Search

Two-dimensional numerical simulations are performed to study the effect of height of control plates placed at the downstream location in a vertical direction of a channel for flow past two inline square cylinders. The numerical method used is the lattice Boltzmann method (LBM) and Reynolds number is kept constant at Re = 150 (Re = Uinf d/ν) for fixed gap spacing g = 3.5 (g = s/d). The height of the control plate varies from h = 0.1 to 8. First, the validity of code for the present problem and suitable grid points are checked along the selection of uniform inflow velocity. After that verification, the numerical simulations are performed to obtain the results under the effect of height of control plates using the passive control method. Calculated results are, in terms of vorticity contour, drag and lift coefficients, pressure contour, energy spectrum, and force statistics. Under the discussion of vorticity contour visualization, we obtained six types of flow mode and named them according to flow behavior and their characteristics. In pressure contour, it is visualized that the maximum pressure occurred at near the upstream position of the first cylinder as well as the upper and lower edge of the main cylinder as compared to the downstream cylinder and control plate, but at some position inside the domain, pressure contour contained negative values as well in between the gap of two square cylinders and control plate. In force statistics, we computed the values of Cdmean, Cdrms, Clrms, and St. Mean drag coefficients for both upstream cylinder (C1) and downstream cylinder (C2) are increased as the height of the control plate increased from 0.1 to 1.5, but as the height of the control plate reached its maximum value, the value of Cdmean turned toward the decline. The maximum value of Cdmean obtained at h = 2.25 and the minimum value occurred at h = 8. For cylinder C2, negative values for some selected heights such as h = 0.1–1.25 and h = 4–8 are also observed because of thrust. The value of the Strouhal number for both the cylinders C1 and C2 are similar at h = 0.1–4.5, but at h = 5, the values of the Strouhal number differ for both cylinders, but at the largest values of control plate's height, i.e., h = 6–8, the Strouhal number contains zero values for both cylinders C1 and C2. At these zero values of the Strouhal number, in vorticity contour jet type of flow mode is observed at h = 6–8, as a new flow mode, which is not identified before in the passive control method for flow past two or more square cylinders. [ABSTRACT FROM AUTHOR]

A numerical study is performed to analyze the effect of small control cylinders on fluid force reduction and vortex shedding suppression on the flow past three inline square cylinders using the lattice Boltzmann method. The Reynolds number Re = 160 is fixed while the spacing between the cylinders is taken in the range of 1.0D [less than or equal to] [g.sup.*] [less than or equal to] 5.0D (where D is the size of the main cylinder) and the control cylinder size is varied from 0.1D to 0.5D. To systematically understand the effect of control cylinders on the forces, a detailed analysis of Strouhal number (St), mean drag coefficient ([C.sub.Dmean]), and root mean square values of the drag and lift coefficients is presented in this paper. In this study, it is observed that the average mean drag coefficient ([C.sub.Dmeanaverage]) and Strouhal number reached either maximum or minimum values at different values of separation ratio ([g.sup.*]) and small control cylinder size (d). It is found that at ([g.sup.*], d) = (5.0, 0.0) and (1.0, 0.5), the average [C.sub.Dmean] attains its maximum ([C.sub.Dmeanaverage] = 0.7813) and minimum ([C.sub.Dmean] = 0.0988) values. Furthermore, at ([g.sup.*], d) = (5.0, 0.3) and (2.0, 0.0) the average St attains its maximum (St = 0.1780) and minimum (St = 0.041) values. It is found that the flow regimes completely change in the presence of control cylinders. In particular, at [g.sup.*] = 4.0 there is a critical flow regime when the size of the control cylinder changes from 0.1 to 0.5. The sudden jump in the mean drag coefficient and Strouhal number for the middle cylinder with their maximum and minimum values also confirms the critical flow regime. The effect of control cylinders within tandem square cylinders has not been studied before. Key words: cylinder, root, drag coefficient, mean-square, lift coefficient. Nous utilisons la methode de Boltzmann sur reseau dans une etude numerique pour examiner les effets de petits cylindres de controle sur la reduction des forces sur le fluide et sur la suppression de vortex dans le flot passant trois cylindres carres en ligne. Le nombre de Reynolds est fixe a Re = 160, et la distance entre les cylindres varie dans le domaine 1.0D [less than or equal to] [g.sup.*] [less than or equal to] 5.0D (ou D est la grandeur du cylindre principal), alors que la grandeur des cylindres de controle (devant chaque cylindre) varie de 0.1D et 0.5D. Afin de comprendre de facon systematique l'effet des cylindres de controle sur les forces, nous presentons une analyse detaillee du nombre individuel de Strouhal (St), du coefficient moyen de trainee individuel ([C.sub.Dmean]), de la moyenne quadratique de la trainee et du coefficient de portance. Nous observons ici que la moyenne sur les cylindres du coefficient de trainee ([C.sub.Dmeanaverage]) et le nombre de Strouhal atteignent soit un maximum soit un minimum a differentes valeurs du rapport de separation ([g.sup.*]) et de la grandeur des petits cylindres de controle (d). Nous trouvons qu'a ([g.sup.*], d)= (5.0, 0.0) et (1.0, 0.5), [C.sub.Dmean] atteint ses valeurs, maximale avec ([C.sub.Dmeanaverage] = 0.7813)et minimale avec ([C.sub.Dmean] = 0.0988). De plus, a ([g.sup.*], d)= (5.0, 0.3) et (2.0, 0.0), la moyenne de St atteint ses valeurs maximale (St = 0.1780 et minimale (St = 0.041). Nous trouvons que le regime de flot change completement en presence des cylindres de controle. Particulierement, a [g.sup.*] = 4.0, il y a un changement critique du flot lorsque la grandeur des cylindres de controle change de 0.1 a 0.5. Le saut brutal du coefficient moyen de trainee et du nombre de Strouhal pour le cylindre du milieu, avec leur valeur respective maximale et minimale, confirme egalement le regime critique de flot. L'effet de cylindres de controle a l'interieur d'un tandem de cylindres carres n'at jamais fait l'objet d'une etude. [Traduit par la Redaction] Mots-cles: cylinder, racine, coefficient de trainee, moyenne quadratique, coefficient de portance., 1. Introduction In the past few decades, flow past multiple bluff bodies has attracted the attention of researchers to reduce the wake unsteadiness and the forces on the bodies. Various [...]

The flow past three square cylinders of unequal size placed in an inline arrangement is studied using the lattice Boltzmann method at different Reynolds numbers [Re = (u∞ d)/ν] within the range of Re = 120, 150, 160, 175, and 200 for various gap spacings (g = s/d), ranging from 1 to 6. This study focused on the symmetric examination of flow behavior for various gap spacing within the three unequal-sized square cylinders. The main objective of this study was to investigate the effects of Reynolds numbers and gap spacing for flow structure mechanism and vortex shedding suppression in between the gap and down-stream position of all three cylinders. Results are obtained in terms of vorticity contours visualization, drag and lift coefficients, Strouhal number, and physical parameters. In vorticity contour visualization, different flow behaviors are observed, known as flow regimes, and are named according to their characteristics, and they are (i) steady flow regime, (ii) shear layer reattachment flow regime (SLR), (iii) fully developed vortex shedding flow regime, (iv) two-row fully developed vortex shedding flow regime, and (v) fully developed irregular vortex shedding flow regime. The present study also includes a discussion on aerodynamic forces, namely the mean drag coefficient (Cdmean), root mean square of the lift coefficient (Clrms), and Strouhal numbers (St) for three cylinders with sizes d = 20, d1 = 15, and d2 = 10, respectively. The maximum value of Cdmean for the first cylinder (C1) is obtained at (Re, g) = (200, 3) that is, 1.5156, where the existing flow regime is the SLR flow regime, while for C2 and C3, the maximum Cdmean values are examined at critical flow behaviors, where the existing flow regime is a fully developed irregular vortex shedding flow regime. Negative values of Cdmean are also examined for cylinders C2 and C3 at some combinations of (Re, g), attributed to the effect of thrust. Furthermore, it is noticed that the values of Strouhal number are increased with an increment in values of gap spacing. The highest value of the Strouhal number for all three cylinders is observed for C1 at (Re, g) = (120, 5), reaching 0.1556 along with a two-row fully developed flow regime. Furthermore, it is investigated from the present problem that the position of unequal sized square cylinders strongly influenced the flow structure mechanism. The information found and discussed in this study could be effective for structure designing arrangement in the case of three square cylinders of unequal size placed in a horizontal arrangement. [ABSTRACT FROM AUTHOR]

Numerical simulations are performed to examine the effect of size of control rods (d1) and spacing ratio (g) on flow around a square rod with upstream and downstream control rods aligned in-line using the lattice Boltzmann method (LBM). The Reynolds number (Re) is fixed at Re = 160, while the spacing between the main rod and control rods is taken in the range 1 ≤ g ≤ 5 and the size of the control rod is varied between 4 and 20. Seven different types of flow mods are observed in this study at different values of g and d1. Variation in force statistics, like mean drag coefficient (Cdmean), Strouhal number (St), root mean square values of drag (Cdrms) and lift coefficients (Clrms), and percentage reduction in mean drag coefficient is discussed in detail. It was examined that vortex shedding completely suppressed at (g, d1) = (1, 12), (2, 12), and (2, 16) where steady flow mode exists. Moreover, it was found that at large gap spacing, where g = 5, the effect of control rods on the main rod vanishes. Due to this strong vortex shedding produced and as a result, maximum value of Cdmean is found at (g, d1) = (5, 8). The negative values of mean drag force are also observed at some gap spacing and size of control rods are due to the effect of thrust. Furthermore, the maximum percentage reduction in Cdmean is 121%, found at (g, d1) = (2, 20). [ABSTRACT FROM AUTHOR]