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
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
– T64K
– Tstag = 370K
– P4693.8Pa
• 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
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– Dn=2.54 cm
– Cavity Dimensions Investigated• Length, L
• Lip radius, r
• Diameter, D
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
L/D Experimental Results
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)
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
tonset=1.46 sec
tonset=1.79 sec
• 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
Lip Radius Experimental Results
• 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
• 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
Animation
Animation
mean*
4L
*0 LLwheref RT
osc
Lip Radius Numerical Results
Lip Radius Mean Heat Flux
tonset=1.5 sec
tonset=3.6 sec
Geometry 24 Geometry 29
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
Diameter Experimental Results
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/(Dn-D) = 0.25, L/D = 4.0
• D = 0.762 cm, 1.27 cm, 1.778 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
0
1
2
3
4
5
6
7
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
D/Dn
Ab
lati
on
On
set
Tim
e (
sec)
Numerical, Tinit=100K
Numerical, Tinit=163KExperimental
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
D/Dn
CD
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/(Dn-D)=0.25, D/Dn = 0.5
