The effect of freestream noise on roughness-induced transition was studied on a blunt 7 deg half-angle cone in the Boeing and U.S. Air Force Office of Scientific Research Mach-6 Quiet Tunnel. Temperature-sensitive paints were used to visualize the wake of an isolated roughness element at 0 and 6 deg angles of attack. Transition onset was determined from the rise in centerline temperature downstream of the roughness. Transition was always delayed under quiet flow compared with noisy flow. For example, at 0 deg angle of attack, transition was up to 6.3 times further downstream from the trip location. The difference in transition location between quiet and noisy flow conditions was significantly reduced when an effective trip height was reached. However, quiet flow still delayed transition by a factor of 2.4 in those cases. Because quiet flow delays transition behind a roughness element, the height of trips sized in a conventional noisy tunnel should be increased for quiet flightlike conditions. On the other hand, any naturally occurring roughness that causes transition under noisy flow might not cause transition in a quiet environment.
Bibliographical noteFunding Information:
This work could not have been completed without the help of many individuals. The Ludwieg Tube Group (Matt Borg, Michael Hannon, Tom Juliano, Brad Wheaton, and especially Erick Swanson) provided advice and suggestions throughout the project. Thanks are also extended to Roger Kimmel for help in planning the HIFiRE BAM6QT experiments. This work was funded in part by the National Defense Science and Engineering Graduate Fellowship, Sandia National Laboratories, and the U.S. Air Force Office of Scientific Research.