Effect of Relative Wind on Notch Back Car with Add-On Parts

Transcription

1 Effect of Relative Wind on Notch Back Car with Add-On Parts DEBOJYOTI MITRA * Associate Professor & Head Department of Mechanical Engineering Sir Padampat Singhania University Udaipur , Rajasthan. Phone No: debojyoti.mitra@spsu.ac.in, dmitra2k@yahoo.com Abstract The behavior of different add-on parts to a basic car model has been studied experimentally.experiments were conducted with different add-on parts, like rear end spoiler & front end spoiler in order to find out how these add-on parts influence the drag and lift coefficients of a basic car model. The experiment was done in Jadavpur University low turbulence subsonic closed circuit wind tunnel at different Reynolds number.the results from the experiment indicated that the addition of different add-on parts like front end spoiler and rear end spoiler to a basic car model reduces the lift coefficient to a considerable amount while the drag coefficient is reduced by a small amount. It was concluded that the addition of these add-on parts like rear-end spoiler etc increases the aerodynamic stability of a basic car model and hence they can be used in real life which will be an added advantage. Keywords Notchback, Front Spoiler, Rear-End Spoiler, Drag coefficient, Lift Coefficient. Introduction The constant need for better fuel economy, greater vehicle performance, reduction in wind noise level and improved road holding and stability for a vehicle on the move, has promoted vehicle manufacturers to investigate the nature of air resistance or drag for different body shapes under various operating conditions, thus giving birth to a new branch of research, namely Surface Vehicle Aerodynamics. Surface vehicle aerodynamic research is mostly concentrated upon drag reduction (Sovran, 1983). Drag, in surface vehicle aerodynamics, is the measure of the aerodynamic force, which resists the forward motion of the vehicle (Bernard, 1996). A low drag coefficient implies that the vehicle body can move easily through the surrounding viscous air with minimum air resistance, whereas a high negative lift coefficient indicates more stability and less chances of skidding. Researchers throughout the world are carrying out extensive research works to lower the drag coefficient and increase the negative lift coefficient by properly designing the shape of the vehicles. The aerodynamics of surface vehicles and their design based on experiments are becoming popular day-by-day keeping the fuel economy and the aerodynamic balance of the vehicle in mind. Palowski may be considered the pioneer in this field having explored the wind resistance on automobiles way back in Carr (1968) has commendable contributions in the field of aerodynamics of road vehicles and its dependency on vehicle shapes. Reduction in drag implies less fuel consumption: it has been rightly pointed out by Sovran et al. (1983). Bernard (1996), Hucho (1998), Heinz (2002) and Julian (2002) are a few more who have dedicated work on this field in the recent past. The flow field of a car is the result of its shape, its driving speed and the speed and the direction of the ambient wind. The factor that is discussed here is, how different add-on parts to a car model can influence the drag and lift of the car. The first area of concern deals with the forces and moments imposed on a car by its flow field. In many cases e.g. for almost all passenger cars, drag and the lift is relevant to the aerodynamic quality of the car. A compromise has to be found between the low drag and high negative lift. The optimum balance of these forces is not only dependent on the specific car but also depends on the parts ISSN:

2 that are added to it. For different types of vehicles with varying shapes different types of add-on parts must be employed to make the vehicle more aerodynamic stable. Selection of car model A notchback type of car model, having a stepped rear end body profile in which the passenger compartment rear window is inclined downward to meet the horizontal rearward extending boot(trunk) lid, is used as a standard car model. With this design the air flows over the rear roof edge and follows the contour of the downward sloping rear screen for a short distance before separating from it ; however, the downwash of the airstreams causes it to re-attach itself to the body near the rear end extended boot lid. Thus the base wake area will virtually be the vertical rear boot and light panel; however, standing vortices will be generated on each side of the body just in board on the top surface of the boot lid and will then be projected in the form of trailing conical vortices well beyond the rear end of the boot. Vortices will also be created along transverse rear screen to boot lid junction and across the rear of the panel light. In this experiment the notchback type of car model is made of plywood. The car model is 138 mm high, 310 mm long, 130 mm wide. Pressure tappings are provided along the surface of the body with inside protrusions for connecting flexible pipes leading to the SENSYM make pressure sensor through a small opening provided at the back face of the car model. Experimental Procedure The experiment was carried out in the lower test section of Jadavpur University Low Turbulence Subsonic Closed Circuit Wind Tunnel at different Reynolds numbers. The notchback car model is placed in the test section with the front facing the flow. At first the basic car model without any added parts is placed in the wind tunnel and the experiment is done. Surface pressure readings were taken from the pressure tappings. From the readings the drag and the lift coefficients were determined for the basic car model. Then a rear end spoiler is added to the basic car model (Fig.1) and the experiment is carried out in the similar fashion. The drag coefficients and the lift coefficients are obtained for this configuration. Then the rear end spoiler is removed and a front spoiler or a air dam is attached to the vehicle (Fig.2). The experiment is performed with this arrangement and the readings are obtained. Drag and lift coefficients are determined for this type of arrangement. Then with both the front spoiler and the rear end spoiler attached to the car the experiment is carried on and the pressure readings were taken. The drag and lift coefficients are determined for this type of arrangement. The readings that are obtained from the experiments are compared with each other to show the effect of different add-on parts to a basic car. Fig. 1 Three Dimensional View of Notch Back Car with Rear End Spoiler ISSN:

3 Fig. 2 Three Dimensional View of Notch Back Car with Front End Spoiler Results and Discussions Fig.3 & 4 show a detailed variation of Drag Coefficient and Lift Coefficient for a Notchback car model with and without add-on parts. They indicate that the drag coefficient almost remains unaffected with the addition of rear-end spoiler. While the addition of front spoiler increases the drag coefficient, the addition of both the rear end and front spoilers also increases the drag coefficient significantly. However, it is seen that the addition of rear end spoiler considerably reduces the lift coefficient. Front spoilers also reduces the lift coefficient but not as that of rear end spoilers. The addition of both the spoilers brings down the lift coefficient to a large extent. The addition of rear end spoiler increases the pressure in the rear part of the vehicle, while the addition of front spoiler increases the pressure on the upper surface of the vehicle. As there is a air dam in front of the vehicle a low pressure region is created underside, and with the increased pressure on the upper surface of the vehicle, gives rise to a increment in negative lift. ISSN:

4 Drag Coefficient Simple Car Rear Spoiler Front Spoiler Both rear &front Spoiler x E x E x E5 1.4 x E5 Reynolds Number Fig 3. Variation of Drag coefficient with Reynolds Number with Different Add-On Parts to a Notchback Car Lift Coefficient Simple Car Rear Spoiler Front Spoiler Both rear &front Spoiler.822 x E x E x E5 1.4 x E5 Reynolds Number Fig 4. Variation of Lift Coefficient with Reynolds Number with Different Add-On Parts to a Notchback Car Conclusions The present work indicates that modifications of a basic car model by addition of different add-on parts can produce significant changes in the drag and lift coefficients. From the experiments it is seen that the addition of rear-end spoilers to a basic car model considerably reduces the lift coefficient whereas the drag coefficient almost remains the same. By addition of front spoiler or air dam, there is a little reduction in the lift coefficient, whereby the drag coefficient increases a bit. The addition of both the parts reduces the positive lift than that of a basic car whereas the drag coefficient increases by a small amount. So it can be concluded that the addition of these add-on parts like rear-end spoiler, air dam etc increases the aerodynamic stability of a basic car model and hence they can be used in real life which will be an added advantage. That ISSN:

5 is why racing cars have inverted wings to increase the road adhesion which is a modified form of rear-end spoiler. References [1] Palowski, F.W. (1930); Wind resistance of automobiles ; SAE Journal, Vol. 27. [2] Barnard, R H 1996, Road Vehicle Aerodynamic Design, Longman, ISBN [3] Heisler Heinz, 2 nd Edition 2002, Advanced vehicle Technology, pp [4] Happian-Smith Julian, 2002, An introduction tomodern Vehicle Design, pp [5] Carr, G.W. (1968). The aerodynamics of basic shapes for road vehicles,part2,saloon car bodies, MIRA,report no. 1968/9 [6] Hucho,W.H, (1998). Aerodynamics of Road Vehicles:from Fkuid Mechanics to Vehicle Engineering,4 th edition, S.A.E., ISBN [7] Sovran, G. (1983); Tractive-energy-based formulae for the impact of aerodynamics on fuel economy Over the EPA Driving Schedules, SAE Paper No ISSN: