11.5 Doppler Global Velocimetry
251
Fig. 11.12 Schematic
representation of laser-optics
setup for DGV
2D field. Alternatively, the laser sheet can be rotated about its axis or three laser sheets
can be used, each at an angle, however limiting measurements to mean quantities
only.
11.6 Particle Image Velocimetry
11.6.1 Basic Principle of Planar PIV
Particle Image Velocimetry (PIV) was implemented in the 90s and has since become
one of the most common techniques used in fluid mechanics research. The basic principle is analogous to LDV where fluid flow is measured by sensing the displacement
of artificially injected particles convected by the flow. In its most primitive set-up as
shown in Fig. 11.13, an experiment using PIV consists of a laser sheet (about 1 mm
thick) expanded by a cylindrical rod which illuminates the measurement plane. The
laser is pulsed continuously at a very short time delay to light-up the particles seeded
into the flow. A camera is positioned normal to the plane of the laser sheet and records
the images of the particles displacement, at two time instants. The images are then
processed to determine the displacement of the particles within the 2D flow field for
the measurement of two velocity components in the plane of the laser sheet.
The quality of the measurements is dictated by the choice of particles which
are transported passively by the flow. This behaviour can be quantified by Stokes
number, which is the ratio of the characteristic time of particle (the time constant
in the exponential decay of the particle velocity due to drag) to the characteristic
time of the flow. For good quality and reliable PIV measurements the Stokes number
of 0.05–0.1 is usually recommended. This pre-requisite on size and particle types
has serious implications on the signal-to-noise ratio of the captured image. In water
it is possible to use solid particles with a diameter of the order of 100 µm and a
251
Fig. 11.12 Schematic
representation of laser-optics
setup for DGV
2D field. Alternatively, the laser sheet can be rotated about its axis or three laser sheets
can be used, each at an angle, however limiting measurements to mean quantities
only.
11.6 Particle Image Velocimetry
11.6.1 Basic Principle of Planar PIV
Particle Image Velocimetry (PIV) was implemented in the 90s and has since become
one of the most common techniques used in fluid mechanics research. The basic principle is analogous to LDV where fluid flow is measured by sensing the displacement
of artificially injected particles convected by the flow. In its most primitive set-up as
shown in Fig. 11.13, an experiment using PIV consists of a laser sheet (about 1 mm
thick) expanded by a cylindrical rod which illuminates the measurement plane. The
laser is pulsed continuously at a very short time delay to light-up the particles seeded
into the flow. A camera is positioned normal to the plane of the laser sheet and records
the images of the particles displacement, at two time instants. The images are then
processed to determine the displacement of the particles within the 2D flow field for
the measurement of two velocity components in the plane of the laser sheet.
The quality of the measurements is dictated by the choice of particles which
are transported passively by the flow. This behaviour can be quantified by Stokes
number, which is the ratio of the characteristic time of particle (the time constant
in the exponential decay of the particle velocity due to drag) to the characteristic
time of the flow. For good quality and reliable PIV measurements the Stokes number
of 0.05–0.1 is usually recommended. This pre-requisite on size and particle types
has serious implications on the signal-to-noise ratio of the captured image. In water
it is possible to use solid particles with a diameter of the order of 100 µm and a