Fluid mechanics of fibre suspensions related to papermaking - DiVA

2. FLUID MECHANICS 9
where G is rate of strain and Dr is the rotary diffusivity that can be expressed
as the Brownian diffusion coefficient according to
Dr = kBTa
, (2.8)
ζr
where ζr is the rotational friction constant, kB Boltzmann’s constant and Ta
absolute temperature.
Looking in more detail at the Reynolds number, viscous forces dominate
when Re ≪ 1. This leads to a simplification of the Navier-Stokes equations,
where at every point in the fluid, a balance between the local pressure and the
viscous forces has to be achieved. In the limit Re → 0, the inertial forces are so
small that they can be neglected, and the resulting equations are linear. This
flow regime is known as creeping flow. There are two interesting features worth
mentioning here regarding to creeping flow:
• The flow is reversible, see for example the flow around a sphere by
Schlichting (1999).
• Long-range interactions, are possible for example when particles influence
each other over distances larger than their size. This is observed
among settling spheres.
Newtonian fluid flow can be of two distinct types–laminar or turbulent.
Laminar flow is characterised by a deterministic flow-field, which can be time
dependent. Due to the physics, a laminar flow can become unstable resulting
in a turbulent flow. This behaviour can also be deduced from the non-linearity
of the equations, where the Reynolds number is the controlling parameter. For
low Reynolds numbers, laminar flows can exist but as Re increases most flows
undergo transition to turbulent flow.
The following brief summary of the phenomenon of turbulence has been
provided by Tritton (1988): “each time a flow changes as a result of an instability,
one’s ability to predict the details of the motion is reduced. When
successive instabilities have reduced the level of predictability so much that it
is appropriate to describe a flow statistically, rather than in every detail, then
one says that the flow is turbulent”
Turbulence or turbulent flow, involves a wide range of scales and Kolmogorov
(see Pope (2000)) presented a theory for turbulence, based on a number
of hypotheses, taking into consideration the energy production range (PR),
the inertial range (IR), and the dissipative range (DR) of scales, see Figure 2.1.
These hypotheses states that high-Reynolds-number turbulence can be seen as
locally homogenous and isotropic if energy is constantly fed into the system and
that the turbulent velocity field can be seen as eddies of different sizes. The
large eddies in (PR) becomes smaller and smaller until the smallest eddies dissipate
in (DR). The energy cascades from large eddies to small eddies through
the intermediate region and there is no energy build up at any scales. So the
intermediate eddies can be characterised by their size and velocity, or length
and time scale. Kolmogorov showed on dimensional grounds that these eddies