aqwa-intro 14.5 l04 articulation and multibody

© 2012 ANSYS, Inc. May 8, 2013 1 Release 14.5

14.5 Release

Lecture 04

ANSYS Aqwa Suite Articulations & Multi-body

Problems


A. Articulations & Constraints

Introduction

• A constraint can be applied to any degree of freedom. This has the effect of stopping the calculation of forces or moments, and stopping motion in the specified constrained degrees of freedom.

• Great care must be exercised if degrees of freedom are de-activated in a structure which is articulated either to another structure or to a fixed point. It is recommended that this should not be done.

• Structures in an Aqwa can be freely floating, moored or connected to other structures by points of articulation. There are four different types of articulation available. These are as follows:

• Ball and Socket → Free to rotate about all axes

• Universal → Free to rotate about two axes transmitting a moment about the third axis at right angles to the first two

• Hinged → Transmitting a moment about two axes and free to rotate about the third axis at right angles the first two

• Locked → Transmitting a moment about all axes and not free to rotate at all

• Sliding articulations are currently not supported in Aqwa


...Articulations & Constraints

Constraints in Aqwa (Deck 12)

• When constraints are defined, the usual specification of starting positions for the analysis (deck 15) is different

• Defined COG positions of attached structures may not be geometrically compatible with position of constraints

• Rather than enforce geometric compatibility at definition time, Aqwa will take the position of the first structure in an articulation group, then connect the remaining structures using only the orientation of these

• Closed connection loops are permissible

• Only one fixed constraint (to ground) is permissible within an articulation group

• Redundant systems of articulation will generally result in a failure since as a rigid body solver the program cannot determine the distribution of loads in the redundant connections



Constraints in Aqwa (Deck 12)

• Mechanical Articulations between Structures

• Relative translational motion is not allowed, but relative rotational motion is possible.

DCON Nt Ns1 Nd1 (Nd3) Ns2 Nd2 (Nd4)

Nt: number of D.O.F. being locked by this constraint.

Nt=0: Ball and Socket, rotation in 3 D.O.F.

Nt=1: Universal joint, rotation in 2 D.O.F.

Nt=2: Hinge, rotation in 1 D.O.F.

Nt=3: Rigid connection, no rotation.

Ns1

Nd3 Nd1, Nd2

Nd4

Ns2



Aqwa-Intro_14.5_WS04.1_Articulations_and_Constraints.pptx

AQWA-Intro_13.0_WS04.1_Articulations_and_Constraints.pptx




B. Multi-body problems

Introduction

• Aqwa can be used to solve problems that involve multiple bodies with up to 20 structures with full hydrodynamic interaction.

• The Response Amplitude Operators (RAOs) for each of the hydrodynamically interacting structures will be different from those that would have resulted if each of these structures were on its own. The RAOs are not a physical property of a structure but, as can be seen from the equations of motion, depend on the radiation and diffraction forces. The radiation as well as the diffraction forces change in the case of hydrodynamic interaction and therefore the RAOs of the structures in question will also change.

• When hydrodynamic interaction is employed, special attention is needed when the user moves the structures relatively to each other (using the MSTR card). In Aqwa, the RAOs are always evaluated relative to the FIXED REFERENCE AXIS (FRA). That means, if the user defines different positions of one or more hydrodynamically interacting structures in two consecutive Aqwa-LINE runs, the results between these two runs will not be comparable.

• For restrictions in the usage of hydrodynamic interactions please refer to Aqwa Theory Manual.


...Multi-body problems

Symmetry (Deck 2)

• SYMX/SYMY Cards: Aqwa can assume that the analysis is symmetric about the FRA (X/Y).

• This allows time-saving shortcuts to be used in the solution.

• These options are not available on WB yet.

• RMXS/RMYS Cards: Removes symmetry, creating a full model (even though the model may still be a symmetric structure).

• It only applies to T/QPPL elements, not to other elements or nodes.

• In general, with multi-body interaction symmetry must be removed on the individual bodies

• MSTR Card: Moves the structure to a new definition position.

• It only applies to elements and associated nodes, not to all nodes listed under the STRC card. This has an impact on nodes associated with moorings, fenders and articulations.

• It actually moves the nodes and elements in the FRA.

• It is NOT the same as the POS card in Deck 15.


...Multi-body problems Use of Lids (Deck 2)

• ILID: to remove irregular frequencies inside structures

• Can be automatically generated

• VLID: to reduce standing waves between structures

• Has to be defined by user


...Multi-body problems

Aqwa-Intro_14.5_WS04.2_Multi-body_interactions.pptx

AQWA-Intro_13.0_WS04.2_Multi-body_interactions.pptx





aqwa-intro 14.5 l04 articulation and multibody

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