Hypersonic Boundary-Layer Transition on a Blunt Ogive: Measuring Controlled Nose Tip Roughness

Prediction of boundary-layer transition is a critical element of hypersonic vehicle design

due to the impact transition has on boundary-layer separation, heat transfer, and aerodynamic

control. Transition is affected by many factors including surface roughness. The

roughness on a hypersonic vehicle can cause a boundary-layer to become turbulent. However,

there is a limited understanding of the impacts that roughness can have, and the conditions

under which it is important.

The rocket-sled track at Holloman Air Force Base was selected as a ground-test facility

for transition measurements. The present work is about understanding the mechanism of

transition on blunt ogives or blunt cones with moderate nose radii, as it appears that nosetip

roughness affects boundary-layer transition on the afterbody for moderate nose radii. A

single test-track shot is to be executed for a blunt ogive to determine if the test track can

make useful measurements of boundary-layer transition.

Initially, the present research used a boundary-layer solver to estimate target roughnesses

that would be applied to the nose tip. Preliminary analysis was conducted on test cases for

sharp cones and blunt cones. However, due to time constraints, the target roughnesses were

then estimated with a higher fidelity code by Brad Wheaton of JHU APL. Two separate

roughness targets were established for the upper and lower sides of the hemispherical nosetip.

The focus of this work then shifted to measurements of the roughness that was applied

by others to the hemisphere nose tip for a blunt ogive. Utilizing the Zygo ZeGage 3D optical

profiler, roughness scans were collected both directly under the profiler head and indirectly

using rubber molds. Profilometer measurements were also recorded. Twelve iterations were

completed to allow the polisher to develop appropriate procedures for applying the roughness,

given the material and curvature. The first five iterations involved roughness applied to

cylindrical-shaped test areas. After achieving the target roughnesses on these test areas,

the hemispherical ends of test specimens were then polished and measured until both the

rough and smooth halves met the roughness target. During this time, the three roughness measurement

techniques were refined until good agreement was reached between them. When the roughness-application and

roughness-measurement techniques were sufficiently mature,

the actual blunt-ogive nose tip was then polished until the roughness target was achieved.