Weak Vortex Research Articles

Characteristics of pulsed surface arc discharge actuator and its verification of flow control over a Delta wing

A actuation system composed of actuators, circuit board, and microsecond pulse power has been designed to conduct pulsed surface arc discharge (PSAD) actuation. The instantaneous impulse energy of PSAD is higher than that of typical dielectric barrier discharge. Schlieren imaging and particle image velocity (PIV) technique have been used to characterize the flow field changes caused by actuation in quiescent air. A localized pressure wave is induced by every actuation, propagating in the air almost at the speed of sound. Special emphasis is given on the hot air mass observed in schlieren imaging, generating thermal convection disturbances in the flow field. A pair of a weak reverse vortex is generated after discharge for each actuator, with the maximum propagation velocity of 0.1 m/s. The induced burst perturbations indicate that the mechanism of plasma flow control based on PSAD is instantaneous heat release generated by PSAD actuation. Wind tunnel experiments were conducted on a 30° sweep delta wing to investigate the aerodynamic control effects by using PSAD actuation. The 12-channel PSAD actuation can effectively improve the aerodynamic performance of the delta wing, delay the angle of attack from 14° to 18° and increase the maximum lift coefficient by 12.9%. In this article, PSAD actuation has the ability to control low speed fluid flow separation by releasing very strong discharge power.

Weak linkage of winter surface air temperature over Northeast Asia with East Asian winter monsoon during 1993–2003

A colder winter over Northeast Asia (NEA) has traditionally been attributed to the stronger East Asian winter monsoon (EAWM) on interannual timescale. However, the present work proposes this robust linkage becomes weak during 1993–2003, along with the changes of the underlying condition in the last three decades. During 1981–1992 and 2004–2016, the loss of autumn Arctic sea ice condensation (SIC) leads to the stratospheric warming and the polar vortex weakening, which results in the enhancement of the Siberian High (SH) and the colder surface air temperature (SAT) over NEA. In particular, the persisting anomalies of snow cover near the Mongolian Plateau from autumn to winter could amplify the influences of Arctic SIC on the EAWM during 1981–1992. However, during 1993–2003 both the EAWM and the SAT over NEA are modulated by the tropical SSTAs in the western Pacific warm pool, corresponding to a weak association between the EAWM and the SAT over NEA. These tropical warm SSTAs could excite a meridional teleconnection over East Asia to generate a barotropic anticyclonic anomaly in situ. It not only weakens the SH and EAWM but also decreases the SAT over NEA. Therefore, the predictor of winter SAT over NEA is altered from the cryosphere to the tropical SSTAs in this episode. The changes of interdecadal background may contribute to the distinct interannual relationship between EAWM and winter SAT over NEA. This will increase the uncertainty in seasonal forecast on the winter climate over East Asia.

Diversity of the Wintertime Arctic Oscillation Pattern among CMIP5 Models: Role of the Stratospheric Polar Vortex

AbstractThe wintertime Arctic Oscillation (AO) pattern in phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models displays notable differences from the reanalysis. The North Pacific center of the AO pattern is larger in the ensemble mean of 27 models than in the reanalysis, and the magnitude of the North Pacific center of the AO pattern varies largely among the models. This study investigates the plausible sources of the diversity of the AO pattern in the models. Analysis indicates that the amplitude of the North Pacific center is associated with the coupling between the North Pacific and North Atlantic, which in turn is primarily modulated by the strength of the stratospheric polar vortex. A comparative analysis is conducted for the strong polar vortex (SPV) and weak polar vortex (WPV) models. It reveals that a stronger stratospheric polar vortex induces more planetary waves to reflect from the North Pacific to the North Atlantic and more wave activity fluxes to propagate from the North Pacific to the North Atlantic in the SPV models than in the WPV models. Thus, the coupling of atmospheric circulation between the North Pacific and North Atlantic is stronger in the SPV models, which facilitates more North Pacific variability to be involved in the AO variability and induces a stronger North Pacific center in the AO pattern. The increase in vertical resolution may improve the simulation of the stratospheric polar vortex and thereby reduces the model biases in the North Pacific–North Atlantic coupling and thereby the amplitude of the North Pacific center of the AO pattern in models.

Sub seasonal variations of weak stratospheric polar vortex in December and its impact on Eurasian air temperature

Weak stratospheric polar vortex (WPV) events during winter months were investigated. WPV events were identified as being weakest in December, accompanied by the most dramatic increase in geopotenti...

Two-dimensional vector solitons in a cold atomic gas via electromagnetically induced transparency

We investigate the formation and propagation of vector vortex solitons (VSs) and unipolar solitons (USs) in a cold coherent atomic gas with a Bessel lattice (BL). The system considered is a gas with a tripod level configuration. Owing to the big enhancement of Kerr nonlinearity contributed by the electromagnetically induced transparency (EIT), a weak vector vortex soliton can be effectively formed with ultraslow propagation velocity. We also demonstrate that the characteristics of two-dimensional VSs and USs can be controlled and manipulated by adjusting the parameters of BL. Results obtained may be useful for designing all-optical switches at low light levels.

Control of Convection in High‐Resolution Simulations of Tropical Cyclogenesis

AbstractThree idealized high‐resolution simulations of tropical storm formation from a weak vortex are analyzed. The three simulations include a case using warm rain microphysics, a similar case in which surface friction is omitted, and a case in which ice microphysics is used. The goal is to understand the mechanisms controlling the intensity and distribution of convection in the formation process in each of these cases. Simulations of convection in weak temperature gradient convective models show that a combination of low to middle tropospheric moist convective instability, the saturation fraction or column relative humidity, and the surface moist entropy flux explain a high percentage of the variance in precipitation and lower tropospheric vertical mass flux. Tropical cyclones differ from other convective environments in that intense frictional convergence occurs in the boundary layer. Adding a measure of convective inhibition to account for this process enables the lower tropospheric mass flux to be predicted even in the core regions of the simulated tropical cyclones. These results are pertinent to the development of more accurate convective parameterizations for large‐scale models.

Interaction of two-dimensional spots with a heat releasing/absorbing shock wave: linear interaction approximation results

The canonical interaction between a two-dimensional weak Gaussian disturbance (entropy spot, density spot, weak vortex) with an exothermic/endothermic planar shock wave is studied via the linear interaction approximation. To this end, a unified framework based on an extended Kovásznay decomposition that simultaneously accounts for non-acoustic density disturbances along with a poloidal–toroidal splitting of the vorticity mode and for heat release is proposed. An extended version of Chu’s definition for the energy of disturbances in compressible flows encompassing multi-component mixtures of gases is also proposed. This new definition precludes spurious non-normal phenomena when computing the total energy of extended Kovásznay modes. Detailed results are provided for three cases, along with fully general expressions for mixed solutions that combine incoming vortical, entropy and density disturbances.

Effect of near-wake jet on the lock-in of a freely vibrating square cylinder

We present a numerical study of a freely vibrating square cylinder with steady bleeding at its base side. In particular, we focus on the suppression of vortex-induced vibration (VIV) and the reduction of drag force for the elastically mounted square cylinder at a laminar flow condition via near-wake jet. We examine the base bleeding mechanism in the near-wake region of a square cylinder and its influence over the flow dynamics and the wake characteristics for both stationary (nonlock-in) and freely vibrating (lock-in) conditions. We consider the near-wake jet parameter as a function of the bleed coefficient (Cq), which is the ratio of near-wake jet flow velocity to the freestream velocity and depends on the Reynolds number (Re) based on the diameter of the cylinder. Investigations of the hydrodynamic coefficients and the flow features are carried out for the laminar Re range, namely, Re = 40, 60, 100, and 150. A single dominant frequency peak is observed in the lift coefficient spectrum plot for all the Reynolds numbers considered, but two peaks are observed for Re = 150 at Cq = 0.175 and 0.2. Higher Cq values behave like a splitter plate thereby preventing the interaction of alternating shear layers. The variation of the mean drag is associated with the pressure distribution around the cylinder surface and along the streamwise locations. This leads to a thinner wake width, weaker vortices, and higher vortex shedding frequency as observed earlier in the literature. The sharp spikes of pressure coefficient at the base side of the cylinder are observed for Re ∈ [40, 150] due to the near-wake jet, accounting for the fluctuations of drag force coefficient. We demonstrate the formation of multiple vortices at the wake region due to the near-wake jet from our detailed qualitative analysis. We observe counter-rotating pair of recirculating fluids flanking at the near-wake jet location and examine the recovery of base pressure due to the jet flow. We demonstrate the splitting of big circulation fluid bubble into many smaller counter-rotating fluids due to high-velocity jet flow, resulting into the stabilization of flow profiles in the wake region. We extend this investigation to quantify the effect of the near-wake jet on the two-degree-of-freedom cylinder system at the representative Reynolds number Re = 100 and three mass ratios m* = 1, 2, and 3. We demonstrate the reduction of peak transverse VIV amplitude by 90% in comparison to the plain cylinder counterpart.

Eurasian Cold Air Outbreaks under Different Arctic Stratospheric Polar Vortex Strengths

AbstractThis study analyzes the differences and similarities of Eurasian cold air outbreaks (CAOs) under the weak (CAOW), strong (CAOS), and neutral (CAON) stratospheric polar vortex states and examines the potential links between the polar vortex and Eurasian CAOs. The results indicate that the colder surface air temperature (SAT) over Europe in the earlier stages of CAOW events is likely because the amplitude of the preexisting negative North Atlantic Oscillation pattern is larger in CAOW events than in CAON and CAOS events. Marked by the considerably negative stratospheric Arctic Oscillation signals entering the troposphere, the SAT at midlatitudes over eastern Eurasia in CAOW events is colder than in CAON events. A larger diabatic heating rate related to a positive sensible heat flux anomaly in CAOW events likely offsets, to some degree, the cooling effect caused by the stronger cold advection and makes the differences in area-averaged SAT anomalies over northern Eurasia between the CAOW and CAON events look insignificant in most stages. Massive anomalous waves from the low-latitude western Pacific merge over northeastern Eurasia, then weaken the westerly wind over this region to create favorable conditions for southward advection of cold air masses in the earlier stages of all three types of CAOs. This study further analyzes the interannual relationship between the stratospheric polar vortex strength and the intensity of Eurasian CAOs and finds that climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) relative to the reanalysis dataset tend to underestimate the correlation between them. The relationship between them is strengthening under representative concentration pathway 4.5 (RCP4.5) and 8.5 (RCP8.5) scenarios over the period 2006–60. In addition, the intensity of Eurasian CAOs exhibits a decreasing trend in the past and in the future.

The applicability of vortex identification methods for complex vortex structures in axial turbine rotor passages

The complex vortex structures in the flow around turbine rotor passages, with weak or strong, large or small vortices, interacting with each other, often generate most of aerodynamic loss in turbomachines. Therefore, it is important to identify the vortex structures accurately for the flow field analysis and the aerodynamic performance optimization for turbomachines. In this paper, by using 4 vortex identification methods (the Q criterion, the Ω method, the Liutex method and the Ω-Liutex method), the vortices are identified in turbine rotor passages. In terms of the threshold selection, the results show that the Ω method and the Q-Liutex method are more robust, by which strong and weak vortices can be visualized simultaneously over a wide range of thresholds. As for the display consistency of the vortex identification methods and the streamlines, it is shown that the Liutex method gives results coinciding best with the streamlines in identifying strong vortices, while the Ω-Liutex method gives results the most consistent with the streamlines in identifying weak vortices. As to the relationship among the loss, the vortices and the shear, except for the Q criterion, the other three methods can distinguish the vortical regions from the high shear regions. And the flow losses in turbine rotor passages are often related to high shear zones, while there is a small loss within the core of the vortex. In order to obtain the variation of vortices in the turbine rotor passages at different working points, the Liutex method is applied in 2 cases of a turbine with different angles of attack. The identification results show that the strengths of the tip leakage vortex and the upper passage vortex are weaker and the distance between them is closer at a negative angle of attack. This indicates that the Liutex method is an effective method, and can be used to analyze the vortex structures and their evolution in turbine rotor passages.

Extension Omega and Omega-Liutex methods applied to identify vortex structures in viscoelastic turbulent flow

The vortex structure plays a significant role in the investigation of the turbulent drag reduction effect of the viscoelastic turbulent flow. This paper aims to find out an optimal vortex identification method for the viscoelastic turbulent flows, and then studies the turbulent drag reduction mechanism by analyzing the characteristics of the identified vortex structures in the turbulent flows of the viscoelastic fluids. The Q, λ2, Liutex, Omega (Ω) and Omega-Liutex (ΩR) methods are adopted for the identification of vortex structures in the forced homogeneous isotropic turbulence (FHIT) with/without the polymer additive, respectively. The comparison among these five methods shows that the threshold values for the Q, λ2 and Liutex methods should be specially adjusted so as to suitably describe the strong and weak vortex structures in the FHIT of both the Newtonian and viscoelastic fluids, while a fixed threshold value of 0.52 for the Ω and ΩR methods is effective for both the Newtonian and viscoelastic fluids. The comparison between the identified vortex structures in the FHIT with and without the polymer additive indicates that the Ω and ΩR methods are more appropriate for the vortex identification because their dimensionless values with a fixed range from 0 to 1 can avoid the effect of the different ranges of the Q, λ2 and ∣R∣ (for the Liutex method) for the Newtonian and viscoelastic fluids. This also illustrates that the Ω and ΩR methods can be extended to identify the vortex structures in the turbulent flow of the viscoelastic fluid. Finally, the characteristics of the vortex structures in the FHIT of the viscoelastic fluid are analyzed by utilizing the Ω and ΩR methods. The results show that both the strong and weak vortex structures are inhibited by increasing the concentration of the polymer solution and by decreasing the Weissenberg number, especially for the weak vortex structures.

Combined Impact of El Niño–Southern Oscillation and Pacific Decadal Oscillation on the Northern Winter Stratosphere

Using reanalysis and the sea surface temperature (SST) analysis, the combined impact of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on the northern winter stratosphere is investigated. The warm and weak stratospheric polar vortex response to El Niño simply appears during positive PDO, whereas the cold and strong stratospheric polar vortex response to La Niña is preferable during negative PDO in the reanalysis. Two mechanisms may account for the enhanced stratospheric response when ENSO and PDO are in phase. First, the asymmetries of the intensity and frequency between El Niño and La Niña can be identified for the two PDO phases. Second, the extratropical SST anomalies in the North Pacific may also play a role in the varying extratropical response to ENSO. The North Pacific SST anomalies related to PDO superimpose ENSO SST anomalies when they are in phase but undermine them when they are out of phase. The superimposed North Pacific SST anomalies help to increase SST meridional gradient anomalies between tropical and extratropics, as well as to lock the local height response to ENSO. Therefore, the passages for the upward propagation of waves from the troposphere is more unimpeded when positive PDO is configured with El Niño, and vice versa when negative PDO is configured with La Niña.

Leading-edge tubercles delay flow separation for a tapered swept-back wing at very low Reynolds number

Flow separation characteristics for two tapered swept-back wings, one with straight leading-edge (LE) and the other with tubercled LE, were investigated in a water tunnel using time-resolved particle image velocimetry (TR-PIV) technique. The two wings were based on the SD7032 aerofoil profile, with Reynolds number Re = 1.4 × 104, close to the working condition for common underwater gliders. The LE tubercles were designed such that the amplitude decreased linearly from the wing root to wing tip, while retaining constant wavelength. Results indicate that the baseline wing shows significantly separated flow in the outboard region at pitch angle of 10° and 20°, and the flow remains attached in the inboard region due to relatively larger local Reynolds number. Implementation of LE tubercles can mitigate flow separation downstream of both troughs and peaks. At higher pitch angles, the separated flows cover most of the baseline wing surface, whereas flow remains attached downstream most of tubercle peaks. Streamwise aligned counter-rotating vortex pairs (CVPs) formed over the tubercles are significantly tilted and asymmetrical due to the sweep and amplitude difference between the two sides of tubercle. Consequently, weaker vortices in CVPs close to the wing root are rapidly dissipated, allowing the CVPs to evolve into a series of co-rotating vortices (CVs), which exerted significant impact on flow separation characteristics downstream of the tubercles.

Wake dynamics and surface pressure variations on two-dimensional normal flat plates

The relationship between vortex dynamics and surface pressure fluctuations on the leeward face of a two-dimensional normal thin flat plate was studied using Direct Numerical Simulations for incompressible flow at a Reynolds number of 1200. The vortex shedding frequency was observed in the spectra of pressure fluctuations on both faces of the plate, while a lower frequency spectral peak was only evident on the leeward pressure fluctuations. Local sharp peaks of low pressure in the separated shear layers coincide with increases in the leeward face pressure fluctuations. These observations are tied to the vortex dynamics using the invariant Q, commonly used for vortex identification. Q is also proportional to the source term for the Poisson equation for the instantaneous pressure. The pressure, which is the only contributor to the plate drag, varies significantly in response to alterations to the vortex shedding, which can be observed in differences of vortex trajectories. At minimum drag, the pressure fluctuations are small, which is attributed to lower values of Q associated with weaker vortices.

Atmospheric Circulation Response to Anomalous Siberian Forcing in October 2016 and its Long‐Range Predictability

AbstractThe warm Arctic‐cold continent pattern was of record strength in October 2016, providing the opportunity to test its proposed influence on large‐scale atmospheric circulation. We find a record weak polar stratospheric vortex and negative North Atlantic Oscillation in November–December 2016 and link them to increased planetary wave generation associated with cold Siberian anomalies followed by troposphere‐stratosphere dynamical coupling. At the same time the warm Arctic anomalies, in particular those over the Barents‐Kara Seas, do not appear to play an important role in forcing the atmospheric circulation. Long‐range forecasts initialized on 1 October 2016 reproduced both the weak polar vortex and negative North Atlantic Oscillation, as well as their link with the Siberian temperatures. Our results support the stratospheric pathway for atmospheric circulation forcing associated with Siberian surface anomalies and uncover a source of skill for subseasonal forecasts from October to December.

Comparisons and analyses of vortex identification between Omega method and Q criterion

The present paper presents comparisons of the vortex identification between the omega method and the Q criterion based on the data of a classical flow. From the comparisons of the vortex structure together with the flow statistics, some important conclusions are drawn on the validity of the two methods, as follows. The omega method can identify various kinds of vortices with different intensities (e.g., the strong vortex, the medium vortex and the weak vortex). For the Q criterion, due to the subjective threshold selection, only the strong vortex with weak deformations could be identified. Finally, some emerging topics related with the advanced vortex identification methods are briefly discussed.

Springtime North Pacific Oscillation and summer sea ice in the Beaufort sea

We investigate the linkage between the spring (April–June) North Pacific Oscillation (NPO) and the following summer sea ice in the Beaufort sea. A positive NPO is characterized by an intensified and northeastward extended Aleutian Low, a strong Beaufort High, and enhanced easterly winds in the Beaufort sea. Concomitantly, while the deepening East Asian trough steers more cyclones from Siberia to the sub-polar Pacific and thus fewer cyclones into the western Arctic Ocean; a weak polar vortex and weak baroclinicity in the Beaufort sea does not favor cyclone activity. In addition, the East Asian trough enhances warm air advection from the Pacific to the Northwestern America. The spring NPO accounts for 29% of interannual variability of the following September sea ice cover in the Beaufort sea and can be used as a potential predictor for ice melting in the Beaufort sea. During a positive NPO, the strong easterly winds over the Beaufort sea enhance ice advection and reduce ice thickness; on the synoptic scale, there are more occurrences of the pack ice becoming detached from the coast due to the strong easterly winds. Moreover, increases in shortwave radiation and longwave radiations accelerate the ice melting.

Near-future tropical cyclone predictions in the western North Pacific: fewer tropical storms but more typhoons

This study presents forecasts of near-future tropical cyclone (TC) activities over the western North Pacific (WNP) using a TC track-pattern-based prediction model in conjunction with long-term free-run simulations from the National Centers for Environmental Prediction Climate Forecast System. The prediction shows that the East Asian coastal area will be affected by fewer TC landfalls. However, the number of stronger TC landfalls may increase in the near future (2016–2030) compared to the present-day period (2002–2015). The potential candidates for modulating the near-future TC activity are three large-scale climate variability: El Nino–Southern Oscillation (ENSO), the North Pacific sea surface temperature (NPSST) variation, and basin-wide warming of the Pacific. More frequent El Nino episodes in the near future may contribute to the activation of TC formations in the far-eastern tropical ocean, which generates a favorable influence on TC intensification due to longer distances and development periods over the ocean. NPSST variations have contrasting effects, i.e., they have unfavorable effects on TC genesis due to weak vorticity, while creating favorable conditions for TC intensification by providing more humid environments in the subtropics. The impact of overall Pacific warming on the changes in TC activities is low compared to that of the other two factors. Our results physically demonstrate the contributions of three WNP sea surface temperature variability on near-future TC activity, emphasizing the critical roles of ENSO and NPSST rather than the near-term warming effect.

The orographic effect of Reunion Island on tropical cyclone track and intensity

Like Taiwan, the orography of Reunion Island may impact tropical cyclone (TC) track and intensity. A Mann–Whitney test is applied on best‐track data from the Regional Specialized Meteorological Center (RSMC) La Reunion to demonstrate that this effect is detectable at less than 250 km from the island. A set of idealized experiments is carried out to investigate this effect with the French non‐hydrostatic mesoscale numerical model Meso‐NH at 12 and 4‐km horizontal grid spacing. Results show that the island influences TC track and intensity within two radii of maximum winds, defining a distance of influence. The impact is similar to an aspiration of the vortex by the island, accompanied by vortex weakening. An asymmetry is found between TCs passing north or south of the island and can be explained by the presence of the island in the flow.

Weak Stratospheric Polar Vortex Events Modulated by the Arctic Sea‐Ice Loss

AbstractWe characterize the differences in the upward planetary‐scale wave propagation during observed weak polar vortex (WPV) events between heavy‐ and light‐sea‐ice years in the Barents‐Kara Sea based on a composite analysis for the period of 1979–2015. Upward wave propagation during WPV events in heavy‐ice years is dominated by the wavenumber 1 component. In contrast, WPV events occurring in light‐ice years are characterized by stronger wavenumber 2 propagation, which is caused by the tropospheric wavenumber 2 response to sea‐ice reduction in the Barents‐Kara Sea. The above observed features are supported by an Atmospheric General Circulation Model experiment. Thus, under present climate conditions, Arctic sea‐ice loss is a possible factor modulating the wave propagation during the WPV events. We also find that the WPV events in light‐ice years have stronger stratosphere‐troposphere coupling, followed by colder midlatitude surface conditions particularly over Eurasia.