Occurrence Of Vortex Breakdown Research Articles

Time-averaged asymmetries and oscillatory global modes of vortex flows over a slender wing

Previous studies indicate that vortex pairs over slender wings can produce time-averaged asymmetries even before the occurrence of vortex breakdown, but the sources for the asymmetries are still not clear. The present investigation uses numerical simulations and experimental measurements to explore the physical nature for the symmetry breaking of vortices. The results indicate that the time-averaged asymmetries come from a spatial growth of local disturbances (spatial instability) instead of a temporal instability. In numerical simulations, the time-averaged flows around a perfectly symmetric wing exhibit symmetric vortex pairs in which any asymmetric initial perturbation will decay and eventually converge to symmetric flows. However, if a geometric micro-perturbation is asymmetrically placed on the apex, asymmetric vortex pairs can be produced; downstream sectional perturbation energy grows spatially, and the growth rates increase with increasing angle of attack. The spatial perturbation growth mainly comes from antisymmetric modes, whereas symmetric modes have smaller growth rates. The experimental results indicate that artificial apex perturbations can overwhelm natural irregularities on the apex and dominate the orientations of asymmetric vortices. Unsteady aspects of the flows were also studied before vortex breakdown, with the emphasis put on oscillatory global modes by applying dynamic mode decomposition to the entire flow field. It was found that the most energetic modes are related to vortex shedding around trailing edges and instabilities of downstream shear layers, which has significant contributions to fluctuations of aerodynamic forces and moments. The vortex shedding exhibits strong three dimensionality characterized by oblique shedding and a beating phenomenon.

Flow structures in a swirl flow - vortex breakdown condition

This paper presents an experimental and numerical analysis of swirl flow in a circular tube. Swirl flow is important for technical and natural processes, where high heat transfer and good fluid mixing are needed. Flow and vortex structures respecting redistribution of momentum and possible occurrence of vortex breakdown are taken into account. The swirl flow is analysed experimentally by the measurement of the velocity field using Particle Image Velocimetry (PIV) and numerically via the commercial CFD code ANSYS CFX. Three cases are investigated: laminar flow regime (Re = 1,000), intermediate flow regime (Re = 2,000), and turbulent flow regime (Re = 5,000). The redistribution of the velocity field and the decrease of the swirl strength towards the outlet are shown. This redistribution affects the Reynolds number. Concerning the Rossby number, the occurrence of vortex breakdown in the swirl flow is determined. It is shown that the vortex breakdown takes place in the flow with higher Reynolds numbers, where an axial backflow may occur. Changes of the Reynolds numbers Reϕ and Rez along the tube length also confirm this statement. Furthermore, a thermodynamic perspective of vortex breakdown phenomena is presented.

Transitions and blowoff of unconfined non-premixed swirling flame

The present experimental work reports the first observations of primary and secondary transitions in the time-averaged flame topology in a non-premixed swirling flame as the geometric swirl number SG (a non-dimensional number used to quantify the intensity of imparted swirl) is varied from a magnitude of zero till flame blowout. First observations of two transition types viz. primary and secondary transitions are reported. The primary transition represents a transformation from yellow straight jet flame (at SG = 0) to lifted flame with blue base and finally to swirling seated (burner attached) yellow flame. Time-averaged streamline plot obtained from 2D PIV in mid-longitudinal plane shows a recirculation zone (RZ) at the immediate vicinity of burner exit. The lifted flame is stabilized along the vortex core of this RZ. Further, when SG ∼ 1.4–3, the first occurrence of vortex breakdown (VB) induced internal recirculation zone (IRZ) is witnessed. The flame now stabilizes at the upstream stagnation point of the VB-IRZ, which is attached to the burner lip. The secondary transition represents a transformation from a swirling seated flame to swirling flame with a conical tailpiece and finally to a highly-swirled near blowout oxidizer-rich flame. This transition is understood to be the result of transition in vortex breakdown modes of the swirling flow field from dual-ring VB bubble to central toroidal recirculation zone (CTRZ). The physics of transition is described on the basis of modified Rossby number (Rom). Finally, when the swirl intensity is very high i.e. SG ∼ 10, the flame blows out due to excessive straining and due to entrainment of large amount of oxidizer due to partial premixing. The present investigation involving changes in flame topology is immensely important because any change in global flame structure causes oscillatory heat release that can couple with dynamic pressure and velocity fluctuations leading to unsteady combustion. In this light, understanding mechanisms of flame stabilization is essential to tackle the problem of thermo-acoustic instability.

Numerical investigation of an advanced aircraft model during pitching motion at high incidence

An unsteady Reynolds averaged Navier–Stokes (URANS) method combined with a rigid dynamic mesh technique was developed to simulate unsteady flows around complex configurations during pitching motion. First, a test case with the NACA0012 airfoil was selected to validate the numerical methods and our in-house codes. Then, we evaluated the unsteady flows around an advanced aircraft model during harmonic pitching motion at high incidence. The effects of pitching motion on the hysteresis of aerodynamic force, the evolution of the leading-edge vortex, and the distribution of pressure on the model’s surface were analyzed in detail. The roles of several significant parameters such as the reduced frequency and pitching amplitude were revealed. Several conclusions were found: pitching motion would delay the initiation of the leading-edge vortex, strengthen the vorticity, postpone the occurrence of vortex breakdown, and weaken the massively separated flows, thus causing additional aerodynamic force. Two categories of critical reduced frequency have been found, which divide the influence of reduced frequency on aerodynamic force into three stages, called the linear increasing range, slowly increasing range, and constant range. The first-order phase lag between the aerodynamic force and the incidence is a constant that is independent of the amplitude when the reduced frequency is sufficiently high. A scaled maximum value of C L is proposed; it depends only on the reduced frequency (instead of the amplitude), and increases linearly when the reduced frequency is sufficiently low.

A numerical analysis of effects of vortex breakdown on oxygen transport in a micro-bioreactor

Vortex breakdown effects on oxygen concentration distributions in a micro-bioreactor were investigated by a finite volume model. The bioreactor consists of a cylindrical chamber with a rotating bottom wall. The minimum oxygen concentration and volumetric oxygen transfer coefficient were summarized at different Reynolds numbers, Damkohler numbers as well as bioreactor aspect ratios. It was found that vortex breakdown extends the axis region where oxygen concentration is higher. Also vortex breakdown tends to induce a uniform oxygen concentration distribution in the bioreactor. Thus, the occurrence of vortex breakdown is of benefit to animal cell culture. Compared with that in the bioreactor with a rigid lid, vortex breakdown in the bioreactor with free surface may be better for animal cell culture as the bubble has larger size and may occur apart from the axis.

334 円筒容器内渦崩壊の数値計算

334 円筒容器内渦崩壊の数値計算

Numerical Study of Axisymmetric Vortex Breakdown Flows in an Open Cylindrical Container with Rotating Bottom Wall.

Although extended studies have been made on the vortex breakdown inside a cylindrical container by means of numerical simulation, majority of the studies conducted on this flow treated the case of flows inside a closed container with bottom and top disks. The occurrence of vortex breakdown in a cylindrical container is considered to be influenced by the swirling velocity component and the upstream vorticity distribution. Therefore, in the present study, by changing the top disk non-slip boundary condition to the free-slip boundary condition, the influence of the free surface on the emergence of vortex breakdown is investigated in detail. Numerical results are compared with the existing experimental study and novel flow behavior is noticed for container with small aspect ratios.

Vortex Breakdown in Atmospheric Columnar Vortices

Abstract Vortex breakdown occurs in tornadoes and waterspouts. This phenomenon may give information on the state and future behavior of those whirlwinds. Because of the rarity of recorded events, archival sources are consulted for qualitative descriptions from earlier times and compared with contemporary sources. Drawings and eyewitness reports from earlier times, rare photographs, movies, and observations from recent years indicate the occurrence of vortex breakdown in tornadoes and waterspouts near the ground, in the midsection of the funnel, and close to or inside the parent cloud. Since the contour of the whirlwind's funnel is delineated only by markers in the form of condensates, dust, or other debris, these markers may distort or obscure the evidence of vortex breakdown. This is a likely reason for the rare observation and identification of vortex breakdown which might be more common in whirlwinds than has been previously thought. According to the records examined, meteorologists deserve the honor f...