| 邓莹莹,时钟*.稳定分层二层流室内实验早期格栅湍流结构[J].实验力学,2021,36(2):205~222 |
| 稳定分层二层流室内实验早期格栅湍流结构 |
| Laboratory experiment on turbulence structures generated by the grid in a stably-stratified two-layer fluid in the early period |
| 投稿时间：2020-04-07 修订日期：2020-07-23 |
| DOI：10.7520/1001-4888-20-063 |
| 中文关键词: 格栅 湍流结构 室内实验 粒子图像测速 欧拉频谱 |
| 英文关键词:grid turbulence structure laboratory experiment Particle Image Velocimetry(PIV) Eulerian frequency spectrum |
| 基金项目:国家留学基金管理委员会/国家公派高级研究学者项目/留金发[2011]3008号 |
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| 中文摘要: |
| 采用室内实验混合箱和粒子图像测速技术，本文研究了稳定分层无平均剪切二层流(上层淡水、下层盐水)振动湍流结构。对实验录像进行粒子图像测速技术处理，获得垂向二维流场（垂直于格栅平面）瞬时速度和涡量，并用于计算：①时均速度和时均涡量；②均方根速度；③均匀程度和各向同性程度；④平均流强度；⑤时均泰勒的欧拉积分长度尺度；⑥时均湍动能和时均湍动能垂向通量;⑦水平和垂向速度的欧拉频谱。结果显示：(1)格栅方棒处时间平均速度方向垂直向上，而其两侧的时间平均涡量正负交替,表明格栅附近射流结构占据主要位置且存在反向涡旋对。(2)均方根速度随着距离格栅(水平面)高程的增大而减小，并且满足高程的-1.425(接近-3/2)幂次律,表明格栅湍流均方根速度的垂向变化较为剧烈。(3)靠近混合箱边壁处的均匀程度和各向同性程度都大于1,表明靠近混合箱边壁处存在各向异性湍流。(4)格栅反湍流强度的量级非0，但是，相对较小,表明平均流强度较低，故而，本实验结果仍可与无平均流的情况作对比。(5)时均泰勒的欧拉积分长度尺度随着距离格栅(水平面)高程的增大而线性增大,表明随着湍流向上发展，涡的平均尺度增大。(6)时均湍动能和时均湍动能垂向通量随着距离格栅(水平面)高程的增大而减小，而时均湍动能垂向通量为正值，表明远离格栅时均湍动能衰减，但是，始终向上传递。(7)水平和垂向速度的欧拉频谱随着距离格栅(水平面)高程的增大而减小，幂次律介于ω-1和ω-5/3(ω为频率)之间，表明水平和垂向的湍流脉动能量远离格栅均衰减，并受湍流的有限雷诺数效应的影响。 |
| 英文摘要: |
| Laboratory experiment, together with the Particle Image Velocimetry (PIV) technique, was used to study the turbulence structures produced by an oscillating grid in a stably-stratified mean shear-free two-layer fluid (fresh water on salt water). The instantaneous velocity and vorticity of the two-dimensional flow fields (perpendicular to the plane of the grid) were obtained from PIV analyses of recorded video. These data were used to calculate the following physical quantities of the flow fields: ① the time-averaged velocity and time-averaged vorticity; ② the root-mean-square velocity; ③ the degree of homogeneity and isotropy; ④ the strength of mean flow; ⑤ the time-averaged Taylor's Eulerian integral length scale; ⑥ the time-averaged turbulent kinetic energy and time-averaged vertical flux of turbulence kinetic energy. Results show that: (1) the time-averaged velocity above the grid is upward, while the time-averaged vorticity is alternately positive and negative on both sides of the bars, suggesting that jet-like structures are the dominant feature near the grid and counter-rotating vortices pairs are present. (2) the root mean square velocity around the grid decreases rapidly with the increasing height above the grid, satisfying a power law: -1.425 (ca. -3/2)th power of height. (3) the degrees of homogeneity and isotropy close to the two sidewalls of the mixing box are larger than 1, suggesting anisotropic regions exit near the two sidewalls. (4) the magnitude of the inverse turbulence intensity is not 0 but relatively small, suggesting that the present results can be compared with previous results of zero-mean flow due to the lower strength of mean flow. (5) the Taylor's Eulerian integral length scale is approximately linearly related toheight above the grid, which increases with the height, suggesting that the average size of an eddy increases with the upward development of turbulence. (6) the time-averaged turbulent kinetic energy and time-averaged vertical flux of turbulence kinetic energy decrease with the increasing height above the grid and the latter one is positive, suggesting that the time-averaged turbulent kinetic energy decays far away from the grid but always transfers upwards. (7) the Eulerian frequency spectra for the horizontal and vertical velocities decrease with the increasing height above the grid and the power laws range from ω-1 to ω-5/3, suggesting that the horizontal and vertical turbulent fluctuation energy decrease far away from the grid and they are subject to the finite Reynolds number effect of turbulence. |
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