To reduce the shear stress, the velocity gradient must be lowered.
The velocity range of the top plate was the same for all the
tests performed. Therefore, ∂vθ
∂z can only be lowered by making the
local gap height larger. The asymmetric textures increase the gap
height linearly from h to hþD such that the maximum spacing
between the plates occurs only at one point in the texture. The
symmetric texture, on the other hand, increases the gap from h to
hþD over the entire length of the texture and therefore has a
larger effective gap, a lower local shear rate, and thus a lower
shear stress.