Development Of Wake Vortices Research Articles

A Critical Review of CFD Modeling Approaches for Darrieus Turbines: Assessing Discrepancies in Power Coefficient Estimation and Wake Vortex Development

This critical review delves into the impact of Computational Fluid Dynamics (CFD) modeling techniques, specifically 2D, 2.5D, and 3D simulations, on the performance and vortex dynamics of Darrieus turbines. The central aim is to dissect the disparities apparent in numerical outcomes derived from these simulation methodologies when assessing the power coefficient (Cp) within a defined velocity ratio (λ) range. The examination delves into the prevalent turbulence models shaping Cp values, and offers insightful visual aids to expound upon their influence. Furthermore, the review underscores the predominant rationale behind the adoption of 2D CFD modeling, attributed to its computationally efficient nature vis-à-vis the more intricate 2.5D or 3D approaches, particularly when gauging the turbine’s performance within the designated λ realm. Moreover, the study meticulously curates a compendium of findings from an expansive collection of over 250 published articles. These findings encapsulate the evolution of pivotal parameters, including Cp, moment coefficient (Cm), lift coefficient (Cl), and drag coefficient (Cd), as well as the intricate portrayal of velocity contours, pressure distributions, vorticity patterns, turbulent kinetic energy dynamics, streamlines, and Q-criterion analyses of vorticity. An additional focal point of the review revolves around the discernment of executing 2D parametric investigations to optimize Darrieus turbine efficacy. This practice persists despite the emergence of turbulent flow structures induced by geometric modifications. Notably, the limitations inherent to the 2D methodology are vividly exemplified through compelling CFD contour representations interspersed throughout the review. Vitally, the review underscores that gauging the accuracy and validation of CFD models based solely on the comparison of Cp values against experimental data falls short. Instead, the validation of CFD models rests on time-averaged Cp values, thereby underscoring the need to account for the intricate vortex patterns in the turbine’s wake—a facet that diverges significantly between 2D and 3D simulations. In a bid to showcase the extant disparities in CFD modeling of Darrieus turbine behavior and facilitate the selection of the most judicious CFD modeling approach, the review diligently presents and appraises outcomes from diverse research endeavors published across esteemed scientific journals.

Effects of the Topography of Sumatra on Tropical Cyclone Formation over the Indian Ocean

One-quarter of the world’s tropical cyclones (TCs) occur in the Indian Ocean (IO) basin.The mechanisms for TC initiation in the IO are varied, but one recently discovered process involves the flow around the steep topography of Sumatra. When the low-level flow impinges on Sumatra, it is blocked and the flow splits under typical environmental stratification. As a result, wake vortices commonly develop at northern and southern island tips of the island. For the case of easterly flow, these circulationssubsequently move downstream over the IO. The wake vortices emanating from the island tips are counter-rotating, but since Sumatra straddles the equator, the circulations are cyclonic in both hemispheres and thus have the potential for TC development. Using data from2.5yearsof observations from DYNAMO and YOTC, it is found that approximately 25% of the TCsthat occurred overIO basin during that periodwere initiated by Sumatra-induced wake vortices.Additional analysis of vortex statistics for the period 2008-17 has found that vortex counts are highest near Madden-Julian Oscillation (MJO) phase 1 when low-level easterlies are strongest across southern Sumatra. A secondary peak in vortex formation occurs during MJO phase 4 when low-level westerlies exist near the equator west of Sumatra. The latter finding suggests that MJO-related, low-level westerly surges on the equator impinging on Sumatracontribute to an increase in wake vortex development. Numerical simulations have shown that circulations farther upstream such aswestern Pacific remnant TCs and the Borneo vortex can influence the development of Sumatra wake vortices and their growth into TCs over the IO.

Combined effect of bending stiffness and streamwise length of the attached flexible splitter plate on the vortex-induced vibration of a circular cylinder

A flexible splitter plate attached to the rear stagnation point of a circular cylinder is applied to control the vortex-induced vibration. The influence of its bending stiffness on the control effect is experimentally investigated for two streamwise lengths of 1D and 2D (D is the cylinder diameter). Five dimensionless bending stiffnesses are selected: 0.979 (P1), 3.123 (P2), 6.797 (P3), 23.087 (P4), and 47.484 (P5). The characteristics of cylinder vibration, flow field, lift, and plate dynamics are synchronously studied. The results show that the effect of the bending stiffness depends on the streamwise length of the plate. For the flexible splitter plate of 1D, the best suppression effect of cylinder vibration is achieved for the P2 case, where maximum vibration amplitude is reduced by 93%, while the galloping-like vibration is excited for P4 and P5 cases. According to the analysis of the plate dynamics and the wake development, a large plate vibration is expected to suppress the galloping-like vibration. The bending stiffness affects the vibration state of the flexible splitter plate, and consequently, the wake vortex development and dynamic response of the cylinder are changed. For the flexible splitter plate of 2D, the vibration amplitude response of the cylinder can be divided into three categories based on the variation of amplitude versus the reduced velocity. For P2-P5 cases, a beneficial phenomenon that a frequency branch is larger than the natural frequency occurs.

Enhancement of wake vortex decay by air blowing from the ground

It is well-known that the long-lasted aircraft wake vortex above the airport runway jeopardizes flight safety. Therefore, the research on the enhancement of wake vortex decay is pretty much essential. The previous studies suggest that the wake decay process can be improved by introducing artificial perturbations [1]. For example, the plate lines on the ground can trigger the helical secondary vortex structures (SVS), propagating along with the primary vortices. As a result, the vortex decay will be enhanced. In the present research, active flow control methods on wake vortex decay, like air blowing or suction, are studied. Blowing/suction zones are embedded on the ground to generate airflow below the wake vortex tubes. Different sizes and shapes of the blowing/suction zones are tested. The large eddy simulation (LES) method is used to quantitatively investigate the blowing/suction effects on wake vortex decay. Furthermore, the coherent vortex structures observed and the acceleration mechanism on wake decay caused by active flow control are studied carefully. According to the present research, it is found that with a small blowing velocity (1.79 m/s) and a short blowing duration (13.15 s), the aircraft wake strength can be roughly reduced by 50% at t⁎=1. Based on the simulation results, it is revealed that the apparent effect on the wake vortex development is due to the detached vortex generated near the blowing zone rather than the blowing flow itself. Besides, a detached vortex with a scale of 0.2b0 is needed to achieve a better wake decay enhancement.

Experimental study on the development of wake vortices behind screen cylinders

This work examined the development of the large-scale vortex structures in the wake of two screen cylinders with porosities β = 48% and 61% in the intermediate region at a Reynolds number of 7000 using two X-type hot-wire probes. The screen cylinders were made of stainless steel screen meshes rolled into a cylindrical shape. The results were compared with those of a solid cylinder (β = 0%) under the same flow conditions. It was shown that the formation of the large-scale vortices in the screen cylinder wakes involves different mechanisms. For the screen cylinder with β = 48%, they were generated due to wake instability (Kármán vortex), which closely resembled that of a solid cylinder. In contrast, for the screen cylinder with β = 61%, the vortices were formed owing to the shear-layer (Kelvin–Helmholtz) convective instability. These findings were justified by the streamwise evolution of the vortex shedding frequency, a stability analysis in the near wake using the mean streamwise velocity profiles, and the coherent and incoherent wake structures obtained by the phase-averaged analysis at different downstream locations.

Effect of front inclined hole on flow structure around a wall-mounted cube

To control the wake flow around a wall-mounted cube, passive control is performed by drilling a front inclined hole (FIH) from the front surface to the free end of the cube. To study the control features and mechanism, controlled and uncontrolled cubes are experimentally investigated. A circulation water tunnel is used to perform PIV measurements, where the Reynolds number is ReD = 11550 based on the cube side length of D = 70 mm, and the thickness of the boundary layer on the tunnel bottom is δ/D = 0.25. To study the effect of the hole position, three FIHs with inlet heights of 20, 35, or 50 mm are investigated. The time-averaged flow fields show that the velocity of the FIH jet increases with the FIH inlet height. Comparing with the flow structures around standard cube, it is found that the reverse zone and shear layer above free-end are obviously suppressed due to the FIH jet. In addition, the scale of the reverse zone and arch-type vortex in the near-wake is reduced by the FIH jets. Because the FIH jets disrupt the development of wake vortices, the Reynolds shear stress and turbulent kinetic energy in the near-wake decrease. The control effect is most apparent for the cube with an FIH at a spanwise height of 50 mm (the FIH50 cube). In this case the effect of the free-end downwash flow on the wake is weaker, and the shedding frequency of the wake vortices near the free-end increases. Proper orthogonal decomposition analysis indicates that the FIH50 suppresses the alternately arranged flow features and enhances the symmetrically arranged features.

The Development and Mechanisms of the High Pressure Turbine Vane Wake Vortex

The wake vortex is an important origin of unsteadiness and losses in turbines. In this paper, the development and underlying mechanisms of the shedding vortex of a high-pressure transonic turbine vane are studied and analyzed using the delayed detached eddy simulation (DDES) and proper orthogonal decomposition (POD). The goal is to understand the unsteadiness related to the wake vortex shedding and the wake evolution and mixing. Special attention is paid to the development of the wake vortex and the mechanisms behind the length characteristics. Interactions of the wake vortex with the shock wave and pressure waves are also discussed. First, the DDES simulation results are compared with published experimental data, Reynolds Averaged Navier-Stokes, and large eddy simulation (LES) simulations. Then, the development of the vane wake vortex, especially the different length characteristics from the cylinder vortex, is discussed. The reason of stronger pressure-side vortex shedding compared to suction-side vortex shedding is revealed. Wake-shock wave interaction and wake-pressure wave interaction are also investigated. The pressure waves are found to have a stronger effect than the shock wave on the spanwise motion and the dissipation of the wake vortex. An analysis of the losses through the turbine vane passage is carried out to evaluate the contributions of thermal and viscous irreversibilities. Losses analysis also confirms the strong interaction between the wake vortex and pressure waves. After that, POD study of the wake behavior was carried out. The results indicate that the shedding vortex is dominant in the unsteady flow. The phase relation between the pressure side wake vortex (PSVP) and the suction side wake vortex (SSVP) is confirmed.

Control method for aircraft wake vortex based on Rayleigh-Ludwig instability

The aircraft wake vortex is an inherent flow phenomenon due to the lift generation mechanism,which has a negative impact on the flight safety. By adding a set of specially designed spoiler,a rectangular wing was designed to generate a pair of weaker vortices,which had different sizes and opposite direction compared to main wing vortices,thereby constructing a self-destructive four-vortex wake system to induce Rayleigh-Ludwig instability. Under different experimental conditions,in terms of changing the size or shape of the spoiler,towing speed and angle of attack,the wake vortex development of the test model,that both with and without spoiler,as well as the circulation analysis,were acquired particle image velocimetry( PIV) measurements under the low Reynolds number. The study demonstrates that the decrease in circulation was 35% to55% in 45 wingspans when spoilers are introduced,whereas the counterpart of the baseline airfoil,without spoilers,is nearly kept steadily,which reveal the application possibility of Rayleigh-Ludwig instability in alleviating the wake vortex. As the initial circulation ratio equals to- 0. 489 and the initial distance ratio is0. 5,the circulation of the primary vortex reduce most significantly( 55. 9%) in 45 wingspans. Results would provide a scheme in the design of airfoils with weaker vortices.

Three-dimensional flow and surface visualization using hydrogen bubble technique

A modified hydrogen bubble technique is developed for simultaneous three-dimensional flow visualization and surface visualization on bluff bodies. A detailed description of the technique implementation and operating principles is presented. The technique is validated on two geometries: (1) a uniform circular cylinder for ReD = 1,050 and 2,100; and (2) a dual-step cylinder for ReD = 2,100, D/d = 2, and L/D = 0.5. The results demonstrate that flow visualizations effectively capture intricate three-dimensional wake vortex development including the formation of streamwise structures and vortex interactions in the wake of a dual-step cylinder. The surface visualizations capture boundary layer separation and its variation along the span of the models. Moreover, the results reveal complex three-dimensional surface flow patterns, shedding light on the topology of vortical structures forming near dual-step cylinder junctions.

Sound generation in the flow behind a rectangular cylinder of various aspect ratios at low Mach numbers

The development of wake vortices and the associated sound generation in the two-dimensional flow past a rectangular cylinder with the aspect ratio from 0.1 to 5.0 at a Reynolds number of 150 and at Mach numbers below 0.3 were studied by direct Navier–Stokes simulations using a compact scheme. The location of vortex formation moved toward the cylinder with decreasing aspect ratio AR, and the lift dipole due to periodic vortex shedding became the strongest at AR=0.5. For AR⩽0.5, a secondary vortex street was developed in the downstream region, but the associated sound was extremely weak compared to the sound associated with the primary vortex shedding. The sound with the frequency of drag fluctuation was pronounced for AR<1 and became stronger with decreasing AR to approach that of the lift-dipole at AR=0.1. We also found that a monopole sound source, in addition to a dipole sound source, was formed at twice the shedding frequency, and the former became pronounced with increasing Mach number.

Transport Aircraft Wake Influenced by Oscillating Winglet Flaps

The wake vortex development in the near field and extended near field behind a four engined large transport aircraft model fitted with an active winglet is presented. A detailed wind-tunnel investigation is conducted using a half-model focusing on the high-lift case of a typical approach configuration at a Reynolds number of 0.5 x 10 6 based on the wing mean aerodynamic chord and an angle of attack of 6.5 deg. The flowfield is observed using advanced hot-wire anemometry mainly focusing on the crossflow plane at 5.6 spans downstream of the model. Based on the time-dependent velocity components the wake flowfield is analyzed by distributions of mean vorticity, turbulence intensities, and spectral densities. Seven main vortical structures dominating the near-field wing vortex sheet roll up and merge to form the remaining trailing vortex in the extended near field. By the use of oscillating winglet flaps the velocity fluctuations at the core region of the remaining vortex are significantly influenced. Distinct narrowband concentrations of turbulent kinetic energy can be found for the farthest downstream plane documenting the presence of the disturbances generated by the winglet flaps which may result in an amplification of inherent far-field instabilities. The frequencies at which these narrowband energy concentrations occur are, on the one hand, dependent on the oscillation frequency of the winglet flaps; on the other hand, there are also independent energy concentrations within the frequency bands associated with wake instabilities.

Transport Aircraft Wake Influenced by a Large Winglet and Winglet Flaps

Detailed flowfields of a wind-tunnel investigation are discussed presenting the wake vortex development and evolution in the near field and extended near field behind a four-engined large transport aircraft model fitted with a large winglet. The tests use a half-model of 1:32 scale focusing on the high-lift case of a typical approach configuration at a Reynolds number of 0.5 x 10 6 based on the wing mean aerodynamic chord and at an angle of attack of 7 deg. Flowfields are carefully inspected by advanced hot-wire anemometry at seven crossflow planes up to 5.6 spans downstream of the model. Based on the measured time-dependent velocity components, the wake flowfield is analyzed by distributions of mean velocity and vorticity, turbulence intensities, and spectral densities. The near-field wing vortex sheet is dominated by seven main vortical structures, namely the winglet vortex, the wing tip vortex, the outboard flap vortex, the outboard and inboard nacelle vortices, and the vortices shed at the wing-body junction and the horizontal tail plane. In the extended near field, these vortices roll up and merge to form the remaining rolled-up vortex. The deflection of winglet flaps produce additional vortices influencing the wing tip near field and enhancing the overall merging process. Especially for the cases with asymmetrical flap deflection, the remaining rolled-up vortex shows a significant narrowband concentration of turbulent kinetic energy which may result in an amplification of inherent far-field instabilities.

Reconstruction of initial field from experimental data for numerical simulations of realistic aircraft wakes

The development of wake vortex behind an A300 Airbus model in high-lift configuration is computed by solving the unsteady Navier-Stokes equations for an incompressible fluid with a second order accurate scheme in time and space. Both two- and three-dimensional simulations are carried out, which allow us to compare the main vortex trajectory with data measured by three-component LDV system in the Onera facility. The study is focused on the way of initialising the computational entrance section based on the experimental data and the consequential effects of main vortex position up to nine wing spans downstream of the model. Several ways to reconstruct the initial velocity field for the simulations are investigated. The circulation distributions around the main vortex are compared and results obtained show that the circulation level is not a pertinent parameter to define a correct initialisation. Numerical results show that a better agreement with experimental data is obtained when a large domain is covered by experiments at the inlet section. A technique to reconstruct adequate initial data solving an inverse problem is also presented.

Comparative early development of wake vortices behind a short semicircular-section cylinder in two opposite arrangements

As a first step in a more general study of the influence of the body shape upon the initial time-development of wake vortices, we consider the case of a 5.20 aspect-ratio semicircular-section cylinder, fitted with two endplates, and with the rounded side and the flat side in turn facing the oncoming current. The flow structure is analysed by means of a detailed qualitative and quantitative analysis of numerous flow visualization pictures, for Reynolds numbers Re ranging between 60 and 600. Beyond the first phase of development, necessary for the vortex-shedding process to take place (t* [ges ] 6), a change in the flow evolution with Re is found for both body configurations, at a critical Reynolds number Rec whose final value is, within about 5%, 190 and 140 for the rounded and flat forebody respectively. However, this change varies with the body shape. Thus, above Re = 200, it is shown that the reversal of the body compared to the free stream (flat-forebody configuration) implies a clear difference in the wake development with a quasi-symmetrical shifting of the vortex cores to the rear of the recirculating zone and a complete annihilation of the process of vortex shedding, at least during the limited period of time corresponding to the present observation. The consequences of the modifications of the wake behaviour are quantitatively evaluated by considering the time- and Re-evolution of the wake geometrical parameters and of the axial velocity distribution; they are related to the body geometry.