تاثير شكل بستر شني بر توزيع هاي سرعت جريان و تنش برشي

چكيده انگليسي :

AbstractTurbulent flow over an erodible bed is influenced by complex interaction of fluid grain of sediment and bed forms Dunes can form the majority of the deposits ofalluvial rivers They are developed in a wide range of textural materials from silt togravel These bed forms occur at discrete sites of the bed although they commonlyrecur in periodic fashion and therefore can be considered as extension of channels Their formation destruction and stability are highly dependent on localized flowconditions and their presence in turn is known to play a crucial role in governingflow They can affect the spatial and temporal distributions of local velocity and shearstress and as a result local sediment transport rate Understanding the feedback in thiscomplex interaction among fluid flow sediment flux and bed morphology are at thehead of studies of river engineering No theory yet exist which can adequately predictthe formation of dunes and their subsequence effects on the turbulent flow field Sincemost previous investigations have been limited to sand dunes this researchinvestigates the effect of gravel dunes and the change of wavelength on flowstructure The experiments were conducted in a straight flume 30m long 1 5m wideand 0 5m deep with zero bed slope and maximum discharge of 200 lit s in the CivilEngineering Hydraulics Laboratory at the University of Ottawa Its side and bottomconstructed of cement and the base constructed of aluminum The flume recirculatedwater from a large underground reservoir through a series of pipes to the channelentrance where turbulence is dampened by a large setting basin Approximately 1 6ton gravel with the median diameter of 10mm was used to build the dunes theprimary advantage of utilizing fixed bed forms are that they allow detailedmeasurements of velocity without complication of both a migrating and changing bedform and the difficulty of flow measurement in the presence of sediment transport However measurements must be averaged over fairly long period of time to obtainaccurate turbulence statistics and because bed forms migrate the flow is typically notstationary over requisite averaging periods Taking them into account all trials madeon laboratory setup with fixed 2D bed form As it was mentioned above the goal ofthis research is to investigate the wavelength effect on flow structure To achieve thisobjective 3 different sizes of wavelength accompanied a constant height slip faceangle and depth were studied in the test section In total 3 runs were selected Thebed dimensions were chosen on the basis of previous studies and existing empiricalrelationship for dune size In all cases 8 two Dimensional gravel dunes 0 08m highwith a slip face angle of 28 were formed To cast these dunes a wooden templateaccording to dune geometry was made Since equilibrium is reached by about fifthbed form the number of 8 dunes were accepted In the first case considering thehydraulic condition of flume and on the basis of Van Mario and de Ruiter 1988 awavelength of 1 6m height of 0 08m and lee side angle of 28 accompanied by waterdepth of 0 252m were selected In the second and third case the 0 08m height and 28 slip face for lee side was preserved constantly and the chosen wavelengths wererespectively 1 26 and 0 96m according to accepted ratio for dune dimensions Calculations of turbulence characteristics were obtained under constant flow depthover fixed two dimensional dunes using a Nortek Vectrino acoustic Dopplervelocimeter ADV The ADV measures three component flow velocities at 200Hz Velocity were sampled for 120s at each point The lowest point in each velocityprofile was 0 005m above the bed All the measurements in each Runs was made overone dune between crest number six and seven approximately 5m upstream of thetailgate and 10 15 17m respectively for Run1 Run2 and Run3 downstream from the

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تاثير شكل بستر شني بر توزيع هاي سرعت جريان و تنش برشي

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