Small-scale Vortices Research Articles

A dynamic version of the improved delayed detached-eddy simulation based on the differential Reynolds-stress model

A dynamic version of the improved delayed detached-eddy simulation (IDDES) based on the differential Reynolds-stress model (RSM), referred to as the RSM-DynIDDES, is developed by applying the dynamic Smagorinsky subgrid model to the large eddy simulation (LES) branch of the IDDES. The RSM-DynIDDES simulates the periodic hills flow after a basic numerical validation for the decaying isotropic turbulence simulation. Well-predicted velocity profiles and R eynolds stress distributions are obtained by the RSM-DynIDDES in the periodic hills flow. The simulation results indicate that the RSM-DynIDDES can capture more small-scale vortex structures in the LES region away from the wall than the original RSM-based IDDES (RSM-IDDES). The RSM-DynIDDES is also employed in simulating the transonic buffeting of a launch vehicle with a payload fairing. The numerical results have been compared with that of the RSM-IDDES. It is found that the RSM-DynIDDES can improve turbulence resolution in the off-wall region while retaining the advantages of the original RSM-IDDES in simulating the instability process of the free shear layer.

Interaction mechanism between the tip leakage flow and inlet boundary layer in a highly loaded compressor cascade based on scale-adaptive simulation

Based on the scale-adaptive simulation, the interaction mechanism between the tip leakage flow (TLF) and the inlet boundary layer (IBL) and its effects on the tip flow field and aerodynamic performance of the compressor cascade were investigated. The time-averaged results show that the IBL reduces the blade tip load near the leading edge region, decreases the axial momentum of the TLF, and inhibits the development of the TLF to a certain extent. On the other hand, the IBL promotes the coupling of the tip leakage vortex, secondary vortex, and separation vortex and induces breakdown, resulting in a large area of severe flow separation in the corner region, greatly reducing the diffusion capacity and significantly increasing the flow loss in the tip region. The analysis of the unsteady transient flow fields indicates that the unsteady fluctuation in the tip region is mainly caused by the tip leakage vortex and flow separation near the blade trailing edge. The former is suppressed under the influence of the IBL, while the latter is amplified. The high-intensity oscillation due to the breakdown and decomposition of the tip vortex structures plays a critical role in the fluctuation of the cascade performance. By means of proper orthogonal decomposition, it is found that the IBL enhances the fluctuation of small-scale vortex structures related to flow separation and leakage flow and makes the stability of the tip flow field worse.

Reduction of noise generated by cylinder-airfoil interaction using grooved structures on the upstream cylinder

The cylinder-airfoil interaction noise can be reduced by changing the shape of the leading edge of the downstream airfoil. Generally, this way not only can reduce the interaction noise at middle and high frequency, but also can change the peak noise at the low frequency. This study attempts to affect the cylinder-airfoil interaction noise from the perspective of reducing the intensity of the upstream wake shedding vortex. In order to achieve this target, the equally spaced grooves were cut into the upstream cylinder, and the acoustic wind tunnel tests at various incoming velocities (20–60m·s−1) were conducted to compare the interaction noise of cylinder-airfoil (NACA0012) models. It is found that the grooved structure can effectively reduce the peak noise at characteristic frequencies bellow 1000 Hz and the broadband noise in the mid-frequency ranging from 1000 Hz to 3000 Hz, especially for the higher incoming velocity. Thereinto, the peak noise and overall sound pressure level (OASPL) with the grooved cylinder are reduced by 13 dB and 7.2 dB, respectively at the incoming velocity of 60 m·s−1. The numerical simulations based on the large eddy simulation (LES) and Ffowcs Williams–Hawkings (FW-H) acoustic analogy were performed to further reveal the mechanisms of noise reduction when the velocity is 60 m·s−1. The results show that the vortex shedding from cylinder wake is suppressed by the grooved cylinder and the vortex structure at the leading edge of the airfoil is also cut into the small-scale vortex structures by the grooved structure. The pressure fluctuation amplitude and the peak value turbulent kinetic energy in the wake of the grooved cylinder are significantly reduced. In addition, the further spectrum analysis reveals that the weak correlation of the vortex shedding on the grooved cylinder could lead to the suppression of the pressure fluctuation in the cylinder wake, and thereby the interaction noise is significantly reduced.

Effects of different suction angles on flow structures and dissipation in the wake of a circular cylinder

Measurements of a Particle Image Velocimetry (PIV) were carried out in a water tunnel to study the flow structures and dissipation in the cylinder wake without suction and with different suction angles (α) at Reynolds number Re = 4000 and suction flow coefficient Cμ = 2.65. The results show that suction can reduce the wake width, wake dissipation, both strength and formation length of Karman vortices at α = ±90° and α = ±112°. The first and second modes of Proper Orthogonal Decomposition (POD) dominate flow dissipation. The flow fields reconstructed by the two POD modes consist of large-scale alternate vortices (LSAV) without and with suction. The size of LSAV and dissipation strength of the two modes are reduced at α = ±90° and α = ±112°. The dominant modes of Dynamic Mode Decomposition (DMD) correspond to the main frequency and its harmonics. The reconstructed flow fields of these dominant DMD modes without suction present large-scale symmetric vortices, large-scale alternate vortices, small-scale alternate vortices and small-scale symmetric vortices. The alternate vortices replace the symmetric vortices due to suction. In addition, the dissipation of DMD modes in the main frequency band (FB2) is related to large-scale vortices' shedding, and these modes dominate flow dissipation in different cases.

Morphology and dynamical stability of self-gravitating vortices

Context. Theoretical and numerical studies have shown that large-scale vortices in protoplanetary discs can result from various hydro-dynamical instabilities. Once produced, such vortices can survive nearly unchanged over a large number of rotation periods, slowly migrating towards the star. Lopsided asymmetries recently observed at sub-millimetre and millimetre wavelengths in a number of transition discs could be explained by the emission of the solid particles trapped by vortices in the outer disc. However, at such a distance from the star, disc self-gravity (SG) may affect the vortex evolution and must be included in models. Aims. Our first goal is to identify how vortex morphology is affected by its own gravity. Next, we look for conditions that a self-gravitating disc must satisfy in order to permit vortex survival at long timescales. Finally, we characterise as well as possible the persistent self-gravitating vortices we have found in isothermal and non-isothermal discs. Methods. We performed 2D hydrodynamic simulations using the RoSSBi 3.0 code. The outline of our computations was limited to Euler’s equations assuming a non-homentropic and non-adiabatic flow for an ideal gas. A series of 45 runs were carried out starting from a Gaussian vortex-model; the evolution of vortices was followed during 300 orbits for various values of the vortex parameters and the Toomre parameter. Two simulations, with the highest resolution thus far for studies of vortices, were also run to better characterise the internal structure of the vortices and for the purpose of comparison with an isothermal case. Results. We find that SG tends to destabilise the injected vortices, but compact small-scale vortices seem to be more robust than large-scale oblong vortices. Vortex survival critically depends on the value of the disc’s Toomre parameter, but may also depend on the disc temperature at equilibrium. Disc SG must be small enough to avoid destruction in successive splitting and an approximate ‘stability’ criterion is deduced for vortices. The self-gravitating vortices that we found persist during hundreds of rotation periods and look like the quasi-steady vortices obtained in the non-self-gravitating case. A number of these self-gravitating vortices are eventually accompanied by a secondary vortex with a horseshoe motion. These vortices reach a new rotational equilibrium in their core, tend to contract in the radial direction, and spin faster. Conclusions. We propose an approximate ‘robustness criterion’, which states that, for a given morphology, a vortex appears stable provided that the disc’s Toomre parameter overcomes a fixed threshold. Global simulations with a high enough numerical resolution are required to avoid inappropriate decay and to follow the evolution of self-gravitating vortices in protoplanetary discs. Vortices reach a nearly steady-state more easily in non-isothermal discs than in isothermal discs.

Method of variable-depth groove on vortex and cavitation suppression for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag performance of the hydrofoil.

Vortex Sheet Sensitivity to Low-Level Vertical Shear and Airmass Temperature Perturbation

Abstract A theoretical, numerical-modeling-based examination of the sensitivity of vortex sheets along airmass boundaries to the following three characteristics is presented: 1) boundary-normal component of the vertical wind shear, 2) boundary-parallel component of the vertical wind shear, and 3) temperature perturbation within the parent air mass of the boundary. The overall aim of this work is to advance understanding of the sensitivity of micro-α- to meso-γ-scale vortex generation along airmass boundaries to the ambient environment. Density currents are simulated in a 2D domain that does not allow baroclinic generation of near-surface vertical vorticity (ζns) with parameterized latent heating for convection initiated at the associated airmass boundary and Coriolis turned on. Despite the absence of baroclinically generated ζns, with Coriolis turned on and without any boundary-parallel shear, ζns more than two orders of magnitude larger than planetary vorticity is generated along the boundary and located within the cold air. The magnitude of ζns is found to increase with increasing boundary-normal shear with statistically significant intra-experiment separations. Near-surface vertical vorticity ζns is found to scale inversely with boundary-parallel shear with a transition to negative leading-edge ζns in several of the larger boundary-normal shear simulations. An inverse and statistically significant relationship is found between ζns and the temperature perturbation within the parent air mass of the boundary (Δθ), and is a direct consequence of the dependence of boundary propagation speed on Δθ. Significance Statement Research presented in this article aims to contribute to an improved understanding of the environmental controls on the generation of small-scale vortices along airmass boundaries. Vertical shear, both along and across an airmass boundary, as well as temperature of the air mass on the cool side of an airmass boundary are found to regulate the magnitude of near-surface vertical vorticity available to small-scale vortices.

Aeroacoustic simulation of transient vortex dynamics subjected to high-intensity acoustic waves

The transient vortex dynamics within a microsecond-level acoustic cycle were numerically investigated when an orifice–cavity structure, which is a unit component of an acoustic liner, was subjected to high-intensity acoustic waves. Three-dimensional vortex-acoustic coupling fields were determined by solving the compressible linearized Navier–Stokes equations (LNSEs) and considering the nonlinear thermoviscous effect around the micro-orifice. First, the LNSE results were well validated by literature results in terms of the sound absorption coefficient, reflection coefficient, acoustic resistance, acoustic reactance, acoustic impedance, and the spatial features of acoustically induced vortex structures. Subsequent cross correlation analysis demonstrated that attenuated standing-waves were generated inside the back cavity when the incident acoustic wave propagated across the orifice. Aeroacoustic energy analysis revealed that the periodic production of vortex kinetic energy contributed more to the sound attenuation in the orifice structure than the viscous dissipation effect. Then, the acoustically induced vortex dynamics were characterized in terms of a phase-dependent evolution process, and the formation, convection, and dissipation regions were classified. Finally, dynamic mode decomposition analyses were conducted to extract the dominant vortex structures by determining their frequency spectra. The dominant modes contained large-scale vortices around the orifice, while the high-order modes contained a series of small-scale vortices toward the upstream incident tube and downstream cavity.

Stirring the base of the solar wind: On heat transfer and vortex formation

Context.Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere; however, these are likely important in shaping the emerging wave-turbulence spectrum that ultimately heats and accelerates the coronal plasma.Aims.This study strives to make connections between small-scale vortex motions at the base of the solar wind and the resulting heating and acceleration of the coronal plasma.Methods.TheBifrostcode produces realistic simulations of the solar atmosphere which facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. For this study, theBifrostsimulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5 Mm below the photosphere) to the low corona (14.5 Mm above the photosphere), with a horizontal extent of 24 Mm × 24 Mm. The network of magnetic funnels in the computational domain influence the movement of plasma, as well as the propagation of magnetohydrodynamic waves into the low corona.Results.The twisting of the coronal magnetic field by photospheric flows efficiently injects energy into the low corona. Poynting fluxes of up to 2 − 4 kWm−2are commonly observed inside twisted magnetic structures with diameters in the low corona of 1–5 Mm. Torsional Alfvén waves are favourably transmitted along these structures, and subsequently escape into the solar wind. However, reflections of these waves from the upper boundary condition make it difficult to unambiguously quantify the emerging Alfvén wave-energy flux.Conclusions.This study represents a first step in quantifying the conditions at the base of the solar wind usingBifrostsimulations. It is shown that the coronal magnetic field is readily braided and twisted by photospheric flows. Temperature and density contrasts form between regions with active stirring motions and those without. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfvén waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind.

Oceanic Kármán Vortex Streets in the Luzon Strait in the Lee of Didicas Island from Multiple Satellite Missions

The oceanic Kármán vortex street is an important hydrokinetic phenomenon caused by the unsteady separation of sea currents in the wake of an obstacle. This study quantitatively analyzed the characteristics of the small-scale vortex street of Didicas Island in the Luzon Strait based on multimission satellite data. Five groups of vortex streets were captured, and each group was observed by two satellite images within one hour. Based on the displacement and change in the vortex street within one hour, we found that the formation time of a single vortex is longer than 40 min. The vortex propagation speed behind the island is approximately 0.98 m s−1 (ranging from 0.66–1.22 m s−1). The incoming velocity was calculated using the vortex propagation speed and the aspect ratio based on a vortex propagation model, which ranges from 0.72 to 1.47 m s−1 The incoming speed is 1.6–2.3 times the geostrophic speed extracted from AVISO data, implying that the contribution of non-geostrophic flow to the total flow field is comparable to that of geostrophic flow in this region.

A Mixing Process Influenced by Wall Jet-Induced Shock Waves in Supersonic Flow

With the development of hypersonic air-breathing propulsion systems, such as the supersonic combustion ramjet (Scramjet) and rocket-based combined cycle (RBCC) engines, the mixing process of supersonic airstream with fuel in the engine combustor has been drawing more and more attention. Due to the compressibility effects, the mixing process in a supersonic condition is significantly inhibited. In the present paper, the novel strategy of wall-jet induced shock waves (WJISW) is put forward to realize mixing enhancement. The interaction process between WJISW and the supersonic mixing layer is researched and the enhanced-mixing mechanism is revealed, employing large eddy simulation (LES) methods. The fine vortex structures of the flow field are well captured and presented, utilizing the numerical schlieren technique. Detailed visualization results indicate that WJISW in a low frequency condition can result in the ‘region action mode’ (RAM) never reported before. The drastic dynamic behaviors including growth, deformation, and distortion in the interaction region can undoubtedly promote the mixing of upper and lower streams. The Reynolds stress distributions along the streamwise x-direction suggest that more intense fluctuations can be achieved with a low frequency WJISW. Moreover, a sharp increase in mixing layer thickness can be realized in the interaction region. The dynamic mode decomposition (DMD) analysis results show that the mixing layer evolution process is dominated by the mode induced by WJISW, which leads to the coexistence of both large- and small-scale structures in the flow field. The entrainment process corresponding to large-scale vortices and the nibbling process corresponding to small-scale vortices can obviously promote mixing enhancement. It is suggested that the present proposed strategy is a good candidate for enhanced-mixing with application to Scramjet and RBCC combustors.

Interaction between a stationary sphere and turbulent flow in a boundary layer

We investigate the interaction of a stationary sphere with turbulent flows in a spatially developing boundary layer over a flat plate by means of fully resolved direct numerical simulations. The diameter of the sphere D is much larger than the Kolmogorov length scale. The sphere Reynolds number ReD is changed by varying the sphere diameter, while the gap ratio is held constant with G/D=0.1, where G is the distance between the bottom of the sphere and the flat plate. The simulation results indicate that there exist complex interactions between the small-scale vortex in the wake of the sphere and the large-scale coherent structures inside the turbulent boundary layer. The jet-like flow through the gap is deflected away from the wall, and the level of deflection increases with ReD. As a result, asymmetrical flow structures are observed in the recirculation zone. In addition, the mean recirculation length is found to decrease with the increase in ReD. Concerning the turbulence modulation, statistics show that the skin-friction drag and boundary layer thickness are decreased in the recirculation region with respect to the undisturbed flow, whereas the displacement thickness is increased due to the presence of reverse flow. These effects become more pronounced as ReD increases. The budget of the turbulent kinetic energy in the sphere wake is also examined. It is revealed that the turbulent production and viscous dissipation rate are significantly enhanced under the effect of the sphere with size of the same order of magnitude as the local boundary layer thickness.

PIV measurements on flow behavior behind a mooring line with imposed top-end motion

Experimental investigation on the flow behavior of a mooring line with imposed top-end motions were performed in a water flume. The surge and heave motions were driven by a programmable electrical motor at the fairlead position. The Particle Image Velocimetry technique (PIV) was employed to capture the instantaneous flow fields around the mooring line at both the horizontal and vertical planes. Compared with the stationary mooring line, the instantaneous vorticity contours behave much more complicated for the mooring line under the imposed surge and heave motions. The coherent vortices are formed along the curvature of the catenary mooring line, oblique vortex shedding is observed with multiple small scale vortices. The vortex strength behind the mooring line under the surge motion is larger than that under the heave motion.

Evaluation of flow and thermal characteristics for flow through a wall-confined array of pin-fins using large-eddy simulation

In the present work, the flow and thermal characteristics for flow through a wall-confined array of pin-fins are studied using large-eddy simulation (LES). The geometry considered here resembles the trailing-edge portion of a early-stage gas-turbine blade. The wall-confined array of low-aspect ratio cylindrical fins are arranged in a staggered manner, and the direction of the coolant is perpendicular to the axis of the fins. The analysis is performed for a fixed spacing between the fins and for a Reynolds number of 5900. In view of the actual running conditions, large temperature differences, ranging from 25∘C to 300∘C, are considered between the walls and the coolant. Together with this, the effect of higher pressure, ranging from 5 bar to 10 bar, on the cooling performance is analyzed. The numerical solver used is thoroughly validated with the reference experimental and LES data available in the literature. A detailed investigation of the mechanism for heating of the fluid, transport, and diffusion of the heat flux is presented by analyzing Nusselt number, turbulent heat fluxes and vorticity contours. It is observed that, an increase in Nusselt number is due to encapsulation of small-scale vortices around the pin-fin surfaces and a decrease in Nusselt number downstream is due to the lower frequency of high-energy vortex. The localized Nusselt number is higher over the fin surfaces compared to end-walls, and the base vortex is responsible for higher energy extraction at the junction of the pin-fin and the end-walls and its effect increases downstream. The spanwise component of velocity is responsible for localized mixing than the remaining two components, and the coolant gets heated inside the recirculation zone and diffusion of which occurs mostly at the free shear layer. The analysis of higher pressure and temperature difference shows higher Nusselt number at high pressure, whereas higher heat flux at lower pressure.

Coupled Bionic Drag-Reducing Surface Covered by Conical Protrusions and Elastic Layer Inspired from Pufferfish Skin

Inspired by the drag-reducing properties of the cone-like spines and elastic layer covering the pufferfish skin, important efforts are underway to establish rational multiple drag-reducing strategies for the development of new marine engineering materials. In the present work, a new drag-reducing surface (CPES) covered by conical protrusions (sparse "k-type" with rough height k+ = 13-15) and an elastic layer are constructed on copper substrate via a hybrid method, combining the sintering and coating processes. The drag-reducing feature of the prepared CPES biomimetic surface is achieved by rheometer and particle image velocimetry (PIV) experiments. To comprehensively investigate its drag reduction mechanism, the porous copper substrate (PCS), copper substrate (CS), conical protrusion resin substrate (CPRS), and conical protrusion porous copper substrate (CPPCS) were used for a comparative analysis. In laminar flow, we discovered that the conical protrusion structure and wettability of the elastic surface coupling affect the CPES sample's drag-reducing performance (7-8%) and that the interface produced slip to reduce the viscous drag. In turbulent flow, the CPES biomimetic surface exhibits an 11.5-17.5% drag-reducing performance. Such behavior was enabled by two concurrent mechanisms: (i) The conical protrusions as vortex generators enhance the number of vortices and the wake effect, enabling faster movement of downstream strips, reducing viscous drag; (ii) The conical protrusion elements break and lift large-scale vortices to produce numerous small-scale vortices with low energy, effectively weakening perturbations and momentum exchange. Additionally, the elastic layer shows high adhesion and stability on copper substrate after sandpaper abrasion and water-flow erosion tests. The copper substrate surface formed by the sintering method is also covered with dense porous structures, which gives the elastic layer and conical protrusions excellent combined robustness. Our findings not only shed new light on the design of robust drag-reducing surfaces but also provide new avenues for underwater drag reduction in the field of marine applications.

Effect of yaw angle on vibration mode transition and wake structure of a near-wall flexible cylinder

The multi-mode transition and vortex structures in the vortex-induced vibration (VIV) of a near-wall flexible cylinder under different yaw angles are investigated through three-dimensional direct numerical simulation. Yaw angles α = 0°–60°, gap ratio G/D = 0.8, and Re = 500 are adopted. With the increase in α, the dominated vibration mode decreases from the 6th to 1st mode in the in-line (IL) direction and the 3rd to 2nd mode in the cross-flow (CF) direction. For the IL vibration, no mode transition occurs at α = 0°, whereas frequently mode transition is observed at α > 0°, due to the intermittent participation and spanwise competition of different modes, thus showing an intensified traveling-wave characteristic. For the CF vibration, mode transition is not excited at any α case even with spanwise mode competitions, due to the significant weight of the dominated mode, thus showing a strong standing-wave characteristic. The asymmetrical distributions of vibration displacements and force coefficients are established because of irregular energy transfer along the span. The spanwise vortex tubes at α = 0°–30° are separated into several cells associated with the dominated vibration mode, showing a locally parallel vortex shedding. However, positively yawed and negatively yawed vortex shedding are observed at α = 45° and 60°, respectively. The vortex strengths vary along the cylinder, where large-scale and small-scale vortices are observed at the CF anti-node and node planes, respectively. The independence principle is only valid at α < 15° for predicting the multi-mode vibrations and hydrodynamics, significantly reduced from that of α < 45° in the wall-free case or the mono-mode VIV case.

The Dodge City Tornadoes on 24 May 2016: Understanding Cycloidal Marks in Surface Damage Tracks and Further Analysis of the Debris Cloud

Abstract Mobile, polarimetric radar data were collected on a series of tornadoes that occurred near Dodge City, Kansas. A poststorm survey revealed a series of tornadic debris swaths in several dirt fields and high-resolution pictures of the tornado documented the visual characteristics of the tornado and the lofted debris cloud. The main rotational couplet associated with the tornado was identified in the single-Doppler velocities; however, no secondary rotational couplets were resolved in the low-level data performed during two consecutive volume scans. Numerical simulations have suggested that cycloidal damage swaths can result when debris is deposited as the low-level inflow turns upward in the corner region of the updraft annulus of the tornado core. This mechanism can dominate even when suction vortices are present in the simulations and can produce these swaths in the absence of these smaller-scale vortices. It is hypothesized that the observed cycloidal damage swaths were a result of the low-level inflow in the corner region of the tornado and not by the existence of suction vortices. Polarimetric data were combined with photographs of the tornado in order to document the lofted debris cloud and its relationship with the funnel. This analysis provided an opportunity to investigate whether recent findings describing the cross-correlation coefficient ρhv and differential reflectivity ZDR signatures of the lofted debris cloud could be replicated. Regions of low ρhv at the periphery of the funnel cloud suggesting high debris loading and a column of negative ZDR centered on the tornado believed to be produced by common debris alignment were noted. Significance Statement It is well known that some tornadoes produce smaller-scale vortices that rotate around the central axis of the main circulation. In addition, numerous aerial photographs have documented cycloidal debris marks within tornado damage tracks that traverse open fields. The prevailing theory shown in numerous textbooks is that these marks are produced by these vortices. The current study suggests that this widely accepted model for producing these marks may be incorrect. It is suggested that these cycloidal marks are produced by the main tornado circulation and not by the smaller-scale vortices in this case.

An Arbitrarily High-order Spectral Difference Method with Divergence Cleaning (SDDC) for Compressible Magnetohydrodynamic Simulations on Unstructured Grids

This paper reports a recent development of the high-order spectral difference method with divergence cleaning (SDDC) for accurate simulations of both ideal and resistive magnetohydrodynamics (MHD) on curved unstructured grids consisting of high-order isoparametric quadrilateral elements. The divergence cleaning approach is based on the improved generalized Lagrange multiplier, which is thermodynamically consistent. The SDDC method can achieve an arbitrarily high order of accuracy in spatial discretization, as demonstrated in the test problems with smooth solutions. The high-order SDDC method combined with the artificial dissipation method can sharply capture shock interfaces with the oscillation-free property and resolve small-scale vortex structures and density fluctuations on relatively sparse grids. The robustness of the codes is demonstrated through long time simulations of ideal MHD problems with progressively interacting shock structures, resistive MHD problems with high Lundquist numbers, and viscous resistive MHD problems on complex curved domains.

Numerical simulation of passive-suction-jet control of flow over two side-by-side circular cylinders

A passive-suction-jet control scheme was adopted to manipulate the periodic vortex shedding of two side-by-side circular cylinders. The baseline case (both cylinders without control), Case #1 (only the lower cylinder was controlled), and Case #2 (both cylinders were controlled) were numerically simulated at five spacing ratios (SR) of 1.5, 2.0, 2.5, 3.0, and 4.0. The results show that the cylinders in all three cases experience a biased flow pattern at SR = 1.5 and 2.0 and tend to behave as isolated cylinders at SR > 2.0. The flow structures in Case #2 reveal that at SR = 1.5 and 2.0, the jets blown from the pipes roll up to form a large-scale vortex behind one cylinder and a series of small-scale vortices behind the other, eliminating the periodic vortex shedding of the two cylinders completely and yielding low fluctuating lifts. At SR > 2.0, the vortices formed by the jets weaken the influence of the wake vortices on the cylinders, contributing to a 94% reduction in lift fluctuation on average. Moreover, the holes centered at θ = 75°–135° were revealed to play an important role in continuously pouring positive vorticities into the fluid, which manipulate the periodic vortex shedding.

Formation of small-scale vortices in the core of a large merged vortex

The merger of two surface quasi-geostrophic vortices is examined in detail. As the two vortices collapse towards each other in the merging process, they trap their external fronts between them; these fronts are inserted into the final merged vortex, where they form a central, nearly parallel, sheared velocity strip, sensitive to barotropic instability. As a result, this strip breaks up into an alley of small vortices. Subsequently, these small vortices may undergo merger and grow in size in the core of the large merged vortex. The number of small trapped vortices decreases correspondingly. Finally, a single or two small vortices remain. These processes are analysed using a numerical model of the surface quasi-geostrophic equations. The sensitivity of this process to the initial vortex characteristics is explored. A parallel is drawn between this problem and the instability of a rectilinear strip of temperature with a central gap. The application of this problem to the Ocean is discussed.