# gp.force.unbal.add

Syntax

## Vector Access

Add to the current unbalanced force at a gridpoint. Supply a vector, unless an optional component argument i is supplied, in which case it requires a scalar value.

This adds to the current unbalanced force at a grid point. Extreme caution is required when using this function, as instability can result. The current unbalanced force in a grid point is cleared to zero after the equations of motion are applied, near the end of the cycle sequence. They are not cleared at the start of the cycle sequence.

Note that for the purpose of modifiers the current value is treated as a zero vector.

In general it is not necessary to call this function. Instead use either a fish or fish-local multiplier to an applied boundary condition, or the gp.force.load intrinsic.

See also the FISH intrinsics gp.force.unbal and gp.force.unbal.current.

Returns: v - value to add to the current unbalanced force vector or component gp - grid point pointer i - optional vector component, from 1 to 3