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

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 distributionTaw(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– 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

mean*

4L

*0 LLwheref RT

osc

Lip Radius Numerical Results

Lip Radius Numerical Results

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