UFLUID

User subroutine to define fluid density and fluid compliance for hydrostatic fluid elements.

User subroutine UFLUID:

is called for each cavity for which a user-defined fluid constitutive model is being specified;
is called for every fluid element (About surface-based fluid cavities) and for every fluid exchange definition (Fluid exchange definition) connected to a cavity reference node;
requires that the fluid density, $ρ (p, θ)$ , and the fluid pressure compliance, $C_{p}$ , be defined;
requires that the fluid temperature compliance, $C_{θ}$ , be defined if the routine is to be used in a linear perturbation step and the fluid is subjected to a temperature excursion; and
ignores any data specified for the fluid constitutive model outside the user subroutine.

The following topics are discussed:

Density and fluid mass
User subroutine interface
Variables to be defined
Variables that can be updated
Variables passed in for information

Density and fluid mass

At the start of the analysis (prior to the first iteration) the density calculated in user subroutine UFLUID (for the initial pressure, $p_{I}$ , and temperature, $θ_{I}$ ) is used to calculate the fluid mass from the initial cavity volume. During the analysis the expected cavity volume is calculated from the fluid mass and the density.

User subroutine interface

      SUBROUTINE UFLUID(RHO,CP,CT,PNEWDT,ENER,PRESS,DPRESS,PRESSI,
     1 TEMP,DTEMP,TEMPI,TIME,DTIME,KSTEP,KINC,NONUM,FLNAME,LFLAG)
C
      INCLUDE 'ABA_PARAM.INC'
C
      CHARACTER*80 FLNAME
      DIMENSION TIME(2)

      user coding to define RHO, CP, and CT

      RETURN
      END

Variables to be defined

RHO

Fluid density, $ρ$ , at the end of the increment.

CP

Fluid pressure compliance, $C_{p}$ , at the end of the increment. For a linear perturbation step this is the base state compliance. Fluid pressure compliance is defined as

C_{p} = \frac{d ρ^{- 1}}{d p} = - ρ^{- 2} \frac{d ρ}{d p},

where p is the fluid cavity pressure.

CT

Fluid temperature compliance, $C_{θ}$ . This variable is needed only if a fluid temperature excursion occurs in a linear perturbation step and is the base state compliance. Fluid temperature compliance is defined as

C_{θ} = \frac{d ρ^{- 1}}{d θ} = - ρ^{- 2} \frac{d ρ}{d θ},

where $θ$ is the fluid cavity temperature.

Variables that can be updated

PNEWDT

Ratio of suggested new time increment to the time increment being used (DTIME, see below). This variable allows you to provide input to the automatic time incrementation algorithms in Abaqus/Standard (if automatic time incrementation is chosen).

PNEWDT is set to a large value before each call to UFLUID.

If PNEWDT is redefined to be less than 1.0, Abaqus/Standard must abandon the time increment and attempt it again with a smaller time increment. The suggested new time increment provided to the automatic time integration algorithms is PNEWDT × DTIME, where the PNEWDT used is the minimum value for all calls to user subroutines that allow redefinition of PNEWDT for this iteration.

If PNEWDT is given a value that is greater than 1.0 for all calls to user subroutines for this iteration and the increment converges in this iteration, Abaqus/Standard may increase the time increment. The suggested new time increment provided to the automatic time integration algorithms is PNEWDT × DTIME, where the PNEWDT used is the minimum value for all calls to user subroutines for this iteration.

If automatic time incrementation is not selected in the analysis procedure, values of PNEWDT that are greater than 1.0 will be ignored and values of PNEWDT that are less than 1.0 will cause the job to terminate.

ENER

Energy per unit mass stored in the fluid. This variable is used for energy output only and has no effect on the solution.

Variables passed in for information

PRESS: Fluid cavity pressure at the end of the increment. For a linear perturbation step this is the base state pressure.
DPRESS: Fluid cavity pressure increment. For a linear perturbation step this value is zero.
PRESSI: Fluid cavity pressure at the beginning of the analysis.
TEMP: Fluid cavity temperature at the end of the increment. For a linear perturbation step this is the base state temperature.
DTEMP: Fluid cavity temperature increment. For a linear perturbation step this value is zero.
TEMPI: Fluid cavity temperature at the beginning of the analysis.
TIME(1): Current value of step time at the start of the increment.
TIME(2): Current value of total time at the start of the increment.
DTIME: Time increment.
KSTEP: Step number.
KINC: Increment number.
NONUM: Cavity reference node number.
FLNAME: User-specified fluid property name, left justified.
LFLAG: Linear perturbation flag for the step. If this is a linear perturbation step, LFLAG=1. For a general analysis step LFLAG=0.