  In this paper we consider the parabolic-elliptic Patlak-Keller-Segel models
in $\mathbb T^d$ with $d=2,3$ with the additional effect of advection by a
large shear flow. Without the shear flow, the model is $L^1$ critical in two
dimensions with critical mass $8\pi$; solutions with mass less than $8\pi$ are
global and solutions with mass larger than $8 \pi$ with finite second moment,
all blow up in finite time. In three dimensions, the model is $L^{3/2}$
critical and $L^1$ supercritical; there exists solutions with arbitrarily small
mass which blow up in finite time arbitrarily fast. We show that the additional
shear flow, if it is chosen sufficiently large, suppresses one dimension of the
dynamics and hence can suppress blow-up. In two dimensions, the problem becomes
effectively $L^1$ subcritical and so all solutions are global in time (if the
shear flow is chosen large). In three dimensions, the problem is effectively
$L^1$ critical, and solutions with mass less than $8\pi$ are global in time
(and for all mass larger than $8\pi$, there exists solutions which blow up in
finite time).
