
For transient simulations, the governing equations must be discretized in both space and time. The spatial discretization for the timedependent equations is identical to the steadystate case. Temporal discretization involves the integration of every term in the differential equations over a time step . The integration of the transient terms is straightforward, as shown below.
A generic expression for the time evolution of a variable
is given by
where the function incorporates any spatial discretization. If the time derivative is discretized using backward differences, the firstorder accurate temporal discretization is given by
(18.316) 
and the secondorder discretization is given by
where
=  a scalar quantity  
=  value at the next time level,  
=  value at the current time level,  
=  value at the previous time level, 
Once the time derivative has been discretized, a choice remains for evaluating : in particular, which time level values of should be used in evaluating ?
Implicit Time Integration
One method is to evaluate at the future time level:
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This is referred to as "implicit'' integration since in a given cell is related to in neighboring cells through :
(18.319) 
This implicit equation can be solved iteratively at each time level before moving to the next time step.
The advantage of the fully implicit scheme is that it is unconditionally stable with respect to time step size.
Explicit Time Integration
A second method is available when the densitybased explicit solver is used. This method evaluates at the current time level:
(18.320) 
and is referred to as "explicit'' integration since can be expressed explicitly in terms of the existing solution values, :
(18.321) 
Here, the time step is restricted to the stability limit of the underlying solver (i.e., a time step is limited by the CourantFriedrichLewy condition). In order to be timeaccurate, all cells in the domain must use the same time step. For stability, this time step must be the minimum of all the local time steps in the domain. This method is also referred to as "global time stepping''.
The use of explicit time stepping is fairly restrictive. It is used primarily to capture the transient behavior of moving waves, such as shocks, because it is more accurate and less expensive than the implicit time stepping methods in such cases. You cannot use explicit time stepping in the following cases: