Papers by N.or Nallamuthu Rajaratnam
Energy Loss Below Rectangular Sharp-Crested Weirs for Free Flows
38th IAHR World Congress - "Water: Connecting the World"
A lot of researches focused on skimming flow over a stepped chute whereas the hydraulics of jet f... more A lot of researches focused on skimming flow over a stepped chute whereas the hydraulics of jet flow has received less attention. This paper presents the results of an experimental work to gain a better understanding of the hydraulics of aerated jet flow in a stepped chute. A twelve step stepped chute with h/l equal to 0.2 was used as a large model to reduce scale effects. Measurements of air concentration along the stepped chute were made. The flow was found to be fully developed after 3 or 4 steps according to the discharge with development occurring earlier for higher flows. Simultaneous measurements of air concentration and velocity fields were measured along step # 8 and step # 10 for different flow rates. The variation of air concentration with the normalized depth (y/y90) along steps #8 and #10 was found to be approximately similar and fell in a relatively narrow band under various discharges. The energy dissipation for steps #8 and #10 based on the effect of air entrained in the flow was calculated. The flow was found to dissipate more than 80% for the smallest yc/h used and this dissipation decreased as the flow increased. A number of possible mechanisms for energy dissipation are also discussed.
This paper presents the results of an experimental study of turbulent sand jets and sand-water sl... more This paper presents the results of an experimental study of turbulent sand jets and sand-water slurry jets impinging vertically into a stagnant water body. The jets contained silica sand with a median diameter D 50 of 206 m, and with an initial concentration 0.60 by volume for the sand jets, and 0.055-0.124 by volume for the slurry jets. The jets had densimetric Froude numbers between 2.0 and 5.94. The sand concentration and velocity profiles were measured simultaneously using a novel fiber optical probe, up to a distance of 130d o for sand jets, and 65d o for slurry jets, where d o is the jet diameter at the water surface. The jets were found to have self-similar Gaussian profiles. The centerline sand concentration within the jets was found to decrease rapidly, following trends similar to single phase plumes. The centerline sand velocity profile decreased significantly before reaching a plateau region. The "terminal" centerline sand velocity within this region varies somewhat depending upon sand mass flux, and is between 0.32 and 0.43 m/s. The spreading rates of the jets were found to vary with the particle Froude number. Within the sand jets and the higher Froude number slurry jet, the sand concentration had a smaller spreading rate than the velocity. The other slurry jets had equal concentration/velocity spreading rates. The momentum flux of the sand within the jets was found to decrease sharply, followed by a constant flux below a depth of 25 to 30 jet diameters.
This note presents the results of an experimental study of circular sand jets in air from three n... more This note presents the results of an experimental study of circular sand jets in air from three nozzles of diameter of 19.2, 31.1 and 63.8 mm. It was found that the frontal speeds of the sand jets and the steady sand jet velocity accelerate due to gravity with negligible air resistance. The sand velocity does not appear to be affected by sand particle sizes for the three sizes tested. The diameters of sand jets, as they travel downwards, decrease and gradually approach an asymptotic value after a distance of 120 times of the initial jet diameter. The sand concentration in the jet decreases as the distance from the nozzle increases. Waves were observed at the periphery of the sand jet and some preliminary results of wave speed and wavelength are reported.
Air injection systems have a wide range of environmental engineering applications. In this study,... more Air injection systems have a wide range of environmental engineering applications. In this study, we conducted experiments on air injection in a relatively large water tank to investigate the effect of nozzle type, including single/multiple orifice nozzles and a porous airstone, on the characteristics of the bubbles and the induced flow structure. Measurements of bubble characteristics and flow field surrounding the bubble core were obtained using a double-tip optical probe and particle image velocimetry, respectively. The results revealed that bubble velocity did not change significantly with different nozzles, but bubble size decreased significantly while interfacial area, liquid entrainment rate, and kinetic energy of the mean and turbulent flow increased significantly by using the porous airstone instead of nozzles with large orifices. The results for a nozzle with multiple orifices of small diameter are comparable to those for the airstone, which suggests the suitability of its use for systems susceptible to clogging of the pores. Correlations using adequate length and velocity scales are also proposed to describe both bubble and liquid flow characteristics. Finally, applications of the results for different artificial aeration/mixing systems are presented.
Experiments were carried out to investigate the behavior of horizontal gas-liquid injection in a ... more Experiments were carried out to investigate the behavior of horizontal gas-liquid injection in a water tank. Measurements of bubble properties and mean liquid flow structure were obtained. The turbulence in the liquid phase appears to help generating bubbles with relatively uniform diameters of 1 -4 mm. Both bubble properties and mean liquid flow structure depended on the gas volume fraction and the densimetric Froude number at the nozzle exit. It was found that the bubbles strongly affected the trajectory of the water jet, which behaved similarly to single-phase buoyant jets. However, at gas volume fractions smaller than about 0.15, the water jet completely separated from the bubble core. Bubble slip velocity was also found to be higher than the terminal velocity for isolated bubbles reported in the literature. Dimensionless correlations were proposed to describe bubble characteristics and the trajectory of the bubble plumes and water jets as a function of the gas volume fraction and the densimetric Froude number. Finally, applications of the results for aeration/ mixing purposes are presented.
Shallow turbulent wakes behind bed-mounted cylinders in open channels
Journal of Hydraulic Research, 2009
ABSTRACT Flow patterns in the shallow turbulent near-wakes behind bed-mounted cylinders are inves... more ABSTRACT Flow patterns in the shallow turbulent near-wakes behind bed-mounted cylinders are investigated on smooth and rough beds. A wall wake analysis revealed that the flows in the region away from the bed are similar and well described by the plane wake equation. Wall wake similarity was also observed for turbulent kinetic energy and primary Reynolds stress for moderate to deeply submerged cylinders.Wake analyses in the horizontal plane revealed that similarity of mean velocity profiles across the flow exist in the near-wake region at all elevations for slightly submerged and surface piercing cylinders; and at all elevations below the object height close to moderate and deeply submerged cylinders. For slightly submerged and surface piercing cylinders similarity was observed in the near-wake region between the non-dimensional velocity profiles across the wake and the plane wake equation. However, the similarity of the velocity profiles improves if a modified transverse length scale is used for shallow near-wake flows.
Flow around Cylinders in Open Channels
Journal of Engineering Mechanics, 2008
ABSTRACT This paper presents the results of an experimental study of flow around cylindrical obje... more ABSTRACT This paper presents the results of an experimental study of flow around cylindrical objects in an open channel. Cylindrical objects of equal diameter and four heights were tested under similar flow conditions producing four different levels of submergence, including a surface piercing bridge-pier-like cylinder. Different flow elements and their locations were identified using a set of flow visualization tests. Observations made from the flow visualization tests were then verified by measurements of bed-shear stress and deflected flow velocity around the cylinders. Horse-shoe vortex systems were found to appear closer to the submerged cylinders compared to a surface piercing cylinder. The increase in dimensionless bed-shear stress is found to be inversely related to the level of submergence of the cylinders. Bed-shear stress results presented in this paper will be valuable for a qualitative understanding of the scour potential of flow around submerged cylinders. Mean velocity profiles in the deflected flow region were analyzed in terms of the theories of three-dimensional turbulent boundary layer. Submergence of a cylinder has been found to suppress alternate vortex shedding and produce stronger three-dimensional flows in the downstream wake. Perry and Joubert's model was found to be sufficiently accurate to predict the deflected velocity magnitudes around submerged cylinders. Overall, the present study will provide valuable knowledge of hydraulics of flow around submerged structures (e.g., simple fish habitat structures).
Canadian Journal of Civil Engineering, 2009
A simple method has been developed to predict the flow division at the louver, based on the exper... more A simple method has been developed to predict the flow division at the louver, based on the experimental finding of that the specific energy of the flow through a diversion structure housing a louver system is approximately constant. Flow through louver slot is expressed using a consolidated contraction coefficient to account for the lumped effect of flow contraction between the louver slats and the approximations of the proposed theoretical model (e.g., constant specific energy, flow direction through the louver slots). A relationship is developed for the consolidated coefficient from the simulation of experimental results obtained from . It is observed from the simulation results that the consolidated contraction coefficient is best correlated with the relative width of the bypass. Finally, the use of this simple method is demonstrated using a set of numerical experiments within the range of experimental conditions of the louver system.
Air-water bubbly jets are studied experimentally in a relatively large water tank with a gas volu... more Air-water bubbly jets are studied experimentally in a relatively large water tank with a gas volume fraction, C o , of up to 80% and nozzle Reynolds number, Re, ranging from 3500 to 17,700. Measurements of bubble properties and mean axial water velocity are obtained and two groups of experiments are identified, one with relatively uniform bubble sizes and another with large and irregular bubbles. For the first group, dimensionless relationships are obtained to describe bubble properties and mean liquid flow structure as functions of C o and Re. Measurements of bubble slip velocity and estimates of the drag coefficient are also provided and compared to those for isolated bubbles from the literature. The study confirms the importance of bubble interactions to the dynamics of bubbly flows. Bubble breakup processes are also investigated for bubbly jets. It was found that a nozzle Reynolds number larger than 8000 is needed to cause breakup of larger bubbles into smaller bubbles and to produce a more uniform bubble size distribution. Moreover, the Weber number based on the mean water velocity appears to be a better criteria than the Weber number based on the bubble slip velocity to describe the onset of bubble breakup away from the nozzle, which occurs at a Weber number larger than 25.
Diffusion Of Rectangular Wall Jets In Wider Channels
Journal of Hydraulic Research, 1967
Closure to “Characteristics of Skimming Flow over Stepped Spillways” by M. R. Chamani and N. Rajaratnam
Journal of Hydraulic Engineering, 2000
Erosion d'un lit de polystyrène par des jets d'air turbulents circulaires d'incidence oblique
This paper presents the results of an experimental study of the erosion of a cohesionless bed by ... more This paper presents the results of an experimental study of the erosion of a cohesionless bed by obliquely impinging circular turbulent jets. The jet of air, with velocity of 27.3 to 86.8 m/s and diameter at the nozzle of either 6.35 or 12.6 mm, was set to impinge on a bed of polystyrene particles at varying angles of impingement of
Erosion Around Simple Habitat Structures
ABSTRACT
Effectiveness of Surface Aeration and Oxygen Injection System in the Athabasca River
Dissolved oxygen (DO) is generally regarded as a proxy for the overall health of a river from the... more Dissolved oxygen (DO) is generally regarded as a proxy for the overall health of a river from the perspective of organic and nutrient loading. When organic loading is high and DO concentrations become a concern, mitigation options may be required such as artificial aeration through oxygen injection into waste streams. We evaluate the effectiveness of a pilot oxygen injection program
Observations on Sand Jets in Air
Journal of Engineering Mechanics
This note presents the results of an experimental study of circular sand jets in air from three n... more This note presents the results of an experimental study of circular sand jets in air from three nozzles of diameter of 19.2, 31.1 and 63.8 mm. It was found that the frontal speeds of the sand jets and the steady sand jet velocity accelerate due to gravity with negligible air resistance. The sand velocity does not appear to be affected by sand particle sizes for the three sizes tested. The diameters of sand jets, as they travel downwards, decrease and gradually approach an asymptotic value after a distance of 120 times of the initial jet diameter. The sand concentration in the jet decreases as the distance from the nozzle increases. Waves were observed at the periphery of the sand jet and some preliminary results of wave speed and wavelength are reported.
International Journal of Multiphase Flow, 2015
Laboratory experiments were carried out to study the effects of sand particles on circular sand-w... more Laboratory experiments were carried out to study the effects of sand particles on circular sand-water wall jets. Mean and turbulence characteristics of sand particles in the sand-water wall jets were measured for different sand concentrations c o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/ d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand-water wall jets with a higher concentration (c o = 1.5-2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.
+244pp., Dfl. 140.00, U.S. $62.25 (hardback). R.H. French, ,Hydraulic Processes on Alluvial Fans Developments in Water Science 1 (1989) Elsevier,London xi
International Journal of Multiphase Flow, 2008
Air-water bubbly jets are studied experimentally in a relatively large water tank with a gas volu... more Air-water bubbly jets are studied experimentally in a relatively large water tank with a gas volume fraction, C o , of up to 80% and nozzle Reynolds number, Re, ranging from 3500 to 17,700. Measurements of bubble properties and mean axial water velocity are obtained and two groups of experiments are identified, one with relatively uniform bubble sizes and another with large and irregular bubbles. For the first group, dimensionless relationships are obtained to describe bubble properties and mean liquid flow structure as functions of C o and Re. Measurements of bubble slip velocity and estimates of the drag coefficient are also provided and compared to those for isolated bubbles from the literature. The study confirms the importance of bubble interactions to the dynamics of bubbly flows. Bubble breakup processes are also investigated for bubbly jets. It was found that a nozzle Reynolds number larger than 8000 is needed to cause breakup of larger bubbles into smaller bubbles and to produce a more uniform bubble size distribution. Moreover, the Weber number based on the mean water velocity appears to be a better criteria than the Weber number based on the bubble slip velocity to describe the onset of bubble breakup away from the nozzle, which occurs at a Weber number larger than 25.
Erosion of Sand Beds by Oblique Plane Water Jets
Proceedings of the ICE - Water Maritime and Energy, 1995
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Papers by N.or Nallamuthu Rajaratnam