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:
he 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. Skady Stab For analytical simplicity, thermal dispersion modes often a= em that the velocity and temperature fields are in steady ate, i.e., that = -= a 1-t =0 The validity of this assumption depends upon a careful asement of the relative magnitudes of the averaging period, the important time scales of variability in the velocity and temperature fields, and the time 4.4-12 period over which the model is intended to apply. For cxample, the assumption of a steady state clearly cannot be made for a model intended to describe the behavior of a thermal plume in a tidal river or estuary if the averaging period is short compared with the tidal period. If, on the other hand, the average is taken over several tidal cycles, and if the time period of interest is less than the scale of seasonal fluctuations in fresh water flow (river flow), a steady state assumption is reasonable. It is important to realize, however, that in these two examples, the magnitudes of the eddy coefficients differ greatly. For a longer averaging period, larger scales of motion are included in the turbulence, and eddy coefficients are larger. c. Hydrostic Approximatlon In a motionless fluid, the vertical equation of motion reduces to the hydrostatic equation expressing an exact balance between the vertical pressure gradient and the acceletation of gravity, I a g" where p_ is a function of x3 and is the density that would exist in the absence of motion. Whether or not this equation is valid as an approximation to a real flow system depends on the velocity, space, and time scales of concern in the vertical momentum equation. A simple scale analysis indicates that the hydrostatic assumption is generally a valid approximation except possibly in the treatment of high-velocity discharges, in which cae the