Document: NRC Regulatory Guide
Document ID: f4c5fb1d-efb9-4168-9804-5ad3f6f64d06
Document Type: regulatory_guide
Title: Reporting Procedure for Mathematical Models Selected To Predict Heated Effluent Dispersion in Natural Water Bodies
Source: NRC Regulatory Guide Division 4
Source URL: https://www.nrc.gov/docs/ML0037/ML003739535.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-4.4
CFR Part: 
CFR Title: 

Content:
esign parameters, and the flow field and gpometry of the receiving waters. For highly simplfied gpometo' and flow conditions, analytical solutions am polble in some instances. In general, however, mom realistic model require solution by numerical methods. The more common simplifying assmptions are enumerated and dicussed briefly below. a. Eddy CoefI.ients The velocity correlation teom on the right-hand side of Eqs. (M 1) through (3-13) any be (B-IS) The classical approximation is to relate the turbulent mofsentum flux in Eq. (3IS) to aldignts of the mean velocity field, giving rise to the concept of eddy viscosity coefficients. Theme may be introduced through expressions of the form R -PUUj = A(i)5 + A(j) an' (B16) where A(i) and A(j) are interpreted as the lateral eddy viscosity coefficient Ah for j * 3, and the vertical eddy viscosity coefficient A, for j = 3. The anisotropy of turbulent diffusion, discussed in Appendix A, leads to the use of separate coefficients for the horizontal and vertical directions. Similarly, in Eq. (B-14) it is customary to relate the turbulent diffusion of heat energy to gradients of the mean temperature, giving rise to eddy diffusion coefficients Di for heat: aT ujT :DjF (B-17) The turbulent momentum and heat transports shown above are much larger than their molecular counterparts, and the terms representing the latter may be dropped from Eqs. (W-I I) through (B- 14). If the above approximations are uted, the velocity and temperature appear explicitly in the conservation equations only as mean valies., The effects of turbulence are concealed in the eddy coefficients A, A4, D1. This simplification is to a certain extent illusory since the eddy coefficients are functions of the stochastic part of the motion and cannot be determined pdoif. The usefulness of any predictive model using eddy coefficients is severely limited by the reliability with which the magnitudes and spatial variations of thee parametes can be determined beforehand. b.