SELF-SUSTAINED OSCILLATIONS OF A CONFINED JET - A CASE-STUDY FOR THE NONLINEAR DELAYED SATURATION MODEL

This paper is concerned with a dominant feature of transitional separa ted flows, namely their ability to develop and maintain self-sustained oscillations. Experiments performed with an axisymmetric confined jet exhibit very nicely the low frequency, large amplitude oscillation of the reattachment point. The unsteadiness of the reattachment process of the flow at the confining walls is discussed by focusing on the rol e of the permanent backflow in the recirculation bubble. It is conject ured that this convective backflow may lead to a non-linear retarded a ction of the turbulent downstream flow on the upstream shear layer ins tability. This crucial ingredient -the time lag- is implemented in a n ew formalism referred to as the Non-Linear Delayed Saturation Model (N LDS), whose features are precisely discussed in connection with the fl ow. The observed oscillation frequencies of the reattachment point com pare well with the predictions of the model. Although the validity of the model is demonstrated with regard to the confined jet case, it is clear that the NLDS concept may represent a paradigm for unstable sepa rated reattaching flows (flow over backward facing steps, blunt bodies , channel protrusions,...).