RADIOSS FSI at NASA Langley: Water Impact of 20 inch Sphere  - Nasa langley
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RADIOSS FSI at NASA Langley: Water Impact of 20 inch Sphere - Nasa langley

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In 2011 NASA Langley completed the construction of a Hydro Impact Basin next to an existing gantry which allows testing of articles for water impact with both horizontal and vertical velocities. NASA ...

In 2011 NASA Langley completed the construction of a Hydro Impact Basin next to an existing gantry which allows testing of articles for water impact with both horizontal and vertical velocities. NASA Langley engineers use simulation in conjunction with the new testing facilities to evaluate water landings for space vehicles.

To better understand Radioss’s Fluid Structure Interaction (FSI) capabilities, a blind benchmark of a water impact test of a 20 inch sphere was conducted. Both Arbitrary Lagrangian Eulerian (ALE) and Spherical Particle Hydrodynamics (SPH) were considered. Options for material modeling of the water were also evaluated.

The Radioss results were then compared to existing test results looking at decelerations and pressure traces.

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RADIOSS FSI at NASA Langley: Water Impact of 20 inch Sphere - Nasa langley Presentation Transcript

  • 1. NASA Langley Radioss Benchmark Water Impact of 20-inch Sphere Antoine Segnegon - AltairInnovation Intelligence® John Brink - Altair Greg Vassilakos – NASA Langley May 16, 2012
  • 2. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Americas Solver Task ForceFormed in 2011 with the purpose of helping existing and potentialcustomers adopt and migrate to Altair Solver TechnologyTeam Members:John Brink – Director – Radioss BulkAntoine Segnegon – Radioss Block ExplicitAndrew Dyer – MotionSolveJaideep Bangal – AcuSolveAbigail Arrington – AcuSolve
  • 3. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Introduction• In 2011 NASA Langley completed construction of a Hydro Impact Basin• The Hydro Impact Basin is next to an existing gantry
  • 4. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Introduction• Allows water impact testing for both horizontal and vertical velocities• Simulation is used in conjunction with the basin to evaluate water landings for space vehicles
  • 5. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Objective of Study• NASA Langley engineers wanted to determine the suitability of Radioss for water impact simulation• Blind Prediction—test results not available until after simulation completed• The problem given was a 20-inch hemisphere dropped from 5 and 10 ft
  • 6. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Responses of Interest• Deceleration of the sphere• Pressure on the surface of the sphere Pressure Transducer Locations Test Set-Up Test Article Configuration
  • 7. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Options to Consider for Radioss Set-upFor Fluid Structure Interaction (FSI) simulation in Radioss, the followingoptions and parameters need to be considered:• ALE or SPH• Mesh Density and Spatial Discretization• Material Modeling• Initial Conditions• Boundaries• Gap of Interface Between Structure and Fluid• Stiffness of Interface Between Structure and Fluid
  • 8. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Initial Set-up for Blind Prediction• ALE (Arbitrary Lagrangian Eulerian) formulation used • Expected to be less CPU costly based on experience• Mesh Density and Spatial Discretization • Followed guidelines from experience for mesh density and relative mesh sizing • Sphere is modeled 20 mm x 20 mm • Fluid mesh is modeled to have surface half the size of the sphere and half again through the thickness Lagrangian 5 ALE
  • 9. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Initial Set-up for Blind Prediction• Material Modeling • Choose Law 51 (Multi-Material Solid, Liquid, Gas) for fluid which exhibits less diffusivity than older multi-material models Rigid Air Infinite Sphere Boundary • Use Law 1 (Linear Elastic) for sphere• Initial Conditions Air Elements • Added gravity to the problem with /GRAV • Did not include initial hydrostatic pressure• Boundaries Water Elements • Used quarter model with symmetries • Used infinite boundary on top of air surface Quarter Model ~900k elements
  • 10. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Initial Set-up for Blind Prediction• Gap of Interface Between Structure and Fluid • Use Type 18 interface (Penalty-stiffness based, coupled Eulerian-Lagrangian) with recommended Gap = 1.5 Lc, where Lc is the characteristic length of the fluid (Lc=Volume/(Largest surface Area)=10x10x5/10x10)• Stiffness of Interface Between Structure and Fluid • The interface stiffness, Stfac, changes with mesh size and velocity and is recommended to be: Stfac = (ρ * V2 * Sel)/GAP ρ - the highest fluid density in the model V – impact velocity of the Lagrangian mesh Sel - Surface area of Lagrangian impact element
  • 11. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Animation of Results – 5 Foot Drop Animation of Results
  • 12. Observation Regarding Fluid Mesh for ALEThe finite element model representing the entire cylindrical tank showedunacceptable behavior at the contact area (top row of pictures) with the firstmesh attempted. The mesh of the fluid was redone in the impact area to avoid“butterfly” mesh. The second row of images show the re-mesh fixed the issue. Leakage at “Butterfly” Interface mesh Cleaner Response Orthogonal mesh
  • 13. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. Comparison to Test Results – 5 Foot Drop • Accelerations match the test results very closely • The pressure trace does not capture the test magnitude • The simulation ran for just over 2 hours on a 16-CPU machineNote: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation resultsThe pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System
  • 14. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. Comparison to Test Results – 10 Foot Drop • Again, accelerations match the test results very closely • The pressure trace does not capture the test magnitude • The reduced model ran in 45 minutes on a 24-CPU machine (4x faster)Note: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation resultsThe pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System
  • 15. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Investigation of Pressure Profile• J.P.D. Wilkinson’s paper “Study of Apollo Water Impact, Final Report, Volume 4, Comparison with Experiments”, May 1967• Pressure profile for water impact shows very steep pressure gradient and very localized effect
  • 16. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Refine Mesh of 5 Foot Drop• The mesh is refined so that the sphere has ~10mm x 10mm elements• The fluid mesh is also refined to maintain proper mesh size ratios to 5mm x 5mm and 2.5mm in the impact direction
  • 17. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved. Mesh Refinement Results • Accelerations peaks match well • Pressures improve but at larger CPU cost (X 5)Note: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation resultsThe pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System
  • 18. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.Conclusions• Doing a blind prediction resulted in very good correlation using standard established modeling practices for Radioss FSI• Pressure traces, however show a lower peak magnitude in simulations; refining the mesh appears to help, but further refinement is necessary to see if the pressures converge to test results• It is unclear whether it is necessary to capture the exact pressure trace to model the structural response of the test article• Including the effect of the tank in the simulation was unnecessary; using infinite boundaries gave very similar results at much less cost• It is recommended to use an orthogonal mesh in the fluid in the area of impact to avoid fluid to structure contact issues• Radioss can be used confidently for water impact problems