How will the Rolls Royce UltraFan provide reverse thrust?
The flow direction reverses only through the bypass duct pulling by blades with negative pitch. A portion of it will still be sucked by low pressure compressor making 180° turn into the core. Having to counteract the losses in turn will obviously reduce the performance.
As stated in research paper by the team funded by Rolls-Royce plc: "the core mass flow rate was 57% of that at take-off conditions, for bypass duct flow rates of 74% of total take-off mass flow."

Another design by Pratt &amp; Whitney features reversed position of the whole core requiring two turns of the flow for incoming and outgoing air. So, these turns are not a big problem as we see them traded for other pros.