Rotor Blade Erosion Phenomenology

1. Rotor Blade Erosion Phenomenology Mr. Robert Lee Military Systems Technologies, LLC. and Dr. William F. Adler University of California at Santa Barbara International Helicopter Safety Symposium Montreal, Canada September, 2005

2. Motivation • Basic rotor blade erosion due to particulate impact is not well understood. Consequently, the current approach to develop blade erosion protection material via laboratory testing is often costly and performs poorly in the field. • A robust Modeling and Simulation tool is sought to augment the laboratory testing which will improve the developmental time and cost, and ultimately, provide a better protection solution. • A predictive tool will increase the life cycle of rotor blades thereby reducing replacement costs and increase operational readiness of current assets in the theater of operation. • Better understanding of blade rotor erosion will increase the safety of the rotorcraft.

3. Phenomenon Potential damage to window and other components

4. Mechanisms/Processes • Rotor Induced Flowfield Environment • Aerodynamic Load • Trimmed and Articulated Rotor • Particulate Trajectory • define impact conditions • Non-spherical particles • Particulate/Blade Impact Interaction • FEA Analysis • Surface/Substrate Damage initiation -> crack growth -> material removal • Prediction of Surface Erosion • Requires both Modeling and Testing

5. Rotorcraft Aerodynamics

6. Rotorcraft Aerodynamics • Industrial Standard (Potential Theory) • Blade Element Analysis (Prandtl Lifting Line) • Wake Analysis (Vortex-Lattice) • Aeromechanics • Blade Dynamics (pitch,flap,yaw) • Trimmed Rotor • Modern Approach • CFD using Navier-Stokes methodology • Structured-Grid Solver • Overset Grid Solver (OVERFLOW) • Unstructured-Grid Solver (FUN3D, Cobalt, CRUNCH) • Multi-Field Approach for Fluid-Structure Interaction

7. Potential Theory • Blade Element Analysis • Vortex Wake Tracking Biot-Savart Law

8. Vortex-Lattice Method • Based on the solution of incompressible, inviscid flow equations • Good accuracy can be obtained with coarse grid • Difficult to predict detailed tip vortex roll-up • Does not account for thickness, viscous, separation, or compressibility effects • Computational time can be short • Uses either “prescribed wake” (based on experimental data) or “free wake” (wake structure solved for directly at each time step)

9. HELIWAKE Model • Free Wake Analysis Model • An earlier assessment of the HELIWAKE model formulated by Dr. Crimi from Cornell University showed good agreements with experimental data • Rotor tip vortices are tracked in time and the vortex induced velocity field is calculated via Biot-Savart Law • This model was rewritten under the Phase I effort • Model was coupled to the Lagrangian particle solver

10. HELIWAKE Solution Cornell Aeronautical Lab Experiment Case 7% of R below rotor plane 20% of R below rotor plane Comparison between predicted and measured downwash velocity

11. Georgia Tech Single Rotor Case Experimental Set-up Velocity comparison

12. HELIWAKE Hover Solution Periodic Solution Downwash Velocity Contour Tip Vortex Structure from Rotor Blades

13. Wake Solution Hover Mode Forward Flight Mode Hover Solution using HELIWAKE Module

14. Modern CFD Methods Structured Grid Unstructured Grid • Simplifications must be made (e.g. rigid blades) • Requires high grid resolution • high computational cost • Transient simulation is still too costly

15. CFD Solution

16. Numerical Dissipation • Conventional modern CFD schemes preserves fluxes but does not preserve vorticity • Vorticity preserving scheme requires conservation of vorticity transport equation. Morton, K.W., and Roe, P.L., “Vorticity-Preserving Lax-Wendroff Schemes for the System Wave Equation”, J. of Sci. Comp., Vol. 23, No. 1, pp. 170-192, January 2001.

17. Vortex Preserving Solution Standard Roe/TVD Flux Split Scheme Vorticity Preserving Scheme

18. Mechanisms/Processes • Rotor Induced Flowfield • Aerodynamic Load • Trimmed Rotor • Particulate Trajectory • define impact conditions • Particulate/Blade Impact Interaction • FEA Analysis • Surface/Substrate Damage initiation -> crack growth -> material removal • Prediction of Surface Erosion • Requires both Modeling and Testing

19. Trajectory Calculation • Lagrangian Particle Tracking • Weber number correlation for Liquids • Empirical Drag Law simpler models but fast running • EulerianParticulate Solver • Dilute particulate cloud density is assumed • Particulate volumetric effect and particle-particle interaction is ignored Resulting equation is similar to NS equations • DSMC Method (Stochastic) • Particle-Particle Interaction • Particle-Surface Interaction Beyond the scope of current effort

20. Lagrangian Solver Two Bladed Hover Case

21. Eulerian Particles • Assume dilute Gas/Particle mixture • Low volumetric loading but occupy significant mass • Particle-Particle interaction is neglected • Assume spherical particle shape with no break-up or agglomeration • Particle size distribution is represented in discrete bins • Gas/Particle interaction is obtained from viscous drag and heat transfer

22. Eulerian Formulation Gas-Particle Interaction Terms Particle Equation in conservation form

23. Solid Rocket Motor Gas Density Multi-Phase Nozzle Flowfield Simulation

24. Particle Environment • Empirically characterize far-field boundary profile • Sieve Analysis to determine the size profile • Shape characterization • Make equivalent spherical particle profile • Size • Shape • Convect the particles to the Rotor plane • Eulerian • Lagrangian • Populate cells with representative angular particles • Use FEA analysis to simulate particle impacts • LSDYNA

25. Mechanisms/Processes • Rotor Induced Flowfield • Aerodynamic Load • Trimmed Rotor • Particulate Trajectory • define impact conditions • Particulate/Blade Impact Interaction • FEA Analysis • Surface/Substrate Damage initiation -> crack growth -> material removal • Prediction of Surface Erosion • Requires both Modeling and Testing

26. FEA/Solid Mechanics Model • LSDYNA (Lawrence Livermore Lab) • Surface Deformation • Impact Model • Large material database • Generalized Motion • CTH Code (Sandia Lab) • Impact Physics • Multi Material • Large Deformation • Eulerian Framework • Adaptive Mesh Refinement • Commercial Code

27. LSDYNA Model Example Sphere Model, Target Mesh, Size Stochastic impact sites after 10 and 200 events Sphere/Surface Interaction for 10,100,200 impacts

28. Sphere Impacting Plate Cases LSDYNA Code CTH Code

29. Mechanisms/Processes • Rotor Induced Flowfield • Aerodynamic Load • Trimmed Rotor • Particulate Trajectory • define impact conditions • Particulate/Blade Impact Interaction • FEA Analysis • Surface/Substrate Damage initiation -> crack growth -> material removal • Prediction of Surface Erosion • Requires both Modeling and Testing

30. Structural Complexity • Rotor blades incorporate thin walled design concepts • Blades have a complex internal structure to withstand load and service conditions • Honeycomb structures are used in blades and plates Crack Growth

31. Load Spectrum Load Cycle Stress Range Update Crack Length Crack Growth For ΔK Stress Intensity Factor ΔK Empirical Data Crack Growth Analysis

32. Empirical Data Polyurethane 3M 8663 Tape Single Impact Tests Testing Optical and SEM Microphotograph Strain Gages Thermocouple Material Response Characterization Observation and Analysis Mechanics of Failure and Degradation Rotor Erosion Tool Kit Physical and Computational Models

33. Summary of Phase I Effort • A vortex tracking module was written • A Lagrangian particle tracking module was written • Survey of literature was conducted with regard to CFD application of helicopter flowfields • LS-DYNA model was evaluated • Examined particle characterization issues • Angular particles • Shape and Size • Examined Particle/Surface Interaction Models

34. Phase I Efforts (continued) • Initiated empirical work • Silica particles (sieving, shape factor, mechanical and physical property) • 3M 8663 protective tape • Particle Impact Experiment Test Matrix • Formulate Erosion Tool Architecture • Identify necessary component modules • Identify FD-CADRE approach as backbone of tool architecture

35. Aerodynamic Particle FEA Damage Crack Growth Material Removal Empirical Model Computational Architecture Process Manager User Erosion Tool Kit Data Communication

36. Future Work • Aerodynamic Module • Potential Solver (HELIWAKE, PMARC, CHARM/CAMRAD II ?) • CFD Models (Cobalt, OVERFLOW, FUN3D ? ) • Particulate Module • Lagrangian • Eulerian • FEA Module • LS-DYNA • Experimental Matrix ( polyurethane )

37. Spherical Particles Far Field Property mapping • Lagrangian or Eulerian Particle Solver Near Rotor Plane Inverse mapping

38. Shape Characterization Methods • Geometrical • Equivalent Volume • Equivalent Mass • Complex Fourier Descriptor Method • Measurement • Microscopy • Image Analysis • Sieve Analysis • Static Light Scattering

39. Angular Particle Impact on Polyurethane • Graded Silica Particle Definition • Evaluate shape factors • Mechanical and Physical Properties • Particle Impact Experiments • Experimental Set-Up and Diagnostics • Test Matrix

40. Characterization of Particle Impact Damage • Particle Dynamics • Analytical Description of observed interactions • Observation of Damage Modes • Optical Microscopy • Scanning Electron Microscopy (SEM) • Identify onset of failure and damage progression • Identify conditions for material removal

41. Computational Model Development • Angular Particles • Concentration • Size Distribution • Impact Locations • Formulate Multiple particle Impacts • Develop criteria for initial damage • Develop criteria for damage growth for random multiple particle impacts • Develop criteria for material removal

42. Aerodynamic Particle FEA Damage Crack Growth Material Removal Empirical Model Rotor Blade Erosion Tool Balanced Modeling and Empirical Work is planned Acknowledgement The support of Army Research, Development and Engineering Command (RDEC) is gratefully acknowledged.