Fig -3 Multitube manometer setup. Fig -1 Experimental setup

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1 Research Paper COMPARATIVE ASSESSMENT OF DRAG FORCE OF SEDAN CAR MODEL BY COMPUTATIONAL FLUID DYNAMICS AND EXPERIMENTAL METHOD *Bhagirathsinh H Zala, Harilal S Sorathia, Dinesh L. Suthar, Vashant K. Pipalia Address for Correspondence Mechanical Engineering Department, Government Polytechnic College, Bhuj , Gujarat ABSTRACT This paper is Research about the aerodynamic of sedan car model by comparing experimentally and subsequent validation by computational fluid dynamics (CFD) The experimental investigations was performed on an open circuit suction type wind tunnel having a 30 cm x 30 cm x 100 cm test section, and maximum speed of 33 m/s on a geometrically similar, reduced scale (1:0) Aluminium car models.while the three dimensional computational analysis was carried out using with the help of software tools like ANSYS-CFX to simulate the flow of air around the automobiles ANSYS-CFX, CFD code were used to run the simulation. Computer desktop with 3. GHz intel core duo with ram of.00gb is used to create and simulate the flow. The main objective of the study is to predict the drag coefficient experimentally as well as computationally. Several factors that influence the drag coefficient such as flow separation, vortex and the effect of pressure coefficient have been studied. Detail velocity profile and pressure distribution plots around the car envelopes have been presented. KEYWORDS - Aerodynamics, drag force, wind tunnels, hatchback & sedan. I INTRODUCTION Airflow over a vehicle determines the drag forces, which in turn affects the car s performance and efficiency testing equipment has been designed to measure both the vertical and horizontal components of air resistance on a model car. Down force, the vertical component is simply negative lift. When a car goes faster and faster the weight of the car changes due to air resistance. Depending on the direction (upward or downward) this is called either lift or down force, respectively. The horizontal component, the drag of the model cars will be measured. Drag is the amount of force that the air is pushing on the car at a certain speed. In other words, it is the amount of resistance the air imposes on the car in a horizontal direction. For a vehicle to maintain its speed it must overcome friction. The most important source of friction at high speeds is air resistance. If air resistance can be minimized, the car will be more efficient because less energy is lost to friction. Today, with increasing energy costs, efficiency is finally receiving the attention it deserves. If a car can cut through the air with little resistance, it will get better gas mileage. The automotive industry is accordingly devoting more attention and resources to the aerodynamics of their vehicles. However, comprehensible knowledge of lift and drag characteristics for automotive, is of great interest for future designs of the racer & car designer. The latter has been the general motivation for this investigation. A more elaborate description of the two approaches is given in the following. II Wind Tunnel Experiments line of pressure tapings are to be made on the model structure. The test model was reduced to a generic shape by neglecting wheels, wings, etc to isolate the performance from any other aerodynamic influences. Measurement data uncertainty is evaluated before presenting the final results. The results of the Experiment will serve as a method of validation of the CFD analysis. III Experimental set-up and test section Experimentally, tests will be carried out in an open circuit suction type wind tunnel (30cmx 30cm x 100 cm) with a glass window meant for visual observation of flow phenomenon. A variable speed DC motor employed varies air velocities (5-30 m/s). A provision for traversing Pitot tube in horizontal direction was created specially to meet with specific requirement of suggested experimentation methodology. Experimental investigations in wind tunnel Tests will be conducted on a geometrically similar, reduced scale (1:0) aluminium model, differing from actual car only in size and simulating dynamically similar flow situations as it would be enormously expensive to engineer and build a full scale car for wind tunnel testing just to find out if the design is acceptable. In this approach, the model instrumented with pressure tapings along the centerline, over its profile, will be tested at different air velocities. The Static pressure distribution so obtained will be represented in terms of a non dimensional parameter pressure co efficient (Cp) The main part of this experiment is to obtain pressure distribution on the center line of car model Fig -1 Experimental setup Fig -3 Multitube manometer setup Fig - Test Section Lift and drag coefficients are to be determined experimentally on a test model at 33 m/s in a wind tunnel. In order to measure the surface pressure variation in a longitudinal section of the geometry, a Fig -4 car model with pressure tapping III Experimental procedure The experimental procedure is as follows a. Pitot-tube is inserted in side test section. and leveled with spirit level. b. Wind tunnel is Started within range of speed (5 m/s-30 m/s) it is run at particular speed for 5-10

2 minutes to get uniform flow and the wind tunnel test section speed is adjusted and then after data is acquired c. Air speed is calculated from the following equation g water H pitotstatictube u air d. The manometer readings are recorded and Cp is calculated from the following equation p-patm C p = ρ g w a te r H m u ltitu b e m a n o m e te r 1 ρ U 1 ρ v a ir Table 1- Cp of hatchback and sedan car model Speed of car Cp of Cp of sedan car Km/h hatchback car e) graph of Pressure variation along the centerline at a particular air velocity is drawn MODEL Sedan car SPEED (KMPH) DRAG FORCE (N) Fig 8 Graph of variation of forces with speed V Estimation of Drag Force By Computational Fluid Dynamics Fig-5 Pressure variation along the centerline at a particular air velocity(sedan car) Fig 9 The control volume with car Fig 6 - Pressure variation along the centerline at a particular air velocity (hatchback car) Fig -7 Pressure distribution over car profile f) drag force was calculated by using the following equation D ragforce F P C O S A 1 d i i i 1 Where θ is angle between direction of relative velocity and normal pressure force IV Results and discussion Drag force of both sedan car model was found out using above procedure and was tabled and graph was prepared as below Fig 10 Generating of the mesh Fig 11 boundary condition

3 Fig 14 outlet boundary condition Freewalls Boundary type : Wall Location : free walls Boundary details : free slip Fig 1 Domain definition Imparting the boundary condition Inlet It is applied at inlet region with following parameters. Boundary type : Inlet Location : inlet Boundary details Flow regime : Subsonic Mass flow rate : kg/sec Flow direction : normal to boundary condition Turbulence : Default Mass flow rate has been given to apply the velocity for air as inlet The calculation is as below Velocity of air for the case of flow of air at 60 kmph = m/s Cross-section of control volume = = m =47 Mass flow rate = cross-sectional area Velocity = = m3/sec = (density of air) = kg/ sec Fig 15 free wall boundary condition Defining the solver control criteria Advection scheme: High resolution Depends upon the condition of the solver that whether it is converging or not. In some case it has been increased to achieve the residual target and also in some case the solver was terminated before maximum iteration since the target value was achieved. Length scale option: conservative (this is the most stringent criterion) Fig 18 control criteria Writing solver file Last step in the cfx-pre is to write the solver file with *.def and quit the cfx-pre and run the cfxsolver. Monitor the run in Ansys cfx- solver. Monitor the run by checking the target residual values and max no. of iteration have been reached or not. If target value is not getting then increase the iteration and check the convergence. In all case the target value is achieved Fig 13 inlet boundary condition Outlet Boundary type : outlet Location : outlet Boundary details Flow regime : subsonic Mass flow rate : kg /sec Fig 19 Writing solver file

4 Observation of the Result In Ansys Cfx Post After successful completion of run the *.res file will be generated by the solver. Load the result file in cfx-post and observe the result like velocity contours, pressure contours, streamlines, drag and lift forces etc. Checking for mesh independent result To check whether the simulation done with current mesh parameter like element size, edge length, mesh independent analysis has been performed. Observation table SOLUTION NO. OF ELEMENT VALUE OF DRAG FORCE VALUE OF LIFT FORCE PERCENT AGE CHANGE FROM 1 ST SOLUTIO N N N N 3.6 % in drag and 4 % in lift Remarks As shown in the table as mesh is refined i.e. no. of element is increased there is change in the result which is less than 4%. In the nd solution the no. of element is more than double of the 1st solution and solution time taken by available CPU was around 4 days of uninterrupted running of the CPU. As variation in result is very less and time CPU capabilities are limited so the mesh parameters of 1st solution is taken for further simulations. COMPUTATIONAL METHOD & RESULT DISCUSSION 6.1 Introduction Comparative assessment of two popular sedan and hatchback model of car with scale of 1:0 have been simulated in CFD software ansys-cfx for air flow around the car and results like drag force, lift force, pressure and velocity distribution, etc have been investigated for aerodynamic analysis. For this as per methodology illustrated in previous chapter both models are simulated for speed 60 kmph,75 kmph,90 kmph,105 kmph. The other Fig 1 MODEL: sedan with speed 75 kmph Fig MODEL: sedan with speed 90 kmph Fig 3 MODEL: sedan with speed 105 kmph SPEED (KMPH) AERODYNAMIC FORCES (NEWTON) DRAG conditions like boundary conditions, domains etc are same as mentioned in the chapter of methodology. Mass flow rate will be changing as change in the speed of the car and change in the cross section means model of the car. Fig 0 MODEL: sedan with speed 60 kmph Fig 4 drag force of sedan car by computational method computational result discussion Both the forces are increases as the speed increases. For high speed there is more increase in the drag and lift force that implies that aerodynamic forces plays critical role at high speeds. Fuel consumption will be high at high speeds as more drag forces is there at high speeds. sedan car shows the same trend of increasing drag with the speed

5 Comparative assessment Results and discussion Fig 5 drag force of sedan car experimental v/s computational 6. Masaru koike, Tsunehisa nagayoshi, Naoki hamamoto research on aerodynamic drag reduction by vortex generators mitshibishi motors technical paper review 004 no-16 pp Philippe Planquart Integration of CFD and Experimental Results at VKI in Low-Speed 3rd International Symposium on Integrating CFD and Experiments in Aerodynamics 0-1 June 007 U.S. Air Force Academy, CO, USA 8. Pravin Peddiraju, Arthur Papad opoulous, Rajneesh Singh CAE framework for aerodynamic design development of automotive vehicle 3 rd ANSA & μeta International Conference, Halkidiki Greece September 9-11, Perzon S. and Davidson L. On Transient Modeling of the Flow Around Vehicles Using the Reynolds Equations, In ACFD 000 Beijing, Oct , pp 70-77, 10. François D.G, Delnero J.S.Colman J Experimental determination of stationary Aerodynamics loads on a double deck bus 11 th America conference on wind energy -san jaun, Puerto Rico June Patrick Hong Bogdan Marcu Drag Forces Experienced by Two, Full-Scale Vehicles at Close Spacing University of Southern California, California path Research Report February 1998 ISSN p Prof A. S sorathia M.E Thesis flow simulation and investigation of aerodynamic forces of car by ANSYS-CFX, Gujarat university 009 Fig 6 drag coefficient computational v/s experimental V Observation and conclusion Fig 5 & Fig 6 shows correlation between experimental obtained value and computational obtained value. drag force of sedan car obtained computationally has higher value than obtained experimentally by 11%. Drag coefficient decreased initially then with further increase in Reynolds number C D attended almost constant value. Distribution of Pressure obtained experimentally is in agreement with prediction that distribution of The pressure remains low down over nose, the back light and on to the trunk because of the continuing curvature or the streamline shape. The front radiator zone displayed higher pressure contours. The pressure contours obtained by CFD at different velocity are identical with those obtained experimentally and thus validate the experimental procedure and result. REFERENCES 1. Manan Desai, S. Achaniwala, H.J agarserth [1] Experimental and Computational Aerodynamic Investigations of a Car ISSN: Issue 4, Volume 3, October 008 p Philippe Planquart integration of CFD and Experimental Results at VKI in Low- Speed Aerodynamic Design 3rd International Symposium on Integrating CFD and Experiments in Aerodynamics 0-1 June Chainani. A, Perera. N CFD Investigation of Airflow on a Model Radio Control Race Car Proceedings of the World Congress on Engineering 008 ISBN: Vol II WCE 008, July - 4, Daniel favier The Role of Wind Tunnel Experiments in CFD validation Encyclopedia of Aerospace Engineering, ISBN: ,page Prof. P.R. Sonawane, prof. S.P. Sekhawat Prof.K.K.Rajput aerodynamic analysis of car body for minimum fuel consumption journal of information knowledge and research in mechanical engineering ISSN X NOV 10 TO OCT 11 VOLUME 01, ISSUE 0.p.54-57