Search results for: “transition”

X-51A Scramjet Test Flight
The X-51A Waverider hypersonic aircraft had its second test flight earlier this week. Unfortunately, its supersonic combustion ramjet (scramjet) engine failed to transition from its start-up fuel to its primary fuel. According to the US Air Force Research Laboratory:
A US Air Force B-52H Stratofortress released the experimental vehicle from an altitude of approximately 50,000 feet. After release the X-51A was initially accelerated by a solid rocket booster to a speed just over Mach 5. The experimental aircraft’s air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation when the vehicle experienced an inlet un-start. The hypersonic vehicle attempted to restart and oriented itself to optimize engine start conditions, but was unsuccessful. The vehicle continued in a controlled flight orientation until it flew into the ocean within the test range. #
Un-starting is the term used when supersonic flow is lost in an engine or wind tunnel. If the pressure or temperature in the engine deviates too far from the ideal conditions, the upstream mass flow through the engine will be greater than the downstream mass flow and the engine will choke (video). A shock wave forms and travels upstream, leaving subsonic flow in its wake. Loss of supersonic flow inside the engine would likely also result in losing ignition of the fuel/air mixture, resulting in flameout. #
If you haven’t guessed already, engineers like to make up words.
June 16, 2011
Godspeed, Discovery!
The space shuttle, despite three decades of service, remains a triumph of engineering. Although it is nominally a space vehicle, fluid dynamics are vital throughout its operation. From the combustion in the engine to the overexpansion of the exhaust gases; from the turbulent plume of the shuttle’s wake to the life support and waste management systems on orbit, fluid mechanics cannot be escaped. Countless simulations and experiments have helped determine the forces, temperatures, and flight profiles for the vehicle during ascent and re-entry. Experiments have flown as payloads and hundreds of astronauts have “performed experiments in fluid mechanics” in microgravity. Since STS-114, flow transition experiments have even been mounted on the orbiter wing. The effort and love put into making these machines fly is staggering, but all things end. Godspeed to Discovery and her crew on this, her final mission!
February 25, 2011
Instability in a Jet
This photo shows the development of a flow instability in an axisymmetric jet. On the left, the jet is smooth and fully laminar, but, by the center of the photo, disturbances in the jet have grown large enough to distort the laminar profile. The jet is then in transition; by the right side of the frame, it has reached a turbulent state, as evidenced by the increased mixing (which causes the smoke to disperse more quickly) and intermittency of the flow. #
February 8, 2011
Wavy Vortices
Shown above is the flow between two concentric cylinders (Taylor-Couette flow). In the laminar regime, the velocity profile between the two cylinders is linear. As the rate of rotation of the inner cylinder increases, the flow develops toroidal vortices known as Taylor vortices, seen in the video above after 9 seconds or so. This is a fluid instability exhibited by transitional flow. Increasing the rotational rate further can result in wavy Taylor vortex flow. At high enough speeds, the flow will become completely turbulent.
November 29, 2010
Three Flows in One
These plumes of smoke demonstrate the three types of fluid flow: laminar, transitional, and turbulent. At the bottom of the photo, the plumes are smooth and orderly, as is typical for laminar flow. At the top, the smoke’s movement is chaotic and intermittent, full of turbulent eddies. Between these two stages, the flow is in transition; there is still some semblance of order to it, but disturbances in the plume are getting amplified and breaking down into turbulence.
Photo credit: J. Russo
October 20, 2010

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