On the Stability of Oscillatory Pipe Flows

The linear stability of pure oscillatory pipe flow is investigated by solving the linearized disturbance equations as an initial value problem. The importance of the initial conditions on transient dynamics of the flow is analyzed. It is shown that transient growth can play an important role in the development of flow instability. The accuracy of the quasi-steady assumption is assessed. It is shown that the growth rates obtained with this assumption deviate considerably from the results obtained with a direct numerical solution of the linearized initial value problem

Numerical investigation of the interactions between solitary waves and pile breakwaters using BGK-based methods

AbstractThe interactions between a solitary wave, which can be used to model a leading tsunami wave, and a pile breakwater made of circular cylinders are numerically investigated. We use the depth-averaged shallow water equations, which are solved by the finite volume method based on the Bhatnagar–Gross–Krook (BGK) model. The numerical results are compared with the experimental data, which yields very good agreement between them when the ratio of wave height to water depth is small (<0.25). As this ratio exceeds the value of 0.25, the larger the ratio is, the bigger deviation of numerical results from experimental data is observed, the possible reasons for this observation are discussed. Both numerical and experimental results indicate that the transmission of the solitary wave decreases and the reflection of the wave increases with reducing gaps between the adjacent cylinders, and that both transmission and reflection coefficients are not very sensitive to the variation in wave height

A Junction and Drop-Shaft Boundary Conditions for Modeling Free Surface, Pressurized, and Mixed Free Surface-Pressurized Transient Flows

A junction and drop-shaft boundary conditions (BCs) for one-dimensional modeling of transient flows in single-phase conditions (pure liquid) are formulated, implemented and their accuracy are evaluated using two Computational Fluid Dynamics (CFD) models. The BCs are formulated for the case when mixed flows are simulated using two sets of govern- ing equations, the Saint Venant equations for the free surface regions and the compressible water hammer equations for the pressurized regions. The proposed BCs handle all possible flow regimes and their combinations. The flow in each pipe can range from free surface to pressurized flow and the water depth at the junction or drop-shaft can take on all possible levels. The BCs are applied to the following three cases: a three-way merging flow, a three- way dividing flow and a drop-shaft connected to a single-horizontal pipe subjected to a rapid variation of the water surface level in the drop-shaft. The flow regime for the first two cases range from free surface to pressurized flows, while for the third case, the flow regime is pure pressurized flow. For the third case, laboratory results as well as CFD results were used for evaluating its accuracy. The results suggest that the junction and drop-shaft boundary conditions can be used for modeling transient free surface, pressurized, and mixed flow conditions with good accuracy

Blockage detection in networks : The area reconstruction method

In this note we present a reconstructive algorithm for solving the cross-sectional pipe area from boundary measurements in a tree network with one inaccessible end. This is equivalent to reconstructing the first order perturbation to a wave equation on a quantum graph from boundary measurements at all network ends except one. The method presented here is based on a time reversal boundary control method originally presented by Sondhi and Gopinath for one dimensional problems and later by Oksanen to higher dimensional manifolds. The algorithm is local, so is applicable to complicated networks if we are interested only in a part isomorphic to a tree. Moreover the numerical implementation requires only one matrix inversion or least squares minimization per discretization point in the physical network. We present a theoretical solution existence proof, a step-by-step algorithm, and a numerical implementation applied to two numerical experiments.Peer reviewe

Further developments in rapidly decelerating turbulent pipe flow modeling

2013-2014 > Academic research: refereed > Publication in refereed journalAccepted ManuscriptPublishe

Electrical characteristics of metal-insulator-semiconductor Schottky diodes using a photowashing treatment in AlzGa1-xAs/InGaAs (X=0.75) pseudimorphic high electron mobility transistors

MIS Schottky diodes on Al0.75Ga0.25As/In0.2Ga0.8As PHEMTs were produced using both photowashing and H2O2 treatments. The Schottky contact on the GaAs layer with photowashing and H2O2 treatments showed enhancements of the SBH of about 0.11 and 0.05 eV, respectively. However, on the undoped AlGaAs layer, no further improvement in SBH was observed. After the photowashing treatment, the Ga oxide (Ga2O3) was dominantly created. In the mean time, two types of As oxide (As2O3,As5O2) were mainly produced by the H2O2 treatment, which are distributed uniformly on the GaAs surface. The thickness of the oxide layer formed by both treatments was nearly the same. Applying a representative model, formation of Ga oxide after the photowashing treatment effectively enhanced the SBH.open4