Vízügyi Közlemények, 1935 (17. évfolyam)

Kivonatok, mellékletek - Kivonat a 4. számhoz

27 Knowing the water volume, and measuring the depths m

1 and m

2, the discharge coefficient к in equation 3 can be computed for every test. Substituting m

2 in turn with values measured at the ends, (he middle, and the thirds of the contracted channel, we get five values of к for every test. In order that the tests carried out with different water depths may be compared, the values of к are plotted in the term of the ratio m^nij. The five values cf к obtained from the same test, lie on a regular curve, but the position of these curves is dependent on the flow condition of the water. (Fig. о : streaming flow ; fig. 7 : shooting flew ; fig. 6 : transition between the two former.) With regard to matters of principle, m

2 may be measured anywhere in the contracted section, because, knowing the flow condition, the coefficient pertaining to a given ratio of m

2/m 1 may always be taken from the middleline of row of curves drawn in figures 5—7, and by this the water volume may be computed. It is, however, advisable to measure m

2 in the upper third of the contracted section, because above the contraction wrinkle formed in this place the water surface is quiet and smooth. When there is a shooting flow in the contracted section, m

2 is certain to drop to or below the critical depth. In this case it is not necessary to measure the depth m

2, and we can substitute into the equation any corresponding values of к and the ratio taken from figure 7. This means that the reading of the gauge (nij) placed upstream of the contracted section immediately gives the discharge in the function of nij. Dimensions can be chosen so that this case will nearly always present itself. e) The variation of the discharge coefficient is influenced above all by the distribution of velocities and the contraction. In his examinations the author used Prof. Camichel's chronophotographic method, but his observations are not yet completed (see photos 5—6 ; the method is described in the essay cited on page 709 below). Figure 8, a combination of figures 5—7, illustrates the dependence of the coefficient к on the flow condition. The sheaflike divergency of the results points to the fact that the flow condition changes gradually and not suddenly, as was supposed in point a). To characterize the flow condition Engel introduced Boussinesq's number (see references on pages 703 and 705 below). The author's results do not regularly agree with Boussinesq's number, therefore further research is needed to determine the exact discharge coefficient. f) Practical application. 1. If hydraulic jump is formed downstream of the contracted section, it is sufficient to measure only m

1, and equation 3 will take the simple form : Q = 0-556 I

, nij. 2. If there is no hydraulic jump, and the ratio computed with m

2 measured in the upper third, in

2/ni

, > 0-80, the value of к can be taken from figure 5. 3. If in^/n^c 0-87, this indicates the transition zone between the streaming and the shooting flow, and the value of к has to be taken from figure 6 (curve drawn with a thick line). But if 0-87 <m

2/m

1 <0-89, the depth in the lower third of the contracted section (m'

2) has also to be measured. If m'

2/m

1>0-91, the curve of figure 5, otherwise that of figure 6, has to be used. The diagram drawn in figure 9 serves for simplifying the computation. When the ratio between the experimental water volume and the discharge to be measured is known, this diagram, in consequence of hydraulic similarity, can be used for determining the dimensions of Venturi canals.