Narrow Wake Research Articles

Kármán Street-Boundary Layer Interactions In Turbulent Flow

Understanding the intricate interplay between turbulent boundary layers and wake structures caused by obstacles in the flow is key to assessing the local shear stress fields as well as the drag. These types of flows can be relevant to natural phenomena including sediment and nutrient transport. While existing literature predominantly explores the flow downstream of cylinders mounted perpendicular to bounding surfaces or downstream of cylinders positioned outside the boundary layer region, this work experimentally studies the turbulent boundary layer passing over a cylindrical obstacle positioned near and parallel to the surface of an underlying solid wall. Planar particle image velocimetry (PIV) was performed in an open recirculating water channel for three test cases at a friction Reynolds number Reτ = 1320. Key parameters investigated include the cylinder diameter relative to the boundary layer thickness (D/δ = 0.09 and 0.01) and the gap between the cylinder and the wall (G/δ = 0.09), where the range of D/δ used is smaller than most examples found in the literature. Instantaneous and averaged velocity and vorticity fields are examined based on the PIV measurements. The results in Fig. 1 show how von Kármán vortex streets and turbulent boundary layer structures interact to form unique flow features. The larger cylinder generates a clear separation downstream of its axis as well as a wake that remains coherent over a distance δ downstream. The smaller cylinder with D/δ = 0.01 generates a much narrower and weaker wake, but still shows a clear influence on time-averaged velocity profile. Future experiments will evaluate flow conditions further downstream as well as additional values of G/δ for the same D/δ values.

Large-Scale Side By Side Stereo-PIV Measurements In An Automotive Wind Tunnel: Aerodynamic Influence Of Ride Height Variations.

In this investigation, we conducted large-scale Stereo PIV experiments in the flow around a Volkswagen ID.4. within the Aerodynamic and Aeroacoustic Wind Tunnel (AAK) at Volkswagen AG, Wolfsburg. The primary objective of these experiments was to investigate the effects of ride height variation on the near wake of the passenger vehicle using Stereo PIV, while minimizing wind tunnel occupancy. The measurements aimed to create datasets for CFD validation to improve virtual aerodynamic development capabilities of passenger vehicles. The experiment was performed on a 1:1 scale vehicle and multiple cross-sectional planes were scanned along the longitudinal axis of the vehicle. The impact of ride height alterations on the wake flow topology were evaluated. Average velocity vector fields were computed per case, providing comprehensive insights into the aerodynamic effects of diverse vehicle setups. A small change in the ride height has shown to have a significant impact on the wake topology. The results show that the electric SUV ID.4 can yield a wake topology similar to the estate back and fast/notchback type of vehicle, depending on the ride height. The normal ride height of the ID.4 shows to have a broader but lower wake region, with a significant downwash. The PIV results of this configuration show significant similarities to the wake topology of a fast back vehicle. Lowering the ride height by 15 mm has shown already to be enough to cause a global effect on the wake topology and to exhibit a narrower wake and recirculation region, like an estate back vehicle. Additionally, the association between the drag/lift coefficients and the wake topology of both configurations was discussed. Results have shown that sole measurements in the wake are not sufficient to fully explain changes in the determined drag or front lift coefficients. The rear lift coefficients however, indicate a stronger relationship with the near wake topology of the vehicle.

Influence of trailing edge radius of intake struts on excitation force and vibration of rotor blades

Adjusting the trailing edge radius of the intake struts may be advantageous in suppressing the vibration of compressor rotor blades. To explore the impact of the intake struts trailing edge radius on the excitation force and vibration, this study focuses on a compressor with intake struts, and fluid and vibration calculations were conducted. The study revealed that decreasing the trailing edge radius led to a reduction in separation near the strut’s trailing edge, resulting in a narrower wake and weakened mixing near the trailing edge. This caused the total pressure loss region induced by the wake to become more elongated in the low-frequency component, leading to a nonlinear decrease in total pressure amplitude with the reduction of the trailing edge radius. As the trailing edge radius decreased, the excitation force and vibration amplitude of the first-stage compressor rotor blades decreased, while the excitation force distribution showed little variation. The research findings provide insights for the design of intake struts.

Energy harvesting based on flow-induced vibration of a wavy cylinder coupled with tuned mass damper

We investigate the flow-energy harvesting performance of a system consisting of a wavy cylinder and a tuned mass damper (TMD), through experiments, theoretical modeling and CFD simulations. The wavy cylinder is elastically supported, being able to oscillate transversally in cross flows and further enforcing a piezoelectric sheet to generate electricity. Results indicate that the output power strongly depends on wavelength λz, amplitude a, and the tip mass of TMD. The wavy cylinder outperforms the smooth cylinder when λz≥ 3.6Dm and peaks at λz = 6.0Dm and a = 0.25Dm with an improvement of 70%. This superiority also exists in a wider range of reduced velocities, leading to a power increase of 121%. The behavior of the oscillation echoes the observations made on the energy. The flow field results reveal that at λz = 6.0Dm and a = 0.25Dm, hairpin vortexes with legs attaching on the two sides of the node are observed, producing a wide wake near the node while a narrow wake near the saddle. However, an opposite scenario is observed for the case with the worst performance, i.e., λz = 1.8Dm and a = 0.25Dm. The former wake distribution seems to create a larger pressure difference over the wavy cylinder, induce a larger oscillation and hence improve the power output.

Flow-induced vibration (FIV) fatigue damage characteristics of two unequal-diameter flexible cylinders in a staggered arrangement

In offshore engineering, fatigue damage caused by flow-induced vibrations (FIVs) presents a considerable and potential threat to flexible cylindrical systems during their service lifetime. Through FIV experiments, this paper estimates and analyzes the fatigue damage of two unequal-diameter cylinders with diameter ratio d/D = 0.5 (where d and D denote the diameter of small and large cylinders) and the center-to-center spacing ratio P/d = 6 (where P denotes the center-to-center distance between two cylinders) through the S–N curve and fatigue damage accumulation methods in the DNV standard. Ten staggered configurations were studied, including two relative positions and five stagger angles (θ = 15°, 30°, 45°, 60°, and 75°). The fatigue damage evaluation results indicate that the FIV fatigue damage is highly affected by the stagger angle and relative position of the two cylinders. The FIV fatigue behavior of the two unequal-diameter cylinders is quite different from that of the two identical cylinders. When the large cylinder is located upstream, its wake shielding effect on the small cylinder is predominant at small stagger angles, so both the IL and CF fatigue damage of the downstream small cylinder is significantly suppressed, especially when Vr ≤ 16.28. But as the stagger angle is increased, the wake-induced flutter can aggravate the fatigue damage of the downstream small cylinder. Moreover, the CF fatigue damage of the large cylinder is decreased at small angles due to the vortex-shedding frequency suppression of the small cylinder and the fatigue damage of the large cylinder in the IL direction is analogous to that of the isolated cylinder and is negligibly affected by the small cylinder. When the small cylinder is located upstream, the IL FIV fatigue damage of the downstream large cylinder is approximately unaffected due to the narrow wake of the small cylinder. However, the CF fatigue damage of the large cylinder is greatly reduced at small stagger angles.

Experimental study of drag loads on open and closed low-aspect-ratio circular cylinders in steady current

We investigate the drag load on low-aspect ratio circular cylinders, submerged as in infinite fluid and subjected to steady current. A series of experiments are carried out in a circulating water tunnel with three different types of models; open and closed cylinders, and open with a splitter plate inside the middle of the cylinders. The length-to-draft ratio of the models, L/D, varies from 0.15 to 0.6. The Reynolds number, based on the cylinder diameter, varies from 40000 to 105000. The experiments show small Reynolds number dependency. The drag coefficient, CD, varies substantially for the open cylinder case; being around 2 for the lowest aspect ratios L/D≤0.2, while 0.65-0.8 for L/D>0.25. There is an abrupt change of flow pattern for L/D=0.25: for L/D≤0.2 the flow which is separated from the front half of the cylinder goes through the cylinder, producing a wide von Karman type vortex street, while for L/D≥0.25 the flow does not go through the models and, as a consequence, produces a significantly more narrow wake. The wide wake regime is suppressed when the splitter plate is added, with drag coefficients CD=0.6−0.8. The drag coefficients for the closed cylinders are lower than the ones with splitter plate, and also consistent with published data on low-aspect ratio closed cylinders. A rational drag load model is proposed for the high-drag regime, with reasonable results. Tests where the cylinder with splitter plate is sectioned into a front and a rear half are presented, revealing a very small contribution of the drag from the rear half. This is discussed in view of existing studies on two-dimensional flat plates in tandem.

Large eddy simulation of flow around two side-by-side circular cylinders at Reynolds number 3900

This paper investigates the flow dynamics around two circular cylinders in a side-by-side arrangement with different spacing ratios (T/D, T is the center-to-center cylinder spacing and D is the diameter) under a subcritical Reynolds number condition (Re = 3900). A three-dimensional (3D) numerical model was developed with Large Eddy Simulation (LES) technique. The model was well validated against published data of flow around a single cylinder at Re = 3900. Numerical simulations were conducted for flow around two side-by-side circular cylinders with T/D = 1.2, 1.5, 1.75, 2, 2.5, 3, 3.5, and 4. Based on the LES results, three wake regimes were identified: single bluff body regime (T/D = 1.2), biased flow regime (T/D = 1.5–2), and parallel vortex streets regime (T/D = 2.5–4). In the single bluff body regime with T/D = 1.2, the stable deflection of gap flow is also observed which indicates that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flip-flopping phenomenon is found to be related to the merging tendency between gap-side vortices in narrow wake region and free-flow-side vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the in-phase mode and anti-phase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the anti-phase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flip-over transitions is closely related to the spacing ratios and is independent of the Reynolds number.

LES and PIV Investigation of the Flow Past a Cactus-Shaped Cylinder with Four Ribs

The flow past a cactus-shaped cylinder with four ribs is investigated numerically using large eddy simulations (LES) at Reynolds number of 20,000 and experimentally using particle image velocimetry (PIV) at Reynolds number of 50,000. In both approaches, the full range of angle of attack is covered. LES results show a good qualitative and quantitative match of the aerodynamic properties to previous experimental data, although the value of the critical angle of attack is slightly lower. The results confirm that there is no Reynolds number dependency within the investigated range allowing a comparison of the flow fields from the present LES and PIV. Significant variations of the flow patterns with the angle of attack are found and quantified using the recirculation length and wake width. Overall, the observed angle of attack dependence resembles the behaviour of the square cylinder. However, the studied cylinder has a narrower wake at all angular orientations. Proper orthogonal decomposition is used to identify large coherent structures in the flow. At all angles of attack the first two modes remain dominant making it possible to reconstruct the periodic vortex shedding process using a low-order model.

FarmConners wind farm flow control benchmark – Part 1: Blind test results

Abstract. Wind farm flow control (WFFC) is a topic of interest at several research institutes and industry and certification agencies worldwide. For reliable performance assessment of the technology, the efficiency and the capability of the models applied to WFFC should be carefully evaluated. To address that, the FarmConners consortium has launched a common benchmark for code comparison under controlled operation to demonstrate its potential benefits, such as increased power production. The benchmark builds on available data sets from previous field campaigns, wind tunnel experiments, and high-fidelity simulations. Within that database, four blind tests are defined and 13 participants in total have submitted results for the analysis of single and multiple wakes under WFFC. Here, we present Part I of the FarmConners benchmark results, focusing on the blind tests with large-scale rotors. The observations and/or the model outcomes are evaluated via direct power comparisons at the upstream and downstream turbine(s), as well as the power gain at the wind farm level under wake steering control strategy. Additionally, wake loss reduction is also analysed to support the power performance comparison, where relevant. The majority of the participating models show good agreement with the observations or the reference high-fidelity simulations, especially for lower degrees of upstream misalignment and narrow wake sector. However, the benchmark clearly highlights the importance of the calibration procedure for control-oriented models. The potential effects of limited controlled operation data in calibration are particularly visible via frequent model mismatch for highly deflected wakes, as well as the power loss at the controlled turbine(s). In addition to the flow modelling, the sensitivity of the predicted WFFC benefits to the turbine representation and the implementation of the controller is also underlined. The FarmConners benchmark is the first of its kind to bring a wide variety of data sets, control settings, and model complexities for the (initial) assessment of farm flow control benefits. It forms an important basis for more detailed benchmarks in the future with extended control objectives to assess the true value of WFFC.

Giving, or Refugee Love Language

Giving, or Refugee Love Language Lilly U. Nguyen (bio) “In other words, silence is not opposed to language, but as a choice not to verbalize, a will not to say, a necessary interval in an interaction—in brief, as a means of communication of its own . . . In silences, I returned home.” —Trinh Minh-Ha, Elsewhere, Within Here “Con gái biết nhờ.” —Vietnamese saying I am a first-born daughter. Of their daughters, Vietnamese people have an aphorism. In English, it goes something like this. Girls know how to be dependable. Or even, you can trust daughters to love you. But to really give you a sense of the full nuance of the phrase, it is most accurate to say, daughters know how to give with full thanks. I was conceived on top of a shipping container at the moment of first quiet after a long journey. I cannot say how I have come to know this because I belong to the first generation to not know. When I was four years old, I slept in a bed with my mother and comforted myself to sleep by running my hand along the stubble on her head. Whether she enjoyed this I never knew, but she neither pushed my [End Page 35] hand away nor turned her head to avoid my touch. Where her black hair had fallen out, one million shorn remainders prickled the pads of my fingertips. I played them like a tiny keyboard, fast and slow. I drew circles with a full palm and heard ocean waves. This made me laugh out loud. When my hand became tired, I fell asleep with my fingers grazing the top of her head. Her right breast had disappeared and was replaced by a line of staples and dried blood. The metal appeared too sharp against her muted skin. I traced my pointer finger along the raised edges. Each staple anchored itself to her chest with two punctures into her flesh. They formed a rhythm of dots and lines like a glyph, and I followed the message they made towards her heart. But these were metal teeth that did not speak. There was something in there, formless and moving. I wanted to reach inside and grasp at the things that made her hum. I wanted to crawl in and draw the covers of her skin over me, as close as a child could ever be to a mother. I wanted to close my eyes and sleep inside that narrow space and wake when the staples had been replaced by a song. My mother and I shared a large bed. Daisies decorated the sheets in shades of flamboyant neon. On especially cold nights, we burrowed ourselves under a sleeping bag she hand-stuffed and patched over with squares of red gingham and yellow thread. Under such a heavy cover, warmth felt like a punishment. One day, my father clapped his hand against her face. She left him in the car and together we walked home to nap. Inside our bedroom, she stripped the mattress and tidied up the sum of that afternoon into a pile of laundry. My mother lay down and the cool pillow next to her cheek relieved the heat of my father’s rage. I watched the tears slip down her face and disappear into the folds of our bed. I gathered my knees into my chest and nestled into my mother’s heaving frame. My mother never sang Vietnamese lullabies because they were not worth remembering. Instead, she whispered stories in the dark. She did not tell them straight. They trailed in a thick of half-finished sentences and forked at the junctures of chronology. She spoke at the joints of memory and mood, scattering fragments of herself in the dark. She spoke to shed the skin of a world left behind. I listened like dog paddling after her and heard a life across two languages and two continents. A world between prison islands and cul-de-sacs, wet markets and strip malls, desert casinos, [End Page 36] and alleyways filled with the sound of noodle pushcarts and clacking wood blocks. Boat journeys transformed into bus rides to...

Hydrodynamic Characteristics of Two Side-by-Side Cylinders at a Pitch Ratio of 2 at Low Subcritical Reynolds Numbers

Isothermal turbulent flow around circular cylinders arranged side-by-side was numerically simulated on a commercial finite-volumes platform, ANSYS® CFX, version 2020 R2. The turbulence was modeled by using k-ω shear stress transport (k-ω SST). Three different Reynolds numbers were computed, Red = 200, 1000, and 3000, which were based on the cylinder diameter, d, the free stream velocity, U∞, and the kinematic viscosity of the fluid, ν. Sided cylinders were spaced apart from each other, forming a p/d ratio equal to 2, which was kept constant throughout the computations regardless of changes in the Reynolds number. The drag coefficient, Cd, as well as its time traces, was evaluated along with the different wake topologies experienced by the cylinders (wide wake WW and narrow wake NW). The simulations were able to predict the bistable flow over the cylinders and the Cd changes associated with the wakes. Whenever a new wake topology was identified, the shape drag changed in accordance with the instantaneous pressure distribution. A laminar simulation was carried out for the lowest Reynolds number case, showing that the adopted turbulence model did not affect the dynamic response of the flow. The Red = 3000 case was compared to Afgan’s outcomes, whose simulations were carried out in a 3-D mesh using LES (Large Eddy Simulation), showing great agreement with their results.

Ship’s Wake on a Finite Water Depth in the Presence of a Shear Flow

The ship’s wake in the presence of a shear flow of constant vorticity at a finite water depth is investigated by expanding the Whitham-Lighthill kinematic theory. It has been established that the structure of a wave ship wake radically depends on Froude number Fr (in terms of water depth) and especially on the critical Froude number Frcr, which depends on the magnitude and direction of the shear flow. At its subcritical values Fr<Frcr two types of waves are presented: long transverse and short divergent waves inside the wedge area with an angle depending on Fr For the supercritical range Fr>Frcr only divergent waves are presented inside the wake region. Critical Froude number Frcr is variable and mostly depends on collinear with the ship path component of the shear flow: it decreases with the unidirectional shear flow and increases on the counter shear current. The wedge angle of the ship wake expands with an increase in the unidirectional shear flow and narrows in the oncoming flow in the subcritical mode of the ship’s motion Fr<Frcr. Wake angle decreased with Froude number for Fr>Frcr and only divergent waves with the crests almost collinear with ship path are finally presented in the narrow ship wake. For a critical value of the Froude number Fr=Frcr, the ship’s wake has a total wedge angle of 180° with waves directed parallel to the ship’s motion. The presence of a shear flow crossing the path of the ship gives a strong asymmetry to the wake. An increase in the perpendicular shear flow leads to an increase in the difference between the angles of the wake arms.

Low Reynolds Number Effects on the Separation and Wake of a Compressor Blade

Abstract This paper investigates the surface boundary layer and wake development of a compressor blade at a range of low Reynolds number from 45,000 to 120,000. Experiments in a miniature linear compressor cascade facility have been performed with detailed surface pressure measurements and flow visualization to track variations in the separation bubble size. These have been combined with high-resolution pneumatic pressure and hot-wire probe traverses in the downstream wake. High-fidelity direct numerical simulations have been completed on the same compressor blade section across the same range of operating conditions. The results show that large laminar separation bubbles exist on both blade surfaces. As Reynolds number increases, these separation bubbles shorten in length and reduce in thickness. Correspondingly, the downstream wake narrows, although the peak wake loss coefficient remains approximately constant. As the Reynolds number is increased from 45,000 to 120,000, the bubble length on the suction side reduced from 48% to 28% chord and on the pressure side reduced from 35% to 20% chord, while the loss coefficient reduced from 9% to 5%. The flow features are examined further within the high-fidelity computations, which reveal the dependence of the wake turbulence on the laminar separation bubbles. The separation bubbles are found to generate turbulent kinetic energy, which convects downstream to form the outer part of wake. As Re increases, a shorter bubble produces less turbulence in the outer part of the boundary layer leading to a narrower wake. However, the trailing edge separation is largely independent of Reynolds number, leading to the constant peak loss coefficient observed. The overall loss is shown to vary linearly with the total turbulence production, and this depends on the size of the separation bubbles. Overall, this research provides new insight into the connection between the blade surface flow field and the wake characteristics at low Reynolds number. The findings suggest that changes that minimize the extent of the blade separation bubbles could provide significant improvements to both the steady and unsteady properties of the wake.

Study of Flow around a Trapezoidal Model of a Small-Sized UAV into Turbulent Wake

Investigations of the structure of the flow near the surface of a trapezoidal model of a small unmanned aerial vehicle were carried out when it enters a narrow turbulent wake. All experimental data were obtained in a wind tunnel at subsonic flow velocities. A feature of the work was that the study of the flow around the model was carried out at full-scale (flight) Reynolds numbers. Using the soot-oily visualization method, data on the features of the flow around the model were obtained, taking into account such factors as the angle of attack, the presence and absence of a source of external disturbances that generated a turbulent wake. The experiments were carried out in two flow regimes: at a zero angle of attack, when there are local separation bubbles on the wing, and at a large (supercritical) angle of attack of 18 degrees, when there is a global stall of the flow from the leading edge. It was shown that the turbulent wake has a significant effect on the nature of the flow near the model surface in both cases. Local separation bubbles gradually decrease in size with a decrease in the distance between the sources of disturbances and the wing. Large-scale vortices significantly decrease in geometrical dimensions and shift towards the side edges in the event of a global stall of the flow, thereby increasing the region of the attached flow on the model surface.

An asymptotic theory for the high-Reynolds-number flow past a shear-free circular cylinder

We present an asymptotic theory for analytical characterization of the high-Reynolds-number incompressible flow of a Newtonian fluid past a shear-free circular cylinder. The viscosity-induced modifications to this flow are localized and except in the neighbourhood of the rear stagnation point, behave like a linear perturbation of the inviscid flow. Our theory gives a highly accurate description of these modifications by including the contribution from the most significant viscous term in a correctional perturbation expansion about an inviscid base state. We derive the boundary layer equation for the flow and deduce a similarity transformation that leads to a set of infinite, shear-free-condition-incompatible, self-similar solutions. By suitably combining members from this set, we construct an all-boundary-condition-compatible solution to the boundary layer equation. We derive the governing equation for vorticity transport through the narrow wake region and determine its closed-form solution. The near and far-field forms of our wake solution are desirably consistent with the boundary layer solution and the well-known, self-similar planar wake solution, respectively. We analyse the flow in the rear stagnation region by formulating an elliptic partial integro-differential equation for the distortion streamfunction that specifically accounts for the fully nonlinear and inviscid dynamics of the viscous correctional terms. The drag force and its atypical logarithmic dependence on Reynolds number, deduced from our matched asymptotic analysis, are in remarkable agreement with the high-resolution simulation results. The logarithmic dependence gives rise to a critical Reynolds number below which the viscous correction term, counterintuitively, reduces the net dissipation in the flow field.

Computational Study of Three-Dimensional Flow Past an Oscillating Cylinder Following a Figure Eight Trajectory

The paper presents a computational study of three-dimensional flow past a cylinder forced to oscillate in a uniform stream, following a figure-eight trajectory. Flow simulations were performed for Re = 400, for different cases, defined in terms of the oscillation mode (‘counter-clockwise’ or ‘clockwise’), for values of the ratio, F, of the transverse oscillation frequency to the Strouhal frequency close to 1.0. The results demonstrate that, for F ≤ 1.0, counter-clockwise cylinder motion is associated with positive power transfer from the flow to the cylinder, corresponding to excitation; for the clockwise motion, power transfer is negative at intermediate to high amplitudes, corresponding to damping. For the clockwise mode, in the range F = 0.9–1.1, a transition to two-dimensional vortex street is identified for transverse oscillation amplitude exceeding a critical value. This results from the induced suction of vortices, which moves vortex formation and shedding closer to the cylinder surface, thus resulting in a narrower wake, characterized by an effective lower Reynolds number. Both oscillation modes are characterized by higher harmonics in the lift force spectrum, with the third harmonic being very pronounced, while even harmonics are present for the case of clockwise mode, resulting from a wake transition to a “S + P” mode.

Flow development over isolated droplet-inspired shapes

Flow development over isolated, surface-mounted, droplet-inspired three-dimensional obstacles submerged in a laminar boundary layer is investigated at a Reynolds number based on obstacle height of Reh=2070 using particle image velocimetry. Three geometries are considered, a sessile droplet; a droplet on the verge of runback (depinning); and a spherical cap, which serves as a first order approximation of a sessile droplet. For all three models a horseshoe vortex system forms at the leading edge and wraps around the obstacle, with the sessile model producing the most prominent horseshoe system due to its relatively high fore-body bluntness. Shear layer vortices shed from the objects form arch vortices, which produce mean streamwise vortices in wakes of the models. Downstream of the objects, turbulent fluctuations grow in a wedge shape. The spreading mechanisms in the wall-normal and lateral directions are explored through proper orthogonal decomposition of the velocity fluctuations. The analysis shows that wall-normal growth is associated with shear layer vortex breakdown, while lateral spreading is influenced by interactions between the horseshoe vortex system and streamwise wake vortices. Overall, similarity between the chopped and sessile model flow fields suggest the spherical cap is a reasonable first order approximation of a sessile droplet from a vortex dynamics perspective. Shear layer vortices shed from the runback model exhibit more complete merging than for the other two models, perhaps due to the smaller radius of curvature at the object peak. This model also exhibits a more muted horsheshoe vortex system and narrower near-field wake than the sessile model, suggesting less disruption to the incoming boundary layer flow.

Three-dimensional flip-flopping flow around a pair of dual-stepped circular cylinders in a side-by-side arrangement

Three-dimensional direct numerical simulations of flow around a pair of dual-stepped circular cylinders in a side-by-side arrangement and a proximity interference region are presented. The dual-stepped cylinders consist of two coaxial cylinders with a larger-to-smaller diameter ratio of 2. The Reynolds number and the side-by-side gap ratio, based on the larger (smaller) diameter, are 200 (100) and 1 (3), respectively. Two flip-flopping flow regimes A and B are observed behind the dual-stepped cylinders, exhibiting temporal variations of hydrodynamic drag and lift force coefficients. The flow regime B is featured by the flip-flopping, higher vortex-shedding frequencies and greater flow instability than those in the flow regime A. Half-loop and direct connections of vortices take place frequently in the narrower wake, while the vortex reconnection and multiple connection predominate the wider wake. This suggests that the dual-stepped cylinder with the narrower wake behaves similarly to a single dual-stepped cylinder. The dynamic mode decomposition analysis reveals pitchfork and Hopf bifurcation (in-phase and anti-phase synchronization) modes in the larger cylinder wake, with the interaction of these bifurcations originating from the flip-flopping flow pattern. However, the pitchfork mode is not found in the smaller cylinder wake, suggesting that its flip-flopping flow behavior is not self-originated but triggered by the flip-flopping flow mechanism of the larger cylinder.

Influence of the direction of rotation on the wake characteristics of closely spaced counter-rotating vertical-axis wind turbines

Closely spaced counter-rotating vertical-axis wind turbines (VAWTs) exhibit a considerable power improvement compared to the same turbines in isolation. It thus makes sense to study their potential for wind farm production optimisation. With this objective in mind, the wake of an isolated VAWT is compared experimentally to the wake of counter-rotating VAWTs. Because of the unsteady aerodynamics, the wake of an isolated VAWT deflects towards the region behind its upwind moving blade. The direction of rotation thus directly influences the deflection of the wake. For paired configurations, it is possible to use this wake deflection as an advantage. A pair of counter-rotating VAWTs where the upwind moving blades are at the centre of the pair, exhibits a particularly narrow wake. With the benefit of a power improvement and a narrow wake, closely spaced counter-rotating VAWTs exhibit promising characteristics to improve the power density in a wind farm.

On the flip-flopping phenomenon of two side-by-side circular cylinders at a high subcritical Reynolds number of 1.4 × 105

To investigate the flow characteristics of the biased gap flow and the flip-flopping phenomenon at high Reynolds numbers, large-eddy simulation is adopted to simulate the crossflow around two side-by-side circular cylinders at Re = 1.4 × 105. The center-to-center pitch ratio P/D varies from 1.1 to 4. Three flow patterns, i.e., single bluff body regime, biased gap flow regime, and coupled vortex regime, have been successfully simulated. A tri-stable flow pattern is found at P/D = 1.1; namely, a single bluff body regime and a biased gap flow regime deflected to the upper or lower cylinder occur spontaneously and intermittently. In contrast to the results obtained at low Reynolds numbers, two distinct biased gap flow structures are observed at P/D = 1.2 and 1.5, respectively. For P/D = 1.2, the gap-side shear layer separated from the narrow wake cylinder further reattaches to the cylinder; namely, a series of separation bubbles would develop along the gap-side surface of the cylinder. These unsteady separation bubbles induce the inward-directed lift force on the cylinder. For P/D = 1.5, the gap-side shear layer separates directly without further reattachment, and both cylinders experience outward lift forces. It is revealed that the flip-flopping of the biased gap flow is associated with the interaction of the gap vortices and the vortex shed from the outer-side shear layer of the wide wake cylinder. Furthermore, the flip-flopping phenomenon arises initially at one particular cross section of the cylinder, rather than occurring simultaneously along the spanwise direction of the cylinder.