Perturbed? Good!

Brent Cullimore

Being perturbed isn’t much fun if you are a human.

But models love getting perturbed. We don’t perturb them enough, in fact.

OK, perturb has negative connotations. And perturber just sounds wrong in so many ways!

Now your understand why we called the new perturbation feature in Version 5.8 of Thermal Desktop the Model Kicker instead. It sounds more violent, I know, but it was intended to be reminiscent of “kicking the tires before you buy a car.” You should “kick your math model before accepting its results.”

This all started a year or two ago when a friend and I were pondering the question of how we could use the increasing power of computers to produce a better model and not just a bigger one. How we could use Moore’s Law for good instead of evil, given that we’re way past the days when we struggled to afford enough nodes to get acceptable accuracy.

Full disclosure: my friend and I were sampling flights of craft beer at one of the many brew pubs here in Boulder Colorado. (Why we would be discussing Thermal Desktop in that situation can’t be explained. Would it surprise you to know that we were alone, and that the booths next to us had mysteriously cleared out?)

Surprisingly, we came up with some good ideas.

Miraculously, we remembered them.

I was especially perturbed (!) by this thought: we’re building such big models these days that we don’t always understand what is going on inside them.

Big models hide mistakes, and make it harder to run the model many times in order to evaluate various scenarios. You have to spend a lot of time staring at colored contours and plotted transient results in order to figure out what exactly is happening inside your model. You can easily find out where heat is flowing, but it is much harder to figure out exactly why it flows where it does when it can follow so many parallel routes.

How about diverting some of those fast new multi-core CPUs and screaming solution algorithms into helping build your intuition about how the system behaves?

The purpose of computing is insight, not numbers.” 
                                                        - Richard Hamming

Does this sound hard? It isn’t! It turns out the Model Kicker is really easy to set up, ridiculously fast to run, and very easy to intuit relative strengths of connection.

Here is a snapshot (at one time point) of a box full of batteries and electronics, isothermalized by four heat pipes. Can you tell how well the heat pipes are helping? Can you even tell whether they are hooked up correctly? You can certainly see what is hottest and what is coldest, but for all you know you forgot to put in the thermal contact in one spot, or you put it in twice, or you put in the wrong value. Or you forgot to merge the edges of the side walls. (Relax, we’ve all goofed up. We just don’t want other people to find those goof-ups before we do!)

Now take a look at what happens when you “kick” one heat pipe and measure not just temperature, but the strength of connection between that heat pipe and the rest of the model on a scale of 0.0 to 1.0. In other words, 0.0 means “thermally not connected” and 1.0 means “extremely well-connected thermally.”

If the heat pipe had experienced a connection problem (e.g., missing contact conductance), that issue would have jumped out of the screen at you.

Or you can perturb all four heat pipes at once to ask questions such as “Are the heat pipes doing that big cylinder any good?”

To learn how to use the Kicker, you can read about it in the TD User’s Manual. You can also view this class or ask us for the class notes.

You don’t have to use the Kicker to explore your model. You can also make your own perturbations in any inputs (such as the bond line conductance, or a convection coefficient scaling factor) and use the powerful TD postprocessing option to compare the temperature differences between two answers. You’ll be surprised what jumps out at you!

Now, go take a look at the last model you built and get perturbed!


Advanced Pipes in FloCAD
Thursday November 14, 9-10am MT (8-9am PT, 11am-noon ET)
This webinar introduces advanced features for FloCAD pipes in addition to working with complex geometry. Complex geometry includes interior fins and surfaces for heat transfer, flow around enclosed objects, annular flow, concentric pipes, and more. FK Locators and TEEs as modeling objects will also be introduced.
Custom Heat Transfer and Pressure Drops
Tuesday November 19, 2-3pm MT (1-2pm PT, 4-5pm ET)
Do you know what the default assumptions are in FloCAD, and whether or not they apply in your situation? Do you know how far you can go past that starting point? The answer: pretty far. There are numerous mechanisms in FloCAD for adjusting factors, scaling uncertainties, and applying different or supplemental correlations. This webinar summarizes the options available to you to customize your flow models to make sure that they apply to each new situation you encounter.
Heat Exchangers: Detailed and System-level
Thursday November 21, 2-3pm MT (1-2pm PT, 4-5pm ET)
This is two webinars in one. The first explains the use and assumptions behind the FloCAD HX system-level modeling object. The second webinar describes detailed-level modeling of complex heat exchanger passages, including application of Compact Heat Exchanger (CHX) methods.
Starting in 2020, we will begin offering Introduction to Thermal Desktop and Introduction to RadCAD as either in-person training or online training, alternating between online and in-person every three months. The training uses lectures and demonstrations to introduce you to basic Thermal Desktop and RadCAD usage. Hands-on tutorials provide practice building models and interpreting results (tutorials are completed by students outside of the online class time).
The next training class will be an online format in January 2020:
  • Introduction to Thermal Desktop (and SINDA) - A three-part series on January 14, 16, and 21 from 9am to 12pm, Mountain time
  • Introduction to RadCAD - January 23 from 9am to 12pm, Mountain time
For up-to-date schedules, fees, and policies, visit our Product Training page. To register for the class above, complete our registration form and select "Online" for the Training Format.
If you are interested in product training for your company based on your schedule, please contact us to obtain a quote for training between 8-12 attendees. We can come to your facility or the lectures can be presented online. Descriptions of the available classes can be found in our course catalog.
To keep up with our training opportunities, take a look at our new Events and Training Calendar.