Here’s the method I use. I’m using ver. 14 Studio, but it should work in Broadcast ver. 12 if you have Mograph and the SplineWrap deformer.
Make an arrow and subdivide it along its length so it will bend. Put it into a Cloner object, choosing Linear, and Per Step mode. Draw a spline path for it to follow. I usually use the Cubic type, and Uniform as the interpolation method.
Make a Null object, and put a SplineWrap deformer into it, and put the spline you drew in the place marked Spline, so the SplineWrap knows which spline to use as the path. Make the Cloner object a child of the SplineWrap. So you have a hierarchy: Null>SplineWrap>Cloner. Adjust the P.Z parameter in the Cloner to control the spacing of the arrows, and adjust the count to make enough arrows to fill the spline.
Use these settings for the SplineWrap: Axis: +Z, Mode: Fit Spline, End Mode: Clamp, Check Fit Bounding Box. Animate the offset of the SplineWrap to make the arrows move around the spline.
This should do it.
Jack