CAE-Companion-2018-2019

Theory WISSEN CAE

other hand, development of a stable and accurate coupling procedure, something practically impossible for a wide range of FSI problems, is required in the iterative coupling method. ProblemDependency of Algorithmof Choice Depending on the physics of the FSI problem someone should consider carefully the FSI algorithm of choice: „ „ Direct or monolithic coupling In the direct FSI coupling solution method the fluid and solid equations are combined and treated in one system (one stiffness matrix for both problems) and solved using an iterative solver such as the Newton-Raphson method. The direct FSI coupling algorithm offers great robustness when solving very difficult FSI problems, for example, large deformations with soft structures or highly compressible flows around very stiff structures. Due to occurring instabili- ties like the so-called “artificial added-mass effect” and similar these types of problems are difficult to solve using the iterative FSI solution method. „ „ Iterative or two-way coupling In general the Iterative FSI coupling solution method re- quires less memory than the direct FSI Coupling method and therefore may be more applicable to solve very large problems despite occurring instabilities which can be handled, sometimes more sometimes less efficiently, by numerical intervention like e.g. relaxation factors.

Figure 4 a+b: Direct FSI coupling examples: a) Tethered Helium Balloon b) Dirigible Free Flying Helium Lighter than Air Aircraft CAEWissen by courtesy of Dr.-Ing. Thomas Chatzikonstantinou (Aachen)

Figure 3: Iterative two-way FSI coupling example: Ram-Air Parachute

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