CFD investigation on the flatback airfoil effect of 10 MW wind turbine blade

The flatback airfoil effect on the inboard region of a large wind turbine blade was investigated by numerical analysis. Complicated flow phenomena in wind turbine blade with flatback and non-flatback airfoil were captured by Reynolds-averaged Navier–Stokes flow simulation with shear stress transport turbulence model. Although both airfoil blades were designed using blade element momentum theory to produce identical shaft power, results of three-dimensional computational fluid dynamics (CFD) flow analysis indicated that at a specific location of the root area, the flatback airfoil improved the inboard force by approximately 6 % compared with the non-flatback airfoil. We were also able to confirm that by using the flatback airfoil, the overall shaft power throughout the blade increased by 1 %, thereby restraining the bending moment exerted by the thrust force on the hub by 0.5 %. Moreover, numerical analysis results indicated that the flatback airfoil blade reduced the size of the secondary vortex around the blade root area and its progress in the secondary direction in comparison with the non-flatback airfoil blade. The shape of the flatback airfoil on the trailing edge weakened the adverse pressure gradient migrating from the lower to the upper surface. Regardless of the flatback airfoils, the tip vortex core of the outboard region formed on the suction surface leading edge and strongly rolled up by the pressure surface boundary layers due to the large pressure difference between the suction and pressure surfaces in the blade tip region. This remarkable strong tip vortex developed downstream and raked up the boundary layer of the blade trailing edge with low energy.     

Power extraction performance of two semi-active flapping airfoils at biplane configuration

The power extraction performance of two semi-active flapping airfoils at biplane configuration is analyzed in this work through numerical approach. Two NACA0015 airfoils, which are regarded as an energy extraction turbine, are arranged in a biplane configuration to perform forced counter pitching motion and are subsequently induced in a plunging motion. A numerical code based on finite volume method to solve the Navier-Stokes equations coupled with finite center difference method to solve the passive plunging motion governing equation is developed to simulate the interaction between the two semi-active flapping airfoils and fluid. Results show that the semi-active flapping airfoil cannot absorb more power from the fluid with biplane arrangement, but this arrangement is beneficial for the power extraction efficiency of the airfoil. Analysis of the fluid field of the airfoil reveals that the wing-wing interaction can promote the vortex evolution and reduce the vortex magnitude of the suction side of the biplane airfoils with appropriate initial distance (h_s = 2.5d). As a result, the maximum plunging of the biplane airfoils is smaller than that of the single airfoil. The smaller maximum plunging displacement contributes to the increase in power extraction efficiency.

     

Unsteady flow field numerical calculations of a ship’s rotating Weis-Fogh-type propulsion mechanism with the advanced vortex method

The Weis-Fogh mechanism, found in the hovering flight of a small bee, is a unique and efficient lift generation. In this study, we proposed a rotating type propulsion model that applies the principle of the Weis-Fogh mechanism and calculated the unsteady flow field of the propulsion model with the advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels, and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. We investigated the thrust and drag coefficients, pressure field, vorticity field, velocity vector field, and average propulsive efficiency of the propulsion model by changing the rotating angle velocity. The force acting on the wing depended heavily on the directions of the thrust and drag and the thrust and drag coefficients largely fluctuated with the change in the rotating angles. The average thrust increased as the rotating angle velocity increased. The maximum propulsive efficiency was 27.9% at a calculated angle velocity. The flow field of this rotating type propulsion mechanism is unsteady and very complex because the wing rotates and moves unsteadily in the channel. However, using the advanced vortex method, it could be calculated accurately.     

Separation control mechanism of airfoil using synthetic jet

Numerical simulation of separation control using a synthetic jet was performed on NACA23012 airfoil. The computed results showed that stall characteristics and control surface performance could be improved substantially by resizing the separation vortices. It was observed that actual flow control mechanism was fundamentally different depending on the range of synthetic jet frequency. For low frequency range, small vortices due to synthetic jet penetrated to the large leading edge separation vortex flow, and as a result, the size of the leading edge separation vortex remarkably decreased. For high frequency range, however, the small vortex did not grow enough to penetrate into the large separation vortex, but the synthetic jet changed airfoil circulation directly. The synthetic jet conditions for effective lift increase are as follows: the non-dimensional frequency of the synthetic jet is 1; the location of the synthetic jet slot is the same as the separation point; and the jet velocity is large enough to perturb the separated flow. By exploiting these conditions, it was observed that the combination of the synthetic jet with a simple high lift device could be as good as a conventional fowler flap system.     

Simulation of free surface flow over the streamlined weirs

The streamlined weirs are a special type of weirs, designed on the basis of airfoil theory. Because of their particular design, they have some merits compared to the other types of weirs, such as; high discharge coefficient, more stability of overflow and less fluctuations of water free surface. In the present study, a numerical simulation performed using an open source software namely, OpenFOAM, to give details about the flow structure over, up- and downstream of these weirs. Also, an experimentation setup was devised to evaluate the numerical results and determine the best numerical model. Analyzing the results of different turbulence models including; standard k-ε, realizable k-ε, RNG k-ε, k-ω SST, and Reynolds stress LRR, indicated that all the aforementioned models accurately estimate the flow field and hydraulic parameters. However, the k-ω SST model gives more accurate results, very close to the experimental data especially for the Reynolds stresses. Accordingly, the k-ω SST turbulence model was chosen as the best turbulence model for analyzing the flow over the streamlined weirs. Numerical results for different relative eccentricities show that, by increasing the relative eccentricity, the flow velocity over the crest of the weirs increases and accordingly the pressure in such section decreases. For a constant flow discharge upstream of different types of the streamlined weirs, the lowest bed pressure and the most probable potential of cavitation belongs to a circular-crested weir (a streamlined weir with a relative eccentricity of unity). Furthermore, the greatest bed shear stresses and the compressive forces occur downstream of the circular-crested weir. Thus, downstream of a circular-crested weir is responsible for larger potential of bed erosion. This is partly due to the formation of shock waves, reduction of the flow depth, and enhancement of the flow velocity downstream of a circular-crested weir. Moreover, the lowest bed shear stresses were observed upstream of the circular-crested weir. Therefore, upstream of a circular-crested weir shows the greatest potential of sedimentation. Finally, applying the streamlined weirs with an appropriate curvature, diminishes the unfavorable flow conditions, as observed for the circular-crested weir, being the safer and economic hydraulic structures.     

两栖混合驱动机器人单翼动力学分析与试验研究

为了实现足翼混合驱动两栖机器人水中高效、稳定浮游,开展了仿生水翼水动力分析与试验研究.首先针对非常规翼型的三维水翼进行结构简化,采用数值方法建立了单翼拍动水动力学模型,基于该模型分析了拍动周期、拍动幅值和拍动相位差等参数对水翼推进性能的影响;然后采用切片法,从翼尖轨迹特征、翼面压力分布以及尾涡脱落等角度,阐述了水翼水动...     

Feedback mechanism of low-speed edgetones

A feedback mechanism of low-speed edgetones is analyzed by using the jet edge interaction model in which reaction of the edge is regarded as an array of dipoles. From the jet-edge interation model the surface pressure of the edge and the upstream wave are estimated by assuming the downstream disturbance as a sinuously oscillating flow with a constant convection speed. The surface pressure distribution on the edge is found to increase from zero at the tip to a peak value around a quarter wavelength downstream, which may be regarded as an effective source point of the upstream-propagating sound wave. From the condition that the two wave trains should be phase-locked at the nozzle lip, the phase factorp is found to be in the range, −1/2<p<0, in the phase criterion of the form,h/A+h/λ=n+p, where h is the stand-off distance.A and λ the wavelengths of downstream and upstream respectively, andn the stage number. From the existing experimental data the phase factor is estimated on the basis that the convection wavelength is dependent only upon the mean jet velocity and the frequency, but not upon the stage number. The experimental values are in the same range of the theoretical estimation indicating that the phase factor is not a universal constant but varies in the range, −1/2<p<0, depending on the nozzle-edge configuration and the flow condition.     

Effects of advance ratio on elytra-hindwing interaction in forward flying Coleopteran beetle

In the present study, numerical simulations are performed to explore the significance of elytron-hindwing interaction in the forward flying Coleopteran beetle. The study investigates the effects of hindwing stroke amplitude (A/c) and advance ratio (J), (which is defined as the ratio of the incoming air velocity to the wing flapping velocity), on the aerodynamic forces. The wing kinematics of a Coleopteran beetle is constructed by using a combination of translation and rotation motion. The elytron is modeled by using a cambered airfoil that mimics the real geometry of the beetle wing, and the hindwing is modeled by using an elliptical profile. The results indicate that the beetle cruises with a constant velocity at approximately J = 0.3 in the tandem wing arrangement. It is observed that the angle of the net force vector relative to the stroke plane tilts systematically according to the flying speed. The influence of vortex structures on the beetle aerodynamic forces is analyzed. The elytron-hindwing interaction is found to be beneficial to the vertical force generation of hindwing as well as for the elytron when J > 0.0. The vortices interaction is observed during the downstroke period, and the leading edge vortex (LEV) of the elytron is captured by LEV of the hindwing that enhances the total vertical force. During the upstroke translation phase, the combined trailing edge vortex of elytron interacts/merges with the LEV of the hindwing and increases the horizontal force.     

Investigation of different aspects of laminar horseshoe vortex system using PIV

Juncture flow is a classical fluid mechanics problem having wide applications in both aero and hydro dynamics. The flow separates upstream of the obstacle due to the adverse pressure gradient generated by it, with the formation of the vortical structure called “horseshoe vortex.” The current study is carried out for an elliptical leading edge obstacle placed on a flat plate to investigate the horseshoe vortex for a range of Reynolds number (Re_W) based on maximum width (W) for which the incoming boundary layer is laminar. Four different types of horseshoe vortex systems were found: the steady, amalgamation, transition and breakaway. The transition vortex system is one after which the vortex system changes from amalgamation to breakaway. In this phase the vortex system alternatively undergoes both amalgamation and breakaway vortex cycles. The effect of variation in the chord wise shape of the obstacle is investigated. The quantitative measurements of PIV show that the vortex system does not undergo any significant change for different chord lengths of the model with the fixed aspect ratio and maximum width. The most upstream saddle point is also studied for steady horseshoe vortex region and found that it is the “saddle of attachment” where flow attaches to the plate surface instead of separating from it.     

波浪结构对圆柱-翼型湍流干涉噪声的影响研究

为探究波浪翼型的降噪效果,采用大涡模拟（LES）和边界元法（BEM）相结合的混合方法对3种不同波浪翼型进行模拟,并通过试验验证了仿真模型的可行性,进一步分析了3种不同波浪翼型（表面波浪Wavy airfoil-A、前缘波浪Wavy airfoil-B和前缘+表面波浪Wavy airfoil-C）对圆柱-翼型湍流干涉噪声的影响。研究结果表明：3种模型都能在一定程度上降低翼型湍流干涉噪声,其中Wavy airfoil-C模型降噪效果最好,降噪频率范围最广,其垂直流向方向总声压级降噪量可达6.7 dB;Wavy airfoil-C模型不仅能有效地降低翼型表面压力脉动、各截面上的湍流强度、升阻力系数波动、功率谱密度,还能利用其前缘波浪结构有效地减少前缘主声源区域的面积,且能利用其表面波浪结构的导流作用降低翼型后缘的声源振动幅值。     

Measurement of unsteady transition on a pitching airfoil using dynamic pressure sensors

Unsteady boundary layer transition on a pitching OA209 airfoil in a wind tunnel was detected by using pressure fluctuation measurement at different oscillation frequency. Thirty Kulite dynamic pressure transducers flush-mounted on the airfoil surface recorded pressure signatures, and root mean square of pressure signatures were calculated. Results indicated that the criterion of transition for static airfoil defined as the peak of root mean square of pressure fluctuation was still suitable for detection of transition on a pitching airfoil. Fixed transition experiment for pitching airfoil was performed to validate the conclusion. Effect of oscillation frequency on transition was investigated. For small reduced frequency, the hysteresis loop is larger near leading edge. With increasing in the oscillation frequencies, the transition was promoted and relaminarization was enhanced.

     

合成射流对钝尾缘翼型气动特性的影响

研究吸力面存在合成射流的情况下,钝尾缘翼型TR-4000-2000流场结构的变化及其升阻力系数等气动特性参数的变化趋势。在相同射流入口速度条件下,采用计算流体力学软件Fluent对相同来流速度不同攻角情况下翼型流场进行非定常数值模拟计算,分析射流前后翼型升阻力系数变化及翼型表面压力的波动状况;在此基础上,对不同射流频率和不同射流速度情况下翼型流场进行模拟计算,寻求最佳射流参数。结果表明,由于射流及尾缘涡的相互作用导致翼型的升阻力特性不断变化,钝尾缘翼型吸力面合成射流有明显的增升减阻效果,在15°攻角时尤为明显,升力系数提高约40%,阻力系数减小约25%。在量纲一射流速度和量纲一射流频率均为1时,射流对翼型的增升减阻效果最佳。     

The aerodynamic performance of flexible wing in plunge

Inspired by the fact that flexible wing in nature possesses advance aerodynamic performance, a numerical experiment is applied to investigate the aerodynamic performance of flexible wing in plunge motion, where the incompressible Navier-Stokes (N-S) equations coupled with the structural dynamic equation for the motion of the wing is solved. A two-dimensional, elastic and inextensible beam model wing is considered at Re = 1256. The harmonic plunge motion is specified at the leading edge of wing, and the other part of wing is responded passively deforming by the aerodynamic force. By analyzing the flow field, aerodynamic force and energy efficiency of different flexibility wings, it is found that the flexibility influences the aerodynamic characteristics of the plunge wing greatly and when the plunge frequency is less than the structural frequency the flexibility can increase the thrust force and the energy efficiency of the wing, and the maximum energy efficiency is obtained when the wing plunge near the resonance. Moreover, a lighter wing possesses larger energy efficiency than a heavier wing, but it may not be functioning for too light wing. The results obtained in this study will provide physical insight into the understanding of fluid and structure interaction problem.     

钝尾缘风力机翼型气动性能计算分析

钝尾缘风力机翼型有较好的结构和气动性能,是目前多被用于大型风力机叶片靠近轮毂区域的选定翼型.但钝尾缘翼型也有缺点,易产生大的脱流涡,这会降低叶片的气动性能.为了更好地研究钝尾缘翼型的性能,以了解其气动性能的降低能否与其结构性能的优化相匹配.采用计算流体动力学(Computational fluid dynamics,CFD)方法,对薄尾缘翼型S809和改进的钝尾缘翼型S809-100的性能进行模拟和对比,结果表明相对于薄尾缘翼型,钝尾缘翼型可以增大断面的最大升力系数和升力曲线斜率,并可以降低翼型污染对翼型升力影响的敏感度.     

Flow characteristics along and above dimpled surfaces with three different dimple depths within a channel

The effects of dimples in altering time-averaged flow behavior occur mostly within one-half of one dimple print diameter from the surface, and the dimples within the arrays periodically eject a primary vortex pair from each dimple, which exists in conjunction with edge vortex pairs that form along the spanwise edges of staggered dimples regardless of three dimple depths. As the dimple depth increases, deeper dimples eject stronger primary vortex pairs, with hig her levels of turbulence transport due to larger deficits of time-averaged, normalized total pressure and streamw ise velocity as the surfaces with deeper dimples are approached. Primary vortex pair ejection frequencies range about 7–9 Hz, and edge vortex pair oscillation frequencies range about 5–7 Hz forR _eH =20,000, regardless of dimple depths.     

Influence of nozzle exit tip thickness on the performance and flow field of jet pump

The influence of exit tip thickness of nozzle δ _e on the flow field and performance of a jet pump was studied numerically in this paper. It is found that δ _e has influence on the distribution of turbulence kinetic energy k. If δ _e is ignored, k takes the highest value but dissipates rapidly than that of nozzle with a certain tip thickness. δ _e also affect apparently the development of tip vortex, which will occur near the exit tip of nozzle. The bigger the δ _e is, the larger the vortex is. The tip vortex develops with the increase of flow rate ratio q. When q=1 and δ _e =0.6∼0.8mm, a small vortex will be found downstream the tip vortex. And a concomitant vortex happens down the tip vortex in the case of q=1 and δ _e =0.8mm. As q increases to 2, the downstream small vortex disappears and the concomitant vortex becomes bigger. It is also found that the tip vortex might interact with the possible backflow that formed in the throat tube and parts of suction chamber. The center of backflow was affect evidently by δ _e . With the increase of δ _e , the center of backflow under the same q will go downstream. When δ _e =0.4mm, the center of backflow goes farthest. Then, as the further increase of δ _e , the center of backflow will go back some distance. Although, δ _e has relatively great influence on the flow field within the jet pump, it exerts only a little impact on the performance of jet pump. When δ _e =0.2∼0.6mm, the jet pump possess better performance. In most case, it is reasonable to ignore the nozzle exit tip thickness in performance prediction for the purpose of simplicity. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Effect of leading edge boundary layer thickness on dimple flow structure and separation control

This paper concerns the effect of leading edge boundary layer thickness on dimple flow structure and separation control. A flat plate with adverse pressure gradient sufficient for separation was designed. Large eddy simulation (LES) with dynamic Smagorinsky subgrid model was validated. Dimples with R=0.378, 0.994, 1.453 (R is the ratio of leading edge boundary layer thickness to dimple depth) were investigated. The results show that the horseshoe vortex dominates the dimple flow structure. As R increases, the head of the horseshoe vortex rises further away from the wall and moves downstream. Hairpin vortices in the dimple wake are productions of the horseshoe vortex. Both the horseshoe vortex and the hairpin vortices energize the boundary layer by mixing with the free-stream fluid. As R increases, the separation control effectiveness decreases.     

Flow structures around a butterfly-shaped low-aspect-ratio wing

In the present study, we numerically investigate three-dimensional flow structures around a butterfly-shaped low-aspect-ratio wing and their effect on the aerodynamic force at the Reynolds number of 1000 based on the wing chord length and free-stream velocity. When the angle of attack is less than 10°, the flow is steady and fully attached to the upper-wing surface, by which the lift force increases almost linearly with the angle of attack. As the angle of attack further increases, the flow around the wing becomes unsteady and contains the leading-edge, trailing-edge, wing-tip, and hairpin vortices. At these angles of attack, the drag force increases rapidly with increasing angle of attack due to massive separation at the leading edge, but the lift force increases gradually without abrupt fall-off. This is because the wing-tip vortices induce a strong downward flow interacting with the flow separated from the leading edge and delay subsequent vortex roll-up in the downstream. The wing-tip vortices themselves also produce low-pressure regions on the upper-wing surface and thus provide an additional lift force. The flows separated from the leading and trailing edges are eventually combined into pairs of hairpin vortices which travel downstream in the wake. The formation of the hairpin vortices above the upper-wing surface also generates lowpressure regions, and they are another source of the lift force.     

Numerical evaluation of flow and performance of turbo pump inducers

Steady state flow calculations are executed for turbo-pump inducers of modern design to validate the performance of Tascflow code. Hydrodynamic performance of inducers is evaluated and structure of the passage flow and leading edge recirculation are also investigated. Calculated results show good coincidence with experimental data of static pressure performance and velocity profiles over the leading edge. Upstream recirculation, tip leakage and vortex flow at the blade tip and near leading edge are main sources of pressure loss. Amount of pressure loss from the upstream to the leading edge corresponds to that of whole pressure loss through the blade passage. The viscous loss is considerably large due to the strong secondary flow. There appears more stronger leading edge recirculation for the backswept inducer. and this increases the pressure loss. However, blade loading near the leading edge is considerably reduced and cavitation inception delayed.     

Large-eddy simulation analysis of turbulent flow over a two-blade horizontal wind turbine rotor

Unsteady turbulent flow characteristics over a two-blade horizontal wind turbine rotor is analyzed using a large-eddy simulation technique. The wind turbine rotor corresponds to the configuration of the U.S. National Renewable Energy Laboratory (NREL) phase VI campaign. The filtered incompressible Navier-Stokes equations in a non-inertial reference frame fixed at the centroid of the rotor, are solved with centrifugal and Coriolis forces using an unstructured-grid finite-volume method. A systematic analysis of effects of grid resolution, computational domain size, and time-step size on simulation results, is carried out. Simulation results such as the surface pressure coefficient, thrust coefficient, torque coefficient, and normal and tangential force coefficients are found to agree favorably with experimental data. The simulation showed that pressure fluctuations, which produce broadband flow-induced noise and vibration of the blades, are especially significant in the mid-chord area of the suction side at around 70 to 95 percent spanwise locations. Large-scale vortices are found to be generated at the blade tip and the location connecting the blade with an airfoil cross section and the circular hub rod. These vortices propagate downstream with helical motions and are found to persist far downstream from the rotor.