|
|
|
Some Comments on the Behavior of the RELAP5 Numerical Scheme at Very Small Time Steps
Volume 134 · Number 3 · March 2000 · Pages 306-311
Technical Note
Iztok Tiselj, Gregor Cerne
ANS Store: Purchase Article
The behavior of the RELAP5 code at very short time steps is described, i.e.,  t [approximately equal to] 0.01 x/c. First, the property of the RELAP5 code to trace acoustic waves with "almost" second-order accuracy is demonstrated. Quasi-second-order accuracy is usually achieved for acoustic waves at very short time steps but can never be achieved for the propagation of nonacoustic temperature and void fraction waves. While this feature may be beneficial for the simulations of fast transients describing pressure waves, it also has an adverse effect: The lack of numerical diffusion at very short time steps can cause typical second-order numerical oscillations near steep pressure jumps. This behavior explains why an automatic halving of the time step, which is used in RELAP5 when numerical difficulties are encountered, in some cases leads to the failure of the simulation.
Second, the integration of the stiff interphase exchange terms in RELAP5 is studied. For transients with flashing and/or rapid condensation as the main phenomena, results strongly depend on the time step used. Poor accuracy is achieved with "normal" time steps (t [approximately equal to] x/v) because of the very short characteristic timescale of the interphase mass and heat transfer sources. In such cases significantly different results are predicted with very short time steps because of the more accurate integration of the stiff interphase exchange terms.
|
|
|
|
