1. Optimization of an Axial Nose-Tip Cavity for Delaying Ablation Onset in Hypersonic Flow Sidra I. Silton and David B. Goldstein Center for Aeromechanics Research The University of Texas at Austin January 6, 2003

2. Overview Motivation Objectives Methodology –Experimental –Numerical Parameter study Conclusions

3. Motivation Need for Decreased Heating –Hypersonic vehicles –High stagnation point heating –Ablation causes perturbations in flight path Previous Work –Passive method to reduce heating Yuceil – experimental Engblom – numerical Forward-Facing Cavities –Shock oscillations –Decrease in surface heating –Cooling Mechanism

4. Cooling Mechanism

5. Objectives Develop understanding of unsteady flow physics –Effect of different cavity geometries Surface heating Ablation onset

6. Experimental Methodology Wind Tunnel Conditions –T   64K –T stag = 370K –P   4693.8Pa Model Development –Ice fiberglass reinforced frozen in LN2 –Mold and spindle –Shield

7. Wind Tunnel Mounted Model During Tunnel Start After Tunnel Start

8. Numerical Methodology Commercial Codes –INCA –COYOTE Procedure

9. Numerical Procedure Flowfield Code used to determine (pseudo-)steady solution (Mean) heat flux distribution per cycle obtained q(x,y) Flowfield code used to obtain (mean) adiabatic wall temperature distribution T aw (x,y) Heat conduction coefficient distribution calculated h(x,y) HEAT CONDUCTION CODE T(x,y,t)

10. Numerical Methodology Commercial Codes –INCA –COYOTE Procedure Assumptions –Flowfield Emulate experimental conditions 2D axisymmetric Laminar Isothermal wall temperature of 100K –Solid Body 2D axisymmetric Initial uniform temperature of 100K or 163K (benchmark study) Ignored sublimation effects Variable material properties of ice

11. Parameter Study Extensive Experiments –Simulations for geometry showing delayed ablation onset Nose-Tip Geometry –D n =2.54 cm –Cavity Dimensions Investigated Length, L Lip radius, r Diameter, D

12. L/D Parameter Study Experiments –r = 0.795 mm, D = 1.113 cm –r = 1.191 mm, D = 1.031 cm –L/D varied from 2.0 to 5.0

13. L/D Experimental Results

14. L/D Parameter Study Experiments –r = 0.795 mm, D = 1.113 cm –r = 1.191 mm, D = 1.031 cm –L/D varied from 2.0 to 5.0 Numerical Simulations –r = 1.191 mm, D = 1.031 cm, L/D = 2.0 (geometry 8) –r = 1.191 mm, D = 1.031 cm, L/D = 4.0 (geometry 12)

15. L/D Numerical Results Mean bow shock speed decreases with increasing L/D –Oscillation frequency decreases with increased cavity depth –  rms approximately constant Mean surface heating increases with L/D –Ablation onset occurs earlier for L/D=4.0 Shallower cavity may be transitioning in experiments t onset =1.46 sec t onset =1.79 sec

16. Experiments –D = 1.27 cm, L/D=3.5, 4.0, 4.5 –r varied from 1.191 mm to 3.175 mm Lip Radius Parameter Study

17. Lip Radius Experimental Results

18. Experiments –D = 1.27 cm, L/D=3.5, 4.0, 4.5 –r varied from 1.191 mm to 3.175 mm Numerical Simulations –r = 1.191 mm, D = 1.27 cm, L/D = 4.0 (geometry 24) –r = 3.175 mm, D = 1.27 cm, L/D = 4.0 (geometry 29) Lip Radius Parameter Study

19. Pressure waves coalesce into shock –Inside cavity for r = 1.191 mm Waves propagate through heat flux –At cavity lip for r = 3.175 mm Mean bow shock speed decreased with increasing lip radius –Oscillation frequency approximately constant –  mean increased with lip radius –  rms decreased with increased lip radius Lip Radius Numerical Results

22. Lip Radius Mean Heat Flux t onset =1.5 sec t onset =3.6 sec Geometry 24Geometry 29

23. Diameter Parameter Study Experiments –D = 0.762 cm, L/D = 4.0 r = 1.905 mm, 3.175 mm, 4.445 mm –D = 1.27 cm, L/D = 4.0 r = 1.984 mm, 3.175 mm –D = 1.778 cm, L/D = 4.0 r = 1.905 mm

24. Diameter Experimental Results

25. Diameter Parameter Study Experiments –D = 0.762 cm, L/D = 4.0 r = 1.905 mm, 3.175 mm, 4.445 mm –D = 1.27 cm, L/D = 4.0 r = 1.984 mm, 3.175 mm –D = 1.778 cm, L/D = 4.0 r = 1.905 mm Numerical Simulations –r/(D n -D) = 0.25, L/D = 4.0 D = 0.762 cm, 1.27 cm, 1.778 cm (geometries 38, 29, 43)

26. Diameter Numerical Results Mean bow shock speed decreases with increasing diameter –Oscillation frequency decreased with increasing depth (L/D=constant) –  mean and  rms increased with increasing diameter Large Diameter Cavity –Pressure waves coalesce into shock inside cavity –Waves propagate through heat flux Small Diameter Cavity –Very little bow shock movement –Cavity remains cold (T=250K)

27. Diameter Mean Stagnation Temperature

28. Diameter Mean Heat Flux Geometry 43 Geometry 29 Geometry 38

29. Diameter Ablation Onset Times

30. Aerodynamic Drag

31. Conclusions Parameter Study –Experimental parameter study –Computational flow visualization Best experimental configurations –Confirms most experimental findings –Flow may indeed be transitioning for sharper cavities –Optimal nose-tip configuration Delayed ablation onset –constant nose diameter means increasing drag –constant drag means decreasing nose diameter Geometry –L/D=4.0, r/(D n -D)=0.25, D/D n = 0.5