The analysis is based on the method of characteristics, making it suitable for the simulation of pressure fluctuations induced by sudden events such as trains crossing tunnel portals, shafts, cross-passages and other trains. Slower events typical of accelerating and decelerating in underground railways are handled with equal ease.
The analysis is one-dimensional. This has enormous benefits in terms of simplicity of use, robustness of the code and high computational speed, thereby making the program ideally suited for engineering design. It also has considerable benefits for the accuracy of the program predictions.
The main disadvantage of 1-D analyses is that they give no information whatsoever about three-dimensional effects. They assume uniform conditions throughout any particular flow cross-section and so cannot give details of phenomena such as flow separations at junctions or the stratification of hot smoke over colder air. One-dimensional analyses take account of such phenomena in approximate manners that are most effective when used by experienced analysts.
The theoretical bases of ThermoTun/5 and ThermoTun/6 differ in an important respect. The former neglects all changes in entropy whereas the latter allows for entropy changes in a meaningful manner.
In both programs, trains are regarded as a series of individual coaches, each of uniform cross-sectional area. Local loss coefficients can be specified at the front and rear of coaches to allow for flow separations. In the case of sealed trains, pressure histories inside coaches are calculated from pressure histories outside coaches, allowing for both leakage and structural compressibility.
