Shear Vorticity Research Articles

Designing Multishape Dual-Gas Initial Conditions for the Study of Hydrodynamic Instabilities

Abstract The flow resulting from the rotation of a series of thin plates that initially separate two gases of different densities is analyzed using direct numerical simulations (DNSs). The 90-deg plates' rotation forms a vorticity shear layer and a density interface in between the tips of two neighboring plates. Results of this study show that the shape of these layers strongly depends on the plate's tip-based Reynolds number that can be varied thanks to a parametrization of the plates' opening law. Different regimes are identified corresponding to single- or multimode initial interfaces, with or without the occurrence of starting vortices during the formation of the shear layer. The density interfaces resulting from this procedure are particularly well-suited to serve as initial conditions for the study of the Richtmyer–Meshkov (RM) instability-induced mixing. Results of this study also provide a description of vortex formation in stratified flows.

Severe MAC increases shear stresses on particles traversing the mitral valve: an in vitro study

Abstract Funding Acknowledgements Type of funding sources: None. Background Mitral annular calcification (MAC) is a strong predictor of stroke but mechanism(s) are poorly defined. Severe MAC can produce a gradient across the mitral valve (MV) and, when studied in vitro, disturbs normal flow across the valve resulting in increased viscous energy dissipation. Purpose We hypothesized that severe MAC would increase shear stress on particles traveling across the MV into the left ventricle (LV). Given that shear stresses cause platelet activation this might represent a mechanism by which MAC could increase stroke risk. Methods A silicone model MV was created using a 3D TEE dataset. 3D printed calcium phantoms were incorporated into the valve simulate severe MAC. The valve was tested in a left heart duplicator under rest and exercise conditions and compared with a duplicate valve without the calcium phantoms. Fine particles suspended in a water/glycerol blood analogue allowed for measurement of vortex formation and shear stresses using particle image velocimetry (PIV). Particle residence time (PRT) maps were created to assess how long blood particles would remain in the LV. Particle residence index (PRI - ratio of remaining particles in LV/initial number of particles) is a more quantitative measure of how fast particles leave the LV. These calculations were used to approximate viscous shear stresses on blood particles. For each particle the induced viscous shear stress was evaluated for the entire duration of residence in the LV. Results For the normal MV all released particles left the LV by the 3rd cycle; with severe MAC particles completely left the LV shortly after the 7th cycle. PRI measurements confirmed that particles remained longer in the LV in the presence of severe MAC (figure 1). MAC also induced a shift in the accumulated shear stress levels from the high range > 0.4 Pa.s and the low range < 0.1 towards the middle region (0.16-0.32 Pa.s, figure 2). As shear stress is reported for one cycle, one may expect MAC to lead to higher accumulated higher viscous stresses as particles reside in the LV for a longer time. Conclusions In the presence of severe MAC blood particles remain longer in the LV and are exposed to greater cumulative shear stresses vs the normal situation. Given that shear stress is known to cause platelet activation this may be a mechanism by which MAC increases risk of ischemic stroke. Abstract Figure 1 Abstract Figure 2

Near-wall turbulent transport mechanisms in a micro-ribbed channel

Blade internal cooling efficiency is of great significance for gas turbine. Among the blade internal cooling methods, micro perturbation elements show the potential of high heat transfer with low pressure drop but the mechanisms remain unclear. This paper investigated the influence of micro-ribs on turbulent momentum and heat transport in the near-wall region of a micro-ribbed channel flow by Large Eddy Simulation. The micro ribs were placed in the buffer region of the turbulent boundary layer, expected to effectively improve the near-wall turbulent transport. The results show that the micro ribs enhance heat transport and weaken momentum transport on average. The micro-ribs destroy the velocity streaks behind the ribs and inhibit turbulence burst, and so the momentum transport is weakened and the friction resistance falls by 9.4%. The micro ribs also induce spreading shear vortices that evolve into near-wall streamwise vortices downstream, leading to enhancement of regenerated streamwise vortices, and finally the Nusselt number increases by 5.0%.

Wake transitions of six tandem circular cylinders at low Reynolds numbers

Direct numerical simulations for flow over six tandem circular cylinders at Reynolds numbers (Re) from 40 to 180 and gap ratios (G/D) from 0.5 to 18 were conducted. The main aims are to identify vortex transitions from primary to secondary vortices and from secondary to tertiary vortices and to explore the mechanisms for these transitions. In addition, the variations of the force coefficients and Strouhal numbers with the transitions are also examined. Based on the characteristics of vortex shedding frequency, flow patterns, and force coefficients on the cylinders, four flow regimes, namely, no-shedding, primary shedding, secondary shedding, and tertiary shedding regimes, were mapped out in the Re–G/D domain. The three shedding regimes always initiate from the last cylinder. With the increase of Re and/or G/D, the onset locations of the regimes shift upstream. The secondary and tertiary vortices are formed after a region of convective instability characterized by two-row parallel vortex streets or shear layers. Two physical mechanisms for the transition from primary to secondary vortices are identified. The first one is associated with shear layer instabilities developed after the decay of the primary vortices, and the second one is the merging of the primary vortices. The transition from secondary to tertiary vortices involves vortex merging only. Whereas no-shedding or primary shedding regimes occur at smaller gap ratios or lower Reynolds numbers, the three vortex shedding patterns (primary, secondary, and tertiary) can co-exist for larger gap ratios or higher Reynolds numbers, and each pattern dominates a region around the group of cylinders. The variations of the flow patterns, vortex shedding frequency, and force coefficients on the cylinders in each of the flow regimes are quantified and interpreted. When the number of cylinders is increased to more than 6 at a gap ratio of 15, no further transition is observed after the transition to tertiary wake, and the vortex shedding processes on the downstream cylinders become chaotic without a dominant frequency, suggesting that for multiple tandem cylinders, only two transitions, corresponding to the generations of the secondary and tertiary vortices, can be identified. The change in Reynolds number or gap ratio influences the onset locations of the transitions only.

Initial experience with intensity distribution analysis of hemodynamic parameters in the thoracic aorta using four-dimensional magnetic resonance imaging

The purpose of this study was to investigate whether there were significant differences in the intensity distributions of thoracic aorta hemodynamic parameters between groups with different ejection fractions (EF) using four-dimensional flow magnetic resonance imaging and to investigate the relationships between each parameter.A total of 26 patients, 13 each with EF of >60% and <30%, underwent cardiac four-dimensional flow magnetic resonance imaging (EF >60%: mean age: 54 ± 11.6 years, EF <30%: mean age: 49.2 ± 17.2 years). The thoracic aorta was divided into the proximal and distal ascending aorta (AAo), aortic arch, and the proximal and distal descending aorta, and each section was further divided into the anterior wall, posterior wall, lesser curvature, and greater curvature. The intensity distributions of wall shear stress (WSS), energy loss (EL), and vorticity (Vort) (hemodynamic parameters) and the concordance rates between these distributions were analyzed.The concordance rate between the intensity distributions of EL and Vort was high. Only the intensity distributions of EL and Vort in the distal AAo differed significantly between the groups (P < .001). In the EF >60% group, these intensity distributions showed higher values in the greater curvature of the AAo, whereas in the EF <30% group higher values were seen in the lesser curvature of the AAo.Although there was no significant intergroup difference in the WSS intensity distribution, in the EF <30% group the WSS intensity distribution tended to exhibit higher values in the lesser curvature of the distal AAo, and the WSS intensity distribution values for the greater curvature tended to gradually increase from the arch to the proximal descending aorta.The only significant differences between the EF groups were found in the intensity distributions of EL and Vort in the distal AAo. This suggests that the distributions of atherosclerosis may be EF-dependent.

Wake flow patterns and turbulence around naturally deposited and installed trees in a gravel bed river

AbstractLarge wood structures, such as wood fragments, debris jams, or entire trees, create flow and habitat diversity in rivers. A key flow feature associated with such structures is the wake, characterised by a core zone of reduced velocity and shear layers at its margins. Wakes are largely controlled by geometric and structural properties of the wood. In the present study, the flow patterns and turbulence created by different wood structures were compared at two study sites: naturally eroded and fragmented oaks (Site A) and artificial poplar installations (Site B). Flow and turbulence were quantified using pointwise velocity measurements with acoustic Doppler velocimeters (ADVs) and surface particle tracking velocimetry (SPTV). The measured flow patterns exhibited similarities with shallow porous wakes that feature fluid advection through the structure into the wake core downstream. Two additional features of wood structures were identified in the present study: (i) the growth of the shear layers was hindered by bed friction like for shallow mixing layers and (ii) the presence of a tree stem and sediment deposit in the wake centre delayed or even suppressed the interaction of the shear layers and vortex street formation similar to a wake‐splitter plate. Methodologically, the combined ADV/SPTV measurement approach and the use of analytical models for shallow mixing layers proved to be highly valuable to decipher the complex flow patterns around wood structures in the field.

Analysis of the Formation Mechanism of Secondary Tip Leakage Vortex (S-TLV) in an Axial Flow Pump

Studies on the tip leakage vortex (TLV) are extensive, while studies on the secondary tip leakage vortex (S-TLV) are rare. To advance the understanding of the formation mechanism of the S-TLV, turbulent cavitating flows were numerically investigated using the shear stress transport (SST) turbulence model and the Zwart–Gerber–Belamri cavitation model. The morphology and physical quantity distribution of the S-TLV under two cavitation conditions were compared, and its formation mechanism was analyzed. The results reveal that in the lower cavitation number case, there is a low-velocity zone of circumferential flow near the tip in the back half of the blade. The shear vortices formed by the leakage jet gradually accumulate and concentrate in the low-velocity area, which is one of the main sources of the S-TLV. Meanwhile, the radial jet pushes the vortices on the suction surface to the tip, which mixes with the S-TLV. The flow path formed by the radial jet and the leakage jet is in accordance with the rotation direction of the S-TLV, which promotes the S-TLV’s further development. Under the conditions of a small cavitation number and low flow rate, the circumferential velocity and radial velocity of the fluid near the gap have altered significantly, which is conducive to the formation of the S-TLV.

Изучение влияния больших сдвиговых деформаций и вихревого течения металла на формирование равноосной ультрамелкозернистой структуры циркониевого сплава Э110 методом РСП

In this work, the task was to investigate the processes of microstructure change occurring in relatively large bulk bars under the action of large plastic deformations. Such large levels of deformation are usually achievable in high pressure twisting of small flat disks, but are difficult to achieve in large bulk bars. The method of radial shear rolling makes it possible to achieve comparable ultrahigh degrees of deformation (~ 45 mm/mm) in combination with the vortex flow of the metal. Sequential rolling of the E110 zirconium alloy was carried out under extreme conditions on 2 radial shear rolling mills with a total reduction in diameter ε = 185% and a maximum accumulated deformation = 46 mm/mm. To assess the level of deformation and its distribution over the section, FEM modeling was carried out in Deform-3D. The resulting structure was studied by electron microscopy (TEM/SEM). A detailed cross-sectional study of the EBSD structure was performed with a resolution of 1 mm. A gradient structure with a predominance of an equiaxed ultrafine-grained structure was found, which was not very pronounced compared to the use of smaller deformations.

Flow around topological defects in active nematic films.

We study the active flow around isolated defects and the self-propulsion velocity of defects in an active nematic film with both viscous dissipation (with viscosity ) and frictional damping with a substrate. The interplay between these two dissipation mechanisms is controlled by the hydrodynamic dissipation length that screens the flows. For an isolated defect, in the absence of screening from other defects, the size of the shear vorticity around the defect is controlled by the system size . In the presence of friction that leads to a finite value of , the vorticity field decays to zero on the lengthscales larger than . We show that the self-propulsion velocity of defects grows with in small systems where , while in the infinite system limit or when , it approaches a constant value determined by .

Research on vehicle window buffeting mechanism and noise control

For a moving car, buffeting noise is generated when a window is opened alone, which seriously affects the comfort of passengers. In particular, there is still no effective control method for side window buffeting, so it is very important to study the buffeting mechanism and characteristics for noise control. First, this paper analyses the pressure change law during the shear vortex shedding process through simulation, and explores the Helmholtz resonance characteristics of the passenger cabin, so as to reveal the causes of buffeting phenomenon. Secondly, wind tunnel tests are used to study the effects of vehicle speed, window opening size and other factors on buffeting, and the differences in buffeting characteristics between sunroof and side windows are compared, then the reasons for buffeting changes are analysed through simulation. Finally, the passive control and active control are discussed, which have reference significance for the control of vehicle window buffeting.

Greater hemodynamic stresses initiated the anterior communicating artery aneurysm on the vascular bifurcation apex

ObjectiveTo investigate hemodynamic stresses associated with the anterior communicating artery (Acom) aneurysm formation using computational fluid dynamics (CFD) analysis. MethodsThree-dimensional geometries of the anterior cerebral artery (ACA) bifurcations in 20 patients with Acom aneurysms and 20 control subjects were used for CFD analysis to investigate hemodynamic stresses including the total and dynamic pressure, wall shear stress (WSS), vorticity and strain rate. ResultsAt the direct flow impinging center on the bifurcation apex, the total pressure was the maximal but decreased quickly from the impinging center to both daughter branches. The WSS, dynamic pressure, vorticity and strain rate were the minimal at the direct impinging center but increased rapidly and reached the peaks at both daughter branches. The ACA bifurcation angle was significantly (P < 0.001) greater in patients with than without Acom aneurysms (144.2° ± 4.1° vs. 105.1° ± 3.2°). Most aneurysms (70% and 85%, respectively) were deviated to the smaller daughter branch or to the daughter branch forming a smaller angle with the A1 segment of ACA, where the hemodynamic stresses were significantly (P < 0.05) greater than those on the contralateral daughter branch. After aneurysm formation, the hemodynamic stresses on the aneurysm dome were all significantly decreased compared with at the aneurysm initiation site with aneurysm virtual removal (P < 0.001). ConclusionFormation of the Acom aneurysm is closely associated with and is to decrease the locally abnormally enhanced hemodynamic stresses.

Solubilization mechanism and structural properties of high‐denatured peanut protein treated by shearing

AbstractHigh‐denatured peanut protein (PP), which exhibited a surface structure dominated by hydrophobic groups due to the high temperature and pressure in oil extraction, was limited in application with poor hydration property. To improve the interaction between PP and water molecules, high‐speed shearing homogenization (HSH) technique was used to depolymerize the aggregates. The results showed that the concentration of soluble protein was increased from 0.22 to 0.79 mg/mL at 15 min, followed by enhanced water holding ability, decreased particle size, and thermal stability. Otherwise, the decrease of α‐helix structure and the exposure of chromophoric groups indicated that the secondary and tertiary structure were unfolded. The stretching vibration of C–O, C=O, C–H, the increase of β‐turn structure, and the exposure of hydrophilic groups (–OH) were the main reason for the improved hydration property. Furthermore, the increased hydration property facilitated the formation of a stable emulsion via stabilizing the water phase. The enhanced hydration property of PP contributed to extending its application in the preparation of beverages and delivery products such as Pickering emulsion and gel for drugs.Practical applicationsAs one of the non‐thermal processing technologies, HSH technology has been widely used in the field of food processing. HSH treatment depolymerized the materials via producing strong shear force, vortex, and cavitation, which was easy to operate and avoided pollution. This study used the HSH technology to improve the hydration property of PP, which has great research potential for the preparation of drug delivery carriers, nutritional products, tissue scaffolds, and food packaging materials.

Numerical simulation of the effects of sandy water on regulator labyrinth channel in micro-sprinkler systems

Abstract The effects of sandy water on the W-shaped labyrinth channel of micro-sprinkler irrigation systems with large flowrate were investigated using Computational Fluid Dynamics (CFD). Using ANSYS FLUENT software and different inflow conditions (e.g., pressure, velocity, sediment concentration, and sand particle diameter), internal turbulent multiphase flow and sand deposition were simulated by the Eulerian multiphase flow model. Particle erosion in the labyrinth channel was calculated by the Discrete Phase Model (DPM). The results show that vortex movements and shear actions at the boundary layer cause self-flushing in the channel. The location of sand particle deposits and the turbulent dissipation rate are related to the operating pressure, which is optimal at 300 kPa. The erosion rate of the channel wall is proportional to the inflow sediment concentration but has no obvious relationship with inflow velocity. Based on the movement regulation of sand particles in the labyrinth channel, recommendations on filtration requirements and operating pressure of irrigation systems are proposed.

Complex geodynamic indicators for forecasting hydrocarbon deposits in the arctic zone

In the article, in relation to the Arctic zone of the Russian Federation, a new method of forecasting hydrocarbon deposits based on computer geodynamic modelling procedures is considered. It is less expensive compared to field and analytical methods. The approbation of the method on the example of the Laptev Sea shelf zone showed a good interpretability of its results and their compliance with the forecast obtained by other methods. The rationality of using six geodynamic indicators for forecasting: the distributions of vortex structures of the velocity vectors of horizontal shear deformations and vortex structures of normal linear deformations; the values of the velocity vectors of horizontal shear deformations and velocity vectors of horizontal normal linear deformations; the distributions of the anomalous gravitational field in isostatic reduction and the reduced temperature. The sequence of stages in determining the potential of hydrocarbon deposits in the studied territories is described, which is associated with the solution of six interrelated sequential tasks: the choice of local territorial areas of optimal size - the calculation of geodynamic indicators – the allocation of homogeneous territorial clusters – the detailing and improvement of geodynamic indicators – the determination of the potential of oil and gas fields in the cluster – the localization of oil and gas fields in each cluster – construction of a digital forecast map of the location of oil and gas fields in the study area.

Shears and vortices of rotational couette flow in a cylindrical gap with radial injection and suction

We examine the shear states and vortices of rotational Couette flows with radial injection and suction. The gap of finite radius and infinite axial length lying between two concentric cylinders is assumed to be filled with incompressible Newtonian viscous fluids. To this goal, a rectilinear injected Couette flow is briefly reviewed as a preliminary to a rotational injected Couette flow. After making clear the differences among a simple shear, a pure shear, and generic shears, we formulate a single normalized shear parameter along any curve on a two-dimensional plane. By this way, the relative roles of stretch, shear, and vorticity are clarified through a suitable three-part decomposition of a velocity gradient tensor. For both rectilinear and rotating Couette flows, a certain material fluid particle spends a finite time while traveling from one boundary of injection to the other boundary of suction. We have solved material dynamics for both types of Couette flows to show how a certain fluid particle undergoes various shear states along a curved material trajectory. Such varied shear states are then compared with the shear states found from conventional spatial dynamics. In addition, the roles of enstropy and energy-density gradient are freshly examined from the viewpoints of shear states. The resulting relationships among stretch, shear, and vorticity for the injected rotational Couette flow turn out to shed light on what are going on in realistic flow configurations such as fountain flows, sink-hole flows, tokamak plasma, and tumor-cell flows, to name a few.

On the terminal spins of accreting stars and planets: boundary layers

ABSTRACT The origin of the spins of giant planets is an open question in astrophysics. As planets and stars accrete from discs, if the specific angular momentum accreted corresponds to that of a Keplerian orbit at the surface of the object, it is possible for planets and stars to be spun-up to near-break-up speeds. However, accretion cannot proceed on to planets and stars in the same way that accretion proceeds through the disc. For example, the magneto-rotational instability cannot operate in the region between the nearly Keplerian disc and more slowly rotating surface because of the sign of the angular velocity gradient. Through this boundary layer where the angular velocity sharply changes, mass and angular momentum transport is thought to be driven by acoustic waves generated by global supersonic shear instabilities and vortices. We present the first study of this mechanism for angular momentum transport around rotating stars and planets using 2D vertically integrated moving-mesh simulations of ideal hydrodynamics. We find that above rotation rates of ∼0.4−0.6 times the Keplerian rate at the surface the rate at which angular momentum is transported inwards through the boundary layer by waves decreases by ∼1−3 orders of magnitude depending on the gas sound speed. We also find that the accretion rate through the boundary layer decreases commensurately and becomes less variable for faster rotating objects. Our results provide a purely hydrodynamic mechanism for limiting the spins of accreting planets and stars to factors of a few less than the break-up speed.

Index‐ and model‐dependent projections of East Asian summer monsoon in Coupled Model Intercomparison Project Phase 6 simulations

AbstractIn terms of five categories of East Asian summer monsoon (EASM) indices and one classic meridional wind index, we project the EASM intensity change over the 21st century using 27 global climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the mid‐range combined scenarios of Shared Socioeconomic Pathways and Representative Concentration Pathways (SSP2‐4.5). Results show that future changes in the EASM intensity are index‐ and model‐dependent. Relative to the reference period 1985–2014, the EASM intensity strengthens slightly based on the east–west thermal contrast indices but varies little as a whole based on the southwest monsoon indices over the 21st century, with large discrepancies among the models. However, it tends to weaken over time based on the other three categories of indices, including the north–south thermal contrast indices, the shear vorticity indices, and the South China Sea monsoon indices. In terms of the meridional wind index that directly measures the EASM circulation in the lower troposphere and holds valid under future global warming, the EASM intensity strengthens significantly over the 21st century as a consequence of increases in zonal and meridional land–sea thermal contrasts between the East Asian continent and the adjacent oceans of western North Pacific and South China Sea. There are certain relationships between the trends of the meridional wind index and other EASM indices. The projections are overall consistent between the ensemble means of all available 27 models and the 14 good models as selected according to their abilities to simulate the present EASM climatology, but with different magnitudes.

Application of Coagulation-Membrane Rotation to Improve Ultrafiltration Performance in Drinking Water Treatment.

The combination of conventional and advanced water treatment is now widely used in drinking water treatment. However, membrane fouling is still the main obstacle to extend its application. In this study, the impact of the combination of coagulation and ultrafiltration (UF) membrane rotation on both fouling control and organic removal of macro (sodium alginate, SA) and micro organic matters (tannic acid, TA) was studied comprehensively to evaluate its applicability in drinking water treatment. The results indicated that membrane rotation could generate shear stress and vortex, thus effectively reducing membrane fouling of both SA and TA solutions, especially for macro SA organics. With additional coagulation, the membrane fouling could be further reduced through the aggregation of mediate and macro organic substances into flocs and elimination by membrane retention. For example, with the membrane rotation speed of 60 r/min, the permeate flux increased by 90% and the organic removal by 35% in SA solution, with 40 mg/L coagulant dosage, with an additional 70% increase of flux and 5% increment of organic removal to 80% obtained. However, too much shear stress could intensify the potential of fiber breakage at the potting, destroying the flocs and resulting in the reduction of permeate flux and deterioration of effluent quality. Finally, the combination of coagulation and membrane rotation would lead to the shaking of the cake layer, which is beneficial for fouling mitigation and prolongation of membrane filtration lifetime. This study provides useful information on applying the combined process of conventional coagulation and the hydrodynamic shear force for drinking water treatment, which can be further explored in the future.

Observed Near-Inertial Waves in the Northern South China Sea

Characteristics of near-inertial waves (NIWs) induced by the tropical storm Noul in the South China Sea are analyzed based on in situ observations, remote sensing, and analysis data. Remote sensing sea level anomaly data suggests that the NIWs were influenced by a southwestward moving anticyclonic eddy. The NIWs had comparable spectral density with internal tides, with a horizontal velocity of 0.14–0.21 m/s. The near-inertial kinetic energy had a maximum value of 7.5 J/m3 and propagated downward with vertical group speed of 10 m/day. Downward propagation of near-inertial energy concentrated in smaller wavenumber bands overwhelmed upward propagation energy. The e-folding time of NIWs ranged from 4 to 11 days, and the larger e-folding time resulted from the mesoscale eddies with negative vorticity. Modified by background relative vorticity, the observed NIWs had both red-shifted and blue-shifted frequencies. The upward propagating NIWs had larger vertical phase speeds and wavelengths than downward propagating NIWs. There was energy transfer from the mesoscale field to NIWs with a maximum value of 8.5 × 10−9 m2 s−3 when total shear and relative vorticity of geostrophic currents were commensurate. Our results suggest that mesoscale eddies are a significant factor influencing the generation and propagation of NIWs in the South China Sea.

Secondary instability of channel-confined transition around dual-circular cylinders in tandem

The incompressible viscous flows around the dual-circular cylinders in tandem confined in a plane channel were simulated by direct numerical simulations (DNS) with the Reynolds numbers (Re) at 200, 400 and 800. The blocking ratio and cylinder center-to-center distance are set at 1/4 and 2.5. The secondary instabilities and the corresponding vortex evolution in transition were investigated by the dynamics of third-generation vortex identification quantities of liutex scalar and vector, which are governed by the vorticity transport equations, or Helmholtz equations, and the liutex-shear decomposition. The studies, for the first time, discover the influence of liutex stretching effect on the rigid rotating strength, which gives rise to the two-dimensional non-uniform deformations of the liutex isosurface in the secondary transition. The simulations also demonstrate that the liutex vector is mainly produced by the newly-defined shear allocating effect at the early stage of vortex generation. The quasi-streamwise vortices in the entire flow domain are found being generated in the region where the shear presents a strong strength. With the distinctive elliptic instability in nature, the vortices in the gap are confined by the cylinder shear layers for the Re=400 flow and the vortex bulges structures are firstly observed as the start to transit from the 2-D transient vortex to the more complex 3-D vortical structures in gap, namely the secondary instability. In the wake region, the study reported the two prominent developing stages of Mode B vortex pairs corresponding to the shear instability and vortex stretching, which are governed by their own corresponding balancing mechanisms in the liutex vector dynamic equations. The similar two-stages developing processes are also confirmed in the bluff-body wake transitions with the Mode B instability in nature. With the increase of Re, the shear layers start to interact with the gap vortices and the new secondary-instability is found causing by the hairpin vortices, which are originated from the gap and subsequently lead to the rapid three-dimensional transitions in both gap and wake regions. These studies of vortex structures and dynamics push the front of the current understandings regarding the secondary instability and the subsequent transition to fully-developed turbulence.