Transient blade row simulations in turbomachinery are needed either to improve the aerodynamic performance predictions or because the flow interaction we are trying to resolve and predict is unsteady in nature such as aeromechanical, aerothermodynamic or aeroacoustic interactions. Because the blade pitch is not similar between the rows of turbine or compressor, a transient blade row simulation will usually require the modeling of the full wheel (or full geometry). This constraint renders these simulations not practical and in many cases prohibitive as analysis or design tools.
In the past six releases, ANSYS CFX has introduced a range of transient blade row with pitch-change models — read past blogs on TBR advancements here: Aeromechanics and Performance, Blade Row Flow Modeling, Designing Superior Turbomachinery. These methods model the turbomachinery flow in one or few blade passages per row. Therefore, not only do they reduce the model size but the time required to obtain a solution with minimum loss of solution fidelity.
For a typical compressor fan stage, the time required to obtain a solution with the pitch-change method can be an order of magnitude faster than full wheel model. Therefore, the transient pitch-change methods provide viable analysis tools to turbomachinery engineer to explore the flow details and make an educated improvement to the design.
In order to use these transient pitch-change methods as design tools early in the design cycle, and not only as analysis tools in later stages of the design, further reduction in time to obtain a solution is required.
In ANSYS 18, CFX has introduced Harmonic Analysis (HA), a hybrid time-frequency domain solution method. This solution method provides rapid answers to transient periodic flows, such as the one in turbomachinery, with acceptable engineering accuracy. The first release of Harmonic Analysis tackles the blade flutter/aerodamping calculations flow problem — a fluid-structural interaction problem.
A typical blade flutter analysis modeled with pitch-change HA can be one order of magnitude faster than the same problem solved using transient with pitch-change solution method, and about two orders of magnitude faster than full-wheel transient simulations. The ability to obtain a fast solution with good engineering accuracy makes this solution method a viable design tool where repeated calculations are required to explore the design space.
Reconstructed pressure distribution on the 22 blades of NASA Rotor 67 for Nodal Diameter = 6 at some instance in time. The solution was obtained on two blade passages only using Fourier-Transformation pitch-change and the Harmonic Analysis solution method.
Aerodynamic damping for the entire nodal diameter range for Rotor 67 is now easily attainable with the efficient Harmonic Analysis solution method
ANSYS has been developing a comprehensive toolset for transient blade row analysis. At the end of June, I will attend the ASME Turbo Expo in Charlotte, NC, If you are attending this fantastic gathering, I would like to meet you. You can find me and my colleagues either in the ANSYS booth or in the technical sessions and we will be excited to tell you more about HA and ANSYS blade row flow modeling capabilities.
Not attending Turbo Expo? I invite you to read the Harmonic Analysis application brief.