Fluctuations in drag co-efficient
 

I am analysing the drag coefficient of a truck. I have increased the domain size, recreated the geometry and refined the mesh, however the drag coefficient continues to fluctuate. I have included an image of the fluctuation of the drag.

Any help would be great thanks!

I am using the k-epsilon realizable turbulence model, the length of the model is 1100 mm and the inlet velocity is 15.94 m/s.