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

Overview Motivation Objectives Methodology –Experimental –Numerical Parameter study Conclusions

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

Cooling Mechanism

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

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

Wind Tunnel Mounted Model During Tunnel Start After Tunnel Start

Numerical Methodology Commercial Codes –INCA –COYOTE Procedure

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)

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

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

L/D Parameter Study Experiments –r = mm, D = cm –r = mm, D = cm –L/D varied from 2.0 to 5.0

L/D Experimental Results

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

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

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

Lip Radius Experimental Results

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

Pressure waves coalesce into shock –Inside cavity for r = mm Waves propagate through heat flux –At cavity lip for r = 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

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

Diameter Parameter Study Experiments –D = cm, L/D = 4.0 r = mm, mm, mm –D = 1.27 cm, L/D = 4.0 r = mm, mm –D = cm, L/D = 4.0 r = mm

Diameter Experimental Results

Diameter Parameter Study Experiments –D = cm, L/D = 4.0 r = mm, mm, mm –D = 1.27 cm, L/D = 4.0 r = mm, mm –D = cm, L/D = 4.0 r = mm Numerical Simulations –r/(D n -D) = 0.25, L/D = 4.0 D = cm, 1.27 cm, cm (geometries 38, 29, 43)

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)

Diameter Mean Stagnation Temperature

Diameter Mean Heat Flux Geometry 43 Geometry 29 Geometry 38

Diameter Ablation Onset Times

Aerodynamic Drag

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