Flow and Heat Transfer in Underexpanded Air Jets Issuing from the Sonic Nozzle of a Plasma Generator

We present the results of an experimental investigation and numerical simulation of the gasdynamic structure of underexpanded dissociated-air jets and the heat transfer in these strongly nonequilibrium flows under the test conditions realized in the 100-kW electrodeless VGU-4 plasma generator of the Institute for Problems in Mechanics of the Russian Academy of Sciences (IPM RAS). The flow and heat transfer analysis is carried out on the basis of measurements of the static pressure in the plenum chamber, at the sonic nozzle exit, and on the low-pressure chamber wall, the stagnation pressure on the jet axis using a Pitot tube, and the heat transfer at the stagnation points of water-cooled models placed along the jet axis. The numerical simulation, based on complete Navier-Stokes equations, includes the calculation of (1) equilibrium air plasma flows in the discharge channel of the VGU-4 plasma generator; (2) underexpanded nonequilibrium dissociated-air jet outflow into the ambient space; and (3) axisymmetric jet flow past cylindrical models.     

旋流喷嘴中空旋转射流近区域流动的研究

从理论上分析了旋流喷嘴喷出的中空射流近区域的液膜的运动,在只考虑液膜表面张力的作用下,应用质量守恒和动量定理,建立了描述液膜运动的非线性常微分（积分）方程组,该方程组可以用数值方法方便地求解,结果表明,液体离开旋流喷嘴后在自由空间形成的液膜呈葫芦形状,其速度和液膜厚度等都周期性地变化。本结果是在液厝受拓动失称碎成液滴前的最基本运动状态,可以在射流的近区域内实验观察到,也是进一步从理论液膜破碎雾化过     

Vortex Behaviour and Velocity Characteristics of Jets Issuing from Hybrid Inclined Elliptic Nozzles

An experimental study on elliptic nozzles with hybrid flat- and inclined-sections is reported here. The hybrid flat and inclined sections are imposed along either the major- or minor-plane of the nozzles (HIN?A and B nozzles respectively). For HIN?A, results show prevalent pairings between adjacent vortex filaments and induced vortex-loops in the immediate vicinity of the flat-section to produce coalesced vortex roll-ups. Once they detach entirely from the nozzle, they proceed to undergo flow changes resembling that of conventional elliptic jets. HIN?B also leads to near-field vortex pairings but produce discrete inclined vortex roll-ups instead, with accompanying delay in rib structure formations. The roles of induced vortex-loops are significantly more limited in the present elliptic HIN than circular HIN investigated previously, due to the dominance of elliptic braid vortices. HIN?A produces significantly larger centerline velocity decay, as well as higher turbulence levels in the near vicinity of the nozzle exit. Vectoring of axial jet momentum is more apparent for HIN?B, where cross-stream entrainment is also relatively larger. Half-jet width results also demonstrate that both nozzle types eventually produce elliptic jets that undergo axis-switching. Lastly, momentum thickness results suggest that the present nozzle lip-modifications significantly enhance mixing characteristics along the plane upon which they are imposed in both HIN?A and B.     

展向喷嘴湍流横向射流大涡模拟及其统计分析

采用大涡模拟方法数值模拟了展向椭圆喷嘴的湍流横向射流,对其大尺度结构的时空演化和湍流脉动速度场的时间序列分析、频谱分析、PDF分析以及时、空截面上的统计平均特性进行分析.结果表明,在射流出口附近的下游核心区中速度脉动剧烈,显现出明显的湍流特征.除了三维涡环脱落、扭曲、变形、摆动所对应频率之外,还存在很宽的湍流基频,它与在喷嘴出口附近产生的三维涡环的时空演化过程密切相关.由于展向椭圆喷嘴的湍流横向射流中的三维涡环快速脱落和强相互作用导致射流尾迹中的强湍流脉动,展向椭圆喷嘴湍流横向射流的PDF空间演化特征结构复杂.在射流核心区的湍流偏应力变化平缓,其统计平均值分布接近左右对称.展向椭圆喷嘴的湍流横向射流脉动速度场具有极为复杂的统计行为,与流向椭圆喷嘴相比具有更好的掺混能力.     

扭矩转化器（T／C）内部叶片流道流场的二维PIV实验研究

本文利用二维ＰＩＶ技术研究了汽车无级调连装置中扭矩转化器内部叶片流道流场的复杂流动,实验结果表明泵和透平在不同转速下,静子叶片流道中的流场体现出不同的流动特征,文中还对实验误差进行了分析,提出了减少误差的措施,通过叶栅流动理论分析说明了本文结果的可靠性。     

Measurements of gravity and gravity-capillary waves in horizontal gas-liquid pipe flow using PIV in both phases

An experimental study was performed in stratified wavy flow of air and water through a horizontal pipe. The velocity fields in both phases were measured simultaneously using PIV and the interfacial shape was resolved using a profile capturing technique. The objective of the study was to investigate the interfacial characteristics and the velocities of the liquid and gas phases in two wave patterns: ‘3D small amplitude’ and ‘2D large amplitude’ waves. The wave patterns were shown to consist of gravity and gravity-capillary waves, respectively, with substantial differences in the wave characteristics and liquid velocities. Contrary to this, the effect of the waves on the gas velocities was rather similar in both wave regimes, with both wave regimes causing an increase in the velocity fluctuations close to the interface. The current measurements also produced a valuable dataset that can be used to further improve the numerical modeling of the stratified flow pattern.     

Control of the Strain Field Near a Crack in Elastic Layer

In the present paper, we consider the strain analysis near the crack parallel to boundary surfaces of a linear isotropic elastic layer of constant thickness. We pose the problem of minimizing the opening of crack faces when a normal force is applied to the boundary of the layer. The control secondary loading is a normal stress uniformly distributed over the boundary surfaces as to oppose the opening caused by the external applied force. We show that the most dangerous are tangential deformations, and the normal ones are always by many times less dangerous. __________ Published in Neliniini Kolyvannya, Vol. 8, No. 2, pp. 265–277, April–June, 2005.     

Helical Structures in the Near Field of a Turbulent Pipe Jet

We perform a finely resolved Large-eddy simulation to study coherent vortical structures populating the initial (near-nozzle) zone of a pipe jet at the Reynolds number of 5300. In contrast to ‘top-hat’ jets featured by Kelvin-Helmholtz rings with the non-dimensional frequency S t≈0.3?0.6, no high-frequency dominant mode is observed in the near field of a jet issuing from a fully-developed pipe flow. Instead, in shear layers we observe a relatively wide peak in the power spectrum within the low-frequency range (S t≈0.14) corresponding to the propagating helical waves entering with the pipe flow. This is confirmed by the Fourier transform with respect to the azimuthal angle and the Proper Orthogonal Decomposition complemented with the linear stability analysis revealing that this low-frequency motion is not connected to the Kelvin-Helmholtz instability. We demonstrate that the azimuthal wavenumbers m=1?5 contain the most of the turbulent kinetic energy and that a common form of an eigenmode is a helical vortex rotating around the axis of symmetry. Small and large timescales are identified corresponding to “fast” and “slow” rotating modes. While the “fast” modes correspond to background turbulence and stochastically switch from co- to counter-rotation, the “slow” modes are due to coherent helical structures which are long-lived and have low angular velocities, in agreement with the previously described spectral peak at low S t.     

Investigation of the Cross-beam Correlation Algorithm to Reconstruct Local Field Statistics from Line-of-sight Measurements in Turbulent Flows

Optical techniques such as speckle photography and rainbow schlieren deflectometry yield path integrated measurements of deflection angle instead of the local field variable such as density, temperature and/or species concentration. Thus, a reconstruction algorithm is employed to obtain the local properties from the path-integrated measurements. Cross-beam correlation (CBC) algorithm provides the link between path-integrated and local field statistics in time-averaged axisymmetric turbulent flows. Path-integrated statistics are obtained using orthogonal light rays crossing within the turbulent flow. The algorithm assumes local isotropy and negligible correlation between points on orthogonal beams, which is valid strictly in fully turbulent flows. In this study, noise-free synthetic scalar turbulence data are generated and used to determine how different assumptions in the CBC algorithm affect the reconstruction accuracy. Results show that the reconstruction accuracy is excellent for a narrow correlation function (or small integral length scale), while significant errors are incurred in case of a wide correlation function. A procedure to improve the reconstruction accuracy of the CBC algorithm is proposed in this study.     

Study of the Turbulent Velocity Field in the Near Wake of a Bluff Body

An experimental characterization of the turbulent flow structure formed downstream of a vertically mounted circular bluff body is performed. Three components of an instantaneous velocity field are measured using the stereo particle image velocimetry technique at the symmetry plane. The average velocity and the turbulent properties are analyzed. The results indicate a recirculation zone consisting of a toroidal vortex with similar dimensions for all Reynolds numbers. The largest turbulent fluctuations are found at the stagnation point region. The observed anisotropy of the normal Reynolds stress components is associated with the stagnation point flow, whereas the cross-correlation component extreme occurs in high strain rate regions. An analysis of the Reynolds tensor anisotropy using the Lumley triangle is performed, revealing that the largest departures from isotropy occur at high shear regions and also within the vortex.     

Evaluation of unsteady pressure fields and forces in rotating airfoils from time-resolved PIV

The final stages of transitional phenomena in laminar separation bubbles play a key role in their reattachment process, and they condition the boundary layer properties and flow structure after reattachment. In this experimental study, the evolution of the perturbation velocity spectra found in this zone is first presented, showing the nonlinear growth of instabilities in their path to develop fully turbulent spectra. The study of the average flow field allows the scaling of the reattachment region, both in its extension and in the characterization of the integral boundary layer magnitudes. Experimental laws are proposed for the evolution of the momentum thickness and of the shape factor. In addition, a universal, wake-like mean velocity profile is found shortly after the reattachment station. The phase-locked characterization technique allows measurements conditioned to the presence of a fluid event. This technique is used to track the evolution of large-scale structures, whose dynamics is seen to dominate the fluid behavior in the reattachment zone. The simultaneous existence of two vortex blobs is found to characterize this flow region, with the longest lived one being convected toward the wall and stretched. This process results in the fast breakdown of the large-scale vorticity structure and the sudden formation of 3-D, small scales that promote the rapid flow evolution toward a fully developed turbulent state.     

水击谐波流场中乳化油液滴粒径的PIV测量

为了掌握乳化油液滴在水击谐波流场中的碰撞、破裂、聚集和变形等微观形态的变化规律,采用粒子图像测速(PIV)技术对水平方管中乳化油液滴的水击谐波流场进行了测量,分析了在水击谐波流场中,不同激振力作用下乳化油液滴的粒径变化。测量结果表明,在水击谐波流场作用下,乳化油液滴平均粒径的增长率随着激振力的减小而减小,随着作用时间的增大呈增加趋势,直至粒径处于一种动态平衡;乳化液滴随着激振力增大到达波节聚集位置的时间减少,可见增大水击谐波激振力有利于乳化液滴的聚集并合并为大尺度的液滴,从而有效地提高了水击谐波流场作用下的油水分离效果。     

光束穿越不同介质时LDV的测试及其误差分析

本文应用平面几何及光学原理,从理论和实践上分析了应用激光多普勒测速仪(LDV)进行实际测量时,在有窗口的情况下应该注意的几个问题,并给出了测量点位置偏差修正的定量公式以及光轴方向上测速精度相对误差的评估方程,省略了繁琐的理论推导.     

In-situ Full Field Out of Plane Displacement and Strain Measurements at the Micro-Scale in Single Reinforcement Composites under Transverse Load

Experimental Mechanics - Micromechanics damage models applied to composites predict stresses and strains in the matrix and fibers as a function of the microstructure, constituting phases mechanical...     

2D PIV measurements in the near field of grid turbulence using stitched fields from multiple cameras

We present measurements of grid turbulence using 2D particle image velocimetry taken immediately downstream from the grid at a Reynolds number of Re _M ?=?16500 where M is the rod spacing. A long field of view of 14M?×?4M in the down- and cross-stream directions was achieved by stitching multiple cameras together. Two uniform biplanar grids were selected to have the same M and pressure drop but different rod diameter D and cross-section. A large data set (10⁴ vector fields) was obtained to ensure good convergence of second-order statistics. Estimations of the dissipation rate $\varepsilon$ of turbulent kinetic energy (TKE) were found to be sensitive to the number of mean-squared velocity gradient terms included and not whether the turbulence was assumed to adhere to isotropy or axisymmetry. The resolution dependency of different turbulence statistics was assessed with a procedure that does not rely on the dissipation scale η. The streamwise evolution of the TKE components and $\varepsilon$ was found to collapse across grids when the rod diameter was included in the normalisation. We argue that this should be the case between all regular grids when the other relevant dimensionless quantities are matched and the flow has become homogeneous across the stream. Two-point space correlation functions at x/M?=?1 show evidence of complex wake interactions which exhibit a strong Reynolds number dependence. However, these changes in initial conditions disappear indicating rapid cross-stream homogenisation. On the other hand, isotropy was, as expected, not found to be established by x/M?=?12 for any case studied.     

On the Accuracy of Elastic Strain Field Measurements by Laue Microdiffraction and High-Resolution EBSD: a Cross-Validation Experiment

Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10^{? 4}.     

Formation of Regions with High Energy and Pressure Gradients at the Free Surface of Liquid Dielectric in a Tangential Electric Field

The nonlinear dynamics of the free surface of an ideal incompressible non-conducting fluid with a high dielectric constant subjected to a strong horizontal electric field is simulated using the method of conformal transformations. It is shown that in the initial stage of interaction of counter-propagating periodic waves of significant amplitude, there is a direct energy cascade leading to energy transfer to small scales. This results in the formation of regions with a steep wave front at the fluid surface, in which the dynamic pressure and the pressure exerted by the electric field undergo a discontinuity. It has been demonstrated that the formation of regions with high gradients of the electric field and fluid velocity is accompanied by breaking of surface waves; the boundary inclination angle tends to 90?, and the surface curvature increases without bound.     

On the Distribution and Scaling of Convective Wavespeeds in the Shear Layers of Heated Supersonic Jets

The noise generated by supersonic plumes is of growing concern given the enormous peak noise intensity radiated by tactical aircraft engines. A key component of this noise is the enhanced radiation of mixing noise caused by large scale eddies convecting supersonically relative to the surrounding quiescent medium. As very little data exist for eddy convection in high Reynolds number, supersonic plumes, our current ability to develop concepts that alter compressible eddy convection is limited. Herein we present new experimental data of eddy convective wavespeeds in the developing shear layer of supersonic heated jets. A new scaling of the wavespeed in radial similarity coordinates is proposed which takes into account the influence of the ratio of static densities between the jet and ambient streams. In particular, we observe a structural change in wavespeed spectra at the end of the potential core—in addition to high turbulence levels, the potential core breakdown region can have enhanced eddy wavespeeds, increasing noise radiation efficiency. The results provide a first examination of the interplay of density ratio effects and the dynamic breakdown process of the potential core in supersonic jets—physics integral to the noise generation process.     

Stability of Shells of Revolution with Different Gaussian Curvature in the Field of Combined Static Loads

International Applied Mechanics - The stability of equilibrium states of shells of revolution with nonzero Gaussian curvature under combined axisymmetric loads is analyzed. The analysis is based on...