A forced, plane, turbulent mixing-layer - A challenge for the predictor

Measurements were carried out in an isothermal plane mixing-layer in absence of pressure gradient and under controlled initial conditions. Disturbances of known frequency and amplitude were introduced into the flow at its origin while measurements were conducted further downstream. Constant temperature hot-wire anemometers were used to measure the mean velocity, the shear stress and the three components of turbulent intensity for various initial conditions. The flow is not self preserving in the range of Reynolds numbers considered. The initial spreading rate of the mixing layer depends on the amplitude and the frequency of the imposed oscillations. At a given frequency, the rate of spreading increases initially with the amplitude of the imposed oscillation but a saturation region occurs beyond which one can no longer control the flow. The spreading rate in the saturation region is less than for the undisturbed mixing layer. The initial rate of spread of the mixing layer is associated with the growth of large, quasi-two-dimensional eddies which engulf fluid into their fold while being convected downstream.