Wind Tunnel Validation of Computational Fluid Dynamics-Based Aero-Optics Model D. Nahrstedt & Y-C Hsia, Boeing Directed Energy Systems E. Jumper & S. Gordeyev, University of Notre Dame J. Ceniceros, Boeing SVS L. Weaver, AFRL/DE L. DeSandre, Office of Naval Research (formerly with JTO/HEL) T. McLaughlin, US Air Force Academy 20 21 Jun 07
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Outline Phase II Program summary Background: Phase I summary Turret requirements & configuration CFD code, computational grid, & OPD calculation Typical CFD flow solutions & OPD maps Validation Summary & conclusions Page 2
Program Summary Need Validate CFD-based aero-optics model to analyze optical performance of larger, more realistic airborne system Objectives Validate CFD-based OPD model using wavefront sensor data from wind tunnel experiments Exercise CFD model to assess performance of larger, more realistic configuration with conformal window Determine wavefront control system requirements CFD validation Approach Wind tunnel WFS measurements of phase over scaled turret with conformal window Compare with CFD-based model at 1:1 scale including wind tunnel boundaries & inlet flow profile Large scale analysis Assess performance of larger turret with conformal window Evaluate wavefront control requirements Status Program successfully completed Turrets designed & fabricated Wind tunnel tests conducted WFS data collected CFD-based aero model validated Large scale analysis & wavefront control requirements completed Page 3
Phase I Results Wind Tunnel Test 1.5 Diameter Turret with Conformal Window Successful Phase I wind tunnel tests conducted at Notre Dame Configuration: 1.5 turret with conformal window Mach number M0.36, 0.5, 0.6, & 0.68 M0.5 basis for validation Lines-of-sight Azimuth = 0 (overhead pass) Elevation = 30 to 160 Fluid measurements Steady & unsteady pressure Velocity Optical measurements Malley probe (1D phase in flow) 2D Hartmann WFS In Draft Tunnel Inlet Test Section Configuration Test Section Static Pressure Coefficient Pressure Turret Optical Turret Page 4
JTO Aero-Optics Phase I Results CFD Validation Validated Over Realistic Lines-of-Sight Resolved Horseshoe Vortex Updated CFD-based aerooptical model Figures-of-merit Tilt-corrected OPD In-flow phase correlation length Time-averaged mean & standard deviation Unsteady OPD (RMS nm) Increased node density to resolve turbulent bdy layer, free shear layer, & necklace vortex Implemented Partially 80 Averaged Navier-Stokes 70 60 (PANS) technique in k-ε 50 turbulence model Correct Shear Layer Structure CFD Code Wind Tunnel 40 30 20 10 Mach 0.5, 1.5 Inch Turret 0 90 100 110 120 130 Elevation Angle (Deg) Optical Path Difference 140 Phase Correlation Length Page 5
Phase II Turret & Lines-of-Sight 12 Diameter Turret with Conformal Window Configuration: 12 turret with conformal window Mach number M0.35, 0.4, & 0.45 M0.4 basis for validation Lines-of-sight Azimuth = 0 (overhead pass) Elevation = 45 to 130 Fluid measurements Steady & unsteady pressure Velocity Optical measurements Malley probe (200 KHz in-flow 1D phase) 2D Hartmann WFS (10 Hz) USAFA 36 Wind Tunnel Window Port Turret on Wind Tunnel Wall θ elevation = 60, 103, 130 θ elevation = 46, 76, 120 Flow Turret Location Adjusts on Side of Wind Tunnel Flow Turret Optical Turret D turret = 12 H cyl = 4.5 D beam = 5 WT Section Page 6
Beam Train & WFS Configuration Lenslet Array & WF Processing Phase Disturbance Tunnel Back Window Turret Tunnel Front Window Beam Steering Mirror Hartmann WFS System Flow Instruments Beam Splitter Source & BEX Laser Injected into Tunnel Optical Bench System Configuration Laser & beam expander Turret generating phase disturbance WF sensor & processing AFA Implementation Laser beam injected thru wind tunnel front window Turret on opposite vertical wall of wind tunnel Page 7
CFD Analysis Approach Grid Generation, Flow Solution, & Path Integration Generate computational grid Initialize CFD code & run Node array for flow solution Varying zones & grid density Steady-state soln from Navier-Stokes eqns Unsteady flow solution Integrate density variations Interpolate flow soln to OPD array Integrate density to yield OPD(time,LOS) Page 8
Zone & Node Density About Turret Grid Development Zones & Nodes Define Computational Boundaries 52 Zones 2.7M Nodes CFD model includes turret, wind tunnel walls, & inlet flow profile Conformal window flow symmetry allows single grid for all Mach numbers & LOS angles Structured grid Grid density increased in boundary & shear layers Extends 45 upstream to 150 downstream 52 zones 2.7 million nodes Page 9
Flow Solver & Conditions Time Iterative Density/pressure-based Algorithm (TIDAL) Code features Generalized 3D flow solver Finite 3D volume with multi-zone method Structured grid Steady & unsteady flow Dual time stepping for time-accurate calculations Partially Averaged Navier-Stokes (PANS) k-ε model Values 5 μsec time step Solution saved at Δt = 50 μsec 300 frames saved (15 msec) for wavefront analysis ftk = 0.4 ~ 1.0 Freestream: ftk = 0.5 Wall: ftk = 1.0 Variable ftk Page 10
OPD Calculation N X N Beamlet Array Generate beam grid 25 x 25 mesh ~ 12.7 x 12.7 x 120 cm 3 Beam grid extends from turret window to tunnel wall Interpolate flow density to beam grid Integrate density or index along beam direction (grid line) to obtain OPL Use ambient density inside turret & outside of tunnel Page 11
Typical Flow Solutions Solution Shows Required Features Instantaneous realizations of central plane & shoulder plane sections Low pressure aft result of wake Low pressure, circular area forward at base is core of necklace vortex Instability in shear layer rolls into vortices Pressure within vortices shows oscillatory behavior as in PIV Resolution of shear layer vortices is critical to accurate simulation of aero-optical effect Page 12
Fluid Mechanical Validation First Match Fluid Properties 2.0 Location 1 Location 3 Location 5 2.0 2.0 Locations of unsteady pressure sensors Z/H 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 CFD, M=0.4, Case 1 CFD, M=0.4, Case 2 Test, M=0.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 U_mean/U_in Z/H 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 CFD, M=0.4, Case 1 CFD, M=0.4, Case 2 Test, M=0.35 0.0 0.2 0.4 0.6 0.8 1.0 1.2 U_mean/U_in Z/H 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 CFD, M=0.4, Case 1 CFD, M=0.4, Case 2 Test, M=0.35 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 U_mean/U_in CFD-based & measured velocity profiles (Case 1 is selected model) CFD & measured C P over elevation angle Locations of velocity profile sensors Page 13
OPD Maps in Time Piston & Tilt Removed OPD at 60, 90, 120 & 132 Elevation Page 14
JTO Aero-Optics Optical Validation FOM: RMS Wavefront Error & Phase Correlation Length 0.0 50 100 Mean RMS OPD Bin (nm) OPD WT(M0.4,120 Deg) Phase Correlation Length 0.4 0.2 0.1 150 0.0 0 50 100 Mean RMS OPD Bin (nm) Measured & CFD-based PDF for RMS OPD at each elevation CFD Code 80 Wind Tunnel OPD CFD(M0.4,132 Deg) 0.4 OPD WT(M0.4,132 Deg) 0.3 132 0.2 150 0.1 Correl_WT(M0.4,90 Deg) 0.7 0.6 90 0.5 0.4 0.3 0.2 0.1 0 50 100 150 Mean RMS OPD Bin (nm) 0.2 0.1 0.0 2 4 6 8 10 12 14 2 4 6 8 10 12 14 16 Correlation Length Bin (mm) 16 Measured & CFD-based PDF for phase correlation length at each elevation 10 50 40 30 20 10 Mach 0.4, 12-Inch Turret CFD Code 9 Wind Tunnel 8 7 6 5 4 3 2 Mach 0.4, 12-Inch Turret 1 0 60 0 80 0.3 Correlation Length Bin (mm) 0.0 60 70 132 0.4 0 0 70 60 Correl_WT(M0.4,132 Deg) 0.5 0.0 Phase Corr Length (cm; Along Flo Dir) 90 Correl N170CFD(M0.4,90 Deg) 0.8 120 0.3 Correl CFD(M0.4,132 Deg) 0.6 Normalized Freq of Occurrence 0.1 Normalized Freq of Occurrence 0.2 0 OPD CFD(M0.4,120 Deg) 0.5 60 Normalized Freq of Occurrence 0.3 Unsteady OPD (RMS nm) Normalized Freq of Occurrence OPD WT(M0.4,60 Deg) Normalized Freq of Occurrence Wavefront Error OPD CFD(M0.4,60 Deg) 0.4 90 100 110 Elevation Angle (Deg) 120 130 140 70 80 90 100 110 120 130 140 Elevation Angle (Deg) Page 15
Summary & Conclusions Successful Model Validation Aero-optical effects beyond separation point can be a significant performance degrader in airborne systems Wind tunnel testing is expensive, time-consuming, & subject to scaling limitations; exercising CFD is cost effective JTO program successfully validated CFD-based aero-optical model based on fluid & optical FOMs Good agreement using reasonable turret & window configurations over practical range of LOS angles Lessons learned: Grid generation (resolution vs CPU time), turbulence model, & scaling limits WFS data useful for validation in future CFD development Page 16