CFD Analysis on Ahmed Body and the suggested aerodynamic changes to the Ahmed Body

Transcription

1 CFD Analysis on Ahmed Body and the suggested aerodynamic changes to the Ahmed Body Sparsh Sharma School of Mechanical Engineering, VIT University Vellore, Tamil Nadu, India. Yagnavalkya Mukkamala School of Mechanical Engineering, VIT University Vellore, Tamil Nadu, India. Abstract Ahmed Body is a very standard simple bluff body which is basically a very simplified version of an automobile. The body is used many times to validate new CFD simulation software and to compare the accuracy of one software with another CFD software by performing simulations in both and comparing the values for Coefficient of drag (Cd) and lift (Cl) with the experimental obtained values of Cd and Cl for the Ahmed body for a particular slant angle. The Ahmed body s has a variable parameter which is the slant angle at the back, the change in the slant angle (the angle which the slant makes with the horizontal) has an effect on the drag coefficient obtained in simulations, this is due to the difference in the size of the wake area and intensity of the vortices formed. This wake area is a low pressure region of recirculating air which is caused due to the separation in flow as the flow encounters change in curvature and shape of the Ahmed Body. Since the Ahmed s body is a very simplified version of the automobile body, there has been various experiments and simulations aiming to introduce slight aerodynamic and other changes to the base model of the Ahmed s body to compare the result of the new obtained body with the original Ahmed body and try to get a picture if this change can be at some level be applied to a commercial car which can improve the performance of the car with very minor operating costs. This paper explores the addition of certain structural and aerodynamic changes to the original: Ahmed body by looking into diffusers, rear spoilers, front nose and other suggested changes made by previous authors in related projects regarding the Ahmed Body. The paper then compares the final results obtained in simulation software to the values of the original Ahmed Body and comments on the impact of each of these aerodynamic and structural changes. Keywords: Coefficient of Lift and Drag, Ahmed Body, Spoiler, Diffuser, FLUENT, Wind Tunnel Testing Introduction The use of Computational software in today s world has increased sharply, it has provided us with a very cost effective way of carrying out experiments with lots of ease and high accuracy in terms of results, It has reduced the overall cost of many projects and has in fact decreased the time of testing as well, In fact simulation has become so reliant that these days most testing is done virtually through these software s itself. This project also uses a CFD simulation software called FLUENT which is a software concerned mainly about the flow of fluids like air in a user defined domain with user defined boundary conditions, it is mainly used to observe the aerodynamics behind a body such as lift, drag, vortices, pressure and velocity distributions over the body and also to visualise the flow. In this paper we will focus on an automotive application by looking into the simulation of a base Ahmed body and then looking into certain aerodynamic and structural modifications to the base Ahmed body in a bid to produce a significant negative lift without producing excessive drag. The paper also deals with verification of the results by performing of the wind tunnel testing. Purpose and Scope of the Paper The purpose of this project is to visualise and understand very basic aerodynamics associated with cars, by analysing the Ahmed Body we can understand the behaviour of the velocity streamline when it comes in contact with the Ahmed Body. From the project the change in velocity and pressure regions at different lengths along the Ahmed Body can be seen. Moreover the concept of wakes, flow separation, vortices, slant angle, and the definition and relation between all the terms become very clear. Most importantly the effect that certain aerodynamic and structural additions made to the Ahmed Body has on the Coefficient of Lift and Drag, turbulence, wake area for the Ahmed Body can be analysed and compared to the original Ahmed Body and certain inferences can be main regarding how each aerodynamic/structural change has a positive or negative impact. The project also deals with understanding of manufacturing process like 3D printing and learning about which type of material and printing method gives us the best surface finish suitable to our application. The project also deals with Wind Tunnel Testing and there is great scope in learning about wind tunnel testing itself and the various parameters involved. The scope of this paper can be found in the Automobile Industry, this is so because, at a very conceptual level, the Ahmed Body is a very simple version of an automobile, Hence when certain new aerodynamic features and structural 6465

2 changes want to be incorporated into the automobile and the engineer would like to know the impact that this change would have on Coefficient of Drag and lift and other factors like Turbulence, the engineer can first try these changes on the Ahmed body, this is because it will be very easy to incorporate these changes to the simple body and moreover it will take much less time than actually making the change in the actual design of the automobile. Instead, the engineer can see if the proposed changes he wishes to bring in the automobile has a positive impact on the performance of the Ahmed Body, if yes, then he can go ahead and be confident of making changes in the actual automobile design, however if the performance characteristics are not satisfactory, then they can be discarded and hence a lot of simulation time can be saved. Literature Survey In M. Grandemange and his team s paper [1] have performed experiments at industrial scales over the Ahmed geometry, i.e. at a Reynolds number of Re=2.5x10 6 based on the height of the body. In their paper the shape of the square back geometry is first optimized to make an initial substantial drag reduction. The separated flow at the trailing edge is orientated by introducing chamfers at the top and bottom edges. A parametric study based on both chamfered angles leads to an optimized Ahmed geometry having a drag of 5.8% lower than the reference square back model. It is evidenced that this optimized geometry produces 4 intense longitudinal vortices that still contribute significantly to the drag. Another suggested method to limit the formation of the local longitudinal vortices of the optimized geometry is to orient simultaneously the flow from the sides of the geometry in order to retrieve a relative axisymmetric in the after body flow. In Sneh Hetawal and team s paper [2] they talk about 3 different FSAE car models and focus on the certain aerodynamic features such as the addition of front wings to the car. In the paper they show that the addition of these aerodynamic devices have a positive impact on the drag reduction as well as the increase in down force. In the paper Characterization of synthetic jet actuation with application to Ahmed body wake [3] an active system is used rather than making structural changes to the original Ahmed Body. An extensive experimental parametric analysis, based on aerodynamic forces, velocity and pressure measurements, has been conducted on a simplified Ahmed body type geometry in order to investigate and to develop a wake flow control technique by means of a synthetic jet. Using this jet system, implemented in an open-loop strategy, the aerodynamic drag has been successfully reduced. In the paper Coupling active and passive techniques to control the flow past the square [4] the authors have applied both active control techniques like steady jets to control and manipulate the formation of wake areas, this along with certain passive techniques like introducing porous material layers so as to reduce the velocity and form small vortices in the nearby area this leads to decrease the overall drag produced by the Ahmed body and make it suitable to the automotive industry standards. In the paper Effect of aspect ratio on the near-wake flow structure [5], the author has talked about a certain parameter called Aspect Ratio which is a parameter which was not used by others, in the paper he defines what this aspect ratio is and then shows through his simulations that by maintaining a certain aspect ratio, he has found that the overall drag has been reduced in comparison to the original size of the Ahmed Body itself. He justifies this approach by saying that the vast array of vehicle aspect ratios existent in the automotive industry, and the severe implications that slant angle choice coupled with aspect ratio can have on overall vehicle drag characteristics. In the paper Effects of suppressing the 3D separation on the rear slant on the flow [6], the author mainly focuses on the flow separation aspect which happens when the airflow around the car comes in contact with the sudden rear slant, the author suggests that by rounding of that edge from where the roof of the Ahmed body goes into a rear slant, we can reduce the drag by nearly 10%. The author says that by rounding off these edges, the flow becomes more smooth and follows the surface of the Ahmed Body for a further distance before eventually flow separation occurs. This way we can manipulate the wake area slightly. In the paper Study of F1 car aerodynamic rear wing using computational fluid dynamic (CFD) [7], the author performs CFD analysis on 3 NACA profiles which are commonly used as wing structures for spoilers in F1 cars. In the study he performs a 2-D as well as a 3-D analysis on these wings and compares the value of the C D and C L he gets for the different wing profiles. In the paper Best practice guidelines for handling Automotive External Aerodynamics with FLUENT [8], the author takes us through various commonly encountered simulations when it comes to automobiles, the author based on experience, suggests various good practices which should be followed at the time of simulation so as to get most accurate results from ANSYS simulation. Ahmed Body Ahmed Body is a standard bluff body widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features. The Ahmed body at a Reynolds number of 780,000 has a coefficient of drag of 0.28 and a coefficient of lift of 0. Even though the values of coefficient of lift and drag are very well established for the Ahmed Body, the Ahmed body is still simulated using FLUENT so that we may validate the ANSYS mesh we are using on the body. The Ahmed body specifications used for the ANSYS simulation is shown below in Fig1. The Ahmed body was simulated in ANSYS FLUENT at a Reynolds number of 780,000 and the results obtained was a C D of 0.26 and a C L of 0. This means there was a zero percent error for the lift calculation and the drag calculation had an error of nearly 7% which is acceptable and shows that the mesh used for carrying out the CFD analysis in FLUENT is indeed a good and valid mesh considering the size of the Ahmed body and the fluid domain around it. This means that a similar type of mesh can be used for the modified Ahmed body which will roughly have the same size 6466

3 and will be analysed at the same operating conditions as that of the standard Ahmed body. Some of the results obtained from the simulation are shown in the Figures [1-9] below. Figure 5: Residual values obtained in transient simulation Figure 1: Dimensions of the Ahmed Body Figure 6: Velocity vectors on the Ahmed Body Figure 2: Ahmed Body and Meshing Figure 7: Static Pressure Contours on Ahmed body on the symmetry plane Figure 3: Coefficient of Lift C L obtained= 0 Figure 8: Velocity contours on the symmetry plane Figure 4: Coefficent of Drag C D = 0.26 Figure 9: Velocity vectors on the Ahmed Body 6467

4 Aerodynamic and structural changes considered Spoilers A spoiler is basically a wing which has been inverted for the purpose of inducing a down-force or a negative lift force on the aerofoil. The purpose of producing this negative lift is to increase the stability of the car by making sure that the negative lift makes the wheels be in contact with the road at all times and hence helps in handling and acceleration of the car. Spoiler is a very essential feature in race car and sports cars in general. The objective while selecting an ideal spoiler is to first consider the Reynolds number at which you will be operating act (which in turn depends on the speed at which your car is going) and after that also consider the angle of attack which gives you the maximum lift force but at the same time less drag. The negative aspect of too much negative lift is that you end up producing excessive drag, so while selecting the aerofoil, we must consider an optimization between the 2 parameters. Taking into consideration a few common aerofoils, Finally NACA 6409 was selected, the following shows the geometry of the aerofoil and the graphs of C D and C L versus the angle of attack. This is shown in the figures [10-12]. Figure 12: C L vs α for different angles of attack for 2 different Reynolds numbers, purple for Re = 500,000 and green for Re = 200,000 Hence from the figures[10-12], it was concluded that the NACA 6409 aerofoil will be chosen and the will be implemented at an angle of 10 0 to the horizontal as good amount of lift is produced and less drag is produced as well. Figure 10: NACA 6409 [9] Diffuser A diffuser, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower free stream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand (in area, density remains constant at the speeds that cars travel) so that it does not cause excessive flow separation and drag, by providing a degree of "wake infill" or more accurately, pressure recovery. The diffuser itself accelerates the flow in front of it, which helps generate down force. The diffuser shape chosen for car is elliptical in shape because the discharge rate is best for an elliptical curve compared to a circular curve. Figure 11: C D vs α for 2 different Reynolds numbers, purple for Re = 500,000 and green for Re = 200,000. Modified Ahmed Body After implementing the airfoil and the diffuser onto a standard Ahmed Body, the modified Ahmed body is shown in the figures below [13-15]. The solid-works body was then imported into the ANSYS, it was meshed and then solved in the same size domain as the standard Ahmed Body and under the same boundary conditions and solution methods used for the standard Ahmed body. The results obtained from the simulation are shown in the figures bellow [16-20]. 6468

5 Figure 16: Coefficient of transient drag = Figure 13: Modified Ahmed Body side view Figure 17: Coefficient of transient lift = Figure 14: Modified Ahmed Body isometric view Figure 18: Scaled Residuals Figure 19: Contours of velocity magnitude on symmetry wall Figure 15: Modified Ahmed Body front view Figure 20: Path lines by particles around Ahmed body 6469

6 Results The values obtained from the CFD simulations using ANSYS FLUENT for both the standard Ahmed body and the modified Ahmed body are shown in tables 1&2. The values of the coefficient of drag and lift were tabulated at different wind velocities ranging from 10m/s to 40m/s which correspond to different Reynolds numbers and to speeds ranging from 36km/h to 144km/h which are speeds at which commercial passenger cars usually travel at. The graphs were then plotted to compare the lift and drag coefficients obtained for both the bodies. The results can be seen in Fig 21 and Fig 22. Simulation Results Table 1: C D and C L obtained for Ahmed body in FLUENT simulation. S.No. Velocity of Air (m/s) Re CD CL x Table 2: C D and C L for modified Ahmed body in FLUENT simulation. S.No. Velocity of Air (m/s) Re CD CL x Figure 22: C D for the standard Ahmed body and the modified Ahmed body at different wind velocities. Conclusion From the results obtained above we can see that the coefficient of drag for the modified Ahmed body in comparison to the standard Ahmed body has increased from roughly 0.27 to However this increase in drag has come with a significant increase in negative lift from roughly 0 to- 0.5 which is a bigger increase than the increase in drag, which is justified with the addition of the spoiler as well as the diffuser which contribute to increase of drag and negative lift. This result can be interpreted as both a positive and a negative result, negative because of the increase in drag but positive because of the drastic stability due to the negative lift. This trade-off exists in the automotive industry as well and especially racing application where car designers and manufacturers are always trying to increase the stability of the car on the road by increasing negative lift but without increasing the drag too much. Acknowledgment I would like to thank my guide Dr.Yagnavalkya S Mukkamala and my reviewer Professor Kushal Kumar Chode of VIT University, for all their important suggestions and guidance at various different stages of the project, without which I would not been able to write this paper. I would also like to acknowledge the contributions of Mr.Manoj C.N, a fellow student for his help. References Figure 21: C L for the standard Ahmed body and the modified Ahmed body at different wind velocities. [1] M. Grandemange, O. Cadota, A. Courbois, V. Herbert, D. Ricot, T. Ruiz, R. Vigneron (2015), A study of wake effects on the drag of Ahmed s squareback model at the industrial scale, Journal of Wind Engineering and Industrial Aerodynamics,145 (2015), pp [2] Sneh Hetawal,*, Mandar Gophane, Ajay B.K., Yagnavalkya Mukkamala, Aerodynamic study of FSAE car, 12th Global Congress On Manufacturing And Management, GCMM

7 [3] Azeddine Kourta, Cedric Leclerc (2013), Characterization of synthetic jet actuation with application to Ahmed body wake, Sensors and Actuators A: Physical, 192(2013) [4] Charles-Henri Bruneau, Emmanuel Creuse, Delphine Depeyras, Patrick Gillieron, Iraj Mortazavi, Coupling active and passive techniques to control the flow past the square, Computers & Fluids, 2010 pg [5] M.Corallo, J. Sheridan, M.C. Thompson, Effect of aspect ratio on the near-wake flow structure, Journal of Wind Engineering and Industrial Aerodynamics, 147(2015) pg [6] A. Thacker, S. Aubrun, A. Leroy, P. Devinant, Effects of suppressing the 3D separation on the rear slant on the flow, Journal of Wind Engineering and Industrial Aerodynamics, (2012) pg [7] Mohd Shahmal Bin Mohd Shahid, Study of f1 car aerodynamic rear wing using computational fluid dynamic (cfd), Faculty of Mechanical Engineering Universiti Malaysia Pahang. [8] Marco Lanfrit, Best practice guidelines for handling Automotive External Aerodynamics with FLUENT, Fluent Deutschland, Version 1.2 (Feb 9th 2005)