[论文解读] Near-Surface Boundary Layer Turbulence Along a Horizontally-Moving, Surface-Piercing Vertical Wall

Nathan Washuta, Naeem Masnadi+1 more|arXiv (Cornell University)|Sep 16, 2016

Fluid Dynamics and Turbulent Flows参考文献 4被引用 4

一句话总结

本研究利用一种新型实验室装置，通过水平移动的表面穿刺钢带模拟船体，探究近表面边界层中的湍流与空气卷吸现象。研究发现，自由表面破裂与空气卷吸起始分别与约150和3–3.5的韦伯数相关，采用动量厚度和速度脉动作为无量纲化参数，支持Brocchini与Peregrine（2001）提出的无量纲化理论。

ABSTRACT

The complex interaction between turbulence and the free surface in boundary layer shear flow created by a vertical surface-piercing wall is considered. A laboratory-scale device was built that utilizes a surface-piercing stainless steel belt that travels in a loop around two vertical rollers, with one length of the belt between the rollers acting as a horizontally-moving flat wall. The belt is accelerated suddenly from rest until reaching constant speed in order to create a temporally-evolving boundary layer analogous to the spatially-evolving boundary layer that would exist along a surface-piercing towed flat plate. Surface profiles are measured with a cinematic laser-induced fluorescence system and sub-surface velocity fields are recorded using a high-speed planar particle image velocimetry system. It is found that the belt initially travels through the water without creating any significant waves, before the free surface bursts with activity close to the belt surface. These free surface ripples travel away from the belt before appearing to become freely-propagating waves. From sub-surface velocity measurements, it is found that close to the surface, transition to turbulence happens sooner than far from the surface, leading to an overall thicker boundary layer in the vicinity of the free surface. A secondary peak in streamwise velocity fluctuations accompanies this transition to turbulence and this peak reaches a maximum value a short time later before smoothing outward. Using momentum thickness as a length scale and the streamwise velocity fluctuations at the location of this outer peak as a velocity scale, free surface bursting and air entrainment onset are found to depend in some way on Weber number and agreement is found with scaling arguments for air entrainment presented by Brocchini and Pregrine (J. Fluid Mech., 499, 225-254, 2001).

研究动机与目标

• 通过实验研究由移动的表面穿刺垂直壁面生成的边界层中湍流与自由表面的相互作用。
• 在受控的实验室环境中，确定自由表面破裂与空气卷吸发生的条件。
• 评估无量纲化定律（特别是韦伯数与弗劳德数）在预测近自由表面湍流边界层中空气卷吸行为时的有效性。
• 通过变换 x = Ut，构建一个随时间演化的边界层装置，以模拟船体表面空间演化边界层的特征。
• 利用高速PIV与LIF测量，量化自由表面接近度对边界层发展与湍流结构的影响。

提出的方法

• 采用绕两个垂直滚轮旋转的不锈钢钢带作为表面穿刺的水平移动平面壁面，模拟船体。
• 钢带在小于0.7秒内快速加速至恒定速度U，通过x = Ut变换，生成随时间演化的边界层，类比于空间演化边界层。
• 采用电影式激光诱导荧光（LIF）测量表面轮廓，研究自由表面波动与波浪生成。
• 采用高速平面粒子成像测速（PIV）记录亚表面速度场，分析湍流结构与边界层发展。
• 以动量厚度θ作为长度尺度，以流向速度脉动u′rms作为速度尺度，进行无量纲化分析。
• 计算韦伯数We = ρu′rms²θ/σ与Reθ，评估其与空气卷吸及破裂起始的关联性。

实验结果

研究问题

• RQ1 在移动的表面穿刺壁面附近的湍流边界层中，自由表面破裂发生在何种韦伯数下？
• RQ2 靠近自由表面如何影响边界层中湍流的起始与演化？
• RQ3 湍流转变是否更早发生在靠近自由表面区域？若是，这如何影响边界层厚度？
• RQ4 Brocchini与Peregrine（2001）提出的关于空气卷吸的无量纲化理论，在采用动量厚度与速度脉动尺度的实验条件下是否成立？
• RQ5 钢带速度、边界层发展与自由传播表面波起始之间存在何种关系？

主要发现

• 当测量点位于自由表面以下14.0 cm处时，自由表面破裂对应的动量厚度雷诺数约为Reθ ≈ 1500；当测量点位于距表面2.5 cm处时，Reθ ≈ 2500。
• 空气卷吸起始与基于动量厚度和流向速度脉动的韦伯数约为3至3.5相关。
• 自由表面破裂在破裂位置处表现出一致的雷诺数Re_x ≈ 5.7 × 10⁶，与钢带速度无关。
• 流向速度脉动的次级峰值出现在距钢带表面约y = 3 mm处，随后向外平滑扩散并达到最大值。
• 靠近自由表面区域的边界层发展更快，导致自由表面附近边界层厚度大于深层区域。
• 靠近钢带生成的表面波纹向外传播并演化为自由传播波，波速随钢带速度从3 m/s增至5 m/s时提升约25%。