In the transonic speed regime the overall speed of an airplane is less than Mach 1 but some parts of the flow around the aircraft break the speed of sound. The photo above shows a schlieren photograph of flow over an airfoil at transonic speeds. The nearly vertical lines are shock waves on the upper and lower surfaces of the airfoil. Although the freestream speed in the tunnel is less than Mach 1 upstream of the airfoil, air accelerates over the curved surface of airfoil and locally exceeds the speed of sound. When that supersonic flow cannot be sustained, a shock wave occurs; flow to the right of the shock wave is once again subsonic. It’s also worth noting the bright white turbulent flow along the upper surface of the airfoil after the shock. This is the boundary layer, which can often separate from the wing in transonic flows, causing a marked increase in drag and decrease in lift. Most commercial airliners operate at transonic Mach numbers and their geometry is specifically designed to mitigate some of the challenges of this speed regime. (Image credit: NASA; via D. Baals and W. Corliss)
Tag: lambda shockwave
Unsteady Rocket Nozzle
This numerical simulation gives a glimpse of flow inside an unsteady rocket nozzle. The nozzle is over-expanded, meaning that the exhaust’s pressure is lower than that of the ambient atmosphere. A slightly over-expanded nozzle causes little more than a decrease in efficiency, but if the nozzle is grossly over-expanded, the boundary layer along the nozzle wall can separate and induce major instabilities, as seen here. In the first segment of the video, turbulent structures along the nozzle wall boundary layer are shown; note how the boundary layer becomes very thick and turbulent after the primary shock wave (shown in gray). This is due to the flow separating near the wall. The second half of the video shows the unsteadiness this can create. The primary shock wave splits into two near the wall, creating a lambda shock wave, named for the shape of the lower case Greek letter. This shock structure is indicative of strong interaction between the boundary layer and shock wave. (Video credit: B. Olson and S. Lele)
