New propulsive concepts, such as boundary layer ingestion, involve stronger interactions between the engine and its environment, and are thus more complex flows compared to classical architectures. Usual turbomachinery design tools are inadequate, and new numerical methodologies are needed to accurately predict the engine performance with affordable CPU resources. The present paper examines the relevance of a reduced-order modeling approach—the body force modeling (BFM) method—for a low-speed cooling fan with inflow distortion. The formulation itself accounts for the blade metal blockage and compressibility effects, and it relies on a semiempirical loss model, independent of computational fluid dynamics (CFD) calibration. The BFM results obtained in the present work are assessed against full-annulus unsteady Reynolds-averaged Navier-Stokes (URANS) results and experiments. The comparison shows that the BFM approach successfully quantifies the fan stage performance. Furthermore, the distortion transfer across the stage is examined and the flow patterns observed are found to be the same as in the URANS results and in the measurements. Hence, this methodology, coming at a low CPU cost, is well-adapted to the early design phase of an innovative propulsion system.

This paper introduces a semi-analytical approach which enables one to deal with distorted inflow in axial fans or compressors. It is inspired by the classical parallel compressor (PC) theory but relies on a local flow-loading coefficient formalism. It is applied to non-uniform flow conditions to study the aerodynamic behavior of a low-speed fan in response to upstream flow distortion. Experimental measurements and 3D RANS simulations are used to evaluate the prediction of fan performance obtained with the local PC method. The comparison proves that, despite its simplicity, the present approach enables to correctly capture first order phenomena, offering interesting perspectives for an early design phase if different fan geometries are to be tested and if the upstream distortion maps are available.

This work points out a stability bifurcation which appears at low mass flow rates when simulating the flow inside of a compressor rotor alone, using the Body Force Modeling (BFM) approach with the Hall–Thollet formulation. This phenomenon is observed for a small propulsive axial fan, with and without model calibration. It does not have any consequence, since it happens far beyond the surge limit of the fan stage and involves “virtual” operating points which cannot be captured with blade simulations. However, this bifurcation also exists with centrifugal impellers, for which a flow recirculation usually takes place at low mass flow rates, enabling extension of the stable operating range. Thus, this represents a serious limitation of this method. This numerical behavior has not yet been documented in the BFM literature. Given the complexity of this subject and the number of parameters, the intention is not to carry out an exhaustive study here. But since the Hall–Thollet formulation is now quite commonly used in turbomachinery CFD, the objective of this work is to briefly report and describe the dichotomy in the solutions obtained.