LINFLOW has originally been conceived for the study of Aeroelasticity and Aeroelastic phenomena. However, during development heavy emphasis was placed on generality with respect to Fluid-Structure Interaction. It is therefore important that we define Aeroelasticity, Hydroelasticity, Aeroelastic Stability Analysis, and Flutter in a LINFLOW sense. We will start by looking at some definitions found at the web, and then define the terms for the purpose of appreciating LINFLOW:

Definition of Aeroelasticity, Definition of Hydroelasticity

Wikipedia states:

"Aeroelasticity is the science related to the physical phenomena which involve interaction among inertial, elastic, and aerodynamic forces. No aircraft structure is completely rigid, so when it is subjected to ..."

A similar definition can be found here.

In “Handbook of Fluid Mechanics and Fluid Machinery”

the following definition for Aeroelasticity can be found (p. 1684):

"The subject has been defined as encompassing those physical phenomena for which aerodynamic, elastic (structural), and inertial (dynamic) forces interact in a significant way."

This definition is more general, but still not general enough in a LINFLOW sense. When we are talking about Aeroelasticity and LINFLOW, we mean:

"Aeroelasticity (Hydroelasticity) is an important subset of Fluid-Structure Interaction encompassing those physical phenomena for which aerodynamic (hydrodynamic), elastic (structural), and inertial (dynamic) forces influence each other and interact in a significant way."

Discussing and providing definitions like the above may look very unproductive, but it is our experience that most people do not have the proper understanding of this class of problems. More important, they do not realize that the phenomena of flutter and aeroelasticity can be found in many other applications than in aircrafts. It is a sad fact that many people believe that they can study Fluid-Structure Interaction utilizing concepts for acoustics. We therefore see a need for concise descriptions/definitions, and hence do not hesitate in bringing more definitions:

Definition of Aeroelastic and Hydroelastic Stability Analysis

Under Aeroelastic (or Hydroelastic) Stability Analysis, we shall understand

" Aeroelastic (or Hydroelastic) Stability Analysis is a Parameter Study of a system of gas or fluid interacting with a structure with respect to stability; i.e. with respect to physical parameters not exceeding a certain treshold."

It shall be mentioned that this class of studies (flutter and aeroelasticity) is not limited to aircraft structures. Systems that should be subject to such studies are pipes containing fluids, ventilation ducts, bridges, and more.

Definition of Flutter

A term that is linked to Aeroelastic Stability is "Flutter", that can have various meanings. A definition with respect to aircraft technology can be found at "Aeroplane Monthly" that has the following definition of Flutter with respect to aircraft design:

" Flutter. Usually applied to wings or ailerons - the unstable oscillation caused by interaction between aerodynamic forces, elastic reactions in the structure and the force of inertia. Unless damped quickly flutter will break a wing. The cure for flutter is usually to dispose the weights so that the centre of gravity of the wing is as far forward as possible. Ailerons, elevators and rudders are mass-balanced to prevent flutter."

In machine dynamics, the term “flutter” is widely used; e.g. with rotating machinery. For bladed systems like turbine disks or fans, please klick here.

In a LINFLOW context, we define

"Flutter is a repeating movement pattern that is potentially destructive or has the potential of causing damage, usually found above some parameter value or in parameter intervals."

Consequently, the Aeroelastic capabilities of LINFLOW makes it possible for the user to predict whether there is a likelihood that a system is Aeroelasticly Unstable or not, i.e. whether Flutter will occur or not. In practice, there are basically 5 methods for predicting flutter:

Prediction of Flutter:
As stated above, the PK- and VG-methods for aeroelastic stability analysis to predict flutter are implemented in LINFLOW.

Definition of Fluid-Structure-Interaction ("FSI")

Fluid-Structure-Interaction (or "FSI"):
Fluid-structure interaction (often referred to as FSI), is where a moving gas (e.g.air) or fluid (e.g. water) interacts with a structure in such a way that the deformations induced by the medium are of a magnitude such that the flow field is affected. We can (somewhat oversimplified) say that FSI is the struggle between structure and medium to find the most comfortable state. FSI is usually a dynamic process. LINFLOW simulates this dynamic process in the frequency domain.