Download FLOW VISUALIZATION

Elementary Mechanics of Fluids
Lab # 3 FLOW VISUALIZATION
Nd:YAG Laser
System Components
Flume
Laser Beam
Nano sense Camera
Control Unit
Traverse System
Timing Hub
Chiller
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.
Camera/Image
Displacements
Displacement Vectors
Application of
Validation Algorithms
Flow visualization Lab
PIV Processing stages
1
2
3
• Image map input
• Evaluation of correlation plane
• Multiple peak detection
4
• Sub-pixel Interpolation
5
• Vector Output
6
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Vector statistics
Scalar maps
Derivatives
Velocity, Vorticity, Deviations
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 path
Seeding 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)
Note: results are processed
and shown downstream of
the cylinder
Flow visualization Lab
Scalar Map for Vorticity
Note: results are processed
and shown downstream of
the cylinder
Vorticity measures the
“swirl” or the “local spin”
of the flow
Flow visualization Lab
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 methods
3. Use of Guassian window function to eliminate noise due to
cyclic convolution
4. Use of filters to optimize the effectiveness of sub-pixel
interpolation
5. Maximum permissible displacement of particles be 25% of
the IA
6. 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