Opposite Vectors in Velocity Field

[Roja Najafi]

Hi everyone,

I would like to understand why we see opposite vectors in the velocity field, as shown in the attached images. I tested the OpenPIV code (customized for my experiment) using the OpenPIV notebooks.

I have a few questions:

1. Could the opposite vectors be caused by noise?

2. How can we control or reduce this noise?

Thank you for your help.

Best regards,

[Erich's Lab]

Hi,

These "opposite" vectors are spurious vectors, something that occurs in most PIV datasets and especially near the borders of the vector field. Spurious vectors can be caused by a number of things that cause a second noise peak to exceed the main (correct) peak in the correlation plane. These causes can be background noise, loss/gain of particle pairs between images, particles that are not optimal (e.g., not between 2 to 3 pixels in size or non-uniform illumination), or areas with too high or low seeding density for a given interrogation window size. In general, you can expect 2% to 9%  spurious vectors for high quality image setups and these can be delt with using post-processing algorithms to replace these vectors with calculated estimates. To better figure out the cause of these spurious vectors, can you by any chance release an image pair from your dataset for analysis?

All the best,

Erich