# Calculating Relative Velocity of a Bubble

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 August 18, 2011, 04:34 Calculating Relative Velocity of a Bubble #1 New Member   Syed Murtuza Mehdi Join Date: Apr 2011 Posts: 19 Rep Power: 15 Hi All, I would like to know whether the center of mass (i.e. the coordinates of C.M X and Y) of a bubble is available in star-cd as a built in post processing variable or one has to use user coding to calculate the relative velocity of a bubble moving in a flowing fluid domain. Any suggestions on how to calculate the relative velocity of a bubble in a fluid domain can be of great help. Thanks for the help

 August 18, 2011, 10:45 #2 Senior Member   Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17 Yes you can do it in Prostar. Here is sample queries for parcel position & velocity. For further details, read the help on the command *get. *get drnm dset dcount ! parcel number for parcel dcount *get dcg_x DRXC drnm 1 ! parcel x location in csys 1 *get dcg_y DRYC drnm 1 ! parcel y location in csys 1 *get dcg_z DRZC drnm 1 ! parcel z location in csys 1 *get dcg_u DRVX drnm ! parcel u velocity in csys 1 *get dcg_v DRVY drnm ! parcel v velocity in csys 1 *get dcg_w DRVZ drnm ! parcel w velocity in csys 1

 August 28, 2011, 22:11 Fully Developed Flows in STAR-CD #3 New Member   Syed Murtuza Mehdi Join Date: Apr 2011 Posts: 19 Rep Power: 15 Thanks Pauli but right now I am cought up in another issue with STAR-CD. The problem is that I am trying to simulate a fully developed steady laminar pipe flow in a pipe of length 3m and diameter 50mm. I have studied the manuals and they clearly mentioned that to simulate the fully developed flows in ducts and pipes we must impose a uniform pressure boundary condition on the inlet plane of the pipe and a prescribed mass flow rate at the exit plane (Methodology manual chap 05 pg.8) and in User's guide manual (chap 07 pg. 12) they have reported clearly that an OUTLET boundary condition with fixed prescribed mass flow rate can be used to explicitly define the mass flow rate. I have calculated the required value of the inlet pressure from the equation of pressure drop for fully developed laminar flow in pipes delta P = Pin = (f*L*rho*(Vavg)^2/2*D+rho*g*L) for vertical pipes if Pout is atmospheric than (Pout gage = 0) and the corresponding mass flow rate can be calculated from simple continuity equation for incompressible fluids (M = rho*Vavg*A). If I use the prescribed pressure inlet boundary condition and an Outlet boundary condition with fixed mass flow rate defined at exit plane I am unable to get a fully developed flow but I thing I have followed the manuals correctly. Any suggestions can be of great help.

 August 29, 2011, 11:38 #4 Senior Member   Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17 What do you get? Is your pipe length long enough to establish a fully developed profile?