合成射流控制参数对大攻角下翼型气动性能的影响

通过数值模拟的方法,对合成射流控制NACA 0012大攻角下翼型流动分离的参数进行了研究.结果表明:对于射流出口宽度为翼型弦长的0.5％,翼型在18°～24°攻角下的流场,当合成射流作用在翼型头部1％弦长位置,吹气速度比为1,无量纲激励频率在1 附近时,可以达到较好的改善翼型整体气动性能的效果.通过对翼型表面压强系数分...     

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.     

Characteristics of flow past a circular cylinder with a rectangular groove

In the present study, features of the flow past a circular cylinder with single longitudinal groove pattern placed on its surface were investigated. Six different rectangular groove sizes were tested for angular position of the groove from the forward stagnation point of the circular cylinder within 0°≤θ≤150°. The particle image velocimetry (PIV) technique were employed to measure flow field downstream of the cylinder immersed in a uniform flow field with the Reynolds number, Re=5000. Time-averaged flow data such as vorticity, 〈ω〉 streamline, 〈Ψ〉, streamwise, 〈u′u′〉 and transverse, 〈v′v′〉 Reynolds normal stresses, turbulent kinetic energy, TKE and RMS of streamwise, u_rms and transverse, v_rms velocity components were obtained from PIV data to demonstrate flow features. Moreover, frequency of Karman vortex shedding was explored using single point spectral analysis. It is concluded that presence of the groove on a cylinder surface significantly affects the near wake flow structure and turbulence statistics. Karman vortex shedding frequency, f_k strongly depends on the groove size. Moreover, the shear layer instability is induced on the grooved side with additional frequencies.     

Flow structure of the wake behind an elliptic cylinder close to a free surface

The flow fields around an elliptic cylinder of axis ratio AR=2 adjacent to a free surface were investigated experimentally using a water channel. The main objective is to understand the effect of the free surface on the flow structure in the near-wake. The flow fields were measured by varying the depth of cylinder submergence, for each experimental condition, 350 velocity fields were measured using a single-frame PIV system and ensemble-averaged to obtain the spatial distribution of turbulent statistics. For small submergence depths a large-scale eddy structure was observed in the near-wake, causing a reverse How near the free surface, downstream of the cylinder. As the depth of cylinder submergence was increased, the flow speed in the gap region between the upper surface of the cylinder and the free surface increased and formed a substantial jet flow. The general flow structure of the elliptic cylinder is similar to previous results for a circular cylinder submerged near to a free surface. However, the width of the wake and the angle of downward deflection of the shear layer developed from the lower surface of the elliptic cylinder are smaller than those for a circular cylinder.     

Fundamental studies on free stream acceleration effect on drag force in bluff bodies

This paper describes fundamental studies on free stream acceleration effect on drag force in bluff bodies. The flow with gradual velocity increase assumed an accelerated flow. The wind tunnel tests were conducted in order to investigate the difference of aerodynamic characteristics between non-accelerated flow and accelerated flow. The experimental models were a circular cylinder and a square cylinder. In an accelerated flow, the condition of free stream was an acceleration of about 3.6m/s². Experimental Reynolds number varied between form 4.0×10⁴ to 1.64×10⁵. The pressure distributions and the aerodynamic force were measured in both case of nonaccelerated flow and accelerated flow. In case of a circular cylinder, the drag of accelerated flow is lower than that of non-accelerated flow in the low Reynolds number regime. Then, it becomes higher than that of non-accelerated flow in the high Reynolds number regime. On the other hand, in case of a square cylinder, the drag of accelerated flow is higher than that of non-accelerated flow in the whole Reynolds number regime. If a separation point can be movable such as the circular cylinder, the additional momentum due to flow acceleration leads to delay separation and to decrease the drag in the low Reynolds number regime. If a separation point is nearly fixed such as a square cylinder, the additional momentum due to acceleration always affects to increase the drag than that of non-accelerated flow. Based on this research, it is expected that the roof shape of high speed train where the separation point is movable has the advantages to reduce the effect of accelerated flow such as strong crosswind/gust.     

Vortex-induced vibration control by micro actuator

The control of vortex-induced vibration of two side-by-side circular cylinders in a cross flow is carried out experimentally. One cylinder is elastically supported and the other is fix-supported at both ends. The two cylinders vibrate under the action of the unsteady flow-induced forces. A micro actuator is embedded on the surface of each cylinder to perturb the boundary layer. The spacing ratio is set at 1.2. The measurement shows that the structural vibration can be suppressed significantly when the reduced excitation frequency is around 2.655.     

合成射流基微泵的控制律

通过对合成射流流场数值分析,得到合成射流鞍点特征及可控特性。基于对合成射流鞍点特征及其随激励条件变化规律的分析,获得调节合成射流基微泵的控制律。通过对调节控制和未调节控制下微泵的流体力学分析,对微泵控制律进行考核验证。验证结果显示：改变激励条件并相应调节分流板位置,合成射流新产生的“涡对”保持良好,微泵工作于设计状态;而分流板未控制调节下,由于分流板处于合成射流“涡对”形成和发展的关键区域,严重阻碍和破坏了合成射流“涡对”的形成和发展,合成射流对周围流体的卷吸量显著减小,微泵工作远离设计状态,泵流流量显著减小。研究结果表明：分流板控制律对微泵的控制是行之有效的,若激励器振动膜的振动幅值与激励电参数的关系已知,则可由改控制律得到微泵的电参数控制律,从而可以实现微泵的智能化控制。     

射流条件下采用纳米流冷却液对柴油机缸盖冷却的研究

为提高柴油机缸盖的散热性能,提出一种将纳米流冷却液作为冷却工质,利用射流技术提高缸盖进排气门鼻梁处传热系数的方法。试验研究发现,相同射流参数下,不同体积分数的纳米流冷却液较传统冷却液都可有效提升传热系数,但体积分数增加的作用有限。同体积分数的纳米流冷却液会因为不同射流参数而表现出不同的传热性能,同时也会因为其本身黏性的增加而耗费更大的驱动功率。采用纳米流冷却液可以有效提高缸盖鼻梁处的传热系数。     

Numerical investigation of cross-flow around a circular cylinder at a low-reynolds number flow under an electromagnetic force

The effect of the electromagnetic force (or Lorentz force) on the flow behavior around a circular cylinder is investigated by computation. Two-dimensional unsteady flow computation forR _e =10² is carried out using a numerical method of finite difference approximation in a curvilinear body-fitted coordinate system by solving the momentum equations including the Lorentz force as a body force. The effect of spatial variations of the Lorentz forcing region and forcing direction along the cylinder circumference is investigated. The numerical results show that the Lorentz force can effectively suppress the flow separation and oscillation of the lift force of circular cylinder cross-flow, leading to reduction of drag.     

Analysis of the impact of flow characteristics on the separation efficiency and pressure drop of a cyclone-type oil separator

The effects of flow structure on the separation efficiency and pressure drop in a cyclone-type oil separator were investigated via Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES), and the results were compared with experiments. Compared with the RANS simulations, the LES results were more similar to the experimental data as they simulated the complex flow structure more realistically. Swirling flow with strong turbulent kinetic energy (TKE) at the top section hindered the flow of particles toward the separator wall. In addition, a decrease in tangential velocity along the wall at the bottom reduced the centrifugal force, resulting in a decreased separation efficiency as particles were able to flow directly toward the outlet. The LES also predicted the pressure drop slightly better than the RANS simulation did, due to increased pressure drop caused by collision of the flow with the helix and outlet tube, which led to the formation of vortical flow structures with strong TKE.

     

Experimental study on the confined flow over a circular cylinder with a splitter plate

An experimental study was carried out to investigate the effect of a splitter plate on wake flows downstream of a circular cylinder symmetrically placed in a confined channel. A particle image velocimetry (PIV) measurement was applied to visualize the flow structure and analyze changes in the vortex shedding process. The control elements of the splitter plate length, L/D (D is the cylinder diameter) was varied from 0 to 1.5 and Reynolds number, Re_D was considered at 2400 and 3000. The experimental results showed that the splitter plate had an influence on stabilization of wake turbulences in a confined channel. For shorter splitter plate length of L/D=0.5 and 0.75 cases, flow structures were significantly modified and the vortex shedding frequency decreased as compared with bare cylinder cases. For longer splitter plate length of L/D=1, 1.25 and 1.5 cases, the generation of a secondary vortex was observed based on the snapshot proper orthogonal decomposition (snapshot POD) analysis. In addition, turbulent characteristics corresponding to turbulent kinetic energy (TKE) and Reynolds shear stress correlations took the lowest values and the dominant vortex shedding frequency disappeared. There was an optimal value of the splitter plate length at L/D=1 on suppression of velocity fluctuations. Moreover, the stabilizing effect of a splitter plate was more obvious at Reynolds number of Re_D=3000 than that at Re_D=2400.     

2D planar jet control for manipulating the separation bubble behind the vertical fence

The effect of a 2D planar jet was investigated to manipulate the separation bubble behind the vertical fence. The pulsating and continuous blowing jets were generated in front of or behind the fence. The experiments were carried out in a circulating water channel. The parameters used for controlling the jet included the distance between the slit nozzle and fence, the speed of the jet, and the frequency of the pulsating jet. The velocity fields were measured using phase-averaged DPIV method. From the results, the mechanism of reduction of the separation bubble and the quantitative effects of parameters to reduce the separation bubble behind the fence were found. A maximum reduction of 87% compared with the reattachment length of the uncontrolled fence flow was achieved using the front continuous jet. The controlled fence wakes have higher flow fluctuations and less sheltering effect than that of uncontrolled fence wake.     

压电驱动自耦合射流激发器影响因素

利用试验研究和数值模拟的方法对压电驱动的自耦合射流激发器进行研究,通过改变激发器激励参数和几何参数探求自耦合射流的变化规律。借助于动网格技术模拟了压电驱动膜片的周期性运动,实现了自耦合射流腔体内部和外部流动的耦合计算,模拟结果获得了和试验相一致的流动现象。研究结果表明:自耦合射流沿流向中心线速度分布呈现先上升后下降的趋势,在量纲一流向距离z/b=10的地方速度达到最大值;激发器的最佳波形输入为方波,在方波信号下激发器产生的射流速度最大;自耦合射流的速度随着激发器输入电压的增大而增加,但是当电压增大到175V后这种影响趋势变得非常微弱;激发器存在两个谐振频率,分别为340Hz和1kHz,最佳谐振频率为1kHz。振幅的增大,对自耦合射流的速度提高影响显著。在条件允许下,减小腔体高度与腔体直径的比值会得到更高速度的自耦合射流。研究结果为激发器的优化设计提供了一定的理论依据。     

涡流发生器在流体机械流动控制中应用研究进展

从实验测量和数值计算两个方面对涡流发生器流动控制的基础研究和应用研究进行评述,介绍了涡流发生器在流体机械流动控制中的应用及其取得的主要研究成果,最后指出了涡流发生器发展存在的问题和发展方向.     

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。     

Drag force control of flow over wavy cylinders at low reynolds number

Three-dimensional numerical simulations on the laminar flow around a circular cylinder with different diameter along the spanwise leading to a type of sinusoidal waviness, named wavy cylinder are performed at low Reynolds number. A series of wavy cylinders with different combinations of spanwise wavelength and wave amplitude are conducted at a Reynolds number of 100. The optimal range of wavelength and the effect of wave amplitude are obtained. The results show that the 3-D free shear layers from the cylinder are more difficult to roll up to vortex and hence the wake formation lengths of some typical wavy cylinders are larger than that of the circular cylinder and in some cases the free shear layers even do not roll up into vortex behind the cylinder. The mean drag coefficients of the typical wavy cylinders are less than that of a corresponding circular cylinder with the same mean diameter; also the fluctuating lift coefficients are reduced. The reduction of mean drag coefficient and fluctuating lift coefficient of wavy cylinder increases with the value of wavy amplitude. Furthermore, a typical wavy cylinder model at Re=150 is also simulated and found that the control of flow induced vibration by modifing the spanwise wavelength of cylinder has a relationship with the variation of Reynolds number.     

大型储罐清洗喷嘴的数值模拟研究

针对石油储罐机械清洗过程中射流发散严重、有效靶距过短、清洗效果差的问题,现将一种含中心体式喷嘴应用到储罐清洗中。运用Fluent软件对含中心体的储罐清洗喷嘴内外流场进行了数值模拟仿真,过程中选用K-ε紊流模型和SIMPLEC算法进行了数值计算,采用Mixture多相流模型和非结构化网格对含中心体喷嘴和圆柱形喷嘴进行了对比模拟评价。研究结果表明,含中心体喷嘴沿射流方向的轴心速度逐渐增大,射流在计算域内未发散,有效靶距得到了提高。相比于圆柱形喷嘴,含中心体的喷嘴出口截面射流半径缩小了13.7%,聚束性得到了提高,打击力得到了更好的保持,因而能更好地满足大型储罐清洗的要求。     

串列三圆柱绕流的时均压力分布与气动力

运用刚性模型测压风洞试验方法对单圆柱、不同间距串列双圆柱和串列三圆柱绕流的时均压力分布与气动力进行了研究。首先,进行单圆柱模型和不同间距串列双圆柱模型的绕流试验,试验的雷诺数为3.4×104;其次,通过与单圆柱进行对比,讨论了气动干扰对串列三圆柱时均压力分布与时均阻力的影响规律;最后,通过与串列双圆柱进行对比,讨论了圆柱的数量对干扰规律的影响。试验结果发现,串列三圆柱的绕流存在两个完全不同的流态,其切换的临界间距(L/D)cr在3.5～4.0之间,两个流态下的时均压力分布与时均阻力存在明显的差异。本研究可对实际工程中串列圆柱结构的风荷载取值提供参考。     

Numerical study of fluid force reduction on a circular cylinder using tripping rods

A numerical investigation on the effects of small tripping rods on the fluid force reduction on a big structure has been carried out using finite volume method where a configuration of a circular cylinder with two small tripping rods symmetrically placed very near to its front surface is studied. The diameter ratio of the rods and the cylinder is set at 0.08, 0.10 and 0.12, and the gap between the rods and the cylinder is fixed at 0.08 of the cylinder diameter. The angular position of the rods varies from 20° to 60°. The effects of the tripping rods on force reduction, vortex shedding frequency and flow separation have been examined for various arrangements of the rods with Reynolds number focused on 200 for laminar flow and 5.5×10⁴ for a turbulent flow. The results reveal that there exits an optimum position where the time averaged force coefficients acting on the cylinder all reach their minimum values and at the same time Strouhal number meets its maximum. At the optimum position the drag coefficient is reduced by 18% for Re=200 and 59% for Re=5.5×10⁴. Further investigation with tripping rods placed near the separation points is also carried out for Re=5.5×10⁴ and a considerable drag reduction is found.     

考虑压力脉动的消防水炮自适应炮头射流系统参数振动研究

消防水炮自适应炮头可根据流体压力和流量变化自动调整炮口开度,从而显著提升其射流性能。考虑自适应炮头射流系统中流体的压力脉动,结合流体体积弹性模量与刚度间的关系,建立基于时变流体刚度的射流系统参数振动动力学模型,采用多尺度法推导脉动激励频率接近射流系统固有频率和固有频率与脉动频率的组合频率时的共振响应公式,分析射流系统的主共振和组合共振响应。结果表明射流系统发生主共振响应时,激励频率占主导成分,且接近一阶固有频率时系统共振幅值最大;射流系统发生组合共振响应时,激励频率与流体刚度波动频率的组合频率对系统响应的影响与流体脉动频率有关;主共振和组合共振将恶化系统的动力学行为。本项研究可为深入探索自适应炮头射流系统的动力学特性,优化不同工况环境下的设计参数提供理论依据。