Abstract
In this chapter, we consider the full 1D TFM of RELAP5 for bubbly vertical flows and assess its linear stability behavior and material wave propagation capabilities in light of the linear stability analyses of Chap. 5, i.e., the characteristics and the dispersion relation. The incomplete virtual mass implementation is the key to the model’s void propagation velocity fidelity and regularization, i.e., hyperbolization. We also analyze the numerical convergence.
RELAP5/MOD3.3 (Information Systems Laboratories, RELAP5/MOD3.3 code manual, Vol. 1: Code structure, system models, and solution methods, 2003) is a well-known TFM nuclear reactor safety code used for the analysis of Loss of Coolant Accidents (LOCA) and is representative of other codes used by industry. A linear stability assessment of the RELAP5 code for vertical bubbly flow demonstrates that the RELAP5 TFM is almost unconditionally hyperbolic, i.e., locally stable, because of artificial regularization by a simplified virtual mass force. In spite of this artificial device, a comparison with experimental data shows that the TFM preserves the capability to model the kinematic wave speed correctly. This is a necessary condition for the prediction of the global instabilities addressed in Chaps. 6 and 7.
In industrial practice the KH instability is removed by artificial correlations and numerical viscosity, but a filter may be used instead. A low pass filter, which has a precise cutoff wavelength, is proposed to replace numerical FOU regularization. It offers two advantages with respect to FOU; it is not mesh dependent and it allows finer nodalizations so that numerical convergence may be tested under all circumstances. In addition, higher order numerical schemes may be easier to implement.
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Notes
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One could also devise an artificial regularization method utilizing third-order derivatives, i.e., artificial surface tension, or higher derivatives.
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de Bertodano, M.L., Fullmer, W., Clausse, A., Ransom, V.H. (2017). RELAP5 Two-Fluid Model. In: Two-Fluid Model Stability, Simulation and Chaos. Springer, Cham. https://doi.org/10.1007/978-3-319-44968-5_8
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DOI: https://doi.org/10.1007/978-3-319-44968-5_8
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