  We present an experimental realisation of spatial spanwise forcing in a
turbulent boundary layer flow, aimed at reducing the frictional drag. The
forcing is achieved by a series of spanwise running belts, running in
alternating spanwise direction, thereby generating a steady spatial square-wave
forcing. SPIV in the streamwise-wall-normal plane is used to investigate the
impact of actuation on the flow in terms of turbulence statistics, drag
performance characteristics, and spanwise velocity profiles, for a
non-dimensional wavelength of $\lambda_x^+ = 397$. We confirm that a
significant flow control effect can be realised with this type of forcing. The
scalar fields of the higher-order turbulence statistics show a strong
attenuation of stresses and production of turbulence kinetic energy over the
first belt already, followed by a more gradual decrease to a steady-state
energy response over the second belt. The streamwise velocity in the near-wall
region is reduced, indicative of a drag-reduced flow state. The profiles of the
higher-order turbulence statistics are attenuated up to a wall-normal height of
$y^+ \approx 100$, with a maximum streamwise stress reduction of 45% and a
reduction of integral turbulence kinetic energy production of 39%, for a
non-dimensional actuation amplitude of $A^+ = 12.7$. An extension of the
classical laminar Stokes layer theory is introduced, to describe the
non-sinusoidal boundary condition that corresponds to the current case. The
spanwise velocity profiles show good agreement with this extended theoretical
model. The drag reduction was estimated from a linear fit in the viscous
sublayer in the range $2 \leq y^+\leq 5$. The results are found to be in good
qualitative agreement with the numerical implementations of Viotti et al.
(2009), matching the drag reduction trend with $A^+$, and reaching a maximum of
20%.
