Occurrence Of Vortices Research Articles

Intrinsic characteristics of three-dimensional flow around a wall-mounted conical cylinder

To investigate the flow characteristics around a wall-mounted conical cylinder, three-dimensional direct numerical simulations are carried out for a conical cylinder with an end diameter ratio ranging from 0.5 to 1.5 at three low Reynolds numbers (Re = 100, 150, 200). The flow features are examined in terms of time-mean streamlines, singularity points location, topological structure, time-mean streamwise vortices, and instantaneous spanwise vortices denoted with different vortex identification methods. The downwash effect also happens even without a free end. The upwash streamlines clash with the downwash streamlines and then coalesce to the saddle (impingement) point P2. The occurrence of the symmetry plane belongs to a new topology, where the saddle point on the bottom wall is an attachment point (NA), instead of the separation point (SS). The singular point verifies the topological existence of the attachment–attachment combination of the horseshoe vortex system. The “Quadrupole Type,” “Sextupole Type,” and “Octupole Type” are identified. The “Octupole Type” is reported first, consisting of a pair of “time-mean streamwise tip vortices,” two pairs of “time-mean streamwise base vortices” and a pair of “time-mean streamwise bottom vortices.” Moreover, the vorticity magnitude cannot represent the occurrence of vortices. The instantaneous iso-surfaces of Q = 0.2 and λ2 = –0.2 in the wake are similar to the threshold of Ω = 0.52. In contrast, the Liutex/Rortex method is easier to identify the streamwise bottom vortices than the former two methods.

Planning of a Single Flow Channel in Valve Blocks Based on Additive Manufacturing and the Ant Colony Algorithm

As electro-hydrostatic actuator (EHA) technology advances towards lightweight and integration, the demand for enhanced internal flow pathways in hydraulic valve blocks intensifies. However, owing to the constraints imposed by traditional manufacturing processes, conventional hydraulic integrated valve blocks fail to satisfy the demands of a more compact channel layout and lower energy dissipation. Notably, the subjectivity in the arrangement of internal passages results in a time-consuming and labor-intensive process. This study employed additive manufacturing technology and the ant colony algorithm and B-spline curves for the meticulous design of internal passages within an aviation EHA valve block. The layout environment for the valve block passages was established, and path optimization was achieved using the ant colony algorithm, complemented by smoothing using B-spline curves. Three-dimensional modeling was performed using SolidWorks software, revealing a 10.03% reduction in volume for the optimized passages compared with the original passages. Computational fluid dynamics (CFD) simulations were performed using Fluent software, demonstrating that the algorithmically optimized passages effectively prevented the occurrence of vortices at right-angled locations, exhibited superior flow characteristics, and concurrently reduced pressure losses by 34.09%–36.36%. The small discrepancy between the experimental and simulation results validated the efficacy of the ant colony algorithm and B-spline curves in optimizing the passage design, offering a viable solution for channel design in additive manufacturing.

Моделирование гидродинамики жидкого металла в ячейке МГД-сепаратора

Ensuring the purity of metals and alloys is a key problem in both the metallurgical and nuclear industries. The difference in the electrical conductivity of the molten metal and impurity particles allows their separation from each other in a non-contact way using an electromagnetic force. In addition to the implementation of phase separation, such force inevitably sets the liquid metal in motion, changing the velocity distribution in the separation cells. This effect is most pronounced in channels of rectangular geometry and therefore should be studied extensively. In this paper, we numerically study the hydrodynamics in a separation cell in the presence of partitions of various configurations and in the presence or absence of an external weighting electromagnetic force with the aim to determine the cell geometry providing the most efficient separation and accumulation of impurity particles in it during the cyclic pumping of liquid metal. It has been found that the intense force action leads to the occurrence of parasitic vortices in the channel, which reduce the efficiency of the separation process. The initial flow velocity does not practically affect the topology of the flow in the channel, whereas the height and configuration of the baffles inside the channel, together with the magnitude of the external weighting force have a significant effect on the flow of liquid metal. It is shown that in the zones between the baffles, the parasitic vortices that could potentially facilitate the washout of impurity from this space are not formed. Based on the results of numerical study the optimum configuration of the MHD-separator cell is selected. It corresponds to the mode with the most effective separation according to four parameters: partition height, magnitude of the external weighting electromagnetic force, flow velocity, and topology of intermediate partitions.

Numerical analysis of turbulent nitrogen-diluted hydrogen flames stabilised by star-shaped bluff bodies

In this paper, we investigate the flow and flame dynamics within a combustion chamber supplied with nitrogen-diluted hydrogen fuel and air as oxidiser. The chamber is equipped with a bluff body that serves as a flame stabiliser. We focus primarily on the influence of the bluff body shape on the formation of large and small-scale vortical structures generated at the bluff body edge. We analyse how the geometry of the bluff body alters these structures and examine the impact of resulting modifications on flame characteristics, including key features such as flame length and radial extent, temperature and species field, and completeness of the combustion process. We consider four bluff body geometries, namely a typical cylindrical shape, a star-shaped wall, and a small triangular sharp and smooth wavy structure. The research employs the Large Eddy Simulation (LES) method, providing deep insight into the complex physics of unsteady flow. In the cylindrical bluff body configuration, the key mechanism driving the fuel-oxidiser mixing process is the periodic occurrence of large toroidal vortices shed at an exit of the oxidiser duct and on the bluff body edge. They exert the flow towards and away from a recirculation zone formed above the bluff body. When its geometry is irregular, the appearance of the large vortices is suppressed, however, the wall irregularities induce small-scale flow phenomena that enhance mixing in the shear layer formed between the recirculation zone and the oxidiser stream. The mixing efficiency is largely dependent on the shape of the bluff body wall, and it turns out that seemingly small changes (sharp vs. smooth small triangular parts), which in non-reactive regimes have only a minor effect on the flow dynamics, significantly change the flame length and fuel consumption rate. Compared to the cylindrical bluff body configuration, the small triangular parts cause an axial shift of the maximum temperature downstream and slow down fuel consumption. Contrary, large triangular elements in the star-shape bluff body clearly influence the flow both in non-reactive and reactive regimes. They cause the formation of local oxidiser streams directed towards the fuel-rich mixture. Consequently, the injected fuel burns at a rate similar to that observed in the case of the cylindrical bluff body, but the combustion process occurs at an average temperature approximately 200 K lower and its fluctuations are reduced. These factors may influence the thermal reduction of NOx. On the other hand, a negative aspect is the highest level of hydrogen in the closest vicinity of the bluff body, which in practical applications may accelerate the embrittlement of its surface.

Experiments on the evolution process of intermittent air-entraining vortices and its influencing factors

Surface vortices are a common hydraulic phenomenon at water intakes that adversely affect safe engineering operations. The complexity of vortex motion, the diversity of influencing factors, and the limitations of research methods make the formation and development mechanisms of vortices unclear. Some analytical and experimental studies on intermittent air-entraining vortices occurring at the horizontal circular intake have been conducted to explore the mechanism of the dynamic characteristics of vortices. Based on experimental observations, the morphologies and dynamic characteristics of intermittent air-entraining vortices at different moments during the entire evolution process were studied, and the velocity field in the vortices was measured using PIV. The evolution processes of intermittent air-entraining vortices comprised inception, rapid development, stability, decay and regeneration, and the collapse and disappearance stages. The tangential velocity, kinetic energy, and vorticity increased over time, reached a maximum, and subsequently decreased. When they decrease to a certain value, they begin to increase before decreasing owing to the decay and regeneration stages. Additionally, we introduced the frequency of intermittent air-entraining vortex occurrence (P) to analyse the direct effect of the Froude number, intake submergence, and rear wall on the occurrence of air-entraining vortices. Based on the analysis of the test results, the effect of intake submergence on P was more significant than that of the intake Froude number in terms of flow conditions. By considering the effects of the flow conditions and rear wall, a method for predicting the possibility of the occurrence of intermittent air-entraining vortices was proposed.

Seston biomass in plankton assemblages in the Southwestern Atlantic Ocean: spatial, vertical, and temporal variations.

Bioseston is a heterogeneous assemblage of bacterioplankton, phytoplankton, zooplankton, and planktonic debris. A detailed knowledge of biosestons is essential for understanding the dynamics of trophic flows in marine ecosystems. The distributional features of seston biomass in plankton (micro- and mesoplankton) in the Southwest Atlantic Ocean (Rio de Janeiro State, Brazil) were analyzed using stratified samples gathered to a depth of 2,400 m during night time. The horizontal pattern of biomass distribution was analyzed vis-a-vis station depth during both wet and dry periods, with higher values recorded in the continental shelf than in the slope, confirming the terrestrial contribution of nutrient sources to the marine environment. This horizontal variation reinforces the occurrence of seasonal vortices in Cabo Frio and Cabo de São Tomé on the central coast of Brazil. Environmental variables reflect the hydrological signatures of the water masses along the Brazilian coast. The largest seston biomass was related to high temperatures, salinities, and low inorganic nutrient concentrations in tropical and South Atlantic central waters. The observed distribution patterns suggest that seston biomass in plankton in the region may be structured based on partitioned horizontal and vertical habitats and food resources.

Mesoscale Cyclonic Vortices Embedded in the South Atlantic Convergence Zone Associated with Natural Disasters in the State of São Paulo, Brazil

Natural disasters (NDs) have been observed more frequently and with increasing intensities in Brazil. The South Atlantic Convergence Zone (SACZ) is identified as one of the main meteorological systems responsible for the NDs, however, intense rainfall does not occur along its entire length but is restricted to some locations within the band of cloudiness that defines it. Thus, the objective of this study is to analyze occurrences of mesoscale cyclonic vortices (MCV) in SACZ events that were associated with NDs in the state of São Paulo from 2013 to 2017 using data from the ERA5 Reanalysis, as well as to analyze one case study. To account for SACZ events, surface synoptic charts, observed and estimated precipitation data were used. ND events were selected from the Integrated Disaster Information System (S2ID) database. The methodology used by Quadro (2012) was adapted to identify MCV. The results showed 62 SACZ events, of which 28 were associated with NDs, and, of these, 10 presented MCV. The MCVs were separated into two groups: 1) MCVs in the SACZ events that showed precipitation at the location of the MCV and NDs and 2) MCVs in the SACZ events that did not show precipitation at the location of the MCV and NDs. Group 1 events were characterized by convergence at low levels and divergence at high levels of the atmosphere, vorticity values lower than -8 x 10-4 s-1 predominating at low levels (850–900 hPa), demonstrating a relationship with the highest precipitation accumulations and possibly with the occurrence of NDs. In the events of group 2, there was a predominance of negative values of vorticity in medium and high levels, the lack of a pattern in the field of divergence in the atmospheric levels, as well as lower values in the accumulated precipitation compared to the events of group 1. The case study was from January 11 to 15, 2016, associated with NDs in 8 cities. As a result, it was obtained that MCV was coupled in the atmosphere and the precipitation associated with it represented more than 37% of all the precipitation of the SACZ event, making it possible to attribute to the MCV a contribution in the occurrence of NDs caused by precipitation.

Direct numerical simulation of adverse pressure gradient turbulent boundary layer up to [formula omitted

A direct numerical simulation of a moderate adverse pressure gradient turbulent boundary layer flow on a flat plate was performed on a fairly long domain with a Reynolds number from Reθ≃2000 to Reθ≃8000. The mean streamwise velocity profile does not exhibit a clear log region even at the highest Reynolds number. The Reynolds stresses exhibit a well defined peak in the outer region which moves away from the wall before to stabilise at a wall distance y of 0.45 boundary layer thickness after evolving over 20 local boundary layer thicknesses. These outer peaks are the consequences of an excess of production over dissipation and they all scale with the shear stress velocity for wall distance 0.2δ<y<0.85δ. The different terms of the Reynolds stresses equations are analysed in the outer region and the analysis confirms that the leading terms are the same as for a zero pressure gradient turbulent boundary layer except that the production and dissipation are not negligible in the outer region. The most significant contribution of scales smaller than two Taylor scales is on the pressure strain term of the shear stress equation. The probability of the occurrence of small scale vortices conditioned by the presence of an ejection or a sweep shows that the shear stress contained in ejections is associated with the production and transport of near wall vortices away from the wall. The local mean shear generated by the sweeps increases the production of near wall vortices. Lastly, the large scale structures analysed by means of two point correlations of the streamwise fluctuating velocity are shown to be strongly modified by the adverse pressure gradient. The correlation reveals a decoupling between the near wall streaks and the large scale streamwise structures enhanced by the adverse pressure gradient.

Mapping of the flow structure and hydrodynamic properties of a round-ended cylinder

This paper reports the numerical results of flow past a round-ended cylinder with various incidence angles in a low Reynolds number range of Re = 60–160. Mapping of the flow structure and hydrodynamic properties is examined in the incidence angle range of α = 0°–90° with increment of 15°. Three wake patterns are identified, including the steady and symmetric wake without vortex shedding (Pattern I), the Karman vortex street (Pattern II), and the Karman vortex street with the occurrence of subordinate vortex (Pattern III). The reattachment of boundary layers results in the occurrence of a subordinate vortex and hence the non-single-frequency fluctuation of hydrodynamic coefficients (CL and CD, which are lift and drag coefficients, respectively). Elliptical and figure-eight CL–CD curves are observed, depending on the frequency ratios of the two coefficients and their weights. Non-zero time-averaged CL occurs when 0° &amp;lt; α &amp;lt; 90°, due to the asymmetric boundary layer separation. The backward migration of boundary layer separation point contributes to the reduction of frictional drag and the vortex formation length. The shortening of the vortex formation length results in the enhanced fluctuations of CL and CD.

Design and Analysis of a Vertical Electric Ducted Fan for Unmanned Aerial Vehicles

Several types of multirotor unmanned aerial vehicles (UAVs) are widely used in various industrial activities. UAVs with electric ducted fans (EDFs) have been developed to mitigate the risks associated with rotary blades, which are installed on the exterior structure of conventional UAVs and exposed to the ambient environment. Particularly, vertical EDFs have become a focus in the field of UAV design because they require less space for the flow channel compared with horizontal EDFs. In this study, a vertical EDF UAV model was developed. Structural and flow field analysis was performed in Ansys software to determine the optimal design configuration. The results confirmed that the designed model facilitated smooth air flow with favorable inlet and outlet velocity. The vehicle structure required minor adjustment to reduce the frequency of occurrence of vortices. Overall, this study made two research contributions. First, an EDF UAV model was developed to mitigate the risk of rotary blades being exposed to the ambient environment. Second, a vertical EDF system was proposed to reduce the flow channel space and maintain the outlet velocity, thereby improving the takeoff, landing, and thrust performance of UAVs.

Influence of relative thickness on static and dynamic stall characteristics and prediction of airfoils

A study was conducted to investigate the effects of relative thickness on the static and dynamic characteristics of airfoils. A series of NACA symmetric airfoils with relative thickness ratios ranging from 12% to 30% have undertaken a series of sinusoid pitching motion simulations and these airfoils' Glasgow wind tunnel experiment data were analyzed. It was found that the relative thickness of airfoils would affect the stall onset angle of attack (AOA), the severity of the stall, and the dynamic stall model parameters. As the relative thickness of airfoils increases, static and dynamic stall onset AOA decrease and stall occurs earlier. At the static AOA, the thickest airfoil shows a very large reverse flow region. At the dynamic stall onset AOA, the thinner airfoil exhibits a more obvious burst of the laminar separation bubble. When the relative thickness of the airfoil is smaller, a dynamic stall occurs with more severity. On the one hand, the aerodynamic characteristic curve and suction surface pressure curve of airfoils change are more complicated. And the severity of the dynamic stall was quantitatively compared by extracting the difference of lift coefficients corresponding to the same AOA in the upstroke and downstroke stages. On the other hand, vortex occurrence, convection and shedding on the suction surface of the airfoil are more frequent and complicated. This paper also gives a method to solve the problem of the lack of the dynamic stall model parameters of blades section airfoils when predicting the wind turbine's unsteady force.

Flow Control around the UAS-S45 Pitching Airfoil Using a Dynamically Morphing Leading Edge (DMLE): A Numerical Study

This paper investigates the effect of the Dynamically Morphing Leading Edge (DMLE) on the flow structure and the behavior of dynamic stall vortices around a pitching UAS-S45 airfoil with the objective of controlling the dynamic stall. An unsteady parametrization framework was developed to model the time-varying motion of the leading edge. This scheme was then integrated within the Ansys-Fluent numerical solver by developing a User-Defined-Function (UDF), with the aim to dynamically deflect the airfoil boundaries, and to control the dynamic mesh used to morph and to further adapt it. The dynamic and sliding mesh techniques were used to simulate the unsteady flow around the sinusoidally pitching UAS-S45 airfoil. While the γ-Reθ turbulence model adequately captured the flow structures of dynamic airfoils associated with leading-edge vortex formations for a wide range of Reynolds numbers, two broader studies are here considered. Firstly, (i) an oscillating airfoil with the DMLE is investigated; the pitching-oscillation motion of an airfoil and its parameters are defined, such as the droop nose amplitude (AD) and the pitch angle at which the leading-edge morphing starts (MST). The effects of the AD and the MST on the aerodynamic performance was studied, and three different amplitude cases are considered. Secondly, (ii) the DMLE of an airfoil motion at stall angles of attack was investigated. In this case, the airfoil was set at stall angles of attack rather than oscillating it. This study will provide the transient lift and drag at different deflection frequencies of 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, and 10 Hz. The results showed that the lift coefficient for the airfoil increased by 20.15%, while a 16.58% delay in the dynamic stall angle was obtained for an oscillating airfoil with DMLE with AD = 0.01 and MST = 14.75°, as compared to the reference airfoil. Similarly, the lift coefficients for two other cases, where AD = 0.05 and AD = 0.0075, increased by 10.67% and 11.46%, respectively, compared to the reference airfoil. Furthermore, it was shown that the downward deflection of the leading edge increased the stall angle of attack and the nose-down pitching moment. Finally, it was concluded that the new radius of curvature of the DMLE airfoil minimized the streamwise adverse pressure gradient and prevented significant flow separation by delaying the Dynamic Stall Vortex (DSV) occurrence.

Activating magnetoelectric optical properties by twisting antiferromagnetic bilayers

Twisting in bilayers introduces structural chirality with two enantiomers, i.e., left- and right-hand bilayers, depending on the oriented twist angle. The interplay between this global chirality and additional degrees of freedom, such as magnetic ordering and the local octahedral chirality arising from the geometry of the bonds, can yield striking phenomena. In this work, we focus on collinear antiferromagnetic CrI$_3$ twisted homo-bilayers, which are characterized by a staggered octahedral chirality in each monolayer. Using symmetry analysis, density functional theory and tight-binding model calculations we show that layers twisting can lower the structural and magnetic point-group symmetries, thus activating pyroelectricity and the magneto-optical Kerr effect, which would otherwise be absent in untwisted antiferromagnetic homo-bilayers. Interestingly, both electric polarization and Kerr angle are controllable by the twist angle and their sign is reversed when switching from left- to right-twisted bilayers. We further unveil the occurrence of unconventional vortices with spin textures that alternate opposite chiralities in momentum space. These findings demonstrate that the interplay between twisting and octahedral chirality in magnetic bilayers and related van der Waals heterostructures represents an extraordinary resource for tailoring their physical properties for spintronic and optoelectronic applications.

Observation of vortices and vortex stripes in a dipolar condensate

Quantized vortices are a prototypical feature of superfluidity that have been observed in multiple quantum gas experiments. But the occurrence of vortices in dipolar quantum gases—a class of ultracold gases characterized by long-range anisotropic interactions—has not been reported yet. Here we exploit the anisotropic nature of the dipole–dipole interaction of a dysprosium Bose–Einstein condensate to induce angular symmetry breaking in an otherwise cylindrically symmetric pancake-shaped trap. Tilting the magnetic field towards the radial plane deforms the cloud into an ellipsoid, which is then set into rotation. At stirring frequencies approaching the radial trap frequency, we observe the generation of dynamically unstable surface excitations, which cause angular momentum to be pumped into the system through vortices. Under continuous rotation, the vortices arrange into a stripe configuration along the field, in close agreement with numerical simulations.

Generation of nonuniform vortices based on metasurfaces and their feature characterization

In view of the wide applications of vortices and the inevitable occurrence of nonuniform vortex in practice, the generation of nonuniform vortices and their feature characterization are proposed. Nonuniform vortices are different from uniform vortices, and their helical phase does not change uniformly with the azimuthal angle. The possible models of nonuniform vortices including several cases with nonlinear phase in equal intervals, linear phase in unequal intervals, and nonlinear phase in unequal intervals are proposed. The feature characterization for the phase and intensity distributions of nonuniform vortices are performed using the forked fringes and the far-diffraction of forked fringes. These nonuniform vortices are also produced with the help of vortex metalenses consisting of rotated nanoholes. The study about the nonuniform vortices is helpful for understanding and estimating practical vortices.

The interaction analysis between liquid-solid two-phase flow and equipment components in a molten salt pump

Latent heat energy storage is an important issue in the solar energy engineering. As energy storage materials, molten salts release some of latent heat and crystallize. The particles of salt crystals probably collide and rub with the contact parts in the process of movement, and the colliding and rubbing leads to the abrasion or damage of the contact parts. In this study, a dimensionless analysis of the solid-liquid two-phase flow transported by molten salt pump (MSP) was carried out by Buckingham's theorem and the similarity criterion, and the numerical model verified by the theory was used to analyze the flow field in the pump. To accurately evaluate the wear-prone position of the MSP, dozens of monitoring points were arranged on the blade and near the volute tongue. The effects of multiple factors such as the solid particle size, pump inlet particle volume fraction (IPVF), span, and flow rate on the internal flow field and stability of the MSP were studied. The results show that the solid particles mainly concentrate at the inlet and outlet ends of the suction surface of the blades, as well as near the volute tongue. Under low flow rate conditions, the IPVF has a significant influence on the flow trajectory with large-sized particles, whereas has little influence on that of the small-sized particles; when the solid particle size is large, the occurrence of vortex and back flow decreases with the increase of IPVF. The IPVF, curvature of the blade and span have a certain impact on the pressure load, and the greatly affected areas are mainly distributed near the blade head and tail. The research results have certain reference value for improving the anti-wear performance and stable operation of MPSs.

On The Existence, Formation and Stabilisation Of Lyne Vortices In Helically Coiled Reactors At Moderate Reynolds Numbers

The occurrence of Dean and Lyne vortices in curved and coiled tubes is widely known. This paper is on the tracks of vortex structures in more resent helically coiled reactors, namely the Coiled-Flow-Reverser (CFR) and the Coiled-Flow-Inverter (CFI) (see top of figure 1). Time-resolved laser induced fluorescence (LIF) and time-resolved particle image velocimetry (TR-PIV) are used to characterize the existence of Lyne vortices in a range of Reynolds numbers from 500 to 2400 for single phase liquid-liquid mixing flows (see centre of figure 1). In this study parameters like the Reynolds number, the inlet angle and the geometry of the helically coiled reactor are varied and their influence on the vortex occurrence and the mixing coefficient is shown. The results are then compared to two-phase flow structures in helical pipes from Müller et al. (2021), in which secondary and tertiary vortices inside the liquid plugs exist as well. Two transition points for the formation of Lyne vortices in the CFI could be characterised. While Lyne instabilities occur in straight horizontal helix geometries at Reynolds numbers above 1000, in the CFI the first Lyne vortices formed at a Reynolds number of 560. From a Reynolds number of 1600 onwards, these begin to break off, whereby the Lyne vortices continue to occur continuously but no longer steadily. Pseudo-3D reconstructions of the flow in the CF and the CFI are used to show the flow behaviour and the occurrence of Lyne instabilities for different conditions.

Numerical Investigation of the Power Performance of the Vertical-Axis Wind Turbine with Endplates

An H-rotor vertical axis wind turbine (VAWTs) can operate independently in any wind direction, making it aerodynamically efficient and suitable to harness wind energy in low wind speed areas. The aerodynamic efficiency of VAWTs is highly dependent on the blade geometry, especially the blade tip. Tip vortices produced at the blade tips can negatively affect the VAWT’s aerodynamic efficiency. Adding endplates to the blade tips can minimize the effects of tip vortices on VAWTs. In this paper, several endplate designs are used to evaluate the effectiveness in improving the power coefficient, Cp of a VAWT at three different tip speed ratios (TSRs) using three-dimensional computational fluid dynamics (3D CFD) simulation. The power coefficients of VAWTs with endplates are compared with the baseline model with the same geometrical parameters where the baseline VAWT model is based on the experimental model from the literature. Since the focus of this study is on the blade tip design, a simplified 3D VAWT model is used where the supporting shaft and arms of the VAWT are excluded to reduce the needed computational capacity. Among the various endplate designs used in this study, the semi-circular inward endplate (ED3) with a diameter equivalent to 1.2 blade chord length showed the best improvement in the Cp which is by 7.45%, and 5.79% for at the TSRs of 2.19 and 2.58, respectively. The pressure difference on both sides of the blade was also examined. The results revealed that the endplate can prevent the flow from bypassing the blade tip, hence, preventing the occurrence of tip vortices while improving the aerodynamic efficiency near the blade tip, ultimately, improving the overall Cp of a VAWT.

Experimental study of the effect of unheated starting segment upon flame spread over solid fuel under forced airflow

In the present work, to quantify the effects of an unheated starting length on the wind-driven flame spread rates, the flow field structure and flame characteristics over a horizontally placed thermally-thick poly (methyl methacrylate) slab (10 cm long, 10 cm wide, and 1.5 cm thick) are experimentally investigated for different lengths of an unheated segment (0–30 cm), where PMMA flames are stabilized under a concurrent forced airflow velocity of 0.75 m/s. The flow field, flame structure, flame spread rates, and flame standoff distances are recorded, measured, and calculated. The results show that the flow field in a reacting boundary layer is noticeably different for varying unheated lengths, which can be attributed to the reverse flow diffusion and vortex occurrence, which significantly affects the flame structure and flame spread rates. The laminar, reverse diffusion, and streaky flame regions can be easily demarcated with increasing unheated starting lengths. Eventually, the flame standoff distance, burning, and flame spread rates seem to have a non-monotonic dependence on unheated starting lengths. The work presented here reveals the functional mechanism of an unheated segment in reacting boundary layers and presents useful information that could enhance our understanding of the wind-driven flame spread rates.

Observational study on drag reduction of continental-shelf bottom boundary layer

Long-term and high-frequency observing of the oceanic bottom boundary layer was implemented using a seabed-mounted tripod equipped with multiple high-frequency instruments in a semidiurnal tide-dominated channel around the Zhoushan Islands, China. The estimated turbulent parameters, e.g., turbulent intensity, friction velocity, and bottom drag coefficient, varied with quarter-diurnal frequency. The amplitudes of turbulent intensity, bottom stress, and friction velocity were larger in spring tide than those in neap tide. As the Reynolds number increased from 103 to 105, the measured bottom drag coefficient initially increased, peaked at a Reynolds number of approximately 1.4 × 105, and then decreased. The observed drag reduction is similar to the variations of drag coefficient of cross-flow around a circular cylinder and the fluid friction of the pipe flow in the transitional state, which can provide a necessary and important proof for further theoretical and experimental investigations. Several plausible explanations are raised for this reduction: the stratification caused by the gradient of salinity or sediment suspension and the occurrence of recirculation vortices generated by seabed topography.