Unsteady 2-D flow field characteristics for perforated plates with a splitter

Yaragal, Subhash C. Techno-Press 2004 Wind and Structures, An International Journal (WAS Vol.7 No.5

Wind tunnel experiments were conducted under highly turbulent and disturbed flow conditions over a solid/perforated plate with a long splitter plate in its plane of symmetry. The effect of varied level of perforation of the normal plate on fluctuating velocities and fluctuating pressures measured across and along the separation bubble was studied. The different perforation levels of the normal plate; that is 0%, 10%, 20%, 30%, 40% and 50% are studied. The Reynolds number based on step height was varied from $4{\times}10^3$ to $1.2{\times}10^4$. The shape and size of the bubble vary with different perforation level of the normal plate that is to say the bubble is reduced both in height and length up to 30% perforation level. For higher perforation of the normal plate, bubble is completely swept out. The peak turbulence value occurs around 0.7 to 0.8 times the reattachment length. The turbulence intensity values are highest for the case of solid normal plate (bleed air is absent) and are lowest for the case of 50% perforation of the normal plate (bleed air is maximum in the present study). From the analysis of data it is observed that $\sqrt{\overline{u^{{\prime}2}}}/(\sqrt{\overline{u^{{\prime}2}}})_{max}$, (the ratio of RMS velocity fluctuation to maximum RMS velocity fluctuation), is uniquely related with dimensionless distance y/Y', (the ratio of distance normal to splitter plate to the distance where RMS velocity fluctuation is half its maximum value) for all the perforated normal plates. It is interesting to note that for 50% perforation of the normal plate, the RMS pressure fluctuation in the flow field gets reduced to around 60% as compared to that for solid normal plate. Analysis of the results show that the ratio [$C^{\prime}_p$ max/$-C_{pb}(1-{\eta})$], where $C^{\prime}_p$ max is the maximum coefficient of fluctuating pressure, $C_{pb}$ is the coefficient of base pressure and ${\eta}$ is the perforation level (ratio of open to total area), for surface RMS pressure fluctuation levels seems to be constant and has value of about 0.22. Similar analysis show that the ratio $[C^{\prime}_p$ max/$-C_{pb}(1-{\eta})]$ for flow field RMS pressure fluctuation levels seems to be constant and has a value of about 0.32.

Numerical investigation of fluid flow past a square cylinder using upstream, downstream and dual splitter plates

Shams Ul Islam,Raheela Manzoor,Zhou Chao Ying,Mohammad Mehdi Rashdi,A. Khan 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.2

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.

부착된 고정형 분할판을 이용한 원형 실린더 주위 유동 제어

김현식(Hyunsik Kim),최해천(Haecheon Choi) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12

In the present study, we conduct numerical simulation of flow over a circular cylinder with an attached rigid splitter plate at Re = 140000 based on the cylinder diameter (d) and freestream velocity (U). The length of the splitter plate (l) varies from 0.1d to 3d and its thickness is fixed at 0.02d. Unlike the previous studies at Re = O(10⁴), mean drag coefficient monotonically decreases with increasing plate length due to the use of very thin plate. The size of the separation bubble becomes the same as the plate length at l = 3d. The interaction between the shed vortex and the splitter plate yields high lift fluctuations at the plate and the low-frequency lift modulation, which is observed at l ≥ d.

높은 레이놀즈 수에서 분할판을 이용한 원형 실린더 주위 유동 제어

김현식(Hyunsik Kim),최해천(Haecheon Choi) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12

In the present study, an attached splitter plate is used as a passive control device to modify the flow over a circular cylinder at Re = 140,000 based on the cylinder diameter and free-stream velocity. The length of the splitter plate (Ɩ/d) varies from 0.5 to 3.0. Vortical structures in the wake region is highly modified due to the attached splitter plate. As the splitter plate is elongated, Strouhal number is decreased except 0.5<Ɩ/d<1.0 and drag is decreased. The lift fluctuations in time for Ɩ/d=2 show a quasi-periodic motion whose period is much larger than the Kármán vortex shedding period. This behavior is closely related to the flapping of shed vortex interacting with the splitter plate, and increases large rms lift fluctuations.

Drag reduction on a circular cylinder using dual detached splitter plates

Hwang, Jong-Yeon,Yang, Kyung-Soo Elsevier 2007 Journal of wind engineering and industrial aerodyn Vol.95 No.7

<P><B>Abstract</B></P><P>Drag reduction on a circular cylinder using dual detached splitter plates is numerically studied. Two splitter plates with the same length as the cylinder diameter (<I>d</I>) are placed along the horizontal centerline; one is upstream of the cylinder and the other is in the near-wake region, respectively. Their positions are described by the gap ratios <I>G</I><SUB>1</SUB>/<I>d</I>, <I>G</I><SUB>2</SUB>/<I>d</I>, where <I>G</I><SUB>1</SUB> represents the gap between the cylinder stagnation point and the rear edge of the upstream splitter plate, and <I>G</I><SUB>2</SUB> denotes the gap between the cylinder base point and the leading edge of the downstream splitter plate. The drag varies with the two gap ratios; it has the minimum value at a certain set of gap ratios for each Reynolds number. The upstream splitter plate reduces the stagnation pressure by friction, while the downstream one increases the base pressure by suppressing vortex shedding. This combined effect causes a significant drag reduction on the cylinder. In particular, the drag sharply increases past the optimum <I>G</I><SUB>2</SUB>/<I>d</I>; this is related to the restarted vortex shedding in the wake region.</P>

Experimental study on passive flow control of circular cylinder via perforated splitter plate

Serdar Sahin,Tahir Durhasan,Engin Pinar,Huseyin Akilli 한국풍공학회 2021 Wind and Structures, An International Journal (WAS Vol.32 No.6

Present experimental investigation aims to reduce the shedding of vortex in the near wake region of a circular cylinder using a perforated splitter plate. Perforated plates were placed in the wake region of the cylinder and aligned with the streamwise direction. The length of the plates was equal to the diameter of the cylinder. Different plate porosities and locations were examined and obtained results were compared to the baseline cylinder. Flow measurements downstream of the cylinder were performed in a water channel by employing a particle image velocimetry technique (PIV) at a Reynolds number of Re=5×103. It is observed that the effect of the porosity on the flow characteristics of the cylinder depends on the location of the plate. The strength of shear layers and flow fluctuations in the near wake region of the cylinder are considerably diminished by the perforated splitter plate. It is found that the porosity of ε=0.3 is the most effective control element for gap ratio of G/D=0.5. On the other hand, proper gap ratio is determined as G/D=2 for porosity of ε=0.7. It is concluded in the present study that the perforated splitter plate could be used as alternative passive flow control technique in order to reduce vortex shedding of the cylinder.

Numerical Study of Flow Characteristics over Square Cylinders with an Attached Splitter Plate

Nguyen, Van Minh,Koo, Bon-Guk Korean Society of Ocean Engineers 2018 Journal of advanced research in ocean engineering Vol.4 No.2

The fluid flow over structures has been widely investigated by many researchers because its extensive application in offshore structures, skyscrapers, chimneys and cooling towers, brides. In the viewpoint of reducing the drag for offshore structure, it becomes challenging problem in the field of hydrodynamic of offshore structure. The purpose of this study is to investigate a flow over a square cylinder with an attached splitter plate using RANS method. First, RANS turbulent models such as a standard $k-{\omega}$ model, SST $k-{\omega}$ model, RNG $k-{\varepsilon}$ model, realizable $k-{\varepsilon}$ model, standard $k-{\varepsilon}$ model were used for choosing suitable turbulent model which has the best agreement with available experimental result. Drag of single cylinder estimated by using standard $k-{\omega}$ has a good agreement with published experimental result. Therefore, the stand $k-{\omega}$ was selected for simulation for flow over a square cylinder with an attached plate. Second, the numerical results of drag of square cylinder with an attached splitter plate in various length of an attached plate were performed using RANS method in ANSYS Fluent. In this paper, the numerical simulations were conducted at a Reynolds number of 485 and the thickness of the splitter plate is chosen as a constant value about 10% of cylinder width. The numerical results of drag coefficient of square cylinder are compared with experimental result published by other researchers. Finally, the effect of the splitter plate attached to the rear side of the square cylinder has been investigated numerically with a focus on the drag coefficient and flow characteristic. As a result, the drag coefficient decreases with an increase in splitter plate length.

고 레이놀즈 수에서 분리된 분할판을 가진 원주의 유동장 특성

노기덕(Ki Deok Ro),이한균(Han Gyun Lee),이종호(Jong Ho Lee),이정민(Jeong Min Lee),신진호(Jin Ho Shin),천강빈(Kang Bin Cheon) 대한기계학회 2016 大韓機械學會論文集B Vol.40 No.6

본 연구는 고 레이놀즈 영역에서 후류측에 분리된 분할판(Detached splitter plate)을 가진 원주의 유동장 특성을 양·항력측정 실험과 PIV를 이용한 가시화 실험으로 파악한 것이다. 실험파라메터는 원주 한변의 길이에 대한 분할판의 폭비(H/B=0.5~1.5) 및 원주 후면에서부터 분할판의 앞전까지의 거리(G/B=0~2)로 했다. 분할판의 폭비를 고정시킨 경우 원주의 항력감소율은 간격비가 증가할수록 증가한 후 감소하는 특성을 보였다. 또한 같은 간격비에서는 분할판의 폭비가 클수록 원주의 항력감소율이 컸다. 분리된 분할판의 설치에 의해 분할판의 상, 하부에는 서로 반대방향의 볼테스가 발생되었고, 이 볼텍스가 원주 후류측에 역류를 발생시켜, 원주에 작용하는 항력을 감소시켰다. In this study, we investigate the characteristics of the drag reduction of a circular cylinder having a detached splitter plate at the wake side. We measure the fluid force on a circular cylinder and visualize the field using particle image velocimetry (PIV) with a high Reynolds number, Re = 10,000. The experimental paraeters used were the width ratios (H/B = 0.5~1.5) of splitters to the prism width and the gap ratios (G/B = 0~2) between the prism and the splitter plate. The drag-reduction rate of the circular cylinder increased with H/B in the case of the same G/B, and it increased and then decreased with G/B in the case of the same H/B. The vortices of the opposite direction on the upper and lower sides of the detached splitter plate were generated by installing the plate. Reverse flow was caused by the vortices at the wake region of the circular cylinder, and the drag of the circular cylinder was decreased by the reverse flow.

Optimization Research on DC Air Circuit Breaker at High Altitudes Based on Arc Root Stagnation and Arc Reverse Movement Phenomena

Li Jing,Peng Shidong,Yi Chenxi,Huang Hao,Cao Yundong,Liu Shuxin,Zhou Zhu 대한전기학회 2024 Journal of Electrical Engineering & Technology Vol.19 No.5

Urban rail transit in high-altitude areas is developing gradually, but the failure of short circuit breaking of on-board largecapacity DC air circuit breaker (LC-DCCB) at 2–4 km altitudes has become severe and common. To interrupt short circuit current at 2–4 km altitudes of on-board LC-DCCB successfully, multi-physical coupling simulation of short circuit breaking of LC-DCCB at 2–4 km altitudes was conducted in this research based on the magneto-hydro-dynamics (MHD) model, and the simulation results were compared with those at 0 km altitude. The two-dimensional (2-D) transient distribution of physical parameters such as temperature, airfl ow, and electromagnetic fi elds in the arc chamber were obtained. The results indicate that the fundamental reasons for the failure of LC-DCCB to interrupt short circuit current arc at high altitudes are the arc root stagnation and arc reverse movement between splitter plates. Based on the simulation results, the arc chamber structure was optimized by setting U-shaped splitter plates to improve the interruption performance of LC-DCCB at high altitudes. Compared with the previous structure, the improved structure with three intercalary U-shaped splitter plates on both sides of the center line of the arc chamber has the best arc extinguishing performance at high altitudes, which can interrupt 18 kA short circuit current successfully at 3 km altitude, with an arc duration of 19.5 ms. This research reveals the fundamental cause for the diffi culty in interrupting short circuit current arc in LC-DCCB at high altitudes, which can provide theoretical and technical guidance for the research and design of such a product.

사각단면 원주의 공기력에 미치는 칼만 와류의 영향에 관한 연구

이재형,김윤석,마츠모토 마사루 한국풍공학회 2006 한국풍공학회지 Vol.10 No.2

In this study, a series of wind tunnel tests were carried out to understand the complicated bluff body aerodynamics affected by Karman vortex on 2D rectangular cylinders with various side ratios, B/D=0.3~15. Based upon the aerostatic coefficients and aerodynamic derivatives of rectangular cylinders measured from experiments, the effects of the Karman vortex on the bluff body aerodynamics, especially paying attention to the galloping instability, are investigated. For mitigation of Karman vortex, a splitter plate with suitable size was installed in a wake center. By the installation of a splitter plate, the sharp peak in CD -B/D (B: along-wind length, D: cross-flow length) diagrams of stationary drag force was completely disappeared. Also, the value of dCL/dα(0°) was significantly sensitive to the Karman vortex, in particular, even its sign was changed in the range of B/D=0.6 to B/D=0.7 and B/D=3.0 to B/D=4.0, which means the marked role of the Karman vortex on the galloping instability based upon the quasi-steady theory. Furthermore, the flutter derivatives were also drastically affected by Karman vortex in the rather wide range of reduced velocity. In summary, it is considered that the substantial bluff body flow, which is not affected by Karman vortex, can be characterized by the side ratio of cylinders, and the interference of the Karman vortex on the fundamental flow depends on the side ratios of cylinders. 본 연구에서는bluff body의 공기역학적 진동현상들에 대한 칼만와류(Karman vortex)의 간섭현상를 이해하기 위해서 구조적 기본단면인 사각단면(B/D=0.3~15)을 대상으로 정적공기력계수와 플러터계수를 일련의 정적실험과 강제가진실험을 통하여 구하였다. 실험에서는 splitter plate를 단면 후류부에 삽입함으로써 칼만와류의 방출을 억제, 칼만와류로부터 간섭받지 않는 상태의 주위유동 특성을 파악할 수 있었다. 또한 splitter Plate의 유무에 따른 실험결과의 비교연구는 이전에 발표된 다른 연구결과들과 잘 일치했으며 특히 갤로핑현상과 관련된 준정상이론에 근거한dCL/dα의 변화와H1*의 변화를 중심으로 bluff body 주위의 흐름내에서 칼만와류의 간섭을 파악하였다. 결론적으로 B/D=3,4 단면에서 시간평균적으로 단면 측면에 재부착하던 흐름이 splitter plate의 삽입으로 칼만와류의 방출을 억제하면 전연에서 박리한 흐름이 재부착하지 않음으로써 갤로핑현상이 관찰되었다. 즉 칼만와류의 억제는 전단박리층의 곡율을 늘임으로써 재부착점을 연장시키는 것으로 판단된다.