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!VELOCITY¶

Definition of velocity boundary conditions

Parameters¶

TYPE = INITIAL (initial velocity boundary condition)
     = TRANSIT (time-history velocity boundary condition specified with !AMPLITUDE; default)
GRPID = group ID
AMP  = time function name (specified with !AMPLITUDE)
       !AMPLITUDE defines the relationship between time t and coefficient f(t).
       The value obtained by multiplying Value below by coefficient f(t) becomes the prescribed value at that time
       (if omitted: the time-coefficient relationship is f(t) = 1.0).
ROT_CENTER = node number or node group name of the center of the rotational velocity constraint.
             When specified, this !VELOCITY is recognized as a rotational velocity constraint.

** 2nd line and later **

(2nd line) NODE_ID, DOF_idS, DOF_idE, Value

Variable	Attribute	Description
NODE_ID	I/C	Node number or node group name
DOF_idS	I	First constrained degree-of-freedom number
DOF_idE	I	Last constrained degree-of-freedom number
Value	R	Prescribed value (default: 0)

Example¶

!VELOCITY, TYPE=TRANSIT, AMP=AMP1
  1, 1, 1, 0.0
  ALL, 3, 3
  Note: Prescribed value is 0.0
!VELOCITY, TYPE=INITIAL
  1, 3, 3, 1.0
  2, 3, 3, 1.0
  3, 3, 3, 1.0

Unlike displacement boundary conditions, velocity boundary conditions cannot define multiple degrees of freedom together, so DOF_idS and DOF_idE must be the same number.
When TYPE is INITIAL, AMP is disabled.
GRPID is referenced from the BOUNDARY line of !STEP and is used to enable velocity boundary conditions for each step.

Rotational Velocity Constraint (ROT_CENTER)¶

When ROT_CENTER is specified, an angular velocity about the center of rotation can be applied as a velocity boundary condition, in the same way as ROT_CENTER of !BOUNDARY (rotational displacement constraint). It is valid only in dynamic analysis.

Each data-line Value is given as a component (degrees of freedom 1–3) of the angular velocity vector \(\boldsymbol{\omega}=(\omega_x,\omega_y,\omega_z)\) [rad/s]. Each node is subjected to the tangential velocity \(\boldsymbol{v}=\boldsymbol{\omega}\times(\boldsymbol{x}-\boldsymbol{x}_c)\), where \(\boldsymbol{x}_c\) is the center of rotation. When AMP is used, \(\boldsymbol{\omega}(t)=\boldsymbol{\omega}_0 f(t)\).

!VELOCITY, TYPE=TRANSIT, AMP=AMP1, ROT_CENTER=CENTER
  RIM, 1, 1, 0.0        ! ωx
  RIM, 2, 2, 0.0        ! ωy
  RIM, 3, 3, 6.283185   ! ωz [rad/s]

When ROT_CENTER is specified

The node group specified by ROT_CENTER must consist of exactly one node.
Whereas the rotational displacement constraint (!BOUNDARY, ROT_CENTER) is a finite rotation using the Rodrigues formula, the rotational velocity constraint is a tangential velocity obtained from the cross product with the angular velocity vector.