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What Is PIV ?

What Is PIV ?. J. Westerweel Delft University of Technology The Netherlands. Conventional methods (HWA, LDV) Single-point measur em ent Traversing of flow domain Time consuming Only turbulence statistics. Particle image velocimetry Whole-field method Non-intrusive (seeding)

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What Is PIV ?

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  1. What Is PIV ? J. Westerweel Delft University of Technology The Netherlands

  2. Conventional methods (HWA, LDV) Single-point measurement Traversing of flow domain Time consuming Only turbulence statistics Particle image velocimetry Whole-field method Non-intrusive (seeding) Instantaneous flow field Why use imaging? After: A.K. Prasad, Lect. Notes short-course on PIV, JMBC 1997

  3. Coherent structures in a TBL Kim, H.T., Kline, S.J. & Reynolds, W.C. J. Fluid Mech. 50 (1971) 133-160. Smith, C.R. (1984) “A synthesized model of the near-wall behaviour in turbulent boundary layers.” In: Proc. 8th Symp. on Turbulence (eds. G.K. Patterson & J.L. Zakin) University of Missouri (Rolla).

  4. PIV optical configuration

  5. Multiple-exposure PIV image

  6. PIV Interrogation analysis RP RD+ RD- RC+RF Double-exposure image Spatial correlation Interrogation region

  7. PIV result Turbulent pipe flow Re = 5300 100×85 vectors “Hairpin” vortex

  8. Instantaneous vorticity fields

  9. Historical development • Quantitative velocity data from particle streak photographs (1930) • Laser speckle velocimetry; Young’s fringes analysis (Dudderar & Simpkins 1977) • Particle image velocimetry • Interrogation by means of spatial correlation • ‘Digital’ PIV • Stereoscopic PIV; holographic PIV

  10. Definitions for PIV • Source density: • Image density: Ctracer concentration [m-3] Dz0light-sheet thickness [m] M0image magnification [-] dtparticle-image diameter [m] DIinterrogation-spot diameter [m]

  11. Particle trajectory Fluid pathline After: Adrian, Adv. Turb. Res. (1995) 1-19 The displacement field • The fluid motion is represented as a displacement field

  12. Velocity from tracer motion Prob(detect) ~ image density (NI) Low image density NI << 1 Particle tracking velocimetry High image density NI >> 1 Particle image velocimetry

  13. Evaluation at high image density Spatial correlation:

  14. Linear system theory Input Output Impulse response Test signals: • Deterministic • Stochastic

  15. The tracer pattern • G(X,t) represents the random ‘pattern’ of tracer particles that moves with the flow Input Output Impulse response:

  16. The tracer ensemble • Consider the ensemble of all realizations of G(X,t) for given u(X,t) Physical space Phase space Liouville’s theorem (continuity): t= PDF oft Homogeneous seeding: Incompressible flow:

  17. Visualization vs. Measurement

  18. Inherent assumptions • Tracer particles follow the fluid motion • Tracer particles are distributed homogeneously • Uniform displacement within interrogation region

  19. “Ingredients” FLOW sampling seeding quantization Pixelization illumination enhancement Acquisition imaging selection registration correlation Interrogation estimation RESULT analysis validation

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