The instantaneous stream wise velocity of a turbulent flow

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The instantaneous stream wise velocity of a turbulent flow is given as follows: u(x, y, z, t) = u(x, y, z) + u'(x, y, z, t) The time average of the fluctuating velocity u (x, y, z, t) is

1. u'
  2
2. - u
  2
3. zero
4. u
  2

Correct Option: C

Time average of fluctuating velocity is zero.