Observations on Performance of Three-Tube Pitot Probe

This technical note discusses two questions related to three-tube (pitch or yaw) pitot probes, which are used for measuring the velocity vectors in 2D flows. When a pitch probe is used in flows in which the third component of the velocity is small but not zero, it is important to know the error this causes in the measurement of the velocity vector. Calibration experiments on a pitch probe showed that as long as the yaw angle is less than 14°, the relative error in the velocity measured by the pitch probe is less than 3% and the error in the pitch angle is less than 1°. Typically a pitch or yaw probe can only be used when the angle of attack is less than approximately 50°. The second part of this work addresses the practical issues associated with pretilting the probe to measure flows at angles greater than 50°.     

Computational and Experimental Study of Surcharged Flow at a 90° Combining Sewer Junction

Combining sewer junctions with a lateral inflow at 90° angle are commonly used in our sewer systems. A computational fluid dynamics (CFD) model based on Ansys CFX 10.0 was established to simulate fully surcharged flow at a 90° combining sewer junction. The model was carefully assessed by comparing its results with the measurements of detailed physical experiments. Good agreement was obtained between results of the computational model and of the laboratory experiments. The computational model was proved to be capable of simulating surcharged combining junction flow in the aspects of water depth, energy losses, velocity distributions, and turbulence. The verified CFD model was also used to investigate air entrainment and effects of the size of the junction chamber on the flow. Such CFD models can be used to optimize the design of sewer junctions and will also be useful in studying sediment transport at sewer junctions.     

Flow Upstream of Orifices and Sluice Gates

Potential flow solutions of a point/line sink are extended to study the velocity field upstream of a finite-size orifice and sluice gate. It is found that, in the “near field” zones, the iso-velocity surfaces appear to be semiellipsoidal; while in the “far field” zones they become hemispheres. The shape and size of the orifice/sluice gate were found to be of no effect on the flow behavior beyond a certain distance. The development of velocity profile away from the orifice and sluice gate is examined, and the effects of water depth are studied. The results of this study compare very well with other numerical and experimental studies, and provide a general understanding of the flow field upstream of orifices and sluice gates.     

Screen-Type Energy Dissipator for Hydraulic Structures

Laboratory experiments have shown that screens or porous baffles with a porosity of about 40% could be used as effective energy dissipators below small hydraulic structures, either in a single wall or a double wall mode. The experiments were carried out for a range of supercritical Froude numbers F1 from about 4 to 13, and the relative energy dissipation was appreciably larger than that produced by the corresponding classical hydraulic jumps. These screens or porous baffles produced free hydraulic jumps, forced hydraulic jumps, and in some cases submerged jumps. The flow leaving these screens was found to be supercritical with a Froude number approximately equal to 1.65 and a tailwater depth equal to 0.28 times the subcritical sequent depth y2* of the classical hydraulic jump with the same F1. To produce a secondary jump downstream of the screens, the tailwater depth needed was found to be about one half of y2*.     

Nonclassical traffic modeling and performance analysis of cellularmobile networks with and without channel reservation

We present a two-moment performance analysis of cellular mobile networks with and without channel reservation. Unlike classical analysis where handoff traffic is modeled as Poisson, we characterize handoff traffic as a general traffic process and represent it using the first two moments of its offered traffic. We empirically show that handoff traffic is a smooth process under negative exponential channel holding times. We also show how one may determine customer-oriented grade-of-service parameters such as new-call blocking, handoff call blocking, and forced termination probability under the two-moment representation of traffic offered to each cell. We present extensive results validating our analysis. We compare the performance of the proposed two-moment analysis with classical single-moment analysis and simulation results. Our simulation employs five different mobility models. We show that our proposed model outperforms the existing analytical method when compared to simulation results employing all five mobility models     

Hydraulics of Stacked Drop Manholes

This paper presents the results of an experimental investigation on flows inside stacked drop manholes (SDM). An SDM consists of two identical rectangular or square manhole chambers stacked together at an elevation difference. SDMs for different conditions were assessed on their ability to dissipate the energy of the approaching flow and their suitability to perform adequately under different flow conditions. Flow regimes were classified based on the inflow conditions and geometry of the structure in the first chamber and downstream outflows in the second chamber. An analysis based on the integral momentum equation was developed to estimate pool depths and energy losses under critical flow conditions. A fully surcharged stage with inflow and outflow pipes running full was also tested and velocity profiles were measured at a horizontal center plane to the opening connecting both shafts. Additionally, air flow rates were measured to assess the air demand into a large-height SDM.     

Plane Turbulent Wall Jets in Shallow Tailwater

This paper presents a theoretical and laboratory study of plane turbulent wall jets with finite tailwater depth. The main objective was to show that, when the depth of tailwater is finite, the momentum flux of the forward flow in the wall jet decays appreciably with the distance from the nozzle. This decay is due to the entrainment of the return flow, which has negative momentum that requires a depression of the water surface near the gate housing the slot. An extensive set of experiments, with different Froude numbers and tailwater depth ratios, was used to observe and quantify the growth of the wall jet, the decay of the velocity scale and the momentum flux, and the variation of the volume flux. Also, experiments were conducted to measure the length of the surface eddy and the drop in the water surface elevation at the gate. This study contributes to an understanding of the behavior of plane turbulent wall jets when the ambient fluid has a limited extent.