摘要:
通过风洞实验研究了一端固定于壁面、另一端为自由端的三维正方形棱柱气动力特性。实验中模型宽度 d =200mm,高宽比为5,来流风速为U ∞=13m/s。基于U ∞与 d 的雷诺数Re d =1.73×105。研究发现,三维方形棱柱时均阻力系数(CD )与升力系数根方差值(C′L )都明显小于二维方柱的对应值,且三维方形棱柱的涡脱落频率也相对较低。三维方柱绕流与气动力存在2种典型状态:一种是类似卡门涡街的展向涡交替脱离状态,此时柱体阻力较大,且升力出现大幅周期性波动;另一种是展向涡呈准对称状态脱落,此时阻力较小,而升力无明显周期性波动。第一种状态所对应的 C′L 明显大于第二种状态的对应值,在柱体下半部分前者 C′L 为后者2倍左右,2者差异随着向自由端的靠近而逐渐减小。此外,当第一种状态发生时,有限长棱柱气动力的展向相关性也明显强于第二种状态。上述2种典型状态交替随机出现,使有限长棱柱气动力具有明显的非平稳特性。
Abstract:
The aerodynamic forces on a wall-mounted finite-length square cylinder with one end mounted on a wall and the other end free are experimentally investigated.The width of the test model is d =200mm,and the aspect ratio is H/d =5.The incoming flow velocity is U ∞ =13m/s,corresponding to a Reynolds number of 1.73×10 5 based onU ∞ and d .It is found that the time-averaged drag coefficient CD and rms value of the lift coefficient C′L of a 3D square cylinder are both significantly smaller than those of a 2D square cylinder,and the vortex shedding fre-quency of the former is lower.Two typical flow modes occur in the flow around a 3D square cyl-inder:the first mode is characterized by alternating spanwise vortices shedding,similar to Kar-man vortex street,corresponding to higher drag and larger fluctuation of lift;the second mode is characterized by symmetrical vortex shedding,corresponding to lower drag and the lift without periodic fluctuation.At the lower part of the cylinder,C′L of the first mode is about one times lar-ger than that of the second mode,and this difference reduces gradually as approaching the free end.Besides,the spanwise correlation of the aerodynamic force is stronger in the first mode. These two typical flow modes occur randomly,resulting in the nonstationary properties of the aerodynamic forces on the wall-mounted finite-length square cylinder.
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