Lecture 10 Interrogation Window Shift

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Lecture 10 Interrogation Window Shift
Engr 691, Section 36University of
MississippiFall 2005
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Interrogation Window Shift

• Reduced working region for correlation
  interrogation

?r lt ?o
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Interrogation Window Shift

• Discrete window shift (DWS)

Cross-correlation of single exposed PIV recording
pair
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Interrogation Window Shift

• Discrete window shift (DWS)

Auto correlation of double exposed PIV recording
No secondary maximum detected because of noises
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Interrogation Window Shift

• Discrete window shift (DWS)

Cross-correlation of double exposed PIV recording

Secondary maximum appears
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Interrogation Window Shift

• Discrete window shift (DWS)

Begin
DWS Flow Chart
Determine g1(i,j)
Determine initial window shift SWS
Determine f2(i,j)
SWSS
Compute ?(m,n) from g1(i,j) and f2(i,j)
Maximum search to determine S, SSWSS
NO
NO
S small enough?
Too many iterations?
YES
YES
Sub-pixel fit
End
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Interrogation Window Shift

• Continuous window shift (CWS)

SwsIx, Jy I,J integer numbers x and y
decimal numbers and 0?xlt10?ylt 1
Other interpolation methods available
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Interrogation Window Shift

• Continuous window shift (CWS)

Begin
CWS Flow Chart
Determine g1(i,j)
Determine initial window shift SWS
Determine f2(i,j)
SWSS
Compute ?(m,n) from g1(i,j) and f2(i,j)
Maximum search and sub-pixel fitto determine S,
SSWSS
NO
NO
S small enough?
Too many iterations?
YES
YES
End
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Interrogation Window Shift

• Evaluation error distribution of DWS and CWS

? Periodical functions of particle image
displacement of 1-pixel period ? DWS better than
correlation tracking around integer-pixel
displacements but worse around mid-pixel
displacements ? CWS has much lower error level
than DWS and correlation-based tracking
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Homework

• Reading
• Westerweel et al (1997) The effect of a discrete
  window offset on the accuracy of
  cross-correlation analysis of digital PIV
  recordings. Exp Fluids 29 S003-S012
• Gui and Wereley (2002) A correlation-based
  continues window shift technique for reducing the
  peak locking effect in digital PIV image
  evaluation. Exp Fluids 32 506-517
• Programming
• Compute cross-correlation function at the center
  of the two sample images with a 32?32-pixel
  interrogation window and window shift SWS(3,5)
  pixels.