Hi Raju,

What sort of discrepancy are you seeing?

Laser Doppler measurements measure a component of the particle velocity in a direction towards the detector. If motion is not along this axis then there will be a component of the velocity you are missing, or may need to account for using trigonometry.

Errors in analysis can occur if the phase shift is greater than 2pi in your Doppler measurement. Fourier analysis may also be an option for your Doppler measurement.

Alexander Malm

Dear sir

I searched in jounrals , i could not found both technique at a time. so only i guessed there will be mismatch in the velocity field

PIV is a number-weighted method whereas PDPA is intensity-weighted. If your droplets are polydisperse then I would expect PDPA to give a larger size. The low angle of intersection between the two beams for PDPA also bias the size distribution to the higher end.

Per Alexander Malm , it would be useful to know what differences you are experiencing. Example distributions would be helpful.

(I used PDPA quite a bit and never found the sizing aspect via phase difference between detectors to be reliable. The velocity measurement was okay.)

John Francis Miller and Alexander Malm

Dear all

I would like to know the for the same operating paramters, what would be the spray droplet velocity , let say 20 mm or 30 mm downstream of the injector with both diagnostics techniques?

Suppose if you are getting same Velocity or different velocity, could you explain in detail ?

That is impossible to answer without a lot more information.

For the PDPA, do you know if the droplet concentration is low enough so that the instrument observes Doppler bursts from single droplets or if the signal is a composite of many droplets? If the latter, then my previous comment about number vs. intensity-weighting applies.

Another consideration is the location of the sampling volume of the PDPA probe (i.e., where the beams intersect). The region that PDPA analyzes is much smaller than PIV. If you have any velocity gradients then the PDPA results will be very dependent on the location of the beams. You may need to collect data from different regions of the spray.

Here's a paper I co-authored related to this:

Article An Experimental Investigation of the Spray Issued from a pMD...

(I don't have the full text - you'll need to get it from the publisher)

Thank you sir

The PIV evaluates droplet velocity based on interrogation windows and not the single droplet velocity. While the PDPA measures velocity based on the frequency shift of the pre-determined interference pattern of the intersecting lasers. Thus the PDPA is point based as the droplets crosses the intersecting laser beams.

Typically the spray velocity at further locations downstream of the injector exit should decrease based on the entrainment of the ambient air. However, you shouldn’t expect the velocities measured using both methods to be exacting the same but could be close. Try to see Benjamin Simmons in Atomization and Sprays or his dissertation for PDPA as well as Jiang et al, Danh et al for PIV analysis.

Oladapo S Akinyemi

Thank You so much

PIV gives average results based on the interrogation window. PDPA gives the instantaneous velocity at a particular point. One can argue about the definition of the point mentioned here. I would say the interrogation window used for PDPA is very small compared to PIV. Moreover, PIV predicts the velocity based on successive frames which sometimes may lead to false values due to several reasons (Choice of the plane, Laser pulse width, the accuracy of cross-correlation program). IF I have to choose one, I would go for PDPA, it is far more reliable and accurate than other methods, provided the lasers are properly aligned and the experiments are performed correctly.