Inhalation Hole Measurement of a NACA Aerofoil to Confirm CFD - Research Paper Example

Assessment Title and Tasks: Wind tunnel testing of a NACA aerofoil to validate CFD Modelling results using ANSYS Fluent Name Module Title: Computational Fluid Dynamics Tutor Date Abstract ANSYS Fluent software has been to carry out CFD simulation of NACA0012 aerofoil to validate the results. The results looked at the effects of changing angle of attack (α) on lift (CL) and drag (CD) coefficients. This will be done by adjusting these variables both negative and positive ways to have the results of simulation. Validation of the results was done by using TecQuipment’s subsonic wind tunnel for the same aerofoil. The results were compared with CFD results. The results showed that there no significant difference in the results obtained and model results. If the down-force is increased to improve traction results will be recorded and opposite is done. The results of a single NACA0012 aerofoil model simulated using ANSYSCFX have been represented in this case. This simulation will help in wing design as the variables will be known. The CFD simulation has been used in fire design, aerofoil design and many other engineering designs involving velocity and pressure. Table of Contents Abstract 1 Introduction/Background Information 3 CFD Model 3 Model creation and meshing 3 Boundary conditions 4 Generate the 2D geometry and meshing 5 Meshing 5 Wind Tunnel Work 7 Analysis and Discussion 8 Lift and drag coefficients 10 Validate the numerical model 11 Pressure contour plots. 11 Mesh sensitivity study 13 Conclusions 15 References 16 Introduction/Background Information The movement of an aerofoil produces a lift that enables an airplane to take off. An aeroplane wings moves in air using drag force which is exerted by air due to differences in pressure. This is made possible because the relative velocity of air and airplane are different creating a stream flow around the aeroplane in a form of laminar. The drag force that is formed when the air craft is moving is always proportional to the speed of the aeroplane. In order to have large relative speeds a large drag force is exerted due to turbulence of the movement. This is necessary to ensure that an aircraft is kept in a certain direction without changing due to external forces. The streamlines of wind in the boundary layer will change their movement from laminar to turbulent. This means there reaches appoint where the streamlines velocity does not increase but will only increase after traversing streamlines changes. Theoretically, the recorded streamline pressure is expected to drop linearly. The streamline velocity profile in study will assist in one to comprehend how boundary conditions and how they are developed. The validation was done using of the Reynolds number. This was done by comparing results and calculated Reynolds number for either laminar or turbulent. CFD Model Model creation and meshing To begin with a single model will be designed for simulation using the following variable and the diagram thereof is presented below; X (m) 0 0 0.03 0.027 0.024 0.19 0.20 0.30 0.30 0.40 0.40 0.60 0.60 Y (m) 0.06 0.10 0.07 0.12 0.18 0.06 0.19 0.06 0.20 0.06 0.30 0.06 0.302 Table 1: Data for aerofoil construction points The constructed aerofoil construction point’s body from the above data is present below . The assumption is this case is that the aerofoil is moving at speed of 360Km/hr (100m/s), creating a free streamline velocity of the same speed this gives Reynolds Number is 3*106 and turbulence intensity is assumed to be 4%. One of characteristics of the aerofoil model that is of length is assumed to be 1. This means that model’s geometry is normalized by 1 from nose to the back of the aerofoil. Figure 1: Single aerofoil Boundary conditions Mathematical modelling was used in the development of model boundary condition which is common in CDF models. Matrix methods are used in Mathematical modelling for the efficient solution of these equations. In essence, the problem is to be able to predict the streamlines velocity and pressure changes around the of aerofoil in air due to design. The solution requires coupling the streamlines modelling approaches with appropriate drag and lift equations whilst allowing for the changing geometric boundary condition of the attack angle. These are the boundary conditions Generate the 2D geometry and meshing The a 2D single aerofoil model has been produced in this section as well as the mesh for the simulation. The elements of ANSYSCFX used have been defined by 10 nodes with 1o of freedom. The 2D model will enable us simulate the aerofoil and other variables easily. The boundary conditions and geometry of the aerofoil shown in the tables above were determined by input data. Since it is assumed that the aerofoil is moving at speed of 100m/s, creating free stream velocity air. It is therefore simulated to indicate that air is moving to the opposite direction of the aerofoil with the same velocity at both cases that is when the aerofoil is moving or static. All computations will be carried in mind of the air resistance and it done in an environment similar to air domain. Meshing Meshing involves designing cells or open spaces that are used for simulation. Inappropriate meshing often gives wrong answer. Through application of the model it is possible to calculate the streamlines occurring between each cell through use of fundamental physics equations. The number of cells which is used has a considerable effect on the results obtained. In is always desirable to have a large number of cells but his would call for more computer resources which will translate to high expenses. Halving the size of the grid cells in each direction will result in the doubling of the run time for each dimension in space and time. Changing the dimensions a mesh from a resolution of 4 cm to 2 cm will mean better results; however, the reduction in cell size will take sixteen times longer to run. The aerofoil mesh is as shown below; Figure 2a: Meshing of the aerofoil The condition of the wind was specified by using the mesh elements below; where the velocity is. The velocity input is negative because the wind travels in the opposite direction of the aerofoil. The command is also used to set the mesh element in the air domain at around the aerofoil. To reduce the cost of modelling a proper mesh element size and applying appropriate inflation on the boundary will optimize in computational cost and accurate result. The following are details of the inflation layer. The boundary layers in finer mesh were used to capture the effect of changes angles of attack of the aerofoil. This was done using thick layer for purpose of saving time in computation. Then a growth rate of 2 is applied to gives a better layer arrangement in the boundary. This is has been used and the following boundary body has been from. Figure 3: Boundary layer Wind Tunnel Work The available data of NACA0012 aerofoil shows that it has 20 static pressure tapping. This is at all sides of the aerofoil that is at lower surfaces, along its chord and the upper surfaces. Since the purpose is to determine how streamlines affect velocity and pressure distribution in the wind tunnel as well as validate results with CFD results. This requires the considerations of angles of attack of 1o, 5o, 10o, 150, and 20o have been chosen for analysis. The following physical equations are used Where Y is pressure force per unit span which reworked as, This equation will then be non-dimensionalized as Where c is chord and the other is dynamic pressure then, Lift and Drag The lift and drag of the airplane is shown in the figure below Lift and drag are related to the X- and Y- forces as follows: Where Cl the lift, and Cd is drag coefficients It can be noted from the diagram that the aeroplane is tilted when taking off but because of the wings that have aerofoil lift force helps it being lifted upward. Analysis and Discussion The range of α for CFD and wind tunnel There is fallacy about lift force where it is believed that air is reflected downwards to causes a lift. However, according to the law of physics, pressure between the forward of aerofoil is different from the pressure the other side. This means the pressure is low where the speed is very high and high where the speed is low thus pushing the aircraft forward. An aerofoil creates a lift as shown in the figure below. The lines in the figure represent streamlines of air flowing past the aircraft since it has wings that act as wind tunnels. As it can be noted, the streamlines bend due to the deflection of air by wings to assist in upward lift. Applying Newton’s third law of motion whereby conservation of momentum is used we will state that the wind will push the air upward when the aircraft want to land, this causes a lift. If the wing pushes air upwards then an upward lift is generated enabling the air craft perform its functions as required. If this theory is taken on the phase value one would assume that the wing of an aircraft will just bounce of due to air deflection, but energy needs to be applied for this to be made possible. The figure below shows lines presenting streamlines of air flowing past the aerofoil and as it can be noted, the streamlines have bends due to air deflection to enable movement. The effect of air within an ideal aerofoil is assumed to be tightly designed will be negligible but the wind of the same strength will have a significant impact on an aerofoil which is loosely designed resulting in increased and complex airflows. This can lead to failure of airplane. Wind induced internal flows is the flow of wind within the atmospheric boundary layer with its induced pressure load on aerofoil surface. There is substantial variation in wind induced flows, on basis of the type of aerofoil structure. Where the steady wind flow conditions exist, there is a rapid buildup of internal pressure surrounding the aerofoil so as to neutralize the external pressure emanating from the windward direction. There will be a decrease in total pressure with the decreasing height up air. This movement will then result into a vortex formation in back of the aerofoil. The pressure will be highest at the stagnation point on the windward side of the aerofoil. There is a lot effort which is being directed towards the development of physical modeling techniques which can be used in development models to be used in investigating on the impact of wind based on boundary layer Lift and drag coefficients The drag and lift forces as well their coefficients of model are shown in the table below Parameters Values Drag force 15.23[N] Drag coefficient 0.523 Lift Force 6.23 [N] Lift Coefficient 0.0850 Table 1: forces and the coefficients The table indicates that values of the case are higher than those for simulation. However in both cases the lift force is positive meaning that it enables aerofoil stay on the air. If it was negative then aerofoil will be lifted up in the air. Validate the numerical model Numerical model will be used in validating of this study. The validated coefficients and modelling coefficients are shown in the table Current Model Validated Automobile Lift Coefficient (CL) 0.2355 0.483 Drag coefficient (Cd) 0.823 0.83 Table 2 : Theoretical coefficients Vs. modelling coefficients The table above show results of both the validated coefficients and modeling coefficients and it can be noted that the coefficients of models are lower than the ones in the validated. This can be attributed to errors in keying in some data. Pressure contour plots. The pressure contour plot below shows the results from simulation. The figure below shows that the pressure of an airfoil and the pressure of an airfoil are focused on the front part of the airfoil. This means other parts of the airfoil are not under extensive pressure as the front. The pressure on the up face of the airfoil increases the drag coefficient because this pressure is a force on the tail area which makes a down force on the airfoil. The figure below the shows that the level of pressure varies behind the front aerofoil while the back shows that pressure is lower. The pressure range is from -184Pa to 128.Pa for simulation and -120Pa to 1180Pa for validated values. Figure 4: Pressure contour It can be noted that pressure results is not the same as they produced by different models to calculate the values Contours of pressure and velocity fields with the CFD results Figure 5: Contour plot of Velocity magnitude around Airfoil The figures above the shows that the velocity varies, the highest Velocity vector are 114m/s. The path that the inlet air follows, starting from the point of entry, is called a streamline. The streamlines curves and bends, but they cannot cross each other; if they did, the air will change direction. The direction of the air velocity at any point must be tangent to the streamline passing through that point. Mesh sensitivity study The figure below shows that the pressure of airfoil and main pressure is focused on the front part. This means other parts of the airfoil are not under extensive pressure as front. The pressure on the up face of the airfoil increases the drag coefficient because this pressure is a force on the airfoil area. The above indicates that due to the drag and lift force provided by the aerofoil, the drag increased from 5.04N to 101N. The aerofoil moves in air using drag force which are exerted by air due to differences in pressure. This is made possible because the relative velocity of air and aerofoil are different creating a stream flow around the aerofoil in a form of laminar. From the table above it can be noted that when the turbulence and flow on the aerofoil increase the whole aerofoil’s drag coefficient but reduce the lift coefficient; it reduce the lift force in the rear part of the aerofoil which can increase the rear grip and stability of the aerofoil. The turbulence and the flow created by the airfoil is the reason that increases the drag force of the airfoil. The streamlines above the wing are closer together than beneath the wing, showing that the flow speed above the wing is faster than it is beneath. This observation confirms that the pressure is lower above the wing, because where the pressure is lower, the flow speed is faster. “the principle of transit time states that because of the longer path of the upper service of an aerofoil, the air going over the top must go faster in order to catch up with the air flowing around the bottom, i.e. the parcels of air that are divided at the leading edge and travel above and below an aerofoil must rejoin when they reach the trailing edge. Bernoulli’s principle then applies, to conclude that since the air moves faster on the top of the wing the air pressure must be lower. This pressure difference pushes the wing up” is not true because when the air on the upper side goes down it is because it is deflected. In this case Newton’s third law of conservative momentum is applied where if the airfoil pushes downward on air the air also pushes upward. This upward force is considered to be a lift and in the process there is air that is bouncing off the bottom side. If the air exerts a net upward force on the wing, the air pressure must be lower above the wing than beneath the wing. The streamlines above the wing are closer together than beneath the wing, showing that the flow speed above the wing is faster than it is beneath. This observation confirms that the pressure is lower above the wing, because where the pressure is lower, the flow speed is faster. When a air flows the longer part, a boundary layer is almost immediately formed circumferentially around the lower side creating pressure difference. The core of the flow which is in viscid in relation to the moving object is restricted from freely moving due to the growth of a viscous boundary layer. Conclusions In order to investigate the angle of attack effect on drag and lift force, varying angles were simulated where 0o was a control simulation that is under normal conditions with normal atmosphere. The results showed that coefficients were influenced by the angle of attack. When angle is increased coefficients increase. The results are also important in reducing fuel consumption and in the process of increasing the stability of the airplane. Streamlines are closer together where the air movement is faster and farther apart where it flows more slowly. Thus, streamlines help us visualize air flow. The air velocity at any point is tangent to a streamline through that point. References Eleni, D. C., Athanasios, T. I. & Dionissios, M. P., 2011. Evaluation of the turbulence models for the simulation of the flow over a National Advisory Committee for Aeronautics (NACA) 0012 airfoil. Journal of Mechanical Engineering Research Ma L, Chen J, Du G, Cao R, 2010. Numerical simulation of aerodynamic performance for wind turbine airfoils. Taiyangneng Xuebao/Acta Energiae Solaris Sinica, 31: 203-209. Sahul, N. K. & Imam, S. 2015. Analysis of Transonic Flow over an Airfoil NACA0012 using CFD. International Journal of Innovative Science, Engineering & Technology, Silisteanu PD, Botez RM., 2010. Transition flow occurrence estimation new method. 48th AIAA Aerospace Science Meeting. Orlando, Floride, Etats-Unis, Janvier, pp. 7-10 Zhao, M., Cheng, L., & Teng, B., 2007, Numerical Modeling of Flow and Hydrodynamic Forces around a Piggyback Pipeline near the Seabed, Journal of Waterway, Port, Coastal, and Ocean Engineering , pp.286-294. Read More

The elements of ANSYSCFX used have been defined by 10 nodes with 1o of freedom. The 2D model will enable us simulate the aerofoil and other variables easily. The boundary conditions and geometry of the aerofoil shown in the tables above were determined by input data. Since it is assumed that the aerofoil is moving at speed of 100m/s, creating free stream velocity air. It is therefore simulated to indicate that air is moving to the opposite direction of the aerofoil with the same velocity at both cases that is when the aerofoil is moving or static.

All computations will be carried in mind of the air resistance and it done in an environment similar to air domain. Meshing Meshing involves designing cells or open spaces that are used for simulation. Inappropriate meshing often gives wrong answer. Through application of the model it is possible to calculate the streamlines occurring between each cell through use of fundamental physics equations. The number of cells which is used has a considerable effect on the results obtained. In is always desirable to have a large number of cells but his would call for more computer resources which will translate to high expenses.

Halving the size of the grid cells in each direction will result in the doubling of the run time for each dimension in space and time. Changing the dimensions a mesh from a resolution of 4 cm to 2 cm will mean better results; however, the reduction in cell size will take sixteen times longer to run. The aerofoil mesh is as shown below; Figure 2a: Meshing of the aerofoil The condition of the wind was specified by using the mesh elements below; where the velocity is. The velocity input is negative because the wind travels in the opposite direction of the aerofoil.

The command is also used to set the mesh element in the air domain at around the aerofoil. To reduce the cost of modelling a proper mesh element size and applying appropriate inflation on the boundary will optimize in computational cost and accurate result. The following are details of the inflation layer. The boundary layers in finer mesh were used to capture the effect of changes angles of attack of the aerofoil. This was done using thick layer for purpose of saving time in computation. Then a growth rate of 2 is applied to gives a better layer arrangement in the boundary.

This is has been used and the following boundary body has been from. Figure 3: Boundary layer Wind Tunnel Work The available data of NACA0012 aerofoil shows that it has 20 static pressure tapping. This is at all sides of the aerofoil that is at lower surfaces, along its chord and the upper surfaces. Since the purpose is to determine how streamlines affect velocity and pressure distribution in the wind tunnel as well as validate results with CFD results. This requires the considerations of angles of attack of 1o, 5o, 10o, 150, and 20o have been chosen for analysis.

The following physical equations are used Where Y is pressure force per unit span which reworked as, This equation will then be non-dimensionalized as Where c is chord and the other is dynamic pressure then, Lift and Drag The lift and drag of the airplane is shown in the figure below Lift and drag are related to the X- and Y- forces as follows: Where Cl the lift, and Cd is drag coefficients It can be noted from the diagram that the aeroplane is tilted when taking off but because of the wings that have aerofoil lift force helps it being lifted upward.

Analysis and Discussion The range of α for CFD and wind tunnel There is fallacy about lift force where it is believed that air is reflected downwards to causes a lift. However, according to the law of physics, pressure between the forward of aerofoil is different from the pressure the other side. This means the pressure is low where the speed is very high and high where the speed is low thus pushing the aircraft forward. An aerofoil creates a lift as shown in the figure below. The lines in the figure represent streamlines of air flowing past the aircraft since it has wings that act as wind tunnels.

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