Contra-rotating open rotor reverse thrust aerodynamics

Reverse thrust operations of a model scale Contra-Rotating Open Rotor design were numerically modelled to produce individual rotor thrust and torque results comparable to experimental measurements. The aims of this research were to develop an understanding of the performance and aerodynamics of open rotors during thrust reversal operations and to establish whether numerical modelling with a CFD code can be used as a prediction tool given the highly complex flowfield. A methodology was developed from single rotor simulations initially before building a 3D‘frozen rotor’ steady-state approach to model contra-rotating blade rows in reverse thrust settings. Two different blade pitch combinations were investigated (β1,2 =+30°,- 10° and β1,2 =-10°,-20°). Thrust and torque results compared well to the experimental data and the effects of varying operating parameters, such as rpm and Mach number, were reproduced and in good agreement with the observed experimental behaviour. The main flow feature seen in all the reverse thrust cases modelled, both single rotor and CROR, is a large area of recirculation immediately downstream of the negative pitch rotor(s).This is a result of a large relative pressure drop region generated by the suction surfaces of the negative pitch blades. An initial 3D unsteady sliding-mesh calculation was performed for one CROR reverse thrust case. The thrust and torque values were in poor agreement with experimental values and the disadvantages relating to time costs and required computational resources for this technique were illustrated. However, the results did yield a nominal unsteady variation of thrust and torque due to rotor phase position. Overall the work shows that it may be possible to develop a CROR reverse thrust prediction tool of beneficial quality using CFD models. The research also shows that the frozen rotor approach can be adopted without undermining the fidelity of the results.