Large-scale Vortex Flow Research Articles

A preliminary investigation on a novel vortex-controlled flameholder for aircraft engine combustor

This study proposes a novel vortex-controlled flameholder (VCF) to enhance the combustion performance, particularly the lean ignition and blowout characteristics of afterburners in advanced aircraft engines across a wide range of operating conditions. Additionally, both experimental and numerical investigations were conducted to explore the effects of the cavity structure on the flow field, lean ignition, lean blowout, flame propagation characteristics, and outlet temperature rise distribution of the flameholder. Two distinct cavity structures designated closed-cavity (case-1) and open-cavity (case-2) were examined, the findings indicating that both case-1 and case-2 can generate large-scale vortex flow structures within the cavity, contributing to achieving excellent combustion stability. Case-2 demonstrated better lean ignition and blowout performance compared to case-1. Furthermore, both case-1 and case-2 exhibited the same ignition process, which comprised four distinct phases: Phase 1 involved the formation of an effective flame kernel; Phase 2 pertained to the ignition of the entire cavity by the flame kernel; Phase 3 represented the full development of the flame downstream of the cavity; Phase 4 described the formation of a fire tornado that anchors the flame front. Interestingly, the outlet temperature rise in case-1 is lower than that in case-2 at low fuel-to-air ratio (FAR) conditions. As the FAR increases, the difference in the outlet temperature rises between the two cases gradually narrows.

Effectiveness of Air Admission through Short Pipes in Reducing Pressure Fluctuation of Propeller Hydro-turbine Flow Channel

Abstract When the hydro-turbine operates under part-load conditions, a large-scale vortex flow will form in the draft tube. This vortex can cause low-frequency pressure pulsations that endanger the safe operation of the hydro-turbine. By injecting air into the draft tube, the structure of the vortex can be effectively disrupted, reducing the amplitude of the pressure pulsations. In this paper, a propeller hydro-turbine was studied, and the SST k-ω turbulence model based on the Reynolds averaged Navier-Stokes equations (RANS) was utilized to perform a full-channel unsteady numerical simulation on a part-load condition with the guide vane opening (GVO) of 35°. Then, the fast Fourier transform (FFT) was applied to analyze the pressure fluctuations at monitoring points on the draft tube wall, the head cover, and the nose of the spiral casing. The results indicated that injecting air with an appropriate flow rate into the draft tube through short pipes can reduce the pressure fluctuations on the draft tube wall by 51.4% to 72.5% and decrease the pressure fluctuations on the head cover and the nose of the spiral casing by approximately 40% to 42%. This study proposes a new air admission structure that can enhance the operating stability of the propeller hydro-turbine under part-load conditions.

Quasi-analytical solution to the problem of heat transfer by vortex flow based on self-similarity

Highly efficient vortex flow increases fluid flow and simultaneously enhances heat transfer. An accurate vortex flow model is of great importance to analyzing and solving heat transfer problems. The general form of the solution to the vortex flow problem was derived from the self-similarity properties of vortex flow along the radius direction, and its correctness was validated. The temperature distribution function of vortex flow was determined, and the relative thermal conductivity was obtained. A dimensionless number (ρCpvθ′Rk) was defined to characterize the temperature distribution of vortex flow. The rate of heat transfer was also determined. The results indicated that the dimensionless number represents the ratio of convection to thermal conduction, and can be used to accurately describe the temperature distribution of vortex flow. When the temperature at the vortex flow center is close to the average temperature, the difference in temperature phase is significant, and convection plays a dominant role in the heat transfer process. The temperature can be defined by sine function when the radius of vortex is decreased to a certain extent. The results can be applied to the analysis of large-scale vortex flow heat transfer, the optimization design of small-scale vortex flow heat transfer, and the development of vortex flow and temperature sensors.

Large-scale vertical vorticity generated by two crossing surface waves

Two crossed surface waves generate vertical vorticity in a viscous fluid due to hydrodynamic nonlinearity. We find parameters of the induced flow and investigate its excitation and damping dynamics, focussing on the case of large-scale vorticity (in comparison with the wavelength), corresponding to surface waves crossing at a small angle. We also show that a thin insoluble liquid film (possibly covering the fluid surface due to contamination) increases the induced flow intensity not only at the surface, but also in the fluid bulk. We discuss our results in view of recent experimental observations of large-scale vortex flows.

Numerical investigation on flow and heat transfer characteristics of vortex cooling in an actual film-cooled leading edge

This paper numerically investigates the flow and heat transfer characteristics of vortex cooling in an actual film-cooled leading edge. Two internal vortex cooling cases called Case P and Case S are involved in this study. Case P means that the coolant injection nozzles are arranged near the pressure surface side of the blade, and Case S means that the coolant injection nozzles are arranged near the suction surface side. The mainstream region and the swirl chambers of the vortex cooling cases are connected by 6 rows of film holes. Results indicate that the practical mainstream flow generated by the actual blade affects the vortex flow structures and brings about an endwall secondary flow. For internal vortex cooling, the large-scale vortex flow structures in Case P skew off while flow structures in Case S are not distinctly influenced. Two type of flow regimes form near the film hole due to the suction effect, and bring about different local heat transfer augmentation intensity. For film cooling, the endwall secondary flow forces the coolant film move towards or away from the endwall thus decreases the film cooling efficiency. The film hole blowing ratio distribution is affected due to the different influence of mainstream flow on internal vortex flow of Case P and Case S, which results in different film cooling efficiency distribution. The film cooling efficiency of Case S on the pressure surface increases by up to 59.8% compared with Case P.

Penetration of a Vortex Lattice into the Bulk of a Liquid

Penetration of a vorticity lattice formed by waves propagating on the surface at an angle to each other into the bulk of a liquid is studied experimentally. It is shown that along with the Stokes drift it is necessary to regard nonlinear wave interaction in order to explain experimental time dependences of average vorticity. Vorticity distribution over the depth of a liquid is described by a sum of two exponents considering the contribution of both mechanisms. It is determined that formation of large-scale vortex flows leads to deformation of a vortex lattice and a decrease in an average value of the amplitude of lattice vorticity as a result of its drift.

Formation of vortex structures by noncollinear waves on the water surface

The generation of a large-scale vortex flow by gravity waves on the water surface has been experimentally studied. It is shown that the mechanism of vortex system attenuation changes with an increase in the relative pump amplitude. The wave amplitudes have been experimentally determined at which the experiment is not described by a theoretical model. The experimental results agree well with the developed theoretical model.

Impacts of duct inner diameter and standoff distance on macroscopic spray characteristics of ducted fuel injection under non-vaporizing conditions

Ducted fuel injection spray is a new technology for reducing soot formation in heavy-duty diesel engines. In this work, the ducted fuel injection spray characteristics with different duct inner diameters and different standoff distances were investigated and compared with free spray. Duct inner diameter ranged from 1.5 to 4 mm, and standoff distance varied between 0.9 and 4.9 mm. Mie-scattering optical technique was used to characterize spray characteristics under various injection pressures in a constant-volume spray chamber. Ambient gas pressure of up to 6 MPa when spraying. The results showed that ducted fuel injection spray with smaller duct has better spray diffusion compared to those of ducted fuel injection sprays with larger ducts and free spray from the perspectives of spray tip penetration, spray cone angle and spray area. Increasing standoff distance could increase spray velocity. Ducted fuel injection spray with smaller duct formed a mushroom-shaped head and large-scale vortex flow close to the duct outlet. All the advantages of ducted fuel injection spray with smaller duct are interpreted as evidence of improving fuel–gas mixing quality significantly.

Kinetics of large scale vertical flow generation by weakly noncollinear nonlinear gravitational waves

The generation of a large-scale vortex flow by gravity waves on the water surface has been experimentally studied. It is shown that large-scale flows are formed as a result of the interaction between nonlinear low-frequency modes which intersect at small angles. The amplitude of vorticity at large scales increases with increasing angle between pump waves. The experimental data agree well with the developed theoretical model.

Modulation Instability of a Gravity Wave and Generation of a Direct Cascade of Vortex Energy on the Surface of Water

The development of instability of gravity-capillary waves on the surface of water excited by two perpendicular plungers has been experimentally observed. As a result of a four-wave process, waves with a frequency of 8 Hz scatter in pairs into waves with frequencies of 3.92 and 4.08 Hz, as well as 11.98 and 12.02 Hz. The amplitude of low-frequency waves increases exponentially with a characteristic time of about 90 s which exceeds the time of viscous wave damping almost by an order of magnitude. Along with the main pumping mode, the appeared low-frequency harmonics, propagating on the surface of water at an angle of 15° to each other, form large-scale vortex flows on the surface of water. The wave energy is transferred from the pumping region directly to vortices with a size comparable to the length of a bath wall. In a vortex system, a direct energy cascade with the energy distribution close to E(k) ~ k–5/3 is formed from the region of low wave vectors.

Experimental Simulation of the Generation of a Vortex Flow on a Water Surface by a Wave Cascade

The generation of a vortex flow by waves on a water surface, which simulate an energy cascade in a system of gravity waves at frequencies of 3, 4, 5, and 6 Hz, has been studied experimentally. It has been found that pumping is accompanied by the propagation of waves on the surface at different angles to the fundamental mode and by a nonlinear interaction between waves resulting in the generation of new harmonics. It has been shown that large-scale flows are formed by modes of the lowest frequency of 3 Hz intersecting at acute angles. The energy distribution of the vortex motion can be described by a power-law function of the wavenumber and is independent of the energy distribution in a system of surface waves. The energy coming to large-scale vortex flows directly from the wave system is transferred to small scales. A direct rather than inverse energy flux is established in the system of vortices.

Experimental study on the heat transfer enhancement in sub-channels of 6 × 6 rod bundle with large scale vortex flow mixing vanes

Abstract In this study, single-phase heat transfer characteristics for downstream flow in the support grid of 6 × 6 rod bundle were investigated. It is known that turbulence generated by a support grid with split mixing vanes enhances heat transfer in a rod bundle. However, the enhancement is observed in relatively short length. Further, a support grid with large scale vortex flow (LSVF) mixing vanes enhances heat transfer over a relatively longer distance. Based on the results of previous studies, heat transfer experiments were performed at Reynolds numbers of 30,000 and 50,000, and the effects of heat transfer enhancement with both i) the split mixing vanes and ii) LSVF mixing vanes were compared in this study. The results showed that the effects of heat transfer enhancement in the rod bundle region by the split mixing vanes were maintained up to a length of 15 Dh behind the spacer grid. At a Reynolds number of 50,000, it was observed that the effects of heat transfer enhancement by the LSVF mixing vanes were approximately 3% higher than those by the split mixing vanes for a distance ranging in 1–15 Dh behind the spacer grid.

Experimental Study on Heat Transfer Enhancement Effect with Different Shapes of Mixing Vanes in Sub-Channels of 6×6 Rod Bundle

In the present study, single-phase heat transfer characteristics for downstream flow in the support grid of 6×6 rod bundle were investigated. It has been known that a turbulence generation due to a support grid with split mixing vanes enhances heat transfer in rod bundle but its heat transfer enhancement actually affects to relatively shorter distance. On the other hand, it has been also turned out that a support grid with large scale vortex flow (LSVF) mixing vanes results in heat transfer enhancement to a longer distance. Based on the results of literatre survey, single-phase water heat transfer experiments were performed for Reynolds numbers at around 30,000, and the heat transfer enhancement effect with both i) the split mixing vanes and ii) the LSVF mixing vanes was compared in this study. The key results showed that the effect of heat transfer enhancement in rod bundle region by the split mixing vanes was maintained up to the length of 15Dh behind the spacer grid. For the Reynolds numbers at around 30,000, it was also observed that the effect using the LSVF mixing vanes was stronger at about 3% when compared to the case using the split mixing vanes only for the distance ranging from 1 to 15Dh behind the spacer grid.

Expanding and Contracting Coronal Loops as Evidence of Vortex Flows Induced by Solar Eruptions

Abstract Eruptive solar flares were predicted to generate large-scale vortex flows at both sides of the erupting magnetic flux rope. This process is analogous to a well-known hydrodynamic process creating vortex rings. The vortices lead to advection of closed coronal loops located at the peripheries of the flaring active region. Outward flows are expected in the upper part and returning flows in the lower part of the vortex. Here, we examine two eruptive solar flares, the X1.1-class flare SOL2012-03-05T03:20 and the C3.5-class SOL2013-06-19T07:29. In both flares, we find that the coronal loops observed by the Atmospheric Imaging Assembly in its 171 Å, 193 Å, or 211 Å passbands show coexistence of expanding and contracting motions, in accordance with the model prediction. In the X-class flare, multiple expanding and contracting loops coexist for more than 35 minutes, while in the C-class flare, an expanding loop in 193 Å appears to be close by and cotemporal with an apparently imploding loop arcade seen in 171 Å. Later, the 193 Å loop also switches to contraction. These observations are naturally explained by vortex flows present in a model of eruptive solar flares.

Drive of a long-lived vortex-flow pattern by coupling to zonal flows in presence of resonant magnetic perturbations

The working hypothesis for the origin of edge-localized-mode stabilization is that Resonant Magnetic Perturbations (RMPs) increase transport in the pedestal, thus lowering the pressure gradient below the ideal MHD threshold. Large-scale vortex-flows matching the RMP helicity were observed experimentally [N. Vianello et al., Plasma Phys. Controlled Fusion 57, 014027 (2015)]. We derive and solve numerically a 1D model for the generation of long-lived vortex-flows in presence of RMPs. We show that, in presence of RMPs, zonal flows are damped and partially transfer their energy to a resonant vortex-flow pattern. The resulting vortex-flow has a multiscale nature with a fast-varying fine-structure set by zonal flows and a slowly-varying radial envelope with a resonant character. The model predicts that the saturated vortex-flow energy E scales with RMP amplitude as E∼δBrBα with α≃1.9. This novel type of nonlinearly driven non-axisymmetric flow has a radial—streamer like—component, and is therefore a candidate for increased convective transport.

Stabilization of ion temperature gradient driven instability by a vortex flow

Stabilization of ion temperature gradient (ITG) driven mode by a large-scale vortex flow is investigated using a gyrofluid model in slab geometry. Extending a recent work [Z. X. Wang et al., Phys. Rev. Lett. 103, 015004 (2009)], the focus in this work is put on the effect of magnetic shear. It is shown that the vortex flow can effectively stabilize the ITG mode by inducing both radial and poloidal mode couplings. Furthermore, decreasing magnetic shear is identified to weaken the stabilizing role of the vortex flow. The effects of the magnetic shear on the ITG mode structure and on the growth rate in the presence of various shear flows are obtained and the relevant mechanisms are discussed in detail.

Model for dynamic self-assembled magnetic surface structures

We propose a first-principles model for the dynamic self-assembly of magnetic structures at a water-air interface reported in earlier experiments. The model is based on the Navier-Stokes equation for liquids in shallow water approximation coupled to Newton equations for interacting magnetic particles suspended at a water-air interface. The model reproduces most of the observed phenomenology, including spontaneous formation of magnetic snakelike structures, generation of large-scale vortex flows, complex ferromagnetic-antiferromagnetic ordering of the snake, and self-propulsion of bead-snake hybrids.

Diagnosis of the behavior characteristics of the evaporative diesel spray by using images analysis

In this study, the effects of change in injection pressure on spray structure have been investigated on the high temperature and pressure field. To analyze the structure of evaporative diesel spray is important in speculation of mixture formation process. Also emissions of diesel engines can be controlled by the analyzed results. Therefore, this study examines the evaporating spray structure in a constant volume chamber. The injection pressure is selected as the experimental parameter, is changed from 72 MPa to 112 MPa with a high pressure injection system (ECD-U2). The PIV (Particle Image Velocimetry) technique was used to capture behavior variation of the evaporative diesel spray. Analysis of the mixture formation process of diesel spray was executed by the results of flow analysis in this study. Consequentially the large-scale vortex flow could be found in downstream spray and the formed vortex governs the mixture formation process in diesel spray.

Turbulent Separated Flows: Near-Wall Behavior and Heat and Mass Transfer

This paper was presented at the Eleventh Asian Congress of Fluid Mechanics as an invited/keynote lecture. The lecture summarizes development of physical model for transfer processes in turbulent separated flows. Attention is focused on the factors that have led to significant advances in understanding of the mechanisms of transfer processes in separated flows, which differs from those in traditional attached turbulent flows in channels and zero pressure gradient turbulent flows. The physical model of transfer processes in the near-wall region of turbulent separated and reattached flows based on the assumption of the governing role of generated local pressure gradient that takes place in the immediate vicinity of the wall in separated flow as a result of intense instantaneous accelerations induced by large-scale vortex flow structures is discussed. Similarity laws for mean velocity and temperature profiles and spectral characteristics of the transfer processes resulting in the physical model are confirmed by the available experimental data. The physical model has provided explanations of the well-known empirical heat and mass transfer correlations for turbulent separated flows and other types of flows close in their structure to those of turbulent separated flows.

Comparison of thermo-hydraulic performances of large scale vortex flow (LSVF) and small scale vortex flow (SSVF) mixing vanes in 17 × 17 nuclear rod bundle

Spacer grids in the nuclear fuel rod assembly maintain a constant distance between rods, secure flow passage and prevent the damage of the rod bundle from flow-induced vibration. The mixing vanes attached to the spacer grids generate vortex flows in the subchannels and enhance the heat transfer performance of the rod bundle. Various types of mixing vanes have been developed to produce cross flows between subchannels as well as vortex flows in the subchannels. The shapes of the mixing vane have been improved to generate larger turbulence and cross flow mixing. In the present study, two types of large scale vortex flow (LSVF) mixing vanes and two types of small scale vortex flow (SSVF) mixing vanes are examined. SSVF-single is conventional split type and SSVF-couple is split type with different arraying method. LSVF mixing vane has different geometry and arraying method to make large scale vortex. 17 × 17 rod bundle with eight spans of mixing vanes is simulated using the IBM 690 supercomputer. The FLUENT code and IBM supercomputer is employed to calculate the flow field and heat transfer in the subchannels. Turbulence intensities, maximum surface temperatures of the rod bundle, heat transfer coefficients and pressure drops of the four kinds of mixing vanes are compared. LSVF mixing vanes produced higher turbulence intensity and heat transfer coefficient than SSVF mixing vanes. Consequently, LSVF mixing vane increases the thermal efficiency and safety of the rod bundle.