Properties of vortices in the self-similar turbulent jet

Instantaneous, two-dimensional velocity measurements were conducted in the axial plane of a self-similar turbulent axisymmetric jet. The velocity fields were high-pass filtered to expose the vortical structures. An automated method was used to identify the radial and axial coordinates of the vortex centers and rotational sense, and to measure their size, circulation, vorticity, and energy. New insights into turbulent jets are obtained by plotting statistical distributions for vortex properties as functions of Reynolds number and radial position. While the probability of finding a vortex is uniform up to the edge of the jet, the strongest eddies in the high-pass filtered field occur near the jet axis. The average circulation is directly proportional to the vortex size. The Reynolds number strongly affects the average vorticity, circulation, and energy of the eddies. However, the normalized curves show a good collapse implying that the jet is indeed self-similar. Results for the left and right half-planes of the jet are also presented. Interestingly, we find that contrary to customary drawings of jet flows, a substantial number of both clockwise and counter-clockwise rotating eddies exist on both sides of the jet axis, with almost equal numbers of oppositely rotating vortices close to the jet axis. Further, the disparity in the number of oppositely rotating eddies in each half-plane increases with the eddy size. Nevertheless, these results are consistent with the well-known radial vorticity distribution of axisymmetric jets.     

Sound generation by interacting vortices in a subsonic turbulent jet

The results of an experimental investigation of sound generation by the interacting toroidal and oblique vortices formed in a subsonic turbulent jet under the action of saw-tooth finite-amplitude sound waves with simultaneous longitudinal internal and transverse external excitation are presented (the sound pressure level on the jet edge is 165–175 dB). The direct schlieren method with an exposure time −3·10⁻⁷ s was employed. This made it possible to visualize not only the jet and the vortices formed in it but also the sound waves. It was confirmed experimentally that the vortex interaction may be accompanied by sound generation of fairly high intensity. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 54–60, January–February, 2000.     

Population,characteristics and kinematics of vortices in a confined rectangular jet with a co-flow

Vortex behavior and characteristics in a confined rectangular jet with a co-flow were examined using vortex swirling strength as a defining characteristic. On the left side of the jet, the positively (counterclockwise) rotating vortices are dominant, while negatively rotating vortices are dominant on the right side of the jet. The characteristics of vortices, such as population density, average size and strength, and deviation velocity, were calculated and analyzed in both the cross-stream direction and the streamwise direction. In the near-field of the jet, the population density, average size and strength of the dominant direction vortices show high values on both sides of the center stream with a small number of counter-rotating vortices produced in the small wake regions close to jet outlet. As the flow develops, the wake regions disappear, these count-rotating vortices also disappear, and the population of the dominant direction vortices increase and spread in the jet. The mean size and strength of the vortices decrease monotonically with streamwise coordinate. The signs of vortex deviation velocity indicate the vortices transfer low momentum to high-velocity region and high momentum to the low velocity region. The developing trends of these characteristics were also identified by tracing vortices using time-resolved particle image velocimetry data. Both the mean tracked vortex strength and size decrease with increasing downstream distance overall. At the locations of the left peak of turbulent kinetic energy, the two-point spatial cross-correlation of swirling strength with velocity fluctuation and concentration fluctuation were calculated. All the correlation fields contain one positively correlated region and one negatively correlated region although the orientations of the correlation fields varied, due to the flow transitioning from wake, to jet, to channel flow. Finally, linear stochastic estimation was used to calculate conditional structures. The large-scale structures in the velocity field revealed by linear stochastic estimation are spindle-shaped with a titling stream-wise major axis.     

Hydromagnetic capillary instability of a liquid jet with an axial flow

The hydromagnetic capillary instability of a jet of inviscid, impressible fluid of infinite electrical conductivity and subjected to a uniform axial magnetic field is studied, taking into account an axial flow in the jet. The results show that while the axial flow promotes instability due to capillary effects and the axial-flow effects can be completely suppressed by a magnetic field of sufficient strength.     

General solutions of axial symmetrical ring shells

This paper gives the general solutions of axial symmetrical ring shells for all values of slenderness ratio. This solution is newly brought out, and can be used to solve various practical problems, including corrugated tubes, thermal expansion joints, Borden tubes, etc.     

Interaction of a shaped-charge jet with a target possessing an axial orifice

Interaction of a shaped-charge jet with a target possessing an axial orifice is studied experimentally. For an orifice diameter approximately equal to 0.2D, where D is the shaped-charge diameter, the shaped-charge penetration depth is found to be substantially reduced owing to deviation of the shaped-charge jet axis from the shaped charge axis because of imperfections of the manufacturing technology. A diameter of the target orifice providing unconstrained penetration of the shaped-charge jet is determined. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 13–16, November–December, 2008.     

Absolute inviscid instability of a ring jet with back-flow and swirl

The absolute instability of a ring jet with back-flow and swirl has been investigated on the basis of the inviscid linearized theory for incompressible flow. An axisymmetric disturbance mode is found to be only convectively unstable. The first azimuthal mode can become absolutely unstable, if the ring jet has a back-flow on the jet axis, and an additional swirl can increase the instability. However, for large swirl the absolute instability is suppressed. A ring jet without back-flow becomes absolutely unstable only in the presence of swirl.     

Lock-in of vortices in the wake of an elevated round turbulent jet in a crossflow

An experimental investigation of a round turbulent jet emitted perpendicularly from a pipe (or stack) into a crossflow was conducted by means of multi-point simultaneous velocity measurements. The motivation was to extract and characterize the underlying coherent structures within the near wake region of the flow. The velocity signals were obtained with both normal and X hot-wire anemometer probes and analyzed with correlation, spectral and pattern-recognition techniques. The results establish that Kármán-like vortices are shed not only by the stack but also by the jet. These structural features are locked-in and are controlled by the stack diameter.This research was supported by the Natural Sciences and Engineering Research Council of Canada through Grant A-2746. We would also like to thank J.A. Ferré and F. Giralt for the use of their pattern-recognition programs.     

Viscous dissipation of a power law fluid in axial flow between isothermal eccentric cylinders

We study the temperature distribution of a power law fluid in a pressure-driven axial flow between isothermal eccentric cylinders in bipolar cylindrical coordinates. We begin our analysis by writing the equation of energy in bipolar cylindrical coordinates. We then obtain a dimensionless algebraic analytic solution for temperature profiles under a steady, laminar, incompressible and fully developed flow [Eq. (64)]. We find that the dimensionless temperature profile depends upon the radius ratio of the inner to outer cylinders, the eccentricity, the angular position, and the power law exponent n. The temperature is a strong function of the gap between the cylinders. The temperature profiles are flat in the middle of the gap and then, near the wall, suddenly drop to the wall temperature.     

Jet column modes in both a plane jet and a passively modified plane jet subject to acoustic excitation

Use is made of acoustic excitation to enhance the coherent vortical structures that form in both a plane jet and a passively modified plane jet. The Strouhal number for the preferred mode based on the nozzle height of the jet is found to be unaffected by the passive modification. The existence of the jet column mode of stable pairing in the excited plane jet is unambiguously confirmed. Measurements in the excited and modified plane jet show that the passive modification suppresses this mode. Finally, the work shows the existence of a new mode of stable pairing. Both time- and phase-averaged measurements are presented to examine this mode.     

Combination interaction of Taylor–Goertler vortices in a curved shear layer of a supersonic jet

Problems of origination and evolution of streamwise vortex structures in an initial region of the shear layer of a supersonic jet are discussed. Streamwise vortices are generated with the use of artificial microroughnesses on the internal surface of polished nozzles. Results of Pitot pressure distributions measured in a supersonic nonisobaric jet both in the radial and azimuthal directions are presented. Streamline curvature in the initial region of supersonic nonisobaric jets has a significant effect on evolution of streamwise vortex structures. Azimuthal heterogeneity corresponding to streamwise vortices in the shear layer is analyzed with the use of both the Fourier analysis and wavelet analysis. PACS 47.40.Ki, 47.20.Ft, 02.30.Nw     

Seeding of helical modes in the initial region of an axisymmetric jet

Active control of fundamental two- and three-dimensional amplified modes in an axisymmetric jet is presented. This is done by introducing localized acoustic disturbances produced by an azimuthal array of miniature speakers placed in the close proximity of the jet lip on the exit face. The independent control of each speaker output allows different azimuthal amplitude and phase distributions of periodic input pressure disturbances. Coupled with this was the development and use of a circular smoke-wire for visualizing shear layer modes around the complete jet circumference.     

轴向脉冲磁场增强金属射流侵彻穿深能力的因素

为掌握轴向脉冲磁场对金属射流的作用规律,基于破甲弹金属射流对目标的作用原理及磁场与金属射流之间的相互作用关系,设计了励磁线圈对破甲弹金属射流作用的试验系统,并开展了相关实弹试验,突破了金属射流形成和励磁线圈脉冲电流产生之间时序匹配的关键技术,得出了使金属射流发生有效变形的合理储能电容器组电参数和励磁线圈结构参数。试验结果表明,当电容器电压为5 kV、电容为1 200 μF、励磁线圈长度为50 mm时,破甲弹金属射流对目标靶板的侵彻穿深增量最大,破甲效果最佳;各因素对破甲弹金属射流侵彻穿深能力影响程度由大到小分别是电容器组充电电压、电容器组电容、线圈长度。研究成果为破甲弹威力电磁增强技术可行性论证、原理试验及励磁线圈结构设计提供了重要的理论和技术支撑。     

NND schemes and numerical simulation of axial symmetric free jet flows

Through a study on one-dimensional Navier-Stokes equations, it was found that the spurious oscillations occuring near shock waves with finite difference equations are related to the dispersion term in the corresponding modified differential equations. If the sign of dispersion coefficient is properly adjusted so that the sign changes across shock waves, the undesirable oscillations can be totally suppressed. Based on this finding, the non-oscillatory, containing no free parameters and dissipative shheme (NND scheme) is developed. This scheme is one of “TVD”. The axisymmetric free jet flows are simulated numerically using this scheme. The results obtained by the present scheme are compared with the experimental picture. It is shown that the agreement is very good, and that this scheme has advantages of high resolution for capturing shocks and contact discontinuities. Project supported by National Science Foundation of China     

Formation of a jet and vortices behind a shock wave reflected from a concave body

A jet and vortices have been observed when a plane shock wave reflects from a concave body in a shock tube. If the cavity is deep enough then two reflected shocks appear near its edges. Air, carbon tetrafluoride (CF) and dichlorodifluoromethane (CClF) were chosen as test gases. The flow was visualized with the aid of a conventional shadow technique. Pressure measurements at the body surface were also obtained. Numerical studies have been conducted using a two-dimensional inviscid model. There is a good qualitative agreement between the experimental and numerical results. Received 8 February 1996 / Accepted 30 June 1997