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11 Non-intrusive Measurement Techniques
challenging to measure are now quantifiable. In addition, LDV allows more access
to the turbulent quantities through the measurement of Reynolds stress tensors and
measurement in flows within a higher frequency band which was previously restricted
by data acquisition issues and very cumbersome treatment of the signal. However,
hot wires still dominate in some types of measurement due to their unarguably higher
frequency response but the laser-optic techniques are catching up steadily.
Later, PIV, where the basic idea is simple and can be traced back to early days
of flow visualisation when it was subsided as being too labour intensive for manual
treatment. Progress in optics, electronics, computational resource and digital image
processing changed the way flows could be diagnosed through rapid processing of
thousands of particle images in a relatively short period of time. It became commercially available very rapidly and thus found its way easily in many wind tunnels and
fluid mechanics laboratories.
The major advantage of PIV over LDV is the ability to capture the whole instantaneous velocity field at a go, whereas LDV provides the information only at a point
in space and a traversing process is required to resolve the flow within similar spatial
domain as the PIV, leading to expensive and bulkier traversing hardware. Also, it is
very difficult to derive correlations in unsteady flows as the probe has to be shifted
from one point to another and due to the time-varying behaviour of the flow the
two measurements are not representative in time. Coupled with these limitations,
the cost and effort required for alignment contributed to the lack of prioritisation of
LDV over PIV. But still LDV is preferred for finer measurements, such as in boundary
layer traverse where it allows for near-wall measurements of the orders of 100 µm.
Thus allowing access to a region of the flow where a “clinical” understanding of the
mechanism is indispensable.
11 Non-intrusive Measurement Techniques
challenging to measure are now quantifiable. In addition, LDV allows more access
to the turbulent quantities through the measurement of Reynolds stress tensors and
measurement in flows within a higher frequency band which was previously restricted
by data acquisition issues and very cumbersome treatment of the signal. However,
hot wires still dominate in some types of measurement due to their unarguably higher
frequency response but the laser-optic techniques are catching up steadily.
Later, PIV, where the basic idea is simple and can be traced back to early days
of flow visualisation when it was subsided as being too labour intensive for manual
treatment. Progress in optics, electronics, computational resource and digital image
processing changed the way flows could be diagnosed through rapid processing of
thousands of particle images in a relatively short period of time. It became commercially available very rapidly and thus found its way easily in many wind tunnels and
fluid mechanics laboratories.
The major advantage of PIV over LDV is the ability to capture the whole instantaneous velocity field at a go, whereas LDV provides the information only at a point
in space and a traversing process is required to resolve the flow within similar spatial
domain as the PIV, leading to expensive and bulkier traversing hardware. Also, it is
very difficult to derive correlations in unsteady flows as the probe has to be shifted
from one point to another and due to the time-varying behaviour of the flow the
two measurements are not representative in time. Coupled with these limitations,
the cost and effort required for alignment contributed to the lack of prioritisation of
LDV over PIV. But still LDV is preferred for finer measurements, such as in boundary
layer traverse where it allows for near-wall measurements of the orders of 100 µm.
Thus allowing access to a region of the flow where a “clinical” understanding of the
mechanism is indispensable.