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Boundary Layer Transition


Boundary layer @ Wikipedia
In physics and fluid mechanics, the boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. In the atmosphere the boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. On an aircraft wing the boundary layer is the part of the flow close to the wing. The Boundary layer effect occurs at the field region in which all changes occur in the flow pattern. The boundary layer distorts surrounding nonviscous flow. It is a phenonomen of viscous forces. This effect is related to the Leidenfrost effect and the Reynolds number. The aerodynamic boundary layer was discovered by Ludwig Prandtl at the beginning of the twentieth century and represents one of the greatest discoveries in the history of aerodynamics. It is particularly important in aerodynamics because it is directly responsible for the drag experienced by a body immersed in a fluid.

Boundary Layer Transition
Despite being studied by many researchers, the fundamentals of boundary layer transition remain poorly understood. In recent years there has been an increase in the level of interest in the subject because of its widespread applications, from the design of gas turbine blades to that of the thermal protection systems on lifting re-entry vehicles such as the space shuttle. Appropriate transition models are still lacking and many fundamental experiments need to be carried out to obtain a better understanding of this phenomena. In addition to transition inception, the transition zone itself needs further study where recent evidence suggests that turbulent spots are not simply uniform two-dimensional arrow-head structures and could have a significant effect on the surrounding flow.

NASA ponders shuttle repair work during spacewalk
The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, would result in higher downstream heating.

Exclusive: Did Rough Wing Break Columbia?
The three axes of flight are roll (tilting of one wingtip up and the other down), pitch (movement of the nose up or down), and yaw (turning of the nose to the right or left). "Anything to cause increased drag on that left wing would certainly have caused it to yaw," Anderson said. "The shuttle is designed to fly straight. It is not designed to fly sideways. That would have been absolute disaster if something had yawed it so much that it was basically trying to fly sideways." As a space shuttle re-enters Earth's upper atmosphere, the initial movement of air over the wings is smooth and orderly—called laminar flow. At a key point in the flight, called the boundary layer transition, the increasing speed causes the smooth flow to breakup into eddies, becoming "turbulent flow." The shift increases heating and drag on the wings.

Columbia Accident Board Refines Failure Scenarios
Boundary layer transition heating. Adm. (ret.) Harold W. Gehman, Jr., who heads the Columbia accident board, said his group has "begun a line of investigation" into how premature changes in the boundary layer possibly caused by launch debris damage could have increased reentry heating. The board is still investigating whether Columbia experienced an early boundary layer transition (BLT); Aviation Week has reported that such an early transition could dramatically increase thermal effects, especially on any damaged areas of the wing (AW&ST Feb. 24, p. 20). Last week, investigators said left-wing roughness characteristics that had caused earlier premature BLT events on Columbia had been corrected in its 1999 overhaul, but wing-roughness BLT factors associated with the tank debris remain an issue in the thermal analysis.

Boundary layer transition @ Wikipedia
The process of a laminar boundary layer becoming turbulent is known as boundary layer transition. This process is an extroadinarily complicated process which at present is not fully understood. However, as the result of many decades of intensive research, certain features have become gradually clear, and it is known that the process proceeds through a series of stages [receptivity, linear stability, non-linear effects ??].


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