Tornado-like Vortices Research Articles

Study of the effects of translation and roughness on tornado-like vortices by large-eddy simulations

The numerical simulation by LES turbulent model for translating tornadoes and those over roughness was carried out in a Ward type simulator. The tornado translation was modeled by providing a relative motion on the ground and the roughness was simulated through adding a momentum source in the Navier–Stokes equation. The effects of translation and roughness on the flow fields of three typical tornado-like vortices, i.e., vortex breakdown, vortex touching down and multi-vortex, were investigated and the detailed velocity distributions, Reynolds stresses and the pressure on the ground were examined. The similarity of the flow fields after the introduction of translation and ground roughness was also studied. It was found that, at the high elevation, Vc and rc shows the same trend versus the external swirl ratio for stationary and translating tornadoes. However, if the ground is rough, the core radius at high elevation changes greatly. The ground roughness will expand the size of the core. But for the very small swirl ratio cases, the ground roughness shows the effect reducing the core size. The explanation for the evolution of the flow fields due to translation and ground roughness is provided.

非静力学軸対称モデルによる竜巻様渦の遷移に関する数値実験

非静力学軸対称モデルによる竜巻様渦の遷移に関する数値実験

Experimental study of wind pressures acting on a cooling tower exposed to stationary tornado-like vortices

Wind pressures acting on a cooling tower exposed to stationary tornado-like vortices are studied physically. This study focuses on the effects of swirl ratio and the distance between a cooling tower and a stationary tornado vortex on the pressure distribution around a cooling tower. Particular attention is devoted to the differences of pressure distribution and cross-correlation coefficients of pressures in a tornado with that in a conventional boundary-layer-type straight-line wind. The results show that a cooling tower exposed to a tornado experiences combined effects of pressure drop accompanying a tornado and aerodynamic flow-structure interaction. The pressure drop accompanying a tornado dominates the pressure coefficient magnitudes when the cooling tower is located at the tornado core center. The cooling tower experiences maximum wind force when it is located at the tornado core radius. Results show that the tornado-induced wind pressure is significantly different than that in conventional straight-line winds, and highlight the need to study tornado-induced wind loads on structures.

Sensitivity of Real-Data Simulations of the 3 May 1999 Oklahoma City Tornadic Supercell and Associated Tornadoes to Multimoment Microphysics. Part I: Storm- and Tornado-Scale Numerical Forecasts

Abstract Numerical predictions of the 3 May 1999 Oklahoma City, Oklahoma, tornadic supercell are performed within a real-data framework utilizing telescoping nested grids of 3-km, 1-km, and 250-m horizontal spacing. Radar reflectivity and radial velocity from the Oklahoma City WSR-88D are assimilated using a cloud analysis procedure coupled with a cycled 3DVAR system to analyze storms on the 1-km grid for subsequent forecast periods. Single-, double-, and triple-moment configurations of a multimoment bulk microphysics scheme are used in several experiments on the 1-km and 250-m grids to assess the impact of varying the complexity of the microphysics scheme on the storm structure, behavior, and tornadic activity (on the 250-m grid). This appears to be the first study of its type to investigate single- versus multimoment microphysics within a real-data context. It is found that the triple-moment scheme overall performs the best, producing the smallest track errors for the mesocyclone on the 1-km grid, and stronger and longer-lived tornado-like vortices (TLVs) on the 250-m grid, closest to the observed tornado. In contrast, the single-moment scheme with the default Marshall–Palmer rain intercept parameter performs poorly, producing a cold pool that is too strong, and only weak and short-lived TLVs. The results in the context of differences in latent cooling from evaporation and melting between the schemes, as well as implications for numerical prediction of tornadoes, are discussed. More generally, the feedbacks to storm thermodynamics and dynamics from increasing the prognostic detail of the hydrometeor size distributions are found to be important for improving the simulation and prediction of tornadic thunderstorms.

A simple hydrodynamic model of tornado-like vortices

Based on similarity arguments, a simple fluid dynamic model of tornado-like vortices is offered that, with account for “vortex breakdown” at a certain height above the ground, relates the maximal azimuthal velocity in the vortex, reachable near the ground surface, to the convective available potential energy (CAPE) stored in the environmental atmosphere under pre-tornado conditions. The relative proportion of the helicity (kinetic energy) destruction (dissipation) in the “vortex breakdown” zone and, accordingly, within the surface boundary layer beneath the vortex is evaluated. These considerations form the basis of the dynamic-statistical analysis of the relationship between the tornado intensity and the CAPE budget in the surrounding atmosphere.

Application of magnetically driven tornado-like vortex for stirring floating particles into liquid metal

Application of magnetically driven tornado-like vortex for stirring floating particles into liquid metal

Reproducing tornadoes in laboratory using proper scaling

Experimentally simulated tornado-like vortices are related to field tornadoes in order to: (i) establish proper kinematic and dynamic scaling and (ii) attempt to determine a relationship between laboratory parameters and the Enhanced Fujita Scale (EF-Scale). Data from recent in-situ Doppler radar campaigns are analyzed using the Ground-Based Velocity Track Display (GBVTD) method and a unique dataset of three-dimensional axisymmetric tornado flow fields is generated. In parallel, Particle Image Velocimetry (PIV) results of the most recent experimental simulations of tornado vortices performed in the model WindEEE Dome (MWD) are analyzed and then compared with the GBVTD-retrieved full-scale data. Based on these comparisons, the swirl ratio of the full-scale tornadoes, as well as the length and velocity scaling ratios of the simulated tornadoes are identified. It is concluded that the MWD apparatus can generate tornado-like vortices equivalent to EF0 to low-end EF3 rated tornadoes in nature.

Visualization of tornado-like vortex interacting with wide tornado-break wall

A three-dimensional CFD model, based on large eddy simulation, is utilized to numerically simulate a perpendicular interaction between a traveling tornado-like vortex and a wide rigid wall. The tornado-like flow is modeled by the Rankine-Combined Vortex Model. The case, where the wall width is much larger than the tornado vortex core diameter is examined. The main goal of the paper is to improve the understanding of the dynamics of the tornado–wall interaction. The conclusions drawn in this study are supported by various visualizations. During the tornado impact, the structure of the tornado-like vortex is substantially mitigated by the wide wall. The low-level portion of the vortex is disturbed both in front of the wall and behind the wall. When the vortex is approaching the wall, the vortex-induced velocities cause a boundary layer separation from the leading face of the wall. The ejected, from the wall, vorticity patches wrap around the tornado-like vortex causing its weakening. Behind the wall the tornado vortex is subjected to the turbulent flow that further disrupts the tornado vortex structure. The tornado-like vortex mitigation results in a low wind speed region behind the tornado-break wall, where the wind velocities are reduced by more than 50 %.

Numerical study on dynamics of a tornado-like vortex with touching down by using the LES turbulence model

The dynamics of a tornado-like vortex with touching down is investigated by using the LES turbulence model. The detailed information of the turbulent flow fields is provided and the force balances in radial and vertical directions are evaluated by using the time-averaged axisymmetric Navier-Stokes equations. The turbulence has slightly influence on the mean flow fields in the radial direction whereas it shows strong impacts in the vertical direction. In addition, the instantaneous flow fields are investigated to clarify and understand the dynamics of the vortex. An organized swirl motion is observed, which is the main source of the turbulence for the radial and tangential components, but not for the vertical component. Power spectrum analysis is conducted to quantify the organized swirl motion of the tornado-like vortex. The gust speeds are also examined and it is found to be very large near the center of vortex.

Numerical Prediction of Flow Structure and Heat Enhancement with Different Dimple Depth

Aim at finding the relation between flow structure and heat enhancement, this paper change the dimple depth to print diameter from 0.06 to 0.3, with an interval of 0.02, and keep the print diameter constant. Through vortex structure analysis and Nu number integration, find out that: Vortex structure varies with dimple depth. Shallow depth dimple induces horseshoe vortex; middle depth dimple induces symmetric tornado-like vortex; deep depth dimple induces tornado-like vortex. Heat enhancement is closely related to vortex structure. Where spiral separation focus point exists, there the lowest Nu/Nu0 exists. Flow impingement at the windward side of dimple and new viscous layer generation at immediate plane after the dimple enhance heat transfer. The highest Nu/Nu0 exists in the sharp rearward dimple edge. From Fig 6, the deeper the dimple is, the higher the average Nu/Nu0 is, so does the pressure drop. But from 0.06 to 0.2 dimple, heat enhancement increased because of vortex structure changed. From 0.2 to 0.3 heat enhancement increased because of the increased area.

Magneto-Rotational Symmetry in Chiral Magnetoelectrodeposition

The chiral activities of electrodeposited surfaces fabricated under vertical magnetic fields were theoretically examined. Chiral activities arise from the micro-vortexes, called micro-MHD (magnetohych-odynamic) flows (MMF), activated under a tornado-like vortex called vertical MHD flow (VMF). It was concluded that the interaction between MMF and VMF determines the activities.

Hawkmoth flight performance in tornado-like whirlwind vortices

Vertical vortex systems such as tornadoes dramatically affect the flight control and stability of aircraft. However, the control implications of smaller scale vertically oriented vortex systems for small fliers such as animals or micro-air vehicles are unknown. Here we examined the flapping kinematics and body dynamics of hawkmoths performing hovering flights (controls) and maintaining position in three different whirlwind intensities with transverse horizontal velocities of 0.7, 0.9 and 1.2 m s−1, respectively, generated in a vortex chamber. The average and standard deviation of yaw and pitch were respectively increased and reduced in comparison with hovering flights. Average roll orientation was unchanged in whirlwind flights but was more variable from wingbeat to wingbeat than in hovering. Flapping frequency remained unchanged. Wingbeat amplitude was lower and the average stroke plane angle was higher. Asymmetry was found in the angle of attack between right and left wings during both downstroke and upstroke at medium and high vortex intensities. Thus, hawkmoth flight control in tornado-like vortices is achieved by a suite of asymmetric and symmetric changes to wingbeat amplitude, stroke plane angle and principally angle of attack.

Direct numerical simulation of steady state, three dimensional, laminar flow around a wall mounted cube

The topology and evolution of flow around a surface mounted cubical object in three dimensional channel flow is examined for low to moderate Reynolds numbers. Direct numerical simulations were performed via a home made parallel finite element code. The computational domain has been designed according to actual laboratory experiment conditions. Analysis of the results is performed using the three dimensional theory of separation. Our findings indicate that a tornado-like vortex by the side of the cube is present for all Reynolds numbers for which flow was simulated. A horseshoe vortex upstream from the cube was formed at Reynolds number approximately 1266. Pressure distributions are shown along with three dimensional images of the tornado-like vortex and the horseshoe vortex at selected Reynolds numbers. Finally, and in accordance to previous work, our results indicate that the upper limit for the Reynolds number for which steady state results are physically realizable is roughly 2000.

Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells

Abstract A three-dimensional idealized cloud model was used to study the storm-scale differences between simulated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and nontornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated environmental sounding. Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that descend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0–1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations.

Spin-up of a magnetically driven tornado-like vortex

AbstractThe spin-up of a concentrated vortex in a liquid metal cylinder with a free surface is considered experimentally and numerically. The vortex is driven by two flow-independent magnetic body forces. A continuously applied rotating magnetic field provides the source of the angular momentum. A pulse of about one order of magnitude stronger travelling magnetic field drives a converging flow that temporarily focuses this angular momentum towards the axis of the container. A highly concentrated vortex forms that produces a funnel-shaped surface depression. We explore experimentally the duration, the depth and the conditions of formation of this funnel. Additionally, we measure the axial velocity and calculate the axisymmetric flow field of this transient vortex at a lower force magnitude. The spin-up vortex is similar to the corresponding developed time-averaged turbulent vortex driven by the same magnetic forces (Grants et al., J. Fluid Mech., vol. 616, 2008, pp. 135–152). There are two main differences. First, the maximum swirl concentration condition cannot be expressed as a constant ratio of the two driving forces. Second, a much higher degree of swirl concentration is feasible. We explain these differences as due to a much lower turbulence during the spin-up.

Characteristics of internal pressures and net local roof wind forces on a building exposed to a tornado-like vortex

Extreme disturbances in the atmosphere such as tornadoes produce large wind loads on buildings that sometimes result in structural failures. Post-tornado investigations have revealed that building roofs are vital components that often suffer extensive damage. An experimental investigation using the tornado-like flow simulator at Tokyo Polytechnic University was performed to obtain a better understanding of the effects of building location with respect to vortex, for two opening types, on the internal pressures in a building and consequent changes in net local roof wind force. Such studies are significant from the viewpoint of design of components and cladding elements. The lowest minimum peak values of net local roof wind force experienced by a building with 0.1% opening ratio, for building models located well within the vortex core, were 1.4 times the values for a building model with 3.9% opening ratio.

An Explanation for Flow Features of Spike-Type Stall Inception in an Axial Compressor Rotor

The unsteady behavior and three-dimensional flow structure of spike-type stall inception in an axial compressor rotor were investigated by experimental and numerical analyses. Previous studies revealed that the test compressor falls into a mild stall after emergence of a spike, in which multiple stall cells, each consisting of a tornado-like vortex, are rotating. However, the flow mechanism from the spike onset to the mild stall remains unexplained. The purpose of this study is to describe the flow mechanism of a spike stall inception in a compressor. In order to capture the transient phenomena of spike-type stall inception experimentally, an instantaneous casing pressure field measurement technique was developed, in which 30 pressure transducers measure an instantaneous casing pressure distribution inside the passage for one blade pitch at a rate of 25 samplings per blade passing period. This technique was applied to obtain the unsteady and transient pressure fields on the casing wall during the inception process of the spike stall. In addition, the details of the three-dimensional flow structure at the spike stall inception were analyzed by a numerical approach using the detached-eddy simulation (DES). The instantaneous casing pressure field measurement results at the stall inception show that a low-pressure region starts traveling near the leading edge in the circumferential direction just after the spiky wave was detected in the casing wall pressure trace measured near the rotor leading edge. The DES results reveal the vortical flow structure behind the low-pressure region on the casing wall at the stall inception, showing that the low-pressure region is caused by a tornado-like separation vortex resulting from a leading-edge separation near the rotor tip. A leading-edge separation occurs near the tip at the onset of the spike stall and grows to form the tornado-like vortex connecting the blade suction surface and the casing wall. The casing-side leg of the tornado-like vortex generating the low-pressure region circumferentially moves around the leading-edge line. When the vortex grows large enough to interact with the leading edge of the next blade, the leading-edge separation begins to propagate, and then the compressor falls into a stall with decreasing performance.

Tornadogenesis in a Simulated Mesovortex within a Mesoscale Convective System

Abstract The Advanced Regional Prediction System (ARPS) is used to simulate a tornadic mesovortex with the aim of understanding the associated tornadogenesis processes. The mesovortex was one of two tornadic mesovortices spawned by a mesoscale convective system (MCS) that traversed southwestern and central Oklahoma on 8–9 May 2007. The simulation used 100-m horizontal grid spacing, and is nested within two outer grids with 400-m and 2-km grid spacing, respectively. Both outer grids assimilate radar, upper-air, and surface observations via 5-min three-dimensional variational data assimilation (3DVAR) cycles. The 100-m grid is initialized from a 40-min forecast on the 400-m grid. Results from the 100-m simulation provide a detailed picture of the development of a mesovortex that produces a submesovortex-scale tornado-like vortex (TLV). Closer examination of the genesis of the TLV suggests that a strong low-level updraft is critical in converging and amplifying vertical vorticity associated with the mesovortex. Vertical cross sections and backward trajectory analyses from this low-level updraft reveal that the updraft is the upward branch of a strong rotor that forms just northwest of the simulated TLV. The horizontal vorticity in this rotor originates in the near-surface inflow and is caused by surface friction. An additional simulation with surface friction turned off does not produce a rotor, strong low-level updraft, or TLV. Comparison with previous two-dimensional numerical studies of rotors in the lee of mountains shows striking similarities to the rotor formation presented herein. The findings of this study are summarized in a four-stage conceptual model for tornadogenesis in this case that describes the evolution of the event from mesovortexgenesis through rotor development and finally TLV genesis and intensification.

Three-dimensional instabilities in tornado-like vortices with secondary circulations

AbstractTornadoes and other intense atmospheric vortices are known to occasionally transition to a flow structure with multiple vortices within their larger circulations. This phenomenon has long been ascribed to fluid dynamical instability of the inner-core circulation, and many previous studies have diagnosed low-wavenumber unstable modes in tornado-like vortices that resemble the observed structures. However, relatively few of these studies have incorporated the strong vertical motions of the inner-core circulation into the stability analysis, and no stability analyses have been performed using a complete, frictionally driven secondary circulation with strong radial inflow near the surface. Stability analyses are presented using the complete circulations generated from idealized simulations of tornado-like vortices. Fast-growing unstable modes are found that are consistent with the asymmetric structures present in these simulations. Attempts to correlate the structures and locations of these modes with instability conditions for vortices with axial jets derived by Howard & Gupta and by Leibovich & Stewartson produce only mixed results. Analyses of perturbation energy growth show that interactions between eddy fluxes and the radial shear of the azimuthal wind contribute very little to the growth of the dominant modes. Rather, the radial shear of the vertical wind and the vertical shear of the vertical wind (corresponding to deformation in the axial direction) are the primary energy sources for perturbation growth. Relatively weak axisymmetric instabilities are also identified that have some similarity to symmetric oscillations that have been observed in tornadoes.

Large eddy simulations of translation and surface roughness effects on tornado-like vortices

The effects of translation and surface roughness on tornado-like vortices were studied using Large Eddy Simulation (LES) of a laboratory scale tornado. The translation was simulated by employing a moving-wall boundary condition and surface roughness was introduced by physical modeling of roughness element blocks. For lower swirl ratios the translation reduces the maximum mean tangential velocity whereas for high swirl ratios it causes a slight increase in the maximum mean tangential velocity. The introduction of surface roughness causes an overall effect similar to reducing the swirl ratio.