Transcript Document

EXPERIMENTAL TRANSIENT FLOW SIMULATION
OF A DOWNBURST OUTFLOW
Background
C. Doswell ©
Cloud base
During storms, strong
downdrafts can persist and
impinge at the ground. The
outflow stage of a
downburst can have a peak
speed of 67 m/s at 5 m
above ground level [1].
Wind damage to our built
environment is a concern.
Holmes [2001]
Failure of a tower due to a downburst
(b) Expanding
vortex ring
(1) Downdraft column
(a) Travelling
burst front
(3) Intense low-level outflow
(2) Radial
divergence
Key time-dependent outflow features are:
(a) the burst front and
(b) the dominant vortex ring
Objective
Development of a large-scale
test facility to study downburst
outflow wind loading on
detailed aeroelastic models of
transmission line systems.
Research Carried Out
An initial facility is built at
1:7 scale. An air jet expels
from a slot when a gate is
actuated. Gate control is
with a step motor. The slot
jet leads to a velocity profile
as in a downburst outflow.
Step motor
Threaded rod / nut
Link bar
Instantaneous streamwise
velocity [m/s]
Key Findings
(a)
Flow
direction
Clockwise vorticity
z/b = 4.5
(61 mm )
Time elapsed from
gate opening [s]
(b)
The key transient features of a downburst
outflow are observed in the simulation
Further Work
Normal-to-wall
direction, z
13.5 mm
travel
Connecting
rods
2-D wall
jet
Slot
jet
Um
Wind loading on models will
be studied in a larger facility
using the same approach.
REFERENCE:
Gate
Slot height, b
[1] Fujita TT, 1985. The downburst: microburst and macroburst. SMRP Research
Paper #210, University of Chicago, Dept. of Geophysical Sciences.