Computational Study to Predict the Free-Surface Flow over Non-uniform Stepped Spillway Using ANSYS-CFX

Using stepped chutes as a structure for controlling flood discharges is applicable for long time. Measuring the depth of flow over that structure is essential for designing of the side walls. The aim of this paper is to determine the free-surface that flows on spillway equipped with non-uniform step sizes. For that purpose, the two-dimensional software package code of ANSYS-CFX has been utilized to run eight configurations of two moderate slopes (1V:2H and 1V:2.5H) and for four different discharges 1≤dc/hs≤2.2 to determine the effect of flow discharges, chute slopes, and step heights on the position of free surface along the structure over non-uniform stepped cascade. The hexahedral grid size of 0.015 m is selected with inflation technique close to the walls. In addition, the renormalized group of k-ε (RNG) turbulence model is implemented and the numerical volume of fluid software is employed. The results show smoother stream for higher discharges, and the free-surface drops when the slope of chutes increases. Moreover, it is found that the step size has insignificant effect on the depth of water. The results of this study are important because they provide new insight in improving the design of stepped spillways. It is recommended to perform more investigations to evaluate their effectiveness in other flow parameters including pressure distribution and energy dissipation rates.

Design characteristics of perforated pooled circular stepped cascade (PPCSC) aeration system

In the present study, an improvised design over circular stepped cascade (CSC) and pooled circular stepped cascade (PCSC) aerator, named the perforated pooled circular stepped cascade (PPCSC) aerator, has been conceptualized and tested for its suitability as an aerator for small intensive aquaculture ponds. Based on dimensional analysis, dimensionless geometric parameters – ratio of width of consecutive steps (Wi/Wi+1) and ratio of perforation diameter to bottom-most radius (d/Rb) and dimensionless dynamic parameters – Froude (Fr) and Reynolds (Re) number were proposed. Initially, aeration experiments were conducted to optimize the geometric parameters, keeping the dynamic parameters constant. Keeping the optimized values of Wi/Wi+1 = 1.05 and d/Rb = 0.0027 as constants, aeration experiments were further conducted at different discharges (Q) and different bottommost radius (Rb) to study the characteristics of oxygen transfer and power consumption of PPCSC aerator at different dynamic conditions. Based on the optimized results, four prototype PPCSC aerators with Rb = 0.75 m, 0.90 m, 1.05 m and 1.20 m were fabricated for their aeration performances. The results showed that the standard aeration efficiency (SAE) values of the prototype PPCSC aerators based on brake power ranged between 3.36 and 4.98 kg O2/kWh, with the average being 4.45 ± 0.741 kg O2/kWh. This shows that the SAE of the PPCSC aerator is many more folds higher than that of the other available cascade aerators, viz., PCSC (SAE: 2.873 ± 0.342 kg O2/kWh) and CSC (2.470 ± 0.256 kg O2/kWh) aerators. The study clearly indicates that this PPCSC aerator may very well be used as pre-aeration or post-aeration units in water or wastewater treatment plants and small-scale intensive aquacultural ponds, replacing the other existing aerators.

Effect of Barriers Height at Inclined Cascade Aerator

World generate a large amount of water loss due development of carbon dioxide, hydrogen sulphide, methane and various volatile organic compounds responsible for bad taste and odour water causes the large amount of marine animals’ death. Aeration is used to increase to oxygen content of the water. Stepped cascade is one of that aerators one of that aeration which is applied. In this present study were analysed the performance of the Standard oxygen transfer rate (SOTR) and Standard oxygen efficiency (SAE) in water. In each case comparison of the performance of the SOTR at the barrier height 10cm shows different from the barrier height at 6cm. Same as comparison of the performance of the SAE at the barrier height 10cm shows different from the barrier height at 6cm. By varying the cascade height 0.5, 1, 1.5, 2 m and 0, 1, 3, 7 number of barriers at two different barrier height 10cm, 6cm. Found that performance increase with the increase in no of barriers, with the height of barriers and the height of cascade.

Mathematical modeling of aeration efficiency and dissolved oxygen provided by stepped cascade aeration

Mathematical modeling has been a vital tool in the field of environmental engineering. Various models have been developed to simulate the level of aeration efficiency (AE) provided by different aerating structures to raise levels of dissolved oxygen (DO) in streams; one of which is the stepped cascade structure. Three models developed by Gameson et al. WRL, and Nakasone, in addition to Qual2k, a computer program for stream modeling, have been used in this research; values of AEs obtained have been compared to those computed using DO measured from a built model at a WWTP. A stepped cascade structure was installed with different heights to aerate five flowrates with different levels of COD. An adjustment has been made to the Nakasone model to test the effect of pollutant load on the amount of aeration that could be reached. Values of AEs computed using the Gameson model were 30%, 39.5%, and 40% for cascade heights (Hd) 45, 60, and 75 cm respectively for the five flowrates (q) that ranged from 21–66 m3/hr. Values of AEs from WRL model were 32.8%, 42%, and 43% consequently. Values of AEs from Nakasone model ranged from 4.6–7.5%, 6–10%, and 7.6–12% respectively. For the adjusted Nakasone model, values of AEs ranged from 3.2–4.9%, 3.3–5.3%, and 4.1–6.7% respectively. Finally, the AEs computed using the values of downstream DO generated by Qual2k ranged from 4–18%, 2–15%, and 2.5–5.1% correspondingly. Around 80% of the downstream DO values computed using the Nakasone and adjusted Nakasone model were closer to those measured in the field, thus more reliable in cascade design.

Aeration characteristics of a rectangular stepped cascade system

Aeration experiments, maintaining nappe flow conditions, were carried out on a rectangular stepped cascade of total height 3.0 m to determine the total number of steps, slope of the entire cascade and hydraulic loading rate at which maximum overall aeration efficiency occurs, keeping the surface area of individual steps constant. Based on dimensional analysis, the overall aeration efficiency at standard conditions (E20) was expressed as a function of square of total number of steps (N2) and dimensionless discharge (dc/h), where dc and h represent critical depth in a rectangular prismatic channel and individual step height respectively. An empirical equation with E20 as the response and N2 and dc/h as the independent parameters was developed based on the experimental results subject to 36 ≤ N2 ≤ 196 and 0.009 ≤ dc/h ≤ 0.144. The experimental results showed that the overall aeration efficiency (E20) for a particular step height of stepped cascade increases with increase in dc/h up to a certain value and then decreases. This may be due to at higher dc/h, i.e., at higher hydraulic loading rate, the flow approaches the transition zone and thereby aeration efficiency decreases. E20 was also found to increase with number of steps at any hydraulic loading rate, because of the increased surface area of fall. The optimum number of steps, slope of the entire stepped cascade and hydraulic loading rate were found to be 14, 0.351 and 0.009 m2/s respectively producing the maximum value of overall aeration efficiency of 0.90.