Flow Characteristics of Two-Dimensional Supersonic Under-Expanded Coanda-Reattached Jet

When there is a wall near the jet, it deflects and flows while being attached to the wall owing to the Coanda effect. The flow characteristics of the incompressible and two-imensional (2D) Coanda-reattached jets have been considerably explained. However, 2D supersonic under-expanded jets, reattached to side walls, have not been sufficiently investigated. These jets are used in gas-atomization to produce fine metal powder particles of several micrometers to several tens micrometers. In this case, the supersonic under-expanded jets are issued from an annular nozzle, which is set around a vertically installed circular nozzle for molten metal. The jet flow at the center cross-section of the annular jet resembles a 2D Coanda-reattached jet that deflects and attaches on the central axis. In this study, the flow characteristics of a supersonic under-expanded Coanda air jet from a 2D nozzle that reattaches to an offset side wall are elucidated through experiment and numerical analysis. For numerical analysis, we show how much it can express experimental results. The effects of supply pressure P0 on the flow characteristics such as the flow pattern, size of shock cell, reattachment distance, and velocity and pressure distributions, etc. are examined. The flow pattern was visualized by Schlieren method and the velocity distribution was measured using a Pitot tube. These results will be also useful in understanding the flow characteristics of a gas-atomization annular nozzle approximately.

基于Coanda脉冲射流的D型体主动减阻控制研究

D型体是典型钝体之一,其尾缘分离流动及近尾流流动结构与其受到的气动阻力密切相关。本文结合Coanda脉冲射流及遗传算法,对D型体绕流进行主动减阻控制。实验在直流风洞中进行,基于来流速度和D型体高度H的雷诺数为1.8×104;Coanda脉冲射流布置于D型体背部上下两侧,控制参数包括射流的驱动压力、频率和占空比,以及背部上下侧射流相位差。遗传算法的目标函数为D型体时均背压,间接反映D型体所受气动阻力。研究结果表明:遗传算法能够帮助确定Coanda脉冲射流的最优控制参数组合(射流驱动压力为1.94 atm,无量纲射流频率为0.27,射流占空比为37%,上下侧射流相位差为136°),使D型体时均背压提升达61%(对应的减阻率约为23%),对应45%的控制效率;在最优控制参数下,D型体近尾流交替脱落的大尺度旋涡被破坏,脱落频率和相位差被改变。

Numerical investigation of the interaction of the turbulent dual-jet and acoustic propagation

In order to study the interaction between two independent jets, a three-dimensional（3D） transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation（LES） with dynamic Smagorinsky sub-grid scale（SGS） approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams–Hawkings（FW-H） integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level（OASPL） profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.

振荡射流气动矢量喷管参数影响特性的试验研究

基于康达效应的同向流矢量技术控制效率高、推力损失低,具备提高飞行器隐身性能的巨大潜力。为了探究马赫数Ma 0.35主流下二次流类型、二次流阵列个数、康达壁面半径R和终止角θ等参数对同向流矢量控制的影响,利用天平研究了9种不同的喷管模型的力学特性,获得矢量偏转力随次主质量流量比变化的控制规律。结果表明,主喷管高和康达壁面半径的比值H/R是对矢量偏转效率和控制稳定性最重要的因素,H/R越低,控制效率越高,控制曲线线性度越高;使用三个振荡射流阵列作为二次流的条件下,H/R从0.5减小到0.43,控制效率提高近49%,与使用定常射流相比,使用振荡射流作为二次流,显著提高了控制效率,增强了控制稳定性;对比分析显示,使用两个或三个振荡二次流阵列、H/R为0.43且θ为90°时,矢量控制效果最好。进一步地研究表明,H/R和θ对矢量控制效率的影响最大,而θ对控制曲线线性度几乎没有影响,本文研究还发现二次流阵列个数低于两个或H/R高于0.6时,控制特性急剧恶化。本文研究可为同向流矢量喷管的工程设计提供理论支撑。

空气放大器内部流场数值模拟研究

在阐述空气放大器的工作原理及结构特点的基础上,应用计算流体力学软件FLUENT模拟了内部流场,不仅能够清晰观察到射流附壁效应,而且能够计算出引射空气的质量流量。

基于康达效应的超微粉气流分级数值模拟研究

基于不可压缩粘性流体的N-S方程和κ-ε两方程模型,结合固体微粒运动方程,采用有限容积法对射流分级机中微粒的运动进行了数值模拟,得到射流分级机内的压力场、速度场和颗粒运动轨迹,计算得到的粒径分布规律与分级实验结果相吻合。

基于响应面分析的附壁脉冲射流喷嘴优化设计

为了提高附壁脉冲射流喷嘴结构的可靠性,针对某一超深油井井下附壁射流喷嘴,采用中心复合响应面设计与数值模拟方法,以反馈通道直径、附壁夹角和出口直径为优化变量,以射流的压力脉动幅值为优化目标,开展了喷嘴结构的优化设计。通过试验与数值模拟结果对比,验证了数值模拟方法的可靠性。对响应面结果的拟合回归方程、三维立体图和等高线进行了分析,分析结果表明,出口直径的变化对响应值压力脉动幅值的影响最为明显,较小的出口直径能够获得更大的压力脉动幅值。基于响应面分析结果,得到了优化的附壁脉冲射流喷嘴结构。对优化得到的附壁脉冲射流喷嘴内流场开展数值模拟分析,得到其脉动幅值为0.308 MPa,与响应面模型预测值非常接近,进一步验证了采用中心复合响应面法优化得到的附壁脉冲射流喷嘴结构的可靠性。研究方法与结果可为类似结构的优化设计提供参考。

内吹式襟翼控制机理和失速特性

短距起降运输机对增升装置提出了更高要求,常规机械式增升装置已无法满足,内吹式襟翼系统是当今固定翼飞机最有效的动力增升形式。为推动该技术的工程应用,基于雷诺平均N-S方程,对某加装60°偏角无缝襟翼的亚声速翼型在环量控制作用下的流场进行数值模拟,研究了其在不同吹气动量系数下的气动特性及流动形态,分析了不同环量控制阶段增升机理、失速特性和吹气动量系数对失速特性影响规律。结果表明:内吹式襟翼增升控制效率(升力系数增量与吹气动量系数的比值)较高,在临界吹气动量系数下可达70,此时相较于无吹气状态,升力增加约125%;主翼上由于环量增加产生的升力增量是翼型升力增量的主要来源,约占总升力增量的78%;吹气动量系数增加可造成翼型气动中心后移;附面层分离控制区主要通过消除襟翼上的流动分离增加升力,超环量控制区升力的增加是由于尾缘下游的射流效应使流线进一步偏转而实现的;随吹气动量增加,附面层分离控制区的失速迎角提前,超环量控制区失速迎角略微推迟。

海泡石颗粒气固两相射流分级的数值模拟

针对气体和海泡石颗粒组成的气固两相流体建立欧拉-拉格朗日两相颗粒模型,依据射流的附壁效应进行海泡石颗粒分级过程的数值模拟;通过分析康达块壁面区域的海泡石颗粒的运动轨迹、速度和位置等运动特性,确定颗粒的分级区域。结果表明:海泡石颗粒的粒径越小,附壁效应作用越明显,海泡石颗粒的速度在康达块壁面区域衰减很快;根据不同粒径颗粒在水平方向的位置划分了颗粒分级区间,粒径为1~10μm的海泡石细粉的分级区间位于X轴坐标为0~0.14 m处。

条缝型送风口形成的竖壁贴附射流通风模式研究:送风速度的影响

提出了一种介于混合通风、置换通风之间的新型通风模式——条缝型送风口形成的竖壁贴附射流。利用2DPIV研究了该模式下射流送风速度对气流流场的影响,并对极限贴附距离进行了探讨。研究表明,射流送风速度越大,形成的贴附射流距离越长,贴附效果越显著;同时,送风速度越大,射流对周围空气的卷吸能力越强,射流影响区域越大;射流送风速度的改变对极限贴附距离的影响不明显。

附壁脉冲射流喷嘴流场数值模拟和冲击性能试验

目前针对附壁喷嘴的研究主要是常压、低雷诺数条件下的试验或二维数值模拟,研究不够系统,也未涉及高围压下附壁喷嘴的研究。为此,采用数值模拟与试验的方法,研究了某一型号附壁脉冲射流喷嘴的非定常流场特性和冲击性能,通过数值模拟和试验均得到了雷诺数与频率的线性关系式,二者所得结果的最大相对误差为8.34%,验证了数值模拟方法的可靠性。喷嘴内部的附壁摆动特性产生了喷嘴射流中心点的脉冲效果,脉冲周期为喷嘴附壁频率的2倍。附壁喷嘴在围压30 MPa、靶距70 mm、入口流速15 m/s时,冲击面中心点处的等效压力脉动幅值约为1.2 MPa,脉动冲击效果良好。附壁喷嘴射流的最大与最小冲击压力都随着入口流量的增加而增大,随着冲击靶距的增大而减小。研究结果可为脉冲射流的发展和应用提供一定的指导。

无阀压电微泵用平面锥管内部流动附壁效应

为研究锥管内流体流动中产生的附壁效应对其流阻系数的影响,采用数值模拟的方法对平面锥管内部流动附壁效应进行研究.结果表明：雷诺数Re在300～3000,锥管角度在5～40°时,扩散方向流动可以分为3种状态,即稳定状态、附壁状态和射流状态.锥管角度为10～35°时,锥管内流动易于发生附壁效应.Re在300～1 200时,稳定状态扩散流阻系数随着扩散角的增大迅速降低;附壁状态扩散流阻系数随着扩散角的增大缓慢增大;射流状态扩散流阻系数随着扩散角的增大而缓慢降低.Re在1 800～3 000时,附壁状态扩散流阻系数在锥管角度为30°时达到最大值.流阻系数比在稳定状态和射流状态下基本不变,在附壁状态下随着扩散角的增大迅速减小.

清淤射流泵流场数值模拟及其参数对性能影响分析

文章采用基于RANS方程的数值模拟方法研究了清淤射流泵的性能,建立了基于CFD技术的清淤射流泵流场分析方法,重点分析了清淤泵喷口间隙、内部结构、出口半径、进口压力等关键参数与流场之间的规律,给出了射流泵流体流动机理,提出了采用QhVh(底部流量与平均速度的乘积)来评价最佳射流泵及作业距离h。文中的射流泵流场数值模拟方法可为该类射流泵设计提供分析评估手段和实用工具。

Study on the Characteristics of Supersonic Coanda Jet

Techniques using Coanda effect have been applied to the fiuid control devices. In this field, experimental studies were so far performed for the spiral jet obtained by the Coanda jet issuing from a conical cylinder with an annular slit, thrust vectoring of supersonic Coanda jets and so on. It is important from the viewpoints of effective applications to investigate the characteristics of the supersonic Coanda jet in detail. In the present study the effects of pressure ratios and nozzle configurations on the characteristics of the supersonic Coanda jet have been investigated experhoentally by a schlieren optical method and pressure measurements. Furthermore, Navier-Stokes equations were solved numerically using a 2nd-order TVD finite-volume scheme with a 3rd-rorder three stage Runge-Kutta method for time integration. k - ε model was used in the computations. The effects of initial conditions on Coandaflow were investigated numerically As a result, the simulated flow helds were compared with experimental data in good agreement qualitatively.

Aerodynamic Performance Improvement of a Highly Loaded Compressor Airfoil with Coanda Jet Flap

Coanda jet flap is an effective flow control technique,which offers pressurized high streamwise velocity to eliminate the boundary layer flow separation and increase the aerodynamic loading of compressor blades.Traditionally,there is only single-jet flap on the blade suction side.A novel Coanda double-jet flap configuration combining the front-jet slot near the blade leading edge and the rear-jet slot near the blade trailing edge is proposed and investigated in this paper.The reference highly loaded compressor profile is the Zierke&Deutsch double-circular-arc airfoil with the diffusion factor of 0.66.Firstly,three types of Coanda jet flap configurations including front-jet,rear-jet and the novel double-jet flaps are designed based on the 2D flow fields in the highly loaded compressor blade passage.The Back Propagation Neural Network(BPNN)combined with the genetic algorithm(GA)is adopted to obtain the optimal geometry for each type of Coanda jet flap configuration.Numerical simulations are then performed to understand the effects of the three optimal Coanda jet flaps on the compressor airfoil performance.Results indicate all the three types of Coanda jet flaps effectively improve the aerodynamic performance of the highly loaded airfoil,and the Coanda double-jet flap behaves best in controlling the boundary layer flow separation.At the inlet flow condition with incidence angle of 5°,the total pressure loss coefficient is reduced by 52.5%and the static pressure rise coefficient is increased by 25.7%with Coanda double-jet flap when the normalized jet mass flow ratio of the front jet and the rear jet is equal to 1.5%and 0.5%,respectively.The impacts of geometric parameters and jet mass flow ratios on the airfoil aerodynamic performance are further analyzed.It is observed that the geometric design parameters of Coanda double-jet flap determine airfoil thickness and jet slot position,which plays the key role in supressing flow separation on the airfoil suction side.Furthermore,there exists an optimal combination of front-jet and rear-jet mass flow ratios to achieve the minimum flow loss at each incidence angle of incoming flow.These results indicate that Coanda double-jet flap combining the adjust of jet mass flow rate varying with the incidence angle of incoming flow would be a promising adaptive flow control technique.

基于康达效应的电源箱体散热优化研究

随着电力电子技术与集成电路的迅速发展,电源箱体结构变得紧凑复杂,而箱体散热成为亟需解决的问题。文章针对采取强迫风冷式散热的电源箱体,依据康达效应,结合变压器的几何结构修改了底部进风口形状,利用数值模拟的方法,分析了修改前后箱体内主要热源(变压器和IGBT模块)及整个箱体内部流场和温度场,并对比了优化效果。结果表明:改变底部进风口形状后,变压器和IGBT模块平均温度分别降低了约26.2和4.0 K;改变进风口形状,使变压器表面形成贴附射流,提高变压器表面的流速,进而增强了散热效果。

非直轴线射流风机特征参数的数值模拟

为了避免风机出风口的气流与上、下壁面形成康达效应,基于流体动力学仿真模拟软件平台,建立二维仿真模型,研究地铁区间隧道内非直轴线射流风机特征参数影响下的速度场与压力场分布。根据模拟结果可知,在不同进出口角度下,非直轴线射流风机靠近上壁面的出口风速沿隧道纵向呈现二项分布,且风机进出口角度在4?~6?范围内其速度场受壁面摩擦影响较小,排烟效果最佳;相同进出口角度情况下,安装高度越大,出风口风流与隧道上壁面接触的位置越远。

康达喷气叶栅喷气内腔体拓扑优化设计及试验研究

为了减小从康达喷气叶栅内部流出的喷气方向与叶栅主流方向的偏差,进一步提升康达喷气对流动分离的抑制效果,本文基于流体拓扑优化技术及布置导流肋的方法对康达喷气叶片原始喷气内腔结构进行优化设计。采用数值计算和高速风洞试验评估优化的喷气内腔结构对康达喷气叶栅气动性能的改善效果。结果表明:优化设计的带肋喷气内腔结构可以减小康达喷气与主流掺混的偏差角,叶片吸力面尾缘的流动分离区域进一步减小;在来流Ma为0.5、攻角为0°、1.25%喷气量下,采用带肋喷气内腔的康达喷气叶栅总压损失系数较采用原始喷气内腔结构降低了7.15%;采用带肋喷气内腔结构能够使叶栅在不同工况条件下的总压损失小于原始内腔结构,且在最佳喷气量下获得最小的总压损失系数。

INTEGRAL ANALYSIS ON INTERACTION OF DUAL-JET FLOW

This work reports the analysis of the flow characteristics of two plane,parallel jets that merge as they issue into stagnant surroundings.The predicted results based on a simple integral analysis of the flowfield are compared with the experimental data.It shows that the simple analysis predicts the major variables of the dual-jet reasonably well and the spreading parameter σ=15 for plane dual-jet.

自适应康达喷气控制在高负荷压气机中的试验研究

未来航空发动机的发展要求其压缩系统级负荷不断增大,由此将使得压气机内部出现较强的角区分离、附面层流动分离等二次流。提出了一种新型的自适应康达喷气流动控制(ACJC)方法,更加智能且高效地抑制压气机内部流动分离并提升压气机的扩压能力,进而拓宽高负荷压气机稳定、高效运行范围。为构建自适应康达喷气流动控制系统并在高负荷压气机上验证其控制效果,首先,选取了扩压因子为0.66的压气机静叶叶栅为研究对象,并优化设计了单缝康达喷气静叶叶栅;然后,基于数值计算结果采用方差分析法、主成分分析法和神经网络算法建立了单缝康达喷气静叶叶栅来流攻角预测模型和最佳喷气量预测模型;最后,搭建了基于自适应康达喷气流动控制系统的试验平台,验证了其对高负荷叶栅流动分离控制的有效性和准确性。试验结果表明:在不同攻角和不同来流马赫数条件下,自适应康达喷气流动控制系统能够实时准确地预测来流攻角,并瞬间做出喷气量实时调节与反馈。此外,在5°来流攻角下,当来流马赫数为0.4、0.5和0.6时,相比于无康达喷气叶栅,康达喷气的引入使得总压损失系数分别降低了11.5%、9.8%和8.0%。