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Old   April 3, 2008, 15:13
Default Calculating mass flow rate
  #1
cfd novice
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Hi Guys

Can some one tell me please what is the relationship between mass flow rate (Ubar) and friction Reynolds number (Re tau). I need to calculate mass flow rate (Ubar) from Re tau. Thanks in advance
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Old   April 3, 2008, 18:31
Default Re: Calculating mass flow rate
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Basically I am using periodic/cyclic BC at my inlet & outlet. I dont have U or P given, I have only Re tau. How can I calculte velocity from Re tau and then mass flow rate for my channel flow. Thanks in advance for answer.

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Old   April 5, 2008, 07:58
Default Re: Calculating mass flow rate
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sivaramakrishnaiah
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you know Re number value ok ro(density)*velocity*damiter/dynamic viscosity for Re number. now you can calculate density in that density you can calculate mass flow rate know ok bye
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Old   April 5, 2008, 15:19
Default Re: Calculating mass flow rate
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Hi sivaramakrishnaiah

How can I calculate mass flow rate from density? Thank You
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Old   April 6, 2008, 16:12
Default Re: Calculating mass flow rate
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alex
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mass flow rate = rho*U*A (Rho=density, U=velocity, A=cross sectional area)

Re = rho*U*l / mu (l=characteristic length, mu=viscosity)

So Mass flow rate = Re*mu*A / l
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Old   April 7, 2008, 18:45
Default Re: Calculating mass flow rate
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Thank you for your answer alex.

mass flow rate = rho*U*A (1)

Re = rho*U*A/mu (2)

From (2): Re*mu = rho*U*A

So mass flow rate = Re*mu

How did you get above answer?

Thanks
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Old   April 8, 2008, 13:08
Default Re: Calculating mass flow rate
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alex
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Re is based on an length, not the area. Re = rho*U*L / mu

What length you use depends on the problem. L is the characteristic length. You might hear things such as Reynolds number based on pipe diameter for example. Or based on distance from leading to trailing edge of an airfoil.

So... Mass flow = rho*U*A (where A=cross sectional area that the flow is flowing into). --(1)

Re= rho*U*L / mu (where L is the length that you are basing your Reynolds number on) --(2)

Combining (1) & (2) :

Mass flow = Re*mu/L * A

Regards,

Alex
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Old   April 21, 2008, 06:38
Default Re: Calculating mass flow rate
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davoche
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there is a relation (from experimental datas) which can help you to determine the flow rate from re in a turbulent flow. Indeed, according to hinze (1975) : re_tau=1/8.3*(Re)^(7/8)

You could also find more element (in french for the moment sorry) at the following link : http://www.olivierdoche.com/images/T...HESE_DOCHE.pdf
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