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Publication Number: FHWA-HRT-07-036
Date: March 2007
Junction Loss Experiments: Laboratory Report
One concern when conducting small-scale experiments is the scaling issue. Comparing the old base run data to the smaller scale base runs confirmed that small-scale models can be used with reasonable confidence to evaluate and develop the proposed junction loss method. Small-scale tests are much more efficient and reduce many of the physical and geometrical constraints. This is the primary reason why the experiments were able to determine that Ki equals 0.43, Ko equals 0.16, and the coefficient in equation 6 should be equal to 1.0 (i.e., equation 7). These values are remarkably close to Kilgore's values of 0.4 for Ki and 0.2 for Ko. The difference in values produces only minor differences in energy loss for pipe velocities less than 3.05 m/s (10 ft/s). It should also be noted that Kilgore's coefficients slightly overestimate the energy level in the access hole, which makes his coefficients slightly more conservative than the lab-determined values.
This new and revised methodology addresses the problem of supercritical flows in outflow pipes. The use of inlet controlled culvert equations to estimate the initial depth in the access hole for these situations appears to work very well. Kilgore proposed a relatively simple equation to compute additional energy loss for plunging flows that accounts for the proportion of the flow that is plunging and the drop height. The experiments show that the new junction loss method is applicable for plunge-height ratios (i.e., plunge height divided by outlet pipe diameter) up to 10.
Characterizing the kinetic energy in the access hole remains the most rational procedure for estimating energy losses in access holes and distributing those losses among several inflow pipes. The two approaches involving PIV and 3–D numerical modeling to analyze the energy level in the access hole, however, proved too difficult due to the extremely chaotic flow inside the access hole. This was the primary reason that the research focused on the more organized flow in the contracted area of the outflow pipe. The area of maximum velocity near the contraction zone was successfully used as an indirect measure of the energy loss in the outflow pipe (an entrance loss), which was then used to backcalculate the energy loss in the inflow pipe. This procedure showed that the entrance and exit losses predicted by the new junction loss method are remarkably accurate.