Nallamuthu Rajaratnam

Hydraulics of simple habitat structures

Habitat structures are built in rivers to provide feeding and resting areas for fish. At the present time, only rough guidelines are available for the design of these structures. This paper presents the results of a laboratory study on the flow and erosion around simple habitat structures. Hemispheres with diameter D of 74 and 130 mm were placed on smooth, rough as well as erodible beds and Froude number of the approaching flow was in the range of 0.074 to 0.6. The relative depth d/h where d is the depth of and h is the height of the body was found to be the important parameter and was varied from about 0.6 to 4.3. Four different regimes of flow were found, which were classified based on the relative depth. Downstream of the body, there was a recirculation region (closed wake) with a length of about 2D which was followed by an open turbulent wake. The structure of flow in this open wake was analyzed in two layers using the concept of the wall wake. In the plane of symmetry, the inner layer was analyzed using the law of the wall whereas the outer layer was analyzed using the wake equation of Schlichting. The variation of the velocity in the transverse direction was also analyzed using the concept of similar profiles. Further an empirical correlation was found for the velocity scale. The amplification of the bed shear stress near the body, especially for the rough bed was significant. Some observations were also made on the nature of erosion around the hemisphere placed on erodible beds of two sand sizes of 1.11 and 2.1 mm. It was found that the pattern of erosion was different for the different flow regimes. The maximum equilibrium clear water scour depth occurred in front of hemispherical bodies and was approximately equal to 0.67 D.

Structure of flow in hydraulic jumps

A high-speed photographic study was made of the hydraulic jump. It was found that the surface roller was made up of several vortices. These vortices are generated in the early part of the jump and travel downstream. As they move downstream, they grow by pairing. At the same time, water spills down the steepened surface to replenish the toe and subsequently is rolled up into new vortices. A Fourier analysis of the time series of the toe position indicates a cyclic mechanism, the frequencies of which appear to scale with the upstream velocity and downstream depth.

LDA study of flow structure in submerged hydraulic jump

This paper presents the results of a Laser Doppler Anemometry (LDA) study of submerged hydraulic jumps in a horizontal rectangular channel of constant width with the submergence factor S varying approximately from 0.20 to 1.70 and inlet Froude number Fx approximately equal to 3.0, 5.5 and 8JX Measurements include surface profiles, mean velocity components of u and v, turbulence shear stress and turbulence intensities Major flow characteristics of submerged hydraulic jumps are discussed and analyzed. The flow in the fully developed region is found to have some degree of similarity. It is also found that a submerged jump is three dimensional in nature.

HYDRAULICS OF CHANNELS WITH FLOOD-PLAINS

This paper presents the results of an experimental study on the interaction between the flow in a “wide” main channel and the flood-plain. This interaction region has been shown to resemble a shear...

Energy loss at drops

This paper presents a critical analysis of the energy loss at drops. Using the results of an experimental study on three drops, several assumptions made by White and Gill in developing an equation ...

Free jumps, submerged jumps and wall jets

This paper presents a critical study of hydraulic jumps, submerged jumps and plane turbulent wall jets as a class of similar flows. Using the experimental results available in the literature as well as some new results, the hydraulic jump and the wall jet are shown to be the end states with the submerged jumps forming the transition state in between. The three properties considered are the similarity of the velocity distribution, decay of the velocity scale and the growth of the length scale. The velocity distribution is approximately the same for all the flows. Using a longer length scale L, the decay of the velocity scales for the end states has been redefined and for submerged jumps, the decay has been found to be either free-jump-like (FJL) or wall-jet-like (WJL). The variation of the length scale L was found to have distinctive variations for the different flows. Some new results are also presented for the reverse flow field of submerged jumps.

IMPINGEMENT OF AXISYMMETRIC DEVELOPING JETS

The impingement of turbulent jets on solid boundaries has been studied extensively in the past. It has been established that flow parameters in the impingement region are governed by the impingement height, H and the initial kinematic momentum flux of the jet. This results from the fact that the jet can be considered as a point source of momentum, whenever it is fully developed before entering the impingement region. Obviously, this finding is suspect in cases of “small” values of H. An experimental and analytical study of developing jets, impinging on a smooth wall, is presented. Measurements of the velocity field, static pressure in excess of ambient, and wall shear stress are reported. Analysis of the results shows that impingement region flow parameters arc governed by the diameter, d and the intial jet velocity, U() but are independent of H. The impingement region was found to extend 1.2d above the wall and 1.4d radially from the stagnation point. Semi-empirical methods for predicting the velocity fi...

Generalized study of erosion by circular horizontal turbulent jets

Experimental data on erosion by circular wall jets from thirteen sources including the present experimental study were compiled and analyzed. The present experimental study consisted of circular air jets on a bed of canola seeds and circular water jets on sand and gravel beds. The compiled data, consisting of over 350 experiments, cover wide ranges of flow submergence, jet sizes and velocities, bed material size, relative channel width and relative density difference. Similarity of the scour hole was examined both in the unsteady and asymptotic states by determining the ratios between the characteristic dimensions of the scour hole. The effect of jet submergence on the asymptotic characteristic dimensions of the scour hole was found to be pronounced when the densimetric particle Froude number F o is greater than 10. The effect of elevating the jet outlet on scour depth was also investigated. Equations relating the asymptotic characteristic dimensions of the eroded bed to F o have been proposed. The effect...

A numerical study of submerged hydraulic jumps

A standard two-dimensional k-e turbulence model is used to predict the mean flow and turbulence characteristics of submerged hydraulic jumps. An offset control volume method is developed to facilitate computation of the variable free surface. The numerical predictions are compared with experimental measurements under three conditions with supercritical Froude numbers ranging from 3.2 to 8.2 and submergence factors ranging from 0.24 to 0.85. Finally the numerical performance is evaluated and discussed in detail. It is concluded that the model is adequate for predicting the surface profile, mean velocity field and to some extent, the turbulence structure of submerged hydraulic jumps.

Turbulent non-buoyant surface jets

This paper presents the results of an experimental study on the diffusion of plane and bluff turbulent surface jets. For plane surface jets, it has been found that its length scale grows at the same rate as a plane wall jet with its velocity scale being about 0.9 times the corresponding value for the plane free jet. For bluff surface jets, the vertical length scale increases at approximately the same rate as bluff wall jet whereas its transverse length scale grows at about half the rate of the corresponding bluff wall jets. The velocity scale decays inversely with the longitudinal distance, but the constant in the velocity scale relation is somewhat higher than that for the corresponding free circular jet.