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The instantaneous stream-wise velocity of a turbulent flow is given as follows:

$u(x,y,z,t)=\overline{u}(x,y,z)+{u}'(x,y,z,t)$

The time-average of the fluctuating velocity ${u}'(x,y,z,t)$ is

- $\displaystyle{\frac{u'}{2}} \\$
- $\displaystyle{\frac{-\overline{u}}{2}} \\$
- $\text{zero}\\$
- $\displaystyle{\frac{\overline{u}}{2}}$