Mixing by microorganisms in stratified fluids
Mixing by microorganisms in stratified fluids
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分层流体中微生物的混合
DOI：
10.1357/002224014813758940
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发表时间：
2014
影响因子：
0.5
通讯作者：
E. Lauga
中科院分区：
文献类型：
--
作者：
G. Wagner;W. Young;E. Lauga
我们研究了在低雷诺数和Peclet数的微生物在稳定分层的海洋中游泳引起的垂直混合，并表明海洋微泳者对垂直混合的全球贡献可以忽略不计。我们提出了两种方法来估计混合效率，η，或势能产生的速率与微泳者在海洋上工作的总速率的比率。第一种是基于尺度参数，并根据典型生物体大小a与分层流的固有长度尺度之间的比率来估计η，ε = νκ/N 2 1/4，其中ν是运动粘度，κ是扩散率，N是浮力频率。特别是，对于相关海洋界限中的小生物，a/ε 1，我们预测了尺度η ε（a/ε）3。通过将游泳者建模为正则化的力偶极子来求解完全耦合的流动分层问题，并数值计算效率，从而形成η的第二个估计值。我们的计算结果，这是检查所有的比率a/�，验证了在限制a/� 1的标度参数，并进一步预测η <$1.2（a/�）3垂直游泳和η <$0.15（a/�）3水平游泳。这些结果与任何富含生物活性的分层流体相关，意味着海洋中游泳微生物的混合效率最多为8%，并且可能小至少两个数量级。
We examine the vertical mixing induced by the swimming of microorganisms at low Reynolds and Peclet numbers in a stably stratified ocean, and show that the global contribution of oceanic microswimmers to vertical mixing is negligible. We propose two approaches to estimating the mixing efficiency, η, or the ratio of the rate of potential energy creation to the total rate-of-working on the ocean by microswimmers. The first is based on scaling arguments and estimates η in terms of the ratio between the typical organism size, a, and an intrinsic length scale for the stratified flow, � = νκ/N 2 1/4 , where ν is the kinematic viscosity, κ the diffusivity, and N the buoyancy frequency. In particular, for small organisms in the relevant oceanic limit, a/� � 1, we predict the scaling η ∼ (a/�) 3 . The second estimate of η is formed by solving the full coupled flow-stratification problem by modeling the swimmer as a regularized force dipole, and computing the efficiency numerically. Our computational results, which are examined for all ratios a/� , validate the scaling arguments in the limit a/� � 1 and further predict η ≈ 1.2 (a/� ) 3 for vertical swimming and η ≈ 0.15 (a/� ) 3 for horizontal swimming. These results, relevant for any stratified fluid rich in biological activity, imply that the mixing efficiency of swimming microorganisms in the ocean is at very most 8% and is likely smaller by at least two orders of magnitude.