Unsteady Flow and Transient Flow Research Papers (original) (raw)

This paper presents an evaluation and analysis of resistance parameters: friction slope, friction velocity and Manning coefficient in unsteady flow. The methodology to enhance the evaluation of resistance by relations derived from flow equations is proposed. The main points of the methodology are (1) to choose a resistance relation with regard to a shape of a channel and (2) type of wave, (3) to choose an appropriate method to evaluate slope of water depth, and (4) to assess the uncertainty of result. In addition to a critical analysis of existing methods, new approaches are presented: formulae for resistance parameters for a trapezoidal channel, and a translation method instead of Jones' formula to evaluate the gradient of flow depth. Measurements obtained from artificial dam-break flood waves in a small lowland watercourse have made it possible to apply the method and to analyse to what extent resistance parameters vary in unsteady flow. The study demonstrates that results of friction slope and friction velocity are more sensitive to applying simplified formulae than the Manning coefficient (n). n is adequate as a flood routing parameter but may be misleading when information on trend of resistance with flow rate is crucial. Then friction slope or friction velocity seems to be better choice.

- by Magdalena Mrokowska and +1
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- Hydrology, Unsteady Flow and Transient Flow, Unsteady flow, Unsteady Open Channel Flow

The paper reports a study of the water surface profile of an entrapped air cavity while emptying water in an initially filled inclined duct. A one-dimensional (1D) model, which consists of the continuity and momentum equations applicable for open channel flow, pipe flow and air–water interface flow, is developed based on the finite volume method. A pressure drop model is proposed to reproduce a better profile around the cavity front, with a particular focus on air pressure changes inside the confined cavity to simulate a kind of transient flow with an entrapped air cavity. In contrast to the previous studies, the application of the model shows that when the pressure drop is not considered and the air pressure is not changed, the confined cavity soon vanishes. A comparison between the simulated and experimental results shows that the model is able to accurately reproduce the water surface profile of an entrapped air cavity while emptying inclined ducts.

- by Hamid Bashiri
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- Computational Fluid Dynamics, Hydraulics, Free surface flows, Pipeline

The objective of this study was to predict and analyze the dam break flood in a real river valley. A numerical model was developed for simulating dam break flood and applied for analyzing flood situation due to the instantaneous hypothetical failure of the proposed dam in river Dibang, a major Himalayan tributary of the River Brahmaputra. Significant variation in bed slope, bed width and resistance characteristic along the channel length are the typical characteristics of Dibang river. To compute the flood under such dam failure conditions, natural channel is generally represented by a simplified channel. . Such simplification may lead to erroneous estimation of the important parameters such as maximum probable depth, peak arrival, maximum probable velocity and inundated area. Therefore, due emphasis should be given in the selection of an appropriate computational channel while simulating a real dam break flood. Two different approaches for adopting the computational natural channel have been proposed here for predicting the dam break flood. Probable maximum depth of flood, time of peak arrival and the maximum probable flood velocity at different sections up to 63 km downstream of the proposed Dibang dam have been computed . In one approach, the predictions are made by adopting a computational channel, which considers the whole floodplain downstream of the dam when River Dibang enters the plain. The other approach considers only the original simplified river channel of Dibang. The predictions of dam Break Wave Propagation by both the methods have been compared and analyzed. The analysis clearly illustrates the importance of the proper selection of the computational channel in a river valley downstream of the dam to avoid over estimation or underestimation of flood.

- by mimi das saikia
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- Unsteady Flow and Transient Flow

Two fundamental one-dimensional (1D) models are proposed and applied to simulate the transient flows with the propagation of an interface in a water-filled duct. The proposed models are developed to simulate the unsteady open channel flows based on finite-volume method (FVM). The models presented herein are based on the continuity and momentum equations of free surface and pressurized flows and the momentum equation of an interface between both flows. However, the highly simplified marker and cell (HSMAC) method with pressure iteration procedures is applied to the pressurized flow region. The numerical simulations are performed under the hydraulic conditions of previous experiments, and then simulated results were compared with the experimental data. It is pointed out that the solitary wave solution is able to reproduce the air cavity profile. In contrast to the hydrostatic model, results of the Boussinesq model compare reasonably well to the experimental observations.

- by Hamid Bashiri and +1
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- Finite Volume Methods, Hydraulics, Free surface flows, Pipeline

The paper's focus is the calculation of unsteady incompressible 2D flows past airfoils. In the framework of the primitive variable Navier–Stokes equations, the initial and boundary conditions must be assigned so as to be compatible, to assure the correct prediction of the flow evolution. This requirement, typical of all incompressible flows, viscous or inviscid, is often violated when modelling the flow past immersed bodies impulsively started from rest. Its fulfillment can however be restored by means of a procedure enforcing compatibility, consisting in a pre-processing of the initial velocity field, here described in detail. Numerical solutions for an impulsively started multiple airfoil have been obtained using a finite element incremental projection method. The spatial discretization chosen for the velocity and pressure are of different order to satisfy the inf–sup condition and obtain a smooth pressure field. Results are provided to illustrate the effect of employing or not the compatibility procedure, and are found in good agreement with those obtained with a non-primitive variable solver. In addition, we introduce a post-processing procedure to evaluate an alternative pressure field which is found to be more accurate than the one resulting from the projection method. This is achieved by considering an appropriate ‘unsplit’ version of the momentum equation, where the velocity solution of the projection method is substituted. Copyright © 2004 John Wiley & Sons, Ltd.

- by Luca Riviello
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- Computational Fluid Dynamics, Fluid Dynamics, Viscous Flows, Incompressible Flows

In this paper we have studied heat and mass transfer effects on unsteady free convection boundary layer flow past an impulsively started vertical surface with Newtonian heating, where the heat transfer rate from the bounding surface with a finite heat capacity is proportional to the local surface temperature, and which is usually termed as conjugate convective flow. The equations governing the flow are studied in the closed form by using the Laplace transform technique. The effects of various physical parameters are studied through graphs and the expression for skin friction also derived and discussed.

- by Prof. M. Changal Raju
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- Unsteady Flow and Transient Flow

The study of draining processes without admitting air has been conducted using only steady friction formulations in the implementation of governing equations. However, this hydraulic event involves transitions from laminar to turbulent flow, and vice versa, because of the changes in water velocity. In this sense, this research improves the current mathematical model considering unsteady friction models. An experimental facility composed by a 4.36 m long methacrylate pipe was configured, and measurements of air pocket pressure oscillations were recorded. The mathematical model was performed using steady and unsteady friction models. Comparisons between measured and computed air pocket pressure patterns indicated that unsteady friction models slightly improve the results compared to steady friction models.

This research aims to evaluate the influence of air on the water surface and pressure profiles in two-phase (air–water) flow. In previous studies, the authors used the continuity and momentum equations for water in a one-dimensional finite volume model. In the present study, the authors additionally incorporate both the mass conservation and momentum equations for incompressible fluids as basic equations for the air portion. Momentum equations were integrated for the free surface and pressurized flow regions after interface observations made at each time step. To correct velocity and pressure in the pressurized portion, the authors used the highly simplified mark-and-cell method. A pressure drop equation is used at the interface of pressurized and free surface flows to deduce the pressure. The Harten total variation diminishing scheme was used to avoid numerical oscillations. The results were compared with experimental data and numerical simulations without considering the air effect.

- by Hamid Bashiri
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- Computational Fluid Dynamics, Finite Volume Methods, Hydraulics, Free surface flows

Streszczenie. modelowanie zmiennych w czasie oporów hydraulicznych nie jest zagadnieniem łatwym. Jak wykazały liczne badania, naprężenie styczne na ściance przewodu może być wyznaczane jako suma quasi-ustalonego i zmiennego w czasie wyrażenia. zmienne w czasie wyrażenie jest tzw. całką splotową z lokalnego przyspieszenia cieczy i funkcji wagi. oryginalna postać funkcji wagi ma przeważnie skom-plikowaną strukturę, przez co nie nadaje się do efektywnego symulowania przebiegów dynamicznych. Dlatego też w celu umożliwienia efektywnego wyznaczania niestacjonarnego naprężenia stycznego zapre-zentowano nową postać funkcji wagi (będącą skończoną sumą wyrażeń eksponentalnych). w przypadku przepływu turbulentnego wykorzystano procedurę skalowania współczynników efektywnej funkcji wagi zaprezentowaną przez Vitkovskiego i in. nowe postacie funkcji wagowych charakteryzuje duża zbieżność z funkcjami klasycznymi (nieefektywnymi). z wykorzystaniem uprzednio omówionych modeli przepływu kawitacyjnego cSm, cSmG, cSma oraz bcm i powyższych efektywnych funkcji wagi dokonano sze-regu badań symulacyjnych, które wykazały, że wprowadzone w modelach przepływu niestacjonarnego z kawitacją zmiany poprawiają stopień zgodności symulacji z wynikami eksperymentalnymi. Słowa kluczowe: numeryczna mechanika płynów, przepływy nieustalone, kawitacja, zmienne opory hydrauliczne, przewody ciśnieniowe

The paper's focus is the calculation of unsteady incompressible 2D ows past airfoils. In the framework of the primitive variable Navier-Stokes equations, the initial and boundary conditions must be assigned so as to be compatible, to assure the correct prediction of the ow evolution. This requirement, typical of all incompressible ows, viscous or inviscid, is often violated when modelling the ow past immersed bodies impulsively started from rest. Its fulÿllment can however be restored by means of a procedure enforcing compatibility, consisting in a pre-processing of the initial velocity ÿeld, here described in detail. Numerical solutions for an impulsively started multiple airfoil have been obtained using a ÿnite element incremental projection method. The spatial discretization chosen for the velocity and pressure are of di erent order to satisfy the inf-sup condition and obtain a smooth pressure ÿeld. Results are provided to illustrate the e ect of employing or not the compatibility procedure, and are found in good agreement with those obtained with a nonprimitive variable solver. In addition, we introduce a post-processing procedure to evaluate an alternative pressure ÿeld which is found to be more accurate than the one resulting from the projection method. This is achieved by considering an appropriate 'unsplit' version of the momentum equation, where the velocity solution of the projection method is substituted.

- by Andrea Mola and +1
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- Engineering, Computational Fluid Dynamics, Fluid Dynamics, Finite Element

Diese Arbeit untersucht den Einfluss einer Oberflächenrauheit auf den Widerstandsbeiwert eines quer-angeströmten Kreiszylinders mit dem Durchmesser D = 0,08 m mithilfe der Software OpenFOAM. Als Art der Rauheit wird ein Stolperdraht bei einem Winkel von 50° und einem Verhältnis von k/D = 0,004 verwendet. Zuerst werden die Grundlagen der Strömungsmechanik einer Zylinderumströmung und einer instationären Strömungssimulation geschaffen, bevor die Parameter des Rechenfalls bestimmt werden, die für die Korrektheit notwendig sind. Die Berechnung der Strömungs erfolgt mit dem LES-Löser pisoFoam und benutzt das Turbulenzmodell SpalartAllmarasDDES.
Neben einer Studie zum Mittelungsintervall, das von 0,01 s bis 3 s reichen sollte, wird die notwendige Oberflächenauflösung mit 0,391 mm (surfaceRefinement-Level 6) für den glatten Zylinder bestimmt. Beim Zylinder mit Sandrauheit soll die kleinste Zelle außerhalb der Wandschicht Kantenlängen kleiner gleich 0,195 mm aufweisen. Schließlich wird der numerisch ermittelte Verlauf des Widerstandsbeiwertes für Reynoldszahlen zwischen 30.000 und 100.000 angegeben.

- by Andreas Iberl
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- Fluid Mechanics, Unsteady Flow and Transient Flow, Cfd

Owing to the persistent interest in the approximate methods, the order of magnitude analysis is revisited in the present work and a method is proposed for the quick estimation of drop in temperature of a fluid flowing through a circular duct. The principle involves judicious guessing of temperature drop for one of the investigated cases, while the temperature estimates for rest of the cases are deduced from the initial guess. A problem of duct flow with widely varying inlet temperature and mass flow rate conditions, typical of a high altitude simulation ground test facilities for scramjet combustors, re-entry vehicles, etc are considered which vary from 400 K to 1200 K with corresponding mass flow rates of 300 kg/s to 0.5 kg/s. The results of this quick estimation method portray remarkable agreement with the exit temperatures of the fluid as predicted by solving the problem using transient quasi-one dimensional codes and two dimensional CFD techniques over the entire operating regime and hence provide a conservative estimate of vital parameters for any design exercise as well as to rate an existing system at off-design conditions. The significant advantages of the proposed method over the others are viz. (a) simplicity (b) lesser computational effort and (c) reasonable accuracy, will be elucidated in detail in this article.

- by Krishnan Srinivasan
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- Unsteady Flow and Transient Flow

The unsteady magnetohydrodinamics flow of an incompressible viscous electrically conducting fluid between two horizontal parallel non-conducting plates, where the lower one is stretching sheet and the upper one is oscillating porous plate, is studied in the presence of a transverse magnetic field and the effects of Hall current. Fluid motion is caused by the stretching of the lower sheet and a constant suction is applied at the upper plate which is oscillating in its own plane. The stretching velocity of the sheet is assumed to be a linear function of distance along the channel. The expressions relating to the velocity distribution are obtained and effects of different values of various physical parameters are calculated numerically and shown graphically.

Study of slug flow in horizontal, inclined and vertical pipes

In this study, the difference between laminar fast transient ﬂow of shear-thinning liquids and that of Newtonian liquids under similar conditions is numerically studied. Since the literature appears to lack fast transient ﬂow investigation of non-Newtonian ﬂuids, this work addresses features of those ﬂows. In this way, the Newton–Kantorovich method is implemented to linearize nonlinear shear stress term available in the characteristic equations.The veriﬁcation and validation of the solution are carried out in detail. The results show that the non-Newtonian behavior of ﬂuids has signiﬁcant inﬂuence on the velocity and shear stress proﬁles and also on the magnitude of pressure head and wall shear stress.