Session: 34-03 Turbulence modeling, CFD models, and assessment

Paper Number: 153775

A New Body Force-Based Modelling Approach for Turbomachinery Aerodynamics: Formulation, Verification and Implementation Aspects 

Modelling of turbomachinery flows in aero engines is becoming increasingly more important as the next generation, high-bypass ratio engine designs will incorporate compact nacelles and short intakes to mitigate the penalties associated with coupling effects, weight and drag. Resolving the flow-field of coupled configurations or unsteady turbomachinery flows where full annulus simulations are required, utilising conventional RANS/uRANS methods entail excessive computational requirements which can be prohibitive particularly during the design/development phase of a product. Body force methods offer an alternative, computationally efficient approach, capable of effectively reproducing key flow features by replacing the turbomachinery component with a body force field. This dramatically reduces the computational cost as it eliminates the need for detailed discretisation of the bladed domains and near-wall mesh refinements. Losses and flow turning are entirely introduced through source terms which typically are predicted using empirical or semi-empirical, physics based lower-order models.

In the present work, a new body-force modelling approach is developed and verified, while unexplored, obscure modelling aspects underlying most, if not all, body force methods are identified and to the extend possible, addressed. The developed body force model comprises 3 components: a base turning force, a correction force and a viscous force. An inviscid, compressible solver is reformulated in the relative frame of reference (FoR), with metal and aerodynamic blockage, while the fluxes are obtained through Riemann solvers which have been reformulated in the relative FoR. Distributions coefficients are introduced, mathematically describing the spatial distribution of key blade quantities such as deviation and losses. A vector-based curvature definition approach is proposed, enabling the full curvature definition in three dimensions expressed by 3 angles: flow path, lean and blade metal angle. Finally, body force-relevant grids are proposed and investigated, ensuring the formation of continuous body force-fields and achieving a trade-off between numerical stability, accuracy and computational cost. All methods have been implemented in the form of a custom-made code.

Initially, a directional grid sensitivity analysis is performed in an axial compressor rotor and an impeller, indicating a high dependency between the grid resolution and the aspect ratio of the blade. These are generalised providing guidelines (recommended ranges) for axial and centrifugal compressor blade discretisation. The solver reformulated in the relative FoR is validated against analytical solutions and compared against different Riemann solvers.  The curvature definition method is validated against experimentally measured impeller blade angles showing great agreement. In the second part, the validity and limitations of the models are verified through numerical response test cases of axial and centrifugal compressors blades. These aim to quantify the ability of the body force models to impose the prescribed flow turning and pressure loss, the validity of the modified governing equations and solver as well as unveil any “hidden”, subsidiary forces underpinning standard body force practices. Subsidiary forces are identified due to abrupt flow turning at the LE, momentum imbalance introduced via errors in the estimation of the blockage terms, as well as abrupt changes in-between highly blocked and unblocked domains. These are identified, quantified and addressed through the proposition of different approaches. Finally, the validity of the distribution coefficients is assessed, by comparing global performance metrics and spanwise distributions comparing CFD-derived and custom metrics.

Presenting Author: Dimitrios Lamprakis Cranfield University

Presenting Author Biography: Dr. Dimitrios Lamprakis was born in 1991, in Athens, Greece. He obtained his 5-year mechanical engineering diploma in Greece, specialising in energy, aeronautics and environment. In 2019 he obtained his MSc in aerospace propulsion from Cranfield university UK (1st class). Shortly after the completion of his MSc, Dimitrios started a PhD programme, in collaboration with Rolls-Royce US (Indianapolis) on axi-centrifugal compressor surge and stall modelling using body forces and reduced-order methods. After the submission of his PhD in 2022, he joined the Rolls-Royce UTC in Cranfield university as a research fellow. He is currently working on the LH2GT project, conducting CFD analysis on a LH2 pump for aircraft applications. In parallel he is working on turbine subidle aerodynamics and compressor surge and stall modelling. His main area of expertise is turbomachinery aerodynamics, with focus on numerical methods for CFD applications, body force modelling for internal flows and surge and stall modelling.

Authors:

Dimitrios Lamprakis Cranfield University
Mauro Righi Cranfield University
Vassilios Pachidis Cranfield University