Accumulation term

Kushagra

Hi, I am not able to understand the meaning of following statements from the CFX-11 Solver Modeling guide Page 392

1) "For transient For transient runs, if convergence within a timestep is achieved in one coefficient iteration, this can lead to unexpected diagnostics for the accumulation term."

2) By default, the initial guess within a timestep is taken from the end of the previous timestep. For the First Order Backward Euler transient scheme, this leads to an accumulation term of zero, and for the Second Order Backward Euler scheme, the accumulation term has the opposite sign from expectation.

3) However, if the Second Order Backward Euler scheme is used with extrapolation as the initialization option, then the accumulation term is reasonable after one coefficient. Information on initialization setting is available.

I will be greateful to you if you can help me in understanding what do above statements mean?

Thanks so much in advance, Kushagra