A computational study of gas leak jets relevant to offshore structures.
In the present research three dimensional, incompressible
turbulent jet flows from sharp edged rectangular orifices were
numerically simulated. Preliminary studies employing the
standard k-E turbulence model stressed the sensitivity of the
flow to the specification of turbulence quantities at the
inlet. The definition of correct turbulence quantities at the
inflow of large eddy simulations was also shown to be crucial
for realistic simulations. Consequently, a novel
computationally inexpensive boundary condition, that provided
turbulence data with known energy, was developed and
programmed into a commercial finite element code.
The large eddy simulations provided instantaneous data that
demonstrated the emergence of complex flow patterns within a
short axial distance. The upstream geometry from the jet exit
was not included in the present simulations. Therefore, the
swirling motion and vortical structures observed could not be
related to the different rates of contraction upstream of the
discharge plane. In addition, saddle-backed profiles were
observed in the long axis of the jet stream. The saddlebacked
profiles were formed by a combination of the secondary
motions and curvature of the streamlines due to the entrained
fluid entering normally to the main jet stream.
Turbulence quantities were quantified from the large eddy
simulations and showed similar trends to the verifying
The round jet assumption used in the past to study gas leakage
from pipework was found to be invalid in the near field. The
simulations clearly illustrate differences between the flow
field, the turbulence field, and the coherent structures of a
rectangular jet and a round jet.
Regions of high and low mixing between the escaping flammable
gas and the surrounding air were identified in the large eddy
simulations. Furthermore, the coherent structures in the
shear layers that resulted in a flapping motion in the lateral
direction of the jet, were recognised as important entrainment
Numerical techniques have been developed that can be used to
investigate gas leakage in detail. The time dependent
numerical data provides valuable insight into the mixing
mechanisms of the local gas/air mixture thus aiding the safer
design of offshore structures.