October 20~ , Hohai University Nanjing, China 1 VORTEX SHAFT OUTLET. Naples Italy,

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1 October 20~ , Hohai University Nanjing, China 1 VORTEX SHAFT OUTLET Giuseppe Del Giudice 1, Corrado Gisonni 2, Giacomo Rasulo 1 1 Department of Hydraulic and Environmental Engineering - University of Naples Federico II - Via Claudio 21, I Naples Italy, delgiudi@unina.it 2 Department of Civil Engineering - Seconda Università di Napoli - via Roma 29, I Aversa (CE) - Italy, corrado.gisonni@unina2.it Abstract Vortex drop shafts are widely used in practice to connect sewer mains characterized by large elevation difference. These structures conventionally include three key elements: intake structure, vertical shaft and outlet structure, also named dissipation chamber. The latter has not received much attention as compared to the first two parts, and only few experimental investigations are currently available from the literature (Viparelli, 1950; Kellenberger, 1988). Actually some rules of thumb are available as design criteria (ATV, 1998; Hager, 1999), but no systematic hydraulic investigation is available so far. The aim of the present study is to present preliminary results of an experimental campaign conducted at the Department of Hydraulic and Environmental Engineering, University of Naples, Italy. The physical model of a vortex drop shaft allowed the Authors to investigate the main hydraulic features of the dissipation chamber, in order to characterize the performance of various types of outlet structures. Key words Drop structure; Energy dissipation; Hydraulic design; Sewer hydraulics; Vortex shaft. 1 INTRODUCTION Sewage and stormwater drainage systems are a crucial infrastructure for the urban safety. The efficiency of the existing sewer systems is often affected by a combination of factors such as urban growth, enduring increase of impervious surfaces, structural aging, and improper design of sewer appurtenances. Drop structures are often necessary to connect shallow to deep sewers in steep areas; suitable drop manholes are widely used to break steep slopes in order to limit the flow velocity in the sewer pipes, normally below 5 m/s. Vortex drop shafts were introduced by Drioli (1947) as an overflow structure for dams, but currently these structures are mainly used in sewer systems. Figure 1 shows a typical sketch of a vortex drop shaft that is essentially constituted by three main parts: (i) intake or inlet structure, (ii) vertical shaft, and (iii) outlet structure. In addition, a sufficient air circulation has to be provided to prevent chocking phenomena and cavitation damage. Given the complexity of the flow features, the hydraulic behavior of the various parts has been mostly studied experimentally through physical models. General design criteria have been issued, based on experimental investigations and literature review (Hager, 1999), mainly for the intake structure and the vertical shaft. However, few experimental data are available for the outlet structure, essentially referred to specific case studies, so that the design of this portion is generally based on the overall features of existing vortex drops (Kellenberger, 1988).

2 2 16 th IAHR-APD & 3 rd IAHR-ISHS following dimensions St D M 4 Bt D Tt D M M 2 (1) Fig. 1 Vortex drop shaft: 3D view where S t, B t, and T t are the length, width and height Following to this evidence, two immediate questions arise: Is it possible to conceive a simple geometry for practical issues, and How to optimize the hydraulic behavior of the outlet structure in terms of both, energy dissipation and flow control toward the tailwater channel? The preliminary results of this research are therefore presented and have exploratory character. The Authors intended to highlight the interesting findings relating to air-water flows in a sewer system. The main purpose is presenting laboratory model observations and preliminary practical issues for the design of an outlet structure of vortex drops. 2 CURRENT DESIGN ISSUES The most important purposes of the outlet structure are to realize a transition between the upstream annular and the tailwater channel flows, to provide adequate energy dissipation, and to guarantee sufficient aeration/deaeration of the airwater flow. As already mentioned, the outlet structure has not received as much attention as the vortex intake and the drop shaft. A literature review indicates some design guidelines (Hager, 1999; Kellenberger, 1988), that are fundamentally based on the geometric features of existing structures. According to the current design criteria, the geometry of the outlet structure should have the of the outlet structure (Fig. 2), respectively, with D M as the larger size among the shaft diameter D s and the tailwater channel diameter D u. Fig. 2 Scheme of the outlet structure: a) streamwise, and b) transverse section, including (1) the aeration pipe and (2) the constriction element (adapted from Hager, 1999) The outlet chamber may optionally be equipped with constriction elements, such as baffles, sills, a weir or a Venturi flumes, in order to provide an adequate water cushion at the impact zone of the annular flow. Typically, the minimum distance between the constriction element and the shaft axis should be at least equal to 1.5D s. The present authors realized that experimental observations on a physical model can be of interest to the profession because of the costly structures, and their important effect on the sewer efficiency and urban safety. 3 EXPERIMENTS The tests were conducted on a physical model at the Laboratory of the Department of Hydraulic

October 20~23 2008, Hohai University Nanjing, China 3 and Environmental Engineering, University of Naples (Fig. 3). Fig.

3 October 20~ , Hohai University Nanjing, China 3 and Environmental Engineering, University of Naples (Fig. 3). Fig. 3 Experimental set up The experimental set-up, made of Plexiglas, consisted of the following elements, starting from upstream: an approach flow channel 0.14 m wide and 5 m long, a Drioli-type vortex intake, a vertical shaft 1.75 m long with a diameter equal to 0.10 m, a parallelepiped outlet structure 0.17 m wide, 0.50 m high, and 0.70 m long, and a tailwater channel 0.17 m wide, 0.20 m high, and 3.00 m long, with a slope equal to m/m. The outlet section of the tailwater channel was equipped with a vertical gate to control the downstream flow depth. According to the intake geometry, discharges ranged between 9 and 15 l/s, measured by means of a differential pressure discharge-meter, whose accuracy was 0.1 l/s. The main hydraulic features were measured and recorded for each run, mainly consisting in flow depths, air entrainment discharge and the location of flow singularities such as hydraulic jumps or shock waves. Basically, four different configurations were considered for the outlet structure, according to Figure 4: Type I: Basic configuration, without any special appurtenance installed in the chamber; Type II: Inclusion of a bottom bend at the beginning of the chamber (detail 1 in Fig. 4) whose radius of curvature is equal to D s. Its installation intended to improve the hydraulic features of the tailwater channel flow, mainly by dampening the free surface waves at the toe of the vertical shaft; Type III: A configuration similar to Type I, with the addition of a control section constituted by a contraction (detail 2 in Fig 4), whose width was such that critical flow established for the maximum discharge; Type IV: the configuration is similar to Type II, with the addition of a control section (detail 2 in Fig 4), as already considered for Type III. Fig. 4 Scheme of the tested outlet structure: a) streamwise, and b) transverse section, including (1) bottom bend and (2) the control section The hydraulic features of these four configurations were investigated to compare the various designs in terms of (i) energy dissipation induced by the outlet structure, and (ii) free surface oscillations of the flow directed toward the downstream tunnel. Furthermore, the air discharges were also preliminarily measured, but will not be further discussed. 3.1 TEST PROGRAM AND PROCEDURE Flow depths were systematically measured during the runs so that the free surface profiles were

4 4 16 th IAHR-APD & 3 rd IAHR-ISHS recorded along the walls of the outlet structure and along the tailwater channel. Five discharges were considered from 9 to 15 l/s ( maximum intake capacity), with a step equal to 1.5 l/s. For each discharge three different operating conditions were specifically considered: (a) unsubmerged tailwater channel, with a supercritical flow developing downstream of the outlet structure; (b) submerged tailwater channel, inducing subcritical flow with a hydraulic jump ending at the outlet section; (c) submerged outlet structure, with the annular vertical jet impinging on a water cushion, whose height is roughly equal to 0.9 D u. The recorded flow depths allowed the evaluation of the energy head at the inlet of the tailwater channel. Air velocities at the deaeration pipe were also measured using a micro-propeller and a hot-wire anemometer, whose accuracies were equal to 0.05 and 0.01 m/s, respectively. 3.2 TEST RESULTS Drop structures essentially intend to prevent the damage of the sewer structures by dissipating the surplus energy. According to the current knowledge, dropshafts are reported to be highly efficient in dissipating energy (Rajaratnam et al., 1997), generating energy losses between 75 and 95%. The vortex dropshaft is generally able to guarantee the larger energy dissipation rate, roughly equal to 90% (Zhao et al., 2006), as compared to plunging-type drop structures. The hereafter selected experimental results mainly aim to illustrate the different hydraulic features of the various outlet structure types I to IV. Figure 5 shows the hydraulic performance of the outlet structure types I to IV, with the unsubmerged tailwater channel, e.g. condition (a), corresponding to the maximum discharge (15 l/s). An analysis of the photos allows the following comments: Type I (Fig. 5a) induces a wavy flow within the sewer outlet structure, specifically characterized by asymmetrical shockwaves developing at the walls; more specifically the shockwave at the left wall was slightly larger than at the right wall. This effect is possibly due to the impact of the annular jet onto the manhole bottom. Type II (Fig. 5b) provokes a more regular flow toward the tailwater channel with two symmetric shockwaves developing at the walls, whose amplitude is comparable with that measured for Type I. Note that the maximum shock-wave height is located more downstream as compared to Type I. Type III (Fig. 5c) induces a hydraulic jump upstream of the constriction elements, that may contribute to create a water cushion at the toe of the vertical shaft. The maximum flow depth is generated by the swell caused by the flow impacting the constriction elements. Furthermore, the filling ratio of the tailwater channel is increased, as compared to Type I, due to the transition across critical flow. Type IV ( Fig. 5d) shows a flow pattern similar to Type II, with two symmetric shockwaves developing at the walls. Similarly to Type III, a hydraulic jump formed, with an impact region around the constriction elements, where a swell is formed whose height may be larger than the

October 20~23 2008, Hohai University Nanjing, China 5 upstream shockwaves.

is the drop height. 5 d the total energy head at the outlet section of the approach flow channel, upstream of the vortex intake.

Type III is seen to cause the largest energy dissipation, ranging from 94 to 92%, with decreasing Q *.

001 a water cushion developed, whose height was sufficiently large to generate additional head losses due to a plunge pool effect. a) 1.00? E/E o Type I Type II Type III 0.95 Type IV 0.90 b) Q * 0.

5 October 20~ , Hohai University Nanjing, China 5 upstream shockwaves. The energy dissipation induced by the outlet structure was also computed and related to the dimensionless discharge Q * Q (2) gh where Q is the discharge, g is the gravitational acceleration, and H d is the drop height. 5 d the total energy head at the outlet section of the approach flow channel, upstream of the vortex intake. Figure 6 shows the values of E/E o corresponding to the unsubmerged tailwater conditions for all four types of outlet structures. Type III is seen to cause the largest energy dissipation, ranging from 94 to 92%, with decreasing Q *. Types I, II and IV provided a slightly smaller energy dissipation with a minimum value of E/E o for Q * In fact, for Q * larger than a water cushion developed, whose height was sufficiently large to generate additional head losses due to a plunge pool effect. a) 1.00? E/E o Type I Type II Type III 0.95 Type IV 0.90 b) Q * Fig. 6 Dimensionless energy dissipation E/E o versus dimensionless discharge Q* for operating condition (a) (supercritical tailwater flow) c) d) Fig. 5 Flow patterns for supercritical tailwater flow: Type I (a), II (b), III (c), and IV (d). Flow direction from left to right The dimensionless energy dissipation E/E o was plotted as a function of the dimensionless discharge Q *, with E as the total head loss and E o In Figure 7 the values of E/E o are plotted as a function of Q *, for the submerged tailwater condition, with a hydraulic jump fully contained within the outlet structure, e.g. condition (b). For Types I and III, it was impossible to attain the condition (b) for the maximum discharge due to the large total momentum of the flow in the tailwater channel, whose backwater effect submerged completely the outlet structure. For both Types I and III the energy loss was essentially induced by flow impinging on the water cushion, so that the effect of the constriction elements was not significant in terms of additional head loss. Similarly, outlet structure Types II and IV do not indicate differences in terms of dimensionless

6 6 16 th IAHR-APD & 3 rd IAHR-ISHS energy dissipation E/E o. In addition it is wise to highlight the following comments: Types II and IV provoke smaller value of E/E o, as compared to Types I and III, due to the presence of the bottom bend which straightens the flow and reduces the head losses; A combination of the bottom bend and the constriction element (Type IV) allowed to attain the condition (b) also for the maximum discharge. 1.00? E/E o Type I Type II Type III dissipation chambers were considered and the main differences are illustrated, with specific reference to the energy dissipation mechanism. Further experimental results will be presented, with specific reference to the following items of the outlet structure: length and height of the chamber, optimum location of the constriction elements, aeration/deaeration features. Acknowledgements The present study was supported by the Italian Ministry of University and Research - PRIN 2005/07 (project n _001) Type IV REFERENCES 0.90 ATV (1998). Standards for the hydraulic dimensioning and performance verification of special structures in sewers and drains. Standard ATV-A 112. Hennef, Germany Fig. 7 Q * Dimensionless energy dissipation for operating condition (b) (subcritical tailwater flow) For the operating condition (c), e.g. submerged outlet structure, a water cushion formed, whose height was constant and independent of the type of the outlet structure. Consequently, the dimensionless energy dissipation E/E o was not significantly influenced by the type of the outlet structure, and roughly equal to 0.89, almost independently on the dimensionless discharge Q *. Furthermore, an inclusion of the bottom curvature (Types II and IV) generally contributed to regularize the free surface within the chamber, even if this effect was associated with a systematic reduction of the efficiency in terms of energy dissipation. Drioli, C. (1947). Su un particolare tipo di i mbocco per pozzi di scarico. L Energia Elettrica, 24(10), [in Italian]. Hager W.H. (1999). Wastewater Hydraulics - Theory and Practice. Spinger-Verlag. Berlin. Kellenberger, M.H. (1988). Wirbelfallschächte in der Kanalisationstechnik. Mitteilung 98. Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie. ETH- Zurich [in German]. Pica, M. (1970). Scaricatori a vortice. L Energia Elettrica, 47(4), 1-18 [in Italian]. Rajaratnam, N., Mainali A., and Hsung C. Y. (1997). Observations on flow in vertical dropshafts in urban drainage systems. Journal of Environmental Engineering, 123(5): Viparelli, M. (1950). Su un particolare tipo di imbocco e sull efflusso con vortice. L Energia Elettrica, 27(10), [in Italian]. 4 CONCLUSIONS Experimental results on a vortex dropshaft are presented, mainly focusing on the hydraulic behavior of the outlet structure. Four types of Zhao, C.H., Zhu, D.Z., Sun S.K., and Liu Z.P. (2006). Experimental study of flow in a vertical drop shaft. Journal of Hydraulic Engineering, 132(1):

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