# QI am trying to solve two-phase flow equations together with transport equations for two additional variables. The continuity equations have a source term, which depends on the additional variable. I know I have to set velocity components when I set the source term, but it makes no sense to me. How am I supposed to choose values for velocity components? Can I just set something arbitrarily or shall I choose initial values/values at the inlet?AI presume that when you say the continuity equations have a source term you mean that you are setting something under Fluid Sources on a subdomain.If this is the case then the velocity must be that which is associated with the mass being transferred. If you add mass to a phase, then it must have properties associated with it, such as temperature, velocity, composition. For a sink term (i.e. removing mass from a phase) you are presented with Sink Option, which allows you to specify how the value of temperature or mass fraction is to be specified (this is useful for example if you have a multicomponent mixture in which not all components are evaporating at the same rate). For source terms however you have to specify the values explicitly. If you are transferring mass from the other phase, then it may be appropriate to set the velocities equal to those of the other phase, e.g. if your phases are Air at 25 C and Water, you might have a source term as follows:FLOW: DOMAIN:Domain 1 SUBDOMAIN:Subdomain 1 Coord Frame = Coord 0 Location = Assembly FLUID:Air at 25 C SOURCES: EQUATION SOURCE:continuity Option = Fluid Mass Source Source = 0.1 [kg m^-3 s^-1] VARIABLE:velOption = Cartesian Vector Components xValue = Water.Velocity u yValue = Water.Velocity v zValue = Water.Velocity u END END END END END ENDENDTo sum up, you can't create mass out of nothing,

 QI am trying to solve two-phase flow equations together with transport equations for two additional variables. The continuity equations have a source term, which depends on the additional variable. I know I have to set velocity components when I set the source term, but it makes no sense to me. How am I supposed to choose values for velocity components? Can I just set something arbitrarily or shall I choose initial values/values at the inlet?AI presume that when you say the continuity equations have a source term you mean that you are setting something under Fluid Sources on a subdomain.If this is the case then the velocity must be that which is associated with the mass being transferred. If you add mass to a phase, then it must have properties associated with it, such as temperature, velocity, composition. For a sink term (i.e. removing mass from a phase) you are presented with Sink Option, which allows you to specify how the value of temperature or mass fraction is to be specified (this is useful for example if you have a multicomponent mixture in which not all components are evaporating at the same rate). For source terms however you have to specify the values explicitly. If you are transferring mass from the other phase, then it may be appropriate to set the velocities equal to those of the other phase, e.g. if your phases are Air at 25 C and Water, you might have a source term as follows:FLOW: DOMAIN:Domain 1 SUBDOMAIN:Subdomain 1 Coord Frame = Coord 0 Location = Assembly FLUID:Air at 25 C SOURCES: EQUATION SOURCE:continuity Option = Fluid Mass Source Source = 0.1 [kg m^-3 s^-1] VARIABLE:velOption = Cartesian Vector Components xValue = Water.Velocity u yValue = Water.Velocity v zValue = Water.Velocity u END END END END END ENDENDTo sum up, you can`t create mass out of nothing,so any mass that you introduce into the system as a source term must have a velocity associated with it in the place from which it came. It is that velocity that you need to prescribe.