Laminar free-surface flow around emerging obstacles: Role of the obstacle elongation on the horseshoe vortex
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Oscillation (cell signaling)
The article [10] has presented the necessity of the classification into two types: Isolated wakes and coupled wakes. In this report, different types of momentum defect diffusion at the beginning of wake are analyzed for the two types of coupled wake and isolated wake. According to ·numerical results obtained, we analyses the existence - long or short - of the wake establishment zone before the wake established zone, and this zone is very different between isolated wake and coupled wake.
Momentum (technical analysis)
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A low-order panel code, VSAERO, was used to predict the aerodynamic characteristics of helicopter hub fairings. Since the simulation of this kind of bluff body by VSAERO was not documented before, the VSAERO solutions were correlated with experimental data to establish their validity. The validation process revealed that simulation of the aerodynamic environment around a hub fairing was sensitive to several modeling parameters. Some of these parameters are body and wake panels arrangement, streamwise and spanwise separation location, and the most prominent one-the wake modeling. Three wake models were used: regular wake, separated wake, and jet model. The regular wake is a wake with negligible thickness (thin wake). It is represented by a single vortex sheet. The separated wake and the jet model in the present application are wakes with finite thickness (thick wake). They consist of a vortex sheet enclosing a region of low-energy flow. The results obtained with the reqular wake were marginally acceptable for sharp-edged hub fairings. For all other cases under consideration, the jet model results correlated slightly better. The separated wake, which seemed to be the most appropriate model, caused the solution to diverge. While the regular wake was straight-forward to apply in simulations, the jet model was not. It requires the user to provide information about the doublet strength gradient on wake panels by guessing the efflux velocities at the wake shedding location. In summary, VSAERO neither predicts accurately the aerodynamic characteristics of helicopter hub fairings nor was cost effective.
Wake turbulence
Aerodynamic force
Flapping
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The interaction of vortices with fixed boundaries is an important problem in many areas of practical interest. During the approach of the vortices to the wall, vorticity is created at the wall, and then mixing and stirring of primary and secondary vorticity lead to different scenarios that depend on the Re number. This problem is relevant to bursting in turbulent boundary layers, which is responsible for turbulence production. In the near-wall region of turbulent flows, vortex stretching plays a fundamental role. As a prelude to attempting the solution by direct simulation of 3-D Navier–Stokes equations, it is convenient to describe the mechanism associated with 2-D or axisymmetric flows. In the present study, axisymmetric vortex rings and vortex dipoles approaching a nonslip flat wall have been considered. In both cases, experimental results based on flow visualization are available.1,2 The purpose of the present paper is to do a numerical simulation with the same conditions used in the real experiments and follow the flow evolution for a much longer time than has been possible in such experiments. Both for axisymmetric and 2-D cases the numerical simulation shows, as revealed by the experiments, that, at the initial stage, very thin layers of vorticity of sign opposite to that of the primary vortex are generated at the wall. This mechanism is almost independent of Re. This thin and intense layer immediately becomes unstable and rolls up creating a secondary vortex. This vortex interacts with the primary vortex and different flow structures are generated, depending on Re. The location where these structures form depends, in large measure, on whether vortex stretching is present or not. Particularly relevant to the bursting phenomenon is the case at intermediate Re (e.g., Re=1600). Here, a new dipole is created near the wall after multiple rebounding and pairing of secondary vortices. This new structure has sufficient strength to move itself away from the wall. We conclude that mixing and stirring of vorticity are the relevant mechanisms influencing the whole process in a two-dimensional simulation. This ejection of a new structure was previously noticed in the interaction of vortex rings with a wall. From flow visualization, Walker et al., at Re=Γ/ν>3500, observed the occurrence of azimuthal instabilities and, immediately after that, they noticed a new vortex ring was rapidly ejected away from the wall.2 From these observations, those authors argued that the ejection was due to the azimuthal instabilities. In the present paper, axisymmetric calculations have been done in the same range of numbers used in the experiment of Walker et al.2 From the time evolution of the vorticity distribution, we argue that vortex pairing is the main mechanism that contributes to the generation of the new ring rather than azimuthal instabilities. The numerical simulations clearly show that this new ring is created above the primary ring and not at its interior.
Vortex stretching
Burgers vortex
Starting vortex
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A series of experiments were conducted to study the effect of obstacle on the evacuation efficiency of mice under different competitive condition. Firstly, experiments were conducted under low competitive condition. The mice were drove to escape in two typical settings. Setting I is an area of 1.5m wide and 1.5m long and Setting II is a laterally constrained area. For both conditions, tests with and without the obstacle were conducted to study the impact of obstacle on the efficiency of evacuation. At setting I, the average escape time is 305s and 298s respectively for non-obstacle condition and obstacle condition. At setting II, the average escape time is 313s and 298s respectively for non-obstacle condition and obstacle condition. Under low competitive condition, the presence of obstacle has little effect on the evacuation efficiency for both settings. Secondly, experiments were conducted in the same two setting with and without the obstacle under high competitive condition. At setting I, the average escape time is 334s and 297s respectively for non-obstacle condition and obstacle condition. At setting II, the mean escape time is 297s and 328s respectively for non-obstacle condition and obstacle condition. Under high competitive condition, the presence of obstacle reduces the escape time by 11.1% at setting I, whilst it increases the escape time by 10.4% at setting II. The force of mice on the wall close to the exit is recorded to study of impact of obstacle on the pressure.
Obstacle avoidance
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Aircraft wake is a kind of intense air movement, and the study of its generation, development, and dissipation law is of great significance to the flight safety. There are abundant researches on the evolution of aircraft wakes affected by weather and ground effects; however, there are few studies on the influence of a single obstacle on the evolution of aircraft wake. In this article, in order to explore the influence of a single obstacle on the evolution of aircraft wake, firstly, we develop a computational fluid dynamics-based method of simulation of aircraft wake affected by cubic obstacle of different heights in order to obtain the wake intensity changes and position changes before and after being affected by the obstacle. Then, the result data are visualized and analyzed, and we obtain the results of velocity and criterion contours, circulation, and data related to wake vortex structure. CFD simulations are conducted, including the cases of the vertical distance between wake vortex and obstacle which is 20 m, 60 m, 100 m, and no obstacle. The quantitative results indicate that a single obstacle also has a great influence on the evolution of the wake vortex. Obstacle will shorten the time for the wake vortex to enter the fast decay stage, and the smaller the distance the wake vortex is above the obstacle, the faster it enters the fast decay stage. In the same time, the circulation will reduce 20% more under the same calculation time when the wake is 20 m above the obstacle than when the wake is 100 m above the obstacle, and the circulation will reduce 45% more than when there is no obstacle. Single obstacle also leads to the generation of multiple secondary vortices and rotates around the wake vortex, resulting in the increase of wake vortex core radius, wake vortex core spacing, and wake vortex height.
Wake turbulence
Large-Eddy Simulation
Circulation (fluid dynamics)
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The strain-induced evolution of shielded monopolar vortices has been investigated in a stratified fluid. A steady strain flow was generated by an arrangement of four rotating horizontal discs, whereas the monopolar vortex was created by a small spinning sphere. Quantitative information about the flow field was obtained by tracking passive tracer particles. The vortex was observed to deform into a tripolar-like structure, followed by the shedding of the accompanying satellites. During this stage, the remaining vortex core evolved quasi-steadily, which was evident from the functional relationship between the vorticity and the stream function. Furthermore, it was shown that the removal of the surrounding ring of oppositely signed vorticity induces an accelerated horizontal growth of the vortex core. Owing to the diffusive decay of vorticity, the vortex was finally torn apart along the horizontal strain axis. The dynamics of the vortex core appeared to be very similar to that of an elliptic patch of uniform vorticity. The instantaneous vorticity contours at high vorticity levels were close to ellipses with nearly the same aspect ratios and orientations. Moreover, the observed vortex evolution was in qualitative agreement with the calculated motion of an elliptic patch of uniform vorticity. As a second approach, the full two-dimensional vorticity equation was solved numerically by a finite-difference method in order to account for both the non-uniform spatial vorticity distribution of the laboratory vortex and the diffusion of vorticity in the horizontal directions. The numerically obtained vortex evolution was in good agreement with that observed in the laboratory.
Vortex stretching
Burgers vortex
Starting vortex
Circulation (fluid dynamics)
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The flow around a circular cylinder mounted normal to the channel bed is essentially a three-dimensional one. The flow upstream undergoes a separation of the turbulent boundary layer and rolls up to form a flow system, known as horseshoe vortex, which is swept around the cylinder. This vortex system plays an important role, if the bed material is movable. The shear stress of the vortex system is responsible to the bed erosion. The shear stress depends on the velocity gradient, may be presented as the vorticity. This study is done to gain a better understanding of the vorticity around a cylinder, especially where the system of vortices exist. Flow measurements were used to study the vorticity fields on flow with a horse-shoe vortex system around a pile. The velocity vector plots show the presence of a primary vortex upstream of the cylinder. Based on these velocity vectors, the vorticity can be analyzed by using central finite difference approximations. Results of the vorticity calculation are presented as the vorticity contours. Results of study show the greatest value of the positive-vorticity on the horse-shoe vortex system is in the plane upstream of cylinder. This value decreases in the planes downstream, attaining the lowest value in the plane downstream, where the wake vortex established. The strength of the positive vorticity increases at larger Reynolds number. Underneath the field of positive vorticity, stretching around the cylinder, it appears a field of negative vorticity. This negative vorticity near the bed is high in front of the cylinder, proportional to the bed shear stress, which is responsible to the development of local scour at the movable bed. The line of zero vorticity is plotted originated at the bed, being as the separation point. It is also concluded that the maximum positive-vorticity is not necessary coincided with the center of the vortex.Abstrak. Pola aliran di sekitar silinder bulat yang dipasang tegak lurus dasar saluran merupakan aliran tiga dimensi Aliran di hulu silinder membentuk separasi pada lapis batas turbulen, membangkitkan terbentuknya pusaran tapal-kuda yang mengelilingi silinder. Sistem pusaran ini mempunyai peranan penting jika material dasar mudah bergerak. Tegangan geser pada sistem pusaran akan menyebabkan terbentuknya erosi lokal. Besarnya tegangan geser bergantung pada perbedaan kecepatan, yang dapat dipresentasikan dengan nilai vortisitasnya. Studi ini bertujuan untuk mendapatkan pemahaman yang lebih baik tentang vortisitas di sekitar silinder, terutama di lokasi dimana terbentuk system vortex. Data hasil pengukuran kecepatan aliran dipakai untuk mempelajari system vortisitas pada aliran dengan sistem pusaran di sekitar pilar. Gambar vektor kecepatan di sekitar pilar silinder memperlihatkan keberadaan pusaran utama di depan silinder. Berdasarkan vector kecepatan tersebut, dapat dianalisis vortisitasnya dengan menggunakan metode beda hingga. Hasil hitungan vortisitas digambarkan dalam bentuk kontur vortisitas. Hasil studi menunjukkan nilai maksimum dari vortisitas positif (searah jarum jam) dari sistem pusaran tapal kuda adalah pada bidang simetri di hulu silinder. Nilai vortisitas tersebut menurun, mencapai nilai minimum di hilir silinder. Besarnya vortisitas positif meningkat pada angka Reynold yang lebih tinggi. Di bawah bidang vortisitas positif, diperlihatkan terbentuknya system vortisitas negatif. Vortisitas negative di dekat dasar di depan silinder mempunyai nilai yang besar, proporsional dengan nilai tegangan geser di dasar, yang bertanggungjawab pada terbentuknya erosi lokal pada saluran dengan dasar bergerak. Garis yang memberikan nilai vortisitas sama dengan nol, merupakan garis separasi antara vortisitas positif dan negative. Dari hasil kajian juga disimpulkan bahwa posisi dari vortisitas positif maksimum tidak selalu berimpit dengan pusat pusaran tapal kuda.
Burgers vortex
Vortex stretching
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Trailing edge blowing wake management technology was adopted to improve the flow of stator wake.The flow parameters of stator wake were measured by hot wire anemometers(HWA) under different mass injection.Axial velocity profile,spectrum of velocity fluctuating and development rule of the wake width were obtained for pure wake,weak wake,momentumless wake and jet.According to the experimental results,injection up to momentumless wake can eliminate the vortex shedding frequency of the stator,and make the outlet velocity profile of the stator smooth.Meanwhile,the wake characteristic length scale of momentumless wake is in direct proportion to x0.35,while pure wake is in direct proportion to x0.37.
Anemometer
Trailing edge
Wake turbulence
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In this article we described Vortex in Cell method. We used it to research influence of the initial vorticity distribution inside the core of a vortex ring on its dynamics. We showed evolution of a vortex ring in time with two different initial distributions: uniform and Gaussian. The graphs for the theoretical and numerical velocities of a vortex ring were created. From presented results one can find out that the evolution of the vorticity around the core of the ring depends on the parameters of the ring and distribution of the vorticity inside the core.
Burgers vortex
Vortex stretching
Starting vortex
Dynamics
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