Kinds Of Vortices Research Articles

Dynamic Analysis of Cavitation Tip Vortex of Pump-Jet Propeller Based on DES

When a pump-jet propeller rotates at high speeds, a tip vortex is usually generated in the tip clearance region. This vortex interacts with the main channel fluid flow leading to the main energy loss of the rotor system. Moreover, operating at a high rotational speed can cause cavitation near the blades which may jeopardize the propulsion efficiency and induce noise. In order to effectively improve the propulsion efficiency of the pump-jet propeller, it is mandatory to research more about the energy loss mechanism in the tip clearance area. Due to the complex turbulence characteristics of the blade tip vortex, the widely used Reynolds averaged Navier–Stokes (RANS) method may not be able to accurately predict the multi-scale turbulent flow in the tip clearance. In this paper, an unsteady numerical simulation was conducted on the three-dimensional full flow field of a pump-jet propeller based on the DES (detached-eddy-simulation) turbulence model and the Z-G-B (Zwart–Gerber–Belamri) cavitation model. The simulation yielded the vortex shape and dynamic characteristics of the vortex core and the surrounding flow field in the tip clearance area. After cavitation occurred, the influence of cavitation bubbles on tip vortices was also studied. The results revealed two kinds of vortices in the tip clearance area, namely tip leakage vortex (TLV) and tip separation vortex (TSV). Slight cavitation at J = 1.02 led to low-frequency and high-frequency pulsation in the TLV vortex core. This occurrence of cavitation promotes the expansion and contraction of the tip vortex. Further, when the advance ratio changes into J = 0.73, a third type of vortex located between TLV and TSV appeared at the trailing edge which runs through the entire rotational cycle. This study has presented the dynamic characteristics of tip vortex including the relationship between cavitation bubbles and TLV inside the pump-jet propeller, which may provide a reference for the optimal design of future pump-jet propellers.

Suppression of vortex-induced vibration of single-box girder with various angles of attack by self-issuing jet method

Wind tunnel tests were conducted to investigate the effect of the self-issuing jet flow control on the vortex-induced vibrations (VIVs) of a streamline single-box girder at various angles of attack (AOAs). Spanwise periodic passive slits were symmetrically arranged on the windward and leeward lower inclined panels of the girder. The Reynolds number varied from 4.19×104 to 1.14×105, and the AOA varied from −5.0°to 5.0°with increments of 2.5°. The vibration response characteristics of the girder with different slit arrangements were discussed, and the interactions between the shedding Kármán vortices (KVs) and passive jets were investigated. The results show that the self-issuing jet control method could effectively suppress the VIV; the slits on the upper part of the lower inclined panel showed the best effect (Case I) at various AOAs. In addition, the large shedding KV were modified into two kinds of vortices: a reduced shedding KV and standing vortex at the trailing bottom corner of the girder. When the shedding vortex generation was displaced downstream, a second instability occurred in the near-wake. The self-issuing jets worked better at positive AOAs because more than one slit rows controlled the shedding vortices. Finally, the flutter performance of the single-box girder with self-issuing jets was investigated; Case I had a positive effect on the flutter behaviour at AOAs of −2.5°to 5°; the same flutter performance was observed at −5°.

The Heat Transfer Characteristics in Stepped Hole of Traditional Metal Materials Induced by Stepped Film Holes

With the increase of temperature requirement, the application of metal matrix materials is gradually reduced due to its poor temperature resistance. In order to improve the application of traditional metal based materials, new cooling technology must be developed to meet the application of metal based materials. In this paper, the cooling flow field of cylindrical hole and two kinds of stepped film holes based on metal materials is simulated by large eddy method(LES), the development of vortex structure and flow characteristics in the mixing area of coolant flow and mainstream are analyzed and studied. The distribution is strongly effected by stronger thermal convection compared with the new temperature resistant material. The results show that four kinds of vortices form downstream the film hole, namely, horseshoes vortex, shear layer vortex, hairpin vortex and the counter rotate vortex pair (CVP). The CVRP formed on the stepped plane which strongly influences the flow and heat transfer of downstream, which is more evident in metal based materials, especially resulted by its isotropic features. Compared with the cylindrical film hole, the structure of step plane efficiently decreases the coolant flow velocity which results in a decrease of the CVP intensity so that the cooling film is adherent to the wall. The thermal conductivity of metal base material is strong, which has a great influence on the temperature distribution inside the wall. Velocity pulsation tightly influenced by CVP, in where the CVP intensity is strong, velocity pulsation is also strong so that the coolant flow strongly mixes with the mainstream. The flow velocity and velocity pulsation decrease with the increasing of the area ratio of stepped hole. The influence of geometric parameters on the heat transfer performance is mainly due to the high heat transfer performance of the metal matrix material.

Generation of a vortex and helix with square arrays with high-efficiency by the use of a 2D binary phase mask

In this paper, we designed a binary phase mask (PM) with specific phase modulation characteristic and analyzed the spatial spectrum of the beam passing through the PM. In the case where the difference of phase modulation between two lattices of the binary PM is not equal to π, we found the spatial spectrum has the central spot (direct current component, DC component) except for the central eight strong symmetrical spots and many outer weak symmetrical spots. Based on the multiple-beam interference, the propagation-invariant vortex with a square array can be realized by interference of the eight plane waves with the same wave vectors along the optical axis from the central eight symmetrical spots via the modulated phase values of the central eight symmetric spots. The vortex arrays have two kinds of vortex with an opposite topological charge of l=±1. The helix can be formed with a square array by the interference of the vortex array and the plane wave along the optical axis from the DC component. The helix with outstanding helical intensity distributions have two screw directions, which coincide with the phase distribution of the optical vortex with the square array. The energy efficiency of this method can reach more than 80%. The simulation results demonstrate the feasibility of this method.

Effects of continuously increasing pitching and plunging amplitudes on the aerodynamic forces of flapping airfoils

Recently, numerous studies have been conducted to clarify the effects of the increases in pitching and plunging amplitudes of flapping wings on thrust and lift generation. In the present study, the effects of continuously increasing pitching and plunging amplitudes on the aerodynamic performances of a two-dimensional (2D) flapping wing are investigated computationally. Continuously increasing pitching and plunging amplitudes have significant effects on the rate of leading-edge vortex (LEV) development and the time of LEV separation; as a result, the aerodynamic performance is influenced. Lift and thrust are gradually improved with increasing pitching and plunging amplitudes; however, higher amplitudes induce the production of drag forces. Furthermore, to compare the contributions of the pitching and plunging amplitudes, we conducted simulations with pure pitching or plunging amplitude increases while keeping the other factors constant. With the increase in pitching amplitude, the vortex on the upper surface becomes weaker during the downstroke and leads to the production of a vortex on the lower surface. During the upstroke, the effect of the increase in pitching amplitude on the vortex has a symmetric influence against the downstroke. The change in pitching amplitude has little effect on the lift and thrust but leads to the production of drag forces. When the plunging amplitude increases, the LEV and the second kind of vortex, the trailing-edge vortex (TEV), becomes stronger, which will cause a concurrent increase in lift and thrust. The increase in plunging amplitude greatly improves lift and can also enhance thrust.

Numerical study on vortex structure of undulating fins in stationary water

The Gymnarchus Niloticus fish(GNF) with long dorsal-fin undulating generally cruises with high efficiency and extra-ordinal maneuverability while keeping its body for straight line. In the paper the unsteady flow field of undulating fins in stationary water is calculated by Large Eddy Simulation and dynamic grid technique with diffusion-based smoothing model. The thrust produced by undulating fins is measured at wave amplitude of 850, and the unsteady flow field around undulating fins on middle cross section at wave amplitude of 850 at a series of phase angles is measured by phase-locked PIV in the stationary water. The thrust calculated by numerical simulation is in good agreement with hydrodynamics experiments. The streamwise vortex structure and its evolution on cross sections calculated by numerical simulation qualitatively agree well with PIV experiments. The numerical results at wave amplitude 300 indicates there are three kinds of vortexes in the flow field, i.e. streamwise vortex, crescent vortex and toroidal vortex. Vortex characteristic and its temporal and spatial evolution is described, and the vortex structure and its relationship with thrust, heave force is analyzed.

Dominating Unsteadiness Flow Structures in Corner Separation Under High Mach Number

Corner separation is a key factor affecting the aerodynamic performance of compressors, whereas corner separation under a high-Mach-number condition is a scientific problem with practical engineering significance. Using a self-developed high-precision (Roe scheme for convective flux calculation and fifth-order weighted essentially non-oscillatory for reconstruction) delay detached-eddy simulation (DDES) code, a compressor cascade with massive corner separation under a high Mach number is simulated. The primary aim of the simulations is to obtain the dominating fluctuation vortex structures in and out of the blade passage, as well as to analyze the reason for the formation of these structures. The vortex structures are analyzed after verifying the accuracy of the code. An advanced data mining technology (that is, dynamic mode decomposition) was performed to extract the dominating fluctuation flowfield simulated by DDES for the first time. The results prove the credibility of the self-developed code. It is found that the passage vortex and the hairpin vortex are the primary sources of unsteady fluctuations in and out of the passage, respectively. The former is formed by endwall separation, and the position where it is generated is relatively stable; thus, the endwall suction technology is suggested for flow control. The latter is formed by the intermixing of the passage vortex, the trailing-edge shedding vortex, and the concentrated shedding vortex. Owing to the different frequencies of those three kinds of vortices, different hairpin scales are formed. The research results can provide a basis for flow control design.

Comparisons and analyses of vortex identification between Omega method and Q criterion

The present paper presents comparisons of the vortex identification between the omega method and the Q criterion based on the data of a classical flow. From the comparisons of the vortex structure together with the flow statistics, some important conclusions are drawn on the validity of the two methods, as follows. The omega method can identify various kinds of vortices with different intensities (e.g., the strong vortex, the medium vortex and the weak vortex). For the Q criterion, due to the subjective threshold selection, only the strong vortex with weak deformations could be identified. Finally, some emerging topics related with the advanced vortex identification methods are briefly discussed.

Dissimilar heat transfer enhancement by a vortex tube introduced in plane Couette flow and its realization mechanism

The purpose of this study is to find what kind of vortices should be introduced in order to enhance dissimilar heat transfer and to interpret its physical mechanism. We carry out a direct numerical simulation on heat and momentum transfer by a vortex introduced into plane Couette flow. The parameters in this system are the Reynolds number, the initial circumferential velocity, the position, the radius and the inclination angle of the vortex. For dissimilar heat transfer enhancement, if a strong vortex can be generated, a spanwise-oriented anti-cyclonic vortex should be introduced, otherwise, a spanwiseoriented cyclonic vortex should be introduced. The reason why the cyclonic (or anticyclonic) vortex realizes dissimilarity is that the streamwise pressure gradient and the streaklines intensify the difference between the temperature and the streamwise velocity in the upper and lower (or downstream and upstream) regions of the vortex, which makes the Reynolds stress dominate over the turbulent heat flux.

3D flow structure around a piezoelectrically oscillating flat plate

Flow simulations have been conducted to analyze the three-dimensional flow around a vibrating flat plate confined with two endwalls. A smoothing technique for hexahedral grids was implemented without a local re-meshing technique in order to resolve complex flow in a small gap between the fan side and the endwall. Three-dimensional numerical results showed a good agreement with experimentally measured data and the two-dimensional results in a previous work. In time-averaged flow analysis, the flow generated by the plate tends to move toward the center of it so that the velocity magnitude at the center is much larger than other heights near the endwall. It was found that three-dimensional velocity distribution is closely related to the interaction of two kinds of vortices from the fan tip and the fan side. The tip vortex and the side vortex interfere each other near the plate corner and weaken their intensity. In instantaneous flow analysis, an isosurface of λ2 is adopted to analyze a complex vortical structure around the oscillating flat plate. Based on the numerical results, an attempt has been made to understand three-dimensional flow generated by a confined oscillating flat plate.

Structural and mixing characteristics influenced by streamwise vortices in supersonic flow

By utilizing nanoparticle-based planar laser scattering method, the flow structures and mixing characteristics with the existence of large-scale streamwise vortices generated by a lobed mixer are investigated experimentally. The emergence and evolution of streamwise and spanwise vortices and the interaction between these two kinds of vortical structures are clearly exhibited. Fractal analysis reveals that streamwise vortices are strongly convoluted and possess complicated three-dimensional characteristics, which can favor molecular mixing. The turbulent/non-turbulent interface is obviously present, in which drastic turbulent entrainment occurs. Moreover, the emergence of typical T-shaped vortex suggests that this kind of vortex is a usual topological structure in turbulent transition region.

Structures of Turbulent Vortices in Floodplains of Compound Channels

Abstract In turbulent flows, several kinds of vortices and their structures are developed. After physical summation al velocities created by them, they intertwine, and the picture obtained from their measurement chronograms is mostly chaotic. Consequently, these structures become masked. It is therefore very difficult to investigate them and despite many studies performed so far, they have not been sufficiently explored. Favorable conditions for investigations of velocity measurement chronograms occurred when sufficient long intervals were observed, in which the above mentioned masking is minimal. In such intervals, the investigation of turbulent vortices and their structures becomes possible, and it has been performed to some degree. The results of these investigations of the compound channel were already discussed in two previously published articles. The first describes all vortices and their structures in the whole water flow of the compound channel. The second contains a detailed analysis of vortices and their structures in the central vertical of the main channel. This article, discuss investigations of coherent structures formed in the floodplain.

Large eddy simulation of liquid jet primary breakup in supersonic air crossflow

The paper proposed a two-phase flow Large Eddy Simulation (LES) algorithm for atomization in supersonic gas flow. A coupled Level Set and Volume of Fluid (VOF) interface tracking method is used. The supersonic gas flow is solved using a compressible flow solver, and the liquid phase is solved using an incompressible flow solver. Appropriate boundary conditions are specified on the interface for both solvers to correctly capture the interaction between the gas and liquid. The water jet primary breakup in a supersonic air crossflow is well predicted, with the shock waves ahead of the liquid column and drops/ligaments properly captured. Two kinds of vortices respectively relating to surface breakup and column breakup are observed. Surface waves on the windward side of the liquid jet are numerically reproduced, and the wavelength is well correlated to an effective Weber number which is determined by the gas flow behind the shock. As gas flow goes across the strong shock wave ahead of liquid column, the disintegrating aerodynamic force is weakened. Rayleigh–Taylor instability is found to be responsible for the development of surface waves which causes column breakup. It is demonstrated that LES can produce almost the same breakup morphology as an under-resolved DNS.

Superconducting magnetoresistance effect observed in Co/Nb/Co trilayers under a parallel magnetic field: The importance of matching the width of magnetic domain walls of the Co layers with the thickness of the Nb interlayer

Magnetoresistance effects observed in ferromagnet/superconductor (FM/SC) hybrids, FM/SC bilayers (BLs) and FM/SC/FM trilayers (TLs), have attracted much interest. Here, we focus on the stray-fields-based superconducting magnetoresistance effect (sMRE) observed in Co(dCo)/Nb(dNb)/Co(dCo) TLs with sufficiently thick Co outer layers so that out-of-plane magnetic domains (MDs) and MDs walls (MDWs) emerge all over their surface when subjected to a parallel external magnetic field, Hex, equal to the coercive field, Hc. To explore the conditions necessary for maximization of the sMRE, we focus on the different kinds of the stray dipolar fields, Hdip, that emerge at the interior of the out-of-plane MDs and at the boundaries of MDWs; these have a different inherent tendency to create straight and semi-loop vortices, respectively. In the recent literature, the creation of straight and semi-loop vortices has been addressed at some extent both theoretically [Laiho et al., Phys. Rev. B 67, 144522 (2003)] and experimentally [Bobba et al., Phys. Rev. B 89, 214502 (2014)] for the case of FM/SC BLs. Here, we address these issues in FM/SC/FM TLs in connection to the sMRE. Specifically, we focus on an experimental finding reported recently [D. Stamopoulos and E. Aristomenopoulou, J. Appl. Phys. 116, 233908 (2014)]; strong magnetostatic coupling of the FM outer layers is accompanied by an intense sMRE in TLs in which the thickness of the SC interlayer, dSC, matches the width of MDWs, DMDWs. To investigate this finding, we employ simulations-modeling and energy-considerations and propose two quantitative criteria that facilitate the creation of straight vortices over semi-loop ones. The first focuses on the maximization of the stray Hdip that occur at the interior of the out-of-plane MDs. The second enables the estimation of a crossover between the preferable creation of one kind of vortices over the other. Both criteria respond well, when tested against experimental results. These generic criteria on the interference between dSC and DMDWs can assist the design of cryogenic devices based on FM/SC/FM TLs.

Transition scenario of the round jet in crossflow topology at low velocity ratios

We study experimentally a round Jet In CrossFlow (JICF) at low values of the jet-to-crossflow velocity ratio R using instantaneous and time-averaged three-dimensions three-components velocimetry. The difference between instantaneous and time-averaged swirling structures of the JICF is emphasized. Through the analysis of spatial distribution of instantaneous transverse and longitudinal vortices the main transitions of the JICF are characterized for 0.15 < R < 2.2. A new transition at very low velocity ratio is found (R < 0.3). When R is large enough (R > 1.25), the classic JICF topology is recovered. In between, a deformation of the classical JICF topology is observed consisting in a progressive disappearance of the leading-edge vortices, a bending of the jet trajectory toward the wall and thus a strengthened interaction with the boundary layer. Thanks to a state-of-the-art review on the JICF topology and using visualizations of the flow structures extracted from our experimental volumetric velocimetry measurements, this article provides a complete transition scenario of the JICF topology from the high velocity ratios to the lowest ones, and gives the topological transition threshold associated with each kind of vortex.

A Nonlineark-εTurbulence Model Applicable to High Pressure Gradient and Large Curvature Flow

Most of the RANS turbulence models solve the Reynolds stress by linear hypothesis with isotropic model. They can not capture all kinds of vortexes in the turbomachineries. In this paper, an improved nonlineark-εturbulence model is proposed, which is modified from the RNGk-εturbulence model and Wilcox'sk-ωturbulence model. The Reynolds stresses are solved by nonlinear methods. The nonlineark-εturbulence model can calculate the near wall region without the use of wall functions. The improved nonlineark-εturbulence model is used to simulate the flow field in a curved rectangular duct. The results based on the improved nonlineark-εturbulence model agree well with the experimental results. The calculation results prove that the nonlineark-εturbulence model is available for high pressure gradient flows and large curvature flows, and it can be used to capture complex vortexes in a turbomachinery.

Instability Analysis of a Model Pump-Turbine with MGV Based on Nonlinear Partially Averaged Navier-Stokes Methods

Pump-turbines were always running at partial condition with the power grid changing. Flow separations and stall phenomena were obvious in the pump-turbine. Most of the RANS turbulence models solved the shear stress by linear difference scheme and isotropic models, so they could not capture all kinds of vortexes in the pump-turbine well. At present, partially-averaged Navier-Stokes (PANS) model has been found to be better than LES in simulating flow regions especially those with less discretized grid. In this paper, a new nonlinear PANS turbulence model was proposed, which was modified from RNG k-ε turbulence model and the shear stresses were solved by Ehrhardt's nonlinear methods. The nonlinear PANS model was used to study the instability of “S” region of a model pump-turbine with misaligned guide vanes (MGV). The opening of preopened guide vanes had great influence on the “S” characteristics. The optimal relative opening of the preopened guide vanes was 50% for the improvement of the “S” characteristics. Pressure fluctuations in the vaneless space were analyzed. It is found that the dominant frequency at the vaneless space was twice the blade passing frequency, while the second dominant frequency decreased as the preopening increased.

Phase structure and vortex configurations of superconductors coexisting with ferromagnetism

We study the Ginzburg-Landau lattice gauge model that we introduced recently for ferromagnetic superconductors, i.e., superconductors in which the p-wave superconducting (SC) order and the ferromagnetic (FM) order may coexist. We report some interesting results obtained by Monte-Carlo simulations. In particular, we have two types of coexisting states distinguished by the transition temperatures of the SC order TSC and the FM order TFM; (i) homogeneous state for TFM/TSC > 1 and (ii) inhomogeneous state for TFM/TSC < 0.7. In (ii) the two orders appear only near the surface of the lattice as observed in ZrZn2. We also study vortex configurations of SC order parameters. Two kinds of vortices, one for spin-up electron pairs and one for spin-down pairs show different behaviors because of the Zeeman coupling.

Flow Visualization and Frequency Characteristics of Velocity Fluctuations of Complex Turbulent Flow in a Short Elbow Piping Under High Reynolds Number Condition

Flow visualization was performed on a single short elbow piping by means of two-dimensional particle image velocimetry. The piping was designed as a 1/7-scale model of a section of the cold-leg piping of a Japan sodium-cooled fast reactor. This study characterized the periodic motions and flow structures that appeared in and downstream of the elbow and potentially affected flow-induced vibrations. The flow field that related flow separation and frequency characteristics of the flow velocity fluctuation were explored for Reynolds number from 0.3 × 106 to 1.0 × 106, which belonged to the post-critical regime. Experimental results show that flow structures are not strongly dependent on Reynolds number in this range. Frequency analysis for the velocity fluctuation in terms of Strouhal number (St) reveals that there exist not only two kinds of vortices with different shedding periods, but also one periodic flow in the circumferential direction. In the flow separation region, vortices are periodically emitted with St ≈ 0.5, while those with about 1.0 are shed in a shear flow region located between the separation region and the pipe center. Moreover, a periodic motion with St ≈ 0.5 appeared in the circumferential direction in the vicinity near the separation region. These values of St were not strongly dependent on Reynolds number in this study.

Stability and self-adaption character of turbulence coherent structure in narrow-deep river bend

In a meandering river, a certain scale of turbulent vortex dominates the development of river morphology, making the river bend with s particular curvature. This kind of vortex is denoted as “bend-forming vortex”. The coordinated relationship of bend-forming vortex and meandering river channel is then known as “self-adaption feature” of rivers. With these two concepts, this paper investigated the stability and self-adaption character of coherent vortex in the U-shape river bend with a constant curvature. On the basis of fluid mechanics theory and in consideration of turbulent coherent vortex as disturbance, the growth rate and the wave number response range of coherent vortex in meandering rivers with different curvatures were calculated in this paper. Moreover, the responses of different scales of coherent turbulence structure to river bend parameters were analyzed to explain the mechanism of river bend maintenance. These methods could provide a theoretical basis for further investigation on river meandering.