generation of structured grids for computational

INTERNATIONAL JOURNAL OF RESEARCH IN AERONAUTICAL AND MECHANICAL ENGINEERING

ISSN (ONLINE): 2321-3051

Vol.4 Issue 3,

March 2016

Pgs: 20-47

Latha S and Gayathri R 20

GENERATION OF STRUCTURED GRIDS FOR COMPUTATIONAL AERODYNAMIC SIMULATIONS AROUND NACA AIRFOIL 0012

Latha S1 and Gayathri R2 1CAE engineer, Thiruvanmiyur CAD centre, email: [email protected]

2 Assistant professor, Aerospace Engineering in Karunya University, Coimbatore (India), email: [email protected]

ABSTRACT: This project work explains about the efficient structured grid generation over an aerofoil by comparing C-type and O-type grid. The nature of work consists of three basic parts. The first reveals a grid refinement study using the C grid for velocity 70m/s with different angle of attack. Ranging from 0.2 to 28 degrees until the solutions became invariant. T-type he second part involves O type grid generations for the same conditions. Thirdly, the efficient grid was identified based on the CFD results.

1. INTRODUCTION: 1.1 OBJECTIVES

In order to avoid difficulties such as resolving the geometry and computed solution, grid generation plays a major role in CFD simulations. A C-grid can capture the wake flow region very well where as in O-type grid the near wall region is captured very well. But O-grid has a poor grid quality at the trailing edge. The current study was done between O-type and C-type grid with laminar flow conditions. The finer grid was found with good cl and less Cd between O-type and C-type grid. GRID: The arrangement of the discrete points throughout the flow field is called grid.

1.2 GRID GENERATION: Process of generating these grids are called grid generation. CASE1: FLOW ANALYSIS OF C-TYPE GRID OVER AN AIRFOIL GAMBIT PROCEDURE: Step 1: Creating the airfoil geometry: Launch gambit. Once gambit is open make certain the solver is set for the appropriate Output, Document must now be imported into gambit. This is done by selecting file → import → Vortex data. They we will get the all the points of the air foil.

Step 2: create a edge,

mailto:[email protected]

mailto:[email protected]



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Step 3: creating the edge merge the edge.

Step 4: Create a c-type boundary:

Step 5: Create an edge mesh



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Step 6: Create a face mesh:

Step 7: Create a group:



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1.3 FLUENT PROCEDURE: Launch fluent Start > programs > fluent ink > fluent Select 2ddp from the list of options and click run. Import file Main menu > file > read > case... Navigate to your working directory and select the airfoil.msh file. Click ok. Analyse grid Grid > info > size Display > grid Select all surfaces and click ok. Define properties Define > models > solver... Under the solver box, select pressure based and clicks ok. Define > models > viscous Select in viscid under model. Define > models > energy The speed of sound under sol conditions is 340 m/s so that our free stream Mach number is around 0.15. This is low enough that we'll assume that the flow is incompressible. So the energy equation can be turned off. Make sure there is no check in the box next to energy equation and click ok. Define > materials Make sure air is selected under fluid materials. Set density to constant and equal to 1.225 kg/m3.click change/create. Define > operating conditions We'll work in terms of gauge pressures in this example. So set operating pressure to the ambient value of 101,325 pas. Click ok. Define > boundary conditions Set the boundary condition The x-component is V*cos (AOA in degree) The y-component is V*sin (AOA in degree) Solve > control > solution Take a look at the options available. Under discretization, set pressure to presto And momentum to second-order upwind. Click ok. Solve > initialize > initialize... Select the inlet as a compute form and click init. Solve > monitors > residual... Now we will set the residual values (the criteria for a good enough solution). Once again, we'll set this value to 1e-06. (Click picture for larger image) Click ok. Solve > monitors > force... Under coefficient, choose lift. Under options, select print and plot. Then, choose air foil under wall zones. Lastly, set the force vector components for the lift. The lift is the force perpendicular to the direction of the free stream. So to get the lift coefficient, set x to -sin ( ) and y to cost ( ).



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Similarly, set the force monitor options for the drag force. The drag is defined as the force component in the direction of the free stream. So under force vector, set x to cost ( ) and y to sin ( ) turn on only print for it.

Report > reference values Now, set the reference values to set the base cases for our iteration. Select inlet under compute from. Click ok. Main menu > file > write > case... Save the case file before you start the iterations. Solve > iterate Main menu > file > write > case & data... Save case and data after you have obtained a converged solution. 2. C-GRID RESULTS: 1.Velocity=70m/s, angle of attack=0.2

Pressure contour:

Velocity contour:

vector velocity:



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2.Velocity=70m/s, angle of attack=0.4

Pressure contour:

velocity contour:

Vector velocity:



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3. Velocity =70m/s, angle of attack=0.6

Pressure contour:

Velocity contour:

Vector velocity:



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4.Velocity=70m/s, aoa=0.8

Pressure contour:

Velocity contour:

Velocity vector:



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5. Velocity =70m/s, angle of attack=1.2

Pressure contour:

Velocity contour:

Vector velocity:



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6.Velocity=70m/s, angle of attack=1.8

Pressure contour:

Velocity contour:

vector velocity:



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7. Velocity =70m/s, angle of attack =2

Pressure contour:

Velocity contour:

Vector velocity:



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8.Velocity =70m/s, angle of attack=15 Pressure contour:

Velocity contour:

vector velocity:



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9.Velocity=70m/s, angle of attack=24 Pressure contour:

Vector velocity:

Velocity contour:



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2.1 CASE2: FLOW ANALYSIS OF O-TYPE GRID OVER AN AIRFOIL GAMBIT STEPS: Step 1: Creating the air foil geometry: Launch gambit. Once gambit is open make certain the solver is set for the appropriate Output. The coordinate Document must now be imported into gambit. This is done by selecting file → import → Vortex data. They we will get the all the points of the air foil.

Step 2: create a edge,

Step 3: creating the edge merge the edge.

Step 4: Create an O-type boundary:



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Step 5: Create an edge mesh:

Step 6: Create a group:

STEPS FOR FLUENT ANALYSIS:



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Set up problem in fluent: Launch fluent Start > programs > fluent ink > fluent 6.3.26 Select 2ddp from the list of options and click run. Import file Main menu > file > read > case... Navigate to your working directory and select the airfoil.msh file. Click ok. Analyse grid Grid > info > size Display > grid Select all surfaces and click ok. Define properties Define > models > solver... Under the solver box, select pressure based and clicks ok. Define > models > viscous Select in viscid under model. Define > models > energy The speed of sound under sol conditions is 340 m/s so that our free stream Mach number is around 0.15. This is low enough that we'll assume that the flow is incompressible. So the energy equation can be turned off. Make sure there is no check in the box next to energy equation and click ok. Define > materials Make sure air is selected under fluid materials. Set density to constant and equal to 1.225 kg/m3.click change/create. Define > operating conditions We'll work in terms of gauge pressures in this example. So set operating pressure to the ambient value of 101,325 pas. Click ok. Define > boundary conditions Set the boundary condition The x-component is V*cos( )

The y-component is V*sin( )

Solve > control > solution Take a look at the options available. Under discretization, set pressure to presto! And momentum to second-order upwind. Click ok. Solve > initialize > initialize... Select the inlet as a compute form and click init. Solve > monitors > residual... Now we will set the residual values (the criteria for a good enough solution). Once again, we'll set this value to 1e-06 Click ok. Solve > monitors > force... Under coefficient, choose lift. Under options, select print and plot. Then, choose air foil under wall zones. Lastly, set the force vector components for the lift. The lift is the force perpendicular to the direction of the free stream. So to get the lift coefficient, set x to -sin ( ) and y to cost ( ).Similarly, set the force monitor options for



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the drag force. The drag is defined as the force component in the direction of the free stream. So under force vector, set x to cost ( ) and y to sin ( ) turn on only print for it.

Report > reference values Now, set the reference values to set the base cases for our iteration. Select inlet under compute from. Click ok. Main menu > file > write > case... Save the case file before you start the iterations. Solve > iterate Iterate for 300 times. Main menu > file > write > case & data... Save case and data after you have obtained a converged solution. 3. O-GRID RESULTS:

1. Velocity =70m/s, aoa=0.2deg Pressure contour:

velocity contour:

vector velocity:



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2. Velocity=70m/s, aoa=0.4deg pressure contour:

velocity contour:

vector velocity:



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3.velocity =70m/s, aoa=0.6 deg

pressure contour:

velocity contour:

vector velocity:



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4.velocity=70m/s, aoa=0.8deg pressure contour:

velocity contour:

velocity vector:



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5.velocity =70m/s, aoa=1.2deg pressure contour:

velocity contour:

vector velocity:



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6.velocity=70m/s, aoa=1.6deg

pressure contour:

velocity contour:

vector velocity:



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7.velocity =70m/s, aoa=1.8deg

pressure contour:

velocity contour:

vector velocity:



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8.velocity=70m/s, aoa=2deg

pressure contour:

velocity contour:

vector velocity:



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RESULT DISCUSSION:

TABLE: 1 Comparison of lift coefficient between O-grid and C-grid: Angle

of attack

O-grid lift coefficient

C-grid lift coefficient

0.2 0.022700 0.011817 0.4 0.038410 0.024537 0.6 0.064417 0.036624 0.8 0.082590 0.048499 1.2 0.125750 0.070332 1.6 0.168570 0.091041 1.8 0.189210 0.10215 2 0.211350 0.11202 4 0.39785 0.21526

15 - 0.62348 24 - 0.66941 28 - 0.3505



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Graph 1: AOA vs. Cl for O-grid

Graph 2: AOA vs. Cl for C-grid TABLE 2: Comparison of drag coefficient between O-grid and C-grid:

Angle of attack O-grid drag coefficient C-grid drag coefficient 0.2 0.003355 0.016728 0.4 0.003345 0.016825 0.6 0.003249 0.01656 0.8 0.003268 0.01709 1.2 0.003319 0.017654 1.6 0.003504 0.018464 1.8 0.003628 0.01892 2 0.003759 0.01947 4 0.006167 0.02741



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15 - 0.14377 24 - 0.27146 28 - 0.3192

Graph 3:AOA vs. Cd for O-grid Graph 4: AOA vs. Cd for C-grid

From the table if the angle of attack increases, the lift and drag coefficient will also increase. The above graph shows the lift curve and drag curve for an O-type and C-type grid at different angles

of attack. O-grid is converged quickly than the C-type grid during the flow analysis over an airfoil at different

angles of attack. Till the angle of attack of 4 degree, the O-type grid yields the faster results after that it is taking long

time to converge.

Graph 5: angle of attack vs lift coefficient graph

Lift coefficient value at an angle of attack 2= 0.20 (from the graph 5) Lift coefficient value at an angle of attack 2= 0.2110 (for O-type grid) Lift coefficient value at an angle of attack 2= 0.11202 (for C-type grid)



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At an angle of attack 2 degree the actual value matches with the analytical value of O-grid.

4. CONCLUSION: Lift coefficient was found to be higher for O-type boundary grid than the C-type boundary grid for the same angle of attack. Drag coefficient was found to be higher for C-type boundary grid than the O-type boundary grid for the same angle of attack. Actual value of O-grid lift coefficient matches with the experimental value at an angle of attack 2. From the results and analysis of O-type will give the better results than the C-grid.

5. REFERENCES 1. Sengupta, Tapan K.,” High Accurancy Computing Methods Fluid Flows And Wave Phenomena”, New York.,Cambridge University Press, 2013. 2. T.Cebeci J.Rshao, F. Kafyeke E. Laurendeau.,”Computational Fluid Dynamics For Engineers”, California , Horizon Publishing., 2005. 3. Michael J.Bouey Jr.,Computational Fluid Dynamics Project,”Generating A Grid That Wraps Around An Airfoi”l., ,2011. 4. Hoffmannn, Klaus A., Chiang Steve T. .,Fourth Edition.,”Computational Fluid Dynamics Vol.1”, Kansas, Engineering Education System, 2000. 5. Siladic,Mato F., B.Sc., “Numerical Grid Generation And Potential Airfoil Analysis And Design”, dean for research and professional development air force institute of technology.,1988. 6. Anderson, John D., JR.” Computational fluid dynamics”,. The basics with applications.,university of Maryland.,1995

generation of structured grids for computational

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