Wheels aerodynamics - wheel ventilation drag
Jaroslav Kme?
Business Leader, Director General, ex-Government CIO, ex-Deputy Minister of Transport, Telco and Post | Ex HP, CSC, DXC, Oracle | 20+ years of experience in Public sector, IT, Telco and Banking | LION >26k connections.
Reducing fuel consumption and thus CO2 emissions is a very topical issue. There have been a number of studies of the aerodynamic performance of this area, which have shown that wheels and wheel-housing flows generate a significant part of the aerodynamic drag on a passenger car and can relate to as much as 25% of it. In this analysis we will focus on passenger vehicle aerodynamics and more specifically on the area of the wheels and wheelhouses. There will be also shown the relative importance of rim design in having better aerodynamic characteristics.
Michal Sura [email protected]
Figure 1.
From experiments in both research (Cogotti 1983) and development (Mercker et al 1991) it can be concluded that wheels and wheel wells contribute approximately half the drag of a low-drag car (1). Wheels and wheelhouses account for approximately 25% of passenger car aerodynamic drag (Figure1) (2). Because the underbody and wheelhouses are responsible for a larger part of the aerodynamic drag force, reducing the aerodynamic drag of these parts would reduce consumption of propulsion energy and thus extend the range of vehicles.
Ventilation drag coefficient
There is possible to see in Figure 2 moment Mvent that acting on the rotating wheel. The moment Mvent is called ventilation moment and it occurs due to the wheel rotation in the airflow. Ftract is traction force that is responsible for overcoming ventilation resistance tracking force Fvtract, which acts against the traction force Ftract exerted by the propulsion systems. It is possible to replace ventilation resistance moment Mvent with equivalent resistance force Fvent counteracting vehicle movement.
Figure 2.
Ventilation resistance moment Mvent is considered to be a result of three effects acting simultaneously:
Fvent = Fvtract
The ventilation resistance force Fvtrac can be expressed as (3):
Fvtrac = ? CD(vent) ρ V∞2 A
Where Fvtrac is the ventilation resistance force that correspondent to ventilation resistance force Fvent , CD(vent) is ventilation resistance coefficient, analogous to aerodynamic resistance drag coefficient, ρ is air density, V∞? is a free stream velocity and A is reference area.
CD(vent) ventilation resistance coefficient can be expressed as:
CD(vent)? = Fvtrac / ? ρ V∞2 A
Tested rims.
In order to measure ventilation resistance in the wind tunnel there were used wheels with different rim designs (4). Rims were tested in Volvo aerodynamic wind tunnel. Each of the wheels was a five spoke, 17′′ aluminum rim (Figure 3).
Figure 3.
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Ventilation resistance results
In Figure 4 one can see ventilation resistance versus vehicle speed for the 6 selected configurations. CD(vent) ?is presented as a percentage difference from the reference design: the High drag profile rim design, Figure 3 (k). The configurations compared are Fan blade out, Thick outer radius cover, Fully covered rim, Base spokes and Star cover, see Figure 3 (e, f, b, i, l) respectively.?
Figure 4.
Figure 5
The best design in terms of ventilation resistance moment was the thick outer radius cover (f) (Figure 5), since it had the lowest CD(vent)? throughout almost the entire velocity range.?
Figure 6
The second best configuration, fan blade out (e) (Figure 6), had aerodynamically shaped wheel spokes designed to pump the air out from the wheelhouse, thereby reducing pressure inside. The smooth corners on the leading edge of the spokes may have also contributed to lower ventilation resistance moment.?
Figure 7
High drag profile (k) (Figure 7) was proved to be the worst design.
Figure 8
The surprise was that fully covered rims (b) (Figure 8) that are known for very low aerodynamic drag force, but in terms of ventilation resistance, they performed slightly below average. It looks like fully covered rims prevent the air getting out from the wheelhouse and it causes increased surface friction inside of the wheelhouse.
References:
#OpenForConsultancy
2 年Thanks for sharing, very interesting post. Looking forwsrd for deeper ones.
Turbomachinery | Waste Heat Recovery | Heat Pumps | ORC | Energy storage | Fuel cells | Cryogenics
3 年Would be good to look at covered wheels (wheel spats). Even 2CV had them ??
IT technology analytik
3 年Next nice analysis ????
Global Roaming Manager
3 年When I do longer trips I tend to increase the tire pressure. Thank you on the insights on Figure 8. I saw people in the RZOC (Renault Zoe Owners Club) searching for that and it is interesting it works against the intended goal.
CEO at AirShaper, aerodynamics expert
3 年Interesting article Jaroslav Kme? We've helped optimize various wheel designs and indeed, closing the outer diameter was by far the most efficient solution. But very often it's a compromise between aesthetics, cooling, safety and cost. So if you still need individual spokes (and openings for cooling), then using large radii in the right places to avoid local separation is very important to lower the ventilation drag. Still, it's important to note that in the end it's the total vehicle drag that matters: a higher ventilation drag may lower the drag elsewhere on the car.