Vortex Merging Research Articles

Effect of tip speed ratio on coherent dynamics in the near wake of a model wind turbine

The near wake of a small-scale wind turbine is investigated using particle image velocimetry experiments at different tip speed ratios ( $\lambda$ ). The wind turbine model had a nacelle and a tower mimicking real-scale wind turbines. The near wake is found to be dominated by multiple coherent structures, including the tip vortices, distinct vortex sheddings from the nacelle and tower, and wake meandering. The merging of the tip vortices is found to be strongly dependent on $\lambda$ . A convective length scale ( $L_c$ ) related to the pitch of the tip vortices is defined that is shown to be a better length scale than turbine diameter ( $D$ ) to demarcate the near wake from the far wake. The tower induced strong vertical asymmetry in the flow by destabilising the tip vortices and promoting mixing in the lower (below the nacelle) plane. The nacelle's shedding is found to be important in ‘seeding’ wake meandering, which, although not potent, exists close to the nacelle, and it becomes important only after a certain distance downstream ( $x>3L_c$ ). A link between the ‘effective porosity’ of the turbine and $\lambda$ is established, and the strength and frequency of wake meandering are found to be dependent on $\lambda$ . In fact, a decreasing trend of wake meandering frequency with $\lambda$ is observed, similar to vortex shedding from a porous plate at varying porosity. Such similarity upholds the notion of wake meandering being a global instability of the turbine, which can be considered as a ‘porous’ bluff body of diameter $D$ .

Vortex-induced particle capture in a micro cross-shaped channel

Vortex-induced particle capture has attracted widespread attention as an effective tool for particle preconcentration or manipulation. In this work, the motion and capture of particles in confined vortices in a micro cross-shaped channel are explored by a high-speed camera within Reynolds numbers ranging from 10 to 450. Micro Laser-Induced Fluorescence (μLIF) and numerical simulation are used to visualize flow structures. Results demonstrate that a buoyant particle undergoes an attenuation oscillation, finally arriving a periodic orbit (i.e., a limit cycle) inside the vortex. The formation of the orbit is closely associated with the development of recirculating flows in the microchannel. With an increase in the number of captured particles, the average distributions of particles show particle exchange corridors between capture regions and mainstream, which is caused by interactions between particles or flow instability. Once the particle density exceeds the fluid density, the particle periodic orbits disappear, and particles move directly or spirally to the outlets. Moreover, oscillation induced by vortex merging and splitting influences the quantities and distributions of capture regions, as well as the migration paths of buoyant particles. In addition to particle capture at the channel center, a portion of captured particles is observed near channel walls.

Wake flow structure and hydrodynamic characteristics of flow around a C-shaped cylinder with variable attack angle at low Reynolds numbers

A numerical investigation is conducted on the flow over a C-shaped cylinder in the low Reynolds number range of Re = 40–160. The effect of attack angle (α) ranging from 0° to 180° is examined simultaneously. Wake evolution and vortex structure as well as the hydrodynamic characteristics are analyzed. Seven flow patterns are identified based on the location of boundary layer separation points and the evolution of near-wall vortices. The boundary layer separation points lock on the two ends of the C-shaped cylinder, resulting in the typical Karman vortex street (Pattern I). A separation point shifts to the curved surface in Pattern II-1 and Pattern II-2, and a quasi-stagnation vortex (QS) is formed within the groove in Pattern II-2. In Pattern III-1 and Pattern III-2, the QS fills the groove. The subordinate vortex is observed in the groove close to the lower end (Pattern IV). The complicated vortex merging occurs around the lower end in Pattern V. The separation points lock on the two ends, exhibiting a pair of counter-rotating vortex shedding downstream of the two ends (Pattern VI). No vortex shedding is found in Pattern VII. Additionally, the characteristic parameters and the hydrodynamic coefficients are related, and they are associated with the flow pattern partition. Four types of vortex street are identified in the wake of the C-shaped cylinder, including no vortex street, 2S vortex mode and decayed vortex street, 2S vortex mode and secondary vortex street (2S-SVS), and P + S vortex mode and secondary vortex street in vortex evolution (P + S-SVS).

Analysis of Vortex Dominated Flow over Double Delta Wing

The maneuverability of fighter aircraft designed using double delta wing configuration at low subsonic speed in addition to under high angle of attacks are highly appealing and fascinating to comprehend. Flow pattern formed by double delta wing help to facilitate lifting force under high angle of attack and at such low subsonic speed. In comparison to delta wing, vortex formations by strake part of wing play an important role to create a persistent vortex core capable of delaying stalling. In this investigation, force coefficients of 75°/45° double delta wing configuration is extracted both from wing tunnel experiment and numerical calculation between 0° to 30° angles of attack at Reynolds number of 1.4 × 105 based on root chord length. It has also been attempted to investigate the flow pattern formation over wing leeward surface, interaction between vortices formed by strake/wing based on Q-criterion and pressure distribution along propagation path of vortices obtained from CFD (Computational Fluid Dynamics) calculations. Implications of interplay between vortices and pressure distribution in lift generation have also been analyzed. It has been found that around 15° angle of attack strake and wing vortices merge and gets more intensified. With higher angle of attack the lift generation significantly diminishes because of vortex bursting

Effect of spacing ratios on coupling mechanism of three tandem cylinders in planar shear flow

Using four-step semi-implicit characteristic-based split method, flow-induced vibrations of three tandem cylinders in planar shear flow are numerically simulated at Re = 100. The vibration responses are more complicated than that of a single cylinder due to the gap development and mutual interference with each other. The spacing ratio (L/D) between three tandem cylinders of equal size ranges from 2.5 to 5.5, with a mass ratio (mr) of 2.0 attributed to each one. The reduced velocity (Ur) spans from 3 to 30, and the shear ratio (k) varies as 0, 0.05 and 0.1 respectively. The numerical results show that spacing ratio and shear ratio have an influence on the dynamic responses, performances of frequency, phase and energy for three tandem cylinders. The vibration responses of each cylinder can be clearly divided in three major branches, which first increase then decrease with increasing Ur. The gap flow inside cylinders at small L/D heightens the probability of vortex merging, intensifying mutual interference and consequently amplifying the amplitude. It is worth noting that as k increases to 0.1, downstream cylinders exhibit different vibration responses at L/D ≥ 4.0. Specifically, a secondary increase occurs after Ur > 18 at L/D = 4.0, and the amplitude is completely suppressed after Ur > 25 at L/D = 5.5. With the help of other performances, the differences in vibration responses of three tandem cylinders at different k and L/D are revealed.

Flow-induced vibrations of ten tandem cylinders at low Reynolds number

Flow-induced vibrations of ten cylinders in tandem arrangement are numerically investigated by using the immersed boundary method with a low Reynolds number (Re = 100). Seven spacing ratios L/D (where L = center–center spacing between tandem cylinders and D = diameter of the cylinder) are selected from 1.1 to 4.0, and the reduced velocity Ur ranges from 2.0 to 13.5 with an increment of 0.5. Small (L/D < 2.0) and large (L/D > 2.0) spacing ranges are identified, both including two types of responses: wake-induced vibration (WIV; Ur = 2.0–9.0∼10.5 for a small L/D and Ur = 2.0–6.0∼6.5 for a large L/D) and wake-induced galloping (WIG; Ur > 9.0∼10.5 for a small L/D and Ur > 6.0∼6.5 for a large L/D). The largest vibration amplitude of each cylinder is obtained in the WIG region for the small L/D condition. The presence of downstream cylinders suppresses the vortex shedding of upstream cylinders and thus postpones the vibration of upstream cylinders at a small Ur, whereas the downstream cylinder enhances the vibration at a larger Ur due to the wake interference. For a small L/D, three flow regimes with the extended-body, reattachment, and co-shedding patterns are successively presented as Ur increases. For a large L/D, four types of flow regimes, namely, EB-2S (the extended-body with “2S” pattern), RT-2S (the reattachment with “2S” pattern), TR-2S (two-row vortex street with “2S” pattern), and CS-VS (co-shedding with variation shedding), are classified. Two new vortex shedding patterns, “2G (two counter-rotating vortices shed from each side per vibration cycle)” and “2C (two co-rotating vortices shed from each side per vibration cycle),” have been identified. In the WIV region, there is only one dominant vibration frequency for upstream cylinders (C1–C7), while a sub-harmonic frequency emerges and dominates C8–C10 when L/D is large. The fluctuating lift force spectra show a broad-band frequency distribution due to the irregular positions of the vortex generation and merging, and the dominant frequency in the WIG region decreases consecutively from C1 to C10.

Revealing excited states of rotational Bose-Einstein condensates

Rotational Bose-Einstein condensates can exhibit quantized vortices as topological excitations. In this study, the ground and excited states of the rotational Bose-Einstein condensates are systematically studied by calculating the stationary points of the Gross-Pitaevskii energy functional. Various excited states and their connections at different rotational frequencies are revealed in solution landscapes constructed with the constrained high-index saddle dynamics method. Four excitation mechanisms are identified: vortex addition, rearrangement, merging, and splitting. We demonstrate changes in the ground state with increasing rotational frequencies and decipher the evolution of the stability of ground states.

Investigation of unsteady engulfment flows in a cross‑shaped mixer by particle image velocimetry

An experimental study was conducted to characterize flow fields in a cross-shaped mixer by particle image velocimetry technique (PIV). Instantaneous flow fields describe the formation and development of secondary flows at 20 ≤ Re ≤ 400, with focus on the dynamic evolution of vortices for periodic unsteady engulfment flows. Three stages are detected during one flow period, i.e., vortex merging, growing, and splitting. Vortex characteristics including populations, distributions, and strengths, are evaluated by a vortex identification and vorticity distributions. Reversal flow regions triggered by vortex breakdown are identified in merging and splitting vortices, which periodically shed downstream, thus causing an axial oscillation. The statistical properties and proper orthogonal decomposition (POD) modes of unsteady engulfment flows are analyzed. The peak of turbulent kinetic energy appears near the chamber corners where two co-rotating vortices are observed, and it decreases with Re. The second and third-order POD modes describe large-scale coherent structures in unsteady engulfment flows.

Numerical investigation of the flow characteristics of supercritical carbon dioxide in a high-speed rotating annular gap

The flow characteristics of Taylor–Couette–Poiseuille flow induced by supercritical carbon dioxide in an annular gap play a pivotal role in determining the overall performance of the rotating machinery. To accurately design the structural components of rotating machinery and enhance its efficiency, this study employs the large eddy simulation method to investigate the flow behavior of Taylor–Couette–Poiseuille flow with supercritical carbon dioxide within an annular gap. The results reveal that vortices are predominantly generated near the inner wall. Initially, the flow exhibits small swirl vortices, spiral ring vortices, and annular vortices along the flow direction. As the flow progresses, these small vortices at the inlet region transition into hairpin swirl vortices. Finally, turbulent flow disturbances lead to the fragmentation and merging of spiral and annular vortices, resulting in a flow field characterized by high-frequency hairpin swirl vortices and small vortices with strong randomness. An increase in the swirl number causes the initial position of the Taylor vortex to shift toward the inlet, while the turbulent kinetic energy is more active on the outer wall side than the inner wall side. Along the flow direction, the vortices experience a developmental process involving stabilization, diffusion, and mixing. Varying the radius ratio affects the magnitude of vorticity, reduces velocity fluctuations in a regular pattern, and alters the distribution of helicity bands from wide and sparse to compact and dense groupings. As the axial Reynolds number increases, the magnitude of vortices grows, leading to more severe velocity fluctuations and the transformation of the helicity bands from a regular annular pattern to fluctuating vortices bands, accompanied by a decrease in helicity.

Assessing the Ensemble Assimilation System for Forecasting the Genesis of Tropical Cyclone Sonca (2017) in the Vietnam East Sea using a Lagrangian Approach

: With the aim of evaluating the cyclogenesis predictability of Sonca (2017) using an ensemble prediction system from the local ensemble transform Kalman filter (LETKF) to model WRF, the study proposed a dynamical assessment method with a Lagrangian approach that is stable and independent of time and reference systes. With this approach, the study can clearly distinguish between cases of tropical cyclone (TC) formation and non-formation in the ensemble forecast members. By evaluating the thermodynamic conditions of the low-level circulation between the forming and non-forming groups, the study found that the ocean-atmosphere interaction and the vortex merger process play imnportant roles in the formation of Sonca. The evaluation of the formation location showed that the detected vortex centers from the forming members tended to concentrate near the monsoon trough axis, indicating that this is a highly favorable region for vortex formation in the Vietnam East Sea.
 
 

Cyclic Analysis of In-Cylinder Vortex Interactions Based on Data-Driven Detection and Characterization Framework

Abstract The complex vortex flow interactions are critical to affect the fuel–air mixing and combustion stability in direct-injection engine. However, due to the strong cyclic variations inside engine, the multiscale swirl flow characteristics with cyclic details are difficult to be sufficiently revealed. Therefore, a vortex detection and characterization framework, including physical and data-driven methods, is implemented to elucidate the cyclic vortex interaction process. In this study, a high-speed time-resolved particle image velocimetry is applied to record the spatiotemporal flow behavior under three different swirl ratio conditions. First, the presence of vortex motion is detected at each crank angle for each engine cycle. Results show that the vortex interaction processes under different swirl ratio conditions exhibit distinctive characteristics. The presence of multiple vortices and their interactions are found to trigger dramatic changes and variations in swirl flow behavior. Then, the individual-cycle analysis of the vortex interaction effects on flow characteristics is conducted. The vortex characteristics including vortex location, strength, and size are examined with cyclic detail using data-driven unsupervised clustering. Results indicate that the vortex merging is the main source inducing the vortex characteristics variations. Furthermore, the occurrence and duration of the vortex merging process are found to be closely related to the intake swirl ratio and valve lift profile. Increased swirl ratio and valve lift cause vortex to merge earlier and reduce the merging duration. This finding provides a potential idea to alleviate the cyclic variation issue by controlling the vortex merging process.

Experimental and numerical analysis of the aerodynamics and vortex interactions on multi-swept delta wings

The flow field around a generic multi-swept delta wing configuration is investigated under transonic flow conditions, both experimentally and numerically. A special focus is on the analysis of vortex/vortex and vortex/shock interactions at moderate angles of attack. In the present study, the Mach number is varied between textrm{Ma} = {0.50} and textrm{Ma} = {1.41} and the angle of attack is varied between alpha = 8^circ and alpha = 28^circ. Numerical results are validated using experimental surface pressure data from pressure taps, as well as forces and moments based on strain gauge measurements. For selected cases, velocity field data from particle image velocimetry (PIV) measurements are available as well. Over a broad range of angle of attack and Mach number, strong vortex/vortex interactions, including vortex braiding and vortex merging, occur. The location of vortex merging is moving downstream with increasing angle of attack and increasing Mach number. Additionally, at textrm{Ma} = {0.85}, vortex/shock interaction occurs above the wing. For moderate angles of attack, shock-induced vortex breakdown is observed.

Experimental Investigation of the Active Flow Control over a Cranked-Delta Wing

A series of experiments was carried out to investigate the effects of active flow control on the aerodynamic characteristics of a cranked-delta-wing model at a subsonic speed. A dielectric barrier discharge (DBD) plasma actuator was used to improve the aerodynamic performance of the half-span cranked-delta-wing model at various angles of attack. The model had inboard and outboard sweepback angles of 57 and 38 deg, respectively. The surface pressure distribution was obtained at five streamwise stations and at two ray stations. All data were obtained at a constant Reynolds number of 0.27×106 and at various angles of attack: 0 deg≤α≤36 deg. The results showed that the pulsed DBD plasma actuator improved the aerodynamic performance of the cranked-delta-wing model by preventing flow separation at high angles of attack. The deep stall angle of attack was delayed up to 6 deg, and the poststall suction coefficient was increased approximately 30% at an angle of attack of about α=34 deg when the pulsed DBD plasma was applied at its optimum condition (F+≈1 and duty cycle=10%). It was found that the pulsed plasma actuator suppressed the wake flow in the forward portion of the wing at poststall angles of attack, thus delaying the flow separation. However, the effects of the DBD actuator at very high angles of attack on the flow reattachment process at the rear portion of the wing for the present DBD setting were seen to be minimal. The induced flow by the pulsed DBD plasma actuator influenced the vortex merging phenomena and delayed the onset of the outer vortex burst at prestall angles of attack.

Experimental research on wake characteristics and vortex evolution of side-by-side circular cylinders placed near a wall

The wake characteristics and the vortex evolution in flow past the side-by-side circular cylinders placed near the wall are investigated with particle image velocimetry (PIV), with the objective of clarifying the interaction between the side-by-side cylinder wake and the wall boundary layer flow. The Reynolds number based on the cylinder diameter D is 1.5×103, and the gap ratio is G/D=1 where G is the gap size, and three spacing ratios (T/D=3, 1.5, and 1.1) are selected for comparison at δ/D=0.63 where δ is the boundary layer thickness without the cylinder. The vortex identification method λci and the virtual dye visualisation are applied to capture vortex evolution. Firstly, the wake characteristics of the side-by-side cylinder without the effects of wall are investigated, and the cylinder wake can be classified into three typical flow states: double wake for T/D=3, deflected gap flow for T/D=1.5, and micro gap flow for T/D=1.1. Secondly, the wake characteristics and vortex evolution of the side-by-side cylinders placed near the wall are investigated in detail. At T/D=3 and G/D=1, the interaction between the wakes of the two cylinders is weak, and two Karman vortex streets are formed, and the secondary vortex is induced periodically by the wake vortex of lower cylinder. As T/D decreases, the lower wake vortex of upper cylinder directly interacts with the lower cylinder wake vortices, and there is the vortex merging occurred in lower wake vortices of two cylinders, meanwhile, the lifting up of the secondary vortex is delayed for small T/D. In addition, the wake vortex shedding is affected by the wall and the T/D, and the deflection of the gap flow between the two cylinders is the reason for the variation of Strouhal number. As T/D decreases from 3 to 1.5, the magnitudes of Reynolds stresses are decreased, and the distributions of Reynolds stresses are asymmetric about the centreline of cylinder due to the deflection of the gap flow.

Vortex evolution and merging mechanism across 180° sharp bend for laminar inflow

The present study deals with the fundamentals of vortex evolution and merging across a 180° sharp bend for laminar inflow (Re = 800). A three-dimensional flow and pressure distribution across the bend are analyzed by using particle image velocimetry measurements. The results show that the sudden flow redirection and adverse pressure gradient create a recirculation region in the vicinity of the divider wall. The vorticity and turbulent kinetic energy distribution highlight the presence of a shear layer between the recirculation and mainstream turning flow. A Rayleigh instability analysis shows the existence of centrifugal instabilities outside the shear layer in the mainstream turning flow. The imbalance of centrifugal and pressure forces act in unison to produce a net force that creates Dean instability across the mainstream turning flow. Consequently, two pairs of counter-rotating Dean vortices develop after the first turn, possibly merging into a single pair after the subsequent 90° turn. Finally, the evolution and merging of Dean vortices are explained with the help of a conceptual diagram illustrating the force interaction and the formation of counter-rotating currents.

Contraction of infinite-long air sheet at capillary-inertial regime

The contraction of a two-dimensional infinite-long flat air sheet in a liquid environment at a capillary-inertial regime with an Ohnesorge number Oh ⩽ 0.01 was investigated numerically. The contraction velocity first increases to a maximum and then decreases and follows a scaling U*∼t*−0.2. In this range of Oh number, pinch-off and oscillation were always observed, and two pinch-off mechanisms were identified as end-pinching (Oh ⩽ 0.006) and oscillation-induced pinch-off (Oh ⩾ 0.007), which also correspond to two oscillation regimes of the rim. Different modes of vortex shedding and merging, co-rotating, and separation of vortices are observed; the interaction between the rim and the vortices is responsible for the complicated contraction dynamics.

Adjoint shape optimization of Ahmed body to improve aerodynamics

Analyses are done on the bluff body of a simple ground vehicle's wake structure. A 4 million Reynolds number depending on model length. Results for various geometric configurations of the Ahmed body's flow visualization were achieved. There was a flexible geometric element called the base slant angle. Drag research found that body drag makes up about 85% of all drag. The body's rear end is where most drag is created or developed. Horseshoe vortices build a three-deck structure in wake flow. The base slant angle affects these vortices' strength, existence, and merging. Examined is the applicability of these phenomena to actual ground vehicles. To reduce drag and enhance aerodynamics, the body's shape was modified using the adjoint approach and the adjoint shape optimization tool in ANSYS Fluent. In this study, we found that by changing the tail-end vortex structure and flow separation at the rear part of the vehicle, the optimized back shape or rear slant design of the body provides better flow separation that results in better pressure recovery and reduces drag by roughly 7 to 10 percent, respectively. The comparison of experimental CD values for the Ahmed body at different rear slant angles, including 12.5o, 25o, and 30o. Since it had less variance from empirical data, the computationally computed CD for a 30° base slant angle was chosen to do adjoint shape optimization. Mesh cells of less than three lakhs were used in the CFD process, and skewness quality was checked. 60 m/s of velocity was the boundary condition for the velocity inlet.

A multifidelity deep operator network approach to closure for multiscale systems

Projection-based reduced order models (PROMs) have shown promise in representing the behavior of multiscale systems using a small set of generalized (or latent) variables. Despite their success, PROMs can be susceptible to inaccuracies, even instabilities, due to the improper accounting of the interaction between the resolved and unresolved scales of the multiscale system (known as the closure problem). In the current work, we interpret closure as a multifidelity problem and use a multifidelity deep operator network (DeepONet) framework to address it. In addition, to enhance the stability and accuracy of the multifidelity-based closure, we employ the recently developed “in-the-loop” training approach from the literature on coupling physics and machine learning models. The resulting approach is tested on shock advection for the one-dimensional viscous Burgers equation and vortex merging using the two-dimensional Navier–Stokes equations. The numerical experiments show significant improvement of the predictive ability of the closure-corrected PROM over the un-corrected one both in the interpolative and the extrapolative regimes.

The onset of turbulence in decelerating diverging channel flows

This work investigates the stability and transition to turbulence in a diverging channel subjected to a time-varying trapezoidal-shaped inflow boundary condition. Numerical simulations are performed for different deceleration rates and Reynolds numbers while maintaining a constant acceleration rate. The flow transition begins with two-dimensional primary instability with the formation of inflectional velocity profiles, followed by local separation and the emergence of an array of shear layer vortices. We divide simulation cases systematically into three categories based on the onset of secondary instability and the generation of streamwise vorticity. At low and medium Reynolds numbers (type I), the spanwise vortex rolls formed by inflectional instability remain two-dimensional and diffuse at the channel centre without exhibiting further instabilities. At high Reynolds numbers and deceleration rates (type II), the rolled shear layer exhibits secondary instability during the zero mean inflow phase, followed by local incipient turbulent structure formation. The streamwise vorticity that develops over the shear layer structures causes oscillations with a spanwise wavelength similar to those associated with the elliptic instability in a counter-rotating vortex pair. Using the Lamb–Oseen approximation of vortices in conjunction with the dynamic mode decomposition algorithm of the three-dimensional flow field, we captured successfully the characteristics of the secondary instability. The third category (type III) is characterized by periodic unsteady separation, secondary instability, and merging of shear layer vortices, which occurs when Reynolds numbers are high and deceleration rates are low.

Applying reinforcement learning to mitigate wake-induced lift fluctuation of a wall-confined circular cylinder in tandem configuration

The flow around two tandem circular cylinders leads to significant lift fluctuation in the downstream cylinder owing to periodic vortex shedding. To address such research issues, we present herein a numerical study that uses deep reinforcement learning to perform active flow control (AFC) on two tandem cylinders with a low Reynolds number of 100, where the actuator causes the rotation of the downstream cylinder. First, the cylinder center spacing ratio L* varies from 1.5 to 9.0, and the variation of L* leads to the quasi-steady reattachment regime (L*≤3.5) and the co-shedding regime (L*≥4.0). The fluctuating lift of the downstream cylinder is maximum when L*=4.5. Next, we train an optimal AFC strategy that suppresses 75% of the lift fluctuation in the downstream cylinder. This approach differs from using direct-opposition control to change the vortex-shedding frequency or strength, as reported in previous studies. This strategy modifies the phase difference between the lift fluctuations of the two cylinders by delaying the merging with the upstream cylinder wake and accelerating the formation of recirculating bubbles after the vortex merging. With the new phase difference, the effect of the additional lift from the upstream cylinder is significantly mitigated. The results of the dynamic mode decomposition show that the vortices surrounding the downstream cylinder in mode 1 that contribute to the lift fluctuation are weakened. To the best of our knowledge, this investigation can provide new ideas and physical insights into the problem of AFC under disturbed incoming flow.