Advancing Fluid-Structure Interaction R&D A recent area of focus for ATA has been developing fluid-
structure interaction simulation methods to evaluate engine nozzle behavior. Because of the wealth of knowledge and data available, the SSME was used as an example to validate these. Previous research at NASA’s Marshall Space Flight Center had never been able to accurately simulate the effect of side loads on the nozzle’s shape. Te analysis always “uncoupled” the fluid dynamics from the structural dynamics because compu- tational fluid dynamics (CFD) took so much more computing power. To work around that, analysts would make certain
CFD model in CHEM, which updates the fluid mesh to reflect the new deformed shape of the nozzle and reanalyzes the fluid mesh accordingly. Tis back-and-forth cycle repeats for every step in the simulation. ATA ran two FSI simulations in sequence to gain a com-
plete look at nozzle dynamics. Te first run determined the shape of the nozzle moments aſter SSME ignition. “We didn’t have the computational resources to simulate those initial seconds when not much was going on, but we needed to get the shape of the engine when things started to get interesting,” said Blades.
“The better your tools are for estimating dynamic loads, the lower the safety factor and the lighter the weight you can achieve.”
assumptions, such as the nozzle being completely rigid and perfectly round. Tese assumptions did not predict the shape of the nozzle during engine firing and did not lead to an ac- curate prediction of the total structural response of the nozzle. ATA decided to test a comprehensive methodology that
would couple structural and fluid dynamics into a full FSI analysis to arrive at a more realistic picture of rocket nozzle dynamics. Te engineers employed the SIMULIA Co-Simula- tion Engine (CSE) capability to link their Abaqus FEA solvers with a flow solver—in this case the Loci/CHEM CFD devel- oped by Mississippi State University (MSU), a code already used by NASA. Starting from a geometric description of the nozzle and a finite element model provided by Rocketdyne, ATA could call upon the wealth of shuttle rocket nozzle data already available from NASA to fine-tune their structural and fluid models. Te CSE was the soſtware “glue” that tightly integrated the
two solvers with mechanisms that provide data synchronization and transfer information for several solvers running concur- rently. It has a mapper to transfer information where the sur- face- and volume-based data overlap, and it has an application programming interface (API) for client simulation programs. Te FEA structural model included the rocket engine
gimbals, the gimbal actuators, the stiffener bands around the nozzle, the main combustion chamber, and the nozzle itself. Te fluid model was based on values for a “single-species gas,” which represented the combined characteristics of the hydro- gen and oxygen in rocket fuel. Te mesh for the fluid domain was created with a mesh generator developed at MSU. During an FSI simulation, the CHEM CFD solver analyzes
its fluid mesh, computes the forces of rocket propulsion on the mesh, and transfers the results of those computations through the SIMULIA CSE to the Abaqus structural solver. Abaqus analyzes the engine structural mesh and generates the structural displacements due to the loads from CHEM. Tese displacements are transferred back through the CSE to the
Te result from the first simulation determined the initial
shape of the nozzle for the second simulation, which gave NASA an idea of how the nozzle deforms, the amount of de- formation, and the side loads involved. Te simulations of the low-frequency structural response of the nozzle were consis- tent with earlier physical observations of the SSME during test and operation. “All of our predictions were in line with what NASA was expecting,” Blades noted.
A First in Rocket Nozzle Simulation “Tese simulations using Abaqus and CHEM CFD early
in the engine-startup sequence represent the first-ever fully coupled, time-accurate, 3D FSI simulation of a rocket engine nozzle,” said Blades. “And while the coupled simulations were more expensive than the CFD-alone simulations NASA currently performs, they weren’t prohibitively so. Te CPU requirements are within reason.” ATA’s next step is to fully validate their FSI simulations
to demonstrate the accuracy of the co-simulation methodol- ogy. “NASA and others in the rocket-engine community may want to use this technology to better represent the physical phenomena in aerospace applications,” said Blades. Te chances of that happening are good: ATA was recently
selected to continue their work within NASA’s SLS program. “Next-generation launch systems will require propulsion systems that deliver high thrust-to-weight ratios, increased trajectory-averaged specific impulse, reliable overall vehicle systems performance, low recurring costs, and other inno- vations to achieve cost and crew-safety goals,” said Blades. “Te existing SSME, on which we based our recent work, will be used for the main stage of this rocket, and NASA is also looking to use our FSI simulation methodology to design the upper-stage engines.” ✈
Article edited by Yearbook Editor Michael Anderson from information provided by Dassault Systèmes.
Aerospace & Defense Manufacturing 2013 89
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