Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

when the buoyancy is removed, for example, in the microgravity environment. Thus, quantitative data on the spatial and temporal
evolutions of the instability, length and time scale of the oscillating mode and its effects on the mean flow and breakdown of the
potential core are needed in normal and microgravity to delineate gravitational effects in buoyant jets. In momentum dominated
low-density jets, the instability is speculated to originate in the potential core. However, experiments have not succeeded in identifying
the direct physical cause of the instability. For example, the theory predicts an oscillating mode for Sis less than 0.62 in the
limit of zero momentum thickness, which contradicts with the experimental findings of Kyle and SReenivasan. The analyses of
momentum-dominated jets neglect buoyancy effects because of the small Richardson number. Although this assumption is
appropriate in the potential core, the gravitational effects are important in the annular region surrounding the jet, where the density
and velocity gradients are large. This reasoning provides basis for the hypothesis that the instability in low Richardosn number
jets studied by Kyle and SReenivasan and Monkewitz et al. is caused by buoyancy. The striking similarity in characteristics of
the instability and virtually the identical conclusions reached by Subbarao and Cantwell in buoyant (Ri>0.5) helium jets on one
hand and by Kyle and SReenivasan in momentum-dominated (Riis less than 1x10(exp -3)) helium jets on the other support this
hypothesis. However, quantitative experiments in normal and microgravity are necessary to obtain direct physical evidence of
buoyancy effects on the flow instability and structure of momentum-dominated low-density jets. The primary objective of this
new research project is to quantify how buoyancy affects the flow instability and structure in the near field of low-density jets.
The flow will be described by the spatial and temporal evolutions of the instability, length and time scales of the oscillating mode,
and the mean and fluctuating concentration fields. to meet this objective, concentration measurements will be obtained across the
whole field using quantitative Rainbow Schlieren Deflectometry, providing spatial resolution of 0.1mm and temporal resolution
of 0.017s to 1ms. The experimental effort will be supplemented with linear stability analysis of low-density jets by considering
buoyancy. The first objective of this research is to investigate the effects of gravity on the flow instability and structure of low-density
jets. The flow instability in these jets has been attributed to buoyancy. by removing buoyancy in our experiments, we seek
to obtain the direct physical evidence of the instability mechanism. In the absence of the instability, the flow structure will undergo
a significant change. We seek to quantify these changes by mapping the flow field (in terms of the concentration profiles) of these
jets at non-buoyant conditions. Such information is presently lacking in the existing literature. The second objective of this
research is to determine if the instability in momentum-driven, low-density jets is caused by buoyancy. At these conditions, the
buoyancy effects are commonly ignored because of the small Richardson based on global parameters. by eliminating buoyancy
in our experiments, globally as well as locally, we seek to examine the possibility that the instability mechanism in self-excited,
buoyant or momentum-driven jets is the same. to meet this objective, we would quantify the jet flow in normal and microgravity,
while systematically decreasing the Richardson number from buoyancy-driven to momentum driven flow regime. The third
objective of this research is to perform a linear stability analysis of low-density gas jets by including the gravitational effects. The
flow oscillations in these jets are attributed to an absolute instability, whereby the disturbance grows exponentially at the site to
ultimately contaminate the entire flow field. We seek to study the characteristics of both convective and absolute instabilities and
demarcate the boundary between them.
Author (revised)
Buoyancy; Flow Stability; Gas Jets; Gravitational Effects; Microgravity; Momentum; Low Density Flow
20010024954 Florida State Univ., Tallahassee, FL USA
Lattice-Boltzmann Methods for Multiphase Flows in Microgravity Environments
Chella, R., Florida State Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference;
December 2000, pp. 1193-1195; In English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only as
part of the entire parent document
This is a newly funded project to use lattice-Boltzmann techniques to provide a computationally efficient and robust approach
to study multiphase flow and transport in microgravity environments. The project objectives and approach are summarized, and
some preliminary results provided. Phase Separation Under Externally Applied Flows - Study of the influence of externally
applied flows in retarding phase separation and stabilizing non-equilibrium morphologies in binary fluids undergoing spinodal
decomposition and nucleation and growth of droplets. Analysis of time-dependent shear and normal stresses developed in these
complex fluids. Phase Separation In Confined Geometries - Study of the role of short-range and long-range interactions at solid
surfaces on phase separation of binary fluids in confined geometries. Quantitatively, to study the range of parameters where the
transition from one morphology to the other occurs, and the scaling behavior of the structure factor and the growth law of the
characteristic length scale of the growing domains. Contact Line Dynamics - Study of droplet, spreading, in particular the evolution
of the contact angle in droplet spreading, influence of the short-range and long-range solid-fluid interactions on the equilibrium
droplet shape and scaling relations for the slip velocity in terms of the surface tension, viscosity of the advancing fluid, and
the deviation of the contact angle from its equilibrium value. Dynamics of the contact line at a moving solid surface, in particular,
the evolution of the advancing and receding contact, angles, and their dependence on the interfacial tension, fluid viscosities, and
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