CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

blowing boundary condition

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   September 12, 2013, 07:03
Question blowing boundary condition
  #1
New Member
 
Join Date: Sep 2013
Posts: 7
Rep Power: 3
Jochen is on a distinguished road
Hi guys,

i am currently working on a problem and am kind off stuck at the boundary condition.

i am computing a super sonic Ma=2.6 2d flow over a flat plate with a slot from which a subsonic cooling film is emmitted using a WENO scheme and a flow solver not written by me.

the problem is the boundary condition at the slot: a constant massflow rate should be blown into the boundary layer.
the mass flow is defined as follows:

massflow rate dot m=rho_c*v_c*A (1)

therein rho_c*v_c=F_c (2) is a given as the blowing rate F_c. the blowing rates used are small F_c=0.065
also the temperature of the coolant gas is give T_c the area of the slot A is also known.

the pressure is taken from the flowfield above the slot p_c
and is used to compute the density rho_c as such:
rho_c=p_c/T_c (3)

for the implementation of the boundary condition i need
the pressure p_c which is taken from the flowfield an is not a constant
the density rho_c which is also not a constant see (3)
the velocity v_c=F_c/rho_c=F_c/p_c

my questions/problems are:
(a) when i compute the solution using these boundary conditions do i get a constant massflow rate?
(b) also by using the pressure from the flow field the density rho_c and the velocity v_c change and cause oscillation how can this be avoided?
(c) how would you guys implement a constant massflow rate?
(d) does it make sense from a physics standpoint to prescribe the pressure p_c instead of getting it from the flowfield?

thanks a lot for your help

Last edited by Jochen; September 12, 2013 at 09:14.
Jochen is offline   Reply With Quote

Old   September 13, 2013, 06:15
Exclamation further explanation
  #2
New Member
 
Join Date: Sep 2013
Posts: 7
Rep Power: 3
Jochen is on a distinguished road
so a day has gone by and nobody was able to help me

anyway to further elaborate on my problem:
the boundary conditions in the solver i am using are prescribed for the slot from which the coolant is emmitted as follows:
the wall is situated between to cells 4 and 5
the wall normal vector points in y direction (north);
to get the pressure from the flowfield the following is used:
p[0]=p[9]
p[1]=p[8]
p[2]=p[7]
p[3]=p[6]
p[4]=p[5]

this essentially says dp/dy=0

the density is defined as the pressure from the flowfield divided by the given coolant temperature T_c:
rho[0]=p[9]/T_c
.
.
.
rho[4]=p[5]/T_c

so now the density is a function of the flowfieldpressure.

finally the velocity is set using the given blowing rate F_c:

v[0]=F_c/rho[0]
.
.
.
v[4]=F_c/rho[4]

all variables are double precision.

if i then compute the massflux:

dot m/A[0]= v[0]*rho[0]=(F_c/rho[0]) *rho[0] it should equal F_c.

running this code the computed dot m/A shows a difference from F_c in the order of F_c-dot m/A=10^-13
after ~ 20,000 iteration steps the computed d m/A equals F_c.

is the formula for the massflux correct?
is it physically correct that the rho[0] cancels out for the analytical solution?

thanks again and any comment is welcome.

Last edited by Jochen; September 13, 2013 at 07:19.
Jochen is offline   Reply With Quote

Reply

Tags
boundary, condition, filmcooling, massflow

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Radiation interface hinca CFX 15 January 26, 2014 18:11
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 06:28
An error has occurred in cfx5solve: volo87 CFX 5 June 14, 2013 17:44
Setting outlet Pressure boundary condition using CAFFA code Mukund Pondkule Main CFD Forum 0 March 16, 2011 04:23
How to set boundary condition in Fluent for the fo Peiyong FLUENT 1 November 10, 2006 12:44


All times are GMT -4. The time now is 16:39.