  Direct numerical simulations are performed to contrast turbulent boundary
layers over a concave wall without and with free-stream turbulence. Adverse
pressure gradient near the onset of curvature leads to sharp decrease in skin
friction and intermittent separation. The presence of free-stream turbulence
reduces the probability of reverse flow, accelerates the recovery of the
boundary layer in the downstream zero-pressure gradient region, and leads to a
sustained and appreciable increase in the skin friction. The forcing also
promotes the amplification of coherent G\"ortler structures in the logarithmic
layer of the curved-wall boundary layer. Statistically, the spanwise and
wall-normal Reynolds stresses intensify and the radial distance between their
peaks increases downstream as the G\"ortler structures expand. The Reynolds
shear stress coefficient also increases in the logarithmic layer in contrast to
a decrease when a flat-plate boundary layer is exposed to free-stream
turbulence. In addition, the more coherent and energetic roll motions in the
forced flow promote mixing of free-stream and boundary-layer fluids, where the
former is seen more often deep within the buffer layer.
