| |
By Thomas Paul, Lockheed Martin Space Operations, Houston, TX
View the pdf of this article
Soon after the Space Shuttle Columbia accident occurred last year, a
group of CFD analysts from NASA centers and private industry was
organized to help determine the cause of the accident. This group
was under the direction of the Applied Aeroscience and CFD Branch of the
Aeroscience and Flight Mechanics Division at the Johnson Space Center.
For external flow simulations, non-commercial CFD codes that specialize in
hypersonic or high Mach number flows were used to determine heating
rates, pressures, and temperatures for a large number of vehicle damage
scenarios. Lockheed Martin Space Operations was called upon to provide
CFD support in the area of internal flows within the shuttle wing cavity,
and for these simulations, FLUENT 6.1 was chosen. Two large-scale, simplified
models were run to understand the flow patterns once a breach of the
internal wing cavity was initiated. The results were primarily used to visualize
flow patterns within the wing cavity.
Pathlines inside the wing cavity,
showing impingement on the wheel
well wall at left by the jet entering
through a breach in the wing
Temperature contours show the heating
caused on the wheel well wall (left) by
the hot, high speed jet entering through
the wing breach
The first CFD model included the entire left wing without the wheel
well cavity. The purpose of the first model, which did not include the reinforced
carbon-carbon (RCC) cavity along the wing leading edge, was to
visualize the flow field within the wing cavity immediately after the leading
edge spar breach. This model assumed that the flow coming into the wing
cavity was normal to the spar. It included all of the primary vents that allow
for flow between the main cavities of the wing. A six-inch diameter hole
was modeled in the spar at the approximate location where the spar
breach was judged to have occurred, which was between RCC panels 8
and 9. The results of the modeling showed that at this location, the high
temperature, high velocity gas stream entering the wing cavity impinged
on the outboard wheel well cavity. Instrumentation in the shuttle wheel
well cavity registered abnormal temperatures during reentry, so the
FLUENT results helped support the conclusion of the accident investigation
team that the spar breach was in the RCC panel 8-9 area, and that the initial
spar breach was likely entering the wing cavity normal to the spar. This
model also showed that the flow entering the wing cavity tended to swirl
within the cavity just outboard of the wheel well, and did not initially penetrate
further into the rear cavities of the wing.
2D simulation showing the path of the
jet entering through the RCC breach on
the outside of the vehicle, then through
the spar breach, and into the wing
cavity, ultimately hitting the wheel
well wall
The second CFD model was a 2-D simulation of the left wing cavity and
the RCC cavity. It was used to visualize the flow through the RCC breach,
through the wing spar breach, and into the wing cavity directly outboard
of the wheel well. The purpose of this model was to verify whether or not
it was possible for the flow to come into the wing cavity normal to the
leading edge spar. The results from this 2-D model showed that the internal
structure behind the RCC panels probably deflected the flow entering
the RCC cavity so that it impinged normal to the spar. As in the 3-D model,
this deflected flow stream was found to impinge on the wheel well outer
wall. This model again supported the conclusions regarding the location of
the spar breach and how the flow behaved inside the wing cavity.
Reference:
Columbia Accident Investigation Board Final Report, Volume 5, Book 3, part 13,
section 5.3.6.3, CFD of Simplified Internal Wing Geometry, p.467-480, 2003.
|
|
|
