From: Wolfgang Bangerth Date: Tue, 7 Apr 2020 16:49:57 +0000 (-0600) Subject: Document the mental model of DiscreteTime. X-Git-Tag: v9.2.0-rc1~266^2 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2d5ef340127b0544bfc3cb48a0325a6deda67983;p=dealii.git Document the mental model of DiscreteTime. --- diff --git a/include/deal.II/base/discrete_time.h b/include/deal.II/base/discrete_time.h index 409face8a2..3427ed7f03 100644 --- a/include/deal.II/base/discrete_time.h +++ b/include/deal.II/base/discrete_time.h @@ -21,13 +21,21 @@ DEAL_II_NAMESPACE_OPEN /** - * Provides a means to keep track of the simulation time in a time-dependent - * simulation. It manages stepping forward from an initial time to a final - * time. It also allows adjusting the time step size during the simulation. - * It is guaranteed that at all times the current simulation time is in the - * closed interval between the start time and the end time. + * This class provides a means to keep track of the simulation time in a + * time-dependent simulation. It manages stepping forward from a start time + * $T_{\text{start}}$ to an end time $T_{\text{end}}$. It also allows adjusting + * the time step size during the simulation. It is guaranteed that at all times + * the current simulation time is in the closed interval between the start time + * and the end time. * - * You can loop over all of the time steps by using a for loop + * The model this class follows is that one sets a *desired* time step length + * either through the constructor or using set_desired_next_step_size() + * function. This step size will then be used in all following calls to the + * advance_time() function, but may be adjusted slightly towards the end of the + * simulation to ensure that the simulation time hits the end time exactly. The + * adjustment is useful for the following reasons: + * + * Let's say that you loop over all of the time steps by using a for loop * @code * for (DiscreteTime time(0., 1., 0.3); * time.get_current_time() != time.get_end_time(); @@ -36,13 +44,39 @@ DEAL_II_NAMESPACE_OPEN * // Insert simulation code here * } * @endcode + * In the above example the time starts at $T_{\text{start}} = 0$ until + * $T_{\text{end}}=1$. Assuming the time step $dt = 0.3$ is not modified inside + * the loop, the time is advanced from $t = 0$ to $t = 0.3$, $t = 0.6$, $t = + * 0.9$ and finally it reaches the end time at $t = 1.0$. Here, the final step + * size needs to be reduced from its desired value of 0.3 to $dt = 0.1$ in order + * to ensure that we finish the simulation exactly at the specified end time. In + * fact, you should assume that not only the last time step length may be + * adjusted, but also previously ones -- for example, this class may take the + * liberty to spread the decrease in time step size out over several time steps + * and increment time from $t=0$, to $0.3$, $0.6$, $0.8$, and finally + * $t=T_{\text{end}}=1$ to avoid too large a change in time step size from one + * step to another. + * + * The other situation in which the time step needs to be adjusted (this time to + * slightly larger values) is if a time increment falls just short of the final + * time. Imagine, for example, a similar situation as above, but with different + * end time: + * @code + * for (DiscreteTime time(0., 1.21, 0.3); + * time.get_current_time() != time.get_end_time(); + * time.advance_time()) + * { + * // Insert simulation code here + * } + * @endcode + * Here, the time step from $t=0.9$ to $t=1.2$ falls just short of the final + * time $T_{\text{end}}=1.21$. Instead of following up with a very small step of + * length $dt=0.01$, the class stretches the last time step (or last time steps) + * slightly to reach the desired end time. * - * In the above example the time starts at $t = 0$. Assuming the time step - * $dt = 0.3$ is not modified inside the loop, the time is advanced to - * $t = 0.3$, $t = 0.6$, $t = 0.9$ and finally it reaches the end time at - * $t = 1.0$. Note that the final step size is automatically reduced to - * $dt = 0.1$ in order to ensure that we finish the simulation exactly at - * the specified end time. + * The examples above make clear that the time step size given to this class is + * only a *desired* step size. You can query the actual time step size using the + * get_next_step_size() function. * * @author Reza Rastak, 2019 */ @@ -79,7 +113,11 @@ public: get_end_time() const; /** - * Return the size of the step from current time step to the next. + * Return the size of the step from current time step to the + * next. As discussed in the introduction to the class, this is the + * *actual* time step, and may differ from the *desired* time step + * set in the constructor or through the + * set_desired_next_step_size() function. */ double get_next_step_size() const;