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Aeromechanics and Performance Simulation for Turbomachinery

Prediction of blade row performance and aeromechanics is important to turbomachinery development because turbomachinery blading lies at the heart of all types of turbomachines: pumps, fans, compressors, turbines etc. Improving the aerodynamic (or hydrodynamic) and structural aspects of the blading is essential to meeting modern requirements for performance and durability. If we consider that the fluid mechanics primarily influences performance, including fuel efficiency, then the structural mechanics is more responsible for durability. But of course, the two are closely related, and that is particularly true as the operating temperature increases, as in the hot section of modern gas turbines. Most end users do not want to sacrifice durability for energy efficiency, but rather demand both.

Operating range and duty are other important machine attributes. Modern automotive turbochargers must operate reliably from engine idle to full speed, and enable fast acceleration with minimal “turbo lag.” The operating “map” — the distance between compressor surge and choke — must be wide, yet efficiency is important. Industrial gas turbines used for power generation now cycle more frequently to compensate for the presence of intermittent power sources such as wind turbines and solar collectors. Wide range is important for pumps. Their ability to avoid damaging cavitation is also a valued property.

Let’s consider aerodynamic performance prediction methods. These methods have evolved with the machines themselves. While one-dimensional and throughflow methods remain important and essential, effective use of CFD is mandatory for design of competitive machines. Steady-state CFD remains the workhorse tool, but unsteady CFD is becoming more common, as engineers seek a more realistic representation of the true unsteady flow.

Adjacent blade rows contain unequal numbers of blades; therefore, in principle, a proper simulation requires solution of all blades in each row. However, ANSYS has developed a suite of tools that enables more efficient solution for a number of analysis types. The key attribute of these tools is that the full wheel solution can be obtained by solving only one or at most a few blades per row. ANSYS 17.0 extends the available methods, including multistage simulation, multi-frequency disturbance and asymmetric geometries, which enable efficient CFD simulation of multistage axial turbines, a blade row subjected to upstream and downstream disturbances, and an aircraft engine fan subject to a cross-flow, respectively.

ANSYS 17.0 Transient Blade Row extensions enable efficient multi-stage CFD simulation.

Now let’s look at the challenges of aeromechanics. Whereas the aerodynamicist generally prefers designs with very thin blades, the structural engineer prefers thick blades to minimize stress and optimize vibration characteristics. Those interested in material cost and weight would no doubt side with the aerodynamicist, whereas those responsible for honoring the machine warranty would favor the structural viewpoint. Achieving agreement requires a balance, and that is where the field of aeromechanics comes in.

Aeromechanics is by no means new. What is new is the fidelity with which engineers can practically consider both the fluid mechanics and the structural aspects of the solution. The real behavior of rotating blades is indeed very complex, and the mechanical loads are very high. For example, a single low-pressure steam turbine blade rotating at operating speed generates a load of several hundred tons! Long, thin blades are susceptible to vibration. Engineers strive to design blades whose natural frequencies do not coincide with the disturbances that arise due to operating speed, etc. That is complicated enough, but there are also periodic disturbances that can originate from more distant blade rows or aerodynamic effects.

Figure 2: Modal analysis provides the mechanical modes of blade vibration required for flutter analysis.

In the past, analysis of fluids and structural dynamics was mostly separate and simplified. But for some time, at least in principle, the ability to perform high-fidelity coupled analysis has been available. In reality, solving for time-dependent, three-dimensional fluid-structure interaction is very time-consuming and expensive, even on today’s high performance computing systems. Engineers have opted for more practical, usually disconnected and often lower-fidelity analysis methods. Recently, practical yet high-fidelity multiple physics solution methods have emerged, and ANSYS 17.0 enhances these methods.

Prediction of aerodynamic blade damping, or “flutter,” is one such enhanced method. The procedure is to first solve for the mechanical modes of vibration, and then feed that information to the CFD simulation. The unsteady CFD simulation deforms the blade in the presence of the flow field and predicts whether the blade is aerodynamically damped, and hence stable, or not. This high-fidelity approach is practical because it provides a solution to the full wheel (all of the many blades in a given row) by solving only for one or at most a few blades in the blade row of interest. Cyclic symmetry is the enabling structural technology here, while the Fourier Transformation method, one of several available ANSYS Transient Blade Row methods, is key on the CFD side. Tightly linking these two efficient methods provides great advances in computing fidelity and speed.

Predicting forced response is essentially the inverse workflow to flutter. First, the unsteady fluid dynamic loads are predicted, and made available to the structural solver. After a mechanical harmonic response simulation, the engineer evaluates the results for acceptable levels of blade displacement, strain and stress.  

This brief summary introduces the possibilities for enhanced turbomachinery design provided by ANSYS 17.0.

Figure 3: ANSYS 17.0 speeds processing of cyclic harmonic mode-superposition analysis with parallel processing.