Applying an Extended Non Linear Harmonic Method to a CROR Configuration With Emphasis on the Acoustic Signature

Abstract

Contra Rotating Open Rotor (CROR) propulsion concepts offer the possibility of gaining a significant step in the reduction of the specific fuel consumption (SFC) of aeroengines. Next to their advantages in this area, CROR configurations pose serious challenges to aircraft designers in terms of their inherently high and unsteady pressure fluctuations, which are not shielded by any sort of casing. When compared to ducted fans, these pressure fluctuations can lead to an unacceptably high acoustic signature on the environment as well as to a high frequency stress load on the airplane structure. It is therefore necessary to capture and understand these unsteady flow effects early in the design phase, which in turn makes unsteady CFD calculations mandatory. The paper presents the application of a highly efficient frequency based unsteady CFD method, the so called Non Linear Harmonic (NLH) method on a tractor (puller) CROR configuration. The Non Linear Harmonic Method returns unsteady results within at least two orders of magnitude less CPU time compared to the conventional approach of traditional unsteady sliding grid methods. The NLH method has first been introduced by [1] and significantly extended by [2–4] in order to take more of the interacting blade frequencies into account. The NLH method used herein does not require any scaling of the geometry or changes in the blade count. In order to apply the method on CROR configurations, it has been further developed [5] and is fully capable to account for the different rotational speeds of all components of a CROR configuration. The novel rank-2 extension enables the unsteady effects of the first rotor to pass through the rotating flow field of the second rotor.