Table 3: Methods of measuring flow.

Measurement methodBasis of the methodProsConsRef.

Particle image velocimetry (PIV), including micro-PIV (μPIV)Monitoring the displacement of small seeded particles in a region of interest of fluid medium via double-pulsed laser beam(i) Can be used through an in vitro investigation
(ii) Noninvasive method
(iii) High spatial resolution
(iv) Simultaneously determination of velocities of two different phases without disturbing the flow
(i) Almost impossible for in vivo experiments
(ii) Requiring undistorted optical access to the area of interest for both an excitation laser and an imaging system
(iii) Limited of temporal resolution
(iv) Requiring appropriate particles to eliminate the differences between solid particles and local fluid velocities

Holographic PIV (HPIV)Record the particle image field using a reference beam to project the hologram, followed by a 2D plane detector moved through the projected hologram (i) Can also record a 3D instantaneous flow field
(ii) Being user friendly
(i) Reduction of speckle noise
(ii) Not handling huge quantities of data
(iii) Cannot extract 3D velocity in presence of large gradients/fluctuations
(iv) Complexity of system
(v) Requiring large depth of focus that affects the measurement accuracy

Particle tracking velocity (PTV)Measuring particle velocities using video camera recording (i) Easily determination of even small displacement of particles without confusing them with neighboring ones
(ii) Measurement of velocity at the location of a particle, without requiring an averaging over a grid (compared to PIV)
(i) Requiring many individual particles to be reconstructed in space and identified in successive frames
(ii) Lower spatial resolution
(iii) Time consuming

Laser Doppler anemometry (LDA) or laser Doppler velocimetry (LDV)Measuring of scattered laser light by particles that pass through a series of interference fringes (a pattern of light and dark surfaces)(i) High spatial and temporal resolution (typically in the order of 1 kHz)
(ii) Nonintrusive method
(iii) No calibration required
(iv) Recording one, two, or three velocity components simultaneously
(v) Also applicable in reversing flows
(i) Cannot simultaneously measure the velocities of different phases
(ii) Time consuming
(iii) Difficult to analyzing the discrete data stream from the flow
[9497, 99]

Acoustic Doppler velocimeter (ADV)Measuring the velocity of particles in a remote sampling volume based on the Doppler shift effect using one transmitter and three receivers Simultaneously recording nine values with each sample: three velocity components, three signal strength values, and three correlation values(i) Only suitable for flow conditions with relatively low turbulence level
(ii) Required postprocessing of data
[99, 100]

Holographic correlation velocimetry (HCV)Measuring 3D velocity fields of a fluid at high speed combining a correlation-based approach with in-line holography(i) Very efficient with regard to the use of light, as it does not rely on side scattering
(ii) Very high quality system at a modest cost
(iii) Appropriate for high-speed flows and low exposure times
(iv) Simple calibration
(v) Using relatively low powered lasers
(vi) Direct measurement of the velocity field at all depth locations
(vii) Nonintrusive technique especially in cell culture procedures
Requiring a separate method to extract velocity data from holographic images [94, 95]