stagnation pressure on a water droplet

mformazhar · March 7, 2018, 06:04

Hi,
I am modeling a free falling of a water droplet through air. the issue is that i want to estimate the deformation rate and for that it seems like i need to calculate the stagnation pressure. by looking at the equation (wiki) it is not clear to me what values to choose. for example should i choose the density of air or density of water droplet in that equation, similarly what velocity should i consider. is it the velocity of water droplet or the air? i appreciate if you could help me route me to a correct forum. thanks!

Best regards
Hussain

piu58 · March 7, 2018, 06:24

You have to use the density of the air. Formula given in textbooks are valid for solid bodies too, which don't deform.

The velocity is the relative one between the droplet and the air. It doesn't play any role if the air is moving or the droplet. It is the difference hat counts.

mformazhar · March 7, 2018, 07:19

HI Pilz
Thanks for a quick reply. how about the static pressure in the equation. can i just use the atmospheric pressure?

Regards
Hussain

piu58 · March 7, 2018, 07:32

> can i just use the atmospheric pressure?

I think so. I need your reference to check this.

mformazhar · March 7, 2018, 07:40

The droplet is falling through the air at atmospheric pressure of 1 atm. do you have any reading suggestions regarding my problem because i want to estimate the deformation rate of the droplet while it is falling?
thanks!

Regards
Hussain..

piu58 · March 7, 2018, 08:27

In this case I would simulate an aerial flow around the droplet. With changing the speed the droplet gets deformed (more or less slightly), but the cf values change fast. Your solution is the one where the drag equals the gravity.

mformazhar · March 7, 2018, 08:55

Thank you Pilz. I will try to dig down more on that suggestion of yours.

regards
Mazhar..

March 7, 2018, 06:04	stagnation pressure on a water droplet	#1
mformazhar New Member Hussain Join Date: Mar 2018 Posts: 4 Rep Power: 8	Hi, I am modeling a free falling of a water droplet through air. the issue is that i want to estimate the deformation rate and for that it seems like i need to calculate the stagnation pressure. by looking at the equation (wiki) it is not clear to me what values to choose. for example should i choose the density of air or density of water droplet in that equation, similarly what velocity should i consider. is it the velocity of water droplet or the air? i appreciate if you could help me route me to a correct forum. thanks! Best regards Hussain

March 7, 2018, 06:24		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	You have to use the density of the air. Formula given in textbooks are valid for solid bodies too, which don't deform. The velocity is the relative one between the droplet and the air. It doesn't play any role if the air is moving or the droplet. It is the difference hat counts. mformazhar likes this. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

March 7, 2018, 07:32		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	> can i just use the atmospheric pressure? I think so. I need your reference to check this. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

March 7, 2018, 08:27		#6
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	In this case I would simulate an aerial flow around the droplet. With changing the speed the droplet gets deformed (more or less slightly), but the cf values change fast. Your solution is the one where the drag equals the gravity. mformazhar likes this. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Periodic flow using Cyclic - comparison with Fluent	nusivares	OpenFOAM Running, Solving & CFD	30	December 12, 2017 05:35
How to patch a water droplet with smooth surface?	Hossein1	FLUENT	0	January 13, 2017 09:25
CFX Solver stopped with error when requested for backup during solver running	Mfaizan	CFX	40	May 13, 2016 06:50
stagnation pressure boundary problem	mbgnfrx2	STAR-CD	2	November 25, 2012 23:55
what the result is negatif pressure at inlet	chong chee nan	FLUENT	0	December 29, 2001 05:13

March 7, 2018, 07:19		#3
mformazhar New Member Hussain Join Date: Mar 2018 Posts: 4 Rep Power: 8	HI Pilz Thanks for a quick reply. how about the static pressure in the equation. can i just use the atmospheric pressure? Regards Hussain

March 7, 2018, 07:40		#5
mformazhar New Member Hussain Join Date: Mar 2018 Posts: 4 Rep Power: 8	The droplet is falling through the air at atmospheric pressure of 1 atm. do you have any reading suggestions regarding my problem because i want to estimate the deformation rate of the droplet while it is falling? thanks! Regards Hussain..

March 7, 2018, 08:55		#7
mformazhar New Member Hussain Join Date: Mar 2018 Posts: 4 Rep Power: 8	Thank you Pilz. I will try to dig down more on that suggestion of yours. regards Mazhar..