Supersonic flow characteristics in laser grooving

The gas flow structure in laser grooving has been evaluated by numerical simulation and experiments in this study. A nozzle designed with a specific arrangement of compound jets for laser grooving has been tested. For this nozzle configuration the compressible flow structure of a shock wave induced by a supersonic side jet has been predicted in a numerical simulation using FLUENT, a computational fluid dynamics code and visualized in the experiment where the wall pressure on the grooving zone was evaluated and measured for jets impinging on the substrate at varying attacking angles of the side jet. The results obtained numerically and experimentally were comparable. In addition, the relationships between the material removal efficiency and the flow structure have also been established. It transpired that the attacking angle made a significant and dramatic improvement on the flow structure and grooving appearance and that a large wall pressure with a clear grooving profile can be obtained for attacking angles between 40° and 50° in the present test setup.     

Aerodynamic design of a supersonic three-dimensional inlet

The results of designing and numerical gas-dynamic modeling a supersonic three-dimensional inlet of a new type are considered. A ramp of external compression of this inlet is the V-shaped body forming an initial plane oblique shock wave and a subsequent isentropic compression wave. The inlet incorporates an entrance section of internal compression, where also a plane oblique shock wave and a subsequent isentropic compression wave are formed by a cowl. The designed three-dimensional inlet has small inclination angles of compression surfaces, which ensures its low wave drag. According to the estimates of inlet efficiency in terms of the compression ratio and the total pressure recovery factor, it is close to the optimal two-dimensional shocked inlet of external compression considered by Oswatisch as well as Petrov and Ukhov. The flow in the inlet was computed with the use of the Euler and Navier — Stokes codes provided by the commercial package “FLUENT”. The flow in the inlet throat in the design regime computed under the inviscid flow approximation is uniform. The most substantial effect of the flow viscosity in this regime manifests itself in the interaction of the shock wave from the cowl with the boundary layer on the V-shaped compression body in the inlet internal duct. According to computed data, the boundary layer separation does not occur in this case; however, due to viscosity effects, reflected shock waves are formed here which results in significant deviations of flow structure as compared to the computed inviscid flow.     

Flow characteristics of hypersonic inlets with different cowl-lip blunting methods

Under hypersonic flight conditions,the sharp cowl-lip leading edges have to be blunted because of the severe aerodynamic heating.This paper proposes four cowl-lip blunting methods and studies the corresponding flow characteristics and performances of the generic hypersonic inlets by numerical simulation under the design conditions of a flight Mach number of 6 and an altitude of 26 km.The results show that the local shock interference patterns in the vicinity of the blunted cowl-lips have a substantial influence on the flow characteristics of the hypersonic inlets even though the blunting radius is very small,which contribute to a pronounced degradation of the inlet performance.The Equal Length blunting Manner(ELM)is the most optimal in that a nearly even reflection of the ramp shock produces an approximately straight and weak cowl reflection shock.The minimal total pressure loss,the lowest cowl drag,maximum mass-capture and the minimal aeroheating are achieved for the hypersonic inlet.For the other blunting manners,the ramp shock cannot reflect evenly and produces more curved cowl reflection shock.The Type V shock interference pattern occurs for the Cross Section Cutting blunting Manner(CSCM)and the strongest cowl reflection shock gives rise to the largest flow loss and drag.The cowl-lip blunted by the other two blunting manners is subjected to the shock interference pattern that transits with an increase in the blunting radius.Accordingly,the peak heat flux does not fall monotonously with the blunting radius increasing.Moreover,the cowl-lip surface suffers from severe aerothermal load when the shear layer or the supersonic jet impinges on the wall.     

自由旋涡气动窗口全流场的数值模拟

建立自由旋涡气动窗口全流场仿真模型,对大密封压比气动窗口的全流场展开数值研究,得到自由旋涡气动窗口的流场结构,发现大密封压比气动窗口形成的自由旋涡射流在光束输出通道内无明显的波系结构.根据模拟结果对自由旋涡气动窗口的性能进行优化,对自由旋涡喷管上壁面型线进行二次粘性修正.优化自由旋涡射流场后,激光器输出光束通道内压力分布稳定上升;增加扩压器外端壁吹气1.19MPa、内端壁吹气1.68MPa时,自由旋涡射流总能提高,气动窗口密封压力从37.5torr降低至6torr.该研究结果对自由旋涡气动窗口技术的发展具有参考意义.     

Formation of laser jet thrust in the supersonic regime

Basic methods for obtaining laser jet thrust in the supersonic regime corresponding to the supersonic flow in the jet nozzle are analyzed. It is shown that the method based on the interaction of a laser ablative jet with the supersonic flow is promising. In this case, laser thrust is formed due to additional acceleration of the flow behind the ablation region. Numerical simulation of the flow in a parabolic nozzle is employed to demonstrate the possibility of effective formation of laser thrust at a level of 3 × 10^?3 N/W.     

Experimental Research on Wet Steam Flow With Shock Wave

The article presents research on a wet steam transonic flow in a Laval half-nozzle with a shock wave. The motivation for this research was to investigate the shock wave/liquid phase interaction in the transonic wet steam flow. This phenomenon is responsible for the lack of good correspondence between experimental data and computational fluid dynamics results. For the tests, the geometry of the half-arc nozzle was used. The shock wave formation at the divergent section of the nozzle is caused by too high back-pressure. The observed instabilities in the flow are mainly initiated by the shock wave/liquid film (boundary layer) interaction. The numerical calculations were compared with experimental results with respect to the static pressure distribution along the nozzle.     

Visualization of impinging supersonic free jet on a tilt plate by LIF and PSP

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.     

保护气对切割弧特性影响的模拟研究

通过比较两种不同结构切割炬所产生的等离子体流场,发现保护气对等离子体的温度和速度分布影响很小.垂直保护气在切割炬喷口形成阻碍作用,造成切割炬内的压强有所升高,但是增加不大.两种结构保护气对切割弧的影响只是在炬喷口外的激波附近.加入保护气后激波的强度会减弱.相对于没有保护气的情况,保护气增加冷却作用,弧电压会略有升高.当改变保护气的成分时,发现弧柱区的氧气含量不受影响,所以保护气成分的改变不会影响到弧电压.计算发现轴线处氧气和周围气体的混合很少,在喷口下游10mm处,氧气的摩尔分数仍在90%以上. 关键词： 等离子体切割弧 保护气 数值模拟     

Structure and parameters of the shock layer formed when a supersonic underexpanded jet interacts with a counterpropagating hypersonic flow in the transition regime

The method of molecular dynamics (the Bird system) has been used to mathematically model a planar, strongly underexpanded supersonic jet that encounters a hypersonic flow of rarefied gas. Particular attention is paid to the structure and parameters of the shock layer close to the plane of symmetry. The results of calculations are presented for currents of a monatomic gas, simulating argon, with a Mach number of the external flow of M_∞=5.48, a Mach number at the nozzle edge of M_a=1, a ratio of the density at the nozzle edge to the density of the unperturbed flow equal to 130, and various stagnation temperatures of the external flow and of the jet. The evolution of the structure and the parameters of the shock layer as the Knudsen number Kn_∞ varies from 0.02 to 0.35 is considered. The results are compared with the data calculated for the shock layer when argon flows around thermally insulated cylinders. The main features and regularities of the relaxation of the translational degrees of freedom of the gas for external and jet flows are considered. Data are presented on the form of the distribution function over velocities and its evolution as gas moves through the shock layers. Zh. Tekh. Fiz. 68, 13–18 (July 1998)     

Interaction of shock waves with a jet wake during gas injection into supersonic stream

In the present paper, we report the results of an experimental study of the interaction region of a planar compression shock produced by a wedge in stream with the wake formed behind a cocurrent gas jet (H₂, air, or Ar) injected into the flow. Depending on the gas jet parameters, three modes of interaction could be distinguished: a strong interaction, observed when the flow velocity in the wake was subsonic; a moderate interaction, observed when a subsonic flow region, bounded by a shock of almost conical shape, formed in the vicinity of the compression shock; and a neutral interaction. Three-dimensional non-stationary Euler equations were solved to numerically examine the interaction of an axisymmetric jet with an oblique shock wave. The obtained interaction regimes were found to be in a reasonable agreement with experimental data.     

激波/边界层干扰对等离子体合成射流的响应特性

利用高速纹影系统和数值模拟方法研究了激波/边界层干扰对逆流喷射的等离子体合成射流的响应特性,并揭示了流动控制机理.实验在来流马赫数Ma=3.1的风洞中进行,测试模型采用钝头体和压缩斜坡的组合模型,等离子体合成射流激励器安装在钝头体头部.纹影系统捕捉了放电频率为f=1 kHz和f=3 kHz的激励对附体激波形态和分离激波运动的控制效果.等离子体合成射流使压缩斜坡激波/边界层干扰区域的起始点向下游移动,分离泡尺寸减小,附体激波强度减弱,发生弯曲,再附点移向上游,与此同时分离激波向附体激波逼近.与f=3 kHz激励相比,f=1 kHz激励的射流流量更大,对激波/边界层干扰的影响范围更广、控制效果更好.通过数值模拟,揭示了射流与来流相互作用对下游流场的作用机理:射流与来流相互作用诱导出大尺度旋涡,大尺度旋涡耗散发展增强了近壁面流场的湍流度;压缩斜坡上游近壁面的流场性质发生变化,进而导致了压缩斜坡激波/边界层干扰区域流动的变化.     

Pulsating jet control for manipulating the separation bubble behind the fence

Abstract  

An optimization study of a pulsating jet was performed to manipulate the separation bubble behind the fence. The experiments were carried out in a circulating water channel and the vertical fence was submerged in the turbulent boundary layer. The parameters used for controlling the pulsating jet included the frequency, speed and velocity profile of jet, and the geometries including angle and location of nozzle. In addition, we investigated the effect of upstream boundary layer changed by continuous suction in front of the jet nozzle. Each test case was divided into 20 phases and phase-averaged DPIV method was applied to measure the velocity field. From the results, the main mechanism of reduction of bubble length was the vortex shedding from the recirculating region. The specific jet condition and nozzle geometries which were effective to reduce the separation bubble behind the fence were found.     

矩形喷口欠膨胀超声速射流对撞的实验研究

在不同喷口间距和射流压力下开展了矩形喷口欠膨胀超声速射流对撞实验并与自由射流进行了对比. 实验表明:超声速射流对撞的辐射噪声中存在四种不同的啸音模式, 且随喷口距离和射流压力的变化在不同模式间切换. 在射流压力大于0.5 MPa且喷口间距小于50 mm时, 射流对撞面在两个喷口外形成两道正激波之间, 啸音基频维持在3 kHz左右. 随喷口间距的增大或射流压力的降低, 射流对撞面在一侧喷口外的弓形激波与另一侧喷口外的正激波之间. 对撞面也有可能出现在两个弓形激波之间, 对应的啸音基频约为9 kHz, 但容易受扰动而回到喷口一侧或是在喷口之间大幅度振荡. 当射流压力小于0.36 MPa且喷口间距大于70 mm后, 对撞面在两个喷口之间大幅度振荡, 产生基频在1 kHz左右并随射流压力的降低和喷口间距的增大而降低的啸音. 关键词： 超声速射流 啸音 射流对撞 激波     

Jet produced by collapse of conical free surface of a metal by plane shock wave

Theoretical and experimental investigations of metal-jets, produced by interaction of a plane shock wave with a conical cavity at free surface of metals like aluminium, copper and mild steel, have been carried out and an attempt has been made to correlate the jet velocity with shock parameters. The velocity of the jet, obtained from the cavity of different half angles have been mostly measured with oscillographic technique. The experimental results agree with theory within experimental variations. The velocity of the jet increases monotonically with the particle velocity but decreases with increase in the angle of the cavity.     

Characteristics of a magnetogasdynamic diffuser under different modes of electric current switching

This paper elaborates upon a previous investigation into the influence of external electric and magnetic fields on a flow through a supersonic diffuser. The aim of the present study is to correlate a change in the configuration of a shock wave emerging near the diffuser inlet at magnetohydrodynamic interaction with the amount of force and energy actions and with total pressure losses. For this purpose, the main parameters of the shock wave structure and the total pressure are measured at the diffuser outlet when the flow is subjected to magnetic and electric fields of various strengths at different routes of current passage. In the experiments, a shock tube with a supersonic nozzle is employed. The shock tube forms a flow behind the shock wave reflecting from the end of the tube, which terminates in the nozzle. The diffuser is located directly downstream of the nozzle. The investigation is carried out in xenon. The flow is subjected to external fields at the inlet of the diffuser. The shock wave structure is visualized by frame sweeping of Schlieren patterns of the flow. The total pressure is measured with a piezoelectric transducer located at the end of the channel. The results obtained make it possible to optimize the action on the flow in terms of power consumption and total pressure losses for a given design of the diffuser.     

Flow structures around an inclined substrate subjected to a supersonic impinging jet in laser cutting

In laser cutting, the flow structure around a substrate significantly affects the material removal rate, the cutting depth and the surface finish of the cutting front. In this paper, the phenomena of shock wave that is induced by a supersonic impinging jet emanating from a straight nozzle onto a substrate with varying inclined angles has been simulated numerically and visualized experimentally. The numerical model offers fairly good prediction in comparison with the experiments. It transpires that the angle of inclination has a significant and dramatic effect on the flow structure and that a large wall pressure with a steep gradient can be built up when the angle is large.     

Control of spray spot in cold spray technology. Part 1. Gas dynamic aspects

This paper presents a study of supersonic jets formed by approaches that are new for cold spray technique: the main flow is swirled, the nozzles with permeable profiles and with exit slots on the supersonic section are engineered. The flow swirling achieved in the nozzle prechamber retains downstream to substrate surface. The system of vortices created within the permeable nozzles changes the shock wave features of the overexpanded jet and the geometry of the bow shock wave ahead of the substrate surface. These new features of flow may affect particle motion and particlesubstrate interaction under the conditions of cold spray process; this offers tools for obtaining the necessary shape of a spray spot.     

Numerical investigation of flow over two sweepback wedges at M = 4 and 6

Results of numerical simulation are discussed: simulation was carried out for a configuration of two wedges with sweepback leading edges placed on a pre-compression ramp in a way that skewed surfaces of the wedges deflect the compressed flows in the opposite directions. It was demonstrated that this configuration produces a flow with irregular interaction in the plane of symmetry for shock waves produced by sweepback wedges. The shock waves formed by the skew wedges induce 3D boundary layer separations along sweepback leading edges of the wedges. Flows in the separation zones are directed toward the plane of symmetry of this configuration; they interact and produce in the central part a “swollen” zone of separation flow with a typical S-shaped profile of velocity. Simulation data was obtained for the free stream flow with Mach number M = 4 and 6 and based upon Navier—Stokes equations and k-ωSST turbulence model using FLUENT computation code. Inviscid flow described by Euler equations was considered as well.     

不同来流马赫数下激光能量形成高超声速进气道虚拟唇口的特性

用基于三维非定常可压缩雷诺平均Navier-Stokes方程的有限体积法计算了马赫数低于设计值6时一种高超声速进气道的性能参数,发现其性能存在明显下降。为提升进气道性能,将功率为15kW的激光能量注入进气道固体唇口前的流场中,形成虚拟唇口,马赫数为4.5,5.0和5.5时,计算得到来流捕获率分别提高了34%,20.6%和15.6%。绘制了不同马赫数下来流捕获率达到峰值时的流场压强云图,说明了虚拟唇口的特性及形成机制。结果表明:来流马赫数越低,来流捕获率越小,但相对于无能量注入时的来流捕获率的提升程度越明显;在不同来流马赫数条件下,通过改变激光能量引致的激波结构和位置,可达到最优状态,即激波与进气道前缘斜激波相交后的透射波打在进气道肩部位置的状态。     

Near-field acoustic characteristics of screech jet exhausted from a nozzle with a hard reflecting plate

The standing wave in the near field of the screech jet exhausted from a nozzle with a hard plate works on the jet flow as the forcing wave by the location of a reflecting plate, and then jet flow is considered to be changed. Moreover, the reflector location from the nozzle changes the sound pressure contours of the near field. Intensity maps of the screech tone which indicate the propagation to the jet axial direction or the radial direction of the jet by the presence of the reflector plate have not been explored. In the present paper, acoustic characteristics in the near field of the screech tone with the reflecting plate are studied using an optical wave microphone, which can measure the sound propagating for both vertical and horizontal directions to the jet axis. As a result, the standing wave in the near field of the screech jet with the reflector has two types: One is the standing wave between the hydrodynamic pressure fluctuation propagating jet downstream and the sound pressure propagating upstream, and the other is the standing wave by the difference between the wavelength of the sound wave and the wavelength at the place close to the jet.