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Shock-induced vortex breakdown, which occurs on the delta wings at transonic speed, causes a sudden and significant change in the aerodynamic coefficients at a moderate angle-of-attack. Wind-tunnel tests show a sudden jump in the aerodynamic coefficients such as lift force, pitching moment and centre of pressure which affect the longitudinal stability and controllability of the vehicle. A pneumatic jet operated at sonic condition blown spanwise and along the vortex core over a 60° swept delta-wing-body configuration is found to be effective in postponing this phenomenon by energising the vortical structure, pushing the vortex breakdown location downstream. The study reports that a modest level of spanwise blowing enhances the lift by about 6 to 9% and lift-to-drag ratio by about 4 to 9%, depending on the free-stream transonic Mach number, and extends the usable angle-of-attack range by 2°. The blowing is found to reduce the magnitude of unsteady pressure fluctuations by 8% to 20% in the aft portion of the wing, depending upon the method of blowing. Detailed investigations carried out on the location of blowing reveal that the blowing close to the apex of the wing maximises the benefits.

The effects of an axial magnetic field on both the vortex breakdown process and fluid layers development in a cylindrical container filled with a conducting viscous fluid are numerically analyzed by using the Generalized Integral Transform Technique (GITT) with a stream function-only formulation. A temperature gradient is imposed in the axial direction on the swirling flow which is advanced by the rotation of the bottom disk under the stabilizing effect of the external magnetic field. Flows are studied for a range of parameters: the Richardson number, Ri, 0 ≤Ri ≤2.0; and three values of the Prandtl number are investigated, Pr = 0.025 (liquid Mercury), 0.032 ( PbLi 17 alloy), and 0.065 (the molten lithium). Three combinations of aspect ratios (H/R) and Reynolds numbers are compared: (case A: Re=1500, H/R=1.5); (case B: Re=1855, H/R=2.0) and (case C: Re=2400, H/R=2.5). The results reveal that the increase in the values of Hartmann number, Ha suppresses the vortex breakdown in the isothermal case and reduces the number of fluid layers in the layering case. The stability diagram (Hacr–Ri) corresponding to the transition from the multiple fluid layers zone to the one fluid layer zone for increasing Prandtl number is obtained‎.

AbstractIn this study, the aim is to exhibit vortical behaviors of flow on double delta wings having 70° strake sweep angle and kink angles of 150°, 160° and 165° using dye visualization technique in a water channel. Experiments were performed at Reynolds numbers based on the chord length Re = 10,000 and 25,000 for angle of attack in the range 5° to 35°. The visualizations were performed for both end-view and cross-flow planes. The results revealed that the kink angle has a significant role on the interaction of vortices and the strake vortex breakdown locations. The interaction between the strake vortex and the wing vortex is dominant on the flow behaviors at α ≤ 10°. The flow behavior is affected by the kink angle. Two interaction mechanisms which are spiral and enveloping are observed. The spiral interaction alternates to enveloping interaction with increasing Reynolds number. Moreover, the trajectory of the strake vortex core moves outboard with increasing Reynolds number at α = 10°. For α ≥ 15°, Reynolds number is less effective on the strake vortex breakdown location and also the vortex breakdown locations move the apex gradually with increasing angle of attack. Wake-alike flow structure takes place after occurrence of the vortex breakdown since vortex core splits into disorganized small-scale vortices. On the other hand, development of the wing vortex is more complex than the strake vortex since it collapses near the vicinity of the junction.Graphical abstract

Three-dimensional numerical simulations are conducted to investigate the origin of flow unsteadiness and its associated unsteady flow phenomena in a transonic compressor rotor. The predicted results are compared with the available experimental data and a good agreement is achieved. The numerical monitoring results and further analyses of the flow field indicate that flow unsteadiness is detected in the passage with the operating condition approaching the stability limit, and the highest oscillating region is at the leading edge of the blade pressure surface; the tip leakage vortex breakdown is not a decisive factor for the flow unsteadiness, and the shock oscillation is a unsteady flow phenomenon resulted from the vibration of the recirculation region; a U-type vortex emerges in the tip leakage vortex breakdown region, and its periodic impingement on the pressure surface of the adjacent blade is treated as a trigger that leads to the flow unsteadiness.

Tip vortices and their breakdown are analyzed for a model wind turbine by means of the vortex particle-mesh solver SailFFish. The objective is to compare SailFFish with a well established high-fidelity DDES using the finite-volume code FLOWer. To minimize the influence caused by the SailFFish setup, boundary influence and grid convergence study is conducted resulting in an radial extent of the computational domain of 1.5 · R and an average grid size of Δ/R = 0.015. The time-averaged axial velocity shows a good agreement between the SailFFish and the DDES results for z/R for z/R ≥ 0.5. For z/R ≤ 0.5 there is a significant difference between both simulation methods caused by comparatively strong root vortices in SailFFish, which are caused by the blade modeling in SailFFish. In addition, the radial expansion of the wake is ≈ 3% lower for SailFFish compared to FLOWer. The tip vortex circulation shows good agreement to the FLOWer results. When comparing the Q-criterion iso-surfaces, SailFFish is able to capture both, short- and long-wavelength instabilities. The tip vortex breakdown position is similar compared to FLOWer. However, SailFFish overestimates the short-wavelength instabilities compared to the DDES leading to similar but not identical tip vortex breakdown behavior. Overall, the results demonstrate a strong potential for SailFFish for preliminary investigations of wind turbine wakes, considering the runtime benefit of three orders of magnitude compared to FLOWer.

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

The features of developing a counterflow zone (bubble-mode vortex breakdown or vortex explosion) at the center of an intensively swirled flow produced in a liquid-filled cylindrical container with a rotating endwall have been studied. The observation showed that the scenario of developing a bubble-mode breakdown zone with generation of counterflow is the same for cylinders with low or high aspect ratio, and it remains independent of stationary-nonstationary transition boundary for the main vortex flow.

In this paper, we address the question of whether total helicity is conserved through viscous reconnection events in topologically complex vortex flows. To answer this question, we performed direct numerical simulations (DNS) focused on two complex vortex flow problems: (1) a trefoil knot and (2) a two-ring link, both simulated for various vortex core radii. The DNS framework relies on a block-structured adaptive mesh refinement (AMR) technique. A third simulation of a colliding pair of unlinked vortex rings, which exhibit no total helicity change, is also performed to serve as a reference case. The results show that a well-defined total helicity jump occurs during the unknotting/unlinking events of cases (1) and (2), which arises from the annihilation of the local helicity density content in the reconnection regions. Changes in total helicity become steeper as thinner core radii are considered for both cases (1) and (2). Finally, an analytical derivation based on the reconnection of two infinitesimal anti-parallel vortex filaments is provided that quantitatively links helicity annihilation and viscous circulation transfer processes, which unveils the fundamental hydrodynamic mechanisms responsible for production/destruction of total helicity during reconnection events.

The multiscale nature of tropical cyclone (TC) intensity change under moderate vertical wind shear was explored through an ensemble of high-resolution simulations of Hurricane Gonzalo (2014). Ensemble intensity forecasts were characterized by large short-term (36-h) uncertainty, with a forecast intensity spread of over 20 m s −1 , due to differences in the timing of rapid intensification (RI) onset. Two subsets of ensemble members were examined, referred to as early-RI and late-RI members. The two ensemble groups displayed significantly different vortex evolutions under the influence of a nearby upper-tropospheric trough and an associated dry-air intrusion. Mid-to-upper-tropospheric ventilation in late-RI members was linked to a disruption of inner-core diabatic heating, a more tilted vortex, and vortex breakdown, as the simulated TCs transitioned from a vorticity annulus toward a monopole structure. A column-integrated moist static energy (MSE) budget revealed the important role of horizontal advection in depleting MSE from the TC core, while mesoscale subsidence beneath the dry-air intrusion acted to dry a deep layer of the troposphere. Eventually, the dry-air intrusion retreated from late-RI members as vertical wind shear weakened, the magnitude of vortex tilt decreased, and late-RI members began to rapidly intensify, ultimately reaching a similar intensity as early-RI members. Conversely, the vortex structures of early-RI members were shown to exhibit greater intrinsic resilience to tilting from vertical wind shear, and early-RI members were able to fend off the dry-air intrusion relatively unscathed. The different TC intensity evolutions can be traced back to differences in the initial TC vortex structure and intensity.

The 2015/16 Northern Hemisphere winter stratosphere appeared to have the greatest potential yet seen for record Arctic ozone loss. Temperatures in the Arctic lower stratosphere were at record lows from December 2015 through early February 2016, with an unprecedented period of temperatures below ice polar stratospheric cloud thresholds. Trace gas measurements from the Aura Microwave Limb Sounder (MLS) show that exceptional denitrification and dehydration, as well as extensive chlorine activation, occurred throughout the polar vortex. Ozone decreases in 2015/16 began earlier and proceeded more rapidly than those in 2010/11, a winter that saw unprecedented Arctic ozone loss. However, on 5–6 March 2016 a major final sudden stratospheric warming (\"major final warming\", MFW) began. By mid-March, the mid-stratospheric vortex split after being displaced far off the pole. The resulting offspring vortices decayed rapidly preceding the full breakdown of the vortex by early April. In the lower stratosphere, the period of temperatures low enough for chlorine activation ended nearly a month earlier than that in 2011 because of the MFW. Ozone loss rates were thus kept in check because there was less sunlight during the cold period. Although the winter mean volume of air in which chemical ozone loss could occur was as large as that in 2010/11, observed ozone values did not drop to the persistently low values reached in 2011.We use MLS trace gas measurements, as well as mixing and polar vortex diagnostics based on meteorological fields, to show how the timing and intensity of the MFW and its impact on transport and mixing halted chemical ozone loss. Our detailed characterization of the polar vortex breakdown includes investigations of individual offspring vortices and the origins and fate of air within them. Comparisons of mixing diagnostics with lower-stratospheric N2O and middle-stratospheric CO from MLS (long-lived tracers) show rapid vortex erosion and extensive mixing during and immediately after the split in mid-March; however, air in the resulting offspring vortices remained isolated until they disappeared. Although the offspring vortices in the lower stratosphere survived longer than those in the middle stratosphere, the rapid temperature increase and dispersal of chemically processed air caused active chlorine to quickly disappear. Furthermore, ozone-depleted air from the lower-stratospheric vortex core was rapidly mixed with ozone rich air from the vortex edge and midlatitudes during the split. The impact of the 2016 MFW on polar processing was the latest in a series of unexpected events that highlight the diversity of potential consequences of sudden warming events for Arctic ozone loss.