Vortex Strength Research Articles

On the aerodynamic loads and flow statistics of airfoil with deformable vortex generators

The aerodynamic performances and flow statistics for a Delft University-91-W2-250 airfoil with deformable vortex generators (DVG) were experimentally studied in a wind tunnel across various angle of attacks and wind speeds. A high-resolution force sensor was used to measure the time-averaged lift force, while a planar particle image velocimetry system was applied to characterize the mean velocity and vortex shedding over airfoil surface. The results highlighted that, similar to conventional rigid vortex generators (RVG), DVG can effectively enhance lift coefficient after the stall angle of airfoil with clean surface under low incoming winds. However, the deformation of DVGs increased with the growth of wind speed; this suppressed the effectiveness of wake mixing where the aerodynamic performance of DVGs gradually converged to clean surface configurations. The flow measurements demonstrated that the deformation of DVG can lead to significant decrease in near-wall flow velocities close to the airfoil trailing edge and generate more dispersed vorticity distributions. To further investigate the linkage between DVG deformation and its wake mixing effectiveness, complementary tomographic particle image velocimetry measurements were conducted. The results indicated that the vorticity strength presented monotonic decay with the bending angle of DVG within both near and intermediate wake regions. The capability of DVGs passively adjusts their bending angle, and therefore, the airfoil lift coefficients provide a novel approach to reduce aerodynamic load fluctuations for aircraft within unsteady flows.

Unsteady forced convection flow and heat transfer past a blunt headed semicircular cylinder at low Reynolds numbers

A numerical analysis is performed to study the forced convective fluid flow and heat transfer past an unconfined semicircular cylinder. The two-dimensional simulations are carried out for Reynolds number ranging 10 ≤ Re ≤ 200 and operating fluid is air (Pr = 0.71) for Newtonian constant property fluid. Continuity, Navier-Stokes, and energy equations with appropriate boundary conditions are solved using computational fluid dynamics solver Ansys Fluent. Various parameters flow such as lift, drag, skin friction coefficient, pressure coefficient, Nusselt (Nu) number, Strouhal (St) number, and vortex strength are calculated. The transition from steady to time periodic flow occurs between Re = 60 and 80. The effect of Reynolds number on heat transfer is discussed. Finally, a developed correlation of Nu and St numbers are presented.

Experimental study of flow structure impact on the fluid parameters in saccular aneurysm models

Aneurysms are abnormal expansion of weakened blood vessels which can be debilitating or fatal upon rupture. Previous studies have shown that hemodynamics play a role in aneurysm formation, growth and rupture. Therefore, the objective of this work is to investigate the impact of flow structures on vortex strength, impinging location, and wall shear stress for two aneurysm models. For this study, experiments were performed on two different, idealized, rigid, and saccular aneurysm models characterized by BF ratios of 1.0 and 1.6. The vortical structures formation and movement were altered by changing peak Reynolds number (Rep) from 50 and 270 and Womersley number (α) from 2, and 5 on both aneurysm models. Velocity field measurements were performed on the mid-plane location of the aneurysm models using Particle Image Velocimetry (PIV). The results showed presence of vortical structures which formed, grew, and dissipated at different phases in the flow cycle. The behavior of these vortical structures impacted the impinging locations where increased frequency of impingement points at the distal neck could be seen for α=5 than α=2 flow scenarios. The wall shear stress results showed that the distal neck experienced elevated stresses during the vortex formation phases of the flow cycle for BF=1.0 and 1.6 models.

Numerical prediction of the influence of free surface vortex air-entrainment on pump unit performance

Free surface vortex (FSV) is a complex gas-liquid two-phase flow, which is harmful to the pump unit. In this paper, FSV in the pump sump with a double-suction centrifugal pump is numerically simulated by two-phase flow method, which is verified by experimental data and agreed well with theoretical models. The purpose of this paper is to study the influence of FSV on the performance of pump, the variation characteristics of several paramount paraments (FSV evolution process, vortex strength, air-laden content, head-drop, flow stability, pressure amplitude) was revealed for the first time. The vortex strength affects the air-laden capacity of FSV, the maximum air content in this impeller can reaches 17.1%. The air destroys the stable structure of the flow field in the impeller, affects the flow rate, and the mean head-drop by 36.8%, meanwhile inducing irregular pressure pulsation. The duration and content of suction air of FSV determine the performance-drop amplitude. The change of unit performance is proportional with the air-entrainment content and vortex strength, which shows that the larger the FSV air-entrainment is, the worse the pump performance. It is clear that the predicted results are consistent with the existing problems of the unit on site, which can provide a method to predict the influence of the FSV on the unit.

Aerodynamic Modelling of the Airfoil Immersed in Two-Dimensional Jet Flow

In order to study the aerodynamic interaction of the airfoil and jet flow, the free streamline model and the panel method are combined to develop a fast calculation method for the airfoil in two-dimensional inviscid jet flow. The vortex strength and position of the jet boundary are determined by using the free streamline model and the constant total pressure difference assumption, the circulation of the airfoil is solved by the vortex panel method, and the whole process is coupled by relaxation iteration. Firstly, the convergence and effectiveness of the present method are verified. Next, the influence of the length ratio of jet height to airfoil chord, the velocity ratio of jet velocity to freestream velocity, and the ground effect on airfoil aerodynamics are studied. The results show that the aerodynamic characteristics of the airfoil in finite width jet flow and in freestream have a large difference, and it is important to consider the jet deflection for jet/airfoil interaction. In jet flow, the velocity ratio can be regarded as an aerodynamic similarity parameter for the airfoil. When the jet flow is deflected, the airfoil will not only generate lift but also drag, and the ground effect can be used to decrease drag. The developed method in this paper can not only capture the jet deflection but also has higher calculation efficiency than Computational Fluid Dynamic (CFD), which is beneficial for the preliminary design of a powered-lift device.

Intra-Seasonal Variations and Frequency of Major Sudden Stratospheric Warmings for Northern Winter in Multi-System Seasonal Hindcast Data

This study investigates intra-seasonal variations and frequency of major sudden stratospheric warmings (MSSWs) in Northern winter seasonal hindcasts of six systems from 1993/1994 to 2016/2017, in comparison to the Japanese 55-year Reanalysis data. Results show that, over all, all systems reproduce precursory signals to the MSSWs well, such as the increase in the planetary wave heat flux in the extratropical lower stratosphere and the anomalous planetary wave patterns in the troposphere. Some systems are suggested to underestimate or overestimate the mean MSSW frequency. Such differences in the frequency of the MSSWs among the systems are related to those in the mean strength of the stratospheric polar vortex, and also may be partly contributed by those in the frequency of notable heat flux events. The hindcast data exhibit a weaker mean vortex and an increased MSSW frequency for a warm phase than for a cold phase of El Niño/Southern Oscillation, and for an easterly phase than for a westerly phase of the Quasi-Biennial Oscillation. These are qualitatively consistent with reanalysis results, except for a lower MSSW frequency for the warm phase in the reanalysis data. The reanalysis teleconnection results are larger in magnitude than the hindcast results for most ensemble members, although they are included near the edge of the distributions of the ensemble members. The changes in the MSSW frequency with the two external factors are correlated to those in the mean vortex strength among the ensemble members and also the ensemble means for some systems.

Seasonal Variability in the Mechanisms behind the 2020 Siberian Heatwaves

Abstract Record-breaking heatwaves and wildfires immersed Siberia during the boreal spring of 2020 following an anomalously warm winter. Springtime heatwaves are becoming more common in the region, with statistically significant trends in the frequency, magnitude, and duration of heatwave events over the past four decades. Mechanisms by which the heatwaves occur and contributing factors differ by season. Winter heatwave frequency is correlated with the atmospheric circulation, particularly the Arctic Oscillation, while the frequency of heatwaves during the spring months is highly correlated with aspects of the land surface including snow cover, albedo, and latent heat flux. Idealized AMIP-style experiments are used to quantify the contribution of suppressed Arctic sea ice and snow cover over Siberia on the atmospheric circulation, surface energy budget, and surface air temperature in Siberia during the winter and spring of 2020. Sea ice concentration contributed to the strength of the stratospheric polar vortex and Arctic Oscillation during the winter months, thereby influencing the tropospheric circulation and surface air temperature over Siberia. Warm temperatures across the region resulted in an earlier-than-usual recession of the winter snowpack. The exposed land surface contributed to up to 20% of the temperature anomaly during the spring through the albedo feedback and changes in the ratio of the latent and sensible heat fluxes. This, in combination with favorable atmospheric circulation patterns, resulted in record-breaking heatwaves in Siberia in the spring of 2020.

Evaporation dynamics of a surrogate respiratory droplet in a vortical environment

HypothesisVortex droplet interaction is crucial for understanding the route of disease transmission through expiratory jet where several such embedded droplets continuously interact with vortical structures of different strengths and sizes. ExperimentsA train of vortex rings with different vortex strength, quantified with vortex Reynolds number (Re′=0,53,221,297) are made to interact with an isolated levitated droplet, and the evolution dynamics is captured using shadowgraphy, particle image velocimetry (PIV), and backlight imaging technique. NaCl-DI water solution of 0, 1, 10 and 20 wt% concentrations are used as test fluids for the droplet. FindingsThe results show the dependence of evaporation characteristics on vortex strength, while the crystallization dynamics was found to be independent of it. A reduction of 12.23% and 14.6% in evaporation time was seen in case of de-ionized (DI) water and 1% wt NaCl solution respectively in presence of vortex ring train at Re′=221. In contrast to this, a minimal reduction in evaporation time (0.6% and 0.9% for DI water and 1% wt NaCl solution, respectively) is observed when Re′ is increased from 221 to 297. The mechanisms for evaporation time reduction due to enhancement of convective heat and mass transfer from the droplet and shearing away of vapor layer by vortex ring interaction are discussed in this work.

Validation of Calibrated Model Parameters for Jet-in-Crossflow

Previous efforts determined a set of calibrated, optimal model parameter values for Reynolds-averaged Navier–Stokes (RANS) simulations of a compressible jet in crossflow (JIC) using a turbulence model. These parameters were derived by comparing simulation results to particle image velocimetry (PIV) data of a complementary JIC experiment under a limited set of flow conditions. Here, a model using both nominal and calibrated parameters is validated against PIV data acquired from a much wider variety of JIC cases, including a realistic flight vehicle. The results from the simulations using the calibrated model parameters showed considerable improvements over those using the nominal values, even for cases that were not used in the calibration procedure that defined the optimal parameters. This improvement is demonstrated using a number of quality metrics that test the spatial alignment of the jet core, the magnitudes of multiple flow variables, and the location and strengths of vortices in the counter-rotating vortex cores on the PIV planes. These results suggest that the calibrated parameters have applicability well outside the specific flow case used in defining them and that with the right model parameters, RANS solutions for the JIC can be improved significantly over those obtained from the nominal model.

Numerical study of the flow field around hydrokinetic turbines with winglets on the blades

Design methods and concepts that can increase the energy conversion efficiency of marine and river current turbines are of great importance for the development and wider utilization of this emerging renewable energy technology. This work is aimed to improve hydrodynamic performances of the hydrokinetic turbines to get as close as possible to the theoretical energy conversion efficiency limitation of 59.3%, known as the Lanchester–Betz–Joukowsky limit. The winglets are integrated at the turbine blade tips to reduce the effect of the tip vortex. The winglet design concept is adopted from the jet aircraft and adjusted to the hydrokinetic turbine application. The numerical investigation of the winglets impact on the hydrodynamic performances was performed using computational fluid dynamic (CFD). The results indicate that winglets subdue the strength of tip vortices, which is reflected through the increase in the power extracted. It has been confirmed that winglet height has a great influence on the power coefficient increase, which for higher winglets reaches the value above 50%. The analysis of the wake regions behind turbines pointed out that the turbines with winglets form stronger vortices in the far wake zone which may influence the back-flow turbine installed in row array.

Rossby Waves in Total Ozone over the Arctic in 2000–2021

The purpose of this work is to study Rossby wave parameters in total ozone over the Arctic in 2000–2021. We consider the averages in the January–March period, when stratospheric trace gases (including ozone) in sudden stratospheric warming events are strongly disturbed by planetary waves. To characterize the wave parameters, we analyzed ozone data at the latitudes of 50°N (the sub-vortex area), 60°N (the polar vortex edge) and 70°N (inner region of the polar vortex). Total ozone column (TOC) measurements over a 22-year time interval were used from the Total Ozone Mapping Spectrometer/Earth Probe and Ozone Mapping Instrument/Aura satellite observations. The TOC zonal distribution and variations in the Fourier spectral components with zonal wave numbers m = 1–5 are presented. The daily and interannual variations in TOC, amplitudes and phases of the spectral wave components, as well as linear trends in the amplitudes of the dominant quasi-stationary wave 1 (QSW1), are discussed. The positive TOC peaks inside the vortex in 2010 and 2018 alternate with negative ones in 2011 and 2020. The extremely low TOC at 70°N in 2020 corresponds to severe depletion of stratospheric ozone over the Arctic in strong vortex conditions due to anomalously low planetary wave activity and a high positive phase of the Arctic Oscillation. Interannual TOC variations in the sub-vortex region at 50°N are accompanied by a negative trend of −4.8 Dobson Units per decade in the QSW1 amplitude, statistically significant at 90% confidence level, while the trend is statistically insignificant in the vortex edge region and inside the vortex due to the increased variability in TOC and QSW1. The processes associated with quasi-circumpolar migration and quasi-stationary oscillation of the wave-1 phase depending on the polar vortex strength in 2020 and 2021 are discussed.

Hydrodynamic helicity polarization in relativistic heavy ion collisions

We study helicity polarization through the $(3+1)$-dimensional relativistic viscous hydrodynamic models at $\sqrt{{s}_{NN}}=200$ GeV $\text{Au}+\text{Au}$ collisions. Similar to the local spin polarization, we consider the helicity polarization beyond global equilibrium and investigate the contributions induced by thermal vorticity, shear viscous tensor, and the fluid acceleration. We find that the local helicity polarization induced by thermal vorticity dominates over other contributions. It also implies that, in low-energy collisions, the fluid vorticity as part of thermal vorticity may play the crucial role in the total helicity polarization. Such a finding could be useful for probing the local strength of vorticity in rotational quark gluon plasmas by measuring helicity polarization. Our simulation confirms the strict space reversal symmetry, whereas we also compare our numerical results with approximated relations derived from ideal Bjorken flow. Our studies also provide a baseline for the future investigation on local parity violation through the correlations of helicity polarization.

In silico analyses of blood flow and oxygen transport in human micro-veins and valves.

Almost 95% of the venous valves are micron scale found in veins smaller than 300μm diameter. The fluid dynamics of blood flow and transport through these micro venous valves and their contribution to thrombosis is not yet well understood or characterized due to difficulty in making direct measurements in murine models. The unique flow patterns that may arise in physiological and pathological non-actuating micro venous valves are predicted. Computational fluid and transport simulations are used to model blood flow and oxygen gradients in a microfluidic vein. The model successfully recreates the typical non-Newtonian vortical flow within the valve cusps seen in preclinical experimental models and in clinic. The analysis further reveals variation in the vortex strengths due to temporal changes in blood flow. The cusp oxygen is typically low from the main lumen, and it is regulated by systemic venous flow. The analysis leads to a clinically-relevant hypothesis that micro venous valves may not create a hypoxic environment needed for endothelial inflammation, which is one of the main causes of thrombosis. However, incompetent micro venous valves are still locations for complex fluid dynamics of blood leading to low shear regions that may contribute to thrombosis through other pathways.

Numerical Investigation on the Effect of Asymmetry of Flow Velocity on the Wake Vortex of Hydrofoils

The Karman vortex street is a common flow phenomenon. In hydraulic machinery, it is usually located downstream of the guide vanes and the runner blades, which reduces hydraulic performance and may also cause fatigue damage to the structure. The latest research suggested that the difference in velocity gradient on each side of the blade trailing edge may have a significant impact on the strength of the wake vortex. The current work aims to verify the above conclusion and further explore the influence of asymmetry of flow velocity on the wake vortex. A numerical model with the velocity ratio, α, between the two sides of the hydrofoil as the only variable was designed, and the wake characteristics were solved by a computational fluid dynamics (CFD) method based on the finite volume. The unsteady Reynolds-average Navier–Stokes (URANS) equations were numerically solved by coupling with a transitional shear-stress transport (SST) turbulence model. The results showed that with the increase of α, the vortex shedding frequency decreased first, and then increased after reaching the critical velocity ratio αc1 ≈ 1.4. The vortex intensity first gradually decreased, and the vortex street suddenly disappeared after reaching the critical velocity ratio αc2 ≈ 2.2. The value of αc1 was affected by the thickness of the trailing edge, and αc2 was affected by the thickness and the Reynolds number. Besides, the asymmetry of the flow velocity also affected the effectiveness of the trailing-edge trimming. This research can provide references for the design of hydraulic machinery and other submerged structures.

Interactions between the stratospheric polar vortex and Atlantic circulation on seasonal to multi-decadal timescales

Abstract. Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSWs), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation, and cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000 year preindustrial control simulation of the UK Earth System Model to study such interactions, using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary, with spectral power in vortex variability at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response through a pathway involving the equatorial Pacific and quasi-biennial oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use regression analysis to estimate the potential contribution of the 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus.

Three-dimensional direct numerical simulations of two interfering side-by-side circular cylinders at intermediate spacing ratios

Side-by-side arranged circular cylinders show fierce interferences when they are not far apart. In this paper, we present the vibration response and wake characteristics of two elastically mounted circular cylinders in a side-by-side arrangement through three-dimensional direct numerical simulations (3-D DNS). The Reynolds number is Re = 500 and two center-to-center spacings (s) of 2.0D and 2.5D are considered where D is the diameter of the two cylinders. We found that with the increasing reduced velocity, the responses of both s/D cases can be divided into three different regions, which are named desynchronization region (DS), initial branch (IB), and lower branch (LB), with the former being further divided into two subregions (DS-I and DS-II). The vibration, hydrodynamic forces, and spectral features in each region are discussed in detail. According to the characteristics of cylinder oscillation and vortex dynamics, six types of wake patterns are identified, with the deflected (DF) and flip-flopping (FF) patterns at s/D = 2.0 while in-phase (IP), anti-phase (AP), in-/anti-phase (I/AP), and hybrid (HB) patterns at both s/D = 2.0 and 2.5. Furthermore, we investigated the wake three-dimensionality through the evolution of vortical structures, and the statistics and development of the enstrophies. It was observed that the presence of the streamwise vortices modifies the strength and spanwise coherence of the spanwise vortices. Finally, flow physics for the interactions of the two cylinders at different regions is elucidated.

Characterizing aircraft wake vortex position and strength using LiDAR measurements processed with artificial neural networks.

The position and strength of wake vortices captured by LiDAR (Light Detection and Ranging) instruments are usually determined by conventional approaches such as the Radial Velocity (RV) method. Promising wake vortex detection results of LiDAR measurements using machine learning and operational drawbacks of the comparatively slow traditional processing methods motivate exploring the suitability of Artificial Neural Networks (ANNs) for quantitatively estimating the position and strength of aircraft wake vortices. The ANNs are trained by a unique data set of wake vortices generated by aircraft during final approach, which are labeled using the RV method. First comparisons reveal the potential of custom Convolutional Neural Networks in comparison to readily available resources as well as traditional LiDAR processing algorithms.

Interference of internal waves due to two point vortices: linear analytical solution and nonlinear interaction.

In this work, we consider steady two-dimensional interfacial waves in a two-layer stratified fluid, which is induced by a vortex pair located in the lower layer of the fluids. A mathematical model based on the boundary integral equation method and the potential-flow theory is established. The linear analytical solution for the linearized model is given in the form of Cauchy integral and then asymptotic behaviour for large x is presented. The fully nonlinear model is solved by the Jacobian-free Newton–Krylov (JFNK) method numerically. Nonlinear characteristics of wave profiles are identified compared with the linear results under different vortex strengths and the distance between the vortex pair. The amplitude of steady downstream waves is found to vary periodically with respect to the distance of the vortex pair, which can be regarded as the interference between waves produced by each vortex. For equal-strength counter- and co-rotating pairs, the downstream wave heights of linear solutions can be eliminated for some special values of the distance between point vortices, namely, the destructive interference occurs. Meanwhile, the wave only exists between the vortex pair like trapped waves. So does the nonlinear counterpart for counter-rotating pairs, but it could not be diminished with any distance.

Physical model setup and tests on cooling circulation water pumping intake system

Abstract The thermal power plant built by the Taiwan power company intends to use multiple cooling circulation water intake systems to introduce seawater from adjacent sea areas. The water in the front pool of the water inlet is introduced into the pump chamber through the large pump in the circulating water pump sump. The primary function of the pump chamber is to provide a stable and uniform flow condition before the large pump sucks in the seawater. When the hydro-dynamic conditions outside the pumping station are not ideal, and the inflow is relatively unstable, the pumping chamber can provide a buffer space for rectification. In addition, when the flow field near the suction intake of a large pump can maintain a certain degree of stability in space and time, it can reduce the frequency and intensity of the vortex or other harmful factors caused by the flow field. Moreover, a pump sump flow field can also maintain the pump's working efficiency, reducing vibration and maintenance costs. Although the relevant design standards such as the Hydraulic Institute (H.I.) standard in the United States or the British Hydromechanics Research Association (B.H.R.A.) standard in the United Kingdom can be consulted for different types of pumping room geometric layout planning and design recommendations. However, most design references are obtained from the boundary conditions of general cases. Therefore, these design criteria indicate that when the pump room geometry and flow conditions have special considerations or detail design, physical model tests should be conducted to verify it or correct the layout according to the model test results. Because the fluid mechanisms involved in the relevant research are complex, the current theoretical or numerical models are not enough to fully simulate flow fields. Therefore, it is necessary to conduct a physical model test based on the feasibility model scale. Based on the similarity theory to scale down prototype structures, the flow mechanism of preliminary design is reproduced, and the revised plan is also verified in the physical model to satisfy the project objectives. The qualitative flow field visualization method was used to observe the free surface vortex, subsurface vortices, and the swirl angle in the suction pipe under different layout schemes at each physical test case. The tested results show that the flow pattern and vortex strength are both acceptable with designed anti-vortex devices.

Numerical simulation of unsteady flow characteristics in a centrifugal pump during runaway process caused by power failure

Abstract Centrifugal pump is widely used in agricultural irrigation, petrochemical, mining, metallurgy and other industries. A special centrifugal pump with elbow pipe is researched in this paper. The flow rate and rotating speed change rapidly, especially in elbow pipe, causing serious pressure fluctuation and complex vorticity in the centrifugal pump. A user-defined function to solve the angular momentum equation was calculated the pump rotational speed. Unsteady numerical simulation with SST turbulence model is carried out and Short time Fourier transform (STFT) and vorticity strength are used to analyse the pressure fluctuation and vortexes respectively. The results indicate that the rotational speed first decreases to zero and then reversely increases to runaway speed. Large area vorticity appeared at the elbow pipe when the high amplitude frequency of pressure fluctuation increased. The change law of main frequency of pressure fluctuation at the elbow pipe is consistent with the rotational speed change ruler, which is equal to blade passing frequency (BPF). The main source of pressure fluctuation was rotor stator interaction. This study’s results provide a meaningful reference about pump runaway process caused by power failure, leading to the prevention of dangerous situations, for safe operations of the pumping unit.