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
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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 -gt crack growth -gt material removal
• Prediction of Surface Erosion
• Requires both Modeling and Testing

5
Rotorcraft Aerodynamics
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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

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Potential Theory

• Blade Element Analysis
• Vortex Wake Tracking

Biot-Savart Law
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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)

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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

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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
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Georgia Tech Single Rotor Case
Experimental Set-up
Velocity comparison
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HELIWAKE Hover Solution
Periodic Solution Downwash Velocity Contour
Tip Vortex Structure from Rotor Blades
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Wake Solution
Hover Mode
Forward Flight Mode
Hover Solution using HELIWAKE Module
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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

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CFD Solution



elsA OVERFLOW FUN3D
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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.
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Vortex Preserving Solution
Standard Roe/TVD Flux Split Scheme
Vorticity Preserving Scheme
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Mechanisms/Processes

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

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Trajectory Calculation

• Lagrangian Particle Tracking
• Weber number correlation for Liquids
• Empirical Drag Law
• simpler models but fast running
• Eulerian Particulate 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

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Lagrangian Solver
Two Bladed Hover Case
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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

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Eulerian Formulation
Gas-Particle Interaction Terms
Particle Equation in conservation form
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Solid Rocket Motor
Gas Density
Multi-Phase Nozzle Flowfield Simulation
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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

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Mechanisms/Processes

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

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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

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LSDYNA Model Example
Sphere Model, Target Mesh, Size
Stochastic impact sites after 10 and 200 events
Sphere/Surface Interaction for 10,100,200 impacts
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Sphere Impacting Plate Cases
LSDYNA Code
CTH Code
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Mechanisms/Processes

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

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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
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Crack Growth Analysis
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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
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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

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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

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Computational Architecture
Process Manager
User
Erosion Tool Kit
Data
Communication
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Future Work
Aerodynamic Module Potential Solver (HELIWAKE,
PMARC, CHARM/CAMRAD II ?) CFD Models (Cobalt,
OVERFLOW, FUN3D ? ) Particulate
Module Lagrangian Eulerian FEA Module LS-DYNA Expe
rimental Matrix ( polyurethane )
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Spherical Particles
Far Field Property
mapping

• Lagrangian or Eulerian Particle Solver

Near Rotor Plane
Inverse mapping
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Shape Characterization Methods

• Geometrical
• Equivalent Volume
• Equivalent Mass
• Complex Fourier Descriptor Method
• Measurement
• Microscopy
• Image Analysis
• Sieve Analysis
• Static Light Scattering

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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

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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

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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

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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.