Transient flow, also known as unsteady flow, is the condition where fluid properties within the system are changing with time. In many systems, transient flow typically occurs immediately after the system initiates. In some cases, as time progresses, the flow stabilizes. If the flow stabilizes and the fluid properties at all points within the system stop changing with time, the transient state is over. Typical flow properties you may evaluate during transient flow conditions include pressure, momentum, energy, temperature, mass fraction, and volume fraction. There are many examples of transient flow conditions that demonstrate the importance of transient flow simulations. For example, the ocean's tides are an example of transient flow, because the strength and directionality of the flow continuously changes with time. In this scenario, you may want to simulate the stress placed on the pillars of a bridge at low tide versus high tide are hours later. Additional examples include:
When you define a transient flow step for your simulation, your results consist of fluid properties that change with time. However, depending on the flow conditions, some simulations may reach a steady-state flow (that is, the fluid properties do not change with time) within the time period you specified. You can define controls that apply to the entire transient flow step to improve the speed and accuracy of the analysis. These controls include defining how time progresses and defining the stability of information transfer between mesh elements. You can also fine-tune the various equation solvers required for your scenario to adjust each solver's behavior in more specific ways. | ||||||