Rishi Gangadhar said:
This is turbulent because there is a change in pressure with space (the pressure does change as the air moves towards the foil).
Turbulence does occur when you have variation of pressure with space and time.
This is What actually happens in the case of an aerofoil (look at the second diagram).
So what you have shown here are turbulent separation bubbles. Yes, those bubbles contain turbulent flow, but they are not the definition of turbulent flow. Turbulence itself is a much broader concept. Turbulence is characterized by rapid changes in both time and space of the various flow variables like temperature, pressure, and velocity. If you were to measure one point in a turbulent flow, it would be highly erratic and look essentially like fairly large random fluctuations around the mean quantity of whatever you are measuring. In the scope of the whole turbulent flow, the most notable characteristic is that it contains eddies (or vortices) of many different sized, where the larger eddies contain a lot of energy and gradually dump energy into smaller and smaller eddies until it is small enough that viscosity can dissipate the energy. In other words, in the pictures you posted, the important part that indicates turbulence are the crazy, "chaotic streamlines" drawn in the turbulent regions, not the fact that there is an abrupt change in pressure.
In every single fluid flow that is even remotely interesting, pressure changes with space and time. However, many of those flows are laminar. What is important is
how variables like pressure change with space and time. You can certainly have laminar flow over an airfoil or around a sphere, and those certainly involve pressure changes with space and time. In a laminar flow, though, the fluctuations about the mean value are very small and predictable, whereas in a turbulent flow, they are much larger and not practically predictable.
Rishi Gangadhar said:
Lines getting closer does indicate compressibility. Can you tell me what else can possibly indicate compressibility.
Lines getting closer does
not indicate compressibility. When streamlines change their relative distance, it indicates a change of speed. Given the definition of a streamline, you can view them almost as little walls in the inviscid flow. Since the velocity is always tangent to a streamline, there is always no component of velocity normal to a streamline and no mass flow across them. Therefore, two adjacent streamlines can be treated as what is called a streamtube, and conservation of mass applies in a streamtube. So, in subsonic flow, if streamlines get closer together, the velocity is increasing and vice versa.
Compressibility, on the other hand, is generally indicated by the Mach number. Any flow in which ##M \geq 0.3## is typically considered compressible. Any slower than that and compressibility effects are negligible and the flow may be safely treated as incompressible. Streamlines can get closer together or farther apart in either case. The other indicator (or rather, definition) of compressibility is that ##\nabla \cdot \vec{v} \neq 0## in a flow field.