Flow under a gate
When a vertical-lift gate in a conduit is opened and the downstream section of the conduit contains no water, a demand for air arises due to entrainment of air in the issuing jet.
The total air demand consists of two different parts: air entrainment in the water flow as bubbles or larger air pockets in the air/water transition region, and air flowing above the transition zone because of the drag of the flowing mixture. At initial gate openings, the issuing jet is accompanied by spray which entrains a high proportion of air.
To explain differences in air demand, flow has been classified.49The total air
demand for free surface flow in conduits is not normally at a maximum when gates are fully open. Often two maxima exist; one for very small gate openings, when spray flow occurs at 4^8% of gate opening, and a second one usually larger than the first when the gate opening is between 40^70%.
If a hydraulic jump occurs further air entrainment will take place. Kalinske and Robertson50expressed this in terms of the ratio of air flow to water flow. Air
entrainment without jumps has been investigated by a number of researchers.49,51
The suggested design assumption52is:
0:03 Frÿ 11:06 9:13
where ratio of air flow to water flow Fr Froude number V=p gy
V flow velocity at the vena contracta g gravitational constant
y water depth at the vena contracta
The contraction coefficient for a gate with a 45 lip is 0.8. The above formula results in significantly more conservative volumes of air than those arising from the investigation of Kalinske and Robertson.50
40 m/s to prevent excessive pressure loss due to flow resistance in the ducts as well as entrance and exit air flow losses. These cause subatmospheric pressure conditions in the water conduit.
Air flow losses can be calculated from data in the CIBSE Guide.53
Flow over a gate
The requirement to vent the nappe of an overflow gate was stated in Chapter 2. Air is entrained in the falling water and reduces the pressure under the gate unless it is vented. The subatmospheric conditions resulting from air evacuation cause the nappe to oscillate and the water level to fluctuate. This can lead to severe gate vibration.
Venting is effected by flow dividers which break up the nappe locally and admit air through the openings created by divided flow. Ducts leading from pier level to the underside of a gate (Fig. 2.27) are another method of venting. In most cases both means of admitting air have to be used together.
In the extreme case pressure under the gate can reach ÿ10.33 m gauge. This would exert a negative pressure of 10.33 Mpa/m2on the underside of the gate.
Air entrainment is proportional to the velocity head of the nappe. The air demand can be expressed as:
QA Q
where QA air demand
Q flow over the gate
a coefficient depending on height of fall of the nappe, h4,
depth of the nappe, h3, and the Froude number Fr of the
nappe
The depth of the nappe is approximately 0.6h2, where h2is the actual head
above the gate lip. Therefore: h3 0:6 h1ÿv
2 1
2g
where v1is the velocity of the approach flow to the gate and h1is the energy head
above the lip of the gate.
The Froude number of the nappe is: Fr v2= gh3
where v2is the velocity of the overflow.
Fig. 9.21 shows the coefficient of air demand against the ratio h4/h3for two
ranges of Froude numbers.
The air ducts can be sized in a similar manner to those required to satisfy the air demand for underflow gates. Since ducts for overflow gates are usually short compared with those for underflow gates, the duct entry and exit losses will be more significant when the friction loss of the air supply system is calculated. They should therefore be considered.
Hydraulic considerations pertaining to gates
The ducts should be arranged so that the air supply in any position of overflow of the gate is not blocked by the downstream water level, and that the air is admitted under the gate. In order to achieve this, the outlets of the air supply ducts are staggered as shown in Fig. 2.27. It may even be necessary to stagger the termination of the air supply pipes relative to one another in opposite sluiceway walls. It is usual practice to screen the outlets of the vent ducts. The screens must be set back from the face of the sluiceway so that they do not damage the side seal of the gate when it moves over the duct outlets.
References
1. Rouse, H (1949) editor: Engineering hydraulics, Proc. 4th Hydr. Conference, Iowa Institute of Hydraulic Research, John Wiley, p. 540.
2. Metzler, D E (1948): A model study of tainter gate operation, MS thesis, State University of Iowa, in Proc. 4th Hydr. Conference, Iowa Institute of Hydraulic Research, editor Rouse, H.
3. Lewin, J (1980): Hydraulic gates, Journ. I.W.E.S., 34, No. 3, p. 237. 4. Chow, V T (1959): Open channel hydraulics, McGraw-Hill.
5. Franke, P G; Valentin, F (1969): The determination of discharge below gates in case of variable tailwater conditions, Journ. Hydr. Res., 7, No. 4.
6. Young, L R; Fellerman, L (1971): Toomesluices calibrationtests, B.H.R.A., report RR 1105, Jul.
7. Buyalski, C P (1983): Canal radial gate discharge, algorithms and their use, Proc. Speciality Conf. on Advances in Irrigation and Drainage: Surviving external pressures, Jackson, USA, July, editors Borelli, J; Hasfurther, V R; Burman, R D, New York, USA, A.S.C.E., pp. 538^545.
8. Lewin, J (1983): Vibration of hydraulic gates, Journ. I.W.E.S., 37, 165^182. Figure 9.21. Coefficient of air
9. Vrijer, A (1979): Stability of vertically movable gates, 19th I.A.H.R. Congress, Karlsruhe, paper C5.
10. US Army Corps of Engineers: Tainter gates in open channels discharge coefficients (free flow), Hydraulic Design Criteria, sheets 3204 to 3207.
11. Toch, A (1952): Theeffectofalipangleuponflowunderataintergate,Masters thesis, State University of Iowa, Feb.
12. Gentilini, L B (1947): Flow under inclined or radial sluice gates ^ technical and experimental results, La Houille Blanche, 2, p.145.
13. US Army Corps of Engineers: Tainter gates in open channels ^ discharge coefficients (submerged flow), hydraulic design criteria, sheets 320-8 to 320-8/1.
14. US Army Corps of Engineers: Tainter gates on spillway crests ^ discharge coefficients, Hydraulic design criteria, sheets 311-1 to 311-5.
15. US Army Corps of Engineers: Taintergates onspillwaycrests^crestpressures, hydraulic design criteria, sheets 311-6.
16. Milan, D; Habraken, P (1984): Kotmale, report on spillway radial gate, model tests, Neyrpic Thermohydraulic and Hydroelasticity Laboratory. (unpublished). 17. US Army Corps of Engineers: Gated overflow spillways, pier contraction coefficients,
hydraulic design criteria, sheets 111-5, 111-6.
18. Naudascher, E; Locher, F A (1974): Flow-induced forces on protruding walls, Proc. A.S.C.E., Journ. Hydr. Div., Vol. 100, HY2, paper 10347, Feb.
19. White, F M (1979): Fluid mechanics, McGraw-Hill, New York, USA.
20. Muskatirovic, J (1984): Analysis of dynamic pressures acting on overflow gates, I.A.H.R. Symposium on scale effects in modelling hydraulic structures, Essingen am Neckar, Germany, Sept.
21. Rogala, R; Winter, J (1985): Hydrodynamic pressures acting upon hinged-arc gates, Proc. A.S.C.E., Journ. Hydr. Engineering, 111, No. 4, Apr.
22. Pethick, R W; Harrison, A J H (1981): The theoretical treatment of the hydraulics of rectangular flap gates, 19th I.A.H.R. Congress, Karlsruhe, subject B (c), paper 12. 23. Naudascher, E; Rao, P V; Richter, A; Vargas, P; Wonik, G (1986): Prediction and control of downpull on tunnel gates, Proc. A.S.C.E., Journ. Hydr. Engineering, 112, No. 5, May.
24. Weaver, D S; Martin, W W (1980): Hydraulic model study for the design of the Wreck Cove control gates, Canadian Journ. of Civ. Eng., 7 No. 2.
25. Thang, N D; Naudascher, E (1983): Approach-flow effects on downpull of gates, Proc. A.S.C.E., Journ. Hydr. Eng., 109, No. 11, Nov.
26. Kolkman, P A (1984): Phenomena of self excitation, in Developments in hydraulic engineering ^2, editor Novak, P, Elsevier Applied Science Publishers.
27. Hite J E; Pickering, G A (1983): Barkley Dam spillway tainter gate and emergency bulkheads, Cumberland River, Kentucky; hydraulic model investigation, US Army Engineer Waterways Experiment Station, Vicksburg, Miss., technical report HL-83-12, Aug.
28. Grace, J L (1964): Spillway for typical low-level navigation dam, Arkansas River, Arkansas; hydraulic model investigation, US Army Engineer Waterways Experiment Station, Vicksburg, Miss., technical report 2-655, Sept.
29. Daggett, L L; Daggett K G H (1974): Similitude in free-surface vortex formations, Proc. A.S.C.E., Journ. Hydr. Div., 100, Nov.
30. Anwar, H O; Weller, J A; Amphlett, M B (1978): Similarity of free-vortex at horizontal intake, Journ. Hydr. Research, 16, No. 2.
31. Gulliver, J S; Rindels, A J; Lindblom, K C (1986): Designing Intakes to Avoid Free-Surface Vortices, Water Power and Dam Construction, Sept, pp. 24^28.
32. Gordon, J L (1970): Vortices at intakes, Water Power, Apr.
Hydraulic considerations pertaining to gates
Journ. Constr. Div., 89, (CO2).
39. Dickson, R S; Murley, K A (1983): Dartmouth Dam low level outlet aeration ramps, Ancold Magazine.
40. Anastassi, G (1983): Besondere Aspekte der Gestaltung von Grundablassen in Stollen (Design of high-pressure tunnel outlets), Wasserwirtschaft, 73, 12.
41. Ball, J W (1959): Hydraulic characteristics of gate slots, Proc.A.S.C.E.,Journ.Hydr. Div., 85, HY10, 81^114.
42. Galperin, R (1971): Hydraulic structures operation under cavitation conditions, 14th I.A.H.R. Congress, Paris, Vol. 5, pp.45^48.
43. May, R W P (1987): Cavitationinhydraulicstructures:occurrenceandprevention, Hydraulic Research, Wallingford, report SR79.
44. Koch, H J (1982): SchuÞstrahlzusammenfÏhrung bei einem Grundablass mit Nemeneinanderliegenden Segmentschutzen (Confluence of two jets created by two parallel segment gates of a bottom outlet), Wasserwirtschaft, 72, 3.
45. Bruce, B A; Crow, D A (1984): Mrica Hydroelectric Project: hydraulic model study of the culvert control structure, B.H.R.A., report RR2325.
46. Nielson, F M; Pickett, E B (1979): Corps of Engineers experiences with flow- induced vibrations, 19th I.A.H.R. Congress, Karlsruhe, paper C3.
47. Petrikat, K (1979): Seal vibration, 19th I.A.H.R. Congress, Karlsruhe, paper C14. 48. Naudascher, E (1972): Entwurfskriterien fÏr Schwingungssichers Talsperren-
verschlÏsse (Design criteria for avoiding vibration of high head gates), Wasserwirtschaft 62, 112.
49. Sharma, H R (1973): Airdemandforhighheadgated conduits, University of Trondheim, Oct.
50. Kalinske, F; Robertson, R A (1943): Closed conduit flow: Symposium on entrainment of air in flowing water, A.S.C.E., Transactions, paper 2205.
51. Wunderlich, W (1961): Beitrag zur BelÏftung des Abflusses in TiefauslÌssen (Commentary on air demand in conduit gates), Technische Hochschule, Karlsruhe. 52. US Army Corps of Engineers: Air demand, regulated outlet works, Hydraulic Design
Criteria, Sheet 050-1.
53. Chartered Institute of Building Services Engineers: Guide,C4-48 and C4-49, Figure C4.3 Air flow in round ducts, Figure C4.4 Air flow in rectangular ducts.