When I look for the Mach range for transonic flows I never see the same answer. What is the correct range?
You are correct, some people refer to transonic flows as flows at about Mach 1, some give various range of Mach numbers.
Looking at the lower end we can see that the critical Mach number depends on the shape of the body. Hence, supersonic flow would start form on different airfoils at different Mach values. For this reason the lower end of the transonic range is not well defined. In most cases transonic effect can be seen at Mach 0.7-0.8 that is why you often see values close to that range.
The upper range, could be even more problematic.😄 Some stop the upper limit to M= 1, while others discuss transoic flows up to M=1.2. The reason is because early theories to mathematically analyze transonic flows were based on a isentropic assumption. Shocks forming at speed around M=1 produce a small increase in entropy, hence the mathematical models were reasonably valid up to M=1.2.
As general guide, the range of transonic flow is around Mach one. Look for the main transonic physics in the flow to identify whether it is transonic or not.
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