This technique can be used for a number of other things, too. For example stopped chamfers, chamfers ending in lamb's tongues, drilled holes (especially useful for that if you want a countersunk hole), etc.
1. Draw the positive version of the fluting. I decided this would be cut using a 1/2" core box router bit. I started with a vertical rectangle located over the centerline of the flute. I drew it 1/2" high. This was a convenient dimension. The side of this shape will be used as a path for Follow Me and the first segment of the path must be perpendicular to the profile for Follow Me to work properly. I drew radii at the two lower corners of the rectangle to the radius of bit.
2. Then I used Push/Pull to pull the faces out 1/4" from the centerline. I drew a 1/4" radius arc on one end.
3. I selected the edges that need to have the radius and ran Follow Me. This actually gets done once for each side because the profile is really two 1/4 arcs.
4. I moved the profile down into the work. Note there is no black line at the intersection of the sides of the profile and the face of the work. I don't show it but the next step, while the profile is still selected is to run Intersect with model from the Context menu (RMB).
5. Finally, I deleted what I didn't need and reversed the face iinside the flute. It was inisde the profile so it was the inside face.
Note: I scaled the model up by a factor of 100 before running the Follow Me operations. There are some tiny faces generated by the arcs and Follow Me that are much easier to deal with at a larger size. In this case they weren't really obvious so they could have probably been ignored.
By the way, when making more than one flute, just make copies of the positive and resize them as needed.