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:
mperature below the point of maximum water density. Should the effluent form a bottom layer, dispersion can be inhibited by bathymetry and bottom friction. b. Turbulent Diffusion With the decay of jet momentum, heated effluent increasingly becomes subject to external perturbations of the ambient water body. The influence of advective motions such as currents has been discussed in the preceding section, but of greater importance to heat dispersion is the effect of ambient turbulent diffusion. Essentially all water motions or transport processes in natural water bodies can be regarded as turbulent; that is, inertial forces in the water dominate over viscous iwr forces. The transformution from laminar flow to turbulent flow in a fluid Is defined by Reynold's criterion, Re = Pv_ JA (A-4) where v is fluid velocity, I represents a characteristic dimension (typically the water depth), and p is dynamic viscosity. The flow is turbulent if Re, the Reynolds number, exceeds some critical value, which in most natural water bodies proves to be small. Turbulent flow is irregular in time, and any dependent variable of that flow can be characterized by the sum of a mean component and an unsteady component. If the dependent variable happens to be temperature, T = T + T' (A-5) where T is the instantaneous temperature, 'T corresponds to the mean temperature, and T' is the unsteady component. Turbulence is manifested by eddy motions, the size of which may vary up to the characteristic dimension of the turbulent medium. Turbulent diffusion by eddies can be expressed as a variation in the unsteady component of the diffusing property. If the diffusing property is heat, s ds (A-6) where UTT represents the time-averaged product of the unsteady components of velocity and temperature, dTl/ds is the mean temperature gradient, and D, is the eddy diffusion coefficient. All quantities are defined relative to the s direction. The definition for eddy diffusion given in Eq. (A-6) is analogous. to