Fighter Damage by Dytran

To help develop aircraft that are better equipped than ever to face enemy fire, researchers at the Agency for Defense Development (ADD), Daejeon, South Korea, have turned to computational Fluid-Structure Interaction (FSI) to understand airframe survivability under battle conditions.

Wing in the Crosshairs
A broad range of potential damage scenarios face fighter planes, but one of the most critical and perhaps most likely is sustaining damage to the wings. The wings and the fuel tanks that they contain are particularly vulnerable to enemy fire because of their large surface area and the high volatility of jet fuel. For this reason, ADD’s Jong H. Kim, senior engineer, and Seung M. Jun, team leader and principal engineer, have chosen to look closely at battle damage involving the wing. In particular, their research studies a phenomenon known as hydrodynamic ram and how it relates to the wing fuel tanks and adjacent areas. Hydrodynamic ram is the damage process that occurs when a high-velocity projectile, such as enemy fire, penetrates and detonates a structure containing fluid, causing a blast wave. 

Traditionally, testing how an aircraft might react to battle damage requires field experiments that re-create battle conditions. In other words, these experiments require blowing up actual planes or plane components. For obvious reasons, these tests are incredibly costly and time consuming, making it impractical to carry out large numbers. Generally, this reality whittles down the list of actual field tests to situations that are the most likely and most threatening to an aircraft’s survival in battle. Kim and Jun’s work set out to determine whether it was possible to use computer simulation and analysis to gather useful information about battle damage scenarios. If so, computer simulations could eliminate much of the costly field testing required to build better aircraft. 


A Stepped Approach to Analysis
To determine whether accurate damage prediction was feasible, the team proceeded by steps, starting with the analysis of hydrodynamic ram and its effect on a plane’s metal fuel tank, as represented by simple geometry. Even this initial analysis posed a sophisticated physics problem because it had to consider various interrelated factors, including the projectile’s drag, the cavity behind the projectile, the pressure on the fuel in the tank, structural stress, and deformation. Given the high nonlinearity and the very short time span involved, explicit time integration was also a must in the simulation. 

To perform the calculations, the team used Dytran from MSC.Software because it possesses a multi-material Euler solver. This decision allowed researchers to more accurately analyze both the Eulerian regions of fluid and air as well as the Lagrangian region represented by the projectile entering the tank.  The simulation included the contact between the projectile and tank wall using a master-slave contact. At each microsecond time step, analysis verified whether any of the projectile’s grid points had penetrated the tank. For any point where penetration had occurred, a force was applied to the projectile grid in the direction opposite of penetration. Furthermore, an adaptive contact algorithm stopped applying force to any failed element in the contact region, allowing for more accurate simulation of hole formation at the penetration point. One notable aspect of the team’s research was that it simultaneously took into account both the structural rupture to the tank and the accompanying fluid burst. 

To see the results of this sophisticated first portion of the study, Kim used EnSight, a visualization software program from Computational Engineering International (CEI) of Apex, NC. EnSight was particularly well suited for the project because it was capable of discretely handling Euler and Lagrangian elements simultaneously in post-processing, which allowed researchers to look at both fluid results as well as structural results.

Kim next looked at the effects of detonating the projectile once in the tank. The detonation, which transfers shock waves through the fluid to the tank walls, was modeled using three multiple adaptive Euler domains defined as water (which stood in for the fuel), air, and the explosion of the projectile. One of the particularly complex areas of the computations was that the three meshes and the tank’s Lagrangian mesh could easily overlap with one another at any point, depending on how the waves caused by the explosion spread through the fluid to the tank walls. 

Animations with Actual Wing Geometry
Based on the results of the preliminary experiments, Kim and the ADD team then moved on to study the ultimate subject: a projectile detonation inside an intermediate complexity wing (ICW). What made this study particularly realistic was that it not only looked at a simple fuel tank but considered the more complex structure of an ICW. The tanks contained in these wings are divided into multiple cells, which are partitioned by various spars and ribs. The study modeled each of these cells and considered the multiple coupling surfaces as well as the holes which allow fuel to flow between cells. Using the same solver as the previous experimentation, the team ran computational simulations and then loaded the results into EnSight for visualization.  

Initially, Kim used another post-processing tool for the visualization but struggled with it, due to the first application’s limited ability to handle his complex modeling. His decision to move the data to EnSight, however, was a real turning point. “Switching to EnSight, I not only saved time and effort on postprocessing the analysis results,” emphasizes Kim, “but I was also able to show better images and animations to people in reduced time. EnSight is really the right match for FSI (fluid structure interaction) analysis.” 

Kim not only used EnSight to create standard animations but also took advantage of EnSight’s stereo feature to create 3D animations. With the use of passive stereo, polarized 3D glasses, it was easy to share vastly more realistic results of the team’s work, whether with other researchers and engineers or with less-technically oriented decision-makers. 

The results of Kim and Jun’s research shows great promise for future development in this area. Their simulations have proven to be consistent with the physics of hydrodynamic ram, demonstrating that simulations have the potential to supplement costly field testing. Future research in this area will allow for more detailed predictions of penetration and detonation damage, among other advances. 

For more information on the research of Jong H. Kim and Seung M. Jun:  LINK

http://www.worldscinet.com/ijmpb/22/2209n11/S021797920804702X.html
https://www.mscsoftware.com/Solutions/Success-Stories/Detail.aspx?storyid=33
http://www.mscsoftware.com/Submitted-Content/Success-Story/PDF/SS_ADD_A4_w.pdf
http://www.mscsoftware.com/Submitted-Content/Resources/14-ADD_Kim.pdf

CAPTION 1:
A simulation of a fighter plane’s intermediate complexity wing as it explodes from enemy fire.

CAPTION 2:
Enemy fire penetrates the wing fuel tank of a fighter jet, causing hydrodynamic ram effect.

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