Open rotor installation effects

Open rotor engines are more efficient than conventional turbofan engines, but reducing their noise emission when installed on an aircraft is a major challenge.

Contra rotating open rotor (CROR) engines could use up to 20% less fuel compared with modern turbofans. In terms of engine size, propulsive efficiency, and flight speed, these engines are midway between turboprops and turbofans. They have two rows of propellers, as shown in Fig. 1, which rotate in opposite directions. They are noisier than turbofans, because there is no nacelle to contain the noise emission. The counter-rotating blades also leads to additional sources of noise. In order to design CROR engines to be quieter, we need to be able to control their unsteady aerodynamics and to understand how the aerodynamics depend on installation features such as a pylon or tail-plane and operation at an angle of attack to the engine centreline.

At the Whittle Lab, in collaboration with Rolls-Royce plc, and using the Turbostream solver, we have performed full-annulus uRANS simulations of an open rotor with and without an upstream horizontal pylon, at 0° and 12° angle-of-attack, to predict the aerodynamics at take-off conditions. Figure 2 illustrates the computational domain used for the simulations. The complete mesh uses approximately 160 million cells. The rotor-rotor interaction, and that between the pylon and rotors, has been resolved in great detail and the computations have been validated by comparison with results from wind tunnel tests from a high-speed rig test campaign.

The high resolution calculations at 12° angle-of-attack predicted the existence of extra tones, called ‘side-bands’, at multiple frequencies centred on the rotor-rotor interaction tones. These side-bands were also observed in high-frequency measurements of static pressure by probes mounted on the blades, and flush with the blade surface. The agreement between measurements (EXP) and predictions (CFD) is shown for a probe on the rear rotor in Fig. 3. Here, side-bands are caused by a modulation of the size and circumferential position of the front rotor wakes and tip vortices, which dominate the interaction between rotors. Further work is in progress to understand how the upstream pylon modifies the open rotor flow-field and can increase or decrease components of the noise emission.