It is important to distinguish between two types of strain: the "rotational" type and the "irrotational" type.
"Encyclopaedia Britannica, 11th Edition, Volume 9, Slice 2" by Various
One consequence of this is that the particles can have no rotation impressed upon them, and the motion of such a fluid is irrotational.
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 1" by Various
Buchert, Relativistic cosmological perturbation scheme on a general background: scalar perturbations for irrotational dust, Class.
Backreaction in late-time cosmology
In the presence of matter, scalars represent the response of the metric to an irrotational distribution of matter, and generalize Newton’s theory of gravitation.
TASI Lectures on Cosmological Perturbations
Although the velocity in real space remains irrotational even when the (cid:13)ow is non-linear as long as orbit-mixing is absent, vorticity is introduced to the velocity (cid:12)eld in redshift space from second order e(cid:11)ects, which we shall here ignore.
On the Prediction of Velocity Fields from Redshift Space Galaxy Samples
We do not, for example, require the (cid:13)ow to be irrotational, as is required for POTENT.
Estimation of Peculiar Velocity from the Inverse Tully-Fisher Relation
Linear theory ﬂows are irrotational, γ · v ∝ µk , and gravitational redshifts are absent in the direction perpendicular to the oscillation.
Toward Understanding CMB Anisotropies and Their Implications