Question about BOS calibration and thermocouple validation

[MUHAMMAD NOUMAN RAFIQ]

Dear Dr.William,

I hope you are well. I am working on a Background Oriented Schlieren (BOS) experiment to reconstruct temperature in a laminar flame. The displacement field is obtained using PIVlab, and the system calibration is performed using a checkerboard pattern.

In my setup, the background pattern and the flame are located at different planes. The distance from the camera to the background is about 0.7 m, and the flame is about 0.5 m away from the background. When I compare the BOS radial temperature profile with thermocouple measurements at the same axial height. In particular, the BOS reconstruction shows approximately 2× larger radial and vertical spread of the flame compared to the thermocouple measurements.

Could this difference be related to the BOS geometry, since the calibration is referenced to the background plane while the density gradients occur in the flame plane? Could you please guide or recommend a correction for this situation?

Thank you very much for your time.

Kind regards,

Nouman Rafiq

[Gary Settles]

Hello Nouman,

The problem is very likely to be in the BOS geometry: Since BOS requires the camera to focus upon the background, the dimensions you cite put the flame very close to the camera, where it will be far out of focus. Try moving the flame to 0.2 m from the background, where BOS sensitivity will decrease but also your problem may go away. Also, if you have enough space, double the camera-to-background distance while keeping the same proportions just suggested. This will require a longer lens focal length and a bigger background, but it should help if you still have a problem. In general, the flame-to-background distance should be only as large as necessary for adequate sensitivity, in order to keep a reasonably-sharp focus on the flame.

-Gary Settles