Vertical Vorticity Research Articles

A Numerical Simulation of the Development Process of a Mesoscale Convection Complex Causing Severe Rainstorm in the Yangtze River Delta Region behind a Northward Moving Typhoon

In recent years, due to the influence of global warming, extreme weather events occur frequently, such as the continuous heavy precipitation, regional high temperature, super typhoon, etc. Tropical cyclones make frequent landfall, heavy rains and flood disasters caused by landfall typhoons have a huge impact, and typhoon rainstorms are often closely related to mesoscale and small-scale system activities. The application 2020 NCEP (National Centers for Environmental Prediction) final operational global analysis data and WRF (Weather Research and Forecasting model, version 3.9) mesoscale numerical prediction model successfully simulates the evolution characteristics of the mesoscale convective complex (MCC) that caused an extreme rainstorm in the Yangtze River delta region behind a northwards typhoon in this article. The results show that a meso-β-scale vortex existed in the mid- to upper troposphere in the region where the MCC occurred; accompanied by the occurrence of the meso-β-scale vortex, the convective cloud clusters developed violently, and its shape is a typical vortex structure. The simulation-sensitive experiment shows that the development of the meso-β-scale cyclonic vortex is the main reason for the enhancement of MCC. The occurrence and development of the MCC is manifested as a vertical positive vorticity column and a strong vertical ascending motion region in the dynamic field. In the development and maturity stage of the MCC, the vorticity and vertical rising velocity in the MCC area are significantly greater than those in the weakened typhoon circulation, which shows significant mesoscale convective system characteristics. The diagnostic analysis of the vorticity equation shows that the positive vorticity advection caused by the meso-β-scale cyclonic vortex in the mid- to upper troposphere plays important roles in the development of the MCC. Enhanced low-level convergence enhances vertical ascending motion. The convective latent heat release also plays an important role on the development of the MCC, changes the atmospheric instability by heating, enhances the upward movement, and delivers positive vorticity to the upper level, making the convection develop higher, forming a positive feedback mechanism between low-level convergence and high-level divergence. The simulation-sensitive experiment also shows that the meso-β-scale cyclonic vortex formation in this process is related to convective latent heat release.

Generation of stripe-like vortex flow by noncollinear waves on the water surface

We have studied experimentally the generation of vortex flow by gravity waves with a frequency of 2.34Hz excited on the water surface at an angle 2θ=arctan(3/4)≈36∘ to each other. The resulting horizontal surface flow has a stripe-like spatial structure. The width of the stripes L=π/(2ksinθ) is determined by the wave vector k of the surface waves and the angle between them, and the length of the stripes is limited by the system size. It was found that the vertical vorticity Ω of the current on the fluid surface is proportional to the product of wave amplitudes, but its value is much higher than the value corresponding to the Stokes drift and it continues to grow with time even after the wave motion reaches a stationary regime. We demonstrate that the measured dependence Ω(t) can be described within the recently developed model that takes into account the Eulerian contribution to the generated vortex flow and the effect of surface contamination. This model contains a free parameter that describes the elastic properties of the contaminated surface, and we also show that the found value of this parameter is in reasonable agreement with the measured decay rate of surface waves.

Volume Transport by a 3D Quasigeostrophic Heton

Oceanic flows self-organize into coherent vortices, which strongly influence their transport and mixing properties. Counter-rotating vortex pairs can travel long distances and carry trapped fluid as they move. These structures are often modeled as hetons, viz. counter-rotating quasigeostrophic point vortex pairs with equal circulations. Here, we investigate the structure of the transport induced by a single three-dimensional heton. The transport is determined by the Hamiltonian structure of the velocity field induced by the heton’s component vortices. The dynamics display a sequence of bifurcations as one moves through the heton-induced velocity field in height. These bifurcations create and destroy unstable fixed points whose associated invariant manifolds bound the trapped volume. Heton configurations fall into three categories. Vertically aligned hetons, which are parallel to the vertical axis and have zero horizontal separation, do not move and do not transport fluid. Horizontally aligned hetons, which lie on the horizontal plane and have zero vertical separation, have a single parameter, the horizontal vortex half-separation Y, and simple scaling shows the dimensional trapped volume scales as Y3. Tilted hetons are described by two parameters, Y and the vertical vortex half-separation Z, rendering the scaling analysis more complex. A scaling theory is developed for the trapped volume of tilted hetons, showing that it scales as Z4/Y for large Z. Numerical calculations illustrate the structure of the trapped volume and verify the scaling theory.

Spatial Flow Pattern, Multi‐Dimensional Vortices, and Junction Momentum Exchange in a Partially Covered Submerged Canopy Flume

AbstractThis study experimentally investigated the influence of a model‐submerged canopy on three‐dimensional hydrodynamic structures in a partially obstructed flume. Canopy density, water discharge, and water depth were varied to achieve generalized conclusions. With tracer experiments, multi‐dimensional (vertical and horizontal) large‐scale coherent vortices arising from both the top and lateral edges of the canopy were visualized. The results of the mean flow and turbulence field show that the hydrodynamic characteristics are highly three‐dimensional, impacted by canopy density and submergence ratio. Typical shear layers are confirmed by scaled velocity distribution with local characteristic velocity and length scales in both vertical and transverse directions along the canopy edges except for the near‐bed region where horizontal vortices are inhibited. Along the canopy edges, the average effect of the vertical coherent vortices overall outweighs that of the horizontal coherent vortices for dense canopies, the ratio of which can be regressed more precisely with the water‐related Reynolds number. The vertical profiles of the longitudinal velocity in the near‐junction region are characterized by near‐bed velocity deflection, strengthened by the increase in canopy density. The negative velocity gradients are found to be more closely related to the drag length‐related Reynolds number. An in‐depth analysis of secondary flows and near‐junction momentum budget has been conducted to show the mass and momentum exchange processes. The results show that multidimensional coherent vortices and turbulence‐induced secondary flows play different roles in mass and momentum exchange, accounting for near‐bed velocity deflections in the junction region.

Transition of Near-Ground Vorticity Dynamics during Tornadogenesis

Abstract Although much is known about the environmental conditions necessary for supercell tornadogenesis, the near-ground vorticity dynamics during the tornadogenesis process itself are still somewhat poorly understood. For instance, seemingly contradicting mechanisms responsible for large near-ground vertical vorticity can be found in the literature. Broadly, these mechanisms can be sorted into two classes, one being based on upward tilting of mainly baroclinically produced horizontal vorticity in descending air (here called the downdraft mechanism), while in the other the horizontal vorticity vector is abruptly tilted upward practically at the surface by a strong updraft gradient (referred to as the in-and-up mechanism). In this study, full-physics supercell simulations and highly idealized simulations show that both mechanisms play important roles during tornadogenesis. Pretornadic vertical vorticity maxima are generated via the downdraft mechanism, while the dynamics of a fully developed vortex are dominated by the in-and-up mechanism. Consequently, a transition between the two mechanisms occurs during tornadogenesis. This transition is a result of axisymmetrization of the pretornadic vortex patch and intensification via vertical stretching. These processes facilitate the development of the corner flow, which enables production of vertical vorticity by upward tilting of horizontal vorticity practically at the surface, i.e., the in-and-up mechanism. The transition of mechanisms found here suggests that early stages of tornado formation rely on the downdraft mechanism, which is often limited to a small vertical component of baroclinically generated vorticity. Subsequently, a larger supply of horizontal vorticity (produced baroclinically or via surface drag, or even imported from the environment) may be utilized, which marks a considerable change in the vortex dynamics.

Submesoscale Ageostrophic Motions Within and Below the Mixed Layer of the Northwestern Pacific Ocean

AbstractSubmesoscale dynamics below the mixed layer (ML) and their mechanisms are still unclear. By a series of nested simulations in the Pacific Northwest with high horizontal resolution of ∼500 m, this study reveals that there exist strong submesoscale ageostrophic motions in the upper pycnocline of the Kuroshio Extension region. These motions exhibit enhanced lateral buoyancy gradient and vigorous vertical velocity but with weak vertical vorticity distinct from the ML submesoscale activities. The vertical velocity in the high‐resolution simulation reaches tens of meters per day, consistent with recent observations (e.g., SubMESI and OSMOSIS). Our analysis shows that the enhanced vertical velocity is mostly attributed to the along‐isopycnal motions at the Kuroshio front, but in the region nearby the large vertical velocity mostly arises from the wave‐like vertical movement of isopycnals. To understand the mechanisms for the large vertical velocity, this paper further examined the instability of the flow and the frequency‐wavenumber spectra of vertical vorticity, lateral divergence, lateral buoyancy gradient, and vertical velocity. A criterion based on the ratio between divergence and vorticity variance in spectral space is used to roughly identify the upper bound of unbalanced submesoscales. The results suggest that the high‐frequency, high‐wavenumber processes dominate the vertical motions within and below the ML and significantly enhance the net vertical heat transport between the ML and the ocean interior. This study seeks to provide comprehension of the submesoscale ageostrophic motions below the ML and their impacts on the upper ocean.

DNS of the spatiotemporal evolution of the vorticity in (pure) mode B of a circular cylinder’s wake

In the present paper, the spatio-temporal evolution of the vorticity field in the second wake instability, i.e. (pure) mode B is investigated to understand the wake vortex dynamics and sign relationships among the three vorticity components. Direct numerical simulation of the flow past a circular cylinder in the three-dimensional (3D) wake transition is performed, typically at a Reynolds number of 300. According to the time histories of fluid forces and frequency analysis, three different stages are identified. In the fully developed wake (FDW), the spanwise vortex core is almost two-dimensional, while the vortex braid is 3D due to the dominant streamwise interaction. However, streamwise and vertical vorticities owing to the intrinsic 3D instability are already generated first on cylinder surfaces early in the computational transition (CT). The evolution of additional vorticities with the same features as mode B shows that (pure) mode B could already be formed in the late CT. In the FDW, a special sign symmetry of these additional vorticities on the rear surface is observed, which is exactly opposite to that in (pure) mode B. Similarly, the two sign laws found in (pure) mode A are also verified in three typical regions, independent of the Reynolds number, for (pure) mode B. Particularly, the mechanism for the physical origin of streamwise and vertical vortices in the shear layers is the vortex generation on the wall first and then dominant vortex induction just near the wall. The entire process of the formation and shedding of vortices with three components of vorticity is first and completely illustrated. Other characteristics of the evolution of mode B are presented in detail.

Potential vorticity perspective of the genesis of a Tibetan Plateau vortex in June 2016

At midnight on 27–28 June 2016, a Tibetan Plateau (TP) Vortex (TPV) generated over the western TP that subsequently caused a downstream record-breaking rainstorm and extremely severe natural disaster. Based on reanalysis data and satellite imagery, this study investigates the formation of this TPV from a potential vorticity (PV) perspective. Results show that, in late June 2016, a remarkable circulation anomaly occurred over the TP and its peripheral area, with easterly flow in the middle and lower troposphere developing in the subtropical zone, replacing the normal westerly flow there. Its forefront merged with the southwesterly flow from the west and penetrated and converged over the western TP where the surface was warmer than normal, forming a low-level jet and downward slantwise isentropic surfaces in-situ. When the air parcel slid down the slantwise isentropic surface, its vertical relative vorticity developed owing to slantwise vorticity development associated with PV restructuring. At the same time, the penetrating southwesterly flow brought abundant water vapor to the western TP and induced increasing sub-cloud entropy and air ascent there. Low-layer cloud formed and the cloud liquid water content increased. The strong latent heat that was released in association with the formation of cloud produced strong diabatic heating near 400 hPa at night and strong PV generation below. The normal diurnal variation was interrupted and the vortex was generated near the surface. These results demonstrate that, against a favorable circulation background, both adiabatic and diabatic PV processes are crucial for TPV genesis.

Quasi-streamwise vortices and enhanced dissipation for incompressible 3D Navier–Stokes equations

We consider the 3D incompressible Navier–Stokes equations under the following 2 + 1 2 2+\frac {1}{2} -dimensional situation: vertical vortex blob (quasi-streamwise vortices) being stretched by two-dimensional shear flow. We prove enhanced dissipation induced by such quasi-streamwise vortices.

The effects of room length on jet momentum flux

As to mixing ventilation in indoor environments, the turbulent jet plays a major role in driving the air movement, contaminant transport, and heat transfer. The main characteristic of a turbulent jet is its momentum flux. By entrainment of air, the flow of a jet increases and may enhance the flooding of contaminant. In investing the jet’s momentum flux, it is generally regarded that the supply jet collides with the opposing wall and the jet is transformed into a wall jet. However, this is not always true if a jet is not sufficiently strong, or the length of a room is large. Therefore, this study adopted computational fluid dynamics (CFD) to investigate the supply jet development and its momentum flux by varying the room length. Initially, the width of the air supply inlet was the same with that of the room. By defining n as the ratio of room length and height, when n = 3, there is a horizontal a vortex which is the normal behaviour. When the room length increased further, the supply jet was unable to collide with the opposing wall. This investigation got two vertical vortices at the room end which is new. The two new vertical vortices were most pronounced for n = 5. It is possible that increasing the length of the room introduces a gradual transition towards a flow in a rectangular duct. This flow is probably very much governed by the side walls. Therefore, this study reduced the width of the air supply inlet by half and maintained the same flow rate. However, a single vertical vortex was identified at the room end for n = 5. In both scenarios, the supply jet may create new vortices that would enhance the flooding of contaminants.

Vertical Vortex Development in Hurricane Michael (2018) during Rapid Intensification

Abstract The landfall of Hurricane Michael (2018) at category-5 intensity occurred after rapid intensification (RI) spanning much of the storm’s lifetime. Four Hurricane Hunter aircraft missions observed the RI period with tail Doppler radar (TDR). Data from each of the 14 aircraft passes through the storm were quality controlled via a combination of interactive and machine-learning techniques. TDR data from each pass were synthesized using the Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) variational wind retrieval technique to yield three-dimensional kinematic fields of the storm to examine inner-core processes during RI. Vorticity and angular momentum increased and concentrated in the eyewall region. A vorticity budget analysis indicates that the tendencies became more axisymmetric over time. In this study, we focus in particular on how the eyewall vorticity tower builds vertically into the upper levels. Horizontal vorticity associated with the vertical gradient of tangential wind was tilted into the vertical by the eyewall updraft to yield a positive vertical vorticity tendency inward atop the existing vorticity tower, which is further developed locally upward and outward along the sloped eyewall through advection and stretching. Observed maintenance of thermal wind balance from a thermodynamic retrieval shows evidence of a strengthening warm core, which aided in lowering surface pressure and further contributed to the efficient intensification in the latter stages of this RI event.

平織金網近傍の流れ構造の解明

In this study, the aim is to clarify the essential vortex interference structure, the flow structure of the wake of the plain weave wire mesh composed of a simple repeating structure is investigated. The aerodynamic characteristics of the wake caused by the wire mesh are assumed to be due to the vortex structure formed in the wake of the wire mesh, which interferes with the jet flow from the open area, however the relationship between the two is not clear. Therefore, the relationship between the vortex structure and the jet flow from the open area was examined. As a result, it was found that the distribution trend of the slow-velocity region behind the wire is different in the longitudinal sections of the open area wake. In this longitudinal section, the slow-velocity area and the high-velocity area were alternately distributed behind the wire crossing point. On the one hand, in the longitudinal section behind the wire, the jet flow from the side of the wire was found. This jet is guessed to be related to vertical vortex of the wake. And, both jets were confirmed to be homogenized in the wake about 10 times the wire diameter.

Inhomogeneous Couette–Poiseuille shear flow

The paper studies a Couette–Poiseuille exact solution for describing inhomogeneous steady-state flows of a viscous incompressible vertical vortex fluid. The fluid moves in an infinite horizontal layer, this motion being conditioned by the displacement of the lower boundary and the specification of the pressure gradient. An exact solution to the hydrodynamics equations describing the three-dimensional inhomogeneous Couette–Poiseuille shear flow is obtained. It is polynomial in all the coordinates, and it belongs to the Lin–Sidorov–Aristov family. Consideration of the inhomogeneity of the velocity field leads to the recording of counterflows. Pressure variation along the horizontal coordinates results in an additional stationary stagnation point as compared to the isobaric flow of a viscous incompressible fluid.

Numerical Analysis of the Primary Gas Boundary Layer Flow Structure in Laser Fusion Cutting in Context to the Striation Characteristics of Cut Edges

In cutting metals with solid-state lasers, a characteristic cutting edge structure is generated whose formation mechanisms still elude a consistent explanation. Several studies suggest a major contribution of the pressurized gas flow. Particular emphasis must be devoted to the gas boundary layer and its developing flow characteristics, since they determine the heat and momentum exchange between the cutting gas and the highly heated melt surface and thus the expulsion of the molten material from the kerf. The present study applies a CFD simulation model to analyze the gas flow during laser cutting with appropriate boundary conditions. Specifically, the gas boundary layer development is considered with a high spatial discretization of this zone in combination with a transition turbulence model. The results of the calculation reveal for the first time that the boundary layer is characterized by a quasi-stationary vortex structure composed of nearly horizontal geometry- and shock-induced separation zones and vertical vortices, which contribute to the transition to turbulent flow. Comparison of the results with the striation structure of experimental cut edges reveals a high agreement of the location, orientation, and size of the characteristic vortices with particular features of the striation structure of cut edges.

Structure and maintenance mechanisms of the Mascarene High in austral winter

AbstractThe Mascarene High (MH), is a key component of the Asian‐Africa‐Australia monsoon system in austral winter (JJA), spanning over the South Indian Ocean (15°–35°S, 15°–110°E). Its three‐dimensional structures and maintenance mechanisms are examined in this study. It is a low‐level subtropical high dominating the southern Africa and South Indian Ocean, characterized by a north‐westward tilt with height, which is attributed to its spatially inhomogeneous thermal structure. Large‐scale subsidence characterizes the main body of the MH, with the stronger subsidence to the east than to the west. Diagnosis using the complete form of the vertical vorticity tendency equation shows that the anticyclonic structure of the MH, which can be described by the distribution of meridional wind, is maintained mainly by the vertical gradient of diabatic heating, change in static stability, and friction dissipation. In particular, a combination of sensible heating and longwave radiative cooling results in a vertical decreasing gradient of diabatic heating in the lower troposphere. It generates the stronger southerlies over the subtropical South Indian Ocean than over the southern Africa. Meanwhile, over the South Indian Ocean, the increasing static stability as a result of the downward transport of a more stable atmosphere partly offsets the effect of the vertical gradient of diabatic heating, and southerlies still prevail there. Over the southern Africa, topographic friction dissipation induces northerlies, balancing the effect of the vertical gradient of diabatic heating with a stronger magnitude, and northerlies prevail.

Effects of Heterogeneous Surface Roughness on Boundary Layer Kinematics and Wind Shear

Different types of land cover are associated with different surface roughness, which produce variations in the frictional force on the wind. Therefore, mean wind profiles at low levels often differ markedly over short distances where there is a gradient in surface roughness. Horizontal gradients in surface roughness may produce vertical vorticity, circulation, and horizontal divergence. The effect of roughness on vertical wind shear and storm-relative helicity is also qualitatively important and may lead to large gradients in helicity over short distances. Recent studies also suggest an important role of friction in tornadogenesis.
 We show conceptually and theoretically how gradients in surface roughness produce quasi-ambient convergence and vertical vorticity, and gradients in vertical shear and storm-relative helicity. We then present observational data and numerical simulations that demonstrate the effects of surface roughness on the kinematics and shear of boundary-layer flow, for future work examining the importance of these processes for tornadogenesis.

Generation of Localised Vertical Streams in Unstable Stratified Atmosphere

A new model of axially symmetric concentrated vortex generation was developed herein. In this work, the solution of a nonlinear equation for internal gravity waves in an unstable stratified atmosphere was obtained and analysed in the framework of ideal hydrodynamics. The related expressions for the velocities in the inner and outer regions of the vortex were described by Bessel functions and modified zeroth-order Bessel functions. The proposed new nonlinear analytical model allows the study of the structure and dynamics of vortices in the radial region. The formation of jets (i.e., structures elongated in the vertical direction with finite components of the poloidal (radial and vertical) velocities that grow exponentially in time in an unstable stratified atmosphere) was also analysed. The characteristic growth time was determined by the inverse growth rate of instability. It is shown that a seed vertical vorticity component may be responsible for the formation of vortices with a finite azimuthal velocity.

Lifecycle of a Submesoscale Front Birthed from a Nearshore Internal Bore

AbstractThe ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entails complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for approximately 4 h, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of alongshore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Bottom rack intake improvement as a fluid physics application through a computational fluid dynamics model

This research focuses on improving the hydraulic behavior of a traditionally design bottom rack intake, from variations in roughness parameters, free height, and the inclusion of chamfers, establishing a contribution to the contrast between classical physics and the physics that takes over the partial resolution of the Navier-Stokes equations. To make possible the structure in OpenFOAM, it is necessary to use the geometric tool Salome-Meca, as well as a meshing tool (snappyHexMesh), and the InterFOAM solver in the processing stage. In the same way, through the turbulence model (K-E) local effects are evidenced in the Fluid-Structure interaction, as well as the identification of events and the development of the phenomenon of vorticity. The results show the improvement presented in some areas of the structure from the stabilization of the water flow through of the fluid-structure interaction change, the modification of the geometry and roughness, minimizing the presence of vertical vortices, cavitation, and surrounding areas. This allows us to conclude that traditional hydraulic do not consider the real physical flow behavior within the structure and neither the subsequent phenomena that develop, establishing as a starting point the need to rethink the design of the bottom rack intakes.

Formation of Coherent Flow Structures beyond Vegetation Patches in Channel

By using model vegetation (e.g., synthetic bars), vortex structures in a channel with vegetation patches have been studied. It has been reported that vortex structures, including both the vertical and horizontal vortexes, may be produced in the wake in the channel bed with a finite-width vegetation patch. In the present experimental study, both velocity and TKE have been measured (via Acoustic Doppler Velocimeter—ADV) to study the formation of vortexes behind four vegetation patches in the channel bed. These vegetation patches have different dimensions, from the channel-bed fully covered patch to small-sized patches. Model vegetation used in this research is closely similar to vegetation in natural rivers with a gravel bed. The results show that, for a channel with a small patch (Lv/Dc = 0.44 and Dv/Dc = 0.33; where Lv and Dv are the length and width of patch and Dc is the channel width, respectively), both the flow passing through the patch and side flow around the patch have a considerable effect on the formation of flow structures beyond the patch. The results of further analysis via 3D classes of the bursting events show that the von Karman vortex street splits into two parts beyond the vegetation patch as the strong part near the surface and the weak part near the bed; while the middle part of the flow is completely occupied by the vertical vortex formed at a distance of 0.8–1 Hv beyond the vegetation patch, and thus, the horizontal vortexes cannot be detected in this region. The octant analysis is conducted for the coherent shear stress analysis that confirms the results of this experimental study.