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The present experimental work is focused on designing and standardizing a direct method of measuring the drag force on a tapered cylinder in a water tunnel. The model considered in the present study is a tapered cylinder of 70:1 taper ratio (with a mean diameter( D m ) of 8 mm) placed in a water tunnel test section. The drag force is measured directly, which is acquired by using a force transducer setup (strain-gauge-based), augmented with a lever arm to amplify or attenuate the force. Multiple calibrations of the load cells were done to ensure the repeatability of experiments. The taper cylinder model is suspended inside the water tunnel and the drag force is obtained for various Reynolds numbers (based on mean diameter). It was found that the drag coefficient was ≈ 10-20 % less than a circular cylinder. [ABSTRACT FROM AUTHOR]

The use of wind energy, the area of wind farms, and the length of wind turbine blades continuously increase with the increase in energy demand. This study aims to simulate large wind farms in the atmospheric boundary layer with different double-rotor (DR) wind turbine arrangements using large eddy simulation. The effect of four DR wind turbine arrangements on the efficiency of a large wind farm was investigated in this study. All simulations were performed using the OpenFOAM solver. The performance of different turbine arrangements was investigated based on the mean velocity, the extent of the wake region, mean kinetic energy, and turbine power. Results showed that the arrangement of DR turbines with transverse spacing produces higher power than other layouts. Moreover, DR layouts produce the same power as single-rotor layouts with a short blade length. This scenario is important from an economic point of view. [ABSTRACT FROM AUTHOR]

The free surface and pycnocline wake characteristics of SUBOFF were studied by simulations and experiments in a two-layer ocean. The experimental results showed that the simulation method accurately calculated the wake characteristics of the scaling model, and the simulation method was expanded to full-scale prototypes. When typical Froude numbers of flow is from 0.05 to 2.7, the submerged body velocity can influence the wake characteristics of the free surface and pycnocline interface. The wake system of the free surface would transform into a Kelvin wake, and the angles of the V-shaped divergent waves of the pycnocline interface would be reduced with the increasing Froude number. [ABSTRACT FROM AUTHOR]

We study the compression of spatial and temporal features in fluid flow data using multimedia compression techniques. The efficacy of spatial compression techniques, including JPEG and JPEG2000 (JP2), and spatiotemporal video compression techniques, namely H.264, H.265, and AV1, in limiting the introduction of compression artifacts and preserving underlying flow physics are considered for laminar periodic wake around a cylinder, two-dimensional turbulence, and turbulent channel flow. These compression techniques significantly compress flow data while maintaining dominant flow features with negligible error. AV1 and H.265 compressions present the best performance across a variety of canonical flow regimes and outperform traditional techniques such as proper orthogonal decomposition in some cases. These image and video compression algorithms are flexible, scalable, and generalizable holding potential for a wide range of applications in fluid dynamics in the context of data storage and transfer. [ABSTRACT FROM AUTHOR]

Flow structures around a wavy square cylinder with a perturbation wavelength of 5.6D are investigated using large eddy simulation at the Reynolds number of 23 500. The detailed force characteristics and wake flow structures of the wavy square cylinder are captured and compared with a square cylinder. Under the effect of wavy leading edge up to 27% and 98% reduction in the mean drag and the lift fluctuations are achieved, respectively. The 3D mean flow field implies a high shear flow between node and saddle positions, which is associated with the generation of additional streamwise and vertical vortex pairs. These vortex structures are responsible for the three-dimensionality of the wake flow behind the wavy square cylinder. The instantaneous flow patterns suggest that the staggered pattern of von Kármán vortex shedding is suppressed and replaced by symmetric vortex shedding into the near wake region of the wavy square cylinder. This symmetric vortex structure in the near wake plays a role in preventing the upper and lower shear layers from interacting with each other. The time–frequency analysis exhibits a reduction in the dominating vortex shedding frequency for the case of wavy square cylinder, indicating a relatively steady wake flow. This can be attributed to the vortex dislocation behind the wavy square cylinder. In the range of high frequencies, small streaks associated with small-scale fluctuations are enhanced by the wavy leading edge. The present study suggests that the perturbation wavelength obtained from the wavelength of Mode A instability at laminar flow regime can be served as a basis for flow control in turbulent flow regime. [ABSTRACT FROM AUTHOR]

In this study, the interaction between a spherical solid particle descending in a fine-bubble plume and the fine-bubbles around the particle is investigated experimentally. The fine-bubbles, which are formed by water electrolysis and have an average diameter of 0.037 mm, rise via buoyancy to form bubble plumes. The average bubble velocity in the conduit is 0.05 mm/s. A spherical solid particle with a diameter of 11.1 mm and a density of 1130 kg/m3 is dropped into the fully-developed bubble plume. The particle falls in a meandering motion in the fine-bubble plume. In this process, the terminal velocity of the particles falling in the fine-bubble plume is almost identical to that of a particle falling in the quiescent water. Additionally, after the fine-bubbles have been separated from the particle surface, they flow into the wake of the particle, forming a stagnant region. The particle wake diameter increases threefold, as compared with its value in the downstream direction of the fine-bubble plume. [ABSTRACT FROM AUTHOR]

Drawing on folklore, oral history, songs, historic ethnography, literature, and archaeology, this study investigates the economic, social, and material evidence of traditional late nineteenth- to early twentieth-century wakes along coastal Ireland through the lens of an archaeological case study on the island of Inishark. We analyze material practices of "good" and "bad" deaths, and explore the myriad ways that islanders used everyday material culture in mortuary rituals, particularly wakes and funerals. We argue that material culture could hold multiple symbolic valences throughout its history, especially everyday items used in ritual contexts. Finally, we describe the material residues associated with mourning practices on Inishark. [ABSTRACT FROM AUTHOR]

In a large wind farm, the wakes of upstream and downstream wind turbines can interfere with each other, affecting the overall power output of the wind farm. To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence, this work proposes some improvements to the actuator line-large-eddy simulation (AL-LES) method. Based on the dynamic k-equation large-eddy simulation (LES), this method uses a precursor method to generate atmospheric inflow turbulence, models the tower and nacelle wakes, and improves the body force projection method based on an anisotropic Gaussian distribution function. For these three improvements, three wind tunnel experiments are used to validate the numerical accuracy of this method. The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results. After modeling the tower and nacelle wakes, the wake velocity distribution is consistent with the experimental result. The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s, which is about 6% different from the experimental value and improves the prediction accuracy of the tip vortex radial movement. The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction. [ABSTRACT FROM AUTHOR]

With the use of the schlieren visualization and echo sounding method, the spatial structure of disturbances in front of a plate and behind it is investigated for the first time in the flow regime of intense internal-wave generation. In the echograms, fine structures are detected both in the upstream disturbances and in the wake within the solid-motion layer. In the schlieren graph, the fields of upstream disturbances are smooth, and thin interfaces are expressed in the confluent wake. Local peaks in the spatial disturbance spectra are expressed on scales that slightly exceed the length of internal waves, and, to a lesser extent, for lower scales. The thinnest interfaces are observed in the schlieren image of the wake immediately behind the plate. [ABSTRACT FROM AUTHOR]

The numerical study of the wake of full-scale slender devices such as aircraft wings and wind turbine blades requires high-fidelity large eddy simulation tools. The broad spectrum of scales involved entails the use of coarse-grain models for the devices. Actuator disk or line methods have been developed for that purpose, and are to date the most employed in that context. These methods transfer the force from the device to the fluid using 3D mollification functions. A novel immersed lifting and dragging line method is here presented, together with its implementation in a hybrid vortex particle-mesh flow solver. The line models the effect of blades or wings on the flow through the generation of vorticity, in a Lagrangian manner. This has several advantages over the treatment inherent to actuator methods: The absence of mollification in the spanwise direction and the relaxation of the classical Courant–Friedrichs–Lewy condition contribute to keeping the accuracy and the efficiency at a high level. The method is thoroughly verified against theory using results on various airfoils and wings. Finally, the efficiency of the approach is illustrated with the simulation of the wake of an elliptical wing. The role of parasitic drag in the development of wake instabilities is pointed out, by comparing configurations with none, moderate and high profile drag. In the last case, the simulation captures the fine scales vortex dynamics and the establishment of a turbulent wake over a length of 30 wing spans. [ABSTRACT FROM AUTHOR]

This paper provides a review of the general experimental methodology of snow-powered flow visualization and super-large-scale particle image velocimetry (SLPIV), the corresponding field deployments and major scientific findings from our work on a 2.5 MW utility-scale wind turbine at the Eolos field station. The field measurements were conducted to investigate the incoming flow in the induction zone and the near-wake flows from different perspectives. It has been shown that these snow-powered measurements can provide sufficient spatiotemporal resolution and fields of view to characterize both qualitatively and quantitatively the incoming flow, all the major coherent structures generated by the turbine (e.g., blade, nacelle and tower vortices, etc.) as well as the development and interaction of these structures in the near wake. Our work has further revealed several interesting behaviors of near-wake flows (e.g., wake contraction, dynamic wake modulation, meandering and deflection of the nacelle wake, etc.), and their connections with constantly-changing inflows and turbine operation, which are uniquely associated with utility-scale turbines. These findings have demonstrated that the near wake flows, though highly complex, can be predicted with substantial statistical confidence using supervisory control and data acquisition (SCADA) and structural response information readily available from current utility-scale turbines. Such knowledge can be potentially incorporated into wake development models and turbine controllers for wind farm optimization in the future. [ABSTRACT FROM AUTHOR]

Aerodynamic flow around an 1/5 scale cyclist model was studied experimentally and numerically. First, measurements of drag force were performed for the model in a low-speed wind tunnel at Reynolds numbers from 5.5 × 10 4 to 1.8 × 10 5 . Meanwhile, numerical computation using a large eddy simulation method was performed at three Reynolds numbers of 1.1 × 10 4 , 6.5 × 10 4 and 1.5 × 10 5 to obtain the drag coefficients for comparison. Second, flow visualization was made in a water channel and the wind tunnel mentioned to examine the three-dimensional flow separation pattern on the model surface, which could also be realized from the numerical results. Finally, a wake flow survey based on the hot-wire measurements in the wind tunnel showed that in the near-wake region, the flow was featured with the formation of multiple streamwise vortices. The numerical results further indicated that these vortices were evolved from the separated flows occurred on the model surface. [ABSTRACT FROM AUTHOR]

The midbrain dopamine system via the dorsal and ventral striatum regulates a wide range of behaviors. To dissect the role of dopaminergic projections to the dorsal striatum (nigrostriatal projection) and ventral striatum (mesolimbic projection) in sleep–wake behavior, we selectively chemogenetically stimulated nigrostriatal or mesolimbic projections and examined the resulting effects on sleep in rats. Stimulation of nigrostriatal pathways increased sleep and EEG delta power, while stimulation of mesolimbic pathways decreased sleep and reduced cortical EEG power. These results indicate that midbrain dopamine signaling in the dorsal or ventral striatum promotes sleep or wake, respectively. [ABSTRACT FROM AUTHOR]

Relational memory is the ability to flexibly organize and integrate multiple sources of information to produce emergent outcomes. In tests for one type of relational memory—stimulus equivalence—arbitrary stimuli become related in ways not explicitly trained. Little is known, however, about whether stimulus equivalence-based relational memory ability differentially emerges during offline periods of either sleep or wake. In this study, 51 healthy young adults learned a series of interconnected conditional relations involving arbitrary visual images (A-B, A-C, and A-D), and were immediately tested for maintenance of these relations. Following a 12-hour offline period consisting of either sleep or wake, both groups were tested for novel inferences—symmetry (B-A, C-A, and D-A) and equivalence relations (B-C, C-B, C-D, and D-C)—as well as retention of the trained relations. Results from delayed testing, supported by Bayesian statistics, showed that accuracy did not differ between the sleep and wake groups. Potential limitations of this preliminary investigation and directions for future research are discussed. [ABSTRACT FROM AUTHOR]

In this paper, the wake modes behind a circular cylinder under streamwisely forcing oscillating motion are studied at Reynolds number Re = 360–460 which are observed by laser-induced fluorescence flow visualization technique. The forcing frequency f e ranges from 0 to 6.85 f s , where f s is the vortex shedding frequency behind a stationary cylinder, and the forcing amplitude A / d = 0.2 , 0.5 , 1.0 , where d is the cylinder diameter. Both time-invariant and linearly ramping f e are investigated. Following our previous modal notation, the following conclusions can be drawn: firstly, three rarely reported modes in numerical studies, C-I, C-II and S-III, are now confirmed in experiments (though with differentiable appearance in their far wake behaviour) at higher A and/or f e ranges and the envelope lines of S modes and C modes, yielded from a vortex circulation model, are shown to be dependent on the peak relative velocity of the free stream to the cylinder surface. So is the occurrence of the S-II mode. Secondly, near the demarcation of A modes and S-I mode, wake mode undergoes constant transition in a stochastic manner at fixed Re, A and f e . Thirdly, a typical hysteretic effect can be observed when the oscillation frequency of the cylinder ramps up and down in a linear way, and the extent of delay is dependent on the ramping rate k. Finally, mode switching during frequency ramping obeys a unidirectional order. During k < 0 (ramp-down), when S-I (Type-II) mode switches to A-IV mode, or A-IV to A-III, the flow structure downstream is affected by the upstream and the entire wake flow eventually switched, which is classified as slow switches. In contrast, during k > 0 (ramp-up), a clear and abrupt switch can be observed in the wake when A-IV or A-III switch to S-I (Type-II) modes, which are jump switches. [ABSTRACT FROM AUTHOR]

In the present paper, physical mechanism responsible for origin of streamwise vortices in mode A appeared in the three-dimensional (3-D) wake transition of a square-section cylinder is investigated. Direct numerical simulations at a Reynolds number of 180 firstly show that such streamwise vorticity is not originated from lateral surfaces. Then through the analysis of local flow field in the immediate neighborhood of rear surface, based on the theory of vortex-induced vortex, a new physical mechanism is identified. At first, the vertical vorticity on rear surface is generated by the intrinsic three-dimensional instability with the same instability wavelength of mode A. Then the streamwise vorticity at a specific sign is induced by such vertical vorticity, convected and concentrated in the shear layers. Finally, streamwise vortices are formed and shed with alternatively shedding spanwise vortices in the near wake. Moreover, the effect of induced spanwise vorticity on original two-dimensional (2-D) spanwise vorticity is also presented in detail. [ABSTRACT FROM AUTHOR]

As reported in a previous work by Lin et al. (Acta Mech Sin, 2018. 10.1007/s10409-018-0758-z), an interesting phenomenon was discovered based on the analysis of wavy vortex and vorticity distribution in the shear layers and near wake of a peak-perforated conic shroud, and two sign laws were summarized. In the present paper, the theory of a vortex-induced vortex is introduced to explore mechanisms in a wavy vortex and applicable sign laws for uniform and incompressible flow past a fixed bluff body. Based on the analysis of the nearest-wall flow, two vortex-induced models for streamwise and vertical vortex pairs, respectively, are proposed under two boundary cases, denoting the induced vorticity introduced or distributed on and near the walls. As a result, the first sign law, for only streamwise and vertical components of vorticity, and the second sign law, for three components of vorticity, are obtained under their own particular conditions. The first sign law reveals the intrinsic physical relationship between streamwise and vertical vorticities, independent of the distribution of spanwise vortices in the whole flow field. It is also confirmed that the spanwise vortices, as well as the shear layers and wake width, distributed wavily across the span, are attributed to the introduced streamwise or vertical vortices. The two sign laws for vorticity are independent of the disturbed spanwise wavelength and the Reynolds number. Through the analysis of flow past the conic shroud, the two sign laws are successfully used to summarize typical spacial distributions of vorticity in three flow regions: on and near the front cylinder surfaces, the separated shear layers and the near wake. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical investigations on the flow characteristics past a circular cylinder with a jet injection at the rear stagnation point are performed. Transient, incompressible, laminar, and isothermal flow governing equations are solved with finite volume method. Numerical simulations have been carried out at a Reynolds number of 150 with different injection ratios (IR) ranging from 0.5 to 7. Time evolutions of the coefficients of drag and lift, and streamline patterns are plotted. Three different flow pattern ranges are observed, namely wake dominant (IR=0-1.5), transition (IR=1.5-2.5) and jet dominant (IR > 2.5), and they are characterized by the combined effects of vortex shedding, undulation and jet dominant phenomena appearing in the flow downstream. It is also found that Cd decreases slightly with the injection ratio up to 1.5, after that it monotonically increases with the injection ratio. The similar incremental trend is observed in the Strouhal number variation with IR up to 1.5; then, it increases almost linearly till IR=4. When IR is greater than 4, there is a sudden drop in the Strouhal number value equal to zero and it remains constant after that for all the IR values considered in this study. The power spectral density of Cl indicates that the dominant frequency is present for the lower IR up to 4 and that no dominant frequency appears in the higher injection ratio range of 5-7 due to a complete suppression of the vortex shedding behind the cylinder by a dominant jet mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The wake of a circular cylinder coated with porous media was experimentally investigated by hot-wire anemometry, particle image velocimetry (PIV) and smoke-wire visualization. Mean velocity and Reynolds stresses in the streamwise and lateral are eliminated obviously for porous layer coating, especially in the centerline. The vortex formation length is elongated, and the width of the wake region is expanded by coating porous media. The Strouhal number of vortex shedding is 0.162 for the porous cylinder, which is smaller than 0.182 for the bare one due to the widened wake. Additionally, porous layer significantly regulates the flow field, which can be detected distinctly by smoke-wire visualization and transient PIV data. The proper orthogonal decomposition (POD) was further used to analyze PIV data with a view to extract dominant coherent structures in the wake. Moreover, a phase-averaged low-order model was applied to provide a comprehensive description of the dynamics of the first four modes. The first two POD modes, showing the signature of asymmetrical vortex shedding and predominating in the wake of the bare cylinder, are weakened dramatically for porous layer coating and present abrupt degradation in the midline. Meanwhile, mode 3 and mode 4, representing the symmetrical vortex shedding, occupy higher energy for porous cylinder. The study reveals that the porous layer coating delays the roll-up of shear layer and weakens the interaction between them, and thus, the vortex region is shifted further downstream.Graphical abstract: [ABSTRACT FROM AUTHOR]

This paper presents a dynamic study of fluid flow around two circular cylinders placed in a transverse arrangement near a solid wall. The flow equations are solved by finite element method for Reynolds number equal to 200. The effect of geometric ratios representing the distance between the two cylinders () and the distance between the bottom cylinder and the wall () on the hydrodynamic behavior of the fluid is investigated. First, it is found that the mean drag coefficient of the bottom cylinder is larger than that of the top cylinder when () varies. However, for the top cylinder it decreases and this coefficient decreases when () increases. Secondly, it has been observed that () strongly influences the lift coefficient of the bottom cylinder. For small values of (), we observe that the values of CL2 tend to zero compared to those of CL1 with values varying between 0.2 and 0.3. Finally, it is also found that Strouhal number values of the bottom cylinder near the wall vary between 0.03 and 0.06. This is radically different from the case without wall where Strouhal number values vary between 0.16 and 0.23. [ABSTRACT FROM AUTHOR]

Abstract: A passive flow control technique, which is to drill a horizontal hole going from the front surface to the rear surface or an inclined hole going from the front surface to the top surface inside a circular cylinder of an aspect ratio H/D = 1, is proposed to control the rear recirculation region and the vortices near the free end surface. Here, both the diameter D and height H of cylinder are 70 mm. PIV measurement was performed with Reynolds number of 8570 in a water tunnel. Furthermore, in order to consider the effect of the hole position of the front surface, the cylinder models having different hole height of front surface from wall were tested. It is found that the rear recirculation zones of the horizontal hole cylinders are smaller than that of the standard cylinder. The Reynolds shear stresses and the turbulent kinetic energy are evidently reduced by the flow issued from the horizontal hole. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone are broken down into several small-scale vortices with decreasing the height of the horizontal hole. Although the rear separation zone of the inclined hole cylinder increases, the recirculation region near the free end surface decreases. The areas of large Reynolds shear stress and high turbulent kinetic energy increase in the rear recirculation zone. However, with increasing the height of the inclined hole, the Reynolds shear stress decreases.Graphical Abstract: [ABSTRACT FROM AUTHOR]

The influence of the velocity ratio (VR) between the jet and main flow on the wake structure and aerodynamic loss of the trailing edge jet is studied using particle image velocimetry and numerical simulations. Three different velocity ratios, namely, VR = 0.5, 1.0, 1.5, are chosen for this comparative study. The Reynolds number (Reh) based on the slot height (h) and the mainstream velocity (U0) are 3380. Results show that the influence of jet on wake structure is significant such that the wake region shrinks and the turbulent kinetic energy is enhanced as the velocity ratio increases. The distribution area of strong vorticity is enlarged with increasing velocity ratio. By using proper orthogonal decomposition and fast Fourier transfer analysis, the variation of velocity ratio demonstrates significant impact on vortex shedding and turbulent kinetic energy. The aerodynamic loss coefficient is nearly constant between VR = 0.5 and 1.0, but increases by 3.25 % as the velocity ratio increases from 1.0 to 1.5. [ABSTRACT FROM AUTHOR]

Orexin (OX)/hypocretin-containing neurons are main regulators of wakefulness stability, arousal, and energy homeostasis. Their activity varies in relation to the animal's behavioral state. We here tested whether such variation is subserved by synaptic plasticity phenomena in basal conditions. Mice were sacrificed during day or night, at times when sleep or wake, respectively, predominates, as assessed by electroencephalography in matched mice. Triple immunofluorescence was used to visualize OX-A perikarya and varicosities containing the vesicular glutamate transporter (VGluT)2 or the vesicular GABA transporter (VGAT) combined with synaptophysin (Syn) as a presynaptic marker. Appositions on OX-A somata were quantitatively analyzed in pairs of sections in epifluorescence and confocal microscopy. The combined total number of glutamatergic (Syn/VGluT2) and GABAergic (Syn/VGAT) varicosities apposed to OX-A somata was similar during day and night. However, glutamatergic varicosities were significantly more numerous at night, whereas GABAergic varicosities prevailed in the day. Triple immunofluorescence in confocal microscopy was employed to visualize synapse scaffold proteins as postsynaptic markers and confirmed the nighttime prevalence of VGluT2 together with postsynaptic density protein 95 excitatory contacts, and daytime prevalence of VGAT together with gephyrin inhibitory contacts, while also showing that they formed synapses on OX-A cell bodies. The findings reveal a daily reorganization of axosomatic synapses in orexinergic neurons, with a switch from a prevalence of excitatory innervation at a time corresponding to wakefulness to a prevalence of inhibitory innervations in the antiphase, at a time corresponding to sleep. This reorganization could represent a key mechanism of plasticity of the orexinergic network in basal conditions. [ABSTRACT FROM AUTHOR]

The wake flow originated by three-dimensional bluff bodies having the different aspect ratio is experimentally studied in this paper. The turbulent wakes generated by asymmetric- and semi-cylinders were first measured by the high-speed PIV. The time-averaged and instantaneous flow structures were analyzed in various planes. Then, the orthogonal wavelet multi-resolution technique was used to analyze the multi-scale flow structures. The instantaneous vorticity and Reynolds shear stresses of various frequencies were evaluated and compared among the different aspect ratio and body shape. It was found that the asymmetric-cylinder produces the aerodynamic downforce and its separation region is smaller than that of semi-cylinder. The large-scale structure makes the most contribution to vorticities, and the intermediate-scale structure dominates the flow structures near the tip of body. The large-scale structure makes the most contribution to Reynolds shear stress except the tip of body, accounting for about 55.9-79.2%. The maximum value of large-scale Reynolds shear stress appears at the end of separation region, and the small-scale Reynolds shear stress exhibits the maximum value in the shear layer of the body surface. Graphical Abstract: [ABSTRACT FROM AUTHOR]

A first attempt is made for identifying the wake characteristics of circular cylinder confined by a wavy wall at laminar flow regime. Numerical study of flow characteristics past circular cylinder with wavy-wall confinement perpendicular to cylinder axis has been carried out in the range of Reynolds number 20-100. The finite volume-based CFD solver Ansys Fluent (Version 15.0) is used for computations. The results are presented in the form of streamline plots, mean drag co-efficient, flow separation angle and recirculation length. Wavy-wall confinement leads to highly significant changes in the cylinder wake such as the evolution of strong x-plane vortices, enhanced fluid mixing, wake suppression near the crest region and vortex stretching near the trough region on the downstream of the cylinder has been observed. Flow separation angle varies significantly along the axis of the cylinder. Increased wall shear stress on rear surface of the cylinder has also been observed. The part of vorticity magnitude as compared to strain rate has been distinguished and identified using vortex identification methods such as Q-criterion and Lambda-2 criterion. [ABSTRACT FROM AUTHOR]

Visualization experiments for flow around a cylinder with parallel slit placed inside a circular pipe are carried out with water as the working medium. Two different color dyes are employed to visualize the complex vortex formation mechanism behind the bluff bodies. The objective of this study is to explore the potential of cylinder with parallel slit as an improved vortex generator for various practical applications. Wake parameters, such as Strouhal Number, wake width, and vortex formation length, are calculated from the recorded images. Three different slit widths with non-dimensional separation ratio of 0.1, 0.2, and 0.4 are chosen to study the effect on the vortex formation mechanism and the corresponding wake parameters. The Reynolds number range covered in this study is Re = 200-2300. Symmetric vortex formation from the outer surface is observed for lower slit widths, whereas for high slit widths, both in-phase and anti-phase shedding are observed. A separation bubble from each of the inner surface is formed which detaches from the bluff body to form a vortex at higher Reynolds numbers. The separation bubble is found to be both bi-stable and symmetric for lower slit widths. The interaction of the separation bubble with the outer vortices is seen to affect the strength of the primary/outer vortex. The slit width is seen to be an important parameter in determining the strength of the vortices. At higher slit width, the configuration behaves like a dual bluff body and strong crisscross vortex shedding is observed. Such novel kind of vortex formation mechanism can be utilized to improve the performance of numerous practical applications, such as flowmeters, heat exchangers, etc. Graphical abstract: [ABSTRACT FROM AUTHOR]

The appearance of a ground surface can play an important role in the flow structures for the flows past a flat plate. We conduct two-dimensional numerical simulations on viscous flows past a flat plate inclined at an angle of attack of $$20^\circ $$ with ground effects using a finite-volume method. Results show that the effects on the separated flow from the ground are highly dependent on the gap ( G) between the plate and the ground. As the gap decreases, the strength of vortices generated from the trailing edge is restrained, which is consistent with experimental observations. Further decrease in the gap even eliminates the vortex shedding and yields a steady flow. It is also found that the flow between the gap can either be accelerated at large gap ratios ( $${G/L >1}$$ , G is the gap, L is the plate length), or be decelerated at small gap ratios ( $${G/L <1}$$ ). Furthermore, the numerical results show that the wake flow behind the plate can significantly change the distribution of surface shear stress on the ground. Specifically, the mean shear stress on the ground in the downstream region at a gap ratio $$G/L = 2.0$$ is one order of magnitude larger than that at a small gap ratio $$G/L = 0.3$$ , and the length of the downstream region where the shear stress can be effectively changed is much larger than the plate length, which provides a guideline to manipulate the ground wall surface shear stress using an inclined plate in the vicinity of the wall. [ABSTRACT FROM AUTHOR]

Two-dimensional supersonic laminar ideal gas flows past a regular flat lattice of identical circular cylinders lying in a plane perpendicular to the free-stream velocity are numerically simulated. The flows are computed by applying a multiblock numerical technique with local boundary-fitted curvilinear grids that have finite regions overlapping the global rectangular grid covering the entire computational domain. Viscous boundary layers are resolved on the local grids by applying the Navier-Stokes equations, while the aerodynamic interference of shock wave structures occurring between the lattice elements is described by the Euler equations. In the overlapping grid regions, the functions are interpolated to the grid interfaces. The regimes of supersonic lattice flow are classified. The parameter ranges in which the steady flow around the lattice is not unique are detected, and the mechanisms of hysteresis phenomena are examined. [ABSTRACT FROM AUTHOR]

To reveal the possibly existed flow phenomena that buried in a two-dimensional symmetric triangle wake, one-dimensional orthogonal wavelet transform and POD are combined to decompose the fluctuating velocity field into different wavelet components and modes. The features of reconstructed flow fields are analyzed in terms of fluctuating energy, time frequency distribution, space correlation and Reynolds shear stress. It is found that the first two wavelet components and POD modes can give representations to the most energetic large-scale structures, contributing about 77 and 73 % to the total fluctuating energy, respectively. Comparing with the first two wavelet components, the first two POD modes are more appropriate to represent the Karman-like vortical structures. The time-frequency and length scale characteristics of wavelet components suggest that frequency behavior can reflect the spatial-related length scale and the wavelet transform can be used to extract turbulent structures of different scales. Similar to the energy distribution, the most significant contributions to the Reynolds shear stresses comes from the large-scale structures that composed of first two wavelet components and POD modes, accounting for 88 and 80 % of the measured maximum Reynolds shear stress, respectively. Due to different classification criterion of wavelet and POD analyses, the combining use of these two methods tends to be more effective for analyzing multi-scale turbulent structures. Graphical abstract: [ABSTRACT FROM AUTHOR]

The performance and safety of the rear wing and spoiler employed on the National Association of Stock Car Auto Racing (NASCAR) COT (car of tomorrow) racecar are experimentally studied using 10 % scale models in a water channel. Particle image velocimetry is used to qualitatively examine the differences in flow structures between the two downforce-generating devices under 0 and 180-degree yaw cases. The latter is important due to an issue with the COT flipping into the air when at extreme yaw (i.e. during a crash). At zero yaw, it is observed that smaller length scales of the flow structures in the wake of the wing compared to those in the wake of the spoiler, provide more predictable handling for racecars in close proximity and may allow more safe and competitive racing. At 180-degree yaw, it is observed that wake-structure interactions may not allow proper operation of anti-flipping devices (roof flaps) on the winged car. In the extreme yaw case, local flow scales are examined and show much stronger Reynolds number (Re) dependence for the wing than the spoiler. [ABSTRACT FROM AUTHOR]

A two-dimensional flow of a non-Newtonian power-law fluid directed normally to a horizontal cylinder with a square cross section is considered in the present paper. The problem is investigated numerically with a finite volume method by using the commercial code Ansys Fluent with a very large computational domain so that the flow could be considered unbounded. The investigation covers the power-law index from 0.1 to 2.0 and the Reynolds number range from 0.001 to 45.000. It is found that the drag coefficient for low Reynolds numbers and low power-law index ( n ≤ 0.5) obeys the relationship C = A/Re. An equation for the quantity A as a function of the power-law index is derived. The drag coefficient becomes almost independent of the power-law index at high Reynolds numbers and the wake length changes nonlinearly with the Reynolds number and power-law index. [ABSTRACT FROM AUTHOR]

The turbulent flow over a cactus-analogue grooved cylinder at a Reynolds number of 5.4 × 10 was investigated using large eddy simulation integrated in OpenFOAM, with particular emphasis on the flow within grooves and its effect on the near wake immediately behind the cylinder. The baseline configuration of a smooth cylinder was examined for comparison. The wall-pressure fluctuation intensity, wall-pressure spectra, and time-averaged velocity field were in favorable agreement with previous experimental measurements. The fluctuation intensity of the lift coefficient for the cactus-analogue grooved cylinder was approximately 50 % less than for the smooth one. Different flow regions around the cylinder were selected for the proper orthogonal decomposition (POD) analysis, with a view to elucidating the production of the fine vortices by the grooves and their coupling with the dynamics of the near wake. Taking the correlation of the first two POD mode coefficients as the phase indicator, the phase-dependent variations of the flow fields and wall-pressure fluctuations were determined. Finally, the POD analysis of the three-dimensional velocity fields confirmed that the vortical structure on the spanwise direction for the grooved cylinder was more scattered and irregular. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

While treatment options are available, excessive daytime sleepiness (EDS) remains a significant unmet medical need for many patients. Relatively little rodent behavioural pharmacology has been conducted in this context to assess potential pro-vigilant compounds for their ability to restore functional capacity following experimentally induced sleep loss. Male Wistar rats were prepared for electroencephalographic (EEG) recording and subject to 11 h of sleep restriction using a biofeedback-induced cage rotation protocol. A simple response latency task (SRLT) was used to behaviourally index sleep restriction and the effects of pro-vigilant compounds: modafinil, d-amphetamine, caffeine, and the mGlu5-positive allosteric modulator LSN2814617. Sleep restriction resulted in a consistent, quantified loss of non-rapid eye movement (NREM) and REM sleep that impaired SRLT performance in a manner suggestive of progressive task disengagement. In terms of EEG parameters, all compounds induced wakefulness. Amphetamine treatment further decreased SRLT performance capacity, whereas the other three compounds decreased omissions and allowed animals to re-engage in the task. Caffeine and modafinil also significantly increased premature responses during this period, an effect not observed for LSN2814617. While all compounds caused compensatory sleep responses, the magnitude of compensation observed for LSN2814617 was much smaller than would be predicted to result from the prolongation of wakefulness exhibited. Using simple response latencies to index performance, an mGlu5 PAM dramatically increased wakefulness and improved functional capacity of sleep-restricted animals, without eliciting a proportionate compensatory sleep response. This effect was qualitatively distinct from that of amphetamine, caffeine and modafinil. [ABSTRACT FROM AUTHOR]

Experimental measurements were conducted on a plunging Eppler 361 Gurney flapped airfoil to study wake structure and dynamic stall phenomenon in the wake. The heights of Gurney flap were 2.6 and 3.3% chord. All oscillation data were taken at the plunging amplitude of 6 cm and Reynolds number 1.5 × 105. Special attention was focused on the temporal progressions of the plunging wake for the range of initial angle of attack (0° and 12°) in prior and post-stall flow conditions. The velocity in the wake was measured by hot-wire anemometry. Surface pressure measurements, as supplementary data, were also carried out to look into the link between the boundary-layer flow and the shedding vortical flow. It was found that the hysteresis is detected between the plunging wake in the upstroke and down-stroke. The shape and width of the wake hysteresis loops strongly depend on the initial angle of attack and vertical positions of the sensor. At prior static-stall angle of attack, positive camber effects of flapped airfoil shifted wake profiles downward and more velocity deficits were detected. In the post-stall conditions the hysteresis loop widths for lower-than-centerline vertical positions were remarkably more than counterpart upper positions. The energetic dynamic stall vortex shedding was found to be main responsible for large hysteresis and velocity deficit at these positions. Furthermore, the extent and strength of the stalled wake or flow separation were found to increase for the flapped case which results in a significant increase in the hysteresis loop widths. [ABSTRACT FROM AUTHOR]

A quasi-steady-state approach to the prediction of fluid forces acting on an insect wing leads to errors that suggest that flight is impossible. However, because biological flight does occur, the effect of unsteady fluid forces must be important to flight. The propulsive forces of these animals are related to unsteady fluid forces accompanied with the movement of vortices. Therefore, unsteady fluid forces must also be considered when estimating the propulsive force of swimming the front crawl and the flat scull technique in synchronized swimming. The purpose of this study was to investigate the effect of a three-dimensional airfoil shape on three-dimensional vortex structure for a high reduced frequency. The vortex structure and its behavior for a discoid airfoil were investigated. Furthermore, the vortex structure of the discoid airfoil was compared with that of a rectangular. The vortical flow fields were measured in the wake of the discoid airfoil using a stereoscopic particle images velocimetry technique. The three-dimensional vortex structure was depicted by plotting iso-vorticity surface calculated from the reconstructed velocity data. Compared with the rectangular airfoil, a strong vortex exists over a prolonged period of time in the wake of the discoid airfoil. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

A time-domain simulation tool to predict the dynamic power output of wind turbines in an offshore wind farm was developed in this study. A wind turbine model consisting of first or second order transfer functions of various wind turbine elements was combined with the Ainslie's eddy viscosity wake model to construct the simulation tool. The wind turbine model also includes an aerodynamic model that is a look up table of power and thrust coefficients with respect to the tip speed ratio and pitch angle of the wind turbine obtained by a commercial multi-body dynamics simulation tool. The wake model includes algorithms of superposition of multiple wakes and propagation based on Taylor's frozen turbulence assumption. Torque and pitch control algorithms were implemented in the simulation tool to perform max-Cp and power regulation control of the wind turbines. The simulation tool calculates wind speeds in the two-dimensional domain of the wind farm at the hub height of the wind turbines and yields power outputs from individual wind turbines. The NREL 5MW reference wind turbine was targeted as a wind turbine to obtain parameters for the simulation. To validate the simulation tool, a Danish offshore wind farm with 80 wind turbines was modelled and used to predict the power from the wind farm. A comparison of the prediction with the measured values available in literature showed that the results from the simulation program were fairly close to the measured results in literature except when the wind turbines are congruent with the wind direction. [ABSTRACT FROM AUTHOR]

The three-dimensional dune wake flow is evaluated by large eddy simulation at a Reynolds number of 5,534, and validated by time-averaged results of particle image velocimetry measurement at a central streamwise plane. To reveal multi-scale characteristics of vortical structure, the instantaneous velocity, vorticity, and pressure were decomposed into the large-, intermediate- and small-scale components by three-dimensional wavelet multi-resolution technique, the scale of each wavelet component is quantified by two-point autocorrelation function with the central scale of 43, 20 and 9.6 mm, respectively. It is found that large-scale structure determines the formation of separation bubble and makes the most significance to the vorticity. Some intermediate-scale and small-scale streamwise vortices cause the upwelling of vortical structures, and they tend to be more active at the downstream. By visualization of pressure distribution, we can find that pressure distribution is mainly characterized by large-scale structure, and the distribution of small-scale structure can be interfered as a reason why large-scale vortical structure breaks into small vortices at the near outlet region. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]