Elementary Mechanics of Fluids

Lab # 3 FLOW VISUALIZATION

System Components

Nd:YAG Laser

Nano sense Camera

Laser Beam

Timing HubChiller

Control UnitTraverse System

Flume

Flow visualization Lab

PIV Measurements

• PIV is a non-intrusive, whole field optical technology used for obtaining velocity information by suspending ‘seeding’ particles in a fluid in motion.

• Measurement is based on particle displacement over a known time interval.

• The system uses a light source (Laser) and a nano-sense camera which are synchronized.

Flow visualization Lab

PIV Processing stages

Flow visualization Lab

Light source, sheet formation and Seeding particles

Double Cavity Nd:YAG Laser:• Pulses of short duration (5-10 ns)• Vast range of Output energy and repetition rates providing

powerful light flash.• Optic components added for transformation of IR to Visible

light and recombination along same optical pathSeeding Particles: • Hollow glass spheres• Diameter comparable to light source wavelength (in

accordance with Lorenz Mie theory)• Light scattering sideways is of interest

Flow visualization Lab

Flow around a Glass Cylinder

Flow visualization Lab

Clip depicting particle movement

Flow visualization Lab

Correlations• Image is subdivided into Interrogation areas (IA), each IA has

a correlation function• Different types such as Adaptive, Cross and Average

correlations• Calculation of velocity vectors with initial IA, applying

refinement steps and using intermediary results as input for the next IA

• Application of Validation Methods and IA offset scheme• Averaging the correlation to increase the signal-to-noise-ratio

significantly and generating clear correlation peaks• Cross-correlations for single frame images

Flow visualization Lab

Filters

• Average filter used to output vector maps by arithmetic averaging, individual vectors smoothed out

• Substitution of vectors with uniformly weighted average over a user defined area

• To enhance the results of measurement, a coherence filter applied to the raw velocity field to modify the inconsistent vectors

• Application of filters improves the acquired parent data, various vector and scalar maps can be derived

Flow visualization Lab

Vector Statistics Output

Flow visualization Lab

Scalar Map Sqrt (U2 + V2)

Flow visualization Lab

Note: results are processed and shown downstream of the cylinder

Scalar Map for Vorticity

Flow visualization Lab

Note: results are processed and shown downstream of the cylinder

Vorticity measures the “swirl” or the “local spin” of the flow

Typical recommendations for PIV measurements around a cylinder:

1. At least 5 seeding particles per IA to minimize “loss of pairs” 2. Use cross-correlation than auto correlation methods3. Use of Guassian window function to eliminate noise due to

cyclic convolution4. Use of filters to optimize the effectiveness of sub-pixel

interpolation5. Maximum permissible displacement of particles be 25% of

the IA6. Minimize effects of zero velocity biasing

Flow visualization Lab

Conclusion• Time resolved PIV is an effective tool for fluid flow

visualization, determination of velocity and related fluid properties

• Non-intrusive method, high speed data processing, high degree of accuracy

• Can be used fairly easily to depict the flow characteristics around objects such as cylinders and airfoils• Scope for more precision as regards to use of camera and multiple cavity laser technology• Valuable for academic and research purposes

Flow visualization Lab