ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016 Analysis of External Aerodynamics of and Hatch Back Models Having Same Frontal Area by Computational Method 1Sharath Kumar S N, 2Dr. C. K. Umesh 1,2Dept. of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore, India

Abstract design time and cost. Recent developments and competition in the automotive industry Also the use of CFD tools and high end configuration computers has forced them in producing aesthetic, economical and safer . has helped automotive industries in reducing the investment cost The current trend in automotive industry is to convert their most on cars and also to make maximum utilization of modeling and successful Sedan car into Hatch back or vice versa, instead of analysis tool to make necessary modifications in the geometry of designing a whole new car, thereby reducing the design as well the cars, thereby making sure that the cars will be safer and defect production cost. free before the actual production. The major objective of this work is to perform aerodynamic Bhagirath zala et al (2012) [2] in their study compared 1:20 reduced comparison of Sedan and Hatch Back car models having same scale Sedan and Hatch Back car models by experimental method. frontal area by computational method on 1:30 reduced scaled The study showed that Sedan car model was more aerodynamic car model in 0.3m wide, 0.3m high and 1.5m length wind as compared to Square Back car model and had a drag co- tunnel domain. The Sedan car is the scaled down model of the efficient value 42% lesser than the Hatch Back car model. The commercially available car, while the Hatch Back car model is computational work carried out by Bhavini Bijlani et al [2013] newly designed such that both the car models have same frontal [3] also suggests that Sedan car model is more aerodynamic than area. The three dimensional computational analysis was carried Hatch back car model by 42%. out using CATIA V5 as the modeling software, ICEM CFD as It is found from the literature survey that most of the road vehicle the preprocessor (meshing), ANSYS FLUENT as the solver and aerodynamics research work has been concentrated either on Sedan ANSYS CFD- POST as the post processor. cars or bluff bodies such as passenger bus and only few works Computational results suggest that Sedan car is more streamlined has been reported on the aerodynamic analysis of Hatch Back and aerodynamically efficient as compared to that of newly cars. Although there are some works done on the aerodynamic designed Hatch Back car model. Even though both the car model comparison of Sedan and Hatch Back cars [2, 4], there is no work has same frontal area, Hatch back model has approximately 3.5% reported on the direct comparison of Sedan and Hatch Back cars more Drag Co-Efficient as compared to that of Sedan car model. having same frontal area by computational method. Since each car This can be concluded in the other way; conversion of Hatch Back manufacturing company has its own design language and it’s not car into a Sedan car by keeping frontal area same can improve a good idea to compare two different segments of cars i.e., Sedan the Drag Co-Efficient by 3.5%. and Hatch Back cars of different make (company) to come into a conclusion that which among them is aerodynamically better, Keywords unless any of the geometric parameter is kept constant (in the Aerodynamics, CFD, Sedan, Hatch Back, drag co-efficient. present work frontal area is kept constant).

I. Introduction II. Computational Methodology Aerodynamic analysis of car models has helped automotive Computational analysis was carried out for both 1:30 scaled industry in reducing the drag force acting on the cars over the down Sedan and Hatch Back car models having same frontal years considerably. Although drag force acting on the car depends area. Sedan car model was made to look geometrically similar to on several parameters such as drag co-efficient, projected frontal commercially available Sedan Car, while Hatch Back car model area, velocity at which the car is cruising and the atmospheric was newly designed by modifying the 2-D profile of the same condition in which the car operates [density parameter], it’s only Sedan car model. Both the car models were modeled such that the drag co-efficient times the frontal area of the car that can be they have same frontal area but varying only at the rear portion controlled [1]. So, designing the cars aerodynamically helps in i.e., one is modeled as Sedan car model and other as the Hatch reducing the drag co-efficient of the cars, thereby reducing the Back car model. Geometrical model was created using CATIA V5 drag force. modeling software, while the meshing of computational domain In the recent times, automotive industries have adopted a new trend was carried out in ICEM CFD software using blocking strategy. of converting their most successful sedan car version into hatch ANSYS Fluent was used to solve the flow problems and CFD back version and vice versa, where both the cars have same frontal POST was used as post processing software. area. This gives a sense of relaxation for aerodynamics design engineers, where they are put into at most pressure in producing A. Geometrical Model aesthetic, fuel efficient and economical cars. Conversion of Geometric models were modeled using CATIA V5 R21 modeling Sedan car into hatch back or vice versa has a advantage, where software. For the present analysis only 2-D profile of the automotive companies can invest money only in redesigning the commercially available car was used to create the 3-D geometrical rear part of the car instead of designing a whole new car where the model. Side mirrors, spoilers and other attachments were neglected cost would obviously be more and also the aerodynamic design to make the simulation case simple. engineers need not concentrate on the aesthetics of entire car, as For the present work total of four different types of car models it is enough to concentrate only at the rear part, thereby reducing were modeled. First is the Hatch Back car model and the Sedan www.ijrmet.com International Journal of Research in Mechanical Engineering & 55 IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016 ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) car model was divided into 3 categories of same scale i.e., Sedan B. Meshing without wheels, Sedan with wheels attached to body, Sedan with Meshing of the computation domain was done entirely by using gap between body and wheels, because it was important to know blocking strategy in ICEM CFD meshing software. Since, only what percentage of drag force increases due to the consideration 2d profile of the Sedan and Hatch Back car models were chosen of wheels and gap between wheels and body. So, if the drag force for analysis in the present work, 2d planer mesh was generated variation is less, then the car models without wheels can be used initially in the XY plane capturing the 2d profile of the car models in the future computation analysis in order to reduce unnecessary which was later on converted to 3d mesh just by extruding the meshing and computational time, on the other hand if the variation mesh blocks in the Z direction. is large than either the car model with wheels attached or the car model with gap between body and wheels can be chosen depending 1. Grid Independency Study on the accuracy and computational resources available. In the present work grid independency study was carried for 1:30 scaled Sedan car model without wheels in a 0.3m wide, 0.3m The Sedan car model has the dimensions 0.065m wide, 0.049m high and 1.5m long wind tunnel domain. Initially course mesh high and 0.175m long, while Hatch Back car model has the was generated for the car model with approximately 2.5 million dimensions 0.065m wide, 0.049m high and 0.154m long. nodes, later on the mesh was refined by increasing the node count to 1.5 times the previous mesh. The same problem was made to run i.e. at the same velocity but with different mesh size. In the final mesh, node count was limited to 15.8 million as there were no appreciable changes in drag co-efficient results.

Table 1: Variation of Drag Co-Efficient With Number of Nodes

Fig. 1: Sedan Car Model Without Wheels

Grid independency study for the present work was carried out at 20m/s and a total of six different grid resolutions were used. Case A has the coarsest resolution and case G has the finest grid resolution. Grid refinement was done only at some specific location around the car model where steep pressure gradients were expected. Table 1 shows the variation of drag co-efficient with increase in the node count. It can be clearly seen that there are no noticeable changes Fig. 2: Sedan Car Model With Wheels Attached in drag co-efficient after 9.6 million node count. Hence there was no point in choosing finest grid at the cost of huge computational resources for slight increase in accuracy. So, the mesh with 9.6 million node count was finalized for the present computational analysis.

All the meshes generated were taken care such that the minimum angle of the grid would lie above 20 degree and the determinant would lie above 0.5. The dimensions of the computational domain was based on the actual wind tunnel test section dimensions (0.3m wide, 0.3m high and 1.5m long) and the car models were placed in the computational wind tunnel domain exactly similar to that Fig. 3: Sedan Car Model With Gap Between Body and Wheels was mounted in the wind tunnel test section with blockage ratio of 3% [4], as it was important to simulate actual wind tunnel conditions.

Fig. 4: Hatch Back Car Model (a)

56 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016

(a)

(b) Fig. 5: Sedan Without Wheels: (a) Mesh Domain; (b) Surface Mesh

(b) (a) Fig. 8: Hatch Back: (a) Mesh Domain; (b) Surface Mesh

C. Solving Computational car aerodynamics problem was solved using ANSYS FLUENT as the solver for both Sedan and Hatch Back car models at 4 different velocities (10m/s, 15m/s, 20m/s, 25m/s).

Table 2: Solver Setting

(b) Fig. 6: Sedan With Wheels Attached: (a) Mesh Domain; (b) Table 3: Boundary Conditions Surface Mesh

(a)

(b) Fig. 7: Sedan With Gap Between Body and Wheels: (a) Mesh Domain; (b) Surface Mesh Fig. 9: Convergence Plot for Sedan Car Model www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 57 IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016 ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print)

Initially problem was made to run for 7000 iterations at a particular less where the car geometry is streamlined and is more at other velocity. Since there were no appreciable changes in the residue areas. Due to abrupt change in geometry and creation of adverse values above 2000 iterations, all the other flow problems were pressure gradient, flow separation takes place at the mid portion of made to run for 2000 iterations, thereby reducing computational rear wind shield. Due to this flow separation, formation of eddies time. Fig. 9 shows the convergence plot for Sedan car model at and vortices takes place at the rear wind shield of both Sedan and 20 m/s. Convergence absolute criteria for scaled residues was set Hatch Back car models which can be clearly seen in the Fig. 11 to 10-6. As the scaled residues for all the parameters are less than (b) and Fig .12 (b). Slight flow re-attachment takes place at the 10-6 it can be said that solution is converged. boot region in case of sedan car model. In case of Hatch back car model due to absence of boot region no flow re-attachment takes D. Post Processing place. Final flow separation leaves negative pressure area behind All the post processing work was done by using CFD POST both the car modes. software. Various contour plots such as velocity contours, pressure contours, Stream lines etc were plotted at different velocities for both Sedan and Hatch Back car model. All the computational analysis was carried out in Hp Z800 workstation having the following specification, Processors: 16x Intel Xeon 5560 2.8GHz, Memory: 24GB DDR3 RAM, Chipset: Intel 5560, Graphics: ATI FirePro 5700 Graphics

III. Results and Discussions

(a)

(a)

(b) Fig. 11: Sedan Car Model: (a) Pressure Contour; (b) Streamlines

(b) It is found that Pressure co-efficient plot trend remains same at all Fig. 10: Pressure Distributions: (a) Sedan (b) Hatch Back the velocities for both the car models as shown in the Fig. 13 (a) and (b). So, it can be concluded that pressure co-efficient variation Simulative aerodynamic analysis was performed on Sedan and along the centre line of the car models is independent of velocity Hatch Back car models having same frontal area in an attempt to [5]. Also it can be seen that variation of pressure co-efficient along understand their aerodynamic behavior at different velocities. It the centre line of the car model follows similar trend as that of is evident from the Fig. 10 (a) and Fig. 10 (b) that high pressure the pressure, with pressure co-efficient being maximum at the (stagnation point) acts at the front bumper and this pressure increases front bumper with a value close to 1.0 and minimum on the roof. with increase in velocity for both the car models. At the intersection Almost identical value of CP is obtained at every port on the car of hood and front windshield the car models experiences positive model for different velocity ranges. pressure due to the obstruction caused by the front windshield. These are the only two positive pressure areas over the car models, while rest of the areas of car models experiences negative pressure. Also it can be seen that pressure acting over the car models is

58 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016

(a) (b) Fig. 13: Variation of CP: (a) Sedan (b) Hatch Back

From the above computational comparison it is clear that geometry change only at the rear end can produce a drag co-efficient variation up to 3.5%. This can be concluded in the other way, conversion of Hatch Back car into a Sedan car can improve the drag co-efficient by 3.5%. Even though the drag improvement is quite small in terms of magnitude, but this small improvement can make huge impact on aerodynamic efficiency of cars. In general this improvement in drag co-efficient of the Sedan car helps in getting the mileage almost equal to that of Hatch Back car even though weight of Sedan car is more as compared to Hatch Back cars.

(b) Fig. 12: Hatch Back Car Model: (a) Pressure Contour; (b) Streamlines

It is evident from the fig. 14 that drag force increases with increase in velocity over the entire velocity ranges for both the car models, while drag co-efficient decreases slightly with increase in Reynolds number. It is found that Hatch Back car model has around 3.5 % more drag force and drag co-efficient than the Sedan car model even though both the car models have same frontal area. Hatch Back car model experiences more drag force as compared to Sedan car model due to the fact that air flow over the Hatch Back car model detaches much earlier and the negative pressure area left Fig. 14: Variation of Fd with Velocity behind the Hatch Back car model due to flow separation is more as compared to that of Sedan car model. It is interesting to note that drag co-efficient decreases with increase in velocity because of delay in flow separation as the speed increases in both the car models.

Fig. 15: Variation of CD with Velocity

As it was important to know the variation in drag force and drag co-efficient obtained with and without the consideration of wheels (a) and gap between body and wheels, computational analysis was www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 59 IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016 ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) carried out for different types of Sedan car models i.e. Sedan car and Sedan car model by experimental method”, International without wheels, Sedan car with wheels attached, Sedan car with Journal of Advanced Engineering Research and Studies, Vol. gap between body and wheels. Table 4 shows the variation in 1, Issue 3, June 2012, pp. 181-183. drag force and drag co-efficient obtained for different types of [3] Bhavini Bijlani, Pravin P. Rathod, Arvind S. Sorthiya, Sedan car models at 10m/s. It can be seen that Sedan car model “Experimental Investigation of Aerodynamic Forces on with wheels attached has 8% more drag co-efficient as compared Sedan, and Square-Back Car by simulation in to Sedan car without wheels, while the difference in drag co- CFD” Review Study”, International Journal of Emerging efficient between Sedan car without wheels and Sedan car with Technology and Advanced Engineering, Vol. 3, Issue 2, gap between body and wheels is 15%. So it is clear that there is a February 2013. greater dependency of drag co-efficient on exterior profile of the [4] Sharath Kumar S N, Dr. C. K. Umesh,“Analysis of External car model. But on the other hand it can be seen that computational Aerodynamics of Sedan and Hatch Back Car Models Having time required for Sedan car with gap between wheels and body is Same Frontal Area by Experimental Wind Tunnel Method”, around 240 hrs while its only 36 hrs for Sedan car without wheels International Journal of Engineering Development and for each velocity. So it can be concluded that the selection of type Research (IJEDR), Vol. 3, Issue 4, pp. 812-816, December of car models for analysis depends on the computational resources 2015. available and the accuracy range required. [5] Manan Desai, S. A. Channiwala, H. J. Nagarsheth “A comparative assessment of two experimental methods for Table 4: Drag Variation for different types of Sedan car model aerodynamic performance evaluation of a car”, Journal of Scientific and Industrial Research, Vol. 67, July 2008, pp. 518-522. [6] Yunus A Cengel, John M Cimbala,“Fluid Mechanics Fundamentals and Applications”, McGraw-Hill Publications, 2006. [7] Vehicle Aerodynamics: Hybrid Team, College of Engineering, San Diego State University, [Online] Available: http://kahuna.sdsu.edu/~hev/aerodyn. html (Accessed on 12-01-2015). [8] Sharath Kumar S N,“Analysis of external aerodynamics of Sedan and Hatch back car models by Experimental and Computational Methods”, M. E Thesis submitted IV. Conclusion to Department of Mechanical Engineering, University 1. Computational analysis of Sedan and Hatch Back car models Visvesvaraya College of Engineering, Bangalore, India, suggest that Sedan car model has less drag co-efficient and 2015. drag force over the entire velocity range as compared to that [9] Dinesh Dhande, Manoj Bauskar,“Analysis of Aerodynamic of Hatch Back car model so it can be concluded that Sedan Aspects of SUV by Analytical and Experimental Method”, car model is more streamlined and aerodynamically efficient International Journal of Emerging Technology and Advanced compared to Hatch Back car model. Engineering, Vol. 3, Issue 7, July 2013. 2. For both Sedan and Hatch Back car models drag force [10] R. B. Sharma, Ram Bansal,“CFD Simulation for Flow increases with increase in velocity, while drag co-efficient over Passenger Car Using Tail Plates for Aerodynamic decreases slightly with increase in Reynolds number. Drag Reduction”, IOSR Journal of Mechanical and Civil 3. Computational analysis suggests that, as the speed increases Engineering, Vol. 7, Issue 5, August 2013, pp. 28-35. the difference in drag force between Sedan and Hatch Back [11] Pramod Nari Krishnani,“CFD study of drag reduction of car models also increases. a generic sport utility vehicle”, M. S Thesis submitted to 4. Computational analysis carried out on Sedan without wheels, Department of Mechanical Engineering California State Sedan with wheels attached to body and Sedan with wheel and University, Sacramento, 2009. gap between the body shows that there is greater dependency [12] Tank Nilesh R, R. Thundil Karuppa Raj,“Numerical on the exterior profile of the car models. Hence it suggested Simulation of Aerodynamic forces acting on Passenger considering the entire profile of the car for future aerodynamic Vehicle While Overtaking”, Research Journal of Recent analysis based on the computational and experimental Sciences” Vol. 1, December 2012. resources available. [13] Sasitharan Ambicapathy, J. Vignesh, P. Sivaraj, Godfrey Derek Sams, K. Sabarinath, V. R. Sanal Kumar,“3D Numerical V. Nomenclature Studies on External Aerodynamics of a Flying Car”,

CP Pressure Co-Efficient World Academy of Science, Engineering and Technology,

Fd Drag Force, N International Journal of Mechanical, Aerospace, Industrial

CD Drag Co-Efficient and Mechatronics Engineering, Vol. 8, Issue 5, 2014. [14] Manikandan. M, Shiva Prasad U, Ashish Ashok Suvarna, References Anuj Bhat B, Vinayak Nair, Gunda Shivakrishna,“External [1] William H. Bettes,“The Aerodynamic Drag of Road Vehicles: Aerodynamic Analysis of TATA Nano using Numerical Past, Present, and Future”, Engineering & Science, January Tool”, International Journal of Current Engineering and 1982. Technology, Issue 2, February 2014, pp. 393-396. [2] Bhagirath Zala, Dr. Pravin P. Rathod, Sorathiya Arvind S, H. [15] Deepak Kumar Kalyan, A.R. Paul,“Computational Study of I. Joshi,“Comparative assessment of drag force of Hatch Back Flow around a Simplified 2D Ahmed Body”, International

60 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print) IJRMET Vo l . 6, Is s u e 1, No v 2015-Ap ri l 2016

Journal of Engineering Science and Innovative Technology, Vol. 2, Issue 3, May 2013. [16] Rehan Salahuddin Khan, Sudhakar Umale,“CFD Aerodynamic Analysis of Ahmed Body”, International Journal of Engineering Trends and Technology, Vol. 18, No. 7, 2014.

Sharath Kumar S N received his B.E. degree in Mechanical Engineering from Visvesvaraya Technological University, Belgaum, India in the Year 2013. He completed his Post graduation in University Visvesvaraya College of Engineering, Bangalore University, Bangalore, India with specialization in Thermal Science and Engineering in the year 2015. Currently he is working as Assistant professor in Department of Mechanical Engineering, Dr. Ambedkar Institute of Technology, Bangalore, India. His research interests include Vehicle Aerodynamics, Turbine Blade cooling and Heat exchangers.

Dr. C. K. Umesh is currently working as Professor in the Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore University, Bangalore, India, with more than 22yrs teaching experience. He has published 6 national journals and 9 international journals. He has guided more than 20 PG students and 5 Doctorate students.

www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 61