Reconnection of vortex tubes

The mechanism of vortex reconnection is investigated by solving the Navier-Stokes equation numerically starting with a trefoiled closed knotted vortex tube. A new type of vortex reconnection mechanism — bridging — is observed. Small regions of high-vorticity burst out of the vortex tube. grow up and bridge different portions of the tube. A relation between the change of the helicity and the mechanism of the vortex reconnection is discussed.     

Effect of the conical-shape on the performance of vortex tube

The present study focuses on the effect of conical shape in the cold side of the Ranque-Hilsch vortex tube which is shown to have a considerable influence on the system performance. A vortex tube is a simple circular tube with no moving parts which is capable to divide a high pressure flow into two relatively lower pressure flows with temperatures higher and lower than the incoming flow. A three-dimensional computational fluid dynamic model is used to analyse the mechanisms of flow inside a vortex tube. The SST turbulence model is used to predict the turbulent flow behaviour inside the vortex tube. The geometry of a vortex tube with circumferential inlet slots as well as axial cold and hot outlet is considered. Performance curves temperature separation versus cold outlet mass fraction are calculated for a given inlet mass flow rate and varying outlet mass flow rates.     

The effect of vortex angle on the efficiency of the Ranque–Hilsch vortex tube

A Ranque–Hilsch vortex tube is a long hollow cylinder with tangential nozzle placed near one end for injection of compressed air. The flow inside the vortex tube can be described as rotating air, which moves as a helical vortex flow. The peripheral flow moves toward the hot end, where the central part of the tube is blocked by a plug. The axial flow, which is forced back by the central part of the hot end plug, moves in the opposite direction toward the cold end. This paper focuses on the effect of the angle of rotating flow on the performance and efficiency of the Ranque–Hilsch vortex tube. To find the effect of vortex angle, different vortex angle generators were used and the best configuration was found.     

Experimental investigation of the flow characteristics within a vortex tube with different configurations

The energy separation in a vortex tube is a combined result of different factors and its explanation remains debatable. As a classical fluid mechanics phenomenon, understanding of the complex helical flow mechanism within a vortex tube is a necessary foundation. The small scale of an industrial vortex tube and the extremely complex flow conditions are the two main challenges in obtaining the internal flow properties. This paper reports the results of an experimental investigation on the flow behaviour within a confined cylindrical system having different configurations corresponding to the actual flow field in a vortex tube at different conditions. Transparent devices were used to enable flow visualisation and Particle Image Velocimetry (PIV) measurement. The results of the flow visualisation and PIV experiments show that a precessing vortex core is significant only in a specific range of swirling strength. A good agreement between the observed flow characteristics and previously published results was observed.     

Visualization of the flow structure in a vortex tube

The temperature separation in a vortex tube has been investigated for the purpose of exploring the phenomenon and improving the tube performance. Different explanations for the temperature separation have been proposed. However, there has not been a consensus in the hypothesis.This paper reports on a study in progress exploring the flow structure in a vortex tube. Flow visualization, using water as a working fluid, is used to reveal the existence of multiple circulation regions within the vortex tube and a new hypothesis describes the temperature separation mechanism. This research contributes to the understanding of the flow behavior in a vortex tube and supports the previous works that show the generation of the cold component of the flow is the result of the expansion near the cold nozzle and the hot component is produced due to the friction between the layers of flow.     

Vortex flows of a viscous heat-conducting gas in a slightly divergent tube with volume energy supply

The results of a numerical investigation of viscous vortex flow in a slightly divergent tube with thermal energy supplied to the flow are presented. The initial stage of vortex flow development is considered for two different longitudinal velocity distributions simulating the velocity profiles in jet-like and wake-like vortex flows in the vicinity of the vortex axis. The first type of flow can be considered as a model for the near-axis region of the vortex formed in the flow around a delta wing at incidence. The second type can serve as a model for the near-axis region of the trailing vortex downstream of a high-aspect-ratio wing. The development of the two flows is studied for a constant area tube, a slightly divergent tube, and in the case of thermal energy supply from a volume energy source at a constant wall temperature.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 90–97, September–October, 1996.     

Experimental analysis on vortex tube energy separation performance

The present study reports the effect of several operating parameters on the thermal performance of the vortex tube. The experimental results indicate that the inlet pressure and the cold fraction are the most significant parameters influencing the vortex tube performance. The experimental data point out that insulation has minimal effect on the vortex tube performance. The same inlet pressure tests show that energy separation increases as number of inlet nozzle increases.     

A study on the optimization of the angle of curvature for a Ranque–Hilsch vortex tube,using both experimental and full Reynolds stress turbulence numerical modelling

The working tube is a main part of vortex tube which the compressed fluid is injected into this part tangentially. An appropriate design of working tube geometry leads to better efficiency and performance of vortex tube. In the experimental investigation, the parameters are focused on the working tube angle, inlet pressure and number of nozzles. The effect of the working tube angle is investigated in the range of θ = 0–120°. The experimental tests show that we have an optimum model between θ = 0 and θ = 20°. The most objective of this investigation is the demonstration of the successful use of CFD in order to develop a design tool that can be utilized with confidence over a range of operating conditions and geometries, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the field of new applications and industries. A computational fluid dynamics model was employed to predict the performances of the air flow inside the vortex tube. The numerical investigation was done by full 3D steady state CFD-simulation using FLUENT6.3.26. This model utilizes the Reynolds stress model to solve the flow equations. Experiments were also conducted to validate results obtained for the numerical simulation. First purpose of numerical study in this case was validation with experimental data to confirm these results and the second was the optimization of experimental model to achieve the highest efficiency.     

Short-wave stability of a helical vortex tube: the effect of torsion on the curvature instability

In this paper, we study the short-wave stability of a helical vortex tube. The base flow field inside the helical vortex tube is obtained by perturbation expansion assuming that ${\epsilon}$ , the ratio of the core to curvature radius of the helical tube, be small. After reviewing our recent results obtained by the short-wavelength stability analysis, the normal-mode stability analysis is carried out to confirm the results and pursue further details. It is shown that the helical vortex tube suffers from curvature instability found for vortex rings. The growth rate of the curvature instability is obtained both analytically and numerically. The effect of torsion and rotation on the stability appears at the second order of ${\epsilon}$ . It is shown that the effects of rotation on the growth rate found by normal-mode stability analysis converges to those found by short-wavelength stability analysis in the short-wave limit.     

Numerical study of interaction of two vortex rings

Three-dimensional interaction of vortices with finite core is investigated numerically using the Rosenhead-Moore approximation. As the computational scheme in this approximation cannot deal with the structure of the vortex core, a bundle of vortex elements is employed to represent a single physical vortex tube. After the validity of this method was confirmed by comparing the numerical result for a single vortex ring composed of various number of the elements with the analytical solution, two cases of interaction of two vortex rings were studied. The first case is two vortex rings traveling along a common axis, and the second is two vortex rings moving side by side along parallel axes. Comparison with the experiments showed good agreement.     

Topological structures of vortex and helicity analysis

In this paper, we discuss the topological structures of the vortex filaments and vortex tubes with an exact solution of a straight spiral vortex tube. We find that there are some confusions about the calculation of the helicities of a knotted vortex filament and some linked vortex filaments by using different methods. We explain how to unify these methods and give the right results. This work is supported by the National Basic Research Important Project “Nonlinear Science”     

激波与涡对相互作用的实验研究

在方截面激波管中进行了平面运动激波和涡对的二维相互作用实验，研究了激波与同向涡对、激波与反向涡对相互作用的非定常过程．根据实验照片，分析讨论了作用过程中激波的变形，二次激波和三波点的形成、演变，激波与激波的相互作用，以及旋涡结构的变化等．实验表明，激波通过涡核时，激波发生剧烈变形，旋涡强度增大，涡核形状改变．     

Application of PIV to velocity measurements in a liquid film flowing down an inclined cylinder

PIV technique is applied for measurements of instant velocity distributions in a liquid film flowing down an inclined tube in the form of a wavy rivulet. An application of special optical calibration is applied to correct distortion effects caused by the curvature of the interface. A vortex flow of liquid is observed inside a wave hump in the reference system moving with wave phase velocity. Conditionally averaged profiles of longitudinal and transverse components of liquid velocity are obtained for different cross-sections of developed non-linear waves. It is shown that the increase in wave amplitude slightly changes the location of the vortex center. The analysis of modification of vortex motion character due to wavy flow conditions, such as tube inclination angle, film Reynolds number, wave excitation frequency, is fulfilled.     

Dynamics of the vortex structure of a jet impinging on a convex surface

Smoke–wire flow visualization is used to investigate the behavior of a round jet issuing from a straight tube and impinging on a convex surface. Video analysis of the impinging jet shows the initiation and growth of ring vortices in the jet shear layer and their interaction with the cylindrical surfaces. Effects of relative curvature, nozzle-to-surface distance, and Reynolds number on vortex initiation, vortex separation from the surface and vortex breakup are described. Examples of vortex merging are discussed.     

Wing-type vortex generators for fin-and-tube heat exchangers

The effect of wing-type vortex generators on heat transfer and pressure drop of a fin-and-tube heat exchanger element was investigated. Local heat transfer was measured by liquid crystal thermography on the fin in the Reynolds number range of 600–2700. Flow losses were estimated from the measured pressure drop of an element. Delta winglets were used as vortex generators. Four fin-and-tube configurations were tested, an inline and a staggered arrangement, each with plain fins and with fins with a pair of vortex generators behind each tube. For the inline tube arrangement the vortex generators increase the heat transfer by 55–65% with a corresponding increase of 20–45% in the apparent friction factor. Results indicate that the vortex generators have the potential to reduce considerably the size and mass of heat exchangers for a given heat load.     

Thin‐tube vortex simulations for sinusoidal instability in a counter‐rotating vortex pair

A thin‐tube vortex method is developed to investigate the intrinsic instability within a counter‐rotating vortex pair system and the effects from the core size and the wavenumbers (or wavelengths). The numerical accuracy and the advantages of the scheme are theoretically estimated. A nearest‐neighbour‐image method is employed in this three‐dimensional vortex simulation. Agreement with Crow's instability analysis has been achieved numerically for the long‐wave cases. A short‐wave instability for the zeroth radial mode of bending instability has also been found using the thin‐tube vortex simulations. Then, the combinations of long‐ and short‐wave instability are investigated to elucidate the non‐linear effects due to the interactions of two different modes. It is shown that instability is enhanced if both long‐ and short‐wave instabilities occur simultaneously. Although the method used in the paper is not capable of including effects such as axial flow, vortex core deformation and other complicated viscous effects, it effectively predicts and clarifies the first‐order factor that dominates the sinusoidal instability behaviour in a vortex pair. Copyright © 2002 John Wiley & Sons, Ltd.     

Chaotic streamlines in the flow of knotted and unknotted vortices

This paper describes the motion and the flow induced by a thin tubular vortex coiled on a torus. The vortex is defined by the number of turns, p, that it makes round the torus symmetry axis and the number of turns, q, that it makes round the torus centerline. All toroidal filamentary vortices are found to progress along and to rotate round the torus symmetry axis in an almost steady manner while approximately preserving their shape. The flow, observed in a frame moving with the vortex, possesses two stagnation points. The stream tube emanating from the forward stagnation point and the stream tube ending at the backward stagnation point transversely intersect along a finite number of streamlines. This produces a three-dimensional chaotic tangle whose geometry depends primarily on the value of p. Inside this chaotic shell there are two major stability tubes: the first one envelopes the vortex whereas the second one runs parallel to it and possesses the same topology. When p > 2 there is an additional stability tube enveloping the torus centerline.     

Flow and thermodynamic characteristics of energy separation in a double-circuit vortex tube - An experimental investigation

Experimental investigations were made into the vortex energy separation effect of gas flux in tubes known as the Ranque effect. Technical applications of the results are considered. Unlike traditional energy separation tubes, so-called double-circuit vortex tubes were chosen that allowed the creation of a refrigeration device with high thermodynamic efficiency. Different thermodynamic and gas dynamical aspects of the processes taking place in the scheme developed are discussed taking into account the modern vortex interaction hypothesis. The possibility of constructing a double-circuit vortex tube refrigeration machine as efficient as a gas expansion system is demonstrated.     

Interaction of Spontaneous Vortex Structure with a Flame Front

Spontaneous vortex formation in combustion products near a flame front propagating in a combustible mixture down a vertical tube has been studied experimentally. It is shown that under certain conditions, free convection suppresses or stimulates vortex perturbations at the flame front. It is found that periodic changes in vortex intensity depend on oscillations of the flame front. This dependence is controlled by the rate of heat exchange among the flame front, combustion products, and pipe walls. The vortex flow structure depends on the shape of the closed trajectory along which the leading point of the flame moves in the coordinate system attached to the vortex.     

Effect of acoustic resonance on the dynamic lift of tube arrays

The effect of acoustic resonance on the dynamic lift force acting on the central tube in square and normal triangle tube arrays is investigated experimentally. For each array pattern three different tube spacing ratios, corresponding to small, intermediate and large spacing ratios, are tested. The resonant sound field in the tube array is found to cause two main effects. First, it generates a “sound-induced” dynamic lift due to the resonant acoustic pressure distribution on the surface of the tube, and secondly, it synchronizes vorticity shedding from the tubes and thereby enhances the hydrodynamic lift force due to vortex shedding. The combined effect of these two unsteady lift forces depends on the phase shift between them, which is dictated by the frequency ratio of the acoustic mode to the natural vortex shedding frequencies. When the flow velocity is increased during the coincidence resonance range, the phase shift increases rapidly and therefore the effects of the two lift components change from reinforcing to counteracting each other. For the pre-coincidence lock-on range, the frequency ratio remains larger than unity and the two lift components always reinforce each other. Numerical simulations are also performed to compute the sound-induced lift force, and sound-enhancement coefficients are developed to estimate the effect of sound on the vortex shedding forces. The simulation and experimental results are implemented in a simplified design guide, which can be used to evaluate the dynamic lift forces acting on the tubes during acoustic resonances.