Aerodynamic Flutter as an Important Factor in Turbomachinery Design

Dec 4, 2014 -- Flutter is an aero-elastic phenomenon which dictates structural dynamic instability occurring in any elastic structure, causing the structure to vibrate violently with increasing amplitude. This instability occurs due the interaction of elastic and inertial forces of the structure with that of the unstable aerodynamic forces caused due to oscillation of the structure. The energy possessed by air stream is transferred in terms of mechanical vibrations, which when exceeds the damping capacity of the structure, will lead to catastrophic failure. As such, flutter is highly important factor required to be studied in cases when there is an interaction between air flow and elastic structures. Some of the devastating effects of flutter have been demonstrated by NASA in a video shown below:

Improtance of Flutter in Turbomachines:

Historically, flutter has remained a prime study for aircraft applications; however, the concept of aero-elasticity or flutter can also be applied on the study of fluid and structural dynamics. Studying the impact of fluid on the turbine blade structure is crucial, as modern turbines are increasingly designed to have less weight, which are likely to react to the effects of dynamic loading due to unsteady aerodynamics. As such, considering carefully the interaction of fluid and structure in turbomachinery is highly important to avoid destructive failure.

At even low flutter intensities, the blades vibrate under periodic aerodynamic force of neighboring blade rows, causing structural damage to the blades, which can reduce number of useful cycles despite the ability to dampen the vibrating frequencies. However, when the amplitude of these self-exciting vibrations increases over time, the damping ability becomes insufficient and the blades fail completely. The goal of engineers in this case is to develop turbomachines that maintain aerodynamic stability over as wide operating range as possible to ensure flutter-free operation. This requires understanding the flow field comprehensively, while also determining its impact on the blade structure.

Assistance of CFD in studying Flutter:

Various analytical methods have been developed to overcome flutter in turbines, which works well for light loading applications. However, for a more advanced investigation in case of heavy load turbines, these methods fail and are often time-consuming. Experimental investigations on the other hand are costly and become complex when evaluating newer design concepts. For these reasons, computational methods are extremely beneficial as they provide detailed insights on flow-field within short period of time, allowing reduction in prototype trials as well as manufacturing time. Detailed assessment of different blade geometries is possible using sophisticated CFD models which can be linked with finite element analysis to study structural modes. Fully couples fluid structure computations allow studying the blade rows using computational structural dynamics under aerodynamic forcing, while the fluid flow is simulated using Euler or Navier-Stokes solvers.

While modern computational capabilities allow better understanding the aeroelastic behavior of structures in order to design the blade geometries with better damping capability, their experimental validation is certainly important. As such, experimental studies play a vital role in the research phenomenon, providing important information to improve the computational methods a step further.

About Author:

Mehul Patel specializes in handling CFD projects for Automobile, Aerospace, Oil and Gas and building HVAC sectors.