Numerical Simulation of
Stationary Downburst
Phenomena with Impinging Jet
Model
Tze Siang, Sim
Ph.D student
Nanyang Technological University, Singapore
24 January 2013
Content
1. Background
2. Objectives
3. Methodology
4. Results and discussion
5. Conclusion
6. Future work
Background
• What is downburst?
The famous atmospheric scientist, Fujita
(1985), in his report “The DownburstMicroburst and Macroburst”, defined
downburst:
as an intense, transient downdraft of air that
induces an outburst of damaging wind on
or near the earth’s surface.
• Fujita, T. T., (1985): "The Downburst- Microburst and Macroburst," Sattellite and Mesometeorology Research Project (SMRP) Research Paper 210, Dept. of
Geophysical Sciences, Univ. of Chicago, (NTIS PB-148880)
Background
• Where can downburst be found?
United States
Account for about
1/3 of extreme
wind. (2002)
Asia
The “thunderstorm” type
downburst occurs in
cyclonic and noncyclonic areas. (2004)
Australia
About ½ of
downbursts in
thunderstorm
contribute to
extreme wind gust.
(2002)
•
•
Edmund C.C. Choi, (2004),“Field measurement and experimental study of wind speed profile during thunderstorms”, Journal of Engineering and Industrial Aerodynamics 92
(2004) 275-290
C.W. Letchford, C. Mans, M.T. Chay, (2002)“Thunderstorm – their importance in wind engineering (a case for the next generation wind tunnel”, Journal of Wind Engineering
and Industrial Aerodynamics 90 , 1415-1433
Background
•Downburst wind starts off by
travelling vertically downward.
•Upon impinging on the ground,
it spreads out radially along the
earth’s surface as outflow.
•Severe cases:
The strength can be equivalent
to a tornado.
•Implications:
Wind hazard to ground
structures.
Image:
• Sengupta, A., Sarkar, P.P., (2008). Experimental measurement and numerical simulation of an impinging jet with application to thunderstorm microburst winds. Journal of Wind
Engineering and Industrial Aerodynamics 96, 345–365.
Background
• The high speed outflow at earth’s surface is important to
the ultimate load limit of structures.
• This might be a concern for large structures.
• Examples are:
- Offshore wind turbines (2011), transmission towers
(2001).
•
•
Hieu Huy Nguyen, (2011) Lance Manuel, Paul S. Veers, “Wind turbine loads during simulated thunderstorm microburst”, Journal of Renewable and Sustainable Energy 3, 053104
Eric Savory, Gerard A.R. Parke, Mostafa Zeinoddini (2001) Norman Toy, Peter Disney, “Modelling of tornado and microburst-induced wind loading and failure of a lattice
trasmission tower”, Engineering Structures 23 , 365-375
Objectives
• Understand downburst outflow near the
earth surface and investigate on the
interaction with large structures.
Methodology
• Literature from the past 10 years from present
indicates few main methods of
investigating/understanding the outflow near
earth surface.
– 1. Laboratory “Impinging jet model” method
(2007)
– 2. “Cooling source” method (1992)
– 3. Meteorological method (1993)
•
•
•
Kim, J., Hangan, H., (2007). Numerical simulations of impinging jets with application to thunderstorm downbursts. Journal of Wind Engineering and Industrial Aerodynamics
95, 279–298.
Anderson, J.R., Orf, L.G., Straka, J.M., (1992). A 3-D model system for simulating thunderstorm microburst outflows. Meteorology and Atmospheric Physics 49, 125–131
Nicholls M., Pielke. R, Meroney R, (1993)“ Large eddy simulation of microburst winds flowing around a building, Journal of Wind Engineering and Industrial Aerodynamics,
46-47, pp. 229-237
Methodology
• To help us gain a rough understanding of the flow
characteristics.
• Employ the simplest laboratory “impinging jet
model”.
• First proven to match closely with the downburst
outflow characteristics at full scale by Hjemfelt
(1987).
•
Hjelmfelt, M.R. (1988), “Structure and Life Cycle of Microburst Outflows Observed in Colorado”, J. of Applied Meteorology, 27, 900-927
Methodology
• In the research:
– Numerically simulate downburst, with
impinging jet Model in 2D axisymmetric
domain.
– Computational Fluid Dynamics (CFD)
technique. Perform steady-state Reynolds
Averaged Navier Stokes (RANS) and
transient RANS.
– Characterise the flow of a stationary
impinging jet and understanding the flow
features.
Axi-symmetric CFD
Computational
domain
Boundary conditions
D
Z
Edges
Boundary conditions
Dimensions
A
Axis
4D
B
Speed inlet
0.5 D
C
Symmetry (slip wall)
6D
D, E
Pressure-outlet
10 D (edge E)
F
Non-slip wall
10D
C
B
A
E
Ujet
O
F
Assumptions:
-Incompressibility
-Temperature and
buoyancy effect neglected.
r
r
Legend
Re
and
Ujet
D
H
r/D
H/D
Reynolds number based on diameter of B
inlet velocity at B.
Inlet velocity at B (independent variable)
Diameter at B
height of A
kinematic viscosity of air.
normalised radial distance from O.
normalised height of A.
Results and discussion
• Grid and domain independence test
Results and discussion
• Validated with Kim and Hangan (2007)
experimental data of the impinging jet at
different locations along the wall (other
locations are not shown.)
•
Kim, J., Hangan, H., (2007). Numerical simulations of impinging jets with application to thunderstorm downbursts. Journal of Wind Engineering and Industrial Aerodynamics 95, 279–298.
Results and discussion
Velocity magnitude (m/s) plot contour.
High speed flow region (close to 7.57 m/s) encountered at the “inlet” and at the “wall”
region.
Results and discussion
Reynolds number dependency
Re = 20,000 ;
Re=2,000,000
Results and discussion
At very high Reynolds number.
Results and discussion
Changing the diameter D of the “inlet”.
Results and discussion
Effects of changing the height H of the inlet from the wall
surface (only location r/D=1.0 and r/D=2.5 shown).
D
C
H/D=2.0 & 4.0
B
A
O
E
Ujet
F
r
Results and discussion
Effects of gravity (in the negative axial direction)
Results and discussion
Transient RANS simulation
Ring vortical flow
pattern near the “wall”
Conclusion
1) Maximum peak velocity magnitude in the whole
computing domain occurs at r=1D.
2) As reynolds number is increased, the height at
which the maximum velocity is decreased.
3) As reynolds number gets extremely large, the flow
is approximately inviscid, and the flow becomes
more periodic and vortices are more organised
and periodic.
4) Decreasing the height H of the inlet results in
increase of the peak speed of that location.
5) There is no significant effect on the flow due to
gravity and changes in the diameter.
Future works
• Study effects of buoyancy and density
stratification
• Performing a 3-dimensional simulation
using Large eddy simulation (LES)
method to study the vortices.
• Study the interaction effect of the flow
with a obstacles blocking the flow.
• Study the effects due to ocean waves,
where the waves are modelled as
roughness elements.