Figure 3.

Fundamental mechanism causing fluid flow: (a) Contours of pressure and (b) Contours of absolute velocity in x-direction for $a/L=2/7$ and $a/\lambda =1/7$ (wave moving to the right) at $t=0.35t_{\mathrm{ref}}$. Due to the velocity of cilia, regions of positive and negative pressure are established in the channel. The deformed position of the cilia causes a lower pressure gradient in direction of the wave compared to that of in the opposite direction. This leads to a high velocity and a net flow to the left.