• Title/Summary/Keyword: vortex

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STABILITY ANALYSIS OF REGULARIZED VISCOUS VORTEX SHEETS

  • Sohn, Sung-Ik
    • Bulletin of the Korean Mathematical Society
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    • v.53 no.3
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    • pp.843-852
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    • 2016
  • A vortex sheet is susceptible to the Kelvin-Helmhotz instability, which leads to a singularity at finite time. The vortex blob model provided a regularization for the motion of vortex sheets in an inviscid fluid. In this paper, we consider the blob model for viscous vortex sheets and present a linear stability analysis for regularized sheets. We show that the diffusing viscous vortex sheet is unstable to small perturbations, regardless of the regularization, but the viscous sheet in the sharp limit becomes stable, when the regularization is applied. Both the regularization parameter and viscosity damp the growth rate of the sharp viscous vortex sheet for large wavenumbers, but the regularization parameter gives more significant effects than viscosity.

Parametric Study on Shock-Vortex Interaction (충격파-와동 간섭의 파라메터 연구)

  • Chang Keun-Shik;Chang Se-Myong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.29 no.8
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    • pp.921-926
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    • 2005
  • In the idealized model problem of the interaction between a planar travelling shock and a symmetric vortex, the physics of shock distortion and quadrupole sound generation are well known to many researchers. However, the authors have distinguished the weak waves reflected and transmitted by the complicated photograph images obtained from a shock tube experiment. In this paper, we introduces a parametric study based on Navier-Stokes simulation and Rankin vortex model to see the difference of shock deformation shapes. Four combination of the strength of shock and vortex are respectively selected from a parameter plane of shock and vortex strength extended to the strong vortex region. The result shows clearly discernable wave morphology for the main parameters, which is not yet explicitly mentioned by other researchers.

How Shock Wave Interacts with a Vortex ?

  • Chang Keun-Shik;Chang Se-Myong
    • 한국가시화정보학회:학술대회논문집
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    • pp.1-7
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    • 2004
  • When a vortex diffracts upon encountering a vortex, many strong and weak waves are produced in the course of interaction. They are the cause of shock wave attenuation and noise production. This phenomenon is fundamental to understanding the more complex supersonic turbulent Jet noise. In this paper we have reviewed the research on shock-vortex interaction we have carried on last seven years. We have computationally investigated the parameter effect. When a shock is strong, shock diffraction pattern becomes complex since the slip lines from the triple points on Mach stem curl into the vortex, causing an entropy layer. When the vortex is unstable, vortexlets are brought about each of which make shock diffraction of a reduced intensity. Strong vortex produces quadrupole noise as it impinges into a vortex. Elementary interaction models such as shock splitting, shock reflection, and shock penetration are presented based on shock tube experiment. These models are also verified by computational approach. They easily explain production and propagation of the aforementioned quadrupole noise, Diverging acoustics are explained in terms of shock-vortexlet interactions for which a computational model Is constructed.

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Experimental Study on the Horseshoe Vortex Systems Around Surface-Mounted Obstacles (평판 위에 부착된 실린더 주위의 말굽와류 시스템에 관한 실험적 연구)

  • 양준모;유정열
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.16 no.10
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    • pp.1979-1989
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    • 1992
  • An experimental study has been performed to investigate the horseshoe vortex system formed around cylindrical obstacles mounted vertically on the surface over which a boundary layer is formed. To measure the mean velocity of the flow field, a five-hole Pitot tube has been used. In addition, surface static pressure measurements and surface flow visualization were also performed. From the five-hole probe measurements, vorticity distribution was deduced numerically and the streamwise velocity distribution was also examined. To consider the effect of the leading-edge shape on the formation of the horseshoe vortex, a qualitative comparison was made between the three-dimensional flows around a circular cylinder and a wedge-type cylinder. The five-hole probe measurements showed a single primary vortex which exists immediately upstream of the obstacles, and endwall flow visualization showed the existence of a corner vortex. As the vortex passes around the obstacle, the vortex strength is reduced and the vortex core moves radially outward. Due to this horseshoe vortex, the fluid momentum is found to decrease along the streamwise direction. Since the horseshoe vortex formed around a wedge-type cylinder has weaker strength and is confined to a narrower region than that around a circular, the possibility that the secondary flow loss due to the horseshoe vortex can be reduced through a change of the leading- edge shape is proposed.

Experimental Investigations on Upper Part Load Vortex Rope Pressure Fluctuations in Francis Turbine Draft Tube

  • Nicolet, Christophe;Zobeiri, Amirreza;Maruzewski, Pierre;Avellan, Francois
    • International Journal of Fluid Machinery and Systems
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    • v.4 no.1
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    • pp.179-190
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    • 2011
  • The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

A Numerical Investigation of the Main Rotor Tip-vortex and Counter-rotating Vortex during Hovering Flight (주로터 제자리 비행 시 익단 와류와 Counter-rotating Vortex의 수치적 관찰)

  • Jun, Jonghyuk;Chung, In Jae;Lee, Duck Joo
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.41 no.10
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    • pp.761-769
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    • 2013
  • Effects of helicopter wakes on helicopter aerodynamics are serious, but the wake configuration is very complicated and hard to predict. The purpose of this study is the detailed observation of wake using numerical methods. Vortex lattice method and freewake method are used to track the vortices in the wake. In this paper, the wake configuration is observed during hovering flight. In the case of hovering flight at the moderate thrust level, besides tip vortex, counter-rotating vortex can be observed at the inboard part of blade. When the vortices move downward, tip vortex and counter-rotating vortex get close and influence to each other. Therefore, vortices are highly distorted due to their own instability.

Effect of Anti-Vortex Hole Angle on the Flat Plate Film Cooling Effectiveness (반와류 홀의 각도가 평판의 막냉각 효율에 미치는 영향 연구)

  • Park, Soon Sang;Park, Jung Shin;Lee, Sang Hoon;Moon, Young Gi;Kwak, Jae Su
    • The KSFM Journal of Fluid Machinery
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    • v.17 no.5
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    • pp.5-10
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    • 2014
  • In this paper, the experimental study was carried to investigate the effect of angle of the anti-vortex holes on the film cooling effectiveness on a flat plate. The pressure sensitive paint technique was applied to measure the film cooling effectiveness. Two anti-vortex hole angles of $0^{\circ}$ and $15^{\circ}$ with respect to the primary hole were considered, and the simple cylindrical hole case was also tested. The blowing ratio based on the cylindrical hole was 0.5 and the same flow rate was kept for all anti-vortex hole cases. Results showed that the film cooling effectiveness for the anti-vortex hole cases were much higher than that of the cylindrical case. Among the anti-vortex hole cases, $15^{\circ}$ angle anti-vortex hole case showed higher film cooling effectiveness than that by the $0^{\circ}$ angle anti-vortex hole case.

Extinction in a Counterflow Nonpremixed Flame Interacting with a Vortex (와동과 상호작용하는 대향류 비예혼합화염의 소염특성)

  • Oh, Chang-Bo;Lee, Chang-Eon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.27 no.10
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    • pp.1401-1411
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    • 2003
  • A two-dimensional direct numerical simulation was performed to investigate the flame structure of CH$_4$$N_2$-air counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman's detailed chemistry were adopted in this simulation. The characteristic vortex and chemical time scales were introduced to quantify and investigate the extinction phenomenon during a flame-vortex interaction. The results showed that fuel- and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex was extinguished at much larger scalar dissipation rate than steady flame. It was also found that the air-side vortex extinguished a flame more rapidly than the fuel-side vortex. Furthermore, it was noted that the degree of unsteady effect experienced by a flame can be investigated by comparing the above two characteristic time scales, and this analysis could give an appropriate reason for the results of the previously reported experiment.

Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics

  • Cao, Shuyang;Wang, Mengen;Zhu, Jinwei;Cao, Jinxin;Tamura, Tetsuro;Yang, Qingshan
    • Wind and Structures
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    • v.26 no.3
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    • pp.115-128
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    • 2018
  • Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-induced wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time-averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.

Unsteady Response of Counterflow Nonpremixed Flames Interacting with a Vortex (와동과 상호작용하는 대향류 비예혼합화염의 비정상 응답특성)

  • Oh, Chang-Bo;Park, Jeong;Lee, Chang-Eon
    • Journal of the Korean Society of Combustion
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    • v.9 no.2
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    • pp.10-17
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    • 2004
  • A two-dimensional direct numerical simulation is performed to investigate the flame structure of $CH_4/N_2-Air$ counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman#s detailed reaction mechanism are adopted in this calculation. To quantify the strain on flame induced by a vortex, a scalar dissipation rate (SDR) is introduced. The results show that fuel-side and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex is extinguished at much larger SDR than steady flame. It is also found that air-side vortex extinguishes a flame more rapidly than fuel-side vortex. The unsteady effect induced by flame-vortex interaction does not lead to a transient OH overshoot of the maximum steady concentration observed in experiment, while $HO_2$ radical increases more than the maximum steady concentration with increasing SDR. In addition, it is seen that NO and $NO_2$ are not sensitive to the unsteady variation of SDR.

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