• Title/Summary/Keyword: Strouhal Number

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A Study on the Effect of Mud-flap on the Cabin Noise in KTX (고속열차의 객실 소음에 미치는 머드플랩의 영향에 관한 연구)

  • Choi, Seong-Hun;Chung, In-Soo;Seo, Sung-Il
    • Journal of the Korean Society for Railway
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    • v.9 no.5 s.36
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    • pp.550-554
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    • 2006
  • In the early stage of operation of KTX, passengers complained of the excessive cabin noise as the passes the tunnel. The noise is caused partly by wheel-rail contact and partly by airflow around the carbody. In this study, to reduce the cabin noise, the effect of the mud-flaps located between the cars is investigated. A series of tests was conducted to clarify the influences of the type and length of mud-flap, and train speed on the cabin noise. The optimum length of mud-flap was found. The shedding vortices around the mud-flap is thought to be the cause of the aerodynamic noise. Strouhal number and the resonant shedding frequency around the mud-flap correlated well with the cabin noise level.

FLOW-INDUCED FORCES ON AN INCLINED SQUARE CYLINDER (기울어진 정방형 실린더에 작용하는 유체력)

  • Yoon, Dong-Hyeog;Yang, Kyung-Soo;Choi, Choon-Bum
    • Journal of computational fluids engineering
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    • v.14 no.3
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    • pp.9-15
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    • 2009
  • Numerical investigation has been carried out for laminar flow past an inclined square cylinder in cross freestream. In particular, inclination of a square cylinder with respect to the main flow direction can cause sudden shift of the separation points to other edges, resulting in drastic change of flow-induced forces on the cylinder such as Strouhal number (St) of vortex shedding, drag and lift forces on the cylinder, depending upon the inclination angle. Collecting all the numerical results obtained, we propose contour diagrams of drag/lift coefficients and Strouhal number on an Re-Angle plane. This study would be the first step towards understanding flow-induced forces on cylindrical structures under a strong gust of wind from the viewpoint of wind hazards.

Analysis of empirical impedance model using Strouhal number (스트롤 수를 이용한 천공요소의 실험 임피던스 모델의 분석)

  • Lee Seong-Hyun;Ih Jeong-Guon
    • Proceedings of the Acoustical Society of Korea Conference
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    • autumn
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    • pp.375-378
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    • 2001
  • 천공 요소는 유체 기계의 흡기계나 배기계의 소음기에 널리 사용되고 있으며, 스치는 유동 (grazing flow)과 통과하는 유동 (cross flow)을 접하게 된다. 천공 요소의 임피던스는 소음기의 음향학적인 특성에 영향을 미치게 된다 본 연구에서는 유동이 임피던스에 미치는 영향을 고려하기 위하여, 실험을 통하여 얻은 임피던스 값을 유동 레지스턴스 (flow resistance)와 종단 보정 비 (end correction ratio)로 나타내고, 스트롤 수 (Strouhal number)로 무차원 화하여 분석하였다 이를 난류를 고려하여 제안된 선행 임피던스 모델들과 비교하여 난류가 미치는 영향을 분석하였다.

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EFFECTS OF INFLOW ANGLE ON LAMINAR FLOW PAST A TRIANGULAR CYLINDER (삼각봉을 지나는 층류유동에 대한 입구유동각도 변화의 영향)

  • Park, Tse-Seon
    • Journal of computational fluids engineering
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    • v.16 no.3
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    • pp.22-28
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    • 2011
  • Laminar Flow over an equilateral triangular cylinder is studied for several inflow angles. Under an uniform flow of $Re_d$=50,75,100,125,150, the triangular cylinder is rotated by ${\theta}$=$0^{\circ}$,$15^{\circ}$,$30^{\circ}$,$45^{\circ}$,$60^{\circ}$,$75^{\circ}$,$90^{\circ}$,$105^{\circ}$. The governing equations are solved by the PISO algorithm based on the finite volume method of the unstructured grid system. The effects of the inflow angle on the vortex-shedding flows are investigated. The Strouhal number shows a minimum at ${\theta}$=$60^{\circ}$. It is closely related to the variation of pressure and flow structure induced by the movement of separation points.

Numerical Study of a Flapping Flat Plate for Thrust Generation (플랩핑 평판의 추력발생에 대한 수치적 연구)

  • An, Sang-Joon;Kim, Yong-Dae;Maeng, Joo-Sung;Han, Chul-Heui
    • 유체기계공업학회:학술대회논문집
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    • 2006.08a
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    • pp.209-212
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    • 2006
  • Insect and birds in nature flap their wings to generate fluid dynamic forces that are required for the locomotion. Most of the previous published papers discussed mainly on the effect of flapping parameters such as flapping frequency and amplitude on the thrust at a fixed Reynolds number. However, it is not much known on the values of the flapping parameters that the flapping wing requires to generate the thrust at the low Reynolds number flow. In this paper, the onset of the thrust generation is investigated using the lattice Boltzmann method. The wake patterns and velocity profiles behind a flat plate in heaving oscillation are investigated for the heaving amplitude of 0.5C. The time-averaged thrust coefficient value is investigated by changing the reduced frequency from 0.125 to 3.0 for three values of heaving amplitude (h/C=0.25, 0.325, 0.50). It is also found that the critical Strouhal number over which the flat plate starts to produce the thrust is around 0.1 and the thrust is an exponential function of the Strouhal number.

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Heat Transfer Enhancement by Pulsating Flow in a Plate Heat Exchanger (판형 열교환기에서 맥동유동에 의한 열전달 촉진에 관한 실험적 연구)

  • Kim, Do-Kyu;Kang, Byung-Ha;Kim, Suk-Hyun
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.28 no.2
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    • pp.199-206
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    • 2004
  • The heat transfer enhancement by pulsating flow in a plate heat exchanger has been experimentally investigated in this study. The effect of the pulsating flow, such as pulsating frequency and flow rate on the heat transfer as well as pressure drop in a plate heat exchanger has been studied in detail. Reynolds number in cold side of a plate heat exchanger is varied 100∼530 while that of hot side is fixed at 620. The pulsating frequency is considered in the range of 5∼30 Hz. The results of the pulsating flow are also compared with those of steady flow. It is found that the average heat transfer rate as well as pressure drop is increased as flow rate is increased for both steady flow and pulsating flow cases. When pulsating flow is applied to the plate heat exchanger, heat transfer could be substantially increased in particular ranges of pulsating frequency or Strouhal number; St=0.36∼0.60 and pressure drop is also increased, compared with those of steady flow. However, in the region of low pulsating frequency or high pulsating frequency, heat transfer enhancement is in meager. Heat transfer enhancement map is suggested based on Strouhal number and Reynolds number of pulsating flow.

Study on Flow Around Circular Cylinder Advancing Beneath Free Surface (자유표면 밑을 전진하는 원주 주위의 유동에 관한 연구)

  • Yi, Hyuck-Joon;Shin, Hyun-Kyung;Yoon, Bum-Sang
    • Journal of Ocean Engineering and Technology
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    • v.27 no.5
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    • pp.16-21
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    • 2013
  • The flow around a circular cylinder advancing beneath the free surface is numerically investigated using a VOF method. The simulations cover Froude numbers in the range of 0.2~0.6 and gap ratios (h/d) in the range of 0.1~2.0, where h is the distance from the free surface to a cylinder, and d is the diameter of a cylinder at Reynolds number 180. It is observed that the vortex suppression effect and surface deformation increase as the gap ratio decreases or the Froude number increases. The most important results of the present study are as follows. The proximity of the free surface causes an initial increase in the Strouhal number and drag coefficient, and the maximum Strouhal number and drag coefficient occur in the range of 0.5~0.7. However, this trend reverses as the gap ratio becomes small, and the lift coefficient increases downward as the gap ratio decreases.

A Heat Release Model of Turbulent Premixed Flame Response to Acoustic Perturbations (유동 섭동에 의한 난류예혼합화염의 열발생 모델에 관한 연구)

  • Cho, Ju-Hyeong;Baek, Seung-Wook
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.32 no.6
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    • pp.413-420
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    • 2008
  • The unsteady heat release characteristics play a significant role in combustion instabilities observed in low emissions gas turbine combustors. Such combustion instabilities are often caused by coupling mechanisms between unsteady heat release rates and acoustic perturbations. A generalized model of the turbulent flame response to acoustic perturbations is analytically formulated by considering a distributed heat release along a curved mean flame front and using the flame's kinematic model that incorporates the turbulent flame development. The effects of the development of flame speed on the flame transfer functions are examined by calculating the transfer functions with a constant or developing flame speed. The flame transfer function due to velocity fluctuation shows that, when a developing flame speed is used, the transfer function magnitude decreases faster with Strouhal number than the results with a constant flame speed at low Strouhal numbers. The flame transfer function due to mixture ratio fluctuation, however, exhibits the opposite results: the transfer function magnitude with a developing flame speed increases faster than that with a constant flame speed at low Strouhal numbers. Oscillatory behaviors of both transfer function magnitudes are shown to be damped when a developing flame speed is used. Both transfer functions also show similar behaviors in the phase characteristics: The phases of both transfer functions with a developing flame speed increase more rapidly than those with a constant flame speed.

Suppression of Wake Transition and Occurrence of Lock-on Downstream of a Circular Cylinder in a Perturbed Flow in the A-mode Instability Regime (A-mode 불안정성 영역에서 교란유동장에 놓인 원형실린더 후류의 천이지연과 유동공진의 발생)

  • Kim, Soo-Hyeon;Bae, Joong-Hun;Yoo, Jung-Yul
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.8
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    • pp.702-710
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    • 2007
  • Direct numerical simulation (DNS) is performed to investigate suppressed wake transition and occurrence of lock-on in the wake of a circular cylinder disturbed by sinusoidal perturbation at the Reynolds number of 220 (A-mode instability regime). The sinusoidal perturbation, of which the frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. It is shown that the wake transition behind the circular cylinder can be suppressed due to the perturbation of the free stream velocity. This change causes a jump in the Strouhal number from the value corresponding to A-mode instability regime to the value corresponding to retarded wake transition regime (extrapolated from laminar shedding regime) in the Strouhal-Reynolds number relationship. As a result, vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.