Internal fluidisation of granular material

Uncontrolled seepage flow due to defects or imperfect joints in civil engineering structures (such as dams, levees, dry-dock, seepage barriers, and sheet piles interlocking) or from fractured underground pipes is a major concern. The result of such leakage is increased water pressure and hydraulic gradients at the source of leakage making the behaviour of the surrounding material complex. This is because of the interaction of particles with the source of flow, turbulent flow in the surrounding bedding and the potential of developing additional mechanisms at the source of flow. Evidence of the occurrence of internal fluidisation has been reported in a number of fields, and yet not much is known about it.In this fundamental study experimental apparatus and techniques have been developed to investigate the mechanism of internal fluidisation of granular material due to localised flow. A two-dimensional experimental model was created for the study, in which a machined box with variable orifice openings was designed and built to simulate an idealised crack for a localised leak. The box was fitted inside a modified seepage tank to fluidise a bed of granular material. Various parameters were investigated: flow rate, excess pore water pressure in the bed, pressure upstream of the orifice, particle size, particle shape, height of the bed, and orifice size on the observed mechanism. Image analysis techniques based on the Particle Image Velocimetry (PIV) have been developed in this study to monitor the behaviour of the fluidised zone.Results and observations of this study suggest that the mechanism of internal fluidisation in a bed of granular materials is associated with an uplift mechanism of the grains in the active region of the bed. This is attributed to the drag force exerted by seepage flow overcoming the downward bulk weight of the bed. From the results and observations of this study a mathematical model based on the concept of force equilibrium has been proposed to predict the pore pressure at the onset of internal fluidisation. The results show that high pressure heads can be sustained upstream of the orifice without the internally fluidised zone breaking through to the bed surface. They also show that the onset of this mechanism in a bed of granular material is highly dependent on packing properties (grain size, grain shape, height of the bed), seepage velocity and orifice size. <br/

Hydraulic Jump Characteristics Downstream of a Compound Weir consisting of Two Rectangles with a below Semicircular Gate

Weirs are often used in laboratories, industries, and irrigation channels to measure discharge. The discharge capacity of a structure is vital for its safety and plays an important role in the combined gate-weir flow, which is a complicated phenomenon in hydropower. This study carried out experiments on a combined hydraulic structure, which included a compound sharp-crested weir made up of two rectangles along with an inverted semicircular sharp gate. Installed on a straight channel, this structure served as a control instrument. The study aimed to investigate the downstream hydraulic jump characteristics of this combined structure, specifically, the sequent depth ratio (y2/y1), the hydraulic jump height ratio (Hj/y1), the energy loss ratio through the jump (EL/Eu), and the jump length ratio (Lj/y1). The width of the upper rectangle on the weir was set at 20 cm. The width of the lower rectangle (W2) was set at 5, 7, and 9 cm, while its depths (z) were fixed at 6, 9, and 11 cm. The gate's diameters varied between 8, 12, and 15 cm. These measurements were alternated with varying initial Froude numbers (Fn1) ranging between 1.32 and 1.5. The results showed that the dimensions of both the weir and the gate influenced the hydraulic jump characteristics. Empirical formulas were developed to predict y2/y1, Hj/y1, EL/Eu, and Lj/y1 based on the differing dimensions of the combined structure. The findings and analysis of this study are limited to the range of data that were tested

Internal fluidisation of granular material

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Effect of soil on leakage rates in water distribution pipes

This paper describes an experimental and analytical study that examined the effects of soil on leakage rate from defective water pipes. An experimental model was designed and constructed to simulate idealized cracks in defective water pipes discharging into a bed of granular materials. Tests were run on three different materials: LBS-B, LBS-A, and glass ballotini with different characteristics. The results reveal that the presence of bedding material surrounding a leaking pipe causes a significant change in the leakage rate versus free flow conditions. The bedding has a greater impact on larger defect openings. There was a significant drop in the values of the coefficient of discharge (Cd) when the pipe discharged into a soil bed versus free flow conditions; this value was 38% lower for coarse particles and 70% for finer particles. The results also showed that high leakage rates could flow from the defective pipe without fluidization of the bedding materials. An analytical model based on the method of fragments was presented to predict the leakage rate after which the predicted results were compared with values measured experimentally. HIGHLIGHTS An analytical model has been presented to predict the leakage rate, in the presence of soil, that provides reasonable estimates of the leakage rate.; Soil with different characteristics (permeability, particle size, particle shape, and bed height) was tested.; Influence of the surrounding soil on the hydraulic behaviour of leakage was examined.

Prediction of the discharge coefficient for the rectangular notch with different hydraulic and geometric properties

This study aims to investigate experimentally the variation of the coefficient of discharge Cd with the rectangular notch hydraulic and geometric parameters such as water head h, notch height p, notch width B, and notch thickness t. The results show that the coefficient of discharge Cd increases with an increase of h and B while it decreases with t. There are no changes in the variation of actual discharge Qact and consequently the discharge coefficient Cd with h for notch height p more than 6 cm. An empirical formula was developed based on the dimensional analysis principle that can be used to predict the coefficient of discharge Cd value for the rectangular notch with known hydraulic and geometric data (h, B, p, and t). HIGHLIGHTS There are no Qact changes and thus the discharge modulus Cd with h for crack height.; The relationship between the theoretical discharge Qth and vertex h to be constant for the thickness of the rectangle Cd increases with increasing h/p because the actual discharge Qact increases with water head h.; The discharge modulus Cd decreases with increasing slit thickness ratio t/p.