The effects of external grid turbulence in a free stream on the turbulent boundary layer are investigated by means of the direct numerical simulation (DNS). A square turbulence-generating grid, on which the velocity components are set to zero, is located upstream of the flat boundary with a nonslip condition. The fractional step method is used to solve the governing equations. The results show that turbulence intensities and Reynolds stress normalized by the inner parameters are significantly suppressed in the log-law region by the external grid turbulence.

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