Human beings differ according to their passion, desire, and aim therefore the tools that we have used for centuries diverge as to satisfy our demands. This divergence can be seen in every field of daily life. Some of those tools are ushering by trends that I guess you have had lots of them so far. The trend is an illicit drug and we are behind the bar to reach at least a piece of them with our average budgets. Some call it economic harassment. Anyway, having a thorough interest on cars since my infancy, I, though, can say that that bulky plastic and metal tools are one of my illicit drugs, especially coupe and convertible ones. Fluids dynamics is my dearly curiosity that develops my interest making me a meddlesome engineer. Throughout this article, I’m going to investigate the aerodynamics of a convertible can and compare it to the coupe one. Despite the fact that convertibles are summer stuff, a guest from Germany (see figure 1) has broken all my bounds that the driver was trying to go on and on amid the snowy winter conditions. It is bizarre that, of course, windows were fully closed due to winter. It would have been a great bliss that air conditioner was on at 27 degree Celsius. Figure 1 The first question that come to my mind was the velocity at which windshield may negate the effect of snow. What would the conditions both driver and passengers encounter? “The Slow Mo Guys”, a YouTube channel, has examined the effect of rain experimentally. In the video below, they use a convertible BMW 2 series: I cannot interpret anything from the video above since they didn't give any result or comparison among their experimental campaign. Despite, it is funny to observe the physical incident with a slow motion camera where it is hard to find a convenient one for an enthusiast. Another experimental examination regarding convertible car aerodynamics belongs to GraysGrage. I recommend you to follow the channel that Gray shares excellent videos regarding ground vehicles and aerodynamics. He has been carrying out wind tunnel tests as to determine aerodynamics of cars and try to testify effect of several kinds of stuff on aerodynamics such as spoilers at bonnet, roof, and rear. Graysgarage compares convertibles and auxiliary structures such as wind deflector regarding location and size. You may have seen a wind deflector as in the picture below if you encounter someone who has a convertible and tries to get rid of back-flow (?). Figure 2: The wind deflector which is located over back seats It is time to have a deep dive into theory. Technically speaking, the separation throughout the boundary layer results in chaotic flow which is defined as Turbulence, the flow condition hardly ever foreseen. For convertible cars, airflow after windshield cannot stand any surfaces to keep on movement through trailing edge without flow in the reverse direction. The back-flow, flow in the reverse direction, is occurred some distance later windshield and directly influences both passenger and driver, although headrests are available. Hence, the application of a wind deflector provides pleasure driving which reduces consequences of aerodynamics instabilities. Moreover, the difference between the aerodynamics of cars either with wind deflectors or not can be seen in figure 3 as follows: Figure 3: Experimental examination of wind deflector The shape of cars is an exploratory issue that is indeed associated with fuel consumption, acoustic, and stability. The drag coefficient is one of the main factors which defines the degree of frictional force as to specify relation between flow and solid structure. According to Wolf-Heinrich Hucho (2013), the drag coefficient through optimal body to road vehicle can be seen as in graph as below (figure 4). The difference between saloon-, coupe-, and convertible-type cars is also specified as below (figure 5):
Aside from literature, most of them based on studies of enthusiastic, I have conducted some CFD studies to examine difference between coupe and convertible. Besides, I have also carried out studies to further investigate a presence of rear spoiler. For this purpose, I have gotten 2D model of BMW 4 series and generated 4 models with and without rear spoiler. Figure 6: BMW 4 series with rear spoiler and BMW 4 series convertible both open and close The parameters and definitions on numerical study can be concluded as follows:
Models:
As expected, the minimum turbulent kinetic energy among model belongs to model 1 which has rear spoiler. The foil of the spoiler is NACA airfoil 0012 which is selected arbitrary. Figure 7: CFD comparison Results: According to CFD results which can be seen as in Figure 7 and video below, a huge ascent in the turbulent kinetic energy is generated at car with open-roof. Both of open- and close-roof cases, application of spoiler reduces the magnitude of the turbulent kinetic energy. It is obvious that the passenger at the back seats would be effected by turbulence at the velocity of 80 km/h which also causes a poor aerodynamics that results in increasing of fuel consumption and noise. Therefore, application of wind deflector would really reduce the effect of turbulence. Yet rear spoiler has a positive effect to negate the presence of turbulence, it is insufficient to prevent boundary layer separation. Thus, it would have been an affirmative solution to using a rectangular plate over the upper-side of windshield (see the bonus below for details). Here is detailed video with velocity contours: Bonus: Have a further idea, please comment below!
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