the average velocity measured was approximately 0.5 times lower than the actual velocity.

[CS K]

Hello! I research with Soccer travel in the same Lab.

I wrote to you to get some advice.

For the new calibration, We measured the actual length corresponding to 1600 horizontal pixels using a ruler.

Pco.1600 14bit CCD camera is used to get Image Pairs. And, Delta T (time step) is 1.925ms.

We conducted a new calibration using a ruler and checked the results with PIVlab. As expected, the average velocity measured was approximately 0.5 times lower than the actual velocity. 

  To understand the gap between the actual experiment and the measurements, we used a hot-wire anemometer to measure the velocity at 10 points from the bottom along the vertical direction.  

And we plotted the results from PIVlab and anemometer.

The graphs above were plotted from the mat file from PIVlab.

  The graphs were plotted from the values obtained from the hot-wire anemometer.  

I would like to know if the graphs are suitable for analysis and if there are any specific aspects I should check. I'm waiting for your advice.

Thanks in advance.

[William]

Hi, how exactly did you validate that delta t is 1.925ms? Did you use an oscilloscope with a photo diode?

[CS K]

- Validate the time step

When we measured with anemometer, the mean Velocity was about 0.43m/s.

As the reference length was 1600px, the corresponding real distance is 266.2mm. So 1px is equal to 0.166mm.

So I calculate the time step with this proportion equation.

430mm : 1s = 0.166mm : t. 

t was about 0.38ms

 We wanted to observe a movement of approximately 5 pixels per image pair. Therefore, the time step is 1.925ms, which is 5 times 0.38ms.

 - using oscilloscope with a photodiode

We don't have oscilloscope with photodiode..

2024년 7월 18일 목요일 오후 5시 29분 53초 UTC+9에 William님이 작성:

[William]

Hi,

thanks for the information. To debug your experiment, I would reommend to get an oscilloscope. There are cheap usb oscilloscopes available.

PIV calculates a displacement (which cannot be completely wrong when there is a sufficient s2n with low background signal which is the case in your experiment). This (correct) displacement is translated into a velocity by the application of a scaling that you provide. So the error clearly has to be the scaling (maybe your calibration image has a different resolution than your PIV images?), or the anemometer is wrong.

[CS K]

Thank you for your dedicate advice!!

I send you next message after checking with oscilloscope.

2024년 7월 22일 월요일 오후 4시 29분 1초 UTC+9에 William님이 작성: