Vertical Drop Shaft Research Articles

Numerical study on the effect of tangential intake design and inflow discharge on vertical dropshaft assessment using pressure and velocity distributions

Tangential intake structures with vertical dropshafts are commonly employed in stormwater and urban drainage systems. Vortex and plunging flows are the two typical flow patterns observed in dropshafts, with the former being the desired due to its favourable drainage performance and enhanced safety. The transition between these flow patterns is primarily influenced by the structural design and rate of flow discharge. To investigate the interception performance and the transition criteria between vortex and plunging flows, a series of numerical simulations were conducted. The effects of the approach channel slope, contraction ratio, and flow discharge were analyzed by comparing the evolution of the water surface, pressure and velocity distributions, and swirling distance across various scenarios. With a higher flow rate of 20 L/s, obvious water congestion was observed in dropshafts with contraction ratios below 0.4 and horizontal approach channels. However, the inflow channel was not congested in cases with sloping approach channels, indicating higher drainage efficiency of intake structures with slopping channels. Two hydraulic factors, namely, the minimum to maximum wall pressure ratios (Pmin/Pmax) and the minimum Vr/Vz ratios (Vr/Vz)min, were employed to differentiate between vortex flow and nonrotational flow. These two factors consistently led to the same conclusions under both subcritical and supercritical inflow conditions, indicating their validity and their potential application in the design and optimization of intake structures. The criteria of Pmin/Pmax > 0.2 and (Vr/Vz)min > 0.2 can be used as criteria for the formation of stable vortex flow.

Air Pocket Expulsion from a Submerged Dropshaft with Inflow

Entrapped air pockets reduce flood conveying capacity of a surcharged drainage tunnel, and could be released explosively as geysers from vertical dropshafts. Nevertheless, the impact of dropshaft inflow on air pocket expulsion has hitherto not been fully understood. In this work, experiments was performed on a simplified drainage system consisting of a dropshaft with inflow. An air pocket was introduced into the tunnel; its dynamics in the dropshaft was measured with a high speed camera and pressure transducers. A heuristic analysis on the kinematics of the air pocket showed that when the dropshaft inflow is greater than a critical flow dependent on the dropshaft diameter, the increased compression leads to stronger air pocket expulsion, as spewing and/or ejection of air-water mixture. Observed occurrence of the phenomena generally agrees with the prediction.

Analytical solutions for vortex flow at the tangential inlet of a vertical dropshaft

Vertical dropshafts with tangential intake structures are often used in urban drainage systems to route surface flow into underground systems. Vortex flow driven by a tangential inlet is the typical flow pattern in the vertical dropshaft. However, analytical solutions of the key hydraulic transition from chute flow to vortex flow have not been investigated, and theoretical guidelines of the vortex dropshaft are not available. In the present study, a series of theoretical analytical solutions are proposed to determine the circumferential flow generated at the vertical dropshaft inlet. The contraction ratio of the tapering chute, dropshaft size, and flow depth at the joint section determines the vortex flow performance in the vertical dropshaft. Based on the free surface drop height, theoretical criteria are proposed to describe the spiral movement intensity in the vertical dropshaft. Systemic experiments for different vortex dropshaft models are tested, and the experimental observations agree with the theoretical analysis. The present investigation provides a general reference for a tangential dropshaft inlet design that can smoothly route rotational flow down a vertical dropshaft.

Flow Regimes of a Surcharged Plunging Dropshaft-Tunnel System

AbstractIn drainage and sewerage systems, vertical dropshafts are commonly used to convey water flow from high to low elevations. When the underground tunnel is surcharged, the lower part of the sh...

Air entrainment in a vertical dropshaft with limited air supply

Dropshafts are vertical structures widely used in urban drainage systems and buildings for water transportation. In this paper, a physical model study was conducted to investigate the air entrainment in the dropshaft under various flow regimes with and without air ventilation. Observed from the experiments, the air entrainment mechanisms varied with the water flow regimes in the dropshaft. When there was no water plug formed in the dropshaft, air could be supplied directly from downstream. Once the water plug was formed, while without venting, the air was replenished only from downstream intermittently and then in the form of large air bubble traveling upwards to the airspace at the top; while with venting, air was mainly replenished from the dropshaft top and no large air bubble was observed. The experimental results also showed that the amount of entrained air in the dropshaft with venting was greater than that without venting.

Spiraling Downhill: The Study and Design of a Vertical Drop Shaft Complete with Vortex Units and Replacement of an Existing Combined Sewer Interceptor

Spiraling Downhill: The Study and Design of a Vertical Drop Shaft Complete with Vortex Units and Replacement of an Existing Combined Sewer Interceptor

Transitional and Weir Flow in a Vented Drop Shaft with a Sharp-Edged Intake

AbstractAn experimental analysis of a vertical drop shaft with a sharp-edged horizontal intake section and a coaxial venting pipe is undertaken to obtain design criteria given the limited attention provided by literature. Three different flow regimes are observed, namely weir flow and pressurized flow for extreme discharges and transitional flow for intermediate discharges. Particular attention is paid to transitional flow, which is a periodical regime consisting of an alternate switch from weir to pressurized flow, with water heads ranging between a maximum and a minimum. For transitional flow a detailed characterization of water surface oscillations is provided, and nondimensional equations are given describing mean water head, oscillation amplitude and frequency as functions of pipe Froude number and dimensionless plunged length of the coaxial pipe. Also, a region of existence is provided in terms of pipe Froude number and different wave types are identified. For weir flow, a nondimensional head-discha...

Flow regimes in a vertical drop shaft with a sharp-edged intake

The results of an experimental analysis are provided concerning the hydraulic behaviour of a vertical drop shaft with a sharp-edged circular intake, also taking into account a peculiar venting system consisting of a coaxial vertical pipe, projecting within the drop shaft with different plunging lengths. Three different flow regimes are experienced: a “weir flow” for low head values, where an air–water mixture at the atmospheric pressure can be observed; a “pressurized flow” for high head values, where water flows in a pressurized regime along the shaft; a “transitional flow” for intermediate water head values, consisting of a continuous, spontaneous switch between weir flow and pressurized flow with consequent water head variation for a fixed discharge. Pressurized flow regime is analytically explained, so that a head–discharge relation can be provided both for the non-vented and for the vented configurations, with specific assumptions about intake losses.

Choking conditions inside plunging flow dropshafts

Severe rain events can subject our existing drainage systems to capacity. While the conveyance capacity of sewers can be increased by temporarily tolerating a full pipe flow condition, the complex dynamics of water–air mixture occurring in vertical dropshafts can largely restrict the overall conveyance capacity of drainage systems. In this study, the hydraulic performance of plunging flow dropshafts is investigated experimentally with respect to their conveyance capacity. The model dropshaft consisted of an upstream horizontal inflow pipe, a vertical circular shaft, and an outflow pipe discharging to the atmosphere. Five setups were built with Ds/Difrom 1.0 to 3.0 for the range of drop heights H/Ds= 3.0 to 21.0, where Diis the inlet pipe diameter, Dsthe shaft diameter, and H the drop height. A wide range of discharges up to Q* = 25 was tested, where Q* = Q/(gDi5)(1/2). Flow patterns recognized within the shaft (free flow, surface roller, plug flow, slug flow, and full pipe flow) help to assess the two stages of choking: the incipient and fully choked state. Criteria to predict choking occurrences were developed experimentally. The flow is considered to reach its conveyance capacity when an incipient choking state is visible. For a typical dropshaft with Ds/Di= 2, Q* was found to be approximately 5.

Experimental Analysis of a Vertical Drop Shaft

An experimental campaign is undertaken in order to investigate the hydraulic features of a vertical drop shaft, also considering the influence of a venting system consisting of a coaxial vertical pipe, projecting within the drop shaft with different plunging rates. Three different flow regimes are observed: a “weir flow” for very low head values, where the flow profile is subject to the atmospheric pressure; a “full flow” for high head values, where water flows in a pressurized regime along the whole shaft; and a “transitional flow” for intermediate water head values. Weir flow and full flow can be experimentally investigated under steady-state conditions, whereas transitional flow is a pulsating condition, alternately switching from full flow to weir flow. Considering some significant geometric parameters, a head-discharge relation is sought both for the non-vented and for the vented configurations, by means of an energy balance equation, with specific assumptions about intake losses.

Theoretical and Experimental Studies of Hydraulic Characteristics of Discharge Tunnel with Vortex Drop

The velocities at given points in the volute chamber, the contracted section and the vertical dropshaft of a discharge tunnel with vortex drop were measured by a small specially designed L-shaped tube, as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) would not work there due to the special structure of the discharge tunnel with vortex drop. Hydraulic empirical formulas were proposed to predict the velocities and the angles of the velocities made with the vertical direction θ. The theoretical analysis results were in good agreement with experimental data. Therefore, the method proposed in this paper can be used to analyze related characteristics of discharge tunnels with vortex drop. Additionally, different model scales were considered to predict the cavitation characteristics on the wall of a dropshaft in practical engineering.

HYDRAULIC DESIGN ASPECTS FOR SWIRLING FLOW AT VERTICAL DROP SHAFT SPILLWAYS

ABSTRACT Vertical drop shaft spillways are generally provided on either banks of the reservoir. It is a hydraulically efficient way to convey water as a swirling flow down vertical drop shaft. The flow adheres to the shaft wall along its entire length and wall friction is effective in reducing the vertical fall of velocities. Generally, vortex drops are provided for sewers. However, in the recent times it is increasingly adopted for heights varying from a few metres up to 200 m and the total discharge of the order of 2000 cumec. Extensive aeration/de-aeration arrangements would be necessary along the vertical shaft as well as at the junction of vertical shaft and horizontal tunnel so that air-bursting phenomenon could be avoided. This paper discusses the hydraulics of the vortex drop shaft spillway and also the experience gained while evolving the hydraulic design including a horizontal air groove in the shaft.

Experimental Study of Flow in a Vortex Drop Shaft

Model experiments were conducted to investigate the performance of a vortex drop structure with a relatively small height to diameter ratio. Detailed measurements of wall pressure and water thickness of annular jet flow were obtained along the vertical drop shaft, and the rate of air entrainment was measured. The results confirmed the high efficiency of energy dissipation in the vortex drop structure even for a relatively small drop height. The air entrainment rate was found to be significant, and good correlation was observed between the rate of air entrainment and the water jet velocity. The one-dimensional frictional free-vortex flow model was extended to include the effects of pressure forces. While the energy loss in the drop shaft can be simulated by correcting the friction factor, both the frictional model and the extended model significantly underpredict the wall pressure.

An experimental study of Roman dropshaft hydraulics

In Roman aqueducts, series of vertical dropshafts were used to dissipate the kinetic energy of the flow: i.e., the dropshaft cascades. A re-analysis of Roman dropshaft hydraulics is conducted with physical model tests. Three basic flow patterns are observed. The results demonstrate that the vertical dropshafts could be very efficient energy dissipators and re-oxygenation structures, under appropriate flow conditions. The optimum operation of Roman dropshaft is discussed and an analytical model is developed to predict these conditions. Further the performances of aqueduct dropshafts are compared with modern dropshaft designs, and the operation of dropshaft cascades is discussed. Additional material is available upon request.

Hydraulic design of intake structures of deeply located underground tunnel systems

This paper deals with hydraulic effects of intake structures of underground storage or tunnel systems from the aspect of energy dissipation and entrained air control. First, through the literature reviews, the advantage of vortex-flow dropshaft type is confirmed. Next, with the analytical solutions of free-surface swirling flows in vertical dropshaft, the energy dissipation effects of vortex-flow dropshaft are examined and some results of practical use are obtained. Also, the relation between the dropshaft shape and air entrainment is studied and the excellent of the helicoidal-ramp dropshaft is clarified. Based on the above results, the directions of the hydraulic designs of drop structures are proposed.

Hydraulic design of intake structures of deeply located underground tunnel systems

This paper deals with hydraulic effects of intake structures of underground Storage or tunnel systems from the aspect of energy dissipation and entrained air control. First, through the literature reviews, the advantage of vortex-flow dropshaft type is confirmed. Next, with the analytical solutions of free-surface swirling flows in vertical dropshaft, the energy dissipation effects of vortex-flow dropshaft are examined and some resulta of practical use are obtained. AIbo, the relation between the dropshaft shape and air entrainment is studied and the excellence of the helicoidal-ramp dropshaft is clarified. Based on the above results, thte directions of the hydraulic designs of drop structures are proposed.

Observations on Flow in Vertical Dropshafts in Urban Drainage Systems

Exploratory physical-model studies were made of the hydraulics of stormwater dropshafts in the city of Edmonton. Under certain conditions, it is necessary to provide a curved inlet at the top of the dropshaft to increase its carrying capacity. Except for very small discharges, the falling water in the dropshaft does not form a coherent central jet but takes the form of a complex jet with a spreading flow around the periphery of the dropshaft. Such a flow also forms a pool at the bottom of the shaft. The energy loss in the dropshaft is significant. For the case studied, with the diameter of the shaft (Ds) equal to about twice the diameter (D) of the inflow sewer and a fall of 6.6Ds, and over a wide range of flow rates, the relative energy loss in the dropshaft was about 80–95%. Further, the air flow Qa required by the falling water, relative to the water flow rate Q, decreased from about 1.4 to 0.5 as Q* increased from 0.1 to about 1.0. Q*=Q/(gD5).Q= water discharge and g= acceleration due to gravity.

On hydraulic property of vortex flow dropshaft

Hydraulic characteristics of vertical dropshaft with tangential vortex-inlet are treated. First, an analytical solution of a freesurface swirling flows in vertical dropshaft is examined through the comparison of experimental results. With the analytical solution, the relation among momentum flux, head, and discharge is discussed. This structure is certified to be effective to high drop disposal. Also, through hydraulic model experiments, suitable volume of a stilling chamber connected to a dropshaft is proposed as a function of head and discharge.

Air Transport in Vortex‐flow Drop Shafts

The effect of swirl on air transport in a vertical drop shaft is described. Laboratory experiments on a vortex‐flow drop shaft showed that most of the air is entrained in the annular hydraulic jump that is formed when the annular jet flow changed to the pipe‐full flow. A significant portion of the air bubbles converged due to an inward pressure gradient toward the center of the drop shaft, coalesced to form large bubbles, and then rose toward the surface. A large number of air bubbles still were carried downstream for flow conditions where the jump occurred at the location of low circulation. The air transport rate down the drop shaft was virtually zero if the jump formed in the strong circulation flow‐region. The theoretical analysis indicates that the amount of air transported downstream depends upon the swirl number. The air concentration decreases with the increase in the swirl number.

Free‐Surface Swirling Flows in Vertical Dropshaft

Analytical solutions for free‐surface swirling flows in a vertical dropshaft are presented. The swirling flows are governed by the circulation number and the Froude number. Two types of flow problems are analyzed. The transition flow problem, where the shaft diameter varies, is solved using specific energy curves. In the gradually varied‐flow problem, the shaft diameter is kept constant. Some results of practical interest are presented.