scholarly journals Analisis Perbandingan Velocity Dan Shear Stress Perkembangan Boundary Layer Flat Plate Menggunakan Turbulent Model k – ε (Standard, Realizable, RNG)

2017 ◽  
Vol 2 (1) ◽  
pp. 27-37
Author(s):  
Setyo Hariyadi S.P.
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Flat Plate ◽  
Turbulent Model ◽  
Standard Parameter

Pengertian lapisan batas adalah daerah dimana aliran mengalami hambatan karena adanya tegangan geser yang besar pada permukaan benda, sehingga partikel-partikel fluida terpaksa berhenti pada sekitar permukaan benda karena geseran viskos. Aliran fluida sejati mana pun selalu menunjukkan adanya suatu daerah yang alirannya terhambat, yaitu dekat batas yang kecepatannya relatif terhadap batas bervariasi antara nol pada batas hingga suatu harga yang dapat diduga dari solusi aliran potensial di titik yang agak jauh dari situ. Daerah yang alirannya terhambat ini disebut lapisan batas (boundary layer) dan ketebalan lapisan batas itu sendiri dinyatakan dengan δ.Proses pembentukan lapisan batas mungkin poling baik bila divisualisasikan dengan membayangkan aliran di sepanjang sebuah pelat rata. Misalkan ada aliran seragam sebuah fluida tak dapat mampat mendekati pelat dengan kecepatan freestream. Ketika fluida mencapai tepi sebelah depan, tegangan geser yang besar terbentuk dekat dengan permukaan pelat karena partikel-partikel fluida yang tiba di situ terpaksa berhenti dan partikel-partikel yang cukup dekat dan normal terhadap plat dihambat oleh geseran viscous. Lapisan batas menebal dalam arah yang sama dengan arah aliran, akibatnya perubahan kecepatan dari nol dipermukaan pelat hingga jarak tertentu pada jarak δ semakin jauh menjadi semakin besar. Laju perubahan kecepatan tadi menentukan gradient kecepatan di permukaan plat dan karena itu tegangan gesernya juga. Studi numerik telah dilaksanakan untuk menguji kinerja aerodinamis pada plat datar dengan menggunakan beberapa turbulent model k – ε (Standard, Realizable, RNG). Kecepatan freestream yang digunakan yaitu kecepatan 10 m/s dan pada kondisi udara standard. Parameter yang dievaluasi meliputi shear stress dan profil kecepatan. Dari penelitian tersebut didapatkan bahwa dengan penggunaan turbulent model k – ε Realizable menghasilkan yang terbaik dibandingkan turbulent model yang lain.

Effects of an Upstream Circular Manipulator on the Flow Past a Cylinder-Flat Plate Junction

2003 ◽  
Author(s):  
Alan Dow ◽  
George Elizarraras ◽  
Hamid R. Rahai ◽  
Hamid Hefazi
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Flat Plate ◽  
Horseshoe Vortex ◽  
Turbulent Shear ◽  
Plate Surface ◽  
Mean Velocity ◽  
Time Resolved ◽  
Turbulent Shear Stress ◽  
Flow Past A Cylinder

Measurements of three components of mean velocity and simultaneous time-resolved measurements of axial and vertical turbulent velocities and their cross moment were made at three perpendicular planes slightly upstream of the corner and in the downstream interaction region of a cylinder-flat plate junction with and without an upstream circular manipulator. The circular manipulator was a smooth circular cylinder of 1.25 mm diameter, which was placed upstream of the cylinder at X/D = 1.2, within the boundary layer above the flat plate surface. Results show that when the manipulator is in place, there is a decrease in the axial mean velocity and increases in the axial mean squared turbulent velocity and turbulent shear stress at the first plane. There is an expanded region of secondary flow with reduced circulation, indicating that the manipulator has reduced the strength of the horseshoe vortex in this region.

scholarly journals Secondary Flow Development Downstream of a Blade Endwall Corner

1994 ◽  
Author(s):  
A. Karim Abdulla-Altaii ◽  
Rishi S. Raj
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Flat Plate ◽  
Secondary Flow ◽  
Total Pressure ◽  
Static Pressure ◽  
Horseshoe Vortex ◽  
Chord Length ◽  
Trailing Edge

The flow downstream of the corner formed by a blade and a flat plate was investigated experimentally. A single dominant horseshoe vortex was identified which persisted more than one chord length downstream of the blade trailing edge. A smaller and weaker corner vortex was also identified. It dissipated and ceased to exist by a downstream axial location of approximately 0.2C (C= chord length). There was no evidence of stress induced vortices in the region of this investigation. The secondary flow system redistributes the mean flow momentum and distorts total pressure profiles and contours. In planes parallel to the flat plate, total pressure values were found to be higher than the undisturbed two-dimensional boundary layer at that height. Surface static pressure was found to be at its maximum at the blade trailing edge location and it decreased in both the downstream and transverse directions. There was no significant static pressure variation in the spanwise direction. Downstream of the blade trailing edge, under the domain of the horseshoe vortex, local wall shear stress increased to values exceeding the values found in the undisturbed boundary layer at that axial location. However, a 20% reduction in the net wall skin-friction (wall shear stress integrated over the flat plate surface) was observed.

Large-scale contribution to mean wall shear stress in high-Reynolds-number flat-plate boundary layers up to 13650

2014 ◽  
Vol 743 ◽  
pp. 202-248 ◽  
Author(s):  
Sébastien Deck ◽  
Nicolas Renard ◽  
Romain Laraufie ◽  
Pierre-Élie Weiss
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Flat Plate ◽  
Skin Friction ◽  
High Reynolds Number ◽  
Large Scale ◽  
Wall Shear ◽  
High Reynolds

AbstractA numerical investigation of the mean wall shear stress properties on a spatially developing turbulent boundary layer over a smooth flat plate was carried out by means of a zonal detached eddy simulation (ZDES) technique for the Reynolds number range $3060\leq Re_{\theta }\leq 13\, 650$. Some asymptotic trends of global parameters are suggested. Consistently with previous findings, the calculation confirms the occurrence of very large-scale motions approximately $5\delta $ to $6 \delta $ long which are meandering with a lateral amplitude of $0.3 \delta $ and which maintain a footprint in the near-wall region. It is shown that these large scales carry a significant amount of Reynolds shear stress and their influence on the skin friction, denoted $C_{f,2}$, is revisited through the FIK identity by Fukagata, Iwamoto & Kasagi (Phys. Fluids, vol. 14, 2002, p. L73). It is argued that $C_{f,2}$ is the relevant parameter to characterize the high-Reynolds-number turbulent skin friction since the term describing the spatial heterogeneity of the boundary layer also characterizes the total shear stress variations across the boundary layer. The behaviour of the latter term seems to follow some remarkable self-similarity trends towards high Reynolds numbers. A spectral analysis of the weighted Reynolds stress with respect to the distance to the wall and to the wavelength is provided for the first time to our knowledge and allows us to analyse the influence of the largest scales on the skin friction. It is shown that structures with a streamwise wavelength $\lambda _x >\delta $ contribute to more than $60\, \%$ of $C_{f,2}$, and that those larger than $\lambda _x >2\delta $ still represent approximately $45\, \%$ of $C_{f,2}$.

Direct numerical simulation of turbulence in a nominally zero-pressure-gradient flat-plate boundary layer

2009 ◽  
Vol 630 ◽  
pp. 5-41 ◽  
Author(s):  
XIAOHUA WU ◽  
PARVIZ MOIN
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Plate Boundary ◽  
Transitional Region ◽  
Bypass Transition ◽  
Number Range ◽  
Flat Plate Boundary Layer

A nominally-zero-pressure-gradient incompressible boundary layer over a smooth flat plate was simulated for a continuous momentum thickness Reynolds number range of 80 ≤ Reθ ≤ 940. Transition which is completed at approximately Reθ = 750 was triggered by intermittent localized disturbances arising from patches of isotropic turbulence introduced periodically from the free stream at Reθ = 80. Streamwise pressure gradient is quantified with several measures and is demonstrated to be weak. Blasius boundary layer is maintained in the early transitional region of 80 < Reθ < 180 within which the maximum deviation of skin friction from the theoretical solution is less than 1%. Mean and second-order turbulence statistics are compared with classic experimental data, and they constitute a rare DNS dataset for the spatially developing zero-pressure-gradient turbulent flat-plate boundary layer. Our calculations indicate that in the present spatially developing low-Reynolds-number turbulent flat-plate boundary layer, total shear stress mildly overshoots the wall shear stress in the near-wall region of 2–20 wall units with vanishing normal gradient at the wall. Overshoots as high as 10% across a wider percentage of the boundary layer thickness exist in the late transitional region. The former is a residual effect of the latter. The instantaneous flow fields are vividly populated by hairpin vortices. This is the first time that direct evidence (in the form of a solution of the Navier–Stokes equations, obeying the statistical measurements, as opposed to synthetic superposition of the structures) shows such dominance of these structures. Hairpin packets arising from upstream fragmented Λ structures are found to be instrumental in the breakdown of the present boundary layer bypass transition.

The effect of spatial restriction on the inner-layer structure of wall turbulence

1987 ◽  
Vol 177 ◽  
pp. 485-500 ◽  
Author(s):  
Shigeo Maruyama ◽  
Hiroaki Tanaka
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Shear Flow ◽  
Flat Plate ◽  
Outer Layer ◽  
Layer Structure ◽  
Wall Turbulence ◽  
Moving Plate ◽  
Hot Film

Hot-film-anemometer measurements were carried out in a shear flow between a flat plate and a moving plate fitted with an array of tall fences. The effect of spatial restriction by the fences on the inner-layer structure of the boundary layer developing on the flat-plate side was investigated. It was revealed that the inner-layer structure was maintained even when the tips of the fences were passing at a distance y+ = 45 from the flat plate; the flow did not become laminar-like until the tips reached y+ = 25. These results suggested the physical view that the inner layer of wall turbulence has a tough, self-sustaining structure, which is uniquely determined under a given mean wall shear stress and is hardly influenced by outer-layer disturbances provided that its own spatial extent of about 45 ν/u* from the wall is maintained.

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