Fluid dynamics around three cylinders in presence of small control cylinders

2020 ◽  
Vol 98 (11) ◽  
pp. 1060-1076
Author(s):  
Ali Ahmed ◽  
Shams-ul Islam ◽  
Chao Ying Zhou ◽  
Raheela Manzoor
Keyword(s):  
Drag Coefficient ◽  
Flow Regime ◽  
Strouhal Number ◽  
Numerical Study ◽  
Critical Flow ◽  
Square Cylinders ◽  
Tandem Square Cylinders ◽  
Critical Flow Regime ◽  
Force Reduction ◽  
Control Cylinder

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 ≤ g* ≤ 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 (CDmean), 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 (CDmeanaverage) and Strouhal number reached either maximum or minimum values at different values of separation ratio (g*) and small control cylinder size (d). It is found that at (g*, d) = (5.0, 0.0) and (1.0, 0.5), the average CDmean attains its maximum (CDmeanaverage = 0.7813) and minimum (CDmean = 0.0988) values. Furthermore, at (g*, 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* = 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.

scholarly journals The conditions for occurrence of critical flow regime of water in the open stream

2019 ◽  
Vol 97 ◽  
pp. 05006
Author(s):  
Yuliya Bryanskaya ◽  
Aleksandra Ostiakova
Keyword(s):  
Froude Number ◽  
Flow Regime ◽  
Water Flow ◽  
Practical Interest ◽  
Open Water ◽  
Critical Regime ◽  
Critical Flow ◽  
Channel Processes ◽  
Channel Deformations ◽  
Critical Flow Regime

For the solution of engineering problems require increasingly accurate estimates of the hydraulic characteristics of the water streams. To date, it is impossible to consider sufficiently complete theoretical and experimental justification of the main provisions of the theory of turbulence, hydraulic resistance, channel processes. The composition of tasks related to flows in wide channels, turbulence problems are of scientific and practical interest. Various interpretations of the determination of the critical Froude number in wide open water flows based on observations and theoretical transformations are considered. The conditions for the emergence of a critical regime of water flow in an open wide channel are analyzed in order to estimate the critical Froude number and critical depth. Estimates of the critical Froude number for laboratory and field conditions are given. The estimations allow us to consider the proposed approach acceptable for determining the conditions of occurrence of the critical flow regime. The General, physical interpretation of conditions of occurrence of the critical regime of water flow on the basis of phenomenological approach is specified. The results take into account the values of the components of the total specific energy of the section. This shows the estimated calculation. The results obtained theoretically make it possible to compare the above interpretations and determine their applicability, and the results of the analysis can be useful for the estimated calculations of flows in channels and river flows in rigid, undeformable boundaries and with minor channel deformations.

Critical flow regime in a stratified medium

1979 ◽  
Vol 14 (2) ◽  
pp. 289-292
Author(s):  
K. E. Dzhaugashtin
Keyword(s):  
Flow Regime ◽  
Stratified Medium ◽  
Critical Flow ◽  
Critical Flow Regime

Flow Past Square Cylinders of Different Size With and Without Corner Modification

2015 ◽  
Author(s):  
Y. T. Krishne Gowda ◽  
Ravindra Holalu Venkatdas ◽  
Vikram Chowdeswarally Krishnappa
Keyword(s):  
Drag Coefficient ◽  
Flow Velocity ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Research Work ◽  
Tall Buildings ◽  
Convergence Criterion ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Flow Past

In many mechanical engineering applications, separated flows often appear around any object such as tall buildings, monuments, and towers are permanently exposed to wind. Similarly, piers, bridge pillars, and legs of offshore platforms are continuously subjected to the load produced by maritime or fluvial streams. These bodies usually create a large region of separated flow and a massive unsteady wake region in the downstream. The highly asymmetric and periodic nature of flow in the downstream has attracted the attention of physicists, engineers and CFD practitioners. A lot of research work is carried out for a square cylinder but flow past square cylinders with and without corner modification work is not taken up. This motivated to take up the task of flow past two different sized square cylinders, numerically simulated. A Reynolds number of 100 and 200 is considered for the investigation. The flow is assumed to be two dimensional unsteady and incompressible. The computational methodology is carried out once the problem is defined the first step in solving the problem is to construct a geometry on which the simulation is planned. Once the geometry is constructed, proper assignment of its boundaries in accordance to the actual physical state is to be done. The various boundary options that are to be set. After setting the boundary types, the continuum type is set. The geometry is discretized into small control volumes. Once the surface mesh is completed, the mesh details are exported to a mesh file, then exported to Fluent, which is CFD solver usually run in background mode. This helps to prioritize the execution of the run. The run would continue until the required convergence criterion is reached or till the maximum number of iterations is completed. Results indicate, in case of chamfered and rounded corners in square cylinder, there is decrease in the wake width and thereby the lift and drag coefficient values. The form drag is reduced because of a higher average pressure downstream when separation is delayed by corner modification. The lift coefficients of Square cylinder with corner modification decreases but Strouhal number increases when compared with a square cylinder without corner modification. Strouhal number remains same even if magnitude of oscillations is increased while monitoring the velocity behind the cylinder. Frequency of vortex shedding decreases with the introduction of second cylinder either in the upstream or downstream of the first cylinder. As the centre distance between two cylinders i.e., pitch-to-perimeter ratio is increased to 6,the behavior of the flow almost approaches to that of flow past a square cylinder of with and without modification of same condition. When the perimeter of the upstream cylinder with and without modification is larger than the downstream cylinder, the size of the eddies is always bigger in between the cylinders compared to the downstream of the second cylinder. The flow velocity in between the cylinders with and without corner modification are less compared to the downstream of the second cylinder. As the distance increases, the flow velocity in between the cylinders become almost equal to the downstream of the second cylinder. The results are presented in the form of streamlines, flow velocity, pressure distribution. drag coefficient, lift coefficient and Strouhal number.

Effect of Axis Ratio on Fluid Flow Around an Elliptic Cylinder—A Numerical Study

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
S. Kalyana Raman ◽  
K. Arul Prakash ◽  
S. Vengadesan
Keyword(s):  
Fluid Flow ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Numerical Study ◽  
Elliptic Cylinder ◽  
Solid Boundary ◽  
Axis Ratio ◽  
Moderate Reynolds Number ◽  
Wake Formation

The bluff body simulations over canonical forms like circular and square cylinders are very well studied and the correlations for bulk parameters like mean drag coefficient and Strouhal numbers for the same are reported widely. In the case of elliptic cylinder, the literature is very sparse, especially for moderate Reynolds number (Re). Hence, in this work, a detailed study about fluid flow characteristics over an elliptic cylinder placed in a free stream is performed. Simulations are carried out for different Re ranging from 50 to 500 with axis ratio (AR) varied between 0.1 to 1.0 in steps of 0.1. Immersed boundary method is used for the solid boundary condition implementation which avoids the grid generation for each AR and a single Cartesian grid is used for all the simulations. The effect of AR for various Reynolds numbers is also focused on using the in-house code. The influence of AR is phenomenal for all the Re and the values of wake length, drag coefficient, and Strouhal number decrease with decreasing AR for a particular Re. The critical ARs, for vortex shedding and wake formation, are identified for various Re. Detailed correlations for wake length, critical ARs for vortex shedding and wake formation, mean drag coefficient and Strouhal number, in terms of AR, are reported in this work.

scholarly journals Estimation of the nozzle with the stepped divergent section in critical flow regime

2018 ◽  
Vol 1105 ◽  
pp. 012004
Author(s):  
I I Saushin ◽  
N I Mikheev ◽  
A E Goltsman ◽  
D V Kratirov
Keyword(s):  
Flow Regime ◽  
Critical Flow ◽  
Critical Flow Regime

Numerical Analysis of Two Square Cylinders of Different Sizes With and Without Corner Modification

2016 ◽  
Author(s):  
Y. T. Krishne Gowda ◽  
Holalu Venkatdas Ravindra ◽  
Vikram Chowdeswarally Krishnappa
Keyword(s):  
Drag Coefficient ◽  
Flow Velocity ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Research Work ◽  
Convergence Criterion ◽  
Back Side ◽  
Square Cylinder ◽  
Square Cylinders ◽  
Flow Past

Flow past square cylinders has attracted a great deal of attention because of its practical significance in engineering e.g., High rise buildings, monuments and towers. Similarly, bridge pillars, and legs of offshore platforms are continuously subjected to the load produced by maritime or fluvial streams. The presence of separated flows, reattachment, formation the vortices, un steadiness of flow, mass and momentum transfer across shear layer makes the flow field quite complex. Many research work was carried out for a single square cylinder and flow past two square cylinders, but with corner medications in square cylinder of different size arranged in tandem was not taken up. This has motivated to take up the flow past two different sized square cylinders i.e., smaller in upstream and larger in downstream which is numerically simulated by using Fluent software. Reynolds number of 100 and 200 is considered for the investigation. The flow is assumed to be two dimensional, unsteady and incompressible. The computational methodology is carried out once the problem is defined, the first step in solving the problem is to construct a geometry then proper assignment of boundaries are set. After setting the boundary types, the geometry is discretized into small control volumes. Once the surface mesh is completed by using Gambit software, the mesh along with boundary conditions are exported to fluent, which is CFD solver usually run in background mode. The run would continue until the required convergence criterion is reached or till the maximum number of iterations is completed. Results indicate, in case of chamfered and rounded corners in square cylinders of different size, there is decrease in the wake width and thereby the lift and drag coefficient values. The lift coefficients in Square cylinders of different size with corner modifications decreases but Strouhal number increases when compared with a single square cylinder without corner modifications. Frequency of vortex shedding decreases with the introduction of second cylinder either in the upstream or downstream of the first cylinder. As the centre distance between two square cylinders i.e., PPR (pitch to perimeter ratio) with and without corner modifications is increased to 6, the flow velocity almost approaches to flow past a single square cylinder with and without modifications for same condition. When the size of the upstream square cylinder with and without modifications is smaller than that of the downstream square cylinder, the size of the eddies is always smaller in between the cylinders compared to the downstream of the second cylinder. The flow velocity in between the cylinders with and without corner modifications are less compared to the downstream of the second cylinder. Pressure on the downstream side of the cylinder is smaller than that on the upstream side of the cylinder for with and without corner modifications. Also, the front portion of the cylinder is experiencing highest pressure compared to the second cylinder for all the three cases i.e., PPR = 2, 4 and 6. Pressure at the upper side, bottom side and back side of square cylinder with and without corner modifications is of negative pressure, it is because of vortices generated at that surfaces. The downstream cylinder is found to experience higher lift compared to the upstream cylinder. The results are presented in the form of while the downstream cylinder is found to experience higher drag compared to the streamlines, flow velocity, pressure distribution, drag coefficient, lift coefficient and strouhal number.

Modeling of Flow Around a Circular Cylinder in Sub-Critical Flow Regime With the Use of Dynamic Grid Adaptation

2002 ◽  
Author(s):  
Govert de With ◽  
Arne E. Holdo̸ ◽  
Thomas A. Huld
Keyword(s):  
Circular Cylinder ◽  
Flow Regime ◽  
Turbulent Viscosity ◽  
Mesh Refinement ◽  
High Rate ◽  
Critical Flow ◽  
Computational Domain ◽  
Grid Adaptation ◽  
High Gradients ◽  
Critical Flow Regime

In the present study a Dynamic Grid Adaptation (DGA) algorithm is used for predicting flow around a circular cylinder in sub-critical flow regime at a Reynolds number of 1.4·105. The reason for adopting a DGA algorithm is to use the flow field as a driving criteria for mesh refinement rather then the geometry of the computational domain or the judgment of the CFD user as common in conventional mesh. It is demonstrated how DGA reduces the mesh size significantly and also makes time consuming mesh testing unnecessary. The concept being adopted is to concentrate mesh refinement in regions with high gradients and high turbulent viscosity, while in the region further downstream where the flow is fully developed a coarser mesh will develop and turbulence is modeled with the Large Eddy Simulation (LES) turbulence model. The aim of the study is to present an appropriate variable for mesh refinement, which accomplishes a high rate of mesh refinement in the region with high gradients. The new variable is a product of the local mesh cell size and the rate of strain and includes two additional variables to allow control over the refinement behaviour.

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