international engineering research journal experimental...

International Engineering Research Journal Special Edition PGCON-MECH-2017

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International Engineering Research Journal Experimental and computational analysis of flow passed over Bluff Body Rohit R. Vedpathak1, Dr. R.K.Patil2

Student, Department of Mechanical Engineering, JSPM’s RSSOER, Savitribai Phule Pune University, Pune, Maharashtra, ,India

Professor in Mechanical Engineering, Padmabhooshan Vasantdada Patil Institute Of Technology, Savitribai Phule Pune University Pune, Maharashtra, India.

Abstract Investigation of the fluid flow passed over Bluff Body is the fundamental need in fluid mechanics. Because in engineering applications pillars of bridge and underground pipelines can be modeled as cylinder. Using the unsteady incompressible Navier-Stokes equation as the governing equation, SST k-ω turbulence closure model is implemented to investigate the drag forces on the Bluff Body by using ANSYS FLUENT and results have been compared with the results obtained in wind tunnel test for validation purpose. It is seen that the results significantly gets affected by change of shape of the object. The force coefficients obtained in CFD results found to be approximately matching with the corresponding values measured in wind tunnel test Keywords: CFD Simulation, Bluff body, Drag coefficient, Reynolds number, Turbulent flows, ANSYS – FLUENT

1. INTRODUCTION The study of three dimensional flow passed through Bluff Body is the significantly important subject in fluid mechanics. Although computational analysis has proved its importance in fluid mechanics, it is very important to check the computational study results comparing it with the wind tunnel experimental results because there are so many problems encountered during the actual experimental procedure on wind tunnel test such as the type of material used for the object to be tested, surface roughness of the material, manual errors encountering during experimental procedure, limitations of the wind tunnel such as vibrations produced when operating on high velocity. The efficiency and accuracy of the solution directly depends on the type of turbulence model used in computational analysis. For the same object and at the same corresponding velocities the results found to be changed when tested using different turbulence models. Also the solution depends on the type of mesh used for the geometry such as all triangle mesh, all tetra mesh, triangle and tetra mix mesh or hex mesh. Important characteristic of fluid flow passed through Bluff Body is the drag coefficient (𝐶𝑑). It is a dimensionless quantity. It is used to measure the drag of an object in a fluid environment. The relationship between the drag coefficient on an object and the drag force is,

𝐶𝑑 =2𝐹𝑑

𝜌𝑉2𝐴

Where, Fd= Drag force A = the projected cross-sectional area V =the velocity of the fluid ρ = the fluid density

2. LITERATURE REVIEW David.C.Wiggert and Merle Potter of Schaum explains highly valuable knowledge about fluid flow. The flow analysis consists of deep study of flow over Bluff Body passed externally from lower to higher Reynold’s number flow. The flow passed through Bluff Body contains region of separation involving recirculation zone and high viscosity zone which is also known as wakes. The flow is known as stokes flow when Reynold’s number <5. Higher Reynold’s number causes the flow to be turbulent. Zadrkovich (1977, 2003) analyses the flow from laminar to turbulent in regions of the flow field passed through cylinders. Bai analyzed hydrodynamic characteristics of circular cylinders in 2-D (spear in 3D) flows by using FLUENT. Reynolds number is the ratio of inertial forces to viscous forces. Governing equations describing the Flow. Conservation Law: In Computational Fluid Dynamics Navier-Stokes equations are the governing equations. The change of mass in the object is as follows

outin mmdt

dM

If 0 outin mm ,

We have,

0dt

dM , constM


2

And can derive the continuity equation, momentum equation and energy equation applying the mass, momentum and energy conservation as follows. Energy Equation:

V

i

j

ij

IV

i

III

i

i

II

i

i

I

x

U

x

T

x

UP

x

TUc

t

Tc

2

2

I: Local energy change with time II: Convective term III: Pressure work IV: Heat flux V: Irreversible transfer of mechanical energy into heat

If the fluid is compressible, we can simplify the continuity equation and momentum equation as follows.

Continuity Equation:

0

i

i

x

U

Momentum Equation:

j

i

j

ji

j

i

jg

x

U

x

P

x

UU

t

U

2

2

General Form of Navier-Stokes Equation

q

xU

xt i

i

i

When

TU j ,,1 , we can respectively get continuity

equation, momentum equation and energy equation.

3. RESEARCH METHODOLOGY

The wind tunnel test is carries out using the sphere and cube as test objects. The air is used as the fluid passed over Bluff Body. The dimensions for sphere is measured as 10cm diameter and for cube it is 10 cm each face. For CFD analysis ANSYS FLUENT is used as an analysis tool. Using the unsteady incompressible Navier-Stokes equation as the governing equation, SST k-ω turbulence closure model is implemented to investigate the drag forces on the Bluff Body. The pressure based solver is used for the analysis. SIMPLE Scheme second order solution method is used for analysis.

CFD Modeling and Methodology: CFD solve the Navier-Stokes equations numerically for fluid flow using computers. All CFD quotes contain three main elements. (a) The Pre-processor (b) The Solver (c) The Post-processor Pre-processor:

Employed to fully specify a CFD flow problem in a form suitable for the use of the solver. The reason of fluid to be analyzed is called the computational domain and it is made up of a number of discrete elements called the mesh. The users have to define the properties of fluid acting on the domain before the analysis is begin; these include external constraints or boundary conditions, like pressure and velocity to implement realistic situations. Solver: Program that calculate the solution of the CFD problem. Governing equations of solved. This is usually done iteratively to compute the flow parameters of the fluid as time elapses. Convergence is important to produce an accurate solution of the partial differential equations. Post-processor: Used to visualize and quantitative process the results from the solver. In a contemporary CFD package, the analyzed flow phenomena can be presented in vector plots of contour plots to display the Trends of velocity, pressure, kinetic energy and other properties of the flow. The advances in Computer technology over the past decade enables CFD to be applied to Complex flow field and has become a vital tool in applications on engineering study. In CFD study, another aspect of consideration of simulation is the residue of the solutions. The equations describing fluid flow are solved iteratively so residuals appear. Engineering application, schedule is usually targeted between 4 to 6 orders of magnitude of the actual values to achieve convergence of the solution to an acceptable level. Geometrical model:

Figure 1: Modeling of cube

Figure 2: Modeling of Sphere


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Figure 3: Meshing of Sphere

Table 1: For Sphere

Nodes 1213165

Elements 2803945

Figure 4: Meshing of Cube

Table 2: For Cube

Nodes 753566

Elements 820715

Experimental Analysis To validate the results, it is very important to compare the results obtained in CFD with the corresponding cases tested experimentally in wind tunnel. It is used for laboratory research. Also for car, drone and planes. The wind tunnel consists of drive Section, Settling Chamber, Contraction Cone, Test Section and Diffuser. One can measure pressure, temperature, forces, moments, turbulence intensity by the use of wind tunnel.

Figure5: Wind Tunnel

4. PROJECT SIMULATION AND RESULTS

Table 3: For sphere

Re Cd (exp) Cd (CFD)

3.31E+04 1.6389 1.3958

3.98E+04 1.7072 1.3516

4.64E+04 1.6723 1.3906

5.30E+04 1.6005 1.3704

5.97E+04 1.5175 1.2927

6.63E+04 1.434 1.2168

7.29E+04 1.5237 1.182

7.96E+04 1.4226 1.1414

8.62E+04 1.3334 1.1152

9.22E+04 1.2542 1.084

Table 4: For cube

Re Cd (exp) Cd (CFD) 1.33E+05 1.2865 1.2826 1.59E+05 1.3404 1.2877 1.86E+05 1.541 1.2806 2.12E+05 1.5076 1.2822 2.39E+05 1.5883 1.2792 2.56E+05 1.6081 1.2826 2.92E+05 1.5949 1.2789 3.18E+05 1.5633 1.2814 3.45E+05 1.6177 1.2783 3.71E+05 1.641 1.2816

Figure 6: Graph of Re Vs Cd (exp) for sphere


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Figure 7: Graph of Re Vs Cd (CFD) for sphere

Figure 8: Graph of Re Vs Cd (exp) for cube

Figure 9: Graph of Re Vs Cd (CFD) for cube

Ansys Fluent enables to plot contour lines or profiles on the physical domain. Contour lines are lines of constant magnitude for selected variables. The minimum and maximum values contoured are set based on the range of values in the entire domain. The color scale will start at the smallest value in the domain and end at largest value. If blue corresponds to 0 and red corresponds to 10 and the values on surfaces ranges only from 4 to 6, the plot will contains mostly green contours.

Figure 10: Velocity contour for sphere (V=5)



Figure 13: Velocity contour for cube (V=5)


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Figure 14: Velocity contour for cube (V=10)

Figure 15: Velocity contour for cube (V=14) 5. CONCLUSION The values of coefficient of drag obtained in CFD simulations by varying the various Reynolds numbers is compared with the values obtained by wind tunnel test by taking the corresponding values used in CFD simulation for the purpose of validation. For cube as Reynolds number increases the intensity of the turbulence increases near the edges of the cube. A very good match is obtained in terms of Cd with the experimental results . The turbulence model used in the present analysis SST k-ω provides a reasonable agreement with the establish the result however other turbulence models can be verified and compared . This part is left as our near future work REFERENCES 1. Mehmet Ishak and Dalshad Ahmed KAREEM , “A Numerical Analysis of Fluid Flow around Circular and Square Cylinders” journal AWWA 2. Siddhartha Bandyopadhyay, Milan Krishna Singha Sarkar, Debasish Roy, Arunava Chanda ,“Experimental observation of boundary layer of the turbulent flow over bluff body inside rectangular diffuser” procedia engineering 56 (2013) 275-280

1. 3. Sibha Veerendra Singh, M. Nirmith Kumar , “CFD analysis of different bluff bodies” International Journal of Novel Research in Electrical and Mechanical

Engineering vol.2 Issue 3, pp: (139-145), Month: September-December 2015

2. 4. Taeyoung Han, “computational analysis of 3D turbulent flow around bluff body in ground proximity” AIAA JOURNAL vol 27 9 sep 1989

3. 5. Yih Nen Jeng , “some detailed information of low speed turbulent flow over a bluff body evacuated by new time frequency analysis” AIAA 2006-3340

4. 6. R.K.Patil, Dr.B.Garnaik, Dr.M.P.Khond, Dr.L.G.Navale “Environmental Pollution Reduction in Cement Industry for Co-Combustion of Waste Tyre and Coal as a Fuel” published in International Journal of Modern Engineering Research (IJMER) Vol.2, Issue.6, Nov-Dec. 2012 pp-4652-4656 ISSN: 2249-6645

5. 7. R.K.Patil, Dr.M.P.Khond, Dr.L.G.Navale “Heat Transfer Modeling of Rotary Kiln for Cement Plants”accepted for publication in International Journal for Advancement in Technical Research Development (IJATRD) ISSN: 2277-405X, May 2012.

6. 8. R.K.Patil, Dr.M.P.Khond, Dr.L.G.Navale “Some studies on Energy Conservation and Cogeneration in Dry Type Indian Cement Plant ”International Journal of Applied Engineering Research. (IJAER) ISSN: 0973-4562 Volume 6, Number 23, 2011.

7. 9 .R.K. Patil, Khond M.P., Nawale L.G., “The Trends and Practices of Thermal Energy Conservation in Indian Cement Industries.” Proceedings of International Conference on Advances in Mechanical Engineering (ICAME) May 29-31, 2013, COEP, Pune, Maharashtra, India Paper ID. ICAME2013, S15/03.

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