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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