"Optional title"
entype, enid, cid${}_\tau$, cid${}_{vx}$, cid${}_{vy}$, cid${}_{vz}$, $t_{beg}$, $t_{end}$
##### Parameter definition
VariableDescription
entype Entity type
options: P, PS, FS
enid Entity identification number
cid${}_\tau$ ID of a CURVE or FUNCTION defining the shear traction
cid${}_{vx}$ ID of a CURVE or FUNCTION defining reference velocity in x-direction
cid${}_{vy}$ ID of a CURVE or FUNCTION defining reference velocity in y-direction
cid${}_{vz}$ ID of a CURVE or FUNCTION defining reference velocity in z-direction
$t_{beg}$ Start time
default: 0
$t_{end}$ End time
default: 1.0e10
##### Description

This command defines shear traction on a surface. It can be used to model drag forces and prescribed friction loads. The local direction of the traction $\hat{\mathbf t}$ is in that of the reference velocity ${\mathbf v}_{ref}$ vector minus the local velocity vector ${\mathbf v}({\mathbf x})$, projected onto the surface.

$\displaystyle{ \hat{\mathbf t} = \frac{({\mathbf I}-\hat{\mathbf n} \otimes \hat{\mathbf n})({\mathbf v}_{ref}-{\mathbf v})} {\Vert ({\mathbf I}-\hat{\mathbf n} \otimes \hat{\mathbf n})({\mathbf v}_{ref}-{\mathbf v}) \Vert}}$

Here $\hat{\mathbf n} = \hat{\mathbf n}({\mathbf x})$ is the local surface normal direction.

##### Example

The following commands apply shear traction on the part with ID 1. The traction is a linear function of the relative tangential velocity vtang. Note that vtang is a built in variable that can be accessed in FUNCTION. The reference velocity is ramping up from 0 to 100 in x-direction.

*PARAMETER
%C = 2.0