请问出口能设置为固定流量么？

xzks

请问出口能设置为固定流量么？怎么设 ？

phoenicswxh

听说可以，设了出口，就不能设入口了

xzks

我得模型有很多出口很多入口，
只设出口其中一个的固定流量。这样可以么？

phoenicswxh

都要设
(d) Fixed values and fixed fluxes
A large C makes the source term the dominant term in the balance equation for PHI, which for large C becomes,
T*C*(V-phiP)=0.0 ,
ie. a large C has the effect of fixing phiP to V. The commands COVAL(name,PHI,2.E10,V), and COVAL(name,PHI,FIXVAL,V) both have this effect, as FIXVAL is numerically equal to 2.0E10.
A very small C and a very large V set the source equal to T*C*V independently of the value of phiP, for example thus: COVAL(name,PHI,1.1E-10,1.1E10*flux) .
COVAL(name,PHI,FIXFLU,flux) has the same effect, because the variable FIXFLU, as well as being a large number (viz 2.0 E10), is also taken by EARTH as a signal multiply flux by 1./FIXFLU when it adds the COVAL term to the source.

(e) Pressure and mass flow boundary conditions
Mass-flow conditions are supplied by way of pressure boundary conditions, eg. COVAL(name,P1,Cp,Vp), sets a mass source ( inflow is positive ),
T * Cp * (Vp-pressP).
A large coefficient fixes the in-cell pressure, pressP, to Vp; a small Cp and a large Vp set a fixed mass flux equal to T*Cp*Vp.
The value, Vphi, of variable PHI convected in to the domain when this mass source is positive is specified by, COVAL(name,PHI,Cphi,Vphi).
This gives a total source for PHI equal to,

    1----------------------------------------------------------1
    1   T*Cphi*(Vphi-phiP) + [T*Cp*(Vp-pressP)]*(Vphi-phiP).   1
    1----------------------------------------------------------1
The first term represents diffusive inflow; the second represents convective inflow. Usually, the diffusive inflow is neglected, so
Cphi is set to 0.0, but,
COVAL(name,PHI,ONLYMS,Vphi),
signifies the same thing, ie. ONLY MaSs flow.
For cells in which mass outflow occurs ( ie. in which pressP is greater than Vp ), the second term above is absent, in agreement with the upwind convention used for the cell-face fluxes generally. Thus, for outflow, the source is,
T * Cphi * (Vphi-phiP).
The magnitude of Cp needed to fix the pressure may be estimated from the following expression, 1.E3 * ( expected flow rate ) / ( T*Vp ). This gives a relative difference between Vp and the in-cell pressure of order 1.E-3.
When the pressure is to be fixed to zero, as is often done in incompressible flows in which the pressure level is immaterial, Vp is omitted from the above expression, which then gives an estimate of Cp needed to give an absolute difference between Vp (which is 0.0) and pressP of order 1.E-3.
A value of Cp 1000 times bigger than that given by the above expression, would result in an absolute pressure difference between zero and press"" barely representable on a 32-bit machine, and should hence be avoided. The coefficient FIXP has a numerical value of 1.0; it is suitable for fixing pressures subject to its conformity to the above expression. FIXVAL ( =1.E10 ) is usually far too big.
When the pressure is "fixed", small changes resulting from dynamic- head variations can change inflow to outflow and vice versa, with consequent convergence difficulties.
When COVAL is used to prescribe pressure boundary conditions, it should be called with P1 or P2 as the variable argument. In this regard, it is helpful to note that the relationship between pressure and mass flux is of the same kind as that between temperature and heat flux. Thus, for fixed pressure, enter:
COVAL(patch name, P1, FIXP, value of pressure). For a fixed first-phase mass flux, enter: COVAL(patch name,P1,FIXFLU, value of mass flow rate) ; and for fixed second-phase mass flux, enter: COVAL(patch name,P2,FIXFLU, value of mass flow rate) .
IMPORTANT NOTE: The above statement IS correct even though, in the current version of PHOENICS, the second phase shares the same pressure as the first, so that no storage space is ordinarily allocated to it unless specifically requested by the user introducing SOLUTN(P2,Y,N,N,N,N,N) .
In two-phase flows, the formula used for mass outflows (ie. when pressP is greater than Vp ) is multiplied by the phase volume fraction: R1 for phase-1 mass outflow, and R2 for phase-2 mass outflow.