Hydraulics of Long Vertical Conduits and Associated Cavitation

Experimental studies have been undertaken to examine the flow in long vertical conduits with particular reference to the design of storm water drop shafts. A distinguishing characteristic of such flow is the potential cavitation regime. Its existence depends upon the design of the structure. The cavitation regime will develop when the conduit is sufficiently long and the head sufficiently large. It can also be generated at a lower head if a control valve is installed in the supply line so that the net head can be negative. The cavitation region consists of a rather finely divided mixture of water and water vapor at a constant cavitation pressure of about -32.0 ft of water throughout the region and for all discharges. The cavitation region terminates with a shock front whose location is also a function of the discharge. The concentration of vapor, while relatively constant throughout the cavitation region, decreases with increasing discharge. If a small amount of air is introduced into the system, the cavitation region is eliminated, the pressure gradient is more uniform, and the flow consists of a uniform mixture of air and water. This report was submitted in fulfillment of Project Number 11034 FLU, Contract EPA 14-12-861, under the sponsorship of the Water Quality Office, Environmental Protection Agency.