Handbook of Turbomachinery Second Edition Revised - Ventech!

losses, but it may affect to a minimal extent pressure distributions for
attached boundary layers. Figure 20 shows measured surface shear-stress
and corresponding unsteady pressures for an oscillating NACA-65 airfoil in
a low-speed wind tunnel [22]. The shear stress data clearly indicate that the
transition point periodically moves over a very large range on the airfoil
surface during one oscillating period [Fig. 20(a)], but the unsteady pressure
over the major portion of the surface appears to be almost unaffected [Fig.
20(b)]. So for this case, an accurate modeling of the large-scale movement of
the transition point (most likely to be nonlinear) should make a negligible
difference in an aerodynamic loading (aerodamping prediction).
A more complex situation where turbulence/transition modeling can
be important is with a bubble-type boundary-layer separation. Bubble-type
separations are common on both compressor and turbine blades at typical
Reynolds numbers. Depending on the bubble size, this type of flow
separation can affect local as well as global unsteady pressures. A typical
scenario is that a boundary layer separates at a laminar state, and a
transition happens in the separated shear layer. The reattachment point of
separation zone and its movement will strongly depend on the transition and
its modeling. For aerodynamic damping calculations involving blade
vibration at a small amplitude, a simple bubble separation/transition model
developed for steady low-speed flows [36] can be implemented, assuming
that an unsteady laminar separation bubble behaves quasisteadily [37]. In
transonic flows, the passage shock position at a highly loaded condition is
very sensitive to small blockage variations. In these cases, unsteady
transition would certainly influence the unsteady shock boundary-layer
interaction and the shock oscillation. The associated aerodamping can be
thus dependent on the transition modeling [38].
For typical off-design conditions with much thickened and/or
separated boundary layers, it is probably fair to say that none of the
existing turbulence/transition models is shown to be reliable. Given that
viscous effects at off-design conditions are likely to be influenced by large
turbulence eddy structures, which in turn interact with short-scale periodic
unsteadiness, large eddy simulation (LES) looks to be the way forward. The
need to pursue LES can also be viewed from a slightly different angle,
relating to mesh dependence of unsteady solutions. Generally it is more
difficult to get a mesh-independent solution for unsteady flows than for
steady flows. This is because that when we refine meshes (which would
normally be associated with using smaller time steps), we will pick up
disturbances at smaller scales. And this would be a neverending process
(imagine that at some stage down the line, we might start to pick up largescale
‘‘random’’ turbulence!). This fundamental uncertainty of modeling
turbulence while resolving it at the same time needs to be clearly appreciated
Copyright © 2003 Marcel Dekker, Inc.