Plunging Circular Jets: Experimental Characterization of Dynamic Pressures Near the Stagnation Zone

Spillways are a requirement for dams’ safety, mainly preventing overtopping during floods. A common spillway solution involves plunging jets, which dissipate a considerable flow energy in the plunge pool. Energy dissipation has to occur in a controlled manner to avoid endangering the dam foundation and river slopes. Indeed, a scouring process in the downstream riverbed will inevitably develop until equilibrium is reached, otherwise a suitable pre-excavated or concrete lined plunge pool has to be provided. This paper focuses on experimental studies in which particular attention was paid to the dynamic pressures in the plunge pool floor at the vicinity of the jet stagnation zone sampled at 2.4 kHz. A rectangular experimental facility, 4.00 m long and 2.65 m wide, was used as plunge pool. Tests involved a vertical circular plunging jet with velocity ranging from 5 to 18 m/s and plunge pool depth ranging from 4.2 to 12.5 jet diameters. Differences in dynamic pressure measurements are highlighted between transducers located in the inner and outer regions of the jet diameter footprint. Several parameters characterizing the dynamic pressures evidence trends tied with the jet velocity that, to the authors’ knowledge, were not dealt in previous research. These can derive from the coupling effects of consequent recirculating motions and air entrainment in the limited-size plunge pool. Both effects, increasing with velocity, cause an reduction in the efficiency of the diffusing jet shear layer. This aspect deserves further investigation to achieve a better understanding and more complete characterization.

Performance and Design of a Stepped Spillway Aerator

Stepped spillways are frequently limited to specific discharges under around 30 m2/s due to concerns about potential cavitation damages. A small air concentration can prevent such damages and the design of bottom aerators is well established for smooth chutes. The purpose of this study is to systematically investigate the performance of a deflector aerator at the beginning of stepped chutes. Six parameters (chute angle, step height, approach flow depth, approach flow Froude number, deflector angle and deflector height) are varied in a physical model. The spatial air concentration distribution downstream of the aerator, the cavity sub-pressure, water discharge and air discharges are measured. The results describe the commonly used air entrainment coefficient, the jet length, as well as the average and bottom air concentration development to design an aerator. The lowest bottom air concentration measured in all tests is higher than the air concentration recommended in literature to protect against cavitation damages. And, unlike smooth chutes, there appears to be no significant air detrainment downstream of the jet impact. One deflector aerator seems therefore sufficient to provide protection of a stepped spillway.

Numerical Analysis of Pressure Profiles and Energy Dissipation across Stepped Spillways Having Curved Risers

In this study, curved risers stepped spillways models based on the increasing angle of suspension were tested to check for improvement in energy dissipation and pressure distributions. Four fourteen-steps stepped spillway models with a slope 1:0.84 were selected, using Froude’s number non-dimensional similarity. The risers of steps were made curved, based on three angles of suspensions, i.e., 30°, 60°, and 90°. The simulations were performed by FLOW 3D software and by the turbulence model Renormalization Group (RNG) for discharges between 0.020 and 0.068 m3/s followed by the model calibration. The 3D Reynolds-averaged Navier–Stokes equations were solved, which included sub-grid models for air entrainment, density evaluation, and drift–flux, to capture free-surface flow over the stepped spillway. It was estimated that curving the risers increases the energy dissipation up to three percent for lower flow rates, whereas it has no significant impact on energy dissipation for higher flow rates. It was found that in simply stepped spillway lower steps dissipate more energy as compared to curved risers stepped where energy dissipation is shifted to higher steps. On the other hand, curved risers stepped spillways showed lower values of negative pressures as compared to the simply stepped spillway. It was seen that a higher energy dissipating step as experienced more negative pressures as compared to the lower energy dissipating step.

Air Entrainment in Drop Shafts: A Novel Approach Based on Machine Learning Algorithms and Hybrid Models

Air entrainment phenomena have a strong influence on the hydraulic operation of a plunging drop shaft. An insufficient air intake from the outside can lead to poor operating conditions, with the onset of negative pressures inside the drop shaft, and the choking or backwater effects of the downstream and upstream flows, respectively. Air entrainment phenomena are very complex; moreover, it is impossible to define simple functional relationships between the airflow and the hydrodynamic and geometric variables on which it depends. However, this problem can be correctly addressed using prediction models based on machine learning (ML) algorithms, which can provide reliable tools to tackle highly nonlinear problems concerning experimental hydrodynamics. Furthermore, hybrid models can be developed by combining different machine learning algorithms. Hybridization may lead to an improvement in prediction accuracy. Two different models were built to predict the overall entrained airflow using data obtained during an extensive experimental campaign. The models were based on different combinations of predictors. For each model, four different hybrid variants were developed, starting from the three individual algorithms: KStar, random forest, and support vector regression. The best predictions were obtained with the model based on the largest number of predictors. Moreover, across all variants, the one based on all three algorithms proved to be the most accurate.

Reconciling Oxygen and Aerosol Delivery with a Hood on In Vitro Infant and Paediatric Models

This study aimed to evaluate optimal aerosol and oxygen delivery with a hood on an infant model and a paediatric model. A facemask and a hood with three inlets, with or without a front cover, were used. A small-volume nebuliser with a unit-dose of salbutamol was used for drug delivery and an air entrainment nebuliser was used to deliver oxygen at 35%. Infant and paediatric breathing patterns were mimicked; a bacterial filter was connected to the end of a manikin trachea for aerosol drug collection, and an oxygen analyser was used to measure the oxygen concentration. For the infant model, inhaled drug dose was significantly higher when the nebuliser was placed in the back of the hood and with a front cover. This was verified by complementary computational simulations in a comparable infant-hood model. For the paediatric model, the inhaled dose was greater with a facemask than with a hood. Oxygen delivery with a facemask and a hood with a front cover achieved a set concentration in both models, yet a hood without a front cover delivered oxygen at far lower concentrations than the set concentration.

Analysis of elliptical diesel nozzle spray dynamics using a one-way coupled spray model

The elliptical nozzle has the potential ability to increase the air-fuel mixture quality. A one-way coupled spray model and Homogenous Relaxation Model (HRM) was adopted to investigate the spray behaviors and the air-fuel mixture progress in real diesel combustion chamber with the application of elliptical and circular diesel nozzle. The results indicated that the spray cone angle and the air entrainment mass of elliptical nozzle were larger than that of the circular nozzle, while the spray penetration of the elliptical nozzle which the aspect ratio is 1.5 and 2 was shortened by 11% and 8.3% as compared to circular spray respectively. Also, the air entrainment mass of the elliptical spray with a ratio of 1.5 and 2 increased by 60% and 35% as compared with circular spray respectively. Furthermore, the partial equivalent ratio and the high concentration area in the cylinder is reduced for elliptical nozzle, and the air-fuel mixture is more uniform. The fuel evaporation rate of elliptical spray is always higher than that of the circular spray.

From composition to the microstructure and durability of limestone ternary blended cements: A Systematic review

Limestone ternary cements have attracted significant research and commercial attention recently, for technical and environmental reasons. Standardization of these cements is imminent under BS EN197-5. Presently, detailed understanding of the hydration and microstructure evolution of limestone ternary cements from different alumina-rich supplementary cementitious materials (SCMs) exists in the scientific literature; improved reaction kinetics and additional phase assemblages refine the pore structure. However, understanding of the performance of these cements under exposure conditions is less prevalent. In this contribution, we review these data in a way that allows stakeholders to appreciate the capabilities of the different compositions and their performance. We focus our discussion on critically examining the interplay between the cement composition and the microstructure on durability. It is demonstrated that limestone ternary cements offer a pathway for reducing the embodied CO2 of concrete without compromising their performance. The resistance to chloride ingress, sulphate attack and ASR are significantly improved in a manner similar to binary cements. Carbonation and freeze-thaw resistance is generally lower than OPC but adequate air entrainment can offer improvement in freeze-thaw resistance. The challenge to widespread adoption of these cements is evidence of durability under field conditions. To this end, we recommend large-scale field trialling of these cements and understanding of the role of combined exposures on durability and mechanical properties.