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:
imensional Froude number represents the ratio of inertial forces to buoyant forces in the jet. 'G. N. Abramovich, "The Theory of Turbulent Jets," Mamachusetts Institute of Technology Press, Cambridge, Massachumetts, 1963. By definition, for Froude numbers greater than unity, inertial forces (i.e., jet momentum) dominate; for Froude numbers less than unity, buoyancy effects (i.e., density stability) are predominant. This principle has been corroborated by laboratory experiments, although field data3 show that buoyancy effects at the discharge can be significant for initial densimetric Froude numbers as large as about 3. Obviously, in order to maximize thermal dispersion by entrainment, the preferred approach would be to optimize plant engineering characteristics: increase discharge velocity and/or decrease the vertical dimension of the outfall (i.e., increase the densimetric Froude number). Regardless of initial conditions at the discharge port, jet velocity eventually decreases to a point where the local Froude number falls below one. When this happens, vertical mixing is suppressed, and the jet ceases to thicken. However, buoyancy also tends to induce lateral spreading due to the horizontal density gradient, in effect enhancing dilution while broadening the plume. With decreasing thermal plume temperature the normalized density difference, Ap/p, approaches zero, the Froude number increases above unity, and vertical mixing can become significant again. In the presence of an ambient current, the jet discharge and natural flow interact to cause the jet to be deflected towards the direction of natural flow. This cross-flow effect is important if the jet and current are initially perpendicular, becoming less so as the two flow directions approach coincidence. At some distance from the discharge point, the magnitude of which depends upon the ratio of discharge velocity to ambient velocity, the motion of the effluent completely follows that of the ambient current. If the