Slight amounts of forcing have traditionally been used in fluid-mechanic stability experiments. It was usually assumed that forcing provided a regular disturbance above the background noise--making the subsequent development easier to follow--but did not otherwise alter the physical process. Recently it has begun to be appreciated that even small amounts of forcing can have a significant influence, not only upon the instability, but upon the resulting turbulent flow. The present paper will document the effects of forcing for several typical unbounded shear flows having engineering application. The results are illustrative, and for further discussion an extensive review can be found in Ho and Huerre 1 .
