CFD-Post: Pressure and Viscous Drag on Isosurface

Shirock · May 31, 2012, 12:04

Hi,

I am modelling a water droplet motion through a gas channel and I want to calculate the drag force on that droplet. Therefore I need the pressure drag and the viscous drag on the isosurface which indicates the droplet.

The gas flow and hence the droplet movement is in inverse x-direction.

For the pressure drag I calcute areaInt(Total Pressure * Normal X)@Isosurface 1

For the viscous drag I define a new variable as gradient velocity u and calculate areaInt(Variable * 5e-06 [Pa s])@Isosurface 1

My Question is if the two equations are the right ones and if not what do I have to use instead. The problem is, that for different gas velocities the pressure drag is sometime positive sometime negative. The viscous drag is always positiv.

Every help would be highly appreciated.

Best regards.

ghorrocks · May 31, 2012, 19:02

What are you trying to do? If you are talking about isosurfaces defining droplets does that mean you have done a free surface simulation where the droplets are directly modelled and you are trying to extract the drag forces from that?

Also, why do you need the pressure and viscous drags? Isn't the total drag more useful and more accurate to measure?

Shirock · June 1, 2012, 01:14

I am doing Fluent VOF simulations. The isosurface defines the shape of my droplet by showing the cells with the water conten of 0.5.

I need the force on the droplet caused by the gas stream to compare it to the adhesion force. It would also be interesting to identify the rate of pressure to viscous drag on that droplet, to improve water motion.

I found a paper where it is said, that the pressure drag is the integrated pressure over the droplet shape and the viscous drag the integrated velocity gradient * viscousity over the shape. Both in flow direction.

My Question is about CFX-Post and how to calculate the two forces in it.

ghorrocks · June 1, 2012, 02:08

This is the CFX forum, for help on Fluent try that forum.

But in general: Yes, pressure drag is the integral of the pressure over the surface. I prefer to think of the viscous drag as the integral of the wall shear stress over the body. But calculating these for a VOF body is tricky. You probably can calculate the pressure drag using a surface integral of pressure in CFD-Post but I doubt it will be accurate.

I suggest the best thing to do is to look at the total drag by looking at the motion of the centre of mass of the blob over time. you can get the drag force from the acceleration of the blob. This will be much more accurate.

Shirock · June 4, 2012, 01:02

Thank you very much for your replies.

I thought to put this in the CFX forum because of my problems in CFX-Post. I will make a new thread in the Fluent forum.

May 31, 2012, 12:04	CFD-Post: Pressure and Viscous Drag on Isosurface	#1
Shirock New Member Alexander M Join Date: Aug 2011 Location: Germany Posts: 5 Rep Power: 14	Hi, I am modelling a water droplet motion through a gas channel and I want to calculate the drag force on that droplet. Therefore I need the pressure drag and the viscous drag on the isosurface which indicates the droplet. The gas flow and hence the droplet movement is in inverse x-direction. For the pressure drag I calcute areaInt(Total Pressure * Normal X)@Isosurface 1 For the viscous drag I define a new variable as gradient velocity u and calculate areaInt(Variable * 5e-06 [Pa s])@Isosurface 1 My Question is if the two equations are the right ones and if not what do I have to use instead. The problem is, that for different gas velocities the pressure drag is sometime positive sometime negative. The viscous drag is always positiv. Every help would be highly appreciated. Best regards. Last edited by Shirock; May 31, 2012 at 12:53.

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May 31, 2012, 19:02		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What are you trying to do? If you are talking about isosurfaces defining droplets does that mean you have done a free surface simulation where the droplets are directly modelled and you are trying to extract the drag forces from that? Also, why do you need the pressure and viscous drags? Isn't the total drag more useful and more accurate to measure?

June 1, 2012, 01:14		#3
Shirock New Member Alexander M Join Date: Aug 2011 Location: Germany Posts: 5 Rep Power: 14	I am doing Fluent VOF simulations. The isosurface defines the shape of my droplet by showing the cells with the water conten of 0.5. I need the force on the droplet caused by the gas stream to compare it to the adhesion force. It would also be interesting to identify the rate of pressure to viscous drag on that droplet, to improve water motion. I found a paper where it is said, that the pressure drag is the integrated pressure over the droplet shape and the viscous drag the integrated velocity gradient * viscousity over the shape. Both in flow direction. My Question is about CFX-Post and how to calculate the two forces in it.

June 1, 2012, 02:08		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This is the CFX forum, for help on Fluent try that forum. But in general: Yes, pressure drag is the integral of the pressure over the surface. I prefer to think of the viscous drag as the integral of the wall shear stress over the body. But calculating these for a VOF body is tricky. You probably can calculate the pressure drag using a surface integral of pressure in CFD-Post but I doubt it will be accurate. I suggest the best thing to do is to look at the total drag by looking at the motion of the centre of mass of the blob over time. you can get the drag force from the acceleration of the blob. This will be much more accurate.

June 4, 2012, 01:02		#5
Shirock New Member Alexander M Join Date: Aug 2011 Location: Germany Posts: 5 Rep Power: 14	Thank you very much for your replies. I thought to put this in the CFX forum because of my problems in CFX-Post. I will make a new thread in the Fluent forum.