Sains Malaysiana 44(1)(2015): 147–153

 

Threshold Criteria for Incipient Grain Motion with Turbulent Fluctuations on a

Horizontal Bed

(Kriteria Pergerakan Ambang Butiran oleh Fluktuasi Gelora

di atas Dasar Mendatar)

 

W.H.M. WAN MOHTAR*

Department of Civil & Structural Engineering, Faculty of Engineering and Built Environment

Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia

 

Diserahkan: 23 September 2013/Diterima: 31 Mei 2014

 

ABSTRACT

The effect of turbulent fluctuations on the threshold criteria for incipient grain motion over a wide range of sediment size is investigated. In this work, attention is paid to the comparison of the critical Shields parameter θc profile obtained when the near-bed fluid forces induced sediment motion are oscillating-grid turbulence and a single idealised eddy of vortex ring. For experimental work, near-spherical monodisperse sediments were used throughout with relative densities of 1.2 and 2.5 and mean diameters d ranging between 80 and 1087 μm. The measured values of θc on a horizontal bed α = 0 (hence denoted as θc0), were compared to the θc0 profiles obtained by grid turbulence and vortex ring experiments. Although different in magnitude, the θc0 profiles were comparable, that is the θc0 were seen to increase monotonically for hydraulically smooth bedforms and to be approximately constant for hydraulically rough bedforms. However the limit of hydraulically smooth region was found to vary between the oscillating-grid turbulence experiments, where wider smooth region was found when the turbulent fluctuations used to calculate θc0 is not the near-bed velocity.

 

Keywords: Hydraulically smooth; incipient grain motion; oscillating-grid turbulence; rough bedforms; vortex ring

 

ABSTRAK

Kajian ini melihat kesan bentukan gelora ke atas kriteria nilai pergerakan ambang butiran pada satu julat saiz sedimen. Fokus kajian adalah perbandingan profil parameter kritikal Shields θc apabila daya bendalir dekat-dasar bagi sedimen bergerak teraruh oleh turbulens grid-berayun dan pusaran tunggal terunggul cincin vorteks. Uji kaji dilakukan ke atas sedimen seragam berbentuk hampir-sfera dengan ketumpatan relatif sedimen adalah 1.2 dan 2.5 dan diameter purata d adalah antara julat 80 dan 1087 μm. Nilai terukur θc pada dasar mendatar α = 0 (dengan itu ditandakan sebagai θc0) dibandingkan dengan profil diperoleh bagi kedua-dua eksperimen turbulens grid-berayun dan cincin vorteks. Walaupun nilai terukur berbeza daripada segi magnitud, profil θc0 bagi kedua-dua uji kaji adalah sebanding, iaitu θc0 bertambah secara ekanada bagi bentuk dasar kelicinan hidraulik dan hampir malar bagi bentuk dasar kekasaran hidraulik. Namun begitu, batasan kawasan kelicinan hidraulik didapati berubah bagi eksperimen grid-berayun dengan daerah kelicinan didapati lebih lebar apabila bentukan gelora diguna pakai untuk mengira θc0 bukan halaju hampir-dasar.

 

Kata kunci: Bentuk dasar kekasaran; cincin vorteks; kelicinan hidraulik; pergerakan ambang butiran; turbulens grid-berayun

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*Pengarang untuk surat-menyurat; email: hanna@ukm.edu.my

 

 

 

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