Type of Document Master's Thesis Author Akhtar, Kareem Author's Email Address kakhtar@vt.edu URN etd-08132010-092911 Title Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers Degree Master of Science Department Engineering Science and Mechanics Advisory Committee
Advisor Name Title Ragab, Saad A. Committee Chair Cramer, Mark S. Committee Member Hajj, Muhammad R. Committee Member Keywords
- confined jets
- bubbly mixing layers
Date of Defense 2010-07-26 Availability unrestricted Abstract This thesis presents numerical investigations of two-dimensionalsingle-phase turbulent jets and bubbly mixing layers using
Reynolds-Averaged Navier-Stokes (RANS) equations.
The behavior of a turbulent jet confined in a channel depends
on the Reynolds number and geometry of the channel which
is given by the expansion ratio (channel width to jet thickness)
and offset ratio (eccentricity of the jet entrance). Steady
solutions to the RANS equations for a two-dimensional turbulent
jet injected in the middle of a channel have been obtained. When
no entrainment from the channel base is allowed, the flow is
asymmetric for a wide range of expansion ratio at high Reynolds
number. The jet attaches to one of the channel side walls. The
attachment length increases linearly with the channel
width for fixed value of Reynolds number. The attachment length is
also found to be independent of the (turbulent) jet Reynolds
number for fixed expansion ratio. By simulating half of the
channel and imposing symmetry, we can construct a steady symmetric
solution to the RANS equations. This implies that there are
possibly two solutions to the steady RANS equations, one is
symmetric but unstable, and the other solution is asymmetric (the
jet attaches to one of the side walls) but stable. A symmetric
solution is also obtained if entrainment from jet exit plane is
permitted. Fearn et al. (Journal of Fluid Mechanics, vol. 121,
1990) studied the laminar problem, and showed that the flow
asymmetry of a symmetric expansion arises at a symmetry-breaking
bifurcation as the jet Reynolds number is increased from zero. In
the present study the Reynolds number is high and the jet is
turbulent. Therefore, a symmetry-breaking bifurcation parameter
might be the level of entrainment or expansion ratio.
The two-dimensional turbulent bubbly mixing layer, which is a
multiphase problem, is investigated using RANS based models.
Available experimental data show that the spreading rate of
turbulent bubbly mixing layers is greater than that of the
corresponding single phase flow. The presence of bubbles also
increases the turbulence level. The global structure of the flow
proved to be sensitive to the void fraction. The present
RANS simulations predict this behavior, but different turbulence
models give different spreading rates. There is a significant
difference in turbulence kinetic energy between numerical
predictions and experimental data. The models tested include
k-ε, shear-stress transport (SST), and Reynolds stress
transport (SSG) models. All tested turbulence models under predict
the spreading rate of the bubbly mixing layer, even though they
accurately predict the spreading rate for single phase flow. The
best predictions are obtained by using SST model.
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