scholarly journals Dynamical effect of the total strain induced by the coherent motion on local isotropy in a wake

2013 ◽  
Vol 720 ◽  
pp. 393-423 ◽  
Author(s):  
F. Thiesset ◽  
L. Danaila ◽  
R. A. Antonia
Keyword(s):  
Circular Cylinder ◽  
Large Scale ◽  
Bluff Body ◽  
Initial Conditions ◽  
Scale Up ◽  
Wake Flow ◽  
Total Strain ◽  
Coherent Motion ◽  
Square Cylinder ◽  
Local Isotropy

AbstractWe assess the extent to which local isotropy (LI) holds in a wake flow for different initial conditions, which may be geometrical (the shape of the bluff body which creates the wake) and hydrodynamical (the Reynolds number), as a function of the dynamical effects of the large-scale forcing (the mean strain, $ \overline{S} $, combined with the strain induced by the coherent motion, $\tilde {S} $). LI is appraised through either classical kinematic tests or phenomenological approaches. In this respect, we reanalyse existing LI criteria and formulate a new isotropy criterion based on the ratio between the turbulence strain intensity and the total strain ($ \overline{S} + \tilde {S} $). These criteria involve either time-averaged or phase-averaged quantities, thus providing a deeper insight into the dynamical aspect of these flows. They are tested using hot wire data in the intermediate wake of five types of obstacles (a circular cylinder, a square cylinder, a screen cylinder, a normal plate and a screen strip). We show that in the presence of an organized motion, isotropy is not an adequate assumption for the large scales but may be satisfied over a range of scales extending from the smallest dissipative scale up to a scale which depends on the total strain rate that characterizes the flow. The local value of this scale depends on the particular nature of the wake and the phase of the coherent motion. The square cylinder wake is the closest to isotropy whereas the least locally isotropic flow is the screen strip wake. For locations away from the axis, the study is restricted to the circular cylinder only and reveals that LI holds at scales smaller than those that apply at the wake centreline. Arguments based on self-similarity show that in the far wake, the strength of the coherent motion decays at the same rate as that of the turbulent motion. This implies the persistence of the same degree of anisotropy far downstream, independently of the scale at which anisotropy is tested.

Wake Interaction of a Square Cylinder With a Splitter Plate Boundary Layer at a Low Reynolds Number

2015 ◽  
Author(s):  
Smriti Srivastava ◽  
Sudipto Sarkar
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Bluff Body ◽  
Acoustic Noise ◽  
Vortex Formation ◽  
Plate Boundary ◽  
Wake Flow ◽  
Stream Velocity ◽  
Square Cylinder ◽  
Plate Length

One of the most important researches in bluff body aerodynamics is to control the shear layer evolution leading to vortex formation. This kind of research is closely associated with reduction of aerodynamics forces and acoustic noise. Passive and active control of wake-flow from bluff bodies have received a great deal of attention in the last few decades [1–4]. Keeping this in mind, authors investigate the interaction of a square cylinder (side of the square = a) wake with a flat plate (length L = a, width w = 0.1a) boundary layer positioned at various downstream locations close to the cylinder. The gap-to-side ratios are maintained at G/a = 0, 0.5, 1 and 2 (where G is the gap between square cylinder and plate), and the simulation is performed at a Reynolds number, Re = 100 (Re = U∞a/v, where U∞ is free stream velocity and v is kinematic viscosity). Instantaneous flow visualization, aerodynamic forces and vortex shedding frequencies for all cases are described to gain insight about the changes associated with wake of the cylinder when a short plate is kept in its downstream.

The Interaction of Porous Metal Coating With the Near Wake of Bluff Body

2012 ◽  
Author(s):  
Hanru Liu ◽  
Jinjia Wei ◽  
Zhiguo Qu
Keyword(s):  
Circular Cylinder ◽  
Bluff Body ◽  
Porous Metal ◽  
Metal Coating ◽  
Wake Flow ◽  
Navier Stokes ◽  
Aerodynamic Forces ◽  
Near Wake ◽  
Drag Forces ◽  
Flow Induced Noise

The flow around a circular cylinder with porous metal coating (PMC) is numerically investigated based on an approach of unsteady Reynolds Averaged Navier-Stokes (URANS) at subcritical Reynolds number. The model validation is carried out through comparison with some available experimental results in the literatures. It is found that the simulated results in the present work coincide well with the experimental data. The interaction of PMC with the near wake of circular cylinder such as streamline, vorticity and shear stress are studied in detail. The result reveals that PMC has capability of manipulating the wake flow so that the near wake of PMC cylinder is substantially different from that of smooth one. In addition, the fluctuations of aerodynamic forces are mitigated effectively. Varying the thickness of porous metal coating causes various velocity distributions and aerodynamic performance of bluff body. When the thickness is appropriate, the drag forces can be reduced to a certain extent. It is expected that the modification of flow characteristic and aerodynamic forces also produces the suppression of flow-induced noise generated by bluff body. These studies on wake flow and analysis of its relationship to flow-induced noise will be useful to understand the mechanism of controlling bluff body flow-induced noise by using PMC and to optimize the PMC for controlling flow and flow-induced noise.

Tomographic PIV investigation on 3D wake structures for flow over a wall-mounted short cylinder

2017 ◽  
Vol 831 ◽  
pp. 743-778 ◽  
Author(s):  
Hang-Yu Zhu ◽  
Cheng-Yue Wang ◽  
Hong-Ping Wang ◽  
Jin-Jun Wang
Keyword(s):  
Aspect Ratio ◽  
Circular Cylinder ◽  
Dynamic Characteristics ◽  
Large Scale ◽  
Low Frequency ◽  
Streamwise Vortex ◽  
Flow Fields ◽  
Vortex Structures ◽  
Wake Flow ◽  
Wake Field

Tomographic particle image velocimetry (TPIV) measurement with six high-resolution charge-coupled device (CCD) cameras is conducted to investigate flow structures over a finite circular cylinder with an aspect ratio of 2 ($h/d=2$). This short wall-mounted cylinder is fully immersed in a thick turbulent boundary layer ($\unicode[STIX]{x1D6FF}/h=1.025$). Focus is placed on the three-dimensional instantaneous vortex structures and their dynamic characteristics in the wake flow fields. Based on the present results, a refined topological model of the mean wake field behind the finite circular cylinder is proposed, where the spatial locations of the typical vortex structures and their interactions are described in more detail. Among the reported typical vortex structures (i.e. the horseshoe, tip, base, trailing and arch vortex), emphasis is laid on discussion of the tip and arch vortex. The instantaneous 3D M-shape arch vortex and an alternating large-scale streamwise vortex are first found in the present experiment, and their developments are also discussed. Therefore, it is suggested that the instantaneous finite-cylinder wake is dominated by the arch vortex system and the large-scale streamwise vortices. Moreover, in the instantaneous volumetric flow fields, both the antisymmetric and the symmetric wake behaviours are observed. With proper orthogonal decomposition (POD) analysis, the dynamic characteristics of the wake field are clarified. Different from the flow around an infinite cylinder without control, the third and fourth POD modes are characterized by low-frequency symmetric shedding. The low-frequency feature shown in the second mode pair is observed and associated with the occurrence of instantaneous symmetric 3D wake behaviour triggered by the low-aspect-ratio effect and the extension of the separated shear layer. The low frequency seems be attributed to the flapping phenomenon, i.e. oscillation of the recirculation in the backward-facing step flow. It is found that the flapping motion has a modulating effect on the occurrence of the antisymmetric shedding vortex and thus the large-scale streamwise vortex.

Transition to bluff-body dynamics in the wake of vertical-axis wind turbines

2017 ◽  
Vol 813 ◽  
pp. 346-381 ◽  
Author(s):  
Daniel B. Araya ◽  
Tim Colonius ◽  
John O. Dabiri
Keyword(s):  
Velocity Field ◽  
Circular Cylinder ◽  
Large Scale ◽  
Wind Farm ◽  
Bluff Body ◽  
Vertical Axis ◽  
Speed Ratio ◽  
Cylinder Wake ◽  
Vertical Axis Wind Turbine ◽  
Far Wake

We present experimental data to demonstrate that the far wake of a vertical-axis wind turbine (VAWT) exhibits features that are quantitatively similar to that of a circular cylinder with the same aspect ratio. For a fixed Reynolds number ($Re\approx 0.8\times 10^{5}$) and variable tip-speed ratio, two-dimensional particle image velocimetry (PIV) is used to measure the velocity field in the wake of four different laboratory-scale models: a 2-bladed, 3-bladed and 5-bladed VAWT, as well as a circular cylinder. With these measurements, we use spectral analysis and proper orthogonal decomposition (POD) to evaluate statistics of the velocity field and investigate the large-scale coherent motions of the wake. In all cases, we observe three distinct regions in the VAWT wake: (i) the near wake, where periodic blade vortex shedding dominates; (ii) a transition region, where growth of a shear-layer instability occurs; (iii) the far wake, where bluff-body wake oscillations dominate. We define a dynamic solidity parameter, $\unicode[STIX]{x1D70E}_{D}$, that relates the characteristic scales of the flow to the streamwise transition location in the wake. In general, we find that increasing $\unicode[STIX]{x1D70E}_{D}$ leads to an earlier transition, a greater initial velocity deficit and a faster rate of recovery in the wake. We propose a coordinate transformation using $\unicode[STIX]{x1D70E}_{D}$ in which the minimum velocity recovery profiles of the VAWT wake closely match that of the cylinder wake. The results have implications for manipulating VAWT wake recovery within a wind farm.

Drag Reduction of a Bluff Body by Grooves Laid Out by Design of Experiment

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Seong-Ho Seo ◽  
Chung-Do Nam ◽  
Jung-Young Han ◽  
Cheol-Hyun Hong
Keyword(s):  
Optimal Design ◽  
Circular Cylinder ◽  
Drag Reduction ◽  
Bluff Body ◽  
Wake Flow ◽  
Design Parameters ◽  
Mean Velocity ◽  
Smooth Cylinder ◽  
Layer Flow ◽  
Parameter Values

In this study, we used the Taguchi method to derive the optimal design parameters for the grooves formed on the upper surface of a circular cylinder. Using the derived values of the optimal design parameters, we created grooves on diphycercal the surfaces of a circular cylinder and analyzed the wake flow and the boundary-layer flow of the circular cylinder. The streamwise time mean velocity and turbulence intensity of the wake flow field were used as the characteristics. Based on these characteristics, the optimal design parameter values were selected: n = 3, k = 1.0 mm (k/d = 2.5%), and θ = 60 deg. When the grooved cylinder was used, the streamwise time mean velocity in the wake of the cylinder showed 12.3% recovery, the wake width was reduced by 18.4% compared to the results from the smooth cylinder and we had 28.2% drag reduction from that of smooth cylinder. Also, the flow on the smooth cylinder separated at around 82 deg but the flow separation on a grooved cylinder appeared beyond 90 deg, that reducing the drag.

Reduction of Drag Force on a Circular Cylinder and Pressure Drop Using a Square Cylinder as Disturbance Body in a Narrow Channel

2014 ◽  
Vol 493 ◽  
pp. 192-197 ◽  
Author(s):  
Wawan Aries Widodo ◽  
Randi Purnama Putra
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Wind Tunnel ◽  
Circular Cylinder ◽  
Drag Force ◽  
Bluff Body ◽  
Flow Characteristics ◽  
Narrow Channel ◽  
Square Cylinder ◽  
Cross Sectional

Many studies related with characteristics of fluid flow acrossing in a bluff body have been conducted. The aim of this research paper was to reduce pressure drop occuring in narrow channels, in which there was a circular cylindrical configuration with square cylinder as disturbance body. Another goal of this research was to reduce the drag force occuring in circular cylinder. Experimentally research of flow characteristics of the wind tunnel had a narrow channel a square cross-section, with implemenred of Reynolds number based on the hydraulic diameter from 5.21x104 to 1.56x105. Wind tunnel that was used had a 125x125mm cross-sectional area and the blockage ratio 26.4% and 36.4%. Specimen was in the form of circular cylinder and square cylinder as disturbance body. Variation of angle position was the inlet disturbance body with α = 200, 300, 400, 500 and 600, respectively. The results was obtained from this study was Reynolds Number value was directly linear with pressure drop there, it was marked by increasing of Reynolds number, the value was also increasing pressure drop. Additional information was obtained by adding inlet disturbance body shaped of square cylinder on the upstream side of the circular cylinder that could reduce pressure drop in the duct and reduce drag happening on a circular cylinder. The position of the optimum angle to reduce pressure drop and drag force was found on the inlet disturbance body with angle α = 300.

Laminar flow of an incompressible fluid past a bluff body: the separation, reattachment, eddy properties and drag

1979 ◽  
Vol 92 (1) ◽  
pp. 171-205 ◽  
Author(s):  
F. T. Smith
Keyword(s):  
Circular Cylinder ◽  
Large Scale ◽  
Bluff Body ◽  
Reynolds Numbers ◽  
The Body ◽  
Navier Stokes ◽  
Pressure Drag ◽  
Body Scale ◽  
Flow Situation ◽  
Large Scale Flow

The asymptotic theory for the laminar, incompressible, separating and reattaching flow past the bluff body is based on an extension of Kirchhoff's (1869) free-streamline solution. The flow field (only the upper half of which is discussed since we consider a symmetric body and flow) consists of two basic parts. The first is the flow on the body scalel*, which is described to leading order by the Kirchhoff solution with smooth inviscid separation, but with an$O(Re^{-\frac{1}{16}})$modification to explain fully the viscous separation (hereRe([Gt ] 1) is the Reynolds number). The influence of this$O(Re^{-\frac{1}{16}})$modification is determined for the circular cylinder. The second part is the large-scale flow, comprising mainly the eddy and the ultimate wake. The eddy has length scaleO(Rel*), widthO(Re½l*) and is of elliptical shape to keep the eddy pressure almost uniform. The ultimate wake is determined numerically and fixes the eddy length. The (asymptotically small) back pressure from the eddy acts (on the body scale) both in the free stream and in the eddy, and it has a marked effect at moderate Reynolds numbers; combined with the Kirchhoff solution, it predicts the pressure drag on a circular cylinder accurately, to within 10% whenRe= 5 and to within 4% whenRe= 50. Other predictions, for the eddy length and width, the front pressure and the eddy pressure, also show encouraging agreement with experiments and Navier-Stokes solutions at moderate Reynolds numbers (of about 30), both for the circular cylinder and the normal flat plate. Finally, an analysis in the appendix indicates that, in wind-tunnel experiments, the tunnel walls (even if widely spaced) can exert considerable influence on the eddy properties, eventually forcing an upper bound on the eddy width asReincreases instead of theO(l*Re½) growth appropriate to the unbounded flow situation.

A laser-Doppler velocimetry study of ensemble-averaged characteristics of the turbulent near wake of a square cylinder

1995 ◽  
Vol 304 ◽  
pp. 285-319 ◽  
Author(s):  
D. A. Lyn ◽  
S. Einav ◽  
W. Rodi ◽  
J.-H. Park
Keyword(s):  
Circular Cylinder ◽  
Laser Doppler ◽  
Wake Flow ◽  
Shear Stresses ◽  
Square Cylinder ◽  
Base Region ◽  
Flow Topology ◽  
Near Wake ◽  
Flow Features ◽  
High Reynolds

Ensemble-averaged statistics at constant phase of the turbulent near-wake flow (Reynolds number ≈ 21400 around a square cylinder have been obtained from two-component laser-Doppler measurements. Phase was defined with reference to a signal taken from a pressure sensor located at the midpoint of a cylinder sidewall. The distinction is drawn between the near wake where the shed vortices are ‘mature’ and distinct and a base region where the vortices grow to maturity and are then shed. Differences in length and velocity scales and vortex celerities between the flow around a square cylinder and the more frequently studied flow around a circular cylinder are discussed. Scaling arguments based on the circulation discharged into the near wake are proposed to explain the differences. The relationship between flow topology and turbulence is also considered with vorticity saddles and streamline saddles being distinguished. While general agreement with previous studies of flow around a circular cylinder is found with regard to essential flow features in the near wake, some previously overlooked details are highlighted, e.g. the possibility of high Reynolds shear stresses in regions of peak vorticity, or asymmetries near the streamline saddle. The base region is examined in more detail than in previous studies, and vorticity saddles, zero-vorticity points, and streamline saddles are observed to differ in importance at different stages of the shedding process.

Flow Around Finite Circular and Square Cylinders in an Open Channel

2009 ◽  
Author(s):  
S. S. Paul ◽  
M. Agelinchaab ◽  
M. F. Tachie
Keyword(s):  
Circular Cylinder ◽  
Proper Orthogonal Decomposition ◽  
Large Scale ◽  
Saddle Points ◽  
Water Channel ◽  
Orthogonal Decomposition ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Pod Analysis ◽  
Proper Orthogonal

An experimental investigation was performed to study the structure of the flow field around finite circular and square cylinders in a water channel. Detailed velocity measurements using a particle image velocimetry (PIV) system and proper orthogonal decomposition (POD) analysis were conducted to examine the structures in the near-wake region. Iso-contours and profiles of mean velocities and some turbulent quantities at various streamwise locations were reported. The proper orthogonal decomposition was applied to provide insight into the structure of the flow. The results show that the vortex in the square cylinder is bigger and extends to cover part of the free end of the cylinder. There is also an up-wash flow from the base of the cylinders to form saddle points. The saddle point in the square cylinder is closer to the free end than in the circular cylinder. The low order reconstructed turbulent intensities from POD show that the square cylinder has more large scale structures than the circular cylinder.

scholarly journals Reduksi Gaya Drag Silinder Sirkular dengan Penambahan Square Disturbance Body Melalui Simulasi Numerik 2D Unsteady-RANS pada Reynold Number 34800

2017 ◽  
Author(s):  
Ruzita Sumiati ◽  
Rina
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Stress ◽  
Circular Cylinder ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Bluff Body ◽  
Reynold Number ◽  
Square Cylinder ◽  
Unsteady Rans

Ketika fluida mengalir di sekitar Bluff body circular cylinder tunggal akan menghasilkan drag yang cukup besar, hal ini disebabkan karena ia memiliki kelengkungan kontur permukaan dengan karakteristik Andversse Pressure Gradient yang cukup kuat akibat tekanan aliran pada permukaan body. Untuk mengurangi drag tersebut, maka dilakukanlah kontrol aliran, salah satunya dengan menempatkan body pengganggu di depan circular cylinder. Penelitian ini bertujuan untuk membandingkan dan melengkapi penelitian eksperimen pengurangan gaya drag yang telah dilakukan sebelumnya. Penelitian ini dilakukan secara numerik 2D Unsteady-RANS menggunakan CFD software FLUENT 6.3.26 dengan model viscous Turbulence Model Shear-Stress-Transport (SST) k-ω pada saluran sempit. Geometri body yang disimulasikan adalah circular cylinder sebagai main bluff body dan square cylinder sebagai disturbance body yang ditempatkan di depan main bluff body dengan rasio s/D 0.107. Posisi disturbance body divariasikan pada (α) 20⁰, 30⁰, 40⁰, 50⁰ dan 60⁰ dengan jarak gap (δ=0,4mm). Reynolds number berdasarkan diameter silinder, yaitu ReD 3.48x104. Hasil simulasi ini menunjukkan bahwa interaksi fluida antara circular cylinder dengan dua body penganggu dapat meningkatkan transisi lapis batas dari laminer ke turbulent boundary layer sehingga menghasilkan drag yang kecil. Sudut optimum pengurangan gaya drag terjadi pada α = 30º yaitu sebesar 53%.

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