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- EL1.HTM

length scale in a form suitable for parabolic calculations of wall boundary layers min EL1A EL1B YG 0 09 width where YG is measured from the south wall boundary to the grid node and the boundary layer width is computed at each z in subroutine GXLEN The width calculation is made in accordance with the user settings for EL1C the fraction of the maximum velocity difference EL1D the free stream velocity and EL1E which is set to 0 0 for a boundary layer or to the jet discharge velocity for a free jet The coding for the width calculation presumes NX 1 and the user sets EL1A 0 and EL1B 0 41 von Karman s constant EL1 GRND8 selects Nikuradze s mixing length scale in a form suitable for 2 dimensional channel and pipe flows namely 0 14 YVLAST 0 08 YG 2 YVLAST 0 06 YG 4 YVLAST 3 where YG is measured from the symmetry axis to the grid node This form of Nikuradse s expression is suitable for flows with a plane or axis of symmetry and with the wall located at the north boundary The user sets EL1A 0 14 YVLAST EL1B 0 08 YVLAST and EL1C 0 06 YVLAST 3 EL1 GRND9 selects Nikuradse s mixing length scale in a form suitable for 2d channel flows with a wall located at the north AND south boundaries 0 4 YG 0 88 YG 2 YVLAST 0 96 YG 3 YVLAST 2 0 48 YG 4 YVLAST 3 where YG is measured from the south wall boundary to the grid node EL1 GRND10 is used to select all other length scale formulae according to the setting of EL1E as follows EL1E 0 0 selects a length scale suitable for use in the Smagorinsky subgrid scale SGS eddy viscosity model MIN EL1A H EL1B WDIS with H SQRT DX 2 DY 2 DZ 2 3 0 where EL1A is Smagorinsky s constant typically 0 17 H is a representative grid interval DX DY and DZ are the local mesh widths in the different coordinate directions EL1B is a constant typically 0 41 and WDIS is the minimum wall distance the calculation of which is activated by the PIL command DISWAL The SGS model may be activated by TURMOD SGSMOD which is equivalent to ENUT GRND2 EL1 GRND10 GENK T EL1A 0 17 EL1B 0 41 and DISWAL The minimum function is not applied if DISWAL is not set or if EL1B 0 0 in which case the length scale reduces to EL1A H For more details see the PHENC entry SUBGRID SCALE turbulence model EL1E 1 0 selects the mixing length scale of Geary and Rice AIChE Journal Vol 36 No 9 p1339 for bubbly two phase flows CFIPB EL1A rd rd av where CFIPB bubble diameter see the option CFIPS GRND7 of the Encyclopaedia entry CFIPS EL1A is a correction factor for bubble deformation rd is the void fraction of the dispersed phase and rd av is

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/el1.htm (2016-02-15)

Open archived version from archive - ENU.HTM

1 46E 6 110 Ammonia 300 1000K 1 54E 6 472 CO2 300 3500K 1 56E 6 233 CO 300 5000K 1 40E 6 109 Helium all t1 1 52E 6 98 H2 300 2200K 0 65E 6 71 N2 300 5000K 1 39E 6 102 O2 300 2800K 1 65E 6 110 See also the Encyclopedia entry NON Newtonian fluids ENULA PIL real default 1 E 5 group 9 ENULA parameter used in formulae for laminar kinematic viscosity Further parameters of the same kind are ENULB ENULC ENUT PIL real default 0 0 group 9 ENUT sets the turbulent kinematic viscosity Options are ENUT const 0 sets turb kin visc const ENUT GRND1 sets turb kin visc ENUTA ENUTB len1 ENUT GRND2 sets turb kin visc sqrt abs lgen1 len1 2 ENUT GRND3 sets turb kin visc CMU len1 KE 0 5 ENUT GRND4 sets turb kin visc ENUTA len1 ABS average V1 V2 R1 R2 ENUT GRND5 sets turb kin visc CMU KE 2 EP ENUT GRND6 sets turb kin visc CMU KE VOSQ 0 5 ENUT GRND7 sets turb kin visc CMU KE 0 5 OMEGA ENUT ENUT represents the contribution to the effective kinematic viscosity E followed by Greek letter NU made by the local turbulence T a Uniform value settings The simplest representation is made by setting ENUT to a positive constant the value of which can be plausibly estimated from the formula ENUT 0 01 Vs Ls where Vs is a typical velocity and Ls is a typical length for the flow and geometry in question For duct flows ENUT may be estimated from the following formula ENUT 0 035 SQRT FRIC 8 REY ENUL where REY is the duct Reynolds number based on the mass averaged velocity and hydraulic diameter and FRIC is the friction factor which is a function of REY the definition of which entails that REY ENUL is the above mentioned velocity length product The following formula provides a fairly good fit to the data for turbulent flow in smooth pipes FRIC 1 82 LOG10 REY 1 64 2 For the plane mixing layer ENUT may be taken as approximately 0 0012 distance from start of layer velocity difference across it or as 0 0044 width of layer times velocity difference across it For the plane jet in stagnant surroundings ENUT may be taken as approximately 0 015 distance from symmetry plane to jet edge velocity at the symmetry plane For the axi symmetrical round jet in stagnant surroundings ENUT may be taken as approximately 0 01 distance from symmetry axis to jet edge velocity at the symmetry axis b Position dependent values The following ENUT options have been provided in subroutine GXENUT called from GREX and selected as indicated ENUT GRND1 selects turbulent kinematic viscosity equal to ENUTA ENUTB len1 where len1 denotes the mixing length scale that pertains to the first phase fluid calculated by means of a formula selected by setting of EL1 ENUT GRND2 selects turbulent

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/enu.htm (2016-02-15)

Open archived version from archive - ERRORS.HTM

can influence both the time taken to attain the solution and indeed whether the solution is reached at all as has been mentioned in sub Section 5 8 above It is therefore a good rule to seek always to supply initial guesses which are as close as possible to the values to be expected in the final solution Thus they should have the right sign and be of the correct order of magnitude and the velocity fields should satisfy continuity If this is not done PHOENICS will normally attain the correct solution but at a greater cost in computer time than is necessary and sometimes the initial guess can be so far from the solution that divergence occurs g Truncation inaccuracies Computers work to only a limited number of significant figures It is therefore advisable to avoid producing calculated results of the following kinds Enthalpy values of the order of 1 E6 when differences of the order of 1 E1 are significant Pressures of the order of 1 E5 when dynamic heads are below 1 E1 These can be avoided by use of suitable datum levels for the enthalpy or pressure see PRESS0 respectively or by non dimensionalisation h Excessive or inappropriate print out It is hard to know in advance precisely what print out will be required Some users therefore call for much more than they are sure to need just in case it will be found to be useful but in doing so they create needless difficulties burdening themselves with more paper or longer output files than they can possibly inspect Because of the ability easily to restart a PHOENICS computation it is best to call for only a small amount of output at first concentrating perhaps on the residuals and how they diminish with number of sweeps and this should be viewed at the VDU without recourse to the line printer If the results of the computation are saved by way of the SAVE T setting in Group 24 this is the default it will always be possible to elicit desired output by making one more sweep with the appropriate settings in Groups 21 22 and 23 see AUTOPS Thus a series of minimum output runs may be made by a succession of restarts until it is judged that the solution is sufficiently converged to be interesting Then a succession of single sweep runs can be made with various output options until tables and plots containing the desired combination of information items suitably displayed have been elicited Only at the end of this is it appropriate to cause the output actually to be printed When divergence occurs at the start of a run printing out and inspecting the field values for the first two or three sweeps will often permit the cause to be discerned For example it may reveal that some variables have totally unrealistic values even at the start possibly because of errors in coding introduced into GROUND or the occurrence of low pressures near

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/errors.htm (2016-02-15)

Open archived version from archive - ENC.TU63.HTM

hotter the cold fragments are as cold as they can be namely at TU and the hot fragments are as hot as they c be at temperature TB b Calculating the mass fractions of the two fluids Since the temperatures of the two components of the population are fixed a single differential transport equation namely that for the time mean temperature TM suffices for the population distribution to be computed Specifically the mass fractions of cold and hot gas MU and MB are given by MU 1 MB TM TU TB TU c Calculating the reaction rate Of course combustion can not take place at an appreciable rate in the cold fragments because they are too cold nor in the hot fragments because they are alreaady fully burned Since combustion undoubtedly does take place it is supposed that 1 it does so at inter faces between the two types of fragments 2 these occupy only a small proportion of the mixture volume 3 the rate of combustion per unit volume of mixture is proportional to the rate of intermingling of the two types of fragments and finally Calculating the reaction rate continued 4 this rate of intermingling is proportional to MB MU MIXRATE where MIXRATE is proportional to either VELOCITY GRADIENT MIXING LENGTH in the first model Spalding 1971b or to TURBULENCE ENERGY DISSIPATION RATE TURBULENCE ENERGY in a later version Mason and Spalding 1973 d Successes and failures of EBU The model has been successful in explaining certain otherwise inexplicable experimental findings for example the fact that the angle subtended by the flame anchored in a plane walled channel is nearly independent of approach gas velocity However it has no means for expressing the INFLUENCE OF CHEMICAL KINETICS Yet such an influence does exist as witness the fact that

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/turmod/enc_tu63.htm (2016-02-15)

Open archived version from archive - WALL.HTM

be changed by setting WALLCO GRND1 or GRND3 as required see WALLCO WALL DISTANCE See PHENC entries DISTANCE from the WALL and DISWAL WALL friction declaring areas of see WALL command Group 13 WALL function parameter variable for see WALLA real Group 13 WALL Functions See Wall functions entry in Turbulence Models in PHOENICS WALLA PIL real default 0 0 group 13 WALLA GREX variable for wall function parameter For the GRND2 i e LOGLAW and GRND3 i e GENLAW turbulent wall functions it represents the absolute sand grain roughness size for rough walls according to the formula of Nikuradse For GRND5 wall functions it is essential to set a roughness height WALLA represents the effective roughness height The default of 0 0 gives the smooth wall value It is specified in the same units as other lengths WALLB PIL real default 0 0 group 13 Inactive WALLB GREX variable for wall function parameter WALLCO PIL real default GRND2 Group 13 WALLCO indicates whether the GRND1 i e BLASIUS GRND2 i e LOGLAW GRND3 i e GENLAW GRND4 i e ALReLAW or GRND5 i e fully rough logarithmic option is to be used whenever wall friction effects are generated automatically by Earth This occurs in two circumstances namely when EGWF T its default value and when EGWF F and the Satellite generates patches on the boundaries of solids by reason of the CONPOR 1 or CONPOR 0 commands See the help and encyclopaedia entries for WALL and EGWF for further information WALLS WALL is not itself a TYPE However the appearance of WALL as part of the PATCH type name implies that the specified value of the COefficient will be multiplied by the reference kinematic viscosity times the fluid density and divided by the distance of the grid node from the specified face See the types EWALL WWALL NWALL SWALL HWALL and LWALL As implied in the above paragraph wall effects friction and heat transfer etc are represented by sources of U1 U2 V1 V2 W1 and W2 for wall friction of H1 and H2 or TEM1 for wall heat transfer etc The PATCH command is used to locate the wall and COVAL is used to activate the source for the variable indicated in the second argument of COVAL The entries PATCH name EWALL COVAL name H1 1 0 PRNDTL H1 10 0 imply that there is a wall at an east cell face and that the transfer rate of heat from it is equal to 10 0 fluid enthalpy east face area reference kinematic viscosity in the cell fluid density prevailing in the cell distance of cell centre from the wall PRNDTL H1 The additional command COVAL name W1 1 0 30 0 sets the velocity of the wall equal to 30 0 m s in the z direction and supplies the following source of momentum to the W1 variable 30 0 W1 velocity east face area reference kinematic viscosity in the cell fluid density prevailing in the cell

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/wall.htm (2016-02-15)

Open archived version from archive - EARTH-GENERATED WALL FUNCTIONS

of skin friction factors Stanton numbers etc may be generated by setting WALPRN T This print out is controlled by the IXPRF IXPRL IYPRF and IYPRL settings in the usual way Alternatively the user may STORE one or more dependent variables with names SKIN STAN STRS YPLS and HTCO into which will be written for wall cells the skin friction factor the Stanton number the shear stress y the dimensionless distance from the wall and the heat transfer coefficient These will then be printed along with the more usual field values 3 Settings for individual CONPORs or BLOCKAGEs The settings of WALLCO and WALLA are default values used everywhere unless specifically overwritten This can be done by setting SPEDAT commands as follows To change the wall function option for a CONPOR named name SPEDAT SET name WALLCO R GRNDn where n is 1 2 3 or 5 To change the roughness height for a CONPOR named name SPEDAT SET name ROUGH R roughness height To set a heat transfer coefficient in W m 2 K for a CONPOR named name SPEDAT SET name HTCO R heat transfer coeff If the surface of the solid is moving the slide velocities are set by SPEDAT SET name VELX R X direction velocity SPEDAT SET name VELY R Y direction velocity SPEDAT SET name VELZ R Z direction velocity where the surface velocities are all in m s with a default value of 0 0 in which case the settings are not needed In polar co ordinates the meanings of the surface velocities are set as follows SPEDAT SET name IURVAL I iflag where iflag 0 all suface velocities are grid aligned in m s 1 X velocity is angular momentum in m 2 s 1 U velocity is angular velocity in radians

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/egwf.htm (2016-02-15)

Open archived version from archive - TURBULENCE MODELS IN PHOENICS

Increased macro mixing an example of which is that the heat transfer from a hot turbulent fluid to the colder wall of the pipe through which it is flowing is much greater than it would be for a laminar flow Increased micro mixing an example of which is the greatly increased rate of chemical reaction between co flowing streams of fuel and oxidant gases when the velocity is raised to the critical level It is the role of a turbulence model to enable the effects of the eddies to be computed without simulating them in detail whereby it should be stated that immensely more attention has been paid to the macro mixing than to the micro mixing effects Micro mixing is handled in PHOENICS by the Multi Fluid Model The three approaches to simulating turbulence Turbulence models of the macro mixing kind were invented several decades before digital computers but it was only the arrival of such computers which enabled them to be used for engineering purposes It is these which receive the greatest attention in the Turbulence Models in PHOENICS Encyclopaedia articles In the last two decades as digital computers have become more and more powerful increasingly successful attempts have been made to use fine enough grids and small enough time steps to enable the behaviour of even the smallest eddies to be computed numerically This practice has become known as Direct Numerical Simulation or DNS DNS is still practicable only for very simple flows and for modest Reynolds numbers and its expense is far too great to be afforded in engineering practice Nevertheless such results as have been published are beginning to serve the needs of turbulence model developers who require empirical data for the calibration of their models Hitherto empirical has been synonymous with experimental but now its meaning can be extended so as to include DNS generated as well DNS is not dicussed further in the present Encyclopaedia articles A third approach to the problem of predicting the behaviour of turbulence phenomena can be regarded as combining some aspects of both DNS and macroscopic modelling It is called Large Eddy Simulation commonly abbreviated to LES Its nature is this The CFD code is run in stepping through time mode even when turbulence fluctuations apart the flow is steady A macrocopic turbulence model is employed but with modifications intended to represent only the eddies which are small compared with the cells of the computational grid which is in use Although still much more expensive in respect of computer time than wholly macroscopic modelling LES is beginning to be used in engineering practice PHOENICS is equipped with one version of LES the nature of which is explained here A useful source of recent information concerning both DNS and LES and its recent variants is K Hanjalic Y Nagano and S Jakirlic Eds Turbulence Heat and Mass Transfer 6 Begell House 2009 2 Summary list of models in the order of appearance in this Encyclopaedia article Prescribed effective viscosity

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/turmod/enc_tu.htm (2016-02-15)

Open archived version from archive - IF.HTM

features The syntax of the block IF is IF logical expression THEN PIL statements 1 ELSE PIL statements 2 ENDIF For example IF NX GT 1 THEN SOLVE U1 PIL statement ELSE SOLVE V1 PIL statement ENDIF The logical expression has a syntax and capability very similar to the corresponding FORTRAN construct with the following differences There is no precedence on the operators AND OR and NOT it is therefore

Original URL path: http://www.cham.co.uk/phoenics/d_polis/d_enc/if.htm (2016-02-15)

Open archived version from archive

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