Flow Simulation solves the time-dependent Navier-Stokes equations through iterations. If the analysis is unsteady (time-dependent) the each iteration is the time step, which is the same for the whole computational domain. If the analysis is not time-dependent the solution is obtained as steady-state, so to obtain the steady-state solution faster, the time step at the each iteration can be different in the different regions of the computational domain, so the iterations are not connected with the time steps.

CPU time The CPU time elapsed from the beginning of the calculation to the current moment.

Physical time

Physical time for time-dependent analysis. For unsteady (Transient, or Time-dependent) problems Flow Simulation "time marches" the solution from initial conditions for the problem’s physical time that you specify.

Travel The term travel, used together with iterations, is a unit characterizing the calculation duration. It is the calculation period (which can be measured in iterations or some other unit) required for a flow disturbance to cross the computational domain’s fluid region. Therefore, N travels denote the calculation period required for a flow disturbance to cross the computational domain N times. The travel equivalent in iterations is determined as a function of the number of cells just after the calculation starts and can be seen in the Info box when monitoring the calculation (see Information and warnings).

So basically for transient analysis, physical time is the actual time taken for a problem to reach the final solution from the initial condition in actual practice.

From the Online Help:

Iterations.Flow Simulation solves the time-dependent Navier-Stokes equations through iterations. If the analysis is unsteady (time-dependent) the each iteration is the time step, which is the same for the whole computational domain. If the analysis is not time-dependent the solution is obtained as steady-state, so to obtain the steady-state solution faster, the time step at the each iteration can be different in the different regions of the computational domain, so the iterations are not connected with the time steps.

CPU timeThe CPU time elapsed from the beginning of the calculation to the current moment.

Physical timePhysical time for time-dependent analysis.

For unsteady (Transient, or Time-dependent) problems Flow Simulation "time marches" the solution from initial conditions for the problem’s physical time that you specify.

TravelThe term travel, used together with iterations, is a unit characterizing the calculation duration. It is the calculation period (which can be measured in iterations or some other unit) required for a flow disturbance to cross the computational domain’s fluid region. Therefore, N travels denote the calculation period required for a flow disturbance to cross the computational domain N times. The travel equivalent in iterations is determined as a function of the number of cells just after the calculation starts and can be seen in the Info box when monitoring the calculation (see Information and warnings).