Vízügyi Közlemények, 1935 (17. évfolyam)

Kivonatok, mellékletek - Kivonat a 4. számhoz

26 ment of this principle to open canals is of recent origin, and several details are still unsolved. Keeping practical purposes in view, the writer has examined the downflow conditions in contracted profile of straight line shewn in figure 3, and on the basis of his expriments he essays to provide rules for choosing practical coefficients to be used for computing water volume in open canals. a) The influence of profile contraction on the form of water level may be examined by means of Koch's q-curve. If m indicates the depth, v the velocity, and H the height of the energy line, the water volume flowing through the prcfile of unity width is expressed by equation 1 (fig. 1). At the critical depth m = 2/з H the discharge is a maximum ; if m >

2/зН, the flow of water is streaming, and if m < 2/зН, it is shooting. In a channel contraction shown in figure 2, the water volume q

2 to be conducted through the unity width of profile 2, is necessarily greater than that (q j) flowing through profile 1. If the height of the energy-line is constant, in the case of streaming flow, when m

2 >

2/з H, a sinking of the water level in the channel contraction must ensue, while in profile 3 the original level is regained. If the contraction is considerable (fig. 2, below), the water is backeel up and the energyline rises (H'). In the contracted part a depth of m = 2/з H' ensues, then after a shooting flow along a short section the water returns into streaming flow by forming a hydraulic jump. In both cases the difference of the upper and lower water levels is smaller than in the case of a weir. For this reason, the method of profile contraction is especially applicable to constant registration of discharge in canals of slight slope, and another advantage is that it can also be used with dirty water. b) The water volume is computed by the aid e)f Bernoulli's theorem (equation 2 and 3, where F =area, 1 = width). The value of the coefficient к is dependent on the degree and form of the contraction, the flow condition of the water, and chiefly on the place where the depths nij and m

2 are measured. c) Tests for determining the coefficient к are carried out in the contracted channel shown in figure 3. In this figure the places where the depths m

1 and m

2 have been measured (the beginning, the midelle, the end, the upper and lower third of the contraction) are also indicated. The testing canal is 20 cm wide, 28 cm deep, and 4-80 m long ; the contraction is formed of glass (photo 2). The water volumes varying between 0-3—3-15 litres per second have been measured by a weir carefully rated (photo 1). d) In the course of the experiments the most varying downflow conelitions have been produced by employing different volumes of water and changing the height of the tail water. As far as the flow in the contracted channel is laminar, the surface of the water is absolutely smooth ; but at the lower end of the contraction, minute wrinkles and eddies indicate the beginning of turbulent flow (fig. 4/a). With greater velocity, a contraction wrinkle and the swinging (fig. 4/b), then waving (fig. 4/c) of the surface show that the flow has become turbulent. Photo 3 and figure 4/el are characteristic of the streaming flow : a contraction wrinkle reaching to '/з

—

]/2 the contracted channel and almost standing waves at regular distances. When the velocity is still greater, the lower end of the contraction wrinkle appears in the form of hydraulic jump. In this case the height of the tail water determines whether a covered water jump will be formed in the contracted channel, or after shooting flow in a longer section the water will be turned to streaming by a free jump (fig. 4/f, photos 2 and 4).