Check this out,
YouTube - Mythbuster TC Model Drag Test
They also did one that was a small table top canal or channel built like an oval track with walls filled with water.
They used a pump to circulate water through this oval channel and on 1 side was a Plexiglas window so you could see the model car submerged under the moving water.
They put in a hand full of oatmeal so they could see how the water moved around the model car as the oatmeal floated past the car.
So even if you don’t have exact flow #’s, you can still devise away to measure before and after.