Biomedical Engineering Reference
In-Depth Information
amount of insight, depending on the accuracy of the geometry of the model and the algo-
rithm used to solve the governing equations. Also, the extent of assumptions may limit
the computational fluid dynamics results. Using PIV systems, an actual device (such as a
mechanical heart valve, a total artificial heart, or a stent) can be investigated within a
dynamically similar environment. For most cardiovascular simulations, it would be most
appropriate to match the Reynolds number and the Womersley number to reach a dynam-
ically similar solution (as discussed in Chapter 13).
There are a number of variations in PIV systems based on the incident light and/or the
collection method. One of these methods, laser speckle velocimetry (LSV), makes use of
the speckle pattern of particles suspended within the fluid instead of the reflective pattern
as discussed above. LSV is typically used when the exact velocity distribution is wanted at
all locations within the fluid. The fluid is seeded with a high concentration of particles,
whose reflected light would not be able to be distinguished by a PIV computer algorithm
(and therefore the particles themselves could not be differentiated) coupled with a camera
system and one particular bandwidth of incident light. For LSV, a coherent light source is
used which generates a speckle pattern based on the interference of the particles. This
speckle pattern is collected by a digital camera, and more powerful computer processing
techniques are needed to determine the flow profile based on this interference speckle pat-
tern. This is typically used to model the movement of large solids, such as thromboemboli,
through a dynamically similar system.
Another PIV technique, holographic PIV (or HPIV) is used to obtain a three-dimensional
velocity profile by collecting the particle location information on a hologram and then
computationally reconstructing this image. The advantage of HPIV systems is that a true
three-dimensional flow profile image can be obtained instead of projecting the three-
dimensional velocities onto a two-dimensional plane. A recent PIV technique can be used
to investigate microflows, with the use of an epi-fluorescent microscope. Instead of using
reflective particles, fluorescent particles are placed in the flow stream, and data are
recorded through a camera coupled to an epi-fluorescent microscope. Fluorescent particles
are useful because they excite at one particular wavelength of light and emit at a second
wavelength of light. Through the use of appropriate filters, these wavelengths can be dis-
cerned precisely and all other wavelengths of light can be ignored. Also, common reflective
particles are typically too large for studies on the microcirculation, but through the use of
fluorescent particles, images on a smaller scale can be obtained.
14.2 LASER DOPPLERVELOCIMETRY
Laser Doppler velocimetry is primarily concerned with quantifying the microstructures
of flows that are subjected to obstacles within the flow field (similar to PIV techniques).
Direct measurements of the kinematics of the fluid motion can be obtained with this
method. For laser Doppler velocimetry, the measurement of the fluid velocity is made at
the intersection of two laser beams that are focused at a point of interest within the flow
field. Laser light sources are required for this technique because the two beams must be
monochromatic (one wavelength) and be coherent, so that the intersecting beams can cre-
ate an interference pattern. The interference pattern occurs within the fluid and is
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