A study of the coherent structures in the viscous wall region of a turbulent flow

Conditionally averaged measurements of flow in the viscous wall region showed that strong spanwise flows at y(+) = 40 are related to strong spanwise flows at the wall that are of the opposite sign. At y(+) = 20 spanwise flows are always in phase with flows at the wall. Characteristics of the dominant eddies at the wall were explored by performing numerical computations using a fixed cell in space. Close to the edge of the viscous sublayer (y(+) = 15) use of a single spatial harmonic (lambda(+) = 100) was found to be adequate in predicting all important features of turbulence. At the edge of the viscous wall layer two spatial harmonics were found to be necessary to describe the flow field. The longer scale outer flow eddies produce most of the spanwise mixing in the wall region. The smaller scale lambda(+) = 100 eddies can account for momentum transport, Reynolds stress production and creation of mean u squared and mean v squared energy in this region. The physical picture of the flow resulting mean u squared and mean v squared from interaction of the wall with the outer layer is as follows. The mean flow energy is primarily produced in the outer layer by the mean pressure gradient and is subsequently transferred into viscous wall region. All the turbulent kinetic energy is produced by the v-w flow in the wall layer and is concentrated in the streamwise component of the velocity. This energy is convected out of the wall layer into the core region, where part of it is transferred to motion of the y-z plane. The cycle of events completes with the transport of mean v squared energy into the wall region and set-up of the v-w field.