Module 11 - Particle Image Velocimetry

Question 1

Why are two lasers housed in the same unit used in PIV systems?

Answer 1

Reduce the minimum separation time for pulses, as lasers require time to re-energize which typically takes longer than the desired separation time

Question 2

What are 2 desirable properties of particles used in PIV?

Answer 2

1. Neutral buoyancy: avoid buoyancy or settling of particles
2. Small diameter: increase spatial resolution and overcome differences in density more faithfully following the fluid

Question 3

What determines the separation time of pulses in PIV?

Answer 3

• fluid velocity
• if separation time too high for local fluid velocity, particles won’t appear in second image
• if separation time too low, particles won’t appear to move

Question 4

How is a thin laser sheet produced in PIV?

Answer 4

• Use cylindrical lens to spread laser beam into a light sheet
• use spherical lens to reduce thickness to desired thickness

Question 5

What is an interrogation window?

Answer 5

Small section of a field of view in PIV in which a displacement vector is calculated, typically 32x32 pixels. Determines spatial resolution of system

Question 6

What are some sources of error in PIV?

Answer 6

• tracking error: particles are assumed to faithfully follow the flow but may not
• particle image density: particle image density must be high enough to produce a displacement peak since this is required for correlation algorithms
• poor cross-correlation peak because too many particles entered/left the interrogation window
• out of plane motion
• uncertainty in alignment of camera and light sheet
• pulse duration choice
• seeding density or choice of particles

Question 7

What does stereoscopic PIV measure? What is needed for it?

Answer 7

Measures the out of plane velocity component
Requires a second camera or mirror to record the flow from a separate angle which can be used to reconstruct out of plane velocity component

Question 8

Why is PIV referred to as quantitative flow visualization?

Answer 8

• measures velocity vectors (quantitative)
• uses flow markers to tack the fluid flow (qualitative)

Question 9

What limits the spatial resolution of PIV?

Answer 9

• tracer particle density
• size of particles
• interrogation window size
• fluid velocity if separation time is limited

Question 10

How are spurious vectors identified in PIV, how are they dealt with, and what can cause them?

Answer 10

• compare vectors to neighbouring vectors
• remove them then replace with an interpolated vector
• area with low particle image density, high velocity gradients, rotation of flow, differences in particle image intensity