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1
Steiner, T. R., and A. E. Perry. "Large-scale vortex structures in turbulent wakes behind bluff bodies. Part 2. Far-wake structures." Journal of Fluid Mechanics 174 (January 1987): 271–98. http://dx.doi.org/10.1017/s0022112087000120.
Анотація:
An investigation of a selection of high-Reynolds-number bluff-body flows was conducted. Here in Part 2 phase-averaged velocity-field results will be presented for several far-wake flows generated by nominally two-dimensional and three-dimensional bodies. In these far-wake flows the shed vortices have approached a nearly constant convection velocity. Some mean velocity and phase-averaged and global Reynoldsstress measurements are also presented. The turbulent wake of a lift-producing three-dimensional body has been examined. Also included are the phase-averaged wake patterns behind a flapping flag and a windmill. The topological structure of these patterns is discussed and a preliminary classification of wake patterns is presented.
2
Hickey, Jean-Pierre, Fazle Hussain, and Xiaohua Wu. "Role of coherent structures in multiple self-similar states of turbulent planar wakes." Journal of Fluid Mechanics 731 (August 22, 2013): 312–63. http://dx.doi.org/10.1017/jfm.2013.315.
Анотація:
AbstractWe study the nature of archetypal, incompressible, planar splitter-plate wakes, specifically the effects of the exit boundary layer state on multiple approximate self-similarity. Temporally developing direct numerical simulations, at a Reynolds number of 1500 based on the volume-flux defect, are performed to investigate three distinct wake evolution scenarios: Kelvin–Helmholtz transition, bypass transition in an asymmetric wake, and an initially fully turbulent wake. The differences in the evolution and far-wake statistics are analysed in detail. The individual approximately self-similar states exhibit a relative variation of up to 48 % in the spread rate, in second-order statistics, and in peak values of the energy budget terms. The multiplicity of self-similar states is tied to the non-universality of the large-scale coherent structures. These structures maintain the memory of the initial conditions. In the far wake, two distinct spanwise-coherent motions are identified: (i) staggered, segregated spanwise rollers on either side of the centreplane, dominant in wakes transitioning via anti-symmetric instability modes; and, (ii) larger spanwise rollers spanning across the centreplane, emerging in the absence of a near-wake characteristic length scale. The latter structure is characterized by strong spanwise coherence, cross-wake velocity correlations and a larger entrainment rate caused by deep pockets of irrotational fluid within the folds of the turbulent/non-turbulent interface. The mid-sized structures, primarily vortical rods, are generic for all initial conditions and are inclined at ∼$\pm 3{3}^{\circ } $ to the downstream, shallower than the preferential $\pm 4{5}^{\circ } $ inclination of the vorticity vector. The spread rate is driven by the inner-wake dynamics, more specifically the advective flux of spanwise vorticity across the centreplane, which depends on the large-scale coherent structures.
3
Wheeler, Andrew P. S., Robert J. Miller, and Howard P. Hodson. "The Effect of Wake Induced Structures on Compressor Boundary-Layers." Journal of Turbomachinery 129, no. 4 (July 31, 2006): 705–12. http://dx.doi.org/10.1115/1.2720499.
Анотація:
The interaction of a convected wake with a compressor blade boundary layer was investigated. Measurements within a single-stage compressor were made using an endoscopic PIV system, a surface mounted pressure transducer, hotfilms and hotwire traverses, along with CFD simulations. The wake/leading-edge interaction was shown to lead to the formation of a thickened laminar boundary-layer, within which turbulent spots formed close to the leading edge. The thickened boundary-layer became turbulent and propagated down the blade surface, giving rise to pressure perturbations of 7% of the inlet dynamic head in magnitude. The results indicate that wake/leading-edge interactions have a crucial role to play in the performance of compressor blades in the presence of wakes.
4
Bodini, Nicola, Dino Zardi, and Julie K. Lundquist. "Three-dimensional structure of wind turbine wakes as measured by scanning lidar." Atmospheric Measurement Techniques 10, no. 8 (August 14, 2017): 2881–96. http://dx.doi.org/10.5194/amt-10-2881-2017.
Анотація:
Abstract. The lower wind speeds and increased turbulence that are characteristic of turbine wakes have considerable consequences on large wind farms: turbines located downwind generate less power and experience increased turbulent loads. The structures of wakes and their downwind impacts are sensitive to wind speed and atmospheric variability. Wake characterization can provide important insights for turbine layout optimization in view of decreasing the cost of wind energy. The CWEX-13 field campaign, which took place between June and September 2013 in a wind farm in Iowa, was designed to explore the interaction of multiple wakes in a range of atmospheric stability conditions. Based on lidar wind measurements, we extend, present, and apply a quantitative algorithm to assess wake parameters such as the velocity deficits, the size of the wake boundaries, and the location of the wake centerlines. We focus on wakes from a row of four turbines at the leading edge of the wind farm to explore variations between wakes from the edge of the row (outer wakes) and those from turbines in the center of the row (inner wakes). Using multiple horizontal scans at different elevations, a three-dimensional structure of wakes from the row of turbines can be created. Wakes erode very quickly during unstable conditions and can in fact be detected primarily in stable conditions in the conditions measured here. During stable conditions, important differences emerge between the wakes of inner turbines and the wakes of outer turbines. Further, the strong wind veer associated with stable conditions results in a stretching of the wake structures, and this stretching manifests differently for inner and outer wakes. These insights can be incorporated into low-order wake models for wind farm layout optimization or for wind power forecasting.
5
Zhang, Can, Jisheng Zhang, Athanasios Angeloudis, Yudi Zhou, Stephan C. Kramer, and Matthew D. Piggott. "Physical Modelling of Tidal Stream Turbine Wake Structures under Yaw Conditions." Energies 16, no. 4 (February 9, 2023): 1742. http://dx.doi.org/10.3390/en16041742.
Анотація:
Tidal stream turbines may operate under yawed conditions due to variability in ocean current directions. Insight into the wake structure of yawed turbines can be essential to ensure efficient tidal stream energy extraction, especially for turbine arrays where wake interactions emerge. We studied experimentally the effects of turbines operating under varying yaw conditions. Two scenarios, including a single turbine and a set of two turbines in alignment, were configured and compared. The turbine thrust force results confirmed that an increasing yaw angle results in a decrease in the turbine streamwise force and an increase in the turbine spanwise force. The velocity distribution from the single turbine scenario showed that the wake deflection and velocity deficit recovery rate increased at a rate proportional to the yaw angle. The two-turbine scenario results indicated that the deployment of an upstream non-yawed turbine significantly limited the downstream wake steering (i.e., the wake area behind the downstream turbine). Interestingly, a yawed downstream turbine was seen to influence the steering of both the upstream and the downstream wakes. These systematically derived data could be regarded as useful references for the numerical modelling and optimisation of large arrays.
6
Sørensen, Jens N., Robert F. Mikkelsen, Dan S. Henningson, Stefan Ivanell, Sasan Sarmast, and Søren J. Andersen. "Simulation of wind turbine wakes using the actuator line technique." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 373, no. 2035 (February 28, 2015): 20140071. http://dx.doi.org/10.1098/rsta.2014.0071.
Анотація:
The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.
7
Yang, Xiaolei, and Fotis Sotiropoulos. "A Review on the Meandering of Wind Turbine Wakes." Energies 12, no. 24 (December 11, 2019): 4725. http://dx.doi.org/10.3390/en12244725.
Анотація:
Meandering describes the large-scale, low frequency motions of wind turbine wakes, which could determine wake recovery rates, impact the loads exerted on turbine structures, and play a critical role in the design and optimal control of wind farms. This paper presents a comprehensive review of previous work related to wake meandering. Emphasis is placed on the origin and characteristics of wake meandering and computational models, including both the dynamic wake meandering models and large-eddy simulation approaches. Future research directions in the field are also discussed.
8
Fleming, Paul, Jennifer Annoni, Matthew Churchfield, Luis A. Martinez-Tossas, Kenny Gruchalla, Michael Lawson, and Patrick Moriarty. "A simulation study demonstrating the importance of large-scale trailing vortices in wake steering." Wind Energy Science 3, no. 1 (May 14, 2018): 243–55. http://dx.doi.org/10.5194/wes-3-243-2018.
Анотація:
Abstract. In this paper, we investigate the role of flow structures generated in wind farm control through yaw misalignment. A pair of counter-rotating vortices is shown to be important in deforming the shape of the wake and in explaining the asymmetry of wake steering in oppositely signed yaw angles. We also demonstrate that vortices generated by an upstream turbine that is performing wake steering can deflect wakes of downstream turbines, even if they are themselves aligned. We encourage the development of improvements to control-oriented engineering models of wind farm control, to include the effects of these large-scale flow structures. Such a new model would improve the predictability of control-oriented models. Further, we demonstrate that the vortex structures created in wake steering can lead to greater impact on power generation than currently modeled in control-oriented models. We propose that wind farm controllers can be made more effective if designed to take advantage of these effects.
9
Fu, Jiawei, Junhui Wang, Jifei Wu, Ke Xu, and Shuling Tian. "Investigation of the Influence of Wake Field Characteristic Structures on Downstream Targets Using the POD Method." Aerospace 10, no. 9 (September 21, 2023): 824. http://dx.doi.org/10.3390/aerospace10090824.
Анотація:
This research investigated the impact of complex low-speed wake flow structures on the aerodynamic characteristics of objects downstream. It employed the proper orthogonal decomposition (POD) method and the domain precursor simulation method to compare traditional methods and validate this approach. The study generated several flow structures of parallel dual-cylinder wakes with different scales and spacing. The variations in the aerodynamic coefficient of three downstream objects at various times passing through wakes of varying scales were appropriately compared and analyzed. The study established that the wake with a cylinder spacing of G = 1.5 has a more compact and concentrated modal structure than that with a cylinder spacing of G = 0.35. Smaller objects were more responsive to the wake flow structure with a spacing of G = 1.5, whereas larger objects responded more to the flow structure with a spacing of G = 0.35. The achieved results also revealed that the aerodynamic force coefficients of objects passing through the wakefield at different times were closely related to the temporal characteristics of the wake flow structure with different scales.
10
Wang, Lianzhou, Xinyu Liu, Nian Wang, and Mijian Li. "Modal analysis of propeller wakes under different loading conditions." Physics of Fluids 34, no. 6 (June 2022): 065136. http://dx.doi.org/10.1063/5.0096307.
Анотація:
Propeller wakes under different loading conditions obtained by the improved delayed detached eddy simulation method were studied based on the flow decomposition technique. The sparsity-promoting dynamic mode decomposition was used to study the flow physics in the wake of a propeller, with particular emphasis placed on identifying the underlying temporal and spatial scales that play important roles in the onset of propeller wake instabilities. The morphology of flow structures of different modes selected by the sparsity-promoting algorithm at different frequencies characterizes the instability process of the wake system. It shows that the circumferential diffusion of tip vortex structures promotes the approaching of adjacent tip vortices, enhancing the interaction of the vortex pairs, which plays an important role in the instability triggering mechanism of the propeller wake, especially the mutual inductance between neighboring tip vortices. The present study further extends knowledge of propeller wake instability inception mechanisms under different loading conditions.
11
De, Arnab Kumar, and Sandip Sarkar. "Effect of aspect ratio on the wake transition behind a thin pitching plate." Physics of Fluids 35, no. 2 (February 2023): 021704. http://dx.doi.org/10.1063/5.0140038.
Анотація:
We study the influence of Aspect Ratio ( AR) on three-dimensional wake transition past a thin pitching plate at Reynolds number of 1000 by performing computations for the range [Formula: see text] at pitching frequencies [Formula: see text] and maximum pitching angles [Formula: see text]. For all AR, larger θmax and St promote thrust generation. However, higher AR imparts a stabilizing influence in the wake of the drag regime. For the ranges of AR, the drag-producing wake consisting of horseshoe vortices and bridgelets-type vortex structures, whereas twin-jet type bifurcated wakes with entangled vortices are observed for thrust-generating wakes. At higher AR, the wakes show a two-dimensional signature in the drag regime, whereas a spatial wake transition is observed in the thurst regime. The spanwise wake width shows the effect of wake compression for larger St even at [Formula: see text], which is also substantiated by particle tracking showing wake compression for the thrust cases up to [Formula: see text]. The near wake oscillations are prevalent for higher AR, although the core region remains unaffected by the aiding influence of spanwise instability for larger AR. The time average streamwise velocity for both drag and thrust regimes resembles an apparent feature of the reverse von Kármán vortex street.
12
Foti, Daniel, Xiaolei Yang, Lian Shen, and Fotis Sotiropoulos. "Effect of wind turbine nacelle on turbine wake dynamics in large wind farms." Journal of Fluid Mechanics 869 (April 18, 2019): 1–26. http://dx.doi.org/10.1017/jfm.2019.206.
Анотація:
Wake meandering, a phenomenon of large-scale lateral oscillation of the wake, has significant effects on the velocity deficit and turbulence intensities in wind turbine wakes. Previous studies of a single turbine (Kang et al., J. Fluid. Mech., vol. 774, 2014, pp. 374–403; Foti et al., Phys. Rev. Fluids, vol. 1 (4), 2016, 044407) have shown that the turbine nacelle induces large-scale coherent structures in the near field that can have a significant effect on wake meandering. However, whether nacelle-induced coherent structures at the turbine scale impact the emergent turbine wake dynamics at the wind farm scale is still an open question of both fundamental and practical significance. We take on this question by carrying out large-eddy simulation of atmospheric turbulent flow over the Horns Rev wind farm using actuator surface parameterisations of the turbines without and with the turbine nacelle taken into account. While the computed mean turbine power output and the mean velocity field away from the nacelle wake are similar for both cases, considerable differences are found in the turbine power fluctuations and turbulence intensities. Furthermore, wake meandering amplitude and area defined by wake meanders, which indicates the turbine wake unsteadiness, are larger for the simulations with the turbine nacelle. The wake influenced area computed from the velocity deficit profiles, which describes the spanwise extent of the turbine wakes, and the spanwise growth rate, on the other hand, are smaller for some rows in the simulation with the nacelle model. Our work shows that incorporating the nacelle model in wind farm scale simulations is critical for accurate predictions of quantities that affect the wind farm levelised cost of energy, such as the dynamics of wake meandering and the dynamic loads on downwind turbines.
13
COULL, JOHN D., and HOWARD P. HODSON. "Unsteady boundary-layer transition in low-pressure turbines." Journal of Fluid Mechanics 681 (July 1, 2011): 370–410. http://dx.doi.org/10.1017/jfm.2011.204.
Анотація:
This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.
14
Wang, Kun, Li Zou, Aimin Wang, Peidong Zhao, and Yichen Jiang. "Wind Tunnel Study on Wake Instability of Twin H-Rotor Vertical-Axis Turbines." Energies 13, no. 17 (August 20, 2020): 4310. http://dx.doi.org/10.3390/en13174310.
Анотація:
In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An analysis of the wake characteristics of twin turbines forms the basis of array optimisation. This study aimed to examine the instability characteristics of a twin-turbine wake with two rotational configurations. The dynamic evolution characteristics of coherent structures with different scales in the wake were analysed via wavelet analysis. The results show that an inverse energy cascade process occurs after the high-frequency small-scale coherent structures induced by rotation lose their coherence. This self-organising characteristic is more apparent in the quasi two-dimensional wake of a forward-moving counter-rotating turbine (Array 1) than in that of a backward-moving counter-rotating turbine (Array 2). With greater organisation and coherence, the wake of Array 1 exhibits low-frequency instability characteristics dominated by a large-scale coherent structure. In addition, the signals reconstructed using wavelet transform show that asymmetric modes exist between low-frequency large-scale coherent structures. The experimental results provide a new perspective on the instability mechanism of twin-turbine wakes, as well as important data for numerical modelling.
15
Khouygani, M. G., R. F. Huang, and C. M. Hsu. "Flow and Dispersion Characteristics of a Stack-Issued Backward Inclined Jet in Crossflow." Journal of Mechanics 33, no. 6 (September 13, 2016): 841–52. http://dx.doi.org/10.1017/jmech.2016.97.
Анотація:
AbstractThe effects of backward inclination angle on flow characteristics and jet dispersion properties of a stack-issued jet in crossflow were studied by means of instantaneous and long-exposure photography, particle image velocimetry (PIV), and tracer-gas concentration detections at a Reynolds number of 2,400, a jet-to-crossflow momentum flux ratio of 1.0, and the backward inclination angles θ = 0° - 60°. Three characteristic flow patterns featured by different near-wake flow structures were found within the surveyed span of the backward inclination angle: low (θ ≤ 25°), mediate (25° < θ < 50°), and high (θ ≥ 50°). In the range of low backward inclination angle, mushroom vortices appeared in the upwind shear layer. Jet fluids were entrained into the jet- and tube-wakes because the near wake region was characterized by a jet-wake vortex and a downwash flow. In the range of mediate backward inclination angle, forward-rolling vortices were formed in the upwind shear layer. Jet fluids were entrained into the jet wake but not appearing in the tube wake because the near wake was characterized by an isolated tube wake and up-going flows. In the range of high backward inclination angle, small-sized forward-rolling vortices were observed in the upwind shear layer. Jet fluids were not observed in both the jet- and tube-wakes because all flows went forward without reversal or vortex, which was similar to that in a jet in co-flow. Large turbulence intensities occurred around the jet-wake vortex and along sides of the tube wake bifurcation line, therefore the mixing at the low backward inclination angles presented better properties than those at mediate and high backward inclination angles owing to the featured flow structures and turbulence intensities.
16
Slama, Myriam, Camille Choma Bex, Grégory Pinon, Michael Togneri, and Iestyn Evans. "Lagrangian Vortex Computations of a Four Tidal Turbine Array: An Example Based on the NEPTHYD Layout in the Alderney Race." Energies 14, no. 13 (June 25, 2021): 3826. http://dx.doi.org/10.3390/en14133826.
Анотація:
This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.
17
BUCHHOLZ, JAMES H. J., and ALEXANDER J. SMITS. "The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel." Journal of Fluid Mechanics 603 (April 30, 2008): 331–65. http://dx.doi.org/10.1017/s0022112008000906.
Анотація:
Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For ReC = O(104), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9% and 21% were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and ReC = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 < St < 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St > 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St > 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper.
18
SPEDDING, G. R. "The evolution of initially turbulent bluff-body wakes at high internal Froude number." Journal of Fluid Mechanics 337 (April 25, 1997): 283–301. http://dx.doi.org/10.1017/s0022112096004557.
Анотація:
Coherent vortex structures are formed in the late wakes of towed spheres for all values of the internal Froude number, F≡2U/ND∈ [10, 240] (U is the body speed, D its diameter, and N is the buoyancy frequency). The eventual emergence of the long-lived and stable pattern of alternating-signed patches of vertical vorticity is characteristic of all towed-sphere wakes, from those dominated by internal lee waves at F=1, to initially fully turbulent early wakes at F[ges ]4. At late times, the local Froude number is always low, and a characteristic stratified wake structure and dynamics result. These wakes have high mean wake defect velocities compared with non-stratified wakes, but the decay rates of energy and enstrophy are similar. Experimental evidence is presented for the existence of an intermediate non-equilibrium (NEQ) regime with very low decay rates of kinetic energy, that precedes the late wake. The NEQ regime is the period when the initial turbulence reorganizes under the increasingly (relative to the decaying turbulent kinetic energy) powerful influence of the background density gradient, accompanied by conversion of potential to kinetic energy as the wake turbulence collapses. The stable long-lived late-wake structure that eventually emerges has a high degree of order and coherence that reflects the initial wake instability. A universal curve for the energy decay of all stratified drag wakes at high Froude and Reynolds numbers is proposed.
19
Shi, Shaoping, Ismail Celik, Andrei Smirnov, and Ibrahim Yavuz. "Large-Eddy Simulation of Spatially Developing Turbulent Wake Flows." Journal of Ship Research 50, no. 03 (September 1, 2006): 208–21. http://dx.doi.org/10.5957/jsr.2006.50.3.208.
Анотація:
The feasibility of applying the large-eddy simulation (LES) technique in complex high Reynolds number flows has been studied. The focus of the study is on the spatially developing wake flows with an application to ship wakes. The bluff body that generates the wake is excluded from the computational domain. To make this possible, a new random flow generation technique (RFG) is used to provide the turbulent inflow boundary conditions as a function of time. The technique provides an instantaneous velocity field at the inlet boundary in conjunction with the prescribed mean flow field obtained either from RANS (Reynolds averaged Navier-Stokes) simulations or from experimental data. The combined LES-RFG procedure has been validated in previous publications in cases of a flat plate and a mixing layer. At the inflow boundary, turbulence characteristics, including the shear stresses, were reconstructed. The time averaged results showed good agreement with the experiments in the developing wake. The same procedure is used to simulate a ship wake (ship model DTMB 5512) in the near field of 1.5 ship cord length. The LES technique captured both spatial and temporal development of the large coherent structures that play an important role in the evaluation of bubble concentration in the ship wakes. These structures are usually smeared out in RANS simulations.
20
Ivanell, Stefan, Jens N. Sørensen, Robert Mikkelsen, and Dan Henningson. "Analysis of numerically generated wake structures." Wind Energy 12, no. 1 (January 2009): 63–80. http://dx.doi.org/10.1002/we.285.
21
Torsvik, T., T. Soomere, I. Didenkulova, and A. Sheremet. "Identification of ship wake structures by a time–frequency method." Journal of Fluid Mechanics 765 (January 19, 2015): 229–51. http://dx.doi.org/10.1017/jfm.2014.734.
Анотація:
AbstractThe wake of a ship that sails at relatively large Froude numbers usually contains a number of components of different nature and with different heights, lengths, timings and propagation directions. We explore the possibilities of the spectrogram representation of one-point measurements of the ship wake to identify these components and to quantify their main properties. This representation, based on the short-time Fourier transform, facilitates a reliable decomposition of the wake into constituent components and makes it possible to quantify their variations in the time–space domain and the energy content of each component, from very low-frequency precursor waves up to high-frequency signals within the frequency range of typical wind-generated waves. A method for estimation of the ship speed and the distance of its sailing line from the measurement site is proposed, which only uses information available within the record of the ship wake surface elevation, but where it is assumed that the wake pattern does not deviate significantly from the classical Kelvin wake structure. The wake decomposition using the spectrogram method allows investigation of the energy content that can be attributed to each individual component of the wake. We demonstrate that the majority (60–80 %) of wake energy from strongly powered large ferries that sail at depth Froude numbers ${\sim}0.7$ is concentrated in components that are located near the edge of the wake wedge. Finally, we demonstrate that the spectrogram representation offers a convenient way to identify a specific signature of single types of ships.
22
Akar, Mustafa Atakan, Burcu Oguz, Huseyin Akilli, and Besir Sahin. "Flow behavior downstream of two side-by-side circular cylinders in shallow water." Canadian Journal of Physics 94, no. 10 (October 2016): 975–81. http://dx.doi.org/10.1139/cjp-2014-0141.
Анотація:
Investigations of bistable flow structure past a pair of cylinders positioned side-by-side in shallow water is conducted experimentally applying dye observation and the particle image velocimetry (PIV) method. For the gap ratio of G/D = 1.25, the jet-like flow between cylinders deflects asymmetrical flow structures forming a large-scale wake as well as a small-scale wake downstream of cylinders. The small vortices around the right cylinder get closer to each other forming a larger vortex in the large-scale wake region, which leads the jet-like flow to changeover side to side. The small frequency (f = 0.352 Hz) associated with frequency of vortex shedding of cylinder with wider wake and the higher frequency (f = 0.793 Hz) which depicts the smaller wakes frequency.
23
Krautwald, Clemens, Constantin Schweiger, David Schürenkamp, and Nils Goseberg. "EXPERIMENTAL WAKE DYNAMICS OF PILES WITH ARTIFICIAL BIOFOULING IN WAVES." Coastal Engineering Proceedings, no. 37 (September 1, 2023): 39. http://dx.doi.org/10.9753/icce.v37.structures.39.
Анотація:
Offshore structures become colonized by marine orga-nisms after a short period of time, whose common benthic communities depend among others on geographic location, water depth, water temperature, food supply, salinity and oxygen content of the water (Kröncke and Bergfeld, 2003; Shi et al., 2012). While biofouling can be categorized in biological terms as bivalves, kelp, algae, barnacles, tubeworms and other species (van der Stap et al., 2016; Wilhelmsson and Malm, 2008), engineers mostly distinguish between hard and soft marine growth based on the strength of their outer shell alone (Shi et al., 2012; Skaugset and Baarholm, 2008). Due to an increasing demand for sustainable energy, the offshore renewables industry experiences significant growth. However, many uncertainties persist in the consideration of biofouling, specifically when calculating loads accor-ding to the Morison concept, the influence of marine fouling on fatigue reassessment, on the flow velocities around cylinders and the vortex formation under waves. For the first time, the flow around cylinders with different artificial marine biofouling was recorded and analyzed in an extensive experimental study using a comprehensive 4D particle tracking velocimetry (PTV) system.
24
Kopp, G. A., J. G. Kawall, and J. F. Keffer. "The evolution of the coherent structures in a uniformly distorted plane turbulent wake." Journal of Fluid Mechanics 291 (May 25, 1995): 299–322. http://dx.doi.org/10.1017/s0022112095002710.
Анотація:
A plane turbulent wake generated by a flat plate is subjected to a uniform distortion. It is observed that nearly two-dimensional, quasi-periodic coherent structures dominate the distorted wake. Rapid distortion theory, applied to a kinematic vortex model of the coherent structures in the undistorted far wake, predicts many of the effects revealed by a hot-wire anemometry/pattern-recognition analysis of these structures. Specifically, rapid distortion theory predicts reasonably well the observed changes in the ensemble-averaged velocity patterns and the disproportionate amplification of the large-scale coherent structures relative to the smaller-scale ‘isotropic’ eddies. These results are consistent with the view that self-preservation of the distorted wake is not possible because of the selective amplification of the coherent structures, which control the development of the wake. As well, the entrainment rate in the distorted wake increases at a rate greater than that predicted by the self-preservation theory.
25
Ghassan Nasif, A.-M. Shinneeb, Ram Balachandarandar, and Chandra Somayaji. "Turbulent Structures in Gap Flow." CFD Letters 14, no. 2 (March 2, 2022): 24–34. http://dx.doi.org/10.37934/cfdl.14.2.2434.
Анотація:
A computational study was carried out to investigate the effect of the gap between the body and the bed on the wake characteristics in shallow flows. A sharp-edged bluff body with two different clearances from the bed is employed in this investigation. The transient three-dimensional governing Navier-Stokes equations are numerically solved using a finite volume formulation with improved delayed detached-eddy simulation as a turbulence model. The volume of fluid approach is used in the current study as a free-surface modelling technique for locating and tracking the free surface. It is found that the existence and size of the gap influence the wake size and the location where the free surface is restored to its original and flat shape. This study also reveals that the fluid structures lose their symmetry about the center of the wake as the gap size increases.
26
Balachandar, R., M. F. Tachie, and V. H. Chu. "Concentration Profiles in Shallow Turbulent Wakes." Journal of Fluids Engineering 121, no. 1 (March 1, 1999): 34–43. http://dx.doi.org/10.1115/1.2822007.
Анотація:
The present study deals with the noninvasive measurement of concentration in the intermediate shallow turbulent wake region using a video-imaging technique. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. On the basis of the observed behavior, the waves are classified as deep-shallow wakes and shallow-shallow wakes. The topology of the dye concentration distribution in the near and intermediate wake region indicates that the vortex structure tends to be preserved when the flow depth is relatively high and the dominant eddy structures are similar to that noticed in conventional two-dimensional wakes. In shallow-shallow wakes, the conventional Karman vortex street appears to be annihilated or intermittent. The lateral concentration distribution at several axial stations covering the first thirty body widths are considered for analysis. The instantaneous concentrations are observed to be several times higher than the corresponding mean values. Attempts are also made to determine the paths traversed by the vortex cores and the vortex core convection velocity. The axial variation of the wake half-width with depth of flow is also examined. A model is developed to predict the spread of the wake with downstream distance from the test body. A friction length scale is introduced in the model to account for the influence of depth and bed friction on the development of the wake.
27
Mei, Renwei, and Christopher J. Lawrence. "The flow field due to a body in impulsive motion." Journal of Fluid Mechanics 325 (October 25, 1996): 79–111. http://dx.doi.org/10.1017/s002211209600804x.
Анотація:
An asymptotic analysis for the long-time unsteady laminar far wake of a bluff body due to a step change in its travelling velocity from U1 to U2 is presented. For U1 [ges ] 0 and U2 > 0, the laminar wake consists of a new wake of volume flux Q2 corresponding to the current velocity U2, an old wake of volume flux Q1 corresponding to the original velocity U1, and a transition zone that connects these two wakes. The transition zone acts as a sink (or a source) of volume flux (Q2 – Q1) and is moving away from the body at speed U2. Streamwise diffusion is negligible in the new and old wakes but a matched asymptotic expansion that retains the streamwise diffusion is required to determine the vorticity transport in the transition zone. A source of volume flux Q2 located near the body needs to be superposed on the unsteady wake to form the global flow field around the body. The asymptotic predictions for the unsteady wake velocity, unsteady wake vorticity, and the global flow field around the body agree well with finite difference solutions for flow over a sphere at finite Reynolds numbers. The long-time unsteady flow structures due to a sudden stop (U2 = 0) and an impulsive reverse (U1U2 < 0) of the body are analysed in detail based on the asymptotic solutions for the unsteady wakes and the finite difference solutions. The elucidation of the long-time behaviour of such unsteady flows provides a framework for understanding the long-time particle dynamics at finite Reynolds number.
28
EIFF, OLIVIER S., and JAMES F. KEFFER. "On the structures in the near-wake region of an elevated turbulent jet in a crossflow." Journal of Fluid Mechanics 333 (February 25, 1997): 161–95. http://dx.doi.org/10.1017/s0022112096004314.
Анотація:
A pattern-recognition technique, applied to multi-point simultaneous velocity measurements obtained with 45° X-wire anemometer probes, is used to extract and characterize the underlying organized motions, i.e. coherent structures, within the near-wake region of a turbulent round jet discharged perpendicularly from a pipe into a crossflow. This flow has been found to be quite complex owing to its three-dimensional nature and the interactions between several flow regions. Analyses of the underlying coherent structures, which play an important role in the physics of the flow, are still rare and are mostly based on flow-visualization techniques. Using a pattern-recognition technique in conjunction with hot-wire measurements, we recently examined the wake regions of the pipe and jet at levels near the tip of the pipe, and found that Kármán-like vortex structures in the wake of the pipe are locked to similar structures in the jet-wake. In this paper we expand upon our previous work and characterize these structures throughout the wake of the jet up into the region of the bent-over jet – a region where they have not been identified previously. The complex geometry of these structures in the wake of the jet as well as their interaction with the bent-over jet are discussed. The results show that these structures split before they link to similar structures on the opposite side of the symmetry plane in the jet region. The results further suggest that the vorticity due to the structures in the wake of the jet contributes to the motion of the well-known counter-rotating vortex pair.
29
Janocha, Marek Jan, Muk Chen Ong, and Guang Yin. "Large eddy simulations and modal decomposition analysis of flow past a cylinder subject to flow-induced vibration." Physics of Fluids 34, no. 4 (April 2022): 045119. http://dx.doi.org/10.1063/5.0084966.
Анотація:
Large eddy simulations (LES) are carried out to investigate the flow around a vibrating cylinder in the subcritical Reynolds number regime at Re = 3900. Three reduced velocities, Ur = 3, 5, and 7, are chosen to investigate the wake structures in different branches of a vortex-induced vibration (VIV) lock-in. The instantaneous vortical structures are identified to show different coherent flow structures in the wake behind the vibrating cylinder for various branches of VIV lock-in. The combined effects of the frequency and amplitude of the oscillation on the flow pattern in the wake region, the hydrodynamic quantities of the cylinder, and the spanwise length scale of the energetic wake flow structures are discussed in detail. It is found that the typical spanwise lengths of the flow structures are [Formula: see text] at Ur = 5 and [Formula: see text] at [Formula: see text] in the near-wake region and level out at [Formula: see text] further downstream. Furthermore, multiscale proper orthogonal decomposition (mPOD) is used to analyze the dominant flow features in the wake region. With the increasing Ur, the total kinetic energy contribution of superharmonic modes increases and the contribution of subharmonic modes decreases. The dominant flow characteristics associated with the vortex shedding and their super harmonics, and the low-frequency modulation of the wake flow can be captured by the mPOD modes.
30
Kanareykin, A. D., I. L. Sheinman, and A. M. Al’tmark. "Frequency control in wake field waveguide structures." Technical Physics Letters 28, no. 11 (November 2002): 916–18. http://dx.doi.org/10.1134/1.1526882.
31
LI, Hui, Yu ZHOU, Masahiro TAKEI, Yoshifuru SAITO, and Kiyoshi HORII. "Visualization of Turbulent Wake Structures using Wavelets." Proceedings of Conference of Kanto Branch 2002.8 (2002): 407–8. http://dx.doi.org/10.1299/jsmekanto.2002.8.407.
32
Wu, J., J. Sheridan, M. C. Welsh, K. Hourigan, and M. Thompson. "Longitudinal vortex structures in a cylinder wake." Physics of Fluids 6, no. 9 (September 1994): 2883–85. http://dx.doi.org/10.1063/1.868115.
33
TAKAGI, Michitoshi, and Soichiro MIYAUCHI. "362 Unsteady Wake Structures of Motorcycle Helmets." Proceedings of Conference of Tokai Branch 2008.57 (2008): 239–40. http://dx.doi.org/10.1299/jsmetokai.2008.57.239.
34
Zheng, Yan, Lin Dong, and Akira Rinoshika. "Multi-scale wake structures around the dune." Experimental Thermal and Fluid Science 104 (June 2019): 209–20. http://dx.doi.org/10.1016/j.expthermflusci.2019.02.021.
35
Weygandt, James H., and Rabindra D. Mehta. "Three-dimensional structure of straight and curved plane wakes." Journal of Fluid Mechanics 282 (January 10, 1995): 279–311. http://dx.doi.org/10.1017/s0022112095000140.
Анотація:
The formation and evolution of the three-dimensional structure of straight and mildly curved ($b/\bar{R} < 2\%$) flat plate wakes at relatively high Reynolds numbers (Reb = 28 000) have been studied through detailed measurements of the mean and fluctuating velocities. In both cases, the role of initial conditions was examined by generating wakes from untripped (laminar) and tripped (turbulent) initial boundary layers. The curved wake was affected by the angular momentum instability such that the inside half of the wake was unstable, whereas the outside half was stable. In both the straight and curved untripped wakes, large spanwise variations, in the form of ‘pinches’ and ‘crests’, were observed in the contours of mean velocity and Reynolds stresses. Well-organized, ‘spatially stationary’ streamwise vorticity was generated in the near-field region in the form of quadrupoles, to which the spanwise variations in the velocity contours were attributed. The presence of mean streamwise vorticity had a significant effect on the wake growth and defect decay rates, mainly by providing additional entrainment. In the straight wake, the mean streamwise vorticity decayed on both sides of the wake such that it had decayed completely by the far-field region. However, in the curved case, the mean streamwise vorticity on the unstable side decayed at a rate significantly lower than that on the stable side. Despite the decay of mean streamwise vorticity, the spanwise variations persisted into the far wake in both cases. The effects of curvature were also apparent in the Reynolds stress results which showed that the levels on the unstable side were increased significantly compared to those on the stable side, with the effect much stronger in the initially laminar wake. With the initial boundary layers tripped, spatially stationary streamwise vortex structures were not observed in either the straight or curved wakes and the velocity contours appeared nominally two-dimensional. This result further confirms the strong dependency of the three-dimensional structure of plane wakes on initial conditions.
36
Flammang, Brooke E., George V. Lauder, Daniel R. Troolin, and Tyson E. Strand. "Volumetric imaging of fish locomotion." Biology Letters 7, no. 5 (April 20, 2011): 695–98. http://dx.doi.org/10.1098/rsbl.2011.0282.
Анотація:
Fishes use multiple flexible fins in order to move and maintain stability in a complex fluid environment. We used a new approach, a volumetric velocimetry imaging system, to provide the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes. This new technology allowed us to demonstrate conclusively the linked ring vortex wake pattern that is produced by the symmetrical (homocercal) tail of fishes, and to visualize for the first time the three-dimensional vortex wake interaction between the dorsal and anal fins and the tail. We found that the dorsal and anal fin wakes were rapidly (within one tail beat) assimilated into the caudal fin vortex wake. These results show that volumetric imaging of biologically generated flow patterns can reveal new features of locomotor dynamics, and provides an avenue for future investigations of the diversity of fish swimming patterns and their hydrodynamic consequences.
37
Dunlop, Jordan Ashley, and Mark Christopher Thompson. "Reducing Slipstream Velocities Experienced in Proximity to High-Speed Trains." Fluids 7, no. 2 (February 9, 2022): 72. http://dx.doi.org/10.3390/fluids7020072.
Анотація:
Slipstream wake structures generated by the passing of high-speed rail vehicles represent a hazard to passengers and workers in close proximity. In this article, the possibility of reducing peak slipstream velocities through the implementation of angled fins or swirling flow injection is assessed on the basis of improved delayed detached eddy simulations (IDDES). The key to improving slipstream velocities involves redirecting and/or reducing the internal energy, a pair of meandering counter-rotating vortex cores that are associated with large wake slipstream deviations. It is demonstrated that the danger imposed by slipstream wake structures, as measured by the induced velocity measures recorded at a series of test points adjacent to the passing train, could be significantly reduced, with decreases from 10% up to 23%, recorded across a range of sampling locations. The means by which these reductions are generated and the corresponding changes in the flow are also explored through analysis of the modified wakes. As such, these devices show promise at improving the operational safety of high-speed rail vehicles.
38
Robert, Szasz, and Fuchs Laszlo. "1072 THE EFFECT OF ICING ON AIRFOIL WAKE STRUCTURES." Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2013.4 (2013): _1072–1_—_1072–6_. http://dx.doi.org/10.1299/jsmeicjwsf.2013.4._1072-1_.
39
Ehret, T., and H. Oertel. "Calculation of wake vortex structures in the near-field wake behind cruising aircraft." Atmospheric Environment 32, no. 18 (September 1998): 3089–95. http://dx.doi.org/10.1016/s1352-2310(98)00112-5.
40
Zargar, Arash, Ali Tarokh, and Arman Hemmati. "The Steady Wake of a Wall-Mounted Rectangular Prism with a Large-Depth-Ratio at Low Reynolds Numbers." Energies 14, no. 12 (June 16, 2021): 3579. http://dx.doi.org/10.3390/en14123579.
Анотація:
The wakes of wall-mounted small (square) and large (long) depth-ratio rectangular prisms are numerically studied at Reynolds numbers of 50–250. The large depth-ratio significantly alters the dominance of lateral secondary flow (upwash and downwash) in the wake due to the reattachment of leading-edge separated flow on the surfaces of the prism. This changes the wake topology by varying the entrained flow in the wake region and changing the distribution of vorticity. Thus, the magnitude of vorticity significantly decreases by increasing the prism depth-ratio. Furthermore, the length of the recirculation region and the orientation of near wake flow structures are altered for the larger depth-ratio prism compared to the square prism. Drag and lift coefficients are also affected due to the change of pressure distributions on the rear face of the prism and surface friction force. This behavior is consistently observed for the entire range of Reynolds numbers considered here. The wake size is scaled with Re1/2, whereas drag coefficient scaled with Re−0.3.
41
Hodgkin, Amy, Sylvain Laizet, and Georgios Deskos. "Do ambient shear and thermal stratification impact wind turbine tip-vortex breakdown?" Journal of Physics: Conference Series 2265, no. 2 (May 1, 2022): 022061. http://dx.doi.org/10.1088/1742-6596/2265/2/022061.
Анотація:
Abstract Modern wind turbines experience uneven inflow conditions across the rotor, due to the ambient flow’s shear and thermal stratification. Such conditions alter the shape and length of turbine wakes and thus impact the loads and power generation of downstream turbines. To this end, understanding the spatial evolution of the individual wakes under different atmospheric conditions is key to controlling and optimising turbine arrays. With this numerical study we aim to obtain a better understanding of the fundamental physics governing the near-wake dynamics of wind turbines under shear and thermal stability, by examining their tip-vortex breakup mechanisms. Our approach considers scale-resolving simulations of a single turbine wake under a linear shear profile as well as the application of harmonic tip perturbations to trigger flow instabilities. For the subsequent analysis we use the proper orthogonal decomposition (POD) method to extract coherent structures from the flow, and we also calculate mean kinetic energy fluxes to quantify each coherent structure’s contribution to wake recovery. The wake’s helical spiral is found to hinder wake recovery for all studied ambient flow conditions, whereas the mutual inductance instability has positive MKE flux leading to an enhanced wake recovery. Finally, the ambient shear has the largest impact on the local MKE flux with respect to downstream location by changing the shape of the curve and location of extrema, whereas thermal stratification has only a minimal impact on the magnitude of the near-wake local MKE flux distribution.
42
Vigny, U., P. Benard, P. Tene Hedje, F. Houtin-Mongrolle, L. Bricteux, and S. Zeoli. "A new wake detection methodology to capture wind turbine wakes using adaptive mesh refinement and Large Eddy Simulation." Journal of Physics: Conference Series 2265, no. 2 (May 1, 2022): 022005. http://dx.doi.org/10.1088/1742-6596/2265/2/022005.
Анотація:
Abstract The development of turbulent vortical wakes released downstream of wind turbines is crucial as it presents many technological implications for wind farm design and exploitation. The numerical prediction of these wakes constitutes a challenging problem as they involve the shedding of fine vortical structures, their instabilities, and interactions with a turbulent ambient flow. A Large Eddy Simulation (LES) approach allows capturing such flow phenomena, which implies a suitable mesh. Adaptive Mesh Refinement (AMR) is used to refine the mesh in the wind turbine wake to limit the computational cost. A methodology is developed to define and capture the wake envelope adequately. Three main parts of this methodology can be identified: The wind turbine wake detection, the target cell size required and adaptation frequency. The target cell size needed to properly capture the wind turbine wake is investigated in previous work [1], while this paper focuses on wind turbine wake detection. A strategy based on a progress variable with a source term in the rotor region is used to capture the wake. This variable is transported by the flow and thus defines the wake envelope. AMR is used to refine the mesh within this region. To validate the method, a comparison between an adaptive mesh case and a reference mesh case has been performed on a single rotor and a two aligned rotor configuration. For both, the wind turbine wake tracking method is effective. The progress variable is transported correctly and leads to a well-defined wake area. The mesh is refined adequately within it. The physical comparison between cases showed similar results, while the performance comparison showed a computational cost reduction of 30% in the single turbine configuration and 50% in the two turbines configuration. Therefore, our methodology coupled with adaptive mesh refinement can adequately capture wind turbine wake, define an accurate wake envelope and decrease the computational cost for the same physical precision.
43
Hedenström, A., M. Rosén, and G. R. Spedding. "Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel." Journal of The Royal Society Interface 3, no. 7 (October 13, 2005): 263–76. http://dx.doi.org/10.1098/rsif.2005.0091.
Анотація:
The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments.
44
Bomphrey, Richard J., Per Henningsson, Dirk Michaelis, and David Hollis. "Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation." Journal of The Royal Society Interface 9, no. 77 (September 12, 2012): 3378–86. http://dx.doi.org/10.1098/rsif.2012.0418.
Анотація:
Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread.
45
Daisaka, Hiroshi. "N-body Simulations of Planetary Rings." Symposium - International Astronomical Union 208 (2003): 387–88. http://dx.doi.org/10.1017/s0074180900207390.
Анотація:
We present the formation and evolution of a structure in particles obtained from a local N-body simulation of a dense planetary ring like Saturn's ring. Our simulations show in a particle system the spontaneous formation of a spatial structure like wakes, clumps, and a structure which could be induced by the viscous overstability. Such a formation depends on parameters characterizing a ring system: the wake is likely to form in Saturn's ring and the existence of the wake is consistent with observations. The viscous overstability would be a good candidate for the explanation of subring structures in the ring.
46
Li, Han, Qiaogao Huang, Guang Pan, Xinguo Dong, and Fuzheng Li. "Effects of Blade Number on the Propulsion and Vortical Structures of Pre-Swirl Stator Pump-Jet Propulsors." Journal of Marine Science and Engineering 9, no. 12 (December 9, 2021): 1406. http://dx.doi.org/10.3390/jmse9121406.
Анотація:
Reducing the noise of the underwater propulsor is gaining more and more attention in the marine industry. The pump-jet propulsor (PJP) is an extraordinary innovation in marine propulsion applications. This paper inspects the effects of blade number on a pre-swirl stator pump-jet propulsor (PJP) quantitatively and qualitatively. The numerical calculations are conducted by IDDES and ELES, where the ELES is only adopted to capture the vortical structures after refining the mesh. The numerical results show good agreement with the experiment. Detailed discussions of the propulsion, the features of thrust fluctuation in time and frequency domains, and the flow field are involved. Based on the ELES results, the vortices in the PJP flow field and the interactions between the vortices of the stator, rotor, and duct are presented. Results suggest that, though changing the blade number under a constant solidity does not affect the propulsion, it has considerable effects on the thrust fluctuation of PJP. The wakes of the stator and rotor are also notably changed. Increasing the stator blade numbers has significantly weakened the high-intensity vortices in the stator wake and, hence, the interaction with the rotor wake vortices. The hub vortices highly depend upon the wake vortices of the rotor. The hub vortices are considerably broken by upstream wake vortices when the load per rotor blade is high. In summary, the blade number is also vital for the further PJP design, particularly when the main concerns are exciting force and noise performance.
47
Costello, John H., Sean P. Colin, Brad J. Gemmell, and John O. Dabiri. "Hydrodynamics of Vortex Generation during Bell Contraction by the Hydromedusa Eutonina indicans (Romanes, 1876)." Biomimetics 4, no. 3 (July 5, 2019): 44. http://dx.doi.org/10.3390/biomimetics4030044.
Анотація:
Swimming bell kinematics and hydrodynamic wake structures were documented during multiple pulsation cycles of a Eutonina indicans (Romanes, 1876) medusa swimming in a predominantly linear path. Bell contractions produced pairs of vortex rings with opposite rotational sense. Analyses of the momentum flux in these wake structures demonstrated that vortex dynamics related directly to variations in the medusa swimming speed. Furthermore, a bulk of the momentum flux in the wake was concentrated spatially at the interfaces between oppositely rotating vortices rings. Similar thrust-producing wake structures have been described in models of fish swimming, which posit vortex rings as vehicles for energy transport from locations of body bending to regions where interacting pairs of opposite-sign vortex rings accelerate the flow into linear propulsive jets. These findings support efforts toward soft robotic biomimetic propulsion.
48
Hayakawa, Michio, and Fazle Hussain. "Three-dimensionality of organized structures in a plane turbulent wake." Journal of Fluid Mechanics 206 (September 1989): 375–404. http://dx.doi.org/10.1017/s0022112089002338.
Анотація:
This paper describes a quantitative study of the three-dimensional nature of organized motions in a turbulent plane wake. Coherent structures are detected from the instantaneous, spatially phase-correlated vorticity field using certain criteria based on size, strength and geometry of vortical structures. With several combinations of X-wire rakes, vorticity distributions in the spanwise and transverse planes are measured in the intermediate region (10d [les ] x [les ] 40d) of the plane turbulent wake of a circular cylinder at a Reynolds number of 13000 based on the cylinder diameter d. Spatial correlations of smoothed vorticity signals as well as phase-aligned ensemble-averaged vorticity maps over structure cross-sections yield a quantitative measure of the spatial coherence and geometry of organized structures in the fully turbulent field. The data demonstrate that the organized structures in the nominally two-dimensional wake exhibit significant three-dimensionality even in the near field. Using instantaneous velocity and vorticity maps as well as correlations of vorticity distributions in different planes, some topological features of the dominant coherent structures in a plane wake are inferred.
49
Jiang, Suyu, Fei Yan, Jian Zhang, and Bo Song. "Multi-Scale Wake Characteristics of the Flow over a Cylinder with Different V-Groove Numbers." Water 15, no. 4 (February 18, 2023): 805. http://dx.doi.org/10.3390/w15040805.
Анотація:
The near-wake structure resulting from the interaction between the water and the cylinder is researched. The wake characteristics for the smooth cylinder and the cylinder with different numbers of grooves are measured by high-speed particle image velocimetry (PIV) at a Reynolds number of 7400. From the measured data, it is observed that the v-groove increases the Reynolds shear stress and the turbulent kinetic energy; thus, the formation of the Karman-like vortices is prevented and the recirculation region is reduced. The number of grooves has a limited effect on the wake characteristics. In addition, a one-dimensional orthogonal wavelet decomposes the fluctuation velocities in the cylindrical wake region to study the multi-scale wake characteristics behind the v-groove cylinders. The large-scale structures show that the Karman-like vortices of the grooved cylinder are closer to the trailing edge of the cylinder. The intermediate-scale structures show more vortices behind the 32-groove cylinder. The small-scale structures show that the phase difference between the upper and lower oscillation positions has become smaller. Meanwhile, the large-, intermediate- and small-scale Reynolds shear stress distributions suggest that the shear layer instability in the near-wake region creates vortices, which increase the turbulent kinetic energy and narrow the wake region.
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Bohrer, Jan Kai, Vlaho Petrović, Andreas Rott, and Martin Kühn. "LES-based validation of a dynamic wind farm flow model under unsteady inflow and yaw misalignment." Journal of Physics: Conference Series 2767, no. 3 (June 1, 2024): 032041. http://dx.doi.org/10.1088/1742-6596/2767/3/032041.
Анотація:
Abstract This work presents the validation of an extended version of the control-oriented, dynamic wind farm flow solver SPLINTER. The two-dimensional model is applied to use cases of wake steering by yaw misalignment and inflow wind direction variations and the results are compared to large-eddy simulations (LES). While SPLINTER is able to reproduce the antagonal behaviour of decreasing upstream and increasing downstream turbine power under wake deflection, a systematic deviation of the downstream power is detected and quantified, which is connected to underrepresented three-dimensional wake effects. In case of changing inflow wind direction, SPLINTER is capable of computing movement and shape of the bending wakes. The model smooths small-scale turbulent structures and disturbances and does not reproduce wake meandering, but manages to describe the evolution of the mean flow, which is tested by averaging over an ensemble of LES and comparing the resulting flow fields and turbine power time series. Under dynamic inflow conditions, SPLINTER is able to predict at which time intervals and at which rates downstream turbines will be influenced by wakes, which can improve the accuracy of short-term power and load forecasting and enables its application to online model predictive wind farm control.