Encyclopaedia Index

V1/2 VABS V1/2 VAC VAL VALIDATION VALUE VAN-Driest damping model VAR/VARIABLES VARMIN VARMAX VC1 VECTORS VELAD VELOCITY VIEW VIRTUAL MASS VIRTUAL REALITY VISCLM VISTRB V1/2 VLSQ VOL VOLUME VPOR VRE VREDIT VREL VR-Editor VR-Viewer VRV VR Viewer Macro VUP


V1

------------------------------------------

Integer flag; value=5.

V1....standard name used to denote the first-phase velocity component in the iy-direction.

See PHI and NAME for further information.


V1AD

------ PIL real; default= 0.0; group 8 --- -

V1AD....add extra velocity V1AD to V1. See U1AD


V2

------------------------------------------

Integer flag; value=6.

V2....standard name used to denote the second-phase velocity component in the iy-direction.

See PHI and NAME for further information.


V2AD

------ PIL real; default= 0.0; group 8 --- -

V2AD....add extra velocity V2AD to V2. See U1AD.


V2CR

------ PIL character flag; group 7 ---------

V2CR....integer name recognized by EARTH denoting second-phase cartesian YC-directed velocity resolute. See UCRT.


VABS

When STORE(VABS) appears in the Q1 file, and velocities are being computed, PHOENICS will compute and print the 3D-stored variable VABS, which represents the square root of U1**2 + V1**2 + W1**2, or the corresponding cartesian velocities for BFC grids, each of these velocities having the value obtained by averaging the 'staggered' velocity components to the scalar-cell centre.


VAL

VAL is an integer index, usable in subroutines called from GROUND, for accessing the 2D array of values, pertaining to the current IZ-slab, of: 'value' which is used to set the values of linearised-source expressions.


VALIDATION

(1) How reliable are predictions made by a CFD code?

Computer software such as PHOENICS is commonly used for the making of predictions about what flow phenomena, temperature distributions, reactor yields, etc, will actually occur in prescribed circumstances of interest to its user, who usually wishes to be assured that the predictions will turn out to be correct.

Such assurances, when given, are usually based upon arguments of one or both of two kinds, namely:-

  1. inherent probability; and
  2. validation.

(2) Conditions conducive to reliability

High inherent probability of correctness attends predictions which:-

The sufficiency implied in the first three conditions is of course for the user to judge; and often there is no way to be certain except by performing further calculations with, for example:

The appropriateness of the physical models is also sometimes a matter of judgement. Thus PHOENICS may be given the task of solving the velocity-potential equations for flow around a stream- lined object, on the grounds that viscous effects are probably small.

However, whether the neglect of the viscous effects is truly justified also requires checking, for example by performing a further calculation in which the Navier-Stokes equations are solved and allowance is made for turbulence.

If turbulence IS to be activated, the last of the above five considerations assumes importance; for the question to be decided is: which turbulence model should be used?

Science has not advanced far enough for definitive answers to be made to questions of this type.

Uncertainty about which model is best for given circumstances increases when any of the following phenomena make their appearance:

(3) Validation

Uncertainty about the reliability of the predictions made by PHOENICS, in so far as it derives from doubts about the physical models which have been invoked, can be allayed by making comparisons with reliable experimental data.

Usually no data can be found which fit exactly the circumstances in which the user is interested; for, if they did, he would not be seeking to make computer predictions at all. However, the nearer are the conditions of the experiment to those which concern the user, and the more closely the PHOENICS predictions agree with those data, the greater will be the reliance which can be prudently placed on the predictions.

PHOENICS predictions have been subjected to many such validation tests; and the results have been reported in many places, for example in the PHOENICS Journal and other publications. (See the Documentation section of POLIS).

(4) Has PHOENICS been sufficiently validated?

The above question is often asked, for understandable reasons, as it is also about other CFD codes; but it cannot be answered by a simple "yes" or "no". The greater is the range of flow simulations which a code is capable of making, the less is it possible for comparisons with experiment to be regarded as adequate.

PHOENICS can simulate a VERY great range of flows; so it can be regarded as LESS well validated than some codes of much more restricted capability. On the other hand, it has been used and tested by more persons than any other, and over a longer period, the claim can also reasonably be made that it is the BEST-validated CFD code.


VALUE

----- Command; group 13 ---------------

This command is used to declare non-zero values of mass-flow, velocity, pressure and scalar variables associated with INLETs, OUTLETs and WALLs.

The syntax is : VALUE(NAME,PHI,VALUE)

NAME is the name of the boundary condition (as used in the associated INLET, OUTLET or WALL command), PHI is the affected variable, and VALUE is the required value. Mass-flow and pressure are both set by reference to the variable P1. For mass flows (INLET) the value should be the mass flow/unit area, whereas for pressure (OUTLET), it should be the required external pressure.

The function of the command is to set the 4th argument of the COVAL for the variable PHI in the patch NAME to VALUE. The coefficient is left at its previous setting, or is set to 0.0 if no COVAL existed before. See the entries on SOURCE, PATCH and COVAL for further details.


Value

(Surface menu) -------------------------------------- Photon Help ----

[Value] specifies the value of the variable on the surface.


VAN-Driest damping model

See PHENC entry: The VAN-Driest low-Reynolds number mixing-length model


VC1

------------ PIL integer flag

VC1.... name used to denote the first-phase co-located velocity component in the y-direction.


VCRT

------ PIL character flag; group 7 ---------

VCRT....integer name recognized by EARTH denoting first-phase cartesian YC-directed velocity resolute. See UCRT.


VELAD

VELAD is an integer index, usable in subroutines called from GROUND, for accessing the 2D array of values, pertaining to the current IZ-slab, of:
additions to the current slab velocities, at the point in the computation at which the convection fluxes are assembled.


VELOCITY RESOLUTES AT INLETS, boundary conditions for

(see BOUNDARY CONDITIONS FOR VELOCITY RESOLUTES AT INLETS)


Velocity variables

The velocity variables for which PHOENICS solves are always the resolutes of the velocity vector in the direction of the line joining the grid points on either side of the cell faces at which velocities are stored.

For cartesian grids, these variables are therefore the cartesian velocity resolutes. For cylindrical-polar grids they are the axial, radial and circumferential resolutes. For general curvilinear grids they are the resolutes of the velocity vector in the local direction of the lines connecting the nodal points PE, PN and PH. In vector terminology, U, V and W are equal to the scalar products of the velocity vector with the unit vector aligned with the direction PE, PN and PH respectively.


Velocity, adding extra

(see U1AD, U2AD, V1AD, V2AD, W1AD, W2AD, Group 8)


Velocity, print-out of

(see IURPRN integer, Group 21)


VISCLM

Integer used in GXKNVSL to denote laminar kinematic viscosity.


VISTRB

Integer used in GXKNVST to denote turbulent kinematic viscosity.


VLSQ

When STORE(VLSQ) appears in the Q1 file, and velocities are being computed, PHOENICS will compute and print the 3D-stored variable VLSQ, which represents the value of U1**2 + V1**2 + W1**2, or the corresponding cartesian velocities for BFC grids, each of these velocities having the value obtained by averaging the 'staggered' velocity components to the scalar-cell centre.


VOL

VOL is an integer index, usable in subroutines called from GROUND, for accessing the 2D array of values, pertaining to the current IZ-slab, of:
the 'free' cell volumes available for fluid after blockages are deducted.


VOLUME

---- PIL real flag; value= 8.0; group 13 -

VOLUME....is a PATCH type used for setting sources per unit volume by way of COVAL in group 13.


Volume fractions, sweeps in

(see ISWR1 and ISWR2 integers, Group 15)


VPOR

------ PIL integer name; group 7 -----------

VPOR.... indicates which whole-field store will be used for the volume porosity in response to the command STORE(VPOR). Once stored, this field should be initialized in group 11. The field values are set to 1.0 by default.


VRE

\phoenics\d_utils\d_windf\vre.bat is a batch fiile which launches the VR-Editor. It can have optional arguments which are explained here.

VREDIT

------ PIL logical; default=T; group 19 -----------

This logical variable was introduced late in 2012, so as to assist human writers of parameterised Q1 files (i.e. PQ1s); for the supplementary user input to these is supplied via the menus of PHOENICS-Direct instead of via VR-Editor dialogue boxes. VREDITOR=F also renders unnecessary the many SAVEnBEGIN / END lines formerly-required in Q1s (see below).

Its default value is True. therefore it is set to False explicitly in PQ1s.

(a) VREDIT=F

In this case, the Satellite reads the Q1 in order, line-by-line, processing each line as it does so. It writes nothing to that file.

If the Satellite has been activated in 'silent' mode, by the commands s or sil, both of which are equivalent to satexe f, when it reaches the STOP line it reads and processes the Q2 file, if such exists, and then writes:

If the Satellite has been activated by the commands vre or runvr, both of which are equivalent to satexe vre, it reads the Q1 file as far as the STOP sign and then opens the screen which displays, in accordance with any GVIEW commands placed in the Q1, a view of the scenario to be simulated.

The user is then able interactively to change the view-point, and to hide or change the transparency or colour of any object; but he cannot change the relative positions or any other properties of he objects. All the editing facilities of the VR-Editor have thus been disabled; only its display capabilities remain.

If the user of PHOENICS-Direct wishes to change the objects in any way, he must do so through its menu; whereupon the display will be correspondingly updated automatically.

When the PHOENICS 'run' button is pressed, the above actions with regard to Q2, eardat, etc ensue as above.

(b) VREDIT=T

In this case, unless it is running in 'silent' mode, the Satellite both reads from and writes to the Q1. To allow for this, and also for the preservation of user-declared variables, settings and logic, of PLANT statements, and of In-Form statements which may be interactively modified, the 'protected mode' of Satellite running was invented in 2007. This involved the judicious inclusion in the Q1 of SAVEnBEGIN / END lines.

This requires Satellite to read from and write to the Q1 in a more complex way, which gives the appearance, to judge from what is printed in satlog.txt, of processing the Q1 file twice.

There is no need to burden readers with the details, but only to advise them always to include the 'save' lines lest what they have painstakingly edited into the Q1 should be wiped out when the Editor re-writes the file,

Two further points should be noted, namely:
  1. PIl statements appearing in the Q1 below the STOP line (to the surprise of some older PHOENICS users) are processed by the Satellite, whether in silent or interactive mode; and do not need 'save' protection; and
  2. In-Form statements which are not enclosed between 'save' lines will not be wiped out, in 'silent'-mode working; but they will, almost equally disconcertingly, be ignored

VREL

When STORE(VREL) appears in the Q1 file, and ONEPHS=F signifying that two-phase flow is in question, PHOENICS will compute and print the 3D-stored variable VREL, which represents the value of the relative (i.e. 'slip') velocity between the two phases. averaged to the centre of a scalar cell.


VRV

\phoenics\d_utils\d_windf\vrv.bat is a batch fiile which launches the VR-Viewer. It can have optional arguments which are explained here.

VUP

------- PIL logical; default=F; group 6 -----

VUP....should be set to T when boundaries are present which give high curvature of the grid lines of constant I and K, ie. the direction of the V velocity resolute is changing rapidly.

See UUP for further details.



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