Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Chapter 2
2.3.5 Instabilities and Vortex Breakdown in swirling
Flows
In swirl flows, instabilities (unsteady phenomena) have many aspects to be
studied. Generally, in case of mostly laminar swirl flows, a steady flow is
subjected to small unsteadiness/ disturbance and the response of the flow is
observed in the form of whether there is growth or decay. With the presence
of swirl in the flow, the instability associated means that some tangential
velocity distributions consistent with the simple redial equilibrium are
unstable and not achievable in practice (Greitzer et al., 2004). This method is
called the ‘linear stability analysis’ and in case of turbulent flows such
introduced ‘small’ fluctuations are smaller than the turbulent fluctuations
and any conclusion of growth or decay of the disturbance always has the
growth and decay of turbulent fluctuations as the major contributor
(Moene, 2003). Other significant stability studies involve studying vortex
break down, precession of vortex cores (PVC), growth and decay of Reynolds
normal stress components and the anisotropy of the Reynolds stress tensor
etc.
The vortex breakdown is an important aspect of swirling flows. In general
the vortex breakdown has two distinct types: (i) Bubble or axis symmetric
type where there is rapid expansion of vortex core in to a near axis symmetric
bubble shape (Figure 2.9a) and (ii) Spiral type where the vortex core deforms
into a spiral (Figure 2.9b). Sarpkaya (1972) also observed a double helix type.
Benjamin (1962) described vortex breakdown as abrupt and drastic change of
flow structure and Ludwieg (1961) suggested that it might be a finiteamplitude
manifestation of the instability of the core flow. Velte et al., (2010)
define vortex breakdown as explosion or abrupt growth of the slender vortex
core with different changes in the flow topology. So far a lot of experimental
measurements, observations and numerical studies have been conducted to
understand this phenomenon but still the physical mechanism of vortex
breakdown is not well understood. The phenomenon remains largely in the
qualitative, descriptive realm of knowledge (Novak et al., 2000). In case of
confined swirling flows, the experiments conducted by Escudier et al. (1982),
show that the consequence of a vortex breakdown is a profile shape change
for the axial velocity i.e. from jet-like to wake-like with an intermediate
stagnation region. Escudier et al. (1982) discusses that the spatial growth rate
of instability waves is very different in the two axial velocity profile types.
The upstream jet-like profile becomes unstable and instability waves amplify
slowly. Then the core flow undergoes a shock-like transition at some
downstream position and the waves grow rapidly on a low velocity wake-like
profile. Escudier et al. (1982) suggests that the swirl intensity (flow criticality)
is the main parameter that determines the basic characteristic of the
downstream flow and the effect of instability waves are superimposed over it.
Alekseenko at el. (2007) has given pictures of many different vortex
breakdown structures both in confined and unconfined swirling flow
experiments.
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Swirling Flows