Journal of Hydraulic Engineering

The presence of blockages in water supply systems wastes energy, decreases system performance, and may pose safety concerns. Previous research showing that blockages induce a shift in the resonant frequencies (eigenfrequencies) of the pipe system made use of that shift information to develop improved inverse problem solution techniques for blockage detection. This paper studies in more detail the eigenfrequency shift mechanism itself that arises from an interior blockage in a pipe system, and shows that more information could be obtained from understanding the nature and physical basis for the shift mechanism. This improved understanding can improve the computational efficiency of current blockage detection solution techniques. This paper explains the mechanism causing positive, negative, and zero eigenfrequency shifts, and shows how these shifts vary with blockage location, size, and resonant modes. Zero shift occurs if the midlength of the blockage is located at a position where the pressure head and flow harmonics are equal in magnitude, whereas maximum (or significant) shifts occur if the midlength of the blockage is located at either a pressure node (if the shift is negative) or a stagnation point (if the shift is positive), where pressure node and stagnation points are where the pressure and flow harmonics’ magnitudes are zero. It is also shown that the Bragg resonance phenomenon directly influences the direction and magnitude of the observed eigenfrequency under different resonant modes.

Recent studies of measured transient pressure signals showed that eigenfrequencies shift with changes in cross-sectional area of the conduit and used this fact to develop blockage-detection algorithms. However, an understanding of the physical basis for eigenfrequency shift-based algorithms is currently lacking. This paper shows heuristically, analytically, and numerically that a blockage in either unbounded or bounded pipe systems interacts strongly with waves at specific frequencies. These specific interacting frequencies conform precisely to Bragg’s resonance condition. The frequency interval between consecutive Bragg frequencies is proportional to the wave speed divided by the blockage length. In addition, it is found that pipe blockage imposes a distinct signature through Bragg resonance phenomena on the unbounded and bounded pipe systems in exactly the same manner (i.e., they exhibit the same variation pattern). It is further shown that the eigenfrequency shift, currently used without physical basis or explanation in many published papers as a basis for blockage detection methods, is because of the Bragg resonance effect. Examples are used to show how this physical insight into the nature and cause of the eigenfrequency shifts can be advantageously used to design direct blockage detection techniques.

Many flow-meter applications require accurate flow measurement (0.5% of actual flow or better), whereas other applications accept less-accurate flow measurements. Some applications attempt to minimize head loss, whereas a few applications have ample energy such that a flow meter with higher head loss is preferred. In many instances where highly accurate flow measurement and low head loss are required, Venturi flow meters are a viable option. The accuracy of a Venturi for flow measurement is well established and well documented; however, the design of recovery cones and their associated head loss is not. This study uses computational fluid dynamics and laboratory data to demonstrate the relationship between recovery cone angle on the classical Venturi meter design and associated head loss. Results from this study show that the optimum recovery cone angle to minimize head loss is a function of beta ratio and Reynolds number. Whereas Venturi codes provide ranges of acceptable cone angles, this research provides the optimal recovery cone angle to minimize permanent pressure loss.

In mountain streams, riverbeds with a series of coarse gravel steps are common. The sequence flow pattern with a chute and pool is usually formed on series of steps, where a transition region from supercritical to subcritical flow appears between steps. In the transition region, a discontinuous water surface with hydraulic jumps is observed and considerable water surface fluctuations occur. These fluctuations significantly affect the bed material. Accurate measurement of the flow velocities under such violent flow conditions is difficult owing to shallow water conditions, air entrainment, and flow nonlinearity. Therefore, detailed flow structures around the steps were not previously understood. To conduct dynamic investigations of formation/collapse processes of the series of steps, numerical simulations were conducted using the discrete element method (DEM) and the enhanced moving particle semi-implicit (MPS) method. Simulation results suggest that the developed DEM-MPS coupling model will be a promising tool to investigate the morphological dynamics of the step series. Furthermore, the potential relationship between water surface elevation and the collapse of steps is implied by the simulation results.

Laboratory experiments on rosette diffusers for dense concentrate disposal from seawater desalination plants into flowing currents are reported. The dimensionless parameters for riser spacing and current speed were typical of operating diffusers. Three-dimensional laser-induced fluorescence (3DLIF) was used to map tracer concentration fields from which the main geometrical and dilution parameters were obtained. As the current speed increases, dilution and impact distance increase and rise height decreases. The results were insensitive to planform orientation of the risers. The rise heights of the rosette jets were always less than that of a corresponding isolated jet in a stationary environment. Semiempirical equations were presented to predict dilution, rise height, and impact distance, and confidence levels for the predictions were estimated. The results did not become independent of riser spacing in the range tested, in contrast to studies in stationary water where the flow properties became dependent on riser spacing for $s_r/d\mathsf{F}<\sim 2$. For narrow riser spacings and relatively slow current speeds, rosette diffusers have higher dilutions than equivalent conventional multiport diffusers, i.e., diffusers with the same length and number of ports, but for higher current speeds the rosette dilutions are significantly less than for the equivalent multiport diffuser.

Water temperature models of urban lakes may fail to reproduce thermal characteristics if the reduction of wind field attributable to sheltering along the banks is not considered. To quantify the effect of wind sheltering, a new wind sheltering (WST) model was developed and coupled with a vertical heat conduction model to create an urban lake temperature model (ULTM). Based on the relationship between the wind direction and obstacle location, as well as the characteristics of wind variation downwind from the obstacle, the WST model could calculate a time-dependent wind sheltering coefficient ($W_{str}$) to dynamically adjust the wind data for the water temperature model. The proposed model was verified using measurements from Guozheng Lake (central China), and good consistency was reported. The results showed that the enhanced modeling system when combined with the proposed WST model was an effective tool for temperature simulation.