Simscale

Finite element analysis software now comes in three forms: online platforms that run in a browser, desktop packages installed on a workstation, and open-source solvers you build and maintain yourself. The choice decides how large a model you can run, how many variants you can test at once, and what you pay per analysis.

A CHT simulation answers the question a flow-only model cannot: how hot the components themselves get, not just the air moving past them. That is the number every thermal design decision hangs on, from heat sink sizing to whether a housing can be plastic. This walks through the setup end to end, how to check the result against physical test, and what to watch for on lattice and TPMS models.

Onshape has no built-in CFD. Onshape Simulation covers structural linear static and modal analysis, which gives you stress, strain, natural frequencies, and mode shapes. Fluid flow, thermal, and nonlinear structural work happen in a connected simulation tool. SimScale is one of two simulation partners listed in the Onshape App Store. It connects to your Onshape documents directly, so you pick an assembly or part in the SimScale workbench and simulate it without exporting a STEP file.

The bench measurement rarely matches the datasheet. You size a heat sink to 2.1 °C/W, order the extrusion, build the board, and the junction runs 12 °C hotter than the calculation predicted. At that point the design has to be reworked quickly to avoid disrupting the schedule. The difference comes from testing conditions.

Three threshold sets share the Lawson name: the original Lawson criteria, Lawson LDDC, and Lawson 2001. They assign different wind speeds to the same activity, and they judge those speeds at different exceedance probabilities. The same CFD results, scored against the original criteria at 2% and against LDDC at 5%, can put the same plaza in two different comfort categories. Which set applies is usually decided by the local authority, not by the engineer.

ASHRAE 90.4 does not ask whether your data center stays cool. It sets a ceiling on how much energy the mechanical and electrical systems may burn per unit of IT load, then leaves you to demonstrate that your design stays under that ceiling across a full year of weather. Those are different questions, and a design can pass the second one on a spreadsheet while failing the first one in the room, with the top third of every rack running hot because the return path recirculates.

A building can hit every energy target in its spec and still generate complaints from the day it opens. Occupants sit in a draft from a badly placed diffuser, or in the cold spot under a curtain wall, and the whole-building average that looked fine on paper never described the seat they are in. ASHRAE 55 is the standard that turns “too cold” into a number you can design against, and it is the reason thermal comfort gets checked before the ductwork is ordered rather than after the first summer.

Design of experiments in engineering usually runs smaller than planned. Engineers pick a few variables to sweep, drop the rest on judgment, and accept a study that shows how each variable behaves on its own but not how they interact. Compute time is the reason. Covering every combination of five variables at three settings each takes 243 simulations. At four hours a run, that is six weeks of solving on one workstation. KSB ran a pump study of 377 design variants, more than 900 simulations in total.

Most duct design software sizes ducts correctly and still lets a bad system through, because sizing to a friction rate says nothing about what the air does at the takeoff, the elbow, or the plenum. A ductulator gives you a diameter. It does not tell you that the branch three fittings downstream is starved because a mitered elbow is shedding a vortex across the inlet.

When engineers think of computational fluid dynamics (CFD), some typical use cases that spring to mind are internal flow (e.g., piping) or external flow (e.g., aircraft aerodynamics). However, an area that has gained a significant increase in CFD usage is computational wind engineering (CWE), or wind engineering, due to the complexities involved and the ever-increasing availability of computational power.