April
2006 | Contributed by Adam W. Hapij
Weidlinger Associates, Inc. | www.wai.com
In tracking the shock wave over the course of the animation, the
sequence of events is as follows:
- At 0.30msec, much of the shock wave reflects off of the water/bubbly
water interface;
- While the remainder of the unabated shock front proceeds to envelope
the bubble water and test fixture, local cavitations form at discontinuities
either in front or in back of the fixture;
- At 1.40msec, the remnant of the shock front, having propagated through
the lower impedance bubbly water, impacts the specimen plate. A further
abated reflection proceeds to emanate from the steel-bubbly water interface,
creating a local cavitated region;
- At 1.90msec, a primary shock wave reflection from the tank wall impacts
the back side of the test fixture and proceeds to envelope the assembly.
The Researcher
Adam W. Hapij, P.E. is an associate with the Applied Science Division
of Weidlinger Associates, Inc., a New York City-based consultancy specialized
in various aspects of civil and structural engineering. The company's
purpose is to serve architects, developers, contractors, high-tech
manufacturers, and public agencies who are under increasing pressure
to build and restore facilities or bring products to market faster
and at less cost.
The Applied Science Division's mission is to advance the technology
for simulation of blast, shock, impact, and vibration effects on military
and civilian structures. The division's expertise is based on simulation
using computational tools internally developed, maintained and licensed
to other consultants and government agencies. The U.S. Government and
its contractors as well as local and state governments are the recipients
of the division's research.
Isometric view of test fixture. The discontinuities visible in the
animation are the water-backed flange plates of the test fixture. The
orange plate is the air-backed specimen plate and the region beyond the
rectangular space, centrally located, is submerged in water. The discontinuities
in the contour field account for the refraction and reflection of pressure
waves in the presence of the plates that extend beyond the centrally
located voidspace.
Tracking Underwater Shock Waves
Using proprietary analysis software, Weidlinger researchers performed
a simulation to validate the results of a physical test in which a
test fixture was submerged in a water tank of finite radius. The test
was conducted to better enable the company's researchers to understand
the complex features of fluid structure interaction in an underwater
explosion (UNDEX) scenario. Not being able to visualize the various
phenomena would have forced the researchers to resort to the inspection
of transient records of individual finite elements. Although feasible,
this would have proved an arduous and time-consuming task.
The simulation was generated by Elasto-Plastic Shell Analysis (EPSA)
software, a Weidlinger proprietary finite-element code designed to solve
wave propagation problems by solving hyperbolic partial differential
equations of motions using an explicit time step integration scheme.
EPSA, developed under partial funding from the Office of Naval Research
and the Defense Threat Reduction Agency, provides analysis of submerged
and floating structures in the large deformation, inelastic regime of
dynamic structural behavior.
Visualizing the Effects of Blasts on Structures
After simulating the problem using EPSA, researchers used Tecplot to
visualize the results. The plot represents the details of shock wave
propagation in an underwater explosion scenario. The plots often generated
by Weidlinger researchers include contours of pressure in a medium
strain in a particular material as well as structural deformations
in order to better evaluate the response of structure systems subjected
to blast effects in air, water, or underground.
"Tecplot enables us to conduct a detailed graphical review of simulation
results and reassure ourselves that the response makes physical sense," says
Hapij.
This plot was created using a proprietary post-processing utility that
enables the extraction of pressures in the simulated fluid along a single
place from a model that contained almost a half a million elements.
How Tecplot Helps Researchers Get the Job Done Better
Adam believes that the company's use of Tecplot provides them with a better
understanding of how to design structures to withstand such blasts. "Ultimately
the insight that we gain from visualizing the post-processed data with
Tecplot enables us to design structures that are more resilient to extreme
loadings." In addition, Hapij says that the company uses Tecplot
for quality assurance or to "validate that we have accurately implemented
our simulation methodology."
Before using Tecplot, which Hapij has used since 2001, the company's researchers
used what he describes as "awkward conversion schemes" to graphically
visualize data. The shortcomings of this technique, however, lead the team
to only attempt to visualize deformed finite element grids.
Hapij believes that Tecplot's greatest strengths are its simple/straightforward
data input format, easy management of large amounts of data, and its superior
rendering capabilities. In addition, because Tecplot is a general-purpose
visualization package, its terminology and syntax does not give preference
to any particular discipline of engineering. Other standout features, according
to Hapij, include its simple interface with any software platform and its
easy-to-use advanced features.
By using Tecplot, Hapij says, "We save time, and hence money, in getting
a quick snapshot of the behavior and assess if there are errors or flaws
in the implementation of a solution algorithm."
Without Tecplot, Hapij says his team would be limited as to how much data
they would be able to inspect and/or review. "Limiting the graphical
assessment of the simulated response would compromise the quality of the
product we deliver to our client."
"Tecplot software has provided an incredible boost to our productivity in terms of our research output and the insights we are able to obtain." Rajat Mittal, George Washington University
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April
2006 | Contributed by Adam W. Hapij
