有限空间中三维壁面紊动射流流动特性试验研究

为了研究三维壁面紊动射流前方布置的垂直挡板对时均流动特性的影响，采用粒子图像测速技术对不同雷诺数和淹没水深条件下的流速场进行了测量，统计分析了U速度剖面分布、速度半宽值变化和最大速度衰减规律。结果表明:沿射流方向具有3个明显不同的衰减区，依次划分为自由边界下的三维壁面射流区（Ⅰ区）、垂直挡板影响区（Ⅱ区）和近壁区（Ⅲ区）。在重点分析的Ⅱ区内，3个特征物理量的分布规律均与雷诺数无关；在垂直挡板影响下，速度半宽值扩展率明显加快，发展更为充分；在水深H~1.75H范围内，射孔中垂面内速度半宽值扩展率为0.043，同时0.5H的递增对射孔中心水平面内速度半宽值的分布较为敏感，其扩展率由0.205增至0.270；最大速度衰减对水深的改变并不敏感，且在Ⅱ区加速衰减，衰减指数为1.095。通过与半无限空间中三维壁面射流径向型衰减区的流动特征比较，Ⅱ区与之相似，将Ⅱ区视为提前进入径向型衰减区。

An experimental study on the flow characteristics of a three-dimensional turbulent wall jet in a limited space

To examine the effect of a vertical plate positioned in front of a rectangular orifice on the time-averaged flow characteristics of a three-dimensional turbulent wall jet,we measured three important quantities: U velocity profiles,velocity half-width,and maximum velocity decay rate with various Reynolds numbers and submerged depths by using particle image velocimetry (PIV). The jet flow field was found to be divided into three major decay regions in terms of the maximum velocity decay exponents:a natural three-dimensional turbulent wall jet region (region I),a vertical plate affecting region (region II),and a near-wall region (region III). In region II,the focus of the present study,the variations in the three measured quantities with downstream distance were found to be independent of the Reynolds number. Compared with the natural wall jet,the confined jet developed substantially faster,and the spread rates in both the longitudinal and lateral directions were significantly higher,owing to the presence of the vertical plate. The spread rate in the symmetry plane was 0.043 within the range of H—1.75H. The lateral spread rate was slightly sensitive to a water depth interval of 0.5H,and the growth rate correspondingly increased from 0.205 to 0.270. Furthermore,the maximum velocity decayed dramatically in this region,with an exponent value of 1.095,and was independent of water depth. The flow characteristics of region II,compared with the natural case,were similar to those in the radial decay region. Therefore,the flow field in region II appears to have fully developed and entered into the radial decay region in advance.