drag models: WenYu and GidaspowSchillerNaumann

[kakila]

These two drag models (WenYu and GidaspowSchillerNaumann) seem to be exactly the same. their formulas are written slightly different but equivalent.
What's the difference between them?

GidaspowSchillerNaumann:

OpenFOAM-11/applications/modules/multiphaseEuler/interfacialModels/dragModels/GidaspowSchillerNaumann/GidaspowSchillerNaumann.C

Lines 72 to 78 in d5b4d96

	const volScalarField CdsRes
	(
	neg(Res - 1000)*24*(1.0 + 0.15*pow(Res, 0.687))/alpha2
	+ pos0(Res - 1000)*0.44*Res
	);
	return CdsRes*pow(alpha2, -1.65);

WenYu:

OpenFOAM-11/applications/modules/multiphaseEuler/interfacialModels/dragModels/WenYu/WenYu.C

Lines 71 to 77 in d5b4d96

	const volScalarField CdsRes
	(
	neg(Res - 1000)*24*(1.0 + 0.15*pow(Res, 0.687))
	+ pos0(Res - 1000)*0.44*Res
	);
	return CdsRes*pow(alpha2, -2.65);

[kakila]

Also in the reference the model has the exponent for alpha of -4.65, but here -2.65 is used. Is there an explanation for the deviation from the reference?