Absolute instability in the nearwake of twodimensional bluff bodies
Abstract
The linear inviscid instability of a family of mean velocity profiles which is representative of the near wake of twodimensional bluff bodies is studied on a locally parallel basis. Restricting attention to the incompressible case, both the influence of centerline velocity and the ratio of wake width to individual mixing layer thickness are explored. The parameters are identified for which the flow is absolutely unstable locally, i.e. for which instability waves amplifying in the upstream direction exist. This feature plays an important role when the attempt is made to explain the selection of a particular vorex shedding mode, i.e. to predict the shedding frequency, for instance. Different possibilities of feedback loops, such as the one proposed by Koch, as well as other mode section criteria, notably the one used by Pierrehumbert, and their relevance to particular types of wakes are discussed. For tophat wakes bounded by thin mixing layers a new criterion is proposed which properly accounts for the influence of the initial mixing layer thickness. In addition, the effects of a recirculating region near the body and of initial mixing layer curvature on the nearwake stability are considered qualitatively. Finally, the possible bearing of these concepts on the observed mode competition between the Karman and the varicose mode is examined.
 Publication:

Journal of Fluids and Structures
 Pub Date:
 April 1987
 DOI:
 10.1016/S08899746(87)903239
 Bibcode:
 1987JFS.....1..165M
 Keywords:

 Bluff Bodies;
 Flow Stability;
 Near Wakes;
 Two Dimensional Flow;
 Incompressible Flow;
 Inviscid Flow;
 Mixing Layers (Fluids);
 Mixing Length Flow Theory;
 Recirculative Fluid Flow;
 Velocity Distribution;
 Vortex Shedding;
 Fluid Mechanics and Heat Transfer