Afixed geometry device has been developed to promote boundary layer transition and generation of streamwise vorticity, and is capable of withstanding entry heating environments for the Space Shuttle Orbiter. Designed to have a total height above the surface of the same order as the local boundary layer thickness, this device is approximately 0.25 in. (≈0.6 cm) tall and 4 in. (≈10 cm) long for the Orbiter entry application. Because temperature exposure is a key design factor for entry systems, the geometry has been optimized to establish peak heating rates and peak surface temperatures that are close to being spatially consistent on the protuberance. A relatively thin cross-section of 0.4 in. (≈1 cm) provides significant thermal radiation relief via conduction through the aft surface of the geometry. Sufficient mechanical strength to satisfy launch, ascent, entry, and landing conditions has been maintained in the design.
Protuberance geometry generates a flow disturbance that results in a localized region of higher pressure and heating. Fluid mechanical effects lead to a localized region of downstream separated flow and the generation of streamwise vorticity that will persist in the downstream boundary layer. These fluid mechanical effects will lead to downstream initiation of boundary layer transition if the local flow properties and protuberance height are appropriately tailored. The profile and cross-section have been optimized in order to reduce surface heating and to maintain relatively consistent heating over the top of the protuberance. Primary features of the geometry include an asymmetrical cross-section to reduce heating, a 45° cant to the leading edge of the protuberance, a 45° sweep of the protuberance relative to the local flow field, use of a triple radius with matched tangency for the cross-section, and a large radius blend between the leading edge of the protuberance and the maximum height.