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Inverse Design of 3D Multistage Transonic Fans at Dual Operating Points

Semi-inverse design is the automatic recambering of an aerofoil during a computational fluid dynamics (CFD) calculation in order to achieve a target lift distribution while maintaining thickness, hence, “semi-inverse.” In this design method, the streamwise distribution of curvature is replaced by a streamwise distribution of lift. The authors have developed an inverse design code based on the method of Hield (2008, “Semi-Inverse Design Applied to an Eight Stage Transonic Axial Flow Compressor,” ASME Paper No. GT2008-50430), which can rapidly design three-dimensional fan blades in a multistage environment. The algorithm uses an inner loop to design to radially varying target lift distributions, an outer loop to achieve radial distributions of stage pressure ratio and exit flow angle, and a choked nozzle to set design mass flow. The code is easily wrapped around any CFD solver. In this paper, we describe a novel algorithm for designing simultaneously for specified performance at full speed and peak efficiency at part speed, without trade-offs between the targets at each of the two operating points. We also introduce a novel adaptive target lift distribution, which automatically develops discontinuous changes of calculated magnitude, based on the passage shock, eliminating erroneous lift demands in the shock vicinity and maintaining a smooth aerofoil.

Effect of vortex ingestion on transonic fan stability

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Inlet flow distortion can comprise complex flow patterns and significantly affect transonic fan rotor stability. In order to be able to design effcient rotor stabilization technologies it is important to understand which features of distorted flow are most important in stall margin reduction. In this paper, unsteady CFD was used to investigate the effect of vortical inlet distortion on stall margin. Steady wingtip vortices were introduced to a one-third annulus transonic rotor geometry. Using a choked nozzle downstream to control the position on the characteristic, the stall points were calculated for an array of vortex configurations. The spanwise vortex injection position was tested at 25%, 50% and 75% span. Three different vortex core radii, 5%, 10% and 20%, were then tested. Finally the maximum local tangential velocity of the vortex was doubled from 50m/s to 100m/s in a flow with a mass averaged axial inlet velocity of 160m/s. The results showed that in these cases, inlet vortical distortion alone caused no significant change in stall margin and could also result in small increases. The suggestion is that vortical inlet distortion should be given appropriately moderated priority when considering the design of rotor stabilizing technology.

Advances of turbomachinery design optimization

© 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Automatic design methods are changing the approach to and processes involved in gas turbine design. These methods can be used on a global scale to explore design space, on a local scale as tools in the specialized design of engine components, or as supporting methods in the optimization of existing design methods. For these purposes, optimization methods may be deployed as the primary design tool or hybridized with other automatic design methods to find new ways to explore the design space. In this paper, three novel examples are presented to demonstrate each of these ways of designing via automation. In the first study, a state-of-the-art inverse design method is used to design a compressor stage. The calculation achieves multiple design targets of streamwise loading distribution, stage pressure ratio, and stage exit ow angle, all radially varying, in addition to massflow. These targets can be distributed across two operating points without compromising the ability to satisfy them. For the first time, a genetic algorithm is wrapped around this inverse design code to form a hybrid automatic design method which is used to optimize for improved aerodynamic efficiency and stall margin, and demonstrate the potential for useful hybridization of automatic techniques in turbomachinery design. In the second study, a very large-scale eddy simulation is used to simulate the ows around the cutback trailing edges of high-pressure turbine blades. For a given external blade design and mainstream ow, a genetic algorithm was used to control the progress of the optimization, aimed at improving the layout of the internal structures within the blade. The genetic algorithm was run for ten generations, by which time, the parameter of fitness-an idealised measure of film cooling performance-was found to have improved significantly over the initial precursor generation. The third study shows the adjoint based improvement of multi-block structured meshes for CFD in several engine parts. The form of the block structure used for complex domains considerably affects the quality of the mesh, which necessarily has a significant knock-on effect on the quality of CFD design. It is normally unclear which blocking would yield the optimal mesh for a specific geometry. Here, the adjoint methods typically employed in design optimization can be used to decide on the mesh block structure. As well as showing the above examples, the paper finally explores some future aspects of design optimization and in particular how eddy-resolving simulations might be used in design optimization in say the next 10 years.

Inverse Design of 3D Multi-Stage Transonic Fans at Dual Operating Points

Semi-inverse design is the automatic re-cambering of an aerofoil, during a computational fluid dynamics (CFD) calculation, in order to achieve a target lift distribution while maintaining thickness, hence “semi-inverse”. In this design method, the streamwise distribution of curvature is replaced by a stream-wise distribution of lift. The authors have developed an inverse design code based on the method of Hield (2008) which can rapidly design three-dimensional fan blades in a multi-stage environment. The algorithm uses an inner loop to design to radially varying target lift distributions, an outer loop to achieve radial distributions of stage pressure ratio and exit flow angle, and a choked nozzle to set design mass flow. The code is easily wrapped around any CFD solver.In this paper, we describe a novel algorithm for designing simultaneously for specified performance at full speed and peak efficiency at part speed, without trade-offs between the targets at each of the two operating points. We also introduce a novel adaptive target lift distribution which automatically develops discontinuous changes of calculated magnitude, based on the passage shock, eliminating erroneous lift demands in the shock vicinity and maintaining a smooth aerofoil.Copyright

PATHWAYS TO IMPROVED AERODYNAMIC DESIGN

Large-scale unsteady graphical processor unit (GPU) based calculations are used to study the flow in realistic transonic fan geometries and shown to produce useful insights into the complex unsteady flow physics in systems of practical engineering relevance. Inlet flow distortion produces complex flow patterns which can significantly reduce stall margin. In order to design efficient, well targeted fan stabilizing technologies, it is essential to gain understanding into which specific features of distorted flow patterns are important in causing instability. In this paper, the use of large-scale, high-fidelity 3D unsteady Reynolds-Averaged Navier Stokes (URANS) calculations with sliding planes is shown to allow the isolation of a wide range of inlet distortion features and a high throughput of calculations is achieved. This facilitates a comprehensive investigation into stall margin loss and the observation of complex instability processes, which is in turn used to support the development of specialized design solutions. The costs of simulations and turnaround times are given. The potential for mixed fidelity simulations, in particular mixing one-dimensional low order models and large eddy type simulations to generate inlet conditions, is discussed.