Articles written in Sadhana

    • Experimental studies of local scour in the pressurized OCF below a wooden log across the flow

      Soumen Maji Prashanth Reddy Hanmaiahgari Subhasish Dey

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      The proposed study examined and reviewed the published experimental results related to clear water scour below a cylinder across the flow. It also highlighted the limitations of existing methods for estimating the scour depth below a submerged cylinder. In the present study, experiments were performed for 50% and 75% submergences of a 70 mm diameter cylinder in the free surface flow over a uniform sand bed with $d_{50}\ =$ 0.98 mm downstream of an apron. Based on the experimental results, an empirical equation was proposed to estimate the amount of gap flow between the cylinder and the bed for an equilibrium scour for a given flow depth and sediment properties. Measured scour profile consisted of a scour hole and immediately followed by a dune. However, no general sediment transport was occurring away from the cylinder due to the undisturbed bed shear stress less than or equal to the critical shear stress required for the sediment entrainment. Different submergence ratios of the cylinder resulted in different longitudinal and vertical extensions of the scour hole and the dune. The maximum equilibrium scour depth occurred when the cylinder is fully submerged in the unidirectional flow with water depth equals to the cylinder diameter. The non-dimensional measured scour profiles were found to be similar. The characteristic lengths of the scour hole and the dune were computed analytically by approximating the measured scour profile by third degree polynomials. The computed non-dimensional scour profiles compared satisfactorily with the measured profiles. It was found that analytical non-dimensional scour profiles were identical for a given diameter of a cylinder with different submergences for the same flow conditions.

    • Turbulence characteristics of open channel flow over non-equilibrium 3-D mobile dunes


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      This paper reports velocity measurements over mobile dunes using an acoustic Doppler velocimetry (ADV). Experiments were conducted with two different flow conditions resulting in the formation of two different size mobile dunes. Dunes height, wavelength and velocity of dunes found to be increasing with increase in average flow velocity for a constant flow depth. The quasi-stationary bed condition was assumed while measuring the velocity distribution along the depth. The effect of the non-equilibrium mobile dunes on the flow characteristics and turbulence is examined by computing turbulent intensities, turbulent kinetic energy and Reynolds shear stresses using time averaged and time–space averaged velocity measurements. The magnitudes of transverse velocities are approximately 1/10 of streamwise velocities and vertical velocities are approximately half of the transverse velocities. The considerable magnitudes of transverse velocities over mobile bedforms necessitate measurement of 3-D velocity components to analyze the flow field. Computed turbulence intensities are found to be maximum in the region consisting of the trough and the reattachment point of the dunes. It is observed that streamwise turbulence intensities near the bed are twice the transverse turbulence intensities, and transverse turbulence intensities are twice the vertical turbulence intensities. Reynolds stresses (transverse fluxes of streamwise and vertical momentum) are observed to be high on mobile bedforms which shows mobile dunes reinforce the secondary currents. Peak values of turbulent kinetic energy (TKE) and Reynolds stresses are also found in the region consisting of the trough and the reattachment point. It is visually observed in the present experiments that maximum erosion takes place at the reattachment point and eroded sediment is carried as totalload and dropped on the lee slope of the subsequent downstream dune. This phenomenon is caused by flow expansion in the separation zone, and which is also the main reason for mobility of dunes and associated bedload transport. Most importantly, it is found that turbulence anisotropy increases with increase in size of mobile bedforms and anisotropy is extended up to the free surface in the flows over mobile bedforms, which proves the entire depth of flow is being disturbed by the mobile dunes.

    • Identification of partial blockages in pipelines using genetic algorithms


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      A methodology to identify the partial blockages in a simple pipeline using genetic algorithms for non-harmonic flows is presented in this paper. A sinusoidal flow generated by the periodic on-and-off operation of a valve at the outlet is investigated in the time domain and it is observed that pressure variation at the valve is influenced by the opening size of blockage and its location. In this technique, the unsteady (steady oscillatory) pressure time series at only one location is required to identify two blockages. In the proposed methodology, thesolution of the governing hyperbolic PDEs of pipe flow is obtained using the method of characteristics. For any piping system similar to the hypothetical pipe system used in the simulations, generalized best amplitude and best frequency of the valve operation are determined, which give maximum deviation in pressure responses for a specific blockage at different locations for a given constant-head reservoir. The generalized best amplitude and best frequency of the valve operation are also obtained for two blockages. Accuracy of the proposed methodology in identifying blockages in a hypothetical simple pipe system with increased noise in the simulated measurements is studied. A non-dimensional variable is proposed to determine whether the proposed methodology is applicable to isolate partial blockages in a piping system. Finally, the proposed methodology is experimentally validated on a laboratory piping system for a single blockage and two blockages.

    • Estimation and examination of linepack pressures in long liquid pipelines


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      In the past, many researchers have carried out water-hammer pressure analysis using Joukowsky equation. However, it has been observed that the computed pressure surge is no longer applicable based on the equation. The Joukowsky equation cannot be used even within the reflection time of the long pipeline. In such cases, the actual pressure rise due to the sudden closure of a quick acting valve will be several times more than that of the sudden increase in pressure as calculated by the Joukowsky equation. The phenomenon of rising pressure at the upstream of an instantaneously closed valve with the passage of time caused by the pipe friction is commonly called as linepacking. In this paper, various parameters affecting the linepack pressure have been thoroughly investigated. As the relative roughness increases, the resulting non-dimensional linepack pressure (PLP/Po) significantly increases and the proportionality constant was equal to 1.5. The linepack pressure was determined to be decreasing with increasing valve closure time. The dominant parameter that influences the linepack pressure is found to be the Reynolds number as compared to the Mach number, and the relative roughness. Furthermore, the linepack pressure is found to be proportional to frictional head loss (hL / D), and inversely proportional to inlet pressure (Po / γcLo)). Finally, a linear regression equation was developed in terms of non-dimensional variables to estimate the linepack pressure using hand calculations without undergoing numerical modeling procedures. The proposed equation was validated for sudden valve closure pressure histories available in the literature. The proposed method is applicable to long distance water supply pipelines where the linepack pressures are significant.

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