Assess Steam Turbine Blade Flutter Using Fidelity CFD FSI Simulations - Part I

Watch the recorded video of the webinar to learn more about Blade Flutter Assessment of a Steam Turbine with Fidelity CFD.

Steam turbines stand at the core of the energy sector, propelling the industry towards more efficient energy extraction from steam. Innovation has led to the advent of elongated, lighter turbine blades, optimizing energy capture but also elevating the risks of flutter—a dynamic stability problem borne from fluid-structure interactions (FSI) where lengthy blades, often exceeding a meter in length, are susceptible to self-sustained vibration due to low structural frequency and supersonic tip speeds. To predict and prevent these detrimental effects, engineers turn to the capabilities of computational fluid dynamics (CFD) to simulate the complex interplay between fluid forces and structural responses. These simulations are both complex and challenging, encompassing mesh deformation, complex modeling, and unsteady flow. Cadence Fidelity CFD software emerges as an integral solution by offering specialized tools for thorough and efficient FSI analysis.

Fidelity CFD streamlines flutter risk assessment by integrating the nonlinear harmonic method with modal analysis—a strategy signifying considerable reductions in both engineering time and CPU resources. Unlike conventional approaches requiring exhaustive unsteady simulations, Cadence's methodologies afford precise flutter predictions, facilitating the development of safer steam turbines that exhibit superior performance and reliability.

Why Model Fluid-Structure Interaction (FSI)?

FSI describes the interaction of the structure dynamics and the fluid loads acting on the structure. When the fluid applies a load on the structure surfaces, a resulting strain in the solid structure causes deformation. The deformed structure further impacts the surrounding flow field. The structure deformations can be small or damped, but the interaction may lead to destructive oscillation failures. For example, the failure of the Tacoma Narrows Bridge resulted from FSI,  aeroelastic flutter, to be specific.

In turbomachinery, variable blade loading can result in complex FSI problems. To address challenges posed by newer designs featuring thinner and lighter blades, it's essential to consider FSI at the early stages of design to avoid potential issues during the production and maintenance of steam turbines.

Refining FSI Simulations: A Leap with Fidelity CFD

Flutter simulations can be quite intricate and demand a considerable amount of computational power to simulate unsteady phenomena accurately. However, by utilizing nonlinear harmonic (NLH) methods through Fidelity CFD, engineers can significantly reduce CPU usage and engineering time while still maintaining the flow's unsteadiness.

The simulation technique employed in these models utilizes a frequency-domain approach to solve the Navier-Stokes equations, as opposed to the more common time-domain approach. The method involves expressing each variable as a combination of a time-averaged component and a series of periodic perturbations, with each perturbation being further broken down into a sum of harmonics and their associated frequencies. By incorporating a greater number of harmonics, the perturbations can be more accurately modeled.

Static Pressure at 95% Span (Reconstructed in time, including all harmonics).

Foremost among the benefits derived from this method is eliminating the need to simulate each blade passage independently. Embracing flow periodicity and blade passing frequency perturbations enables accurate simulation of unsteady flows by modeling just a singular passage, from which a full reconstruction is extrapolated at a fraction of the common computational cost.

Integration of structural dynamics into FSI simulations is achievable by imposing structural perturbations either at the modal eigenfrequency or harmonizing with blade passing frequencies. For example, simulations of rotor blades can produce sinusoidal functions that represent flutter through blade deformations, computed using CFD. The flow conditions are reconstructed over time, which leads to predictive simulations. Fidelity CFD is versatile enough to accurately represent flutter scenarios. The inter-blade phase angle can either be set to zero for uniform blade deformation or alternated (e.g., 180 degrees for differential patterns).

With continued advancements in CFD and FSI simulations, the energy industry can look forward to more efficient and sustainable energy production. Technologies in Fidelity CFD can help engineers address complex FSI problems and improve the efficiency and safety of steam turbines. Cadence's expertise in computational software can play a significant role in the energy industry's progress towards a more sustainable future.

Stay tuned for the upcoming blog post in this series!

Watch the recorded video of the webinar to learn more about Blade Flutter Assessment of a Steam Turbine with Fidelity CFD