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CFD Simulations for the2nd Aeroelastic Prediction Workshop using EZAIR
AIAA SciTech, January 2016, San-Diego CA
Dr. Tomer RokitaRafael, RD&E Division, Aerodynamics Department
Benchmark Supercritical Wing (BSCW)
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Who are we?• RAFAEL – Advanced Defense Systems
– Develops and manufactures advanced defense systems for the IDF, as well as for foreign customers around the world.
– Innovative solutions at the leading edge of global technology• Air superiority, space, underwater, naval, and ground systems
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Iron Dome SPICE Python-5
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Flow Solver• EZAIR code
– Developed by the Israeli CFD Center (ISCFDC)
– 2nd generation code following the EZNSS code
– Euler/Navier-Stokes finite volume solver
– 3rd order (biased) spatial, 2nd order temporal
– Chimera (overset) suite + 6-DOF motion suite
– Aeroelastic suite (modal approach)
– RANS + Hybrid RANS/LES turbulence models
– Fully parallel MPI/OpenMP
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AePW2 CasesCase 1 Case 2 Case 3 (Optional)
A B C
Mach 0.70 0.74 0.85 0.85 0.85
Angle of attack
3° 0° 5° 5° 5°
DynamicData Type
Forced oscillation
Flutter Unforced Unsteady
Forced Oscillation Flutter
Gas type R134-a gas R12 gas R134-a gas
Notes: • Attached flow solution.
• Oscillating Turn Table (OTT) exp data.
• Unknown flow state.
• Pitch and Plunge Apparatus (PAPA) exp data.
• Separated flow effects.
• Oscillating Turn Table (OTT)experimental data.
• Separated flow effects.
• Oscillating Turn Table (OTT)experimental data.
• Separated flow effects on aeroelastic solution.
• No experimental data for comparison.
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Computational Setup• Grid: single coarse grid (C-O topology, 1.7M)
• Flux construction: HLLC, vanalbada limiter
• Spatial order: Up-wind 3rd (biased) order
• Temporal order: 2nd order with dual-time stepping (basic time step used as 1ms, with 50 sub-iterations)
• Time marching:– Diagonally-dominant alternate direction implicit (DDADI)
– Line Gauss-Seidel with B2 scheme (convergence issues)
• Turbulence model:– RANS: kω TNT (Kok, 2000)
– Hybrid RANS/LES (DES): X-LES (Kok, 2004)
• Aeroelasticity: modal approach – Dynamic: global time step coupling
– Static (at flutter conditions): dynamic with high damping5
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Results – Case 1 (M=0.70, α=3°)• Steady results:
– “Straight-forward, simple” case
– Linear regime: no shocks, limited trailing-edge flow separation
– Good agreement with experimental results
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Results – Case 1 (M=0.70, α=3°)• Unsteady (forced oscillation) results:– Rigid-body prescribed motion (aeroelastic module is not activated)
– Time step (1ms) corresponds to 100 steps per cycle (sensitivity checked for 200 step per cycle)
– Dual-time stepping: 4 orders of magnitude convergence at each time step
– Total time recorded: 1.2sec (12 oscillation cycles) for “complete” periodicity of the signal and statistical analysis
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Results – Case 1 (M=0.70, α=3°)• Unsteady (forced oscillation) results:– Low frequency, almost quasi-steady
(small time-lag in integral forces)
– Large response concentrated at L.E. (expected)
– Non-linear effect (shock) appears at upper surface
– Response not only in the actuation frequency
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Results – Case 1 (M=0.70, α=3°)• Unsteady (forced oscillation) results:– Upper surface: main response at the actuation frequency, “folded”
frequencies appear near shock location
– Reasonable agreement with experimental results. Double-peak magnitude behavior corresponds to shock oscillation
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Results – Case 1 (M=0.70, α=3°)• Unsteady (forced oscillation) results:– Lower surface: “only” response at the actuation frequency (no shocks)
– Good agreement with experimental results. Slight phase deviation near trailing-edge might be associated with T.E. separation
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Results – Case 1 (M=0.70, α=3°)• Unsteady (forced oscillation) results:– Sum up of case 1: time-step sensitivity check showed
convergence/independency
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Results – Case 2 (M=0.74, α=0°)• Steady results:
– “Straight-forward, simple” case (low lift CL≈0.2)
– Linear regime: no shocks, limited trailing-edge flow separation
– Good agreement with experimental results
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• Flutter results: structural modes mapping
– Used provided structural model (NASTRAN)
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Heave mode,
3.32Hz
Pitch mode,
5.25Hz
Results – Case 2 (M=0.74, α=0°)
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Results – Case 2 (M=0.74, α=0°)• Flutter results:– Three simulations ran: Exp. flutter dynamic pressure (168.8psf),
-25% (pre-flutter), +25% (post-flutter)
– Simulation I.C.: rigid solution
– Recorded time: 1.6sec (dt=1ms)
– Frequency and damping calculated based on pitch angle time history
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flutter
Results – Case 2 (M=0.74, α=0°)• Flutter results:– Frequency and damping calculation of each mode response (“V-g-ω”)
Stable
Unstable
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Results – Case 3 (M=0.85, α=5°)• Steady results:
– Non-linear “complicated” case (shock induced flow separation)
– RANS model, kω TNT: convergence issues, used robust LGS+B2 scheme to converge in steady state
– Only fair agreement with experimental results; rear shock location
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Results – Case 3 (M=0.85, α=5°)• “Steady” results:
– Hybrid RANS/LES (DES) model (X-LES): Shock-wave oscillation + spanwise structures (coarse grid is not suitable)
– Better agreement to experimental results.
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• Unsteady (forced oscillation) results:– Shock wave frequency “locked” on actuation frequency
– RANS results in a aft spanwise-uniform shock wave relative to DES (front, spanwise non-uniform)
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kω (TNT) X-LES
Results – Case 3 (M=0.85, α=5°)
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kω (TNT)
X-LES
Results – Case 3 (M=0.85, α=5°)• Unsteady (forced oscillation) results:– RANS: response mainly at actuation frequency
– DES: turbulent spectrum after shock (separation)
– Only fair agreement near shock, good agreement elsewhere
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kω (TNT)
X-LES
Results – Case 3 (M=0.85, α=5°)• Unsteady (forced oscillation) results:– Better agreement for the lower surface
– Shock oscillation is not apparent in the experimental magnitude FRF results
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Conclusions
• EZAIR simulations results for the 2nd AeroelasticPrediction Workshop (AePW2) were presented, for all three cases required
• Overall good agreement with experimental results:
– “Exact” pressure distributions for the “simple” cases (1&2)
– Accurate flutter dynamic pressure prediction (case 2)
– Only fair agreement for the optional case 3 (DES capabilities were demonstrated for massive flow separation; however grid refinement is required for further investigation)
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Thank you!