Particle Measurement in CFD

ANSYS Fluent can track the motion of particles through a fluid using the Discrete Phase Model (DPM).  In the most basic approach to implementing particles in a CFD analysis the user specifies:  injection location, speed, mass flow rate, particle size & material.

Particles can interact with boundary walls with options: reflection, escape, trapping, sliding, or forming a film.

DPM offers an impressive range of advanced capabilities, including:

  • Random effects of small-scale turbulent eddies
  • Particle size distribution
  • Two-way interaction where particles influence flow
  • Particle-particle interaction for dense populations
  • Particle heat transfer and vaporization/boiling of droplets & bubbles

“Particle Tracks” is the primary tool to postprocess particle behavior.  In addition to creating a graphical display of particle paths, this tool offers a powerful option to quantify particle behavior.  From the Particle Tracks menu, activate Report Type -> Summary, and Report To -> Console.  The text interface will report statistics for individual boundaries where particles exit the system, including Particle Count, Elapsed Time and Mass Flow.

 

 

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How to Parameterize Everything in ANSYS

kaan-80x80One of the best ways to get value out of your simulation is to do parametric analysis. With very little marginal work a completed model can be parameterized to simulate scenarios limited only by your computational time and resources (have you considered a Parametric Pack for parallel design solves?). With ANSYS R18’s promise of digital exploration, DesignXplorer is now included with all CFD, Mechanical and Multiphysics bundles. Sophisticated parametric exploration, optimization and robustness is now at your fingertips. While ANSYS Workbench and DesignXplorer manage your parameters in a consistent interface, setting up a parameter is different in each of the software tools and not always obvious. This document is intended to be a quick reference of how to do so, letting you get over this initial hurdle to take full advantage of the promise of Digital Exploration.

Table of Contents (for quick jumping around):

 

SpaceClaim Direct Modeler

SpaceClaim let’s you create a parameter from almost any operation:

Param - SpaceClaim Simple

Your parameters show up in the Groups tab.

For more advanced driving dimensions you need to have a dimension on an annotation plane:

Param - SpaceClaim Advanced

 

DesignModeler

DesignModeler has the familiar checkbox to “promote” parameters out to Workbench:

Param - DesignModelerYou can create relations between promoted parameters and other dimensions in the model with the Parameters Pane.

CAD Systems

With the appropriate associative interface licensed and configured, you can make Workbench aware of CAD parameters.

Param - CADBe sure to either use the DS or ANS prefix or else ANSYS will ignore your parameter. You can modify this with the Parameter Key property shown above.

Mechanical

In Mechanical, anywhere you see the checkbox can be promoted to a parameter for Workbench to use:

Param - MechanicalMechanical Command Snippets

APDL command snippets used in Mechanical can also be parameterized, both as input parameters:

Param - Mechanical APDL In

and output parameters (any variable with the Output Search Prefix will be retrieved):

Param - Mechanical APDL OutBe aware that APDL command snippets are not units aware!

Mechanical APDL (ANSYS Classic)

You can also easily use script files from the older user interface in Workbench, easily. With the Mechanical APDL component, specify the input file (along with supporting files as a reference file) and it will be parsed for all of it’s variables. All that’s left is to specify what’s an output and what’s an input:

Param - MAPDL

Fluent

In Fluent, most places that you can enter a value will have a dropdown that allows you to specify a parameter instead:

Param - Fluent

CFX-Pre

In CFX-Pre you can specify expressions as parameters and use them as inputs in other parts of the model:

Param - CFX

CFD-Post

In CFD-Post you base parameters off of expressions as well, making sure to use the nice right click menu to help with building expressions:

Param - CFD-Post

Maxwell & HFSS

The user interface is similar enough in these tools so that the same instructions apply. When accessing these from Workbench, a DesignXplorer node is created under the Optimetrics portion of the tree. Optimetrics is an EMAG specific optimization tool that is complimentary to DesignXplorer. In most places in Maxwell and HFSS, enter an identifier instead of a number to automatically create a parameter. Promote it out to Workbench in the DesignXplorer node. In the DesignXplorer node, output variables can be created in the Calculate tab.

Param - Maxwell HFSS

Icepak

Similar to the above, in the user interface where you would normally enter a number, instead enter an identifier preceded by $. Then in the Define Trials dialog, expose the parameter to Workbench.

Param - Icepak

And that’s it! Remember that the power of an integrating platform like Workbench allows you to have several of these software tools be connected in the same analysis flow. From CAD Parameter interaction to coupled field analysis, it’s all possible. Now go forth and digitally explore your design space!

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Are Cats a Fluid? Thinking (and Learning) about CFD Through Humor

kaan-80x80The internet has uncovered some intriguing evidence on the rheological state of a common household item. How do physical constants like Deborah’s number, adhesion, relaxation time and catpillary number apply in unusual situations? I came upon this topic from the Improbable Research Podcast and the pictures are from the linked rheology bulletin. Welcome to the internet and read on to learn some CFD terminology as applied to an interesting and ridiculous question.

reheology-of-cats1Deborah Number: Given enough time, everything flows. The ratio of relaxation time to the time of observation can be used to determine if something behaves as a gas, liquid or solid in your timescales of interest. Do older cats have a higher relaxation time? Is the question of whether cats flow reminiscent of the wave-particle duality of light? If so, is it of equal to or greater import?

Solid-Liquid-Gas Continuum: Solids that deform under stress vs liquids that flow to fill a container. Clearly the full spectrum can be observed in certain specimens.

Capillary Bridge: Observed in extensional rheometry experiments of test samples (see 2a). The subject is suspended between arbitrary rigid body surfaces. It remains to be seen how important surface tension and transient strain-hardening are to these CATBER (Capillary thinning and breakup extensional rheometer) experiments and other awesome words.

reheology-of-cats2

Lotus Effect: The substrate pictured in 2c seems to exhibit extreme felidaphobicity, showing high contact angles on the outer surface. Suggests possible applications in the tops of tables and desks.

Yield Stress: The ketchup in a bottle effect is observed (2c) due to the cat being below its yield stress. This is distinct from the free surface flow effects observed above.

reheology-of-cats3Reynolds-Weissenberg Number: The ratio of the relaxation time vs the rate of deformation. As this number increases, secondary chaotic flows emerge. For simple, non-viscous fluids this is Turbulence and depends primarily on inertia. For biologically active materials, their rates of deformation can be dependent on its environment.

Don’t forget that you can always perform your own experiments in silico, without harming any cats, using ANSYS Fluent. In the material properties of a fluid you have a variety of viscosity and turbulence models to simulate all of the non-Newtonian felines that you desire. Happy new year!

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Tech Tip – Fluent Profiles: A Text File as Your UDF

In an advanced software like ANSYS Fluent you often need to specify things in more detail than a constant value. Perhaps you have a radial velocity profile of fully developed flow at an inlet, a time history of mesh motion or an axially varying heat generation. You don’t need to dust off your C++ compiler, though, there is an easier way: profiles.

Say we want to apply the following x displacement over time to a mesh motion:
techtipsept2016_prof1

We can accomplish this with the following text file:

((foil 3 point)
 (time 0 1 5)
 (v_z 0 10 10))

Load it into Fluent with the File -> Read -> Profiles menu item and it can be selected like a UDF in the dropdown menu:

techtipsept2016_prof2

 

It’s that easy.

There are several ways to specify profiles in Fluent but some of the most common are:

A Radial Profile

((<profile_name> radial <n>)
 (r <r1> <r2> <r3> ... <r_n>)
 (<field name 1> <f1_1> <f1_2> <f1_3> <f1_n>)
 (<field name 2> <f2_1> <f2_2> <f2_3> <f2_n>)
 ...
 (<field name m> <fm_1> <fm_2> <fm_3> <fm_n>))

An Axial Profile

((<profile_name> axial <n>)
 (z <z1> <z2> <z3> ... <z_n>)
 (<field name 1> <f1_1> <f1_2> <f1_3> <f1_n>)
 (<field name 2> <f2_1> <f2_2> <f2_3> <f2_n>)
 ...
 (<field name m> <fm_1> <fm_2> <fm_3> <fm_n>))

A Transient Profile

((<profile_name> transient <n> <periodic? (1/0)>)
 (time <time1> <time2> <time3> ... <time_n>)
 (<field name 1> <f1_1> <f1_2> <f1_3> <f1_n>)
 (<field name 2> <f2_1> <f2_2> <f2_3> <f2_n>)
 ...
 (<field name m> <fm_1> <fm_2> <fm_3> <fm_n>))

 

The above are extremely convenient ways to specify spatially or time varying boundary conditions from analytical or empirical equations. You can easily generate these models with either a text editor or an Excel spreadsheet similar to the the pictured below (save as a tab separated text file)

techtipsept2016_prof3

 

If you open the Profiles Manager in Fluent, you can write more complex profiles from your solution data, orient profiles to a certain coordinate system and other such operations.

techtipsept2016_prof4

For more information on profiles in Fluent, check out the documentation section at Fluent -> User’s Guide -> Cell Zone and Boundary Conditions -> Profiles. Good luck on making your simulations more accurate!

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Tips & Tricks - CFD-Post

Tech Tip – Transient Animation Improvements in CFD-Post

ANSYS CFD-Post is a state-of-the-art postprocessing tool used to visualize and quantify results from CFD simulations. At R17, transient flow variations are easier to animate using the new “Timestep Animation” option. This new capability uses ‘music-player’ style controls to play back transient results.

CFD-Post Animation graphical user interface

CFD-Post Animation graphical user interface

Some new user-interface options for Timestep Animation:

  • ‘Specify Range For Animation’ allows to trim the start/stop at specific timesteps instead of playing the entire animation.
  • ‘Advanced Frame Selection Controls’ allows to skip timesteps during the animation. This option is useful for speeding-up creation of animations from large results files.

 

 

 

 

 

 

 

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