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Vortex Shedding and Lock-on for a Rotationally Oscillating Flat Plate
Cheng, Fang Yuan
|關鍵字:||Vortex Shedding;渦漩剝離;Lock-on;Rotationally Oscillating;Flat Plate;互鎖;旋轉振盪;平板||出版社:||機械工程學系||摘要:||
本文以實驗量測及數值模擬探討平板尾流渦漩剝離的特性及互鎖的現象。實驗主要是量測尾流渦漩剝離頻率、表面壓力、平板的受力及尾流渦漩結構，並利用煙線可視化的技巧來觀察近尾流場的結構。數值模擬則選擇旋度與流線函數(vorticity and stream function formulation)型式之二維納維爾-史托克斯方程式，模擬的重點在於瞭解互鎖過程的流場細微變化。
The vortex shedding and lock-on induced by rotational oscillation of a flat plate normal and inclined to a uniform stream in its neutral position have been studied experimentally and numerically. The shedding frequency, lock-on bands, unsteady surface pressures, mean drag and vortical structure of the near wake are described.
The experiments were performed in an open-type wind tunnel with a square test-section of 305 mm305 mm and 1100 mm in length. A total of six flat plate models with blockage ratios in the range K = 4.3- 26.2% were tested in the present study for Reynolds numbers between 3.5103-3.2104. By the use of a correction factor to account for the effect of bevel angle, a universal Strouhal number of 0.1600.003 can be obtained for inclined angles =10-90 for the stationary flat plate. It appears that for the Reynolds numbers presented, the only significant effect of blockage is to produce an increase in the local free-stream velocity. Accordingly, the effect of blockage on drag coefficient and Strouhal number can be corrected by employing the common correction formula. The present study also determines the drag coefficients of the models with significant blockage effect using the momentum defect method.
The onset of lock-on, which depends on the combination of the frequency and amplitude of the forcing, is examined by using spectral analysis of the wake velocity. Three lock-on flow regimes are studied in detail. These include the primary lock-on regime that occurs when the ratio of forcing frequency to natural shedding frequency comes close to unity, and two subharmonic regimes 1/2 and 1/3 harmonics. Moreover, the lock-on state can be reached via a frequency higher or lower than the natural frequency. For a normal flat plate, the route leading to the lock-on state via a higher-frequency approach is found to ‘attract' the vortex shedding frequencies, whereas the lower-frequency approach ‘suppresses' the vortex shedding frequency other than the forcing one. However, for an inclined plate, the route leading to lock-on via higher-frequency or lower-frequency approach is observed opposite to that for the normal plate. Furthermore, the higher and lower frequency approaches to lock-on significantly alter the phase and structure of the shed vortices. Consequently, the base pressures exhibit different distributions in response to the vortex formation immediately behind the plate.
A finite difference simulation of the vortex shedding and lock-on for a rotationally oscillating flat plate at Re = 100 is computed by solving the unsteady two-dimensional Navier-Stokes equations in the vorticity-stream function form. The Strouhal number and vortical structure, which are evaluated for the stationary normal plate at Re=100, 126 and 500, reflect excellent agreement with both the experimental and numerical data available. Behavior of the route to vortex lock-on is also investigated. It is found that both the time from stationary to lock-on situation and vortex formation length decrease with increasing forcing frequency. The character of limits of lock-on regime is found in good agreement with present experimental results. An approach to lock via a higher-frequency is easier than via a lower frequency.
|Appears in Collections:||機械工程學系所|
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