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Flow control of a circular cylinder with O-rings
No full textThe flow around a circular cylinder is controlled by attaching O-rings to reduce drag force acting on the cylinder. Wind tunnel experiments on the flow around a circular cylinder with and without ring type surface protrusions are carried out to investigate the flow characteristics of the controlled wake. Four experimental models are tested in this study; one smooth cylinder of diameter D (60 mm) and three cylinders fitted with longitudinal O-rings of diameters d=0.0167D, 0.05D and 0.067D with various pitches. The drag force, mean velocity and turbulence intensity profiles in the near wake behind the cylinders are measured for Reynolds numbers based on the cylinder diameter in the range of ReD=7.8×10^3 ~ 1.2×10^5. Flow field around the cylinders is visualized using a smoke-wire technique to see the flow structure qualitatively. The results are compared with those for a smooth cylinder having the same diameter. At ReD=1.2×10^5, the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.165D shows the maximum drag reduction of about 9%, compared with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high and it has nearly the same value as that of the smooth cylinder. For the O-ring circular, as the Reynolds number increases, the location of peak turbulence intensity shifts downstream and the peak magnitude is decreased. In addition, the vortex shedding frequency has nearly same value as that of the smooth cylinder up to a Reynolds number of 3.2×10^4. Thereafter, the shedding frequency increases and finally disappears as the Reynolds number increases. The visualized flow for the smooth cylinder does not show distinct spanwise variation of flow pattern. However, the size of vortices and vortex formation region formed behind the O-ring cylinder are smaller, compared with the smooth cylinder. In addition, the instantaneous topological flow image shows spanwise variation of V-shaped flow pattern. Consequently, the simple attachment of O-rings effectively controls the vortex shedding from the cylinder and elongates the vortex formation region, reducing drag force acting on the cylinder at high Reynolds numbers
A numerical study on flow around a triangular prism located behind a porous fence
No full textThe flow characteristics around a two-dimensional triangular-shaped prism located behind a porous wind fence were investigated numerically. The shelter effect of the porous wind fence on wind flow around the triangular prism model was studied by changing the porosity, height, and location of the wind fence. The numerical method developed for the present work is based on the finite volume method with the Quick scheme. The RNG k– turbulence model and orthogonal grid were used and they are found to be stable in terms of numerical convergence for predicting the separated shear flow. The validity of the numerical method developed in the present work has been evaluated by comparing the numerical results with the experimental data. As a result, the numerical predictions show good agreements with the experimental results. The wind fence with a porosity of about 0.3–0.5 seems to be most effective in attenuating the mean pressure around the prism model acting on the prism surface and turbulent kinetic energy
Flow around a cube in a turbulent boundary layer: LES and experiment
Get PDFWe present a numerical simulation of flow around a surface mounted cube placed in a turbulent boundary layer which, although representing a typical wind environment, has been specifically tailored to match a series of wind tunnel observations. The simulations were carried out at a Reynolds number, based on the velocity U at the cube height h, of 20,000—large enough that many aspects of the flow are effectively Reynolds number independent. The turbulence intensity was about 18% at the cube height, and the integral length scale was about 0.8 times the cube height h. The Jenson number Je=h/z0, based on the approach flow roughness length z0, was 600, to match the wind tunnel situation. The computational mesh was uniform with a spacing of h/32, aiding rapid convergence of the multigrid solver, and the governing equations were discretised using second-order finite differences within a parallel multiblock environment. The results presented include detailed comparison between measurements and LES computations of both the inflow boundary layer and the flow field around the cube including mean and fluctuating surface pressures. It is concluded that provided properly formulated inflow and surface boundary conditions are used, LES is now a viable tool for use in wind engineering problems concerning flow over isolated bodies. In particular, both mean and fluctuating surface pressures can be obtained with a similar degree of uncertainty as usually associated with wind tunnel modelling
Bluff bodies in deep turbulent boundary layers: Reynolds-number issues
Get PDFIt is generally assumed that flows around wall-mounted sharp-edged bluff bodies
submerged in thick turbulent boundary layers are essentially independent of the
Reynolds number Re, provided that this exceeds some (2–3) × 104. (Re is based on
the body height and upstream velocity at that height.) This is a particularization of
the general principle of Reynolds-number similarity and it has important implications,
most notably that it allows model scale testing in wind tunnels of, for example,
atmospheric flows around buildings. A significant part of the literature on wind
engineering thus describes work which implicitly rests on the validity of this
assumption. This paper presents new wind-tunnel data obtained in the ‘classical’ case
of thick fully turbulent boundary-layer flow over a surface-mounted cube, covering an
Re range of well over an order of magnitude (that is, a factor of 22). The results are
also compared with new field data, providing a further order of magnitude increase in
Re. It is demonstrated that if on the one hand the flow around the obstacle does not
contain strong concentrated-vortex motions (like the delta-wing-type motions present
for a cube oriented at 45? to the oncoming flow), Re effects only appear on fluctuating
quantities such as the r.m.s. fluctuating surface pressures. If, on the other hand, the
flow is characterized by the presence of such vortex motions, Re effects are significant
even on mean-flow quantities such as the mean surface pressures or the mean velocities
near the surfaces. It is thus concluded that although, in certain circumstances and for
some quantities, the Reynolds-number-independency assumption is valid, there are
other important quantities and circumstances for which it is not
Wind flow over sinusoidal hilly obstacles located in a uniform flow
No full textThe wind flow over two-dimensional sinusoidal hilly obstacles with slope (the ratio of height to half width) of 0.5 has been investigated experimentally and numerically. Experiments for single and double sinusoidal hill models were carried out in a subsonic wind tunnel. The mean velocity profiles, turbulence statistics, and surface pressure distributions were measured at the Reynolds number based on the obstacle height(h=40 mm) of 2.6 104. The reattachment points behind the obstacles were determined using the oil-ink dot and tuft methods. The smoke-wire method was employed to visualize the flow structure qualitatively. The finite-volume-method and the SIMPLE-C algorithm with an orthogonal body-fitted grid were used for numerical simulation. Comparison of mean velocity profiles between the experiments and the numerical simulation shows a good agreement except for the separation region, however, the surface pressure data show almost similar distributions
Flow control of circular cylinders with longitudinal groove surfaces
No full textFlow around circular cylinders with different grooved surfaces (U and V shaped) was investigated experimentally. The drag force, mean velocity, and turbulence intensity profiles of the wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter (D = 60 mm) in the range ReD = 8 × 103 ~ 1.4 × 105 . Flow around the cylinders was visualized using a particle tracer technique to see the flow structure qualitatively. The results were compared with those for a smooth cylinder of the same diameter D. At ReD = 1.4 × 105, the U-type groove surface reduced the drag coefficient by 18.6% compared with the smooth cylinder, whereas the drag reduction of V-type grooves was only 2.5%. For the circular cylinder with a U-grooved surface, as the Reynolds number increases, the location of peak turbulence intensity moves downstream, and the turbulence intensity is decreased. The two longitudinal-grooved surfaces tested in this study show different surface pressure distributions at high Reynolds numbers due to their different flow structures near the cylinder surface. Flow visualization results show that the U-grooved cylinder elongates the vortex formation region more than 50% compared with the smooth cylinder. This is consistent with the results of turbulence intensity distribution measured along the wake centerline. The large-scale longitudinal vortices formed behind the grooved cylinders seem to be broken into smaller ones due to the existence of longitudinal grooves. On average, the scale of longitudinal vortices behind the U-grooved cylinder is much smaller in size compared with the other two cases
Channel flow with large longitudinal ribs
Get PDFWe present data from direct numerical simulations of flow through channels containing large, longitudinal, surface-mounted, rectangular ribs at various spanwise spacings, which lead to secondary flows. It is shown that appropriate modifications to the classical log-law, predicated on a greater wetted surface area than in a plane channel, lead to a log-law-like region in the spanwise-averaged axial mean velocity profiles, even though local profiles may be very different. The secondary flows resulting from the presence of the ribs are examined and their effects discussed. Comparing our results with the literature we conclude that the sense of the secondary flows is largely independent of the particular rib spacing whether normalised by channel depth or rib width. The strength of the secondary flows, however, is shown to depend on the ratio of rib spacing to rib width and on Reynolds number. Topological features of the secondary flow structure are illustrated via a critical point analysis and shown to be characterised in all cases by a free stagnation point above the centre of the rib. Finally, we show that if the domain size is chosen as a 'minimal channel' size, rather than a size which allows adequate development of the usual outer layer flow structures, the secondary flows can be affected and this leads inevitably to differences in the near-rib flows so that for ribbed channels, unlike plain channels, it is unwise to use minimal domains to identify details of the near-wall flow.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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