[tymcme]

Hi,

I am a student at the Georgia Institute of Technology and I am performing research on flow over an airfoil.

For the research we are using two main applications, Pointwise and Fluent. As part of a report, I would like to know include what some of the limitations are to these programs.

I know how to use the programs for exactly what I need but nothing more. Therefore, I know what the program can do and not what it can't do.

If someone could please enlighten me on what some of the limitations are of these applications, it would be much appreciated.

[bhanuday.sharma]

25.1.2.*Limitations


25.1.2.1.*Limitation on the Particle Volume Fraction


The discrete phase formulation used by ANSYS FLUENT contains the assumption that the second phase is sufficiently dilute that particle-particle interactions and the effects of the particle volume fraction on the gas phase are negligible. In practice, these issues imply that the discrete phase must be present at a fairly low volume fraction, usually less than 10–12%. Note that the mass loading of the discrete phase may greatly exceed 10–12%: you may solve problems in which the mass flow of the discrete phase equals or exceeds that of the continuous phase. See Modeling Multiphase Flows for information about when you might want to use one of the general multiphase models instead of the discrete phase model.


25.1.2.2.*Limitation on Modeling Continuous Suspensions of Particles

The steady-particle Lagrangian discrete phase model is suited for flows in which particle streams are injected into a continuous phase flow with a well-defined entrance and exit condition. The Lagrangian model does not effectively model flows in which particles are suspended indefinitely in the continuum, as occurs in solid suspensions within closed systems such as stirred tanks, mixing vessels, or fluidized beds. The unsteady-particle discrete phase model, however, is capable of modeling continuous suspensions of particles. See Modeling Multiphase Flows for information about when you might want to use one of the general multiphase models instead of the discrete phase models.


25.1.2.3.* Limitations on Using the Discrete Phase Model with Other ANSYS FLUENT Models




The following restrictions exist on the use of other models with the discrete phase model:
When tracking particles in parallel, the DPM model cannot be used with any of the multiphase flow models (VOF, mixture, or Eulerian – see Modeling Multiphase Flows) if the shared memory option is enabled (Parallel Processing for the Discrete Phase Model). (Note that using the message passing or hybrid option, when running in parallel, enables the compatibility of all multiphase flow models with the DPM model.)
When using the DPM model with the Eulerian multiphase model, the tracked particles rely only on the primary phase to compute drag, heat, and mass transfer. Also, any DPM related source terms are applied to the primary phase. Particle tracking relative to a secondary phase is not provided.
Streamwise periodic flow (either specified mass flow rate or specified pressure drop) cannot be modeled with steady particle tracks in coupled simulation. It is possible using transient particle tracks.
Only nonreacting particles can be included when the premixed combustion model is used.
Surface injections will be moved with the mesh when a sliding mesh or a moving or deforming mesh is being used, however only those surfaces associated with a boundary will be recalculated. Injections from cut plane surfaces will not be moved with the mesh and will be deleted when remeshing occurs.
The cloud model is not available for unsteady particle tracking, or in parallel, when using the message passing or hybrid option for the particles.
The wall-film model is only valid for liquid materials. If a nonliquid particle interacts with a wall-film boundary, the boundary condition will default to the reflect boundary condition.
When multiple reference frames are used in conjunction with the discrete phase model, the display of particle tracks will not, by default, be meaningful. Similarly, coupled discrete-phase calculations are not meaningful.
An alternative approach for particle tracking and coupled discrete-phase calculations with multiple reference frames is to track particles based on absolute velocity instead of relative velocity. To make this change, use the define/models/dpm/ options/track-in-absolute-frame text command. Note that the results may strongly depend on the location of walls inside the multiple reference frame.
The particle injection velocities (specified in the Set Injection Properties dialog box) are defined relative to the frame of reference in which the particles are tracked. By default, the injection velocities are specified relative to the local reference frame. If you enable the track-in-absolute-frame option, the injection velocities are specified relative to the absolute frame.
Relative particle tracking cannot be used in combination with sliding and moving deforming meshes. If sliding and/or deforming meshes are used with the DPM model, the particles will always be tracked in the absolute frame. Switching to the relative frame is not permitted.
Boundedness of planes is not considered during sampling of particle tracks, which means that all particle tracks crossing the unbounded plane are sampled.

[bhanuday.sharma]

4.4.1.*Limitations of the ImportBGD Feature
There is a known limitation with the ImportBGD feature when importing multiple BladeGen files. If you have imported two or more BladeGen files using separate ImportBGD features, and have turned on shroud clearance for one of these features, then the import process may fail. The workaround is to import the case(s) with shroud clearance first, then import the others.
Furthermore, changing the Blade Design cell Shroud Clearance property from "Relative Layer" or "Absolute Layer" to "None" will have no effect on the ImportBGD feature. In this case, you must change the Shroud Clearance property directly in the ImportBGD feature.

[bhanuday.sharma]

4.5.*Limitations

Many MHD applications involve the simultaneous use of other advanced ANSYS FLUENT*capabilities such as solidification, free surface modeling with the volume of fluid (VOF) approach, DPM, Eulerian multiphase, and so on. You should consult the latest ANSYS FLUENT*documentation for the limitations that apply to those features. In addition, you should be aware of the following limitations of the MHD capability.
As explained in Magnetohydrodynamic Model Theory, the MHD module assumes a sufficiently conductive fluid so that the charge density and displacement current terms in Maxwell’s equations can be neglected. For marginally conductive fluids, this assumption may not be valid. More information about this simplification is available in the bibliography.
For electromagnetic material properties, only constant isotropic models are available. Multiphase volume fractions are not dependent on temperature, species concentration, or field strength. However, sufficiently strong magnetic fields can cause the constant-permeability assumption to become invalid.
You must specify the applied magnetic field directly. The alternative specification of an imposed electrical current is not supported.
In the case of alternating-current (AC) magnetic fields, the capability has been designed for relatively low frequencies; explicit temporal resolution of each cycle is required. Although not a fundamental limitation, the computational expense of simulating high-frequency effects may become excessive due to small required time step size. Time-averaging methods to incorporate high-frequency MHD effects have not been implemented.

Release 14.0 - © 2011 SAS IP, Inc. All rights reserved.

[bhanuday.sharma]

similar to these limitation can be found in ansys fluent help.
just open the fluent help and search the keyword "limitations."

[Amna]

hi everyone

i m working on gas-solid fluidized bed.Geometry is a simple 2D cylindrical column.During simulation as first step , i kept gas velocity equal to zero & selected surface injections in DDPM to inject solids from bottom of column i.e from gas inlet.

i have to inject 800 particles which will give 0.25 m static bed height (as per literature). but in my case static bed height i just get 0.125 m upon injection. Kindly let me know how could i get the required 0.25 m static bed height?

secondly, just to see the phenomena at 0.125 m static bed height , i gave 1.4 m/s gas velocity, i didn't get a proper fluidized bed rather some random solid fraction from inlet-outlet obtained, which isn't matching to any of fluidization stage.

Kindly tell me how could i get a proper fluidized bed by using this euler-Lagrangian approach?

Hope to get a prompt reply ......

[sina_sls]

Originally Posted by bhanuday.sharma

4.5.*Limitations

Many MHD applications involve the simultaneous use of other advanced ANSYS FLUENT*capabilities such as solidification, free surface modeling with the volume of fluid (VOF) approach, DPM, Eulerian multiphase, and so on. You should consult the latest ANSYS FLUENT*documentation for the limitations that apply to those features. In addition, you should be aware of the following limitations of the MHD capability.
As explained in Magnetohydrodynamic Model Theory, the MHD module assumes a sufficiently conductive fluid so that the charge density and displacement current terms in Maxwell’s equations can be neglected. For marginally conductive fluids, this assumption may not be valid. More information about this simplification is available in the bibliography.
For electromagnetic material properties, only constant isotropic models are available. Multiphase volume fractions are not dependent on temperature, species concentration, or field strength. However, sufficiently strong magnetic fields can cause the constant-permeability assumption to become invalid.
You must specify the applied magnetic field directly. The alternative specification of an imposed electrical current is not supported.
In the case of alternating-current (AC) magnetic fields, the capability has been designed for relatively low frequencies; explicit temporal resolution of each cycle is required. Although not a fundamental limitation, the computational expense of simulating high-frequency effects may become excessive due to small required time step size. Time-averaging methods to incorporate high-frequency MHD effects have not been implemented.

Release 14.0 - © 2011 SAS IP, Inc. All rights reserved.

Hello

I want to set sinusoidal value of electric potential (MHD module) for a wall, first i try expression but i faced to error :

Error: wta(2nd) to string_eq

Error Object: __expr__

then i try Profile and UDF profile , but agian i faced to problem , when i use them it consider the wall to insulated.

Could you help me how can i set sinusoidal value for current density ( Electric Potential MHD) ?

Best Regards,