  Vertically bounded fingering double diffusive convection (DDC) is numerically
investigated, focusing on the influences of different velocity boundary
conditions, i.e. the no-slip condition which is inevitable in the lab-scale
experimental research, and the free-slip condition which is an approximation
for the interfaces in many natural environments, such as the oceans. For both
boundary conditions the flow is dominated by fingers and the global responses
follow the same scaling laws, with enhanced prefactors for the free-slip cases.
Therefore, the laboratory experiments with the no-slip boundaries serve as a
good model for the finger layers in the ocean. Moreover, in the free-slip case
although the tangential shear stress is eliminated at the boundaries, the local
dissipation rate in the near-wall region may exceed the value found in the
no-slip cases, which is caused by the stronger vertical motions of fingers and
sheet structures near the free-slip boundaries. This counter intuitive result
might be relevant for properly estimating and modelling the mixing and
entrainment phenomena at free-surfaces and interfaces.
