Influence of afterbody rounding on the pressure distribution over a fastback vehicle

Giacomo Rossitto; Christophe Sicot; Valérie Ferrand; Jacques Borée; Fabien Harambat

doi:10.1007/s00348-016-2120-1

Experiments in Fluids

March 2016, 57:43 | Cite as

Influence of afterbody rounding on the pressure distribution over a fastback vehicle

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Authors and affiliations

Giacomo RossittoEmail author
Christophe Sicot
Valérie Ferrand
Jacques Borée
Fabien Harambat

Research Article

First Online: 27 February 2016

Received: 20 July 2015

Revised: 04 January 2016

Accepted: 07 January 2016

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Abstract

Experimental analyzes were performed to understand the drag evolution and the flow field modifications resulting from afterbody rounding on the Ahmed body, a simplified vehicle model with 25 degrees rear slant. Curvature effects were investigated using balance measurements, flow visualizations, wall pressure, and particle image velocimetry measurements. The rear end of the original well-known Ahmed body has sharp connections between the roof and the rear window as well as squared rear pillars. Similarly to previous studies, rounding the roof/backlight intersection was shown to reduce drag up to 16 %. Surprisingly, additional rear curvature associated with side pillar rounding did not further modify the drag. However, the zero net effect was found to result from opposite drag effects on the slanted and vertical surfaces and to hide strong local modifications on the flow field. The tridimensional organization and vorticity transport in the near wake were analyzed and connected to the observed local increase in the pressure drag on the base. Finally, these results were shown to be generic for a realistic rounded-end car shape.

Keywords

Vorticity Drag Reduction Lift Coefficient Pressure Recovery Longitudinal Vortex

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Notes

Acknowledgments

The authors are thankful to L. Mies who took care of the production process of the models and to Y. Goraguer, the wind tunnel manager, for his precious help during the experiments. We acknowledge A. Glad for reviewing the English of the paper. This work has been performed in the framework of the “OpenLab Fluidics” @Poitiers.

Appendix

The present work pointed out that a small edge radius on the side of a simplified car model modifies the flux and the orientation of the mean vorticity vector. One should not forget that realistic cars tend to have large radii of curvature. This justifies the need to test the effect of a large radius of curvature over a realistic car shape to prove that the \(R_{10}S_{10}\) rear end of the Ahmed model is an intermediate configuration where C-pillar vortices are delayed but still present in a weaker way. The aim of this complementary test was to increase the physical understanding of the phenomena and prove that what was understood thanks to the Ahmed body rounding can be exploited for more realistic models.

The chosen model and its relevant dimensions are exhibited in Fig. 14. Two different rear ends were tested: one with a sharp lateral radius and one with an edge radius corresponding to 40 % of the span of the vehicle. Both rear ends have a rounded edge at the end of the roof to avoid flow separation over the rear window.

Fig. 14
Realistic model and location of the rounded edges

Table 3 reports the variation in the drag and lift coefficients for the rounded rear end compared with the sharp one, (the value of drag and lift is not reported because they are meaningless since this model is a unique prototype). As an order of magnitude, the drag coefficient of the sharp rear end is 35 % less than the reference Ahmed body. The rounded configuration exhibits a slight increase in drag. As for the Ahmed body, this hides major local flow modifications. On the contrary, lift is strongly reduced, which demonstrates the influence of the rounded edge on the C-pillar.

Table 3

Variation in aerodynamic coefficients relative to the sharp case

	\({\Delta }\)Cd	\({\Delta }\)Cl
Sharp	–	–
Rounded	+1.5 %	−8.5 %

When comparing with Fig. 8, similar oil flow visualization interpretations were made for the realistic model in Fig. 15. The sharp configuration, reported on the left side of the picture, clearly demonstrates the presence of a C-pillar vortex. For the rounded configuration, the large curvature does not even generate a delayed C-pillar vortex; all the flow that comes from the side of the model, in fact, enters the rear window region through an inward transverse attached flow.

Fig. 15
Oil visualization. *Left* sharp rear end. *Right* rounded rear end. *Dashed line* is the attachment line of the primary vortex, and *solid line* is the separation *line* of the secondary vortex. *Long arrows* indicate the overall direction of the flow off the surface

Figure 16 highlights the pressure wall measurements over the rear ends. Due to the complexity of the side surface of the model, as for example a realistic gap between the window and the side surface of the model, the C-pillar vortex pressure footprint is less intense than the modified Ahmed body, but still present in the sharp case. The pressure distributions on the side of the rounded edge rear end confirm the absence of the C-pillar vortex. As with the modified Ahmed body, the vertical surface displays a lower pressure coefficient than for the sharp case, Table 4. For the Ahmed body, the loss of pressure on the vertical surface compensates for the gain over the rear window when a rounded edge is present. This is not the case for the realistic model, where the net balance between the loss and the gain is not zero (ΔCd = +1.5 %, see Table 3). This underlines the generic nature of the positive and negative effects over the two zones of the rear end, but points out that every model has its own ratio between the vertical surface and the rear window, which influences drag balance, Howell and Good (2008).

Fig. 16
Wall pressure coefficient \(C_{p}\): left sharp, right rounded

Table 4

Variation in spatially averaged mean Cp over the rear window and vertical surface relative to the sharp case

	Rear window \({\Delta }\)Cp	Vertical surface \({\Delta }\)Cp
Sharp	–	–
Rounded	+10 %	−300 %

Fig. 17
Streamwise velocity \(U^{*}\) at \(Y^{*}=0\). *Left* sharp rear end. *Right* rounded rear end

PIV was also performed in the symmetry plane of the model Fig. 17. It is noticed that the rounding of the C-pillar induces strong effects in the near wake close to the vertical surface. When rounding the C-pillar, the length of the separated region is strongly reduced and no “spanwise” large-scale rotation is detected in the symmetry plane. In fact, the separated zone is now a fully 3D flow. To understand this modification of topology in the near wake, YZ stereo-PIV planes are reported in Fig. 18 at \(X^{*}=0.07\).

Fig. 18
Plane at \(X^{*}= 0.07\). The *dashed lines* represent the geometry of the model, (a) mean streamwise velocity \(U^{*}\), (b) mean spanwise velocity \(V^{*}\), (c) mean normal velocity \(W^{*}\), (d) \({\varGamma }_{2}\)

Figure 18a evidences the presence of the C-pillar vortex in the sharp case due its characteristic streamwise velocity contours, no sign of the vortex is visible on the right side of the picture. On the top of the separated region, the vortex generated from the front of the vehicles, the so-called A-pillars, is visible for both models. The spanwise velocity is reported in Fig. 18b. While for the sharp rear end there are two opposite regions of localized transverse velocity due to the C-pillar vortex, for the rounded rear only an intense region of lateral flow that comes from the side (and is injected in the near wake) is present. The resulting downwash is visible in the right side of Fig. 18c. Contours of \({\varGamma }_{2}\) in Fig. 18d illustrate the global streamwise rotation of the flow, expected to induce a low-pressure region over the vertical base of the vehicle.

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Authors and Affiliations

Giacomo Rossitto
- 1
- 2
Email authorView author's OrcID profile
Christophe Sicot
- 2
Valérie Ferrand
- 3
Jacques Borée
- 2
Fabien Harambat
- 1

1.PSA Peugeot CitroënVelizy-VillacoublayFrance
2.Institut Pprime, UPR-3346 CNRS, ENSMAUniversité de PoitiersPoitiersFrance
3.Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO)Université de ToulouseToulouseFrance

Influence of afterbody rounding on the pressure distribution over a fastback vehicle

Abstract

Keywords

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Acknowledgments

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