In recent years, there has been an increased interest in the old NACA four-digit series when designing wind turbines or small aircraft. One of the airfoils frequently used for this purpose is the NACA 0018 profile. However, since 1933, for over 70 years, almost no new experimental studies of this profile have been carried out to investigate its performance in the regime of small and medium Reynolds numbers as well as for various turbulence parameters. This paper discusses the effect of the Reynolds number and the turbulence intensity on the lift and drag coefficients of the NACA 0018 airfoil under the low Reynolds number regime. The research was carried out for the range of Reynolds numbers from 50,000 to 200,000 and for the range of turbulence intensity on the airfoil from 0.01% to 0.5%. Moreover, the tests were carried out for the range of angles of attack from 0 to 10 degrees. The uncalibrated γ−Reθ transition turbulence model was used for the analysis. Our research has shown that airfoil performance is largely dependent on the Reynolds number and less on the turbulence intensity. For this range of Reynolds numbers, the characteristic of the lift coefficient is not linear and cannot be analyzed using a single aerodynamic derivative as for large Reynolds numbers. The largest differences in both aerodynamic coefficients are observed for the Reynolds number of 50,000.

https://www.mdpi.com/2227-9717/10/5/1004/pdf?version=1653635714

Numerical Study of the Effect of the Reynolds Number and the Turbulence Intensity on the Performance of the NACA 0018 Airfoil at the Low Reynolds Number Regime

Multiphase pumps are used as an important tool for natural gas hydrate extraction owing to their excellent gas–liquid mixing and transport properties. This paper proposes an adaptive response surface-based integrated optimization design method. A model pump is designed based on the axial flow pump design theory. The model pump is numerically simulated and analyzed to obtain its performance parameters. Then the structural and performance parameters of the pump are parameterized to establish a closed-loop input–output system. Based on this closed-loop system, a sensitivity analysis is performed on the structural parameters of the impeller and guide vane, and the parameters that affect the performance of the gas–liquid hybrid pump the most are derived. The Sparse Grid method was introduced to design the experiment and construct the approximate model. The structural parameters of the impeller and guide vane are used as design variables to optimize the pressure increment and efficiency of the pump. After optimization, the pressure increment of the multiphase pump was increased by 10.78 KPa and the efficiency was increased by 0.89% compared to the original model. Finally, we validate the accuracy of the optimized model with tests.

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A Method for the Integrated Optimal Design of Multiphase Pump Based on the Sparse Grid Model

We investigate dynamics of a single cavitation bubble in vicinity to a horizontal wall throughout expansion and collapse using a sharp-interface level-set method. The numerical scheme is based on a finite-volume formulation with low-dissipation high-order reconstruction schemes. Viscosity and surface tension are taken into account. The simulations are conducted in three-dimensional axi-symmetric space. A wide range of initial bubble wall standoff distances is covered. We focus, however, on the near-wall region where the distance between the bubble and the wall is small. We reproduce three jetting regimes: needle, mixed, and regular jets. The needle jets impose a significant load on the solid wall, exceeding the force induced by the collapse of the pierced torus bubble. For intermediate standoff distances the large delay time between jet impact and torus bubble collapse leads to a significant decrease of the imposed maximum wall-pressure. A liquid film between bubble and wall is observed whenever the bubble is initially detached from the wall. Its thickness increases linearly for very small standoff distances and growths exponentially for intermediate distances leading to a significant increase of wall-normal bubble expansion and bubble asymmetry. For configurations where the torus bubble after jet impact reaches maximum size, the collapse time of the cavitation bubble also is maximal, leading to a plateau in the overall prolongation of the cycle time of the bubble. Once the initial bubble is attached to the solid wall, a significant drop of all macroscopic time and length scales towards a hemispherical evolution is observed.

Investigation of cavitation bubble dynamics near a solid wall by high-resolution numerical simulation

• Analysis of velocity magnitude distribution of hydraulic oil inside the filter. • Visualization of anthracite particle trajectories for different flow regimes. • Influence of stream structure on particles’ motion. • Determination of cleaning efficiency of hydraulic oil from different kind of particles. Dynamic filtration using a rotating perforated cylinder can provide excellent cleaning efficiency of liquids from solid admixtures that is closely related to the fluid flow characteristics. The experimental tests and numerical simulation were carried out to study the cleaning efficiency of hydraulic oil AMG-10 from anthracite particles and other kinds of solid admixtures using a rotating perforated cylinder. The hydraulic oil flow structure inside the filter was analysed, and hydraulic oil velocity distributions near the rotating cylinder were obtained. Results show that primary and secondary vortex flows influence the filter cleaning efficiency. A vortex flow near-surface of the rotating cylinder has caused a stagnant zone in which particles accumulate. The comparison of the cleaning efficiency results for 2D and 3D modelling using the SST k- ω turbulence model, previous 3D modelling using the RSM turbulence model and experimental data is presented. The expediency of applying the 2D problem statement using the SST k- ω turbulence model for flow regimes characterized by the dimensionless velocity ratio criterion m > 64 is shown. Based on the numerical model, we determined the cleaning efficiency of some metallic and non-metallic impurities, which can be present in the hydraulic system of the mining machine. 

Study of hydraulic oil filtration process from solid admixtures using rotating perforated cylinder

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

Numerical Study of Effect of Sawtooth Riblets on Low-Reynolds-Number Airfoil Flow Characteristic and Aerodynamic Performance

This paper aims to reveal the influence of a rigid wall with a gas entrapping hole on the characteristics of the dynamic behavior of a laser-induced bubble collapse. A high-speed camera system was used to record the oscillation process of the laser-induced bubble on a rigid wall with a gas entrapping hole. When a bubble is generated by a laser above the wall with a gas entrapping hole, the entire bubble collapse stays away from the wall or splits into two bubbles because of a radial jet induced by bubble contraction. These two distinctive collapse modes are dependent on the distance between the wall and the bubble. The focus of this study is the quantitative analysis of the jet formation, bubble migration, and oscillation period, and compared with the behavior of the bubble near a rigid wall. The results show that unlike the generation of the bubble near a rigid wall, a rigid wall with a gas entrapping hole affects the morphology of the jet and changes the direction of migration of the bubble, and decreases the oscillation period. Thus, the rigid wall with a gas entrapping hole could be effective for reducing cavitation erosion on the wall surface, which is supported by our experiment results.

Cavitation bubble collapse in a vicinity of a rigid wall with a gas entrapping hole

The turbulence dissipation will cause the increment of energy loss in the multiphase pump and deteriorate the pump performance. In order to research the turbulence dissipation rate distribution characteristics in the pressurized unit of the multiphase pump, the spiral axial flow type multiphase pump is researched numerically in the present study. This research is focused on the turbulence dissipation rate distribution characteristics in the directions of inlet to outlet, hub to rim, and in the circumferential direction of the rotating impeller blades. Numerical simulation based on the RANS (Reynolds averaged Navier–Stokes equations) and the k-ω SST (Shear Stress Transport) turbulence model has been carried out. The numerical method is verified by comparing the numerical results with the experimental data. Results show that the regions of the large turbulence dissipation rate are mainly at the inlet and outlet of the rotating impeller and static impeller, while it is almost zero from the inlet to the middle of outlet in the suction surface and pressure surface of the first-stage rotating impeller blades. The turbulence dissipation rate is increased gradually from the hub to the rim of the inlet section of the first-stage rotating impeller, while it is decreased firstly and then increased on the middle and outlet sections. The turbulence dissipation rate distributes unevenly in the circumferential direction on the outlet section. The maximum value of the turbulence dissipation rate occurs at 0.9 times of the rated flow rate, while the minimum value at 1.5 times of the rated flow rate. Four turning points in the turbulence dissipation rate distribution that are the same as the number of impeller blades occur at 0.5 times the blade height at 0.9 times the rated flow rate condition. The turbulence dissipation rate distribution characteristics in the pressurized unit of the multiphase pump have been studied carefully in this paper, and the research results have an important significance for improving the performance of the multiphase pump theoretically.

Effect of Flow Rate on Turbulence Dissipation Rate Distribution in a Multiphase Pump

The pressure pulsation characteristics of the blade’s surface have an impact on the lifespan of the multiphase pump impeller. To explore the influence of the gap matching relation on the pressure pulsation characteristics at blade’s surface of the pump, the axial clearance coefficient (ACC) between the impeller and diffuser and the relative tip clearance of the impeller were defined. By combining of numerical simulation and experiment, the pressure pulsation characteristics at blade’s surface of the single pressurization unit of the pump were studied under different gap matching relations. The results show that the size of tip leakage flow and the strength of rotor-stator interaction caused by different relative tip clearances have a decisive influence on the pressure pulsation characteristics in the impeller. With the increase of tip clearance, the internal flow field distribution law was greatly changed, which had a great impact on the pressure pulsation peak-to-peak value and dominant frequency amplitude of the suction surface. In addition, the pressure pulsation characteristics of the diffuser were mainly affected by the strength of rotor-stator interaction. When the strength of rotor-stator interaction weakened, the amplitude of the dominant frequency pulsation was decreased by degrees. As the ACC increased, the variation of the pressure pulsation peak-to-peak value and the dominant frequency amplitude coefficient were gradually slowed down in the diffuser, and the dominant frequency amplitude of the monitoring points were most sensitive to change in the ACC. The research results can provide a theoretical reference for the optimal design of the multiphase pump blades.

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Effect of the Gap Matching Relation on the Pressure Pulsation Characteristics at Blade’s Surface of the Multiphase Pump

The flow is extremely complex within the main flow passage during the operation of the multiphase pump, which results in constant changes in vortex structure, and interferes with the orderly flow of flow field present, thus, reducing the pump performance. Numerical calculations, supplemented by experimental verification, are used as the main method for investigating influencing factors that affects the vortex structure in impeller passage of pump, and vortex structure present on isosurfaces is selected based on Q criterion to study the evolution of vortex structures under different working conditions. Results indicate that the flow rate affects vortex structure generation on the suction side and trailing edge of the blade. With an increase in flow rate, the vortex becomes complete from fine broken structures. Speed has a greater influence on vortex structures in inlet and outlet areas: the higher the speed, the larger the vortex structure attached to the impeller leading edge. The vortex structure is separated from blade surface at 2/3 of the impeller under gas-liquid two-phase working condition, with an increase in inlet gas void fraction (IGVF) the vortex structrue expands to the center of impeller passage. Therefore, exploring evolutions of the vortex structure can provide a certain theoretical basis for improving the stability of multiphase pump internal flow.

Influence of operating parameters on the vortex structure in the main flow passage of the Helico-Axial multiphase pump

The axial flow screw-type oil-gas multiphase pump is mainly applied to oil and gas transport in the deep sea. In the process of transporting the multiphase medium, the gas volume fraction (GVF) on the gas phase changes from time-to-time, resulting in the performance of the oil-gas multiphase pump being greatly influenced by the gas phase. This paper presents a detailed analysis of the gas-phase distribution law and the vortex distribution in the flow passages within the oil-gas multiphase pump by means of numerical calculations, supplemented by experimental verification. The results show that the gas phase is mainly concentrated in the diffuser at different GVFs, and the gas phase gathering in the diffuser becomes more significant with the increase in the GVF. The gas-phase volume fraction increases gradually from rim to hub, that is, the gas-phase gathering degree increases. The maximum gas-phase volume distribution area is mainly concentrated in the area near the hub of the diffuser inlet and the middle blade height area at the outlet of the diffuser. The flow in the impeller is relatively stable under the different GVFs, while there is a large vortex near the inlet of the diffuser near the hub, and there is a backflow phenomenon between the outlet of the diffuser and the tip clearance of the impeller. The volume fraction of the gas phase near the rim fluctuates more than that near the hub because the gas phase is squeezed by the liquid phase more violently. The research results can provide theoretical guidance for the optimal design of oil-gas multiphase pump blades.

https://www.mdpi.com/2227-9717/9/5/760/pdf

Haigang Wen

Papers

Cavitation bubble collapse in a vicinity of a rigid wall with a gas entrapping hole

Effect of Flow Rate on Turbulence Dissipation Rate Distribution in a Multiphase Pump

Effect of the Gap Matching Relation on the Pressure Pulsation Characteristics at Blade’s Surface of the Multiphase Pump

Influence of operating parameters on the vortex structure in the main flow passage of the Helico-Axial multiphase pump

Effect of Gas Volume Fraction on the Gas-Phase Distribution in the Passage and Blade Surface of the Axial Flow Screw-Type Oil-Gas Multiphase Pump