Summary
Evolving flow structures leave definite but not unique “footprints” of the outer (off-wall, mid-air) flow on the wall. Bifurcating flows, characterized by dynamical systems for the velocity field reveal that two-dimensional separation bubbles together with their three-dimensional bifurcations are “embedded” between fully attaching and completely separating three-dimensional flow structures with a possibility that local flow attachment occurs at a separation line. In this respect we show: 1. An unsteady or steady, incompressible flow field can be completely determined by the knowledge of the wall-shear stress and the wall-pressure field without specifying any outer (farfield) boundary conditions. 2. Nonlinearity or time-dependence or an explicit Reynolds number/viscosity dependence which would reflect properties of the Navier-Stokes equations do not affect the structural changes associated with local flow bifurcations which lead to separation bubbles or near-wake vortex flow separations. Only the continuity equation is fulfilled in every case. Therefore, 3. the question is considered how far away from the bifurcation set of parameters the kinematically possible structures remain structurally stable against Navier-Stokes perturbations. This is studied by direct numerical simulations of the structural changes of separated flows around an ellipsoid at angles of attack. It is shown that flow attachment in a region where a separation line forms is possible in a wide range of angles of attack. The dilemma of defining or locating steady streamwise vortices due to open flow separation is considered.
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Dallmann, U., Gebing, H. (1994). Flow attachment at flow separation lines. In: Schnerr, G.H., Bohning, R., Frank, W., Bühler, K. (eds) Fluid- and Gasdynamics. Acta Mechanica, vol 4. Springer, Vienna. https://doi.org/10.1007/978-3-7091-9310-5_6
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DOI: https://doi.org/10.1007/978-3-7091-9310-5_6
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