Title page for ETD etd-08132010-092911


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-dimensional

single-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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