Numerical investigation on aerodynamic performance of a 2-D inclined hovering wing in asymmetric strokes

A numerical study on the effects of the asymmetry in flapping motions on the aerodynamic performance of a 2-D inclined hovering wing is carried out through the solution of the two-dimensional unsteady Navier-Stokes equations. Asymmetric flapping motions with different flapping durations as well as different angles of attack during downstroke and upstroke are considered, correspondingly the aerodynamic forces, the energy consumption and the flow structures of the wing are examined for the purpose. The results show that the asymmetry either in flapping duration or in angle of attack can effectively alter the aerodynamic characteristics of the wing. It is found that much larger vertical force is generated in the downstroke with smaller duration. As for the angle of attack, an increase in the angle during the downstroke gives rise to a larger vertical force and lifting efficiency while for the upstroke the increase in the angle results in a larger thrust force. Furthermore, a combined model with asymmetry in both flapping duration and angles of attack is also studied and the results show that a combined flapping motion with a faster downstroke and a higher downstroke angle of attack can produce much larger vertical force than a motion with asymmetry simply in stroke duration or angle of attack. The present results provide physical insight into a better understanding of the asymmetric stroke mechanisms in the flapping motion of insects.     

An investigation on the effects of irregular airfoils on the aerodynamic performance of small axial flow fans

In this paper, the effects of airfoils on the aerodynamic performance of small axial flow fans were investigated to reduce airflow turbulence on the blade surface and to improve the aerodynamic performance of small axial flow fans. Irregular airfoils where several convex grooves are bound in the blade pressure surface of the fans have two kinds. The wave-shaped edge is bound to the blade trailing edge of the fans and is designed from the smooth airfoil of the fans. The filtered N-S equations with the finite volume method and the standard k-? turbulence model were adopted to carry out the steady simulation calculation. The large eddy simulation and the FH-W noise models were adopted to carry out the unsteady numerical calculation and aerodynamic noise prediction. The results of simulation calculation are in good agreement with the tests, which proves that the numerical calculation method is feasible. The spectrum characteristics of aerodynamic noise of the smooth airfoil and the two kinds of irregular airfoils were analyzed. Although the fans of the three airfoils are regarded as noise sources, the vortex distribution features in the unsteady flow field are also described. Noise reduction mechanisms of the irregular designs of the airfoils were also discussed. The results of this research may provide proof of the parameter optimization and the structural design of small axial flow fans with low noise.     

An investigation into the effects of installation on the performance of insertion flowmeters

An electromagnetic and a turbine insertion flowmeter were tested in three different flow conditions inside a 0.590-m bore pipe inserted in the National Engineering Laboratory (NEL) large water flow measurement facility. The results were compared with velocity measurements obtained from a laser Doppler velocimeter (LDV). The advantage of using such a reference measurement is that LDV is non-intrusive and does not affect the velocity profile itself.Of the meters tested, one was supplied with a whole meter calibration factor and the other was supplied with a calibration factor for the D/2 position.For both meters, application of the respective manufacturer's blockage correction improved the velocity measurements, reducing the differences between the LDV and corrected insertion meter measurements and the difference between the integrated insertion meter measurements and the gravimetric measurements.Swirling and skew flow profiles were generated by the installation of the NEL designed swirl generator and flow disturber, respectively. Neither of these disturbed profiles affected the performance of either of the meters in terms of accuracy of measurement compared with the LDV readings. The profiles themselves, however, changed the velocities at the D/8 and 7D/8 points, making single point estimates of the mean velocity inappropriate. A complete 13-point traverse, integrated using the method of cubics as described in BS 1042 [1] (Section 2.3: Measurement of fluid flow in closed conduits, 1992), gave acceptable estimates of mean velocity in both swirling and skew flow for both probes.     

Numerical investigation of the aerodynamic performance affected by spiral inlet and outlet in a positive displacement blower

The flow in the positive displacement blower is very complex.The existing two-dimensional numerical simulation cannot provide the detailed flow information,especially flow characteristics along the axial direction,which is unfavorable to improve the performance of positive displacement blower.To investigate the effects of spiral inlet and outlet on the aerodynamic performance of positive displacement blower,three-dimensional unsteady flow characteristics in a three-lobe positive displacement blower with and without the spiral inlet and outlet are simulated by solving Navier-Stokes equations coupled with RNG k-ε turbulent model.In the numerical simulation,the dynamic mesh technique and overset mesh updating method are used.The computational results are compared with the experimental measurements on the variation of flow rate with the outlet pressure to verify the validity of the numerical method presented.The results show that the mass flow rate with the change of pressure is slightly affected by the application of spiral inlet and outlet,but the internal flow state is largely affected.In the exhaust region,the fluctuations of pressure,velocity and temperature as well as the average values of velocity are significantly reduced.This illustrates that the spiral outlet can effectively suppress the fluctuations of pressure,thus reducing reflux shock and energy dissipation.In the intake area,the average value of pressure,velocity and temperature are slightly declined,but the fluctuations of them are significantly reduced,indicating that the spiral inlet plays the role in making the flow more stable.The numerical results obtained reveal the three-dimensional flow characteristics of the positive displacement blower with spiral inlet and outlet,and provide useful reference to improve performance and empirical correction in the noise-reduction design of the positive displacement blowers.     

Numerical investigation of the combined effects of slip and texture on tribological performance of bearing

Slider bearings are used in many applications. An increase in the load support may allow for saving of energy. In this work, in order to enhance the load support and decrease the friction force, a combined textured surface bearing using boundary slip is discussed. A modified Reynolds equation with slip is adopted. With the main goal of evaluating the effects of slip and texture, a parametric analysis is performed. For the given operating conditions, texturing features as well as slip pattern are analysed in detail. The numerical analysis is undertaken under the condition of different gap ratio values and the slip-textured area. The results show that combined techniques of slip and texture have a significant effect on the improvement of the tribological performance of bearing, that is, a high load support but low friction force. The gap ratio of the bearing is shown to have a significant effect on the lubrication behaviour. It is found that even with a smallest gap ratio (parallel gap), a high load support can be produced. However, it is also shown that the gap ratio appears to contribute to the generated friction force and the volume flow rate more than the boundary slip. Further analysis indicates that the optimum slip-text zones for certain gap ratio are highlighted. These findings may provide references for designing hydrodynamic-textured slider bearing considering boundary slip.     

Numerical investigation of the aerodynamic benefits of wing-wing interactions in a dragonfly-like flapping wing

Journal of Mechanical Science and Technology - Numerical simulations are performed to investigate the aerodynamic benefits of wing-wing interactions on a dragonfly-like flapping wing while...     

轴流排汽缸气动性能数值研究与改型优化

为减小轴流排汽缸内部损失,提高排汽缸的气动性能,以杭州汽轮机股份有限公司新开发的轴流排汽缸为研究对象,考虑末级动叶出口径向速度不均匀性对排汽缸的影响,联合末两级整圈低压级组和排缸进行了数值计算。分析了轴流排汽缸内部流动特点,并根据流场对其进行了改型优化。计算结果表明,轴流排汽缸与低压级叶片之间存在相互作用,排汽缸后部出现两个旋流方向相反的涡,并随着气流向下游扩展;通过对轴流排汽缸结构改型,静压恢复系数提高了40.7%,总压损失系数减少了31.4%。通过优化通油管道的截面形状,使静压恢复系数提高了0.13%,总压损失系数减少了2.2%,改善了排汽缸内部流动,显著提高了轴流排汽缸的气动性能。     

Numerical simulation investigation on centrifugal compressor performance of turbocharger

In this paper, the mathematical model of the flow filed in centrifugal compressor of turbocharger was studied. Based on the theory of computational fluid dynamics (CFD), performance curves and parameter distributions of the compressor were obtained from the 3-D numerical simulation by using CFX. Meanwhile, the influences of grid number and distribution on compressor performance were investigated, and numerical calculation method was analyzed and validated, through combining with test data. The results obtained show the increase of the grid number has little influence on compressor performance while the grid number of single-passage is above 300,000. The results also show that the numerical calculation mass flow rate of compressor choke situation has a good consistent with test results, and the maximum difference of the diffuser exit pressure between simulation and experiment decrease to 3.5% with the assumption of 6 kPa additional total pressure loss at compressor inlet. The numerical simulation method in this paper can be used to predict compressor performance, and the difference of total pressure ratio between calculation and test is less than 7%, and the total-to-total efficiency also have a good consistent with test.     

Numerical investigation of prop-rotor and tail-wing aerodynamic interference for a tilt-rotor UAV configuration

We investigated the aerodynamic interference between the prop-rotor and the tail-wing, and its effect on the lateral stability of a tiltrotor UAV configuration during transient flight modes. Unsteady flow calculations were conducted using an unstructured mesh flow solver, coupled with an overset mesh technique. It was shown that as the sideslip angle increases, the interference between the prop-rotor and the tail-wing is enlarged. This interference magnifies the fluctuating amplitudes of the yaw and roll moments. The increased sideslip angle also induces an imbalance of the thrust force generated by the right and left prop-rotors, affecting the lateral stability. The effect of the nacelle tilt angle on the lateral stability was also investigated for the nacelle tilt angles of 80° and 90°. When the nacelle tilt angle was 90°, the interference between the prop-rotor and the tail-wing was reduced.     

高密实度H型风力机气动性能的数值分析

针对高密实度H型风力机在不同叶片数下的气动特性及风场布置等问题,将采用k-ω SST湍流模型进行数值模拟的方法应用到对高密实度H型风力机的研究中.开展了对风力机做功特性的分析,建立了流场与风力机功率之间的关系,提出了单个风力机设计选择叶片数时应综合考虑风力机效率和轴承安全这两个因素;在尾流场分析的基础上对不同叶片数的风力机在风场前后串列布置进行了评价.研究结果表明:叶片在上游(θ=90°附近)的气动性能决定整个风力机的性能;由正常工况点下的流场图显示,叶片数的增加导致流场复杂,以及叶片的内外压差逐渐减小,从而使得功率下降;尾流场流向速度恢复至来流速度的距离随叶片数的增加而减小了28.1％,这对风力机的前后串列布置提供了依据.     

几何参数对层板冷却性能影响的数值模拟研究

对12种不同几何结构尺寸的层板进行了系统的耦合传热数值计算，研究了通道高度、扰流柱直径、孔间距与孔径比等几何参数对层板冷却效果的影响。结果表明，在相同的冷气密流条件下，减小通道高度可显著提高冷却效果，孔间距对冷却效果影响不大。这里的结果可为层板的优化设计提供参考。     

Flapping flight in the wake of a leading insect

Journal of Mechanical Science and Technology - This study explores the wake effects of an upstream leading insect on the flight performance of a following one. The potential-flow based aerodynamic...     

Experimental study on the aerodynamic characteristics of a fighter-type aircraft model in close formation flight

This paper presents on the aerodynamic characteristics of a fighter-type aircraft model in formation flight. The effect of longitudinal and lateral distance changes on the aerodynamic characteristics of a trailing aircraft is investigated by experiment. The wind-tunnel testing is conducted with two-5% scaled small sized fighter-type aircraft models. Aerodynamic forces and moments of a trailing aircraft model are measured at 49 locations on the cross-section behind a leading aircraft model. It is observed that the leading aircraft affects the incremental lift and drag of the trailing aircraft significantly. It is also proved that the aerodynamic characteristics of the trailing aircraft can be improved if positioned at proper locations behind the leading aircraft while in formation flight     

Numerical study on the effect of the tip clearance of a 100 HP axial fan on the aerodynamic performance and unsteady stall characteristics

In this study, a numerical analysis was conducted to investigate the effect of the tip clearance on the aerodynamic performance, internal flow characteristics, and stall region characteristics of an axial fan. Three-dimensional steady and unsteady Reynolds-averaged Navier-Stokes (RANS) calculations were conducted with a shear stress transport (SST) turbulence model. Tip clearance ratios of 0, 0.01, and 0.02 were applied to the impeller. As the tip clearance ratio increased, the aerodynamic performance of the axial fan decreased at both the design and the off-design conditions. The correlation between the tip leakage vortex (TLV) and the flow angle of the velocity triangle was presented for the difference in the tip clearance and flow rate. As the flow rate increased, the differences in the aerodynamic performance induced by the tip clearance ratio decreased. As the tip clearance ratio increased, the size of the TLV increased and gradually moved in the circumferential direction to interfere with the main flow at the low flow rate. Meanwhile, the size of the TLV was similar and gradually moved in the axial direction even if the tip clearance ratio increased at the high flow rate. The pressure fluctuations were observed by the fast Fourier transformation (FFT) analysis to compare and analyze internal flow characteristics at the stall region and design point. The static pressure was converted to the appropriate magnitude. The locations of the highest magnitude were shown to be different at the stall region and the design point, respectively.

     

Experimental investigation into the performance of an aerostatic industrial thrust bearing

Load and flow versus air-gap curves and stability maps have been obtained for an industrial externally-pressurized air-lubricated thrust bearing with a central restrictor. Supply pressure and pocket depth were treated as parameters. Bearing stiffness was estimated by using a computer to fit a polynomial type curve to the test data. The results show that whereas the effect of inserting a pocket into the bearing improves its load capacity, there is no significant improvement in film stiffness at design conditions. The suggestion is made that the diameter of the orifice in the restrictor should be increased. As pocket depth is increased beyond the design value the bearing becomes prone to pneumatic hammer.     

Numerical investigation of the effects of geometric parameters on transverse motion with slanted-groove micro-mixers

Journal of Mechanical Science and Technology - We investigated hydrodynamic phenomena inside several passive microfluidic mixers using a Lattice Boltzmann method (LBM) based on particle mesoscopic...     

涡轮基组合循环发动机/飞行器一体化数值模拟

基于两级入轨高超声速飞行器方案,确定了涡轮基组合循环发动机设计点各参数,设计了飞行器进排气系统,编写了推进系统安装损失计算程序,采用数值模拟技术研究了沿飞行轨迹的涡轮基组合循环发动机/飞行器一体化性能,计算结果显示,跨声速阶段的安装损失最大,溢流阻力在跨声速区域具有主要的影响,在工作模式转换结束后,安装损失会有一定的增长,这是因为喷管喉部气流总压降低,致使流经发动机的气流流量减少,造成溢流阻力和旁路放气阻力的增加所致.     

Numerical predictions of roughness effects on the performance degradation of an axial-turbine stage

This paper describes a numerical investigation on the performance deteriorations of a low speed, single-stage axial turbine due to use of rough blades. Numerical calculations have been carried out with a commercial CFD code, CFX-Tascflow, by using a modified wall function to implement rough surfaces on the stator vane and rotor blade. To assess the stage performance variations corresponding to 5 equivalent sand-grain roughness heights from a transitionally rough regime to a fully rough regime, stage work coefficient and total to static efficiency were chosen. Numerical results showed that both work coefficient and stage efficiency reduced as roughness height increased. Higher surface roughness induced higher blade loading both on the stator and rotor which in turn resulted in higher deviation angles and corresponding work coefficient reductions. Although, deviation angle changes were small, a simple sensitivity analysis suggested that their contributions on work coefficient reductions were substantial. Higher profile loss coefficients were predicted by higher roughness heights, especially on the suction surface of the stator and rotor. Furthermore sensitivity analysis similar to the above, suggested that additional profile loss generations due to roughness were accountable for efficiency reductions.     

Numerical investigation on vibration characteristics of a micro-speaker diaphragm considering thermoforming effects

Micro-speaker diaphragms play an important role in generating desired sound responses, and are designed to have thin membrane shapes for flexibility in the axial direction. The micro-speaker diaphragms are formed from thin polymer film through the thermoforming process, in which local thickness reductions occur due to strain localization. This thickness reduction results in a change in vibration characteristics of the diaphragm and different sound responses from that of the original design. In this study, the effect of this thickness change in the diaphragm on its vibration characteristics is numerically investigated by coupling thermoforming simulation, structural analysis and modal analysis. Thus, the thickness change in the diaphragm is calculated from the thermoforming simulation, and reflected in the further structural and modal analyses in order to estimate the relevant stiffness and vibration modes. Comparing these simulation results with those from a diaphragm with the uniform thickness, it is found that a local thickness reduction results in the stiffness reduction and the relevant change in the natural frequencies and the corresponding vibration modes.