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

Received: 23 September 2013/Accepted: 31 May 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 θ_c 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 θ_c on a horizontal bed α = 0 (hence denoted as θ_c0), were compared to the θ_c0 profiles obtained by grid turbulence and vortex ring experiments. Although different in magnitude, the θ_c0 profiles were comparable, that is the θ_c0 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 θ_c0 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 θ_c 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 θ_c pada dasar mendatar α = 0 (dengan itu ditandakan sebagai θ_c0) dibandingkan dengan profil diperoleh bagi kedua-dua eksperimen turbulens grid-berayun dan cincin vorteks. Walaupun nilai terukur berbeza daripada segi magnitud, profil θ_c0 bagi kedua-dua uji kaji adalah sebanding, iaitu θ_c0 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 θ_c0 bukan halaju hampir-dasar.

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

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*Corresponding author; email: hanna@ukm.edu.my