What happens when things shake, swing, or get slammed.
1. What is it?
Dynamics answers questions where time and motion matter:
Modal ("eigenfrequency"): at what frequencies does this part want to ring, and what do the shapes look like? "Will the motor's 50 Hz excite a resonance in this panel?"
Modal transient / Gen-α: you shake the base or apply a time-varying force — how does the part respond over the next few seconds?
Explicit: very fast events — drop, impact, crash — measured in milliseconds, where waves travel through the material.
2. When do I use which?
You have…
Use
Example
"What are its natural frequencies?"
Modal
avoid resonance with a motor
A recorded shake/table motion vs time
Modal transient (Gen-α)
earthquake base excitation
Millisecond impact, contact, wave propagation
Explicit dynamics
drop test
A frequency sweep (0→N Hz)
Frequency response
find amplification at each frequency
A shock/earthquake spec
Response spectrum
code compliance
Random vibration (PSD)
Random vibration
aerospace qualification
Use cases
Problem Type
Industry
Example
Natural frequency analysis
Aerospace
Aircraft wing modal analysis
Seismic response
Civil/Structural
Building response spectrum analysis
Drop testing
Electronics
Phone drop simulation
Vibration fatigue
Automotive
Engine bracket random vibration
Impact simulation
Aerospace
Bird strike on turbine blade
Rotating machinery
Energy/Turbine
Critical speed analysis
Acoustic resonance
Automotive
Exhaust system modal analysis
3. The flow
Modal
Design — geometry
Boundary — material + supports (no loads needed for modal)
Mesh — generate
Simulate → Modal → choose number of modes
Results — natural frequencies + mode shapes
Modal Transient
Design — geometry
Boundary — material + supports + time-varying load or base motion
Mesh — generate
Simulate → Modal Transient → set time step and duration
Results — displacement vs time, stress vs time
Explicit
Design — geometry
Boundary — material + supports + impact velocity or contact
Mesh — generate (finer than modal — waves need resolution)
Simulate → Explicit → set time step (auto or manual)
Results — deformation sequence, stress waves, energy balance
4. Reading your results
Modal
Result
Meaning
Natural frequency (Hz)
Frequency at which the part wants to ring
Mode shape
The shape it takes at that frequency
Participating mass fraction
How much of the mass moves in that mode
Good: Operating frequency is far from any natural frequency. Bad: Operating frequency matches a natural frequency → resonance → large amplification.
Transient
Result
Meaning
Displacement vs time
How much it moves at each moment
Stress vs time
Peak stress during the event
Energy balance
Kinetic + strain + dissipated = total (should be conserved)
Explicit
Result
Meaning
Deformation sequence
How it crumples or bounces
Stress wave propagation
How the impact travels through the material
Energy balance
Kinetic + internal + contact = total
5. Boundaries of truth
What's validated
Case
Reference
Error
Modal eigen
Analytical (cantilever)
< 1%
Modal transient
Published
< 2%
Response spectrum
Published
< 3%
Random vibration (PSD)
Published
< 5%
Explicit wave propagation
Analytical
< 2%
What's NOT covered
Large-strain plasticity in explicit (limited to small-strain wave propagation)
General impact contact — validated wave only; erosion, damage, restart not production
Frequency-dependent damping — constant damping models only
Multi-body dynamics — single deformable body only
Common mistakes
Mistake
Fix
Too few modes in modal transient
Include modes up to 2× the highest excitation frequency
Time step too large in explicit
Use auto time step or satisfy CFL condition
Coarse mesh for explicit
Waves need ≥ 6 elements per wavelength
Missing mass density for dynamics
Add ρ to material properties
Forgetting participating mass
Check that ≥ 90% mass participates in included modes
See also
[Static Structural](static-structural.md) — when loads are steady
[Vibration, Spectra & Fatigue](vibration-fatigue.md) — for repeated/shock loading