elementary mechanics of fluids lab # 3 flow visualization
TRANSCRIPT
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