Bai model droplet-wall impact / elastic rebound

crevoise · November 15, 2012, 08:05

Hello,

I am dealing with some Lagrangian simulation of droplet injection into a pipe.
For the droplet-wall interaction, I am using the Bai model. I am testing the injection for different case, and I am concerned about cases when the droplets should bounce from the wall after the impact.

Normally, the Bai model is using the following equation for the droplet velocity after rebound (a=after impact, b=before impact), respectively for tangential and normal velocity:
$U_{d,a}=e_{1}U_{d,b}=\dfrac{5}{7}U_{d,b}$
$V_{d,a}=e_{2}V_{d,b}=-eV_{d,b}$
Where e is the restitution coefficient defined by Grant and Tabakoff, regarding the incident angle.

However, when investigating the velocities before and after the rebound, it seems the rebound is modeled as an elastic rebound, so the tangential velocity is not affected and the normal one only change of sign.
In the methodology manual, it is noticed that e1 an e2 are set to 1 in case of elastic rebound.

My question is then the following: how to specify to StarCD to do not use an elastic rebound, but the proper Bai relation? For now, I have no clue how to set up the Bai model properly concerning the rebound.

Thanks a lot for any help

Pauli · November 15, 2012, 11:42

If you want elastic rebound, switch from Bai to Rebound.

You said "it seems" you are getting elastic rebound using Bai. Did you compute (back calculate) the restitution coefficient & see if it agrees with the value given by the incidence angle formula? Maybe your incidence angle is such that the rebound is nearly elastic.

crevoise · November 15, 2012, 12:06

Hi Pauli,

Thank you for your answer.

I actually do not wish to have an elastic rebound, so that I was switching to Bai model to investigate different injection case (Later, I will also investigate the splashing regime).

I checked the incident angle which is around 37° (How to determine the elastic rebound limit incident angle? What is the limitation of the 'rebound model' in the Bai model?), so that the restitution coefficient should be of 0.37.
My incident normal velocity is of 20m/s, which should give a rebound velocity of 7m/s applying the Bai model, but startCD provides a rebound velocity of 20m/s (as for elastic rebound).

Thanks again for your info.

Pauli · November 15, 2012, 15:06

Sounds like you have the makings of a support request.

November 15, 2012, 08:05	Bai model droplet-wall impact / elastic rebound	#1
crevoise Member Join Date: Oct 2011 Location: Thessaloniki, Greece Posts: 75 Rep Power: 15	Hello, I am dealing with some Lagrangian simulation of droplet injection into a pipe. For the droplet-wall interaction, I am using the Bai model. I am testing the injection for different case, and I am concerned about cases when the droplets should bounce from the wall after the impact. Normally, the Bai model is using the following equation for the droplet velocity after rebound (a=after impact, b=before impact), respectively for tangential and normal velocity: $U_{d,a}=e_{1}U_{d,b}=\dfrac{5}{7}U_{d,b}$ $V_{d,a}=e_{2}V_{d,b}=-eV_{d,b}$ Where e is the restitution coefficient defined by Grant and Tabakoff, regarding the incident angle. However, when investigating the velocities before and after the rebound, it seems the rebound is modeled as an elastic rebound, so the tangential velocity is not affected and the normal one only change of sign. In the methodology manual, it is noticed that e1 an e2 are set to 1 in case of elastic rebound. My question is then the following: how to specify to StarCD to do not use an elastic rebound, but the proper Bai relation? For now, I have no clue how to set up the Bai model properly concerning the rebound. Thanks a lot for any help

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November 15, 2012, 11:42		#2
Pauli Senior Member Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17	If you want elastic rebound, switch from Bai to Rebound. You said "it seems" you are getting elastic rebound using Bai. Did you compute (back calculate) the restitution coefficient & see if it agrees with the value given by the incidence angle formula? Maybe your incidence angle is such that the rebound is nearly elastic.

November 15, 2012, 12:06		#3
crevoise Member Join Date: Oct 2011 Location: Thessaloniki, Greece Posts: 75 Rep Power: 15	Hi Pauli, Thank you for your answer. I actually do not wish to have an elastic rebound, so that I was switching to Bai model to investigate different injection case (Later, I will also investigate the splashing regime). I checked the incident angle which is around 37° (How to determine the elastic rebound limit incident angle? What is the limitation of the 'rebound model' in the Bai model?), so that the restitution coefficient should be of 0.37. My incident normal velocity is of 20m/s, which should give a rebound velocity of 7m/s applying the Bai model, but startCD provides a rebound velocity of 20m/s (as for elastic rebound). Thanks again for your info.

November 15, 2012, 15:06		#4
Pauli Senior Member Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17	Sounds like you have the makings of a support request.