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-   -   drag calculations (https://www.cfd-online.com/Forums/phoenics/51777-drag-calculations.html)

 Alex July 12, 2004 06:40

drag calculations

i am a student and have a old pheonics version. I am trying to find the drag for a object, but i do not the vwt add on. Is there a way of finding the drag of an object without this add on?

Cheers

 User July 13, 2004 04:30

Re: drag calculations

It's possible to use pressure diffrence -> dP=ksi*(ro*w^2)/2

 Alex July 13, 2004 04:55

Re: drag calculations

Yeah, but is it possible to put that in to the coding, because it will be very laborious to do that for the whole projectile. I was going to use somthing very similar to that where a Cp is taken every 10 deg and then do this,

Cd=[1/2*(Cp)*cos(0)+(Cp)*cos(10)+(Cp)*cos(20)+.....+(Cp) *cos(170)+1/2*(Cp)cos(180)]pi/180*10

But this will have inaccuracys so i was hoping that there would be a better way. Is it possible to find the dP for an object? and what is ksi and w?

 User July 13, 2004 23:50

Re: drag calculations

I can suggest do it by next way. If there are flow in channel with obstacle, you can get averaged pressure before and after object in some distanse (averaged of channel cross section). The difference of this values will be dP - static pressure difference. For velocity you can use diffterent ways. First, w=G/(ro*F), where ro - dencity of media, F - min(or inlet - this is reference velocity) wetted cross section in channel with obstacle. After this you can find drag coefficient Ksi=2*dP/(ro*w^2). Also you can receive w from volume averaging of velocity in obstacle area.

 STK July 14, 2004 15:05

Re: drag calculations

hello I think that all those suggestions does not guide to accurate results. You have to integrate along the surface of the object. Then you have to find firstly the normal vector corresponding to every surface element and then multiply it by the static pressure. This code will be add-on in Phoenics 3.6 according to cham, or you can request it if you have license for 3.5.1 version. I would suggest you trying fluent 6.1.18! you will be impressed !!!! (if you don't need license for your project,....). good luck

 mike July 15, 2004 09:33

Re: drag calculations

Dear STK,

This is a PHOENICS forum, for PHOENICS users! Telling everyone how wonderful FLUENT is does not do anyone here any favours. Why don't you show a bit of professionalism or better still go and use FLUENT.

Can I say that I have been a PHOENICS user for 15 years and have seen fanastic things modelled with the code.

Here in the UK we have a saying, "a bad workman blames his tools".

Please either input constructively or not at all.

Thanks

 STK July 15, 2004 13:01

Re: drag calculations

Dear Mike,

My tool thanks to me is the powerful solver FLUENT 6.1.18. Phoenics is for my free time and i blame the code because cham cheat people (i hope one day will stop doing that!). I am not unfortunately as you, a professional cfd researcher, i am just a student!What ever you do, i have to tell you that here is a cfd forum not only PHOENICS forum. So nobody can't prevent me or anybody else to tell the truth! I am a user of both cfd codes and i have the right to give some suggestions! I do not enforce anybody to buy Fluent. If you use phoenics for 15 years, you must be very patient!!but yours expectations may not be in a very high level, especially when we are talking for the needs of 21st centurty (dynamic meshes, deforming domains, fluid-solid interactions,....). Stop preventing people from getting info concerning other cfd codes. Thanks

 mike July 16, 2004 08:13

Re: drag calculations

Dear STK,

You are truely pathetic!

Ask yourself this: With contributions like yours, what incentive do experienced CFD users or even CHAM have to support this forum. Thats right None!!

You are ruining it for everyone.

As a student you probably think that the world evolves around you and your PhD problem or whatever it is you are trying to do. But in the real world we are all busy working with real customers, you know? customers who pay the bills! So any help that we might get or give would be gratefully received and down to people going beyond the call of duty to assist.

Lets not put these people off, hey !

 STK July 17, 2004 17:17

Re: drag calculations

Dear Mike,

You have right about the students' world and the professionals' world. I am just trying to inform people about the capabilities of two different cfd codes, as being a user of both codes.

I believe what i said about CHAM Ltd. and i am not afraid to declare it in public. Take a look in the library cases i do not tell you anything else! I know that those things are not usually seen in a cfd forum, because most people do not care to inform others. I am not like the others!

Finally i have helped people in phoenics forum ! I don't hate phoenics users or Cham!You can check this if you do not believe me in cad--cfd problems. Although i help and getting help of course, i don't give any motive to those people to use this code. I hope i straighted out my purposes, i do not have any bad purpose in mind. thanks, **i hope we will cooperate, helping each other one day!!!!!!!

 User July 18, 2004 20:07

Re: drag calculations

STK, About Fluent, it's just your opinion, please, do not disturb another users - we already know your opinion. If we want to see Fluent promotion, we can visit Fluent software website.

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