delta_v is of course correct.
What's confusing about this is mentally we tend to remain "grounded" on solid earth even when we are flying along in a moving airmass, totally disconnected from the ground below. Forget about ground and groundspeed and imagine when you start a turn, off your wing at about the center of the turn radius is a special air molecule that is shining, shining so bright you can keep it in sight. As you make the turn you will see that shining air molecule staying right there around the center of your turn radius. There is no "acceleration", just you, the molecule and the whole air mass behaving as a unit, just the same as if it were dead calm.
