Delayed separation effect

The separation from the top of the wheel is one specific area that is influenced considerably by the wing presence and relative location. Fackrell [34] proposes that separation takes place just downstream of the location of the largest suction at the top of the wheel, where the following small adverse pressure gradient changes into a favourable pressure gradient again. From figure 4.3 it can be derived that separation defined in this way occurs around θ = 274◦ for the isolated wheel at the centreline. Furthermore it can be seen that the separation for the combined case moves downstream with increasing wing ride height, over the crown of the wheel, to θ = 250◦ _{for h/c = 0.458. The other two pressure}

Effect of wing presence on wheel aerodynamics

trend can be observed over the complete width of the wheel. The downstream movement of the separation position at higher ride heights is however even more pronounced for these locations near the side of the wheel than for the centreline.

An alternative way of looking at this effect is by studying the PIV data over the top of the wheel. Figure 4.8 (b) reveals that the separation lines, defined byU = 0m/s, at the centre of the wheel move downstream as well with increasing ride height. This trend is consistent with the one derived from the pressure distributions1_{, even though the actual}

separation location is not visible in this figure, because it is blocked by the cambered wheel surface. The value of the downstream local pressure minimum after separation, between θ = 240◦ and 250◦ in figure 4.3, could be related to the vertical distance between the separation line and the wheel surface. For the isolated wheel it has been proposed that this feature is the time-averaged contribution of unsteady pressure fluctuations as a result of large scale eddies in the recirculation region. The maximum possible size of the eddies determines the suction increment due to this feature in this explanation. It is expected that the eddies will be smaller when the separation line lies closer to the wheel surface for higher ride heights, see figure 4.8 (b), and therefore that the local suction maximum after separation will be less for these cases. Figure 4.3 reveals that the suction maximum downstream of separation becomes smaller compared to the maximum suction upstream of separation for increasing ride heights, which is in agreement with this explanation.

Two different flow mechanisms seem responsible for the delay in separation with in- creasing wing ride height. The primary mechanism is most likely related to the circulation that the upstream-located downforce-producing wing induces on the wheel. In order to generate downforce the wing introduces an anti-clockwise circulation around the y-axis. This is in the same direction as the wheel rotation. The wing circulation induces an opposite circulation on the annulus of flow around the wheel. Just like wheel rotation promotes separation from the top of the wheel, this opposite wing-induced circulation postpones the separation. Figure 4.2 gives a schematic impression of this effect. This wing induced circulation mechanism is most effective when the wing and wheel are closest to each other athF te/D≈0.5 and increasingly loses significance for higher and lower ride

1_{One difference between the two methods is that the PIV separation lines seem to indicate that sep-}

aration forh/c= 0.063 and for 0.106 takes place upstream of the location for the isolated wheel, while

the pressure distributions suggest that separation for the combined wing - wheel case always occurs down- stream of that for the isolated wheel. It needs to be kept in mind though that this is a comparison of two totally different definitions of the separation location and furthermore that the PIV plane was set up vertically instead of perpendicular to the cambered wheel surface.

Effect of wing presence on wheel aerodynamics

heights. Alternatively the increase in flow directed over the top of the wheel for higher ride heights could provide an explanation for the delayed separation as well.

The second mechanism that delays the separation over the top of the wheel results from the trajectory of the upper edge vortex of the wing. CFD simulations have revealed that the trajectory of this upper edge vortex changes from passing on the inside of the wheel for low ride heights to over the wheel for the higher ride heights as shown in figure 4.11. The switch between these two options occurs at the sharp change in wheel drag, which is at hEP t/D= 0.67 for these configuration settings. The upper edge vortex re-energizes

the flow layer around the wheel, when it passes over the top, while the anti-clockwise flow rotation - when looked from behind - of the vortex pushes the flow towards the wheel surface, postponing the separation in this way.

The delayed separation leads to a larger suction over the top of the wheel fromθ= 300◦

to close to 200◦_{, as can be concluded from figure 4.3. The resulting influence on the wheel}

drag is relatively small and most prominent near the boundaries of this region, as can be concluded from the fact that the pressure distribution has to be multiplied by cos(θ) to determine the drag contribution. However, interestingly, this reveals that separation delay is accompanied by an increase in wheel drag. This observation is counter-intuitive to the classical results for infinite circular cylinders [18]. The last paragraph of section 8.1 pays more attention to this paradox. The contribution of the delayed separation to the wheel lift is however considerably larger, making the additional suction over the top of the wheel one of the main factors of influence for the wheel lift. It can be assumed that the side force on the wheel is not very dependent on the location of separation over the top of the wheel. Nevertheless the side force towards the symmetry plane will be larger when the upper edge vortex passes on the inside of the wheel, inducing lower pressures on this wheel surface, than when it passes over the top.