic pressure wave in tunnels and method for determining the amplitude of pressure wave on trains

In this study, the aerodynamic performance of full-scale trains of different lengths going through or crossing each other in a tunnel was investigated using sliding mesh technology and a numerical algorithm developed and verified through a full-scale train test. The waveforms of the fluctuating pressure distribution on the train and in the tunnel were compared and analyzed, and the effect of the train length on the flow field in the tunnel examined. The results show that, because of the significant difference in pressure amplitude, long trains cannot be replaced by short trains when simulating trains going through or crossing each other in tunnels. However, some common regular patterns, such as the distribution of the peak values of the time evolution of pressure on the train and in the tunnel, can still be found in both cases. It was found that, for the evaluation of the fatigue effect induced by pressure on the train body, the equivalent load method based on the Paris formula is more secure and reliable than the root-mean-square equivalent load method. It was also discovered that, while analyzing the effect of the fluctuating aerodynamic pressure, it is better from the viewpoint of safety to consider the maximum pressure on the constant-section part of the train body as the representative parameter.

Research on Ventilation Ontology of Wind Power Vehicle

When the car is running in high speed, the energy consumption of the air resistance is responsible for 60%~80% of the total energy consumption. If the energy consumed by windward side is converted into electric energy and stored in the battery, the goal of energy conservation and environmental protection can be achieved. The wind scooper structure to achieve the goal was designed, the software FLUENT is used for numerical simulation of fluid analysis. The best wind scooper structure is determined.The power coefficients of different number of blades are analyzed by using sliding mesh technology of CFD through 2D transient method.

Numerical analysis of the slipstream development around a high-speed train in a double-track tunnel.

Analysis of the slipstream development around the high-speed trains in tunnels would provide references for assessing the transient gust loads on trackside workers and trackside furniture in tunnels. This paper focuses on the computational analysis of the slipstream caused by high-speed trains passing through double-track tunnels with a cross-sectional area of 100 m2. Three-dimensional unsteady compressible Reynolds-averaged Navier-Stokes equations and a realizable k-ε turbulence model were used to describe the airflow characteristics around a high-speed train in the tunnel. The moving boundary problem was treated using the sliding mesh technology. Three cases were simulated in this paper, including two tunnel lengths and two different configurations of the train. The train speed in these three cases was 250 km/h. The accuracy of the numerical method was validated by the experimental data from full-scale tests, and reasonable consistency was obtained. The results show that the flow field around the high-speed trains can be divided into three distinct regions: the region in front of the train nose, the annular region and the wake region. The slipstream development along the two sides of train is not in balance and offsets to the narrow side in the double-track tunnels. Due to the piston effect, the slipstream has a larger peak value in the tunnel than in open air. The tunnel length, train length and length ratio affect the slipstream velocities; in particular, the velocities increase with longer trains. Moreover, the propagation of pressure waves also induces the slipstream fluctuations: substantial velocity fluctuations mainly occur in front of the train, and weaken with the decrease in amplitude of the pressure wave.

Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank

The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing.

Three-dimensional computational fluid dynamics simulation of valve-induced water hammer

In this study, three-dimensional computational fluid dynamics simulation was adopted to evaluate the valve-induced water hammer phenomena in a typical tank-pipeline-valve-tank system. Meanwhile, one-dimensional analysis based on method of characteristics was also used for comparison and reference. As for the computational fluid dynamics model, the water hammer event was successfully simulated by using the sliding mesh technology and considering water compressibility. The key factors affecting simulation results were investigated in detail. It is found that the size of time step has an obvious effect on the attenuation of the wave and there exists a best time step. The obtained simulation results have a good agreement with the experimental data, which shows an unquestionable advantage over the method of characteristics calculation in predicting valve-induced water hammer. In addition, the computational fluid dynamics simulation can also provide a visualization of the pressure and flow evolutions during the transient process.

Effect on measurements of anemometers due to a passing high-speed train

The three-dimensional unsteady incompressible Reynolds-averaged Navier-Stokes equations and k-<TEX>${\varepsilon}$</TEX> double equations turbulent model were used to investigate the effect on the measurements of anemometers due to a passing high-speed train. Sliding mesh technology in Fluent was utilized to treat the moving boundary problem. The high-speed train considered in this paper was with bogies and inter-carriage gaps. Combined with the results of the wind tunnel test in a published paper, the accuracy of the present numerical method was validated to be used for further study. In addition, the difference of slipstream between three-car and eight-car grouping models was analyzed, and a series of numerical simulations were carried out to study the influences of the anemometer heights, the train speeds, the crosswind speeds and the directions of the induced slipstream on the measurements of the anemometers. The results show that the influence factors of the train-induced slipstream are the passing head car and tail car. Using the three-car grouping model to analyze the train-induced flow is reasonable. The maxima of horizontal slipstream velocity tend to reduce as the height of the anemometer increases. With the train speed increasing, the relationship between <TEX>$V_{train}$</TEX> and <TEX>$V_{induced\;slipstream}$</TEX> can be expressed with linear increment. In the absence of natural wind conditions, from the head car arriving to the tail car leaving, the induced wind direction changes about <TEX>$330^{\circ}$</TEX>, while under the crosswind condition the wind direction fluctuates around <TEX>$-90^{\circ}$</TEX>. With the crosswind speed increasing, the peaks of <TEX>$V_X,{\mid}V_{XY}-V_{wind}{\mid}$</TEX> of the head car and that of <TEX>$V_X$</TEX> of the tail car tend to enlarge. Thus, when anemometers are installed along high-speed railways, it is important to study the effect on the measurements of anemometers due to the train-induced slipstream.

Experimental and numerical studies on aerodynamic loads on an overhead bridge due to passage of high-speed train

When a high speed train passes an overhead bridge structure, the aerodynamic forces acting on the structure change abruptly. This train-induced aerodynamic loading should be taken into consideration for the design of overhead structures. This paper reports on the field experiment and numerical simulation conducted in the study of the aerodynamic loads on such a structure. Wavelet transformation was conducted on the pressure measurements to analyse the surface pressure fluctuation characteristics and to identify the pressure distribution in different frequency bands. The numerical simulation adopted sliding mesh technology with the two-equation k–ε turbulence model, and commercial CFD software FLUENT was used. The distribution of pressure and the relationship between the pressure and train speed were analyzed. Results showed that when the train passed under the bridge structure, complex unsteady turbulent flow arose between the train and the structure. This flow should be taken into consideration in the structural design of the overhead bridge structure.