The typical Airbus with RR engines (two spool I believe?) I've understood uses internal mixing? The only difference from my point of view is that the outer cowling reaches further aft than its CFM counterpart and allows bypass air and core air to mix before exiting rearwards.
At the risk of treading on expert toes LnS, I believe Rolls-Royce have, since the original RB211, produced
three spool engines. Which is what makes them so different from GE or P&W engines.
As for 'mixing', I always understood that the fan air - the bypass air - formed a tube of relatively slow, quite cool air that provided the vast amount of thrust typical in high bypass engines. This tube is coaxial to and surrounds the very hot, fast moving gas from the core engine, which is why the modern high bypass engine is so much quieter than its straight turbojet predecessor. I don't know, but my guess would be that in
any manufacturers engine, the mixing of these two, dramatically different streams of gas happens tens, even hundreds of feet
behind the aeroplane.
In the case of the turboprop engine, the useful thrust of the engine is produced by the propeller - I guess there is some pure jet thrust from the 'generator' - so, in my truly humble opinion
, the volume of air displaced by the propeller compared to the volume of gas coming out of the 'generator/core engine' must represent the bypass ratio?
I'll get me coat.
As an entirely gratuitous aside, did you know that the engine Frank Whittle
really wanted to build was a two spool high bypass engine? He realised that the engine he visualised would only be really powerful and efficient when it produced a very large volume of slow moving air, as opposed to the low volume of noisy, fast moving air that 'jets' do.
Of course he had champagne ideas and beer support
and had to make do with a thrice used, much battered and re-machined centrifugal compressor on a single spool. He did, however, patent water injection; prop/jets; multispool jets; (high) bypass jets and many more ideas long before they were technically possible.
By the time they were possible and fashionable, the patents had all expired and, in any case, the British Government had all but
given the jet engine to the Americans.
As I said, I'll get me coat.
Roger.