Control of spray spot shape in cold spray technology. Part 2. Spraying process

The present paper studies high-velocity heterogeneous flows produced with nozzle designs unconventional for cold spray; the process of coating deposition was studied under these conditions. The possibility of using unconventional gas dynamic tools (swirling of main flow, nozzles with permeable profiles and with slots in the supersonic part of nozzle) for control of particle distribution in the supersonic jet is investigated: this might be useful for obtaining a proper shape of the spray spot. These experiments offered a method of gas-dynamic design for the spray spot shape, which extends the possibilities of cold spray technique.     

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     

On the Global Existence and Stability of a Multi-Dimensional Supersonic Conic Shock Wave

We establish the global existence and stability of a three-dimensional supersonic conic shock wave for a compactly perturbed steady supersonic flow past an infinitely long circular cone with a sharp angle. The flow is described by a 3-D steady potential equation, which is multi-dimensional, quasilinear, and hyperbolic with respect to the supersonic direction. Making use of the geometric properties of the pointed shock surface together with the Rankine–Hugoniot conditions on the conic shock surface and the boundary condition on the surface of the cone, we obtain a global uniform weighted energy estimate for the nonlinear problem by finding an appropriate multiplier and establishing a new Hardy-type inequality on the shock surface. Based on this, we prove that a multi-dimensional conic shock attached to the vertex of the cone exists globally when the Mach number of the incoming supersonic flow is sufficiently large. Moreover, the asymptotic behavior of the 3-D supersonic conic shock solution, which is shown to approach the corresponding background shock solution in the downstream domain for the uniform supersonic constant flow past the sharp cone, is also explicitly given.     

平面定常超音速绕流问题的新的数值解法

用特征线法解平面定常超音速绕流问题虽然有效,但当激波很弱、几乎与特征线平行时则很难处理。用有限差分法计算此问题也比较复杂。本文把作者在文章中^[1]提出的新的数值解法,发展并应用到平面定常超音速绕流的问题。仍然采用了许为厚教授提出的新拉格朗日变量^[2],这使边界条件的提法大为简化。此新的数值解法按变量指标之和,一排排地往下计算,方法简单,可以处理各种形状物体的超音速绕流。本文对向上弯曲的抛物形固壁绕流向题的实例进行了具体计算。算出了激波的形状。当激波没有形成以前,相应的普朗特一迈耶气流是有准确分析解的,把数值解与准确解进行了l比较结果是满意的。     

Broadband shock noise reduction in turbulent jets by water injection

The concept of effective jet properties introduced by the author (Kandula M. Prediction of turbulent jet mixing noise reduction by water injection. AIAA J 2008;46(11):2714-22) has been extended to the estimation of broadband shock noise reduction by water injection in supersonic jets. Comparison of the predictions with the test data for cold and hot underexpanded supersonic nozzles shows a satisfactory agreement. The results also reveal the range of water mass flow rates over which saturation of mixing noise reduction and existence of parasitic noise are manifest.     

Supersonic laser spray of aluminium alloy on a ceramic substrate

Applying a ceramic coating onto a metallic substrate to improve its wear resistance or corrosion resistance has attracted the interest of many researchers during decades. However, only few works explore the possibility to apply a metallic layer onto a ceramic material. This work presents a novel technique to coat ceramic materials with metals: the supersonic laser spraying.In this technique a laser beam is focused on the surface of the precursor metal in such a way that the metal is transformed to the liquid state in the beam-metal interaction zone. A supersonic jet expels the molten material and propels it to the surface of the ceramic substrate.In this study, we present the preliminary results obtained using the supersonic laser spray to coat a commercial cordierite ceramic plate with an Al-Cu alloy using a 3.5 kW CO₂ laser and a supersonic jet of Argon.Coatings were characterized by scanning electron microscopy (SEM) and interferometric profilometry.     

The characteristics of surface arc plasma and its control effect on supersonic flow

The characteristic of surface arc plasma included millisecond and microsecond actuation in supersonic flow is investigated both experimentally and numerically. In the experiment, the discharge characteristic of surface arc plasma in quiescent air and supersonic flow is recorded. The stable oblique shock could be observed with millisecond actuation. And the unstable compressive wave could be also observed with microsecond actuation. In the numerical investigation, plasma actuation is defined as a source term with input power density from discharge V–I characteristic, which is expected to better describe the influence of heating process. The numerical results are coincident with experimental results. In order to confirm the capability of surface arc plasma actuation to control supersonic flow, experimental investigations on control shock induced by ramp and separation of boundary layer induced by impinging shock are performed. All the results demonstrate the control effect of surface arc plasma actuation onto supersonic flow.     

Acoustic disturbance from gas non-uniformities convected through a nozzle

The non-steady flow generated by convection of gas containing non-uniform temperature regions or “entropy spots” through a nozzle is examined analytically as a source of acoustic disturbance. The first portion of the investigation treats the “compact nozzle”, the case where all wave lengths are much longer than the nozzle. Strengths of transmitted and reflected one-dimensional waves are given for supersonic and subsonic nozzles and for one configuration of supersonic nozzle with normal shock at the outlet. In addition to a wave reflected from the nozzle inlet, the supersonic nozzle discharges two waves, one facing upstream and the other facing downstream. For reasonable values of the nozzle inlet Mach number, the pressure amplitude of each wave increases directly as the discharge Mach number.The acoustic perturbations from a supercritical nozzle of finite length, in which the undisturbed gas velocity increases linearly through the nozzle, are analyzed for several inlet and discharge Mach number values and over a wide frequency range. The results which agree with the compact analysis for low frequency, deviate considerably as the frequency rises, achieving pressure fluctuation levels of several times the compact values. It is shown that this result originates in a phase shift between the two waves emitted downstream and that the pressure fluctuations for moderate frequencies may be approximated from the compact analysis with an appropriate phase shift.In all cases, the pressure fluctuations caused by a 2% fluctuation in absolute inlet temperature are large enough to require consideration in acoustic analysis of nozzles or turbine blade channels.     

Compressible Flow and Transonic Shock in a Diverging Nozzle

The paper is devoted to the study of compressible flows and transonic shocks in diverging nozzles in the framework of the full compressible Euler system. Consider a nozzle having a shape as a diverging truncated sector with generic opening angle: if the upstream flow at the entrance is supersonic and is near to an axial symmetric flow, and if all parameters of the upstream flow and the receiver pressure at the exit are suitably assigned, then a transonic shock appears in the nozzle. To determine the transonic shock and the flow in the nozzle leads to a free boundary value problem for a nonlinear partial differential equation. We prove that the receiver pressure can uniquely determine the location of the transonic shock, as well as the flow behind the shock. Such a conclusion was conjectured by Courant and Friedrichs, and is confirmed theoretically in this paper for the divergent nozzles. The main advantage of this paper compared with the previous studies on this subject is that the section of the nozzle is allowed to vary substantially, while the transonic shock is not assumed to pass a fixed point. The situation coincides with the requirement in Courant-Friedrichs’ conjecture. To describe the compressible flow we use the full Euler system, which is purely hyperbolic in the supersonic region and is elliptic-hyperbolic in the subsonic region. Solving the free boundary value problem of an elliptic-hyperbolic problem forms the main part of this paper. In our demonstration some new approaches, including the introduction of a pseudo-free boundary problem and the corresponding relaxation, design of a delicate double iteration scheme, are developed to overcome the difficulties caused by the divergence of the nozzle.     

Influence of the adiabatic index on switching between different types of shock wave reflection in a steady supersonic gas flow

A comprehensive pattern of different types of shock wave reflection in a steady supersonic gas flow is analytically constructed with regard to a new wave configuration found by the authors-negative-angle irregular reflection. A double Mach reflection with a negative reflection angle in a steady supersonic gas flow is numerically obtained for the first time.     

Flow characteristics and micro-scale metallic particle formation in the laser supersonic heating technique

The characteristics of the supersonic flow of the laser heating technique for producing micro-scale metallic particles were investigated in this study. A numerical model was established to predict the flow fields and particle trajectories leaving a spray nozzle with shock wave effects. The compressible flow of the shock waves and the trajectories of particles in diameters of 1–20 μm were simulated and compared with the flow visualization. In the experiment, a pulsed Nd-YAG laser was used as heat source on a carbon steel target within the nozzle, and the carbon steel particles were ejected by high-pressure air. The result shows that the shock wave structures were generated at various entrance pressures, and there is a significant increase in the amount of carbon steel particles and the spraying angles by increasing the entrance air pressure.     

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.     

高风速下海洋飞沫水滴对拖曳系数的影响

海面飞沫水滴改变着海气动量通量的分布, 从而在相当程度上影响着海面拖曳系数. 为了能够精确估计海面飞沫水滴对海面拖曳系数的影响, 推导出新的依赖于风速和海浪状态的海洋飞沫生成函数, 将该飞沫生成函数用在改进的飞沫动量通量计算公式中进行飞沫动量通量的计算, 发现本文提出的飞沫动量通量计算公式对海浪状态具有较强的敏感性, 能够清晰地表达海浪状态对飞沫动量通量的影响. 海面总动量通量包含飞沫动量通量和海气界面动量通量, 基于此理论, 得到高风速下受飞沫水滴影响的海面拖曳系数关系式, 从关系式的理论值可知, 在高风速下, 海面拖曳系数开始衰减, 说明高风速下海面飞沫水滴能够抑制拖曳系数值的增加. 将理论结果与实验室和外海测量值进行对比, 发现海面拖曳系数的理论值变化范围覆盖了测量值, 同时将该海面拖曳系数代入海浪模式进行台风浪模拟, 发现海浪模式能够较好地模拟出有效波高, 说明本文提出的新的海面拖曳系数公式能够合理地用在高风速条件下.     

Research on the mechanics of underwater supersonic gas jets

An experimental research was carried out to study the fluid mechanics of underwater supersonic gas jets. High pressure air was injected into a water tank through converging-diverging nozzles (Laval nozzles). The jets were operated at different conditions of over-, full- and under-expansions. The jet sequences were visualized using a CCD camera. It was found that the injection of supersonic air jets into water is always accompanied by strong flow oscillation, which is related to the phenomenon of shock waves feedback in the gas phase. The shock wave feedback is different from the acoustic feedback when a supersonic gas jet discharges into open air, which causes screech tone. It is a process that the shock waves enclosed in the gas pocket induce a periodic pressure with large amplitude variation in the gas jet. Consequently, the periodic pressure causes the jet oscillation including the large amplitude expansion. Detailed pressure measurements were also conducted to verify the shock wave feedback phenomenon. Three kinds of measuring methods were used, i.e., pressure probe submerged in water, pressure measurements from the side and front walls of the nozzle devices respectively. The results measured by these methods are in a good agreement. They show that every oscillation of the jets causes a sudden increase of pressure and the average frequency of the shock wave feedback is about 5–10 Hz.     

Aerodynamic actuation characteristics of radio-frequency discharge plasma and control of supersonic flow

In this paper, aerodynamic actuation characteristics of radio-frequency(RF) discharge plasma are studied and a method is proposed for shock wave control based on RF discharge. Under the static condition, a RF diffuse glow discharge can be observed; under the supersonic inflow, the plasma is blown downstream but remains continuous and stable.Time-resolved schlieren is used for flow field visualization. It is found that RF discharge not only leads to continuous energy deposition on the electrode surface but also induces a compression wave. Under the supersonic inflow condition, a weak oblique shock wave is induced by discharge. Experimental results of the shock wave control indicate that the applied actuation can disperse the bottom structure of the ramp-induced oblique shock wave, which is also observed in the extracted shock wave structure after image processing. More importantly, this control effect can be maintained steadily due to the continuous high-frequency(MHz) discharge. Finally, correlations for schlieren images and numerical simulations are employed to further explore the flow control mechanism. It is observed that the vortex in the boundary layer increases after the application of actuation, meaning that the boundary layer in the downstream of the actuation position is thickened. This is equivalent to covering a layer of low-density smooth wall around the compression corner and on the ramp surface, thereby weakening the compressibility at the compression corner. Our results demonstrate the ability of RF plasma aerodynamic actuation to control the supersonic airflow.     

Determination of residual stresses within plasma spray coating using Moiré interferometry method

In this paper, residual stresses of the Ni-Cr-B-Si coatings prepared by supersonic plasma spray processing were measured by moiré interferometry and X-ray diffraction method. Moiré interferometry method was used in measuring the distribution of residual stresses of the Ni-Cr-B-Si coatings alongside the specimen thickness direction, then the distribution of residual stresses both in the substrate and the coating was also analyzed. Experimental results showed that residual stresses in the coating and the substrate are tensile and compressive separately; residual stresses of the coating are diminished with the increase of the distance from the coating surface and almost zero at the coating-substrate interface; the maximum of compressive residual stresses of the substrate are present to the vicinity of the coating-substrate interface. It could be concluded that residual stresses in the specimen would result from the dismatch of thermophysical properties between the coating and substrate during the spray process, and the distribution of residual stresses of the substrate would be influenced by the sandblasting prior to spraying.     

一类高压比新概念风扇转子激波结构探索

为适应未来超声速巡航飞机动力系统的要求,作为一种可能的解决方案,本文对于一类来流为轴向超声速、出口为相对亚声速的高压比新概念风扇开展了研究,主要采用三维定常粘性数值方法对转子通道内部激波结构进行了初步探讨,根据流场分布构造了转子三维激波结构,为此类风扇气动设计提供了一定的依据。     

超声速气膜冷却中激波抑制的研究

针对激波破坏超声速气膜冷却的机理,本文提出了一种壁面开孔的结构,数值研究结果表明：一方面,壁面开孔的结构能使激波作用的区域壁面附近的压力分布较均匀,从而使近壁区的马赫数分布比不开孔的壁面要高,有利于超声速气膜冷却。同时在激波的作用下,冷却气体可以通过开孔壁面的孔进入槽道内,而在槽内的下游再从孔里流出,保护下游的壁面,这...     

Dynamic characteristics of gas jets from subsonic and supersonic nozzles for high pressure gas laser cutting

During laser cutting of stainless steels, titanium and aluminum alloys, a coaxial and high pressure inert gas jet is used to improve the cut edge quality. The process normally consumes a large amount of inert gas and has a poor tolerance to variation in process parameters. This is solely because the gas nozzles are mostly of the conical and convergent type in which the gas jets are subsonic. Based on two dimensional steady state gas dynamic theory, computer simulation and shadowgraphic techniques, the gas jet patterns from conical nozzles and the newly designed supersonic nozzles are analyzed. The distribution of pressure, momentum, gas density and existence of shock waves are predicted and mapped. Based on these characteristics, the effect of the gas jets upon the cut quality is explained. It is concluded that a supersonic gas jet offers the best flow characteristics for high pressure laser cutting.     

Vorticity on the Surface of an Axially Symmetric Body behind a Detached Shock Wave

Doklady Physics - The vorticity on the surface of an axially symmetric body streamlined by a steady unswirling homogeneous ideal gas supersonic flow with a detached shock wave has been studied. The...