مجله هیدرولیک
سال ششم شماره 1 (بهار 1390)

Shear stress is one of the important parameters in river flow studies and has a key role on river patternand also estimating the amount of erosion and sedimentation of fluvial material within the river.Therefore, estimating this parameter in different positions of the field flow, especially next to the riverbed and banks, is vital. Despite doing intensive studies by many researchers on the amount of shearstress and its distribution in straight and curved channels, due to the complex flow regime within thebends, more studies have to be carried out. In this research, the results obtained from various methodsof shear stress estimation are compared with those obtained by using of mean velocities and theirfluctuations taken by an ADVP instrument. The compared methods include Reynolds shear stress,Turbulent Kinematic Energy, and Logarithmic Profile, have been previously used and compared instraight channels, but never have been used and compared in a sharp bend. The results obtained fromthe study on a 1930 bend with developed topography showed that unlike the straight channels, thestrong 3D flow changes the Reynolds stress patterns in sharp bend, so that there is no specific order toget a reliable estimation of bed shear stress in this type of channels. Moreover, because of the effectsof strong secondary current, the longitudinal velocity profiles in sharp bends are flatter than those instraight channels, and hence there are some uncertainty regarding using logarithmic profiledistribution. The results of this study showed that in general, the Logarithmic method to estimate thebed shear stress gives better results among the available mentioned methods.

One of the most substantial water resources in water engineering is the groundwater, stored inaquifers. In arid and tropical regions having monsoon climate, the groundwater accumulated under theground and used later during dry seasons. The use of subsurface dams in aquifers to store water behindthe dam is one of the common techniques in recent years. But a weir should be constructed insubsurface dam body to convey the excess water from the aquifer. Without an appropriate weir towithdraw the excess water, groundwater level will reach the ground surface and in addition todisrupting the lands application, will cause inevitable damages to the environment. In this study, theeffects of characteristics of weir together with the aquifer hydrodynamic properties on groundwaterflow discharge are studied. The effects of different hydraulic parameters on the flow rate through theweir of the dam are also investigated. An experimental model of the aquifer, including undergroundreservoir, dam and weir is built and the effects of different parameters such as hydraulic conductivityof the aquifer, head above the weir, height of the weir and weir crest length on the discharge throughthe weir in steady state condition are tested. The results show that increasing the hydraulicconductivity of the aquifer, yields an increase of the flow discharge. By increasing the weir height, thehead above the weir or the weir crest length, the discharge increases. In order to evaluate all theinfluential parameters and analyze their effects on flow discharge through the subsurface weir, theSeep3D software is used. The results of numerical models are verified using the measured data. Thendifferent models of underground dams and were prepared and the effect of each influential parameteron the groundwater discharge through the weir was studied. Results of numerical models showed thatthe hydraulic conductivity of aquifer, head above the weir and the effective width of the aquifer are themost significant parameters to establish a certain groundwater discharge through the weir.

The outer bank in meandering rivers is suitable place for flow diversion. Flow diversion from the outerbank of channel lead to formation of a transverse flow component having an opposite alignment withthe secondary current in bed and consequently transferring a part of the sediment bed load to intake.The diversion of flow will change the flow pattern in the river bend and decrease the energy of thesecondary current in this location. In this paper the effect of flow diversion to sediment entering theintake in a 52 degree diversion angle was investigated. The results have shown that the low ratios ofdiverted flow has a little effect on the secondary current, and also this current causes the repelling ofsediments from the outer bend of the river. Along with the increase of the ratio of diverted flow, theenergy of secondary currents decrease and the ratio of sediment diversion increases. In a specific valueof flow diversion, the secondary flow was fully damped. After that, the sediments entering the intakehave a higher increase compared to the previous situation. The results also showed that the trend ofvariations of diverted sediments with the flow diversion generally follows the S shape.

Investigation on the behavior of plants, when exposed to hydrodynamic forces, is of vital importancein flood analysis and evaluation of its consequent losses, design of channels and water resourcesmanagement. Because of the important effects of flow depth and velocity on plants' bending andrapture point, the evaluation of which is necessary for flood loss estimation and its management acrossthe watershed. Studying the bending of plants and their probable rapture is not easy on account ofcomplicated interaction between plant and flow characteristics. In this paper, an analytical method ispresented for plant rapture in an un-submerged condition and static state and the outcomes arecompared with laboratory results. In this study, the multiplication of depth by flow velocity is used asan index for the analysis of the plant rapture. The differences between the results of the analyticalsolution and laboratory tests for the rapture index and the maximum tensile stress were less than 0.5and 0.1 percents respectively. The results of the analytical solution and laboratory tests reveals thatunder the same hydraulic conditions the rapture point for rigid plants with fixed diameter is located inthe plant's base, but this point will move upward for plants of variable diameter. In this regard, if thereduction of the plant's diameter is relatively high, this point will be closer to the water surface.

This research aims to model the dynamic viscoelastic effects of polyethylene pipes under waterhammerpressures. As known, following the water hammer event, a cyclic loading and unloading dueto the pressure fluctuations occurs. Loading on polymeric materials results in deformations that unlikethe elastic materials do not immediately return after unloading. This retarded behaviour graduallyreduces the wave speed causing to a different transient response. Herein, the viscoelastic effects ofpolyethylene pipes on the transient flow is numerically simulated and studied using the Kelvin-Voigtmodel. The developed model is then verified using some experimental data from the literature.Afterwards, a simple hypothetical reservoir-pipe-valve system is defined to investigate several aspectsof viscoelasticity. It is concluded that in a standard water hammer caused by valve closure, theminimum pressure head is more affected by viscoelasticity than the maximum one. Furthermore, withthe increase of initial flow, viscoelasticity takes part more to damp out the destructive dynamic effectsof water hammer. This makes the viscoelastic properties of polyethylene pipes to be figured out as anadaptive procedure to automatically alleviate the water hammer issues in water supply systems.

Floating breakwaters (FB) represent an alternative solution to protect coastal areas from wave attackand erosion. The major advantages of using floating breakwater in comparison to conventional fixedones are lower cost of construction, using in severe hydrodynamic conditions and high speedinstallation. Therefore, engineers lead to design and use floating breakwaters in those situation inwhich this type of breakwater is more suitable to construct. To predict floating breakwater behavior, itis essential to investigate flow characteristics in the vicinity of structure. In this regard, numericalmodeling is presented based on the solutions of 2DV Reynolds Averaged Navier-Stokes (RANS)equations and the VOF method using in Fluent Software.To generate wave, a sub-model has been developed and hooked to the main software. Results obtainedfrom the numerical model then validated by experimental data cited in the literature to represent thecapability of model in simulation of complicated problems. Numerical modeling shows that the singlefixed FB operates well but, the attached plate in the front part of the FB significantly enhances theefficiency of the structure and with regard to cost-effectiveness, the configuration of the FB with theattached plate should be considered for design purposes. Also, double FBs and trapezoidal ones withinclined faces of different angles are simulated. It is found that the trapezoidal FBs are the mostefficient in comparison to the other types of FBs.

Analysis of turbulent plane channel flow is performed by direct numerical simulation. In this papercommon forms of the nonlinear term are applied to solve Navier-Stokes equations by using pseudospectral method. The Navier-Stokes equations are expanded with Chebychev and Fourier series in PSmethod and commonly used forms of the nonlinear terms
 
 are explained and applied,referred to as divergence, convection, skew-symmetric, rotational, alternating and linearized form. Theresults of channel flow analysis are presented due to those six forms of the nonlinear terms .Thecomputational grids of 128×65×128 in computational domain of 1=[0,46]×[-1,1]×[0,26] are used inthe x, y and z directions, respectively. The friction Reynolds number for channel flow is set to beRe=175.The comparison is made between turbulent quantities such as the turbulent statistics 

   Theresults show that that the rotational and linearized forms are more appropriate than other schemes,especially than skew-symmetric form. But it must be noted that linearized form is not energyconservingso it is not stable and applicable in all situations. Hence the choice of nonlinear termreduces to the rotational form in analysis of pseudo spectral method which satisfies robustness andaccuracy and reduces the computational cost at about 62% (compared to other schemes). Frompractical point of view, different types of nonlinear term do affect in reduction of total running time athigher Reynolds number and greater domain in fluid flow, which provides decreased cost and time indirect numerical simulation of channel flow if final decision rests on economics.