drag force in dragModel.C

Hi, everyone. I am confused that the expression of drag force used in Openfoam, as shown in the following codes. Why there is nu and divided by square of particle diameter. I saw the formula is without nu and only divided by diameter in some literature. Do you have any tip for me? Thanks a lot. By the way, After I deleted pair_.continuous().nu() and only divided by diameter. The results I obtained are totally the same.
Foam::tmp<Foam::volScalarField> Foam::dragModel::Ki() const { return 0.75 *CdRe() *swarmCorrection_>Cs() *pair_.continuous().rho() *pair_.continuous().nu() /sqr(pair_.dispersed().d()); }

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Thanks a lot, Dongyue. Very helpful. I found in SN model CdRe was multiplied by another Re. Thus, the formula is consistent.