  We present numerical simulations of laminar and turbulent channel flow of an
elastoviscoplastic fluid. The non-Newtonian flow is simulated by solving the
full incompressible Navier-Stokes equations coupled with the evolution equation
for the elastoviscoplastic stress tensor. The laminar simulations are carried
out for a wide range of Reynolds numbers, Bingham numbers and ratios of the
fluid and total viscosity, while the turbulent flow simulations are performed
at a fixed bulk Reynolds number equal to 2800 and weak elasticity. We show that
in the laminar flow regime the friction factor increases monotonically with the
Bingham number (yield stress) and decreases with the viscosity ratio, while in
the turbulent regime the the friction factor is almost independent of the
viscosity ratio and decreases with the Bingham number, until the flow
eventually returns to a fully laminar condition for large enough yield
stresses. Three main regimes are found in the turbulent case, depending on the
Bingham number: for low values, the friction Reynolds number and the turbulent
flow statistics only slightly differ from those of a Newtonian fluid; for
intermediate values of the Bingham number, the fluctuations increase and the
inertial equilibrium range is lost. Finally, for higher values the flow
completely laminarises. These different behaviors are associated with a
progressive increases of the volume where the fluid is not yielded, growing
from the centerline towards the walls as the Bingham number increases. The
unyielded region interacts with the near-wall structures, forming
preferentially above the high speed streaks. In particular, the near-wall
streaks and the associated quasi-streamwise vortices are strongly enhanced in
an highly elastoviscoplastic fluid and the flow becomes more correlated in the
streamwise direction.
