In Fluid simulations, Arrow helpers are linked to emitters, colliders, guides, and foam masks to provide a physical direction to follow.
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Create panel > 
(Helpers) > Standard > Object Type rollout > Arrow
Interface
Other than icon size and color, arrow helpers don't have any parameter rollouts or settings.
Procedure
To create an arrow helper:
- From the Create panel or menu, click Arrow and drag in the viewport.
- Adjust the direction of the arrow using transform controls.
Note: When used in Fluid simulations, each emitter object can have its own velocity, and can therefore each have its own arrow. However, if the overrides are off and an arrow is created in global controls, that arrow should be affecting all velocities for all emitters.