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
