  We conducted a three-dimensional numerical simulation of bioconvection
generated by oxygen-reactive chemotactic bacteria. This study investigated the
bioconvection patterns, interference between plumes, and the wavelength of
bioconvection patterns. In addition, we clarified the transport characteristics
of cells and oxygen in the bioconvection. Multiple plumes occur in the
suspension and three-dimensional bioconvection is formed around the plumes by
the cells with vortex rings arising around the plumes. Even if bioconvection at
a high Rayleigh number is disturbed, the bioconvection is strongly stable with
respect to disturbances, and the pattern does not change due to disturbances.
Bioconvection changes depending on the physical properties of bacteria and
oxygen, and, in particular, the rate of oxygen consumption by bacteria
significantly affects the strength of bioconvection. Bioconvection patterns
with different plume arrangements and shapes are formed for different Rayleigh
numbers or initial disturbances of the cell concentration. As a result, the
wavelengths of the patterns also vary. As the Rayleigh number increases,
interference between plumes is strengthened by the shortening of the pattern
wavelength, so the velocities of both upward and downward flows increase. Many
cells are located under the plumes and a strong shear flow occurs in these
regions. As the pattern wavelength decreases, the cells are affected by high
shear stress. Then, the convective transport of the entire suspension
strengthens and the transport characteristics of cells and oxygen improve. When
the chamber boundary is changed to side walls, bacteria adhere to the wall
surface, and plumes are regularly arranged along the side walls.
