The properties of the time- and span-averaged mean wake recirculation
region are investigated in separated flows over several different two-dimensional
bluff bodies. Ten different cases are considered and they divide into two
of circular, elliptic and square cross-sections and the normal plate. A
number range from 250 to 140000 is considered, but in all the cases the
portion of the boundary layer remains laminar until separation. The lower
number data are from direct numerical simulations, while the data at the
Reynolds number are obtained from large-eddy simulation and the experimental
of Cantwell & Coles (1983), Krothapalli (1996, personal communication),
Leder (1991) and Lyn et al. (1995). Unlike supersonic and subsonic
separations with a splitter plate
in the wake, in all the cases considered here there is strong interaction
between the shear layers resulting in Kármán vortex shedding.
impact of this fundamental
difference on the distribution of Reynolds stress components and pressure
to the mean wake recirculation region (wake bubble) is considered.
It is observed that
in all cases the contribution from Reynolds normal stress to the force
balance of the
wake bubble is significant. In fact, in the cylinder geometries this contribution
outweigh the net force from the shear stress, so that the net pressure
force tends to push
the bubble away from the body. In contrast, in the case of normal plate,
owing to the
longer wake, the net contribution from shear stress outweighs that from
stress. At higher Reynolds numbers, separation of the Reynolds stress components
incoherent contributions provides more insight. The behaviour of the coherent
contribution, arising from the dominant vortex shedding, is similar to
that at lower
Reynolds numbers. The incoherent contribution to Reynolds stress, arising
small-scale activity, is compared with that of a canonical free shear layer.
these observations a simple extension of the wake model (Sychev 1982; Roshko
1993a, b) is proposed.