Twin, Simulation, and Model Compared
A model describes behavior, a simulation runs it forward, and a twin keeps a simulation locked to a real asset with live data.
Three related ideas
The words model, simulation, and digital twin are often used loosely. They name three distinct things, and the difference is the degree of coupling to a specific physical object. Getting the vocabulary right prevents overclaiming, which matters for a machine that is not yet built.
Model
A model is a mathematical description of how a system behaves: equations, parameters, and boundary conditions. A magnetohydrodynamic equilibrium code, a lumped thermal network, a fatigue curve for a structural alloy, all are models. A model is a static description; it does not by itself represent any particular unit or any particular moment in time.
Simulation
A simulation is a model executed forward in time or across a parameter sweep to produce predicted behavior. Simulations answer design questions and generate the training data for surrogates. They can be generic, describing a class of machines, and they need no connection to a real asset. Today's Kronos three-dimensional model of Hyperion and the burner is elaborate simulation, not a twin.
Digital twin
A twin is a simulation bound to one specific physical unit through a data assimilation loop. It carries an estimate of that unit's current internal state, updates that estimate as sensors report, and adjusts its own parameters to match observed drift. Two identical machines have two different twins, because each accumulates its own history of wear, calibration, and operating conditions.
A useful test
- Can it describe behavior in equations? It is at least a model.
- Does it run forward to predict? It is a simulation.
- Is it corrected continuously by data from one named unit? It is a twin.
The progression is cumulative. A twin contains models and runs simulations; what makes it a twin is the live reconciliation. See data assimilation for the mechanism that performs that reconciliation, and fidelity levels for how much physics a twin needs to carry.