Turbulent Flow:
When a flow velocity becomes high, the flow becomes unstable and
fluctuating
velocity component appears. This is called "Turbulence" or "Turbulent
Flow". The
turbulence enhances the diffusion and mixing of the fluid and
consequently the
transport of momentum, heat, and mass is enhanced. Heat transfer
enhancement is
beneficial, but at the same time the turbulence causes the higher
pressure loss
as a result of the enhanced momentum transfer which deteriorates overall
heat
transfer performance taking the pressure loss into account.
Large Eddy Simulation (LES):
Turbulence models
express the turbulent transport by adding an extra term to the governing
equation of the averaged field. For example, in the Reynolds averaged
turbulence
model an ensemble average (in reality it is time-average) of the
variables is
used, and the additional term, the Reynolds stress, is introduced to the
Reynolds averaged field. On the other hand, in LES only the sub-grid
scale
turbulence which cannot be expressed in the computation is modeled. In
LES, the
turbulence model is confined only to the sub-grid scale components, and
the
time- and space-wise fluctuating components are reproduced. This feature
gives
the LES higher applicability to the complicated turbulent
fields.
Flow
Visualization: Fluids like air and water are transparent,
and
therefore it is impossible to know which part of the fluid is flowing
fast. In
order to visualize the transparent fluid flow, markers are introduced
into the
flow: the markers can be dye, solid particles, bubbles, etc.
Particle Image Velocimetry(PIV)/Particle Tracking Velocimetry(PTV): In a flow-visualization image with tracer particles, a
small
area is selected, and the corresponding area to it in the next time-step
image
is searched by using the similarity of the tracer pattern between two
areas as
an index. By knowing the corresponding areas between consecutive two
images, the
velocity vectors can be calculated. There is another way to calculate
the
velocity vectors in which each particle's location is used instead of
the tracer
pattern and the corresponding tracer location in the next time-step
image is
searched.
Parallel
Computing Technique: In the numerical analysis of the heat
transfer
phenomena, a system of linear algebraic equations has to be solved. When
the
space and time resolutions become high, the requirements for the
computational
speed and memory storage exceed the presently available computer
ability. In
order to deal with the huge computational load, the computational domain
(or
algorithm) is decomposed into N separate sub-domains, and each
sub-domain's computation is performed on a separate CPU of a parallel
computer
system. The ideal computing time with N CPUs should be reduced by
the
factor of 1/N, but in reality the data transfer between the CPUs
and the
synchronization of the communication are additionally introduced in the
program.
As a result, the speedup ratio of the parallel computing is less than N.
Transient technique:
A method to calculate heat transfer coefficient from measured surface
temperature profile at a certain time. In the calculation, a
theoretical solution of one-dimensional heat conduction problem in
semi-infinite solid is used.
Classical
Molecular
Dynamics Simulation: All materials consist of molecules. In
the
molecular dynamics simulation, the Newton's equation of motion for each
molecule
is solved. "Classical" indicates that the inter-molecular potential
function is
empirical. Inter-molecular force is calculated by differentiating the
inter-molecular potential function.
Quantum Molecular
Dynamics Simulation (Ab Initio Molecular Dynamics Simulation):
"Quantum" indicates that the inter-molecular potential function is not
empirical
but it is exactly solved by using quantum mechanics. The motion of the
electron
is expressed by the wave function following the Schrodinger equation. By
solving
the Schrodinger equation, physically exact inter-molecular potential is
known.
However, in reality, in the multi-electron state the empirical modeling
is used
to a certain extent as in the expression of the inter-electron
interaction and
the electron spin.
