Fluctuating Total Head in a Viscous Sublayer

The unsteady viscous sublayer model proposed by Einstein and Li was used to study the character of the velocity history occurring near a smooth wall in a turbulent flow. It is shown that a distinctly intermittent velocity should exist, alternating between a higher, turbulent type velocity with large fluctuations and a lower, laminar type velocity with smaller fluctuations. A supercritical flow of viscous oil was established in an open channel. With the point of measurement far out in the flow (at all times beyond the viscous layer), the total head (or velocity) records exhibited the random character expected in turbulent flow. With the point of measurement close enough to the wall so as to be covered part of the time by the alternately growing and decaying sublayer, the velocity records took on the intermittent properties predicted by the model, with the percentage of time spent in the turbulent phase depending on the distance of the point of observation from the wall. Autocorrelation curves made from the total head measurements indicated a periodicity of the same magnitude as that predicted by the sublayer model.