4.2 Governing mechanisms
4.2.3 Wake effect
A final area of the wheel on which the wing flow has a dominant effect is the wheel wake. To improve the understanding of the wake physics it is elucidating to divide the wake into two zones: the upper and the lower wake. The upper wake reaches from the top of the wheel down toθ≈190◦, depending on the spanwise position. The upper wake primarily
consists of the arch shaped vortex at the top of the wheel, which has also been found for the isolated wheel. CFD simulations have revealed that this feature exists for lower ride heights2. The lower wake covers the lower region of the wake and extends downwards
to the downstream side of the contact patch. CFD showed again that the lower wake is similar to that for the isolated wheel case at the highest ride heights3, consisting of the two
counterrotating longitudinal vortices near to the ground (see feature ‘H’ in figure 4.11). At
2The arch shaped vortex, see feature ‘E’ in figure 4.11 (b), remains relatively unchanged up toh
EP t/D≈
0.5 and still characterizes the upper wake up till the sudden change in wheel drag athEP t/D= 0.67.
3At the highest ride height of h/c= 0.634, when the lower edge vortex is well above the axis of the
wheel, the two longitudinal vortices are as distinct as for the isolated case, however towards the sudden change in wheel drag the vortex at the inside of the wheel starts weakening.
Effect of wing presence on wheel aerodynamics
the lower ride heights the inside longitudinal vortex disappears, resulting in an asymmetric wake, while its void is filled by the lower edge wing vortex. The high-vorticity ‘bow-wave’ zone (feature ‘D’ in figure 4.11) originating from the upstream side of the contact patch disappears as well for the inside of the wheel at lower ride heights.
Upper wake The features of the upper wake partly overlap with those described for the top of the wheel in the section on delayed separation. Figure 4.3 shows that the upper wake pressures at the centreline are characterized by the recovery from the second local minimum that is caused by the unsteady pressure fluctuations. At the lower boundary the pressures are very similar for each of the ride heights and reach the base pressure experienced in the lower wake. However at the upper boundary the pressures very much depend on the suction over the top of the wheel and therefore ride heights abovehEP t/D= 0.67 imply
more suction over the upper wake wheel surface. The pressure distributions on the sides of the tyre tread, P2 and P4, show a comparable decay in suction with reducingθ, although the pressures do not reach the same value at the lower boundary. Especially for the inside of the wheel it can be concluded that the higher ride heights, above the sudden wheel drag change, lead to more suction at the lower boundary of the upper wake as well.
The flow mechanism that causes this difference in behaviour between the lower and higher ride heights is again primarily the change in trajectory of the upper edge wing vortex. From CFD simulations for various ride heights it could be concluded that the arch shaped vortex in the upper wake (feature ‘E’ in figure 4.11) is replaced by attached flow near the centreline of the wheel at higher ride heights, while the legs (feature ‘F’ in figure 4.11) of this vortex change into two regions with strong vorticity, where the flow along the side spills over the edge of the wheel into the wake. These regions of high vorticity are the reason for the higher suction at the sides of the tyre tread near the lower boundary of the upper wake for higher ride heights. The region on the inside of the wheel (location P4) displays more suction than on the outside, because it is energized by the upper edge wing vortex (feature ‘A’), which passes over the wheel for the higher ride heights. A complex interaction between this region of high vorticity (F), the upper edge wing vortex (A) and the lower edge wing vortex (C), which passes on the inside of the wheel, can occur for certain settings. It is expected that this interaction and the resulting higher suction on the wheel surface in the upper wake is partly responsible for generating the highest wheel drag ath/c= 0.458.
Effect of wing presence on wheel aerodynamics
the wheel as well, the upper wheel wake is similar to that for the isolated wheel. The arch shaped vortex is the most dominant feature of the upper wake under these conditions. Only small increases in suction on the wheel surface are noticeable for increasing ride height and these are directly related to the suction over the top of the wheel. The upper wake effect causes more wheel drag and lift for the higher ride heights, while the side force is again mostly influenced by the location of the upper edge wing vortex.
Lower wake The pressure distributions for the lower wake do not present a very clear consistent trend with ride height change. For the 45◦-segment closest to the ground it seems that the suction, in general, reduces with increasing ride height. In the remaining part of the lower wake this trend can be noticed up till around the ride height of the sudden change in wheel drag, after which the suction grows again with increasing ride height in order to match the value at the upper boundary. This variation is roughly similar for each of the three pressure distribution locations on the tyre tread.
The flow mechanism responsible for the behaviour in the lower wake is difficult to point out. It could be that the suction reduction with increasing ride height in the lower segment is induced by the replacement of the inside longitudinal vortex and ‘bow wave’ zone with the remains of the lower edge wing vortex. At the lowest ride heights this lower edge vortex has burst upstream of the wheel, but it still leaves its marks on the flow field and seems to produce higher velocity flow around the corner of the wheel than when the longitudinal vortex is present. At the lowest ride height the outside longitudinal vortex seems to be missing as well in the CFD results. This influence of the replacement of the longitudinal vortices reduces when the wing moves away from the ground, leading to less suction in this area of the wheel surface.
The increase in suction closer to the middle of the wake for higher ride heights seems to be related to the flow field presented in figure 4.10. The equivalent position of this horizontal plane in the pressure distributions is atθ= 187◦ and 189◦ for respectively the
P4 and P5 location. This is in the region where the higher ride heights show considerably more suction, as can be seen in figures 4.5 and 4.7. The strong recirculation in the PIV data, which can be noticed in the figures 4.10 (d), (e) and (f), only occurs for ride heights above the sudden increase in wheel drag4. The highest wheel drag is experienced when the recirculation zone lies closest to the back of the wheel at h/c= 0.458, inducing the
4The case forh/c= 0.317 has not been included in this figure, but the flow field looks very similar to
theh/c= 0.211 case showing slightly more deflection into the wake, while still no signs of recirculation
Effect of wing presence on wheel aerodynamics
highest velocities on the surface. It is therefore expected that the flow mechanism causing this recirculation is fundamental to the value of the wheel drag. It is proposed here that a complex interaction of the vortices originating from the wing and wheel is responsible for the recirculation in the wheel wake. This mechanism seems to be confirmed by CFD results for the 3D flow domain, but is difficult to prove with the experimental data.
Following the previous discussion, it is to be expected that the influence of the lower wake effect on the wheel force coefficients is complicated. The suction near to the contact patch has little influence on the drag, but has more relevance to the lift. The drag contribution most likely reduces slightly with increasing ride heights when the wing is close to the ground and then grows suddenly when the recirculation occurs. After reaching a maximum it most likely reduces again due to the recirculation moving away from the wheel surface. The wheel lift contribution initially increases due to the reduced suction over the lowest segment, while for higher ride heights it may reduce, but this depends on the location where the recirculation effect is most prominent. The side force increases when the outside longitudinal vortex appears as the wing starts to move away from the ground. The symmetry of the lower wake is restored for the higher ride heights, when both longitudinal vortices are present, and this should lead to a reduction of the side force. No experimental data is available for the outerside of the wheel and therefore it is unknown whether the recirculation zone exists on that side as well.