Both laboratory tests and discrete numerical simulations related to the drag experienced by various objects inside granular flows showed a large increase in drag coefficient when the Froude number of the undisturbed incident flow decreased. The present paper describes an elementary theory that can catch this observed trend. Accounting for a correlation length related to the presence of the obstacle, it is possible to derive a generalized drag coefficient. The careful calibration of the model on existing data about dry and cohesive dense granular flows around obstacles gives crucial information on the correlation length.