energies Article Wake Structures and Surface Patterns of the DrivAer Notchback Car Model under Side Wind Conditions Dirk Wieser *, Christian Navid Nayeri * and Christian Oliver Paschereit Institute of Fluid Dynamics and Technical Acoustics, Technische Universität Berlin, Müller-Breslau-Straße 8, 10623 Berlin, Germany; [email protected] * Correspondence: [email protected] (D.W.); [email protected] (C.N.N.) Received: 4 December 2019; Accepted: 26 December 2019; Published: 9 January 2020 Abstract: The flow field topology of passenger cars considerably changes under side wind conditions. This changes the surface pressure, aerodynamic force, and drag and performance of a vehicle. In this study, the flow field of a generic passenger vehicle is investigated based on three different side wind angles. The study aimed to identify vortical structures causing changes in the rear pressure distribution. The notchback section of the DrivAer model is evaluated on a scale of 1:4. The wind tunnel tests are conducted in a closed section with a splitter plate at a Reynolds number of 3 million. The side wind angles are 0◦, 5◦, and 10◦. The three-dimensional and time-averaged flow field downstream direction of the model is captured by a stereoscopic particle image velocimetry system performed at several measurement planes. These flow field data are complemented by surface flow visualizations performed on the entire model. The combined approaches provide a comprehensive insight into the flow field at the frontal and side wind inflows. The flow without side wind is almost symmetrical. Longitudinal vortices are evident along the downstream direction of the A-pillar, the C-pillars, the middle part of the rear window, and the base surface. In addition, there is a ring vortex downstream of the vehicle base. The side wind completely changes the flow field. The asymmetric topology is dominated by the windward C-pillar vortex, the leeward A-pillar vortex, and other base vortices. Based on the location of the vortices and the pressure distributions measured in earlier studies, it can be concluded that the vortices identified in the wake are responsible for the local minima of pressure, increasing the vehicle drag. Keywords: DrivAer; aerodynamics; wind tunnel; vehicle; flow visualization; PIV; wake structures; side wind; crosswind 1. Introduction Road vehicles operate in an environment in which the inflow direction and the magnitude of the velocity vary permanently. The average natural wind speed on the ground is assumed to be between 2 and 5 km/h [1,2]. Moreover, this value is superimposed by wind fluctuations caused by gust, weather, area topology, and road users. Experimental analyses of vehicles equipped with measuring probes on the road have mainly shown that the inflow direction is mostly in the range of 10 [3] and that the ± ◦ turbulence level is in the range of 2 to 10% [4–7]. Guilmineau et al. [8] documented a drag increase of 20% for the Ahmed test body with a ≈ crosswind of 10◦. The drag increase even doubles when the crosswind angle is increased to 15◦. The same correlation is evident for other vehicle geometries, although it is slightly smaller for streamlined vehicle geometries. For the realistic vehicle geometry of the DrivAer model, a drag increase of 10% at a side wind angle of 10 is evident compared to the frontal direction [9,10]. ≈ ◦ In addition to the vehicle drag, the lift force also experiences a significant change in the yawed flow. 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For For the the crosswind crosswind investigations, investigations, the the model model was was rotated rotated in the in wind the wind tunneltunnel at the at yaw the yaw angle angleβ relativeb relative to to the the inflow. inflow. The vehicle vehicle drag dragCDCforD for the the notchback notchback version version increased increased successivelysuccessively from from 0.258 0.258 at at0◦0toto approximately approximately 0.277 0.277 at at1010◦(D(C∆CD 8%8%) (Figure) (Figure1). The1). DrivAerThe DrivAer model model ◦ ◦ D ≈ ≈ in fastbackin fastback configuration configuration showed showed a a drag drag increase increase of of 10%10% at atb =β =10◦10(not(not shown shown here). here). ≈≈ ◦ 0.28 0.275 0.27 [-] . D 0 265 C 0.26 0.255 0.25 0 2 4 6 8 10 Side wind angle in degree Figure 1. C for the notchback driver model at varying side wind conditions of 0 b 10 and Figure 1. CD forD the notchback driver model at varying side wind conditions◦ of 0◦SW βSW◦ 10◦ and 6 ≤ ≤ ≤ ≤ at Reat=Re3.2= 3.210610. × × Figure2 shows the surface pressure distribution CP and the pressure variation p0 in the notchback Figure2 shows the surface pressure distribution CP and the pressure variation p0 in the notchback configuration of the DrivAer model at side wind angles of 0◦, 5◦, and 10◦. The pressure coefficient configuration of the DrivAer model at side wind angles of 0 −, 5 , and− 10 . The pressure coefficient is defined as ◦ − ◦ − ◦ is defined as dp C = (1) P r 2 dp C = /2 c¥ ADrivAer (1) P ρ · 2 · /2 c ADrivAer where dp is the pressure difference between quiescent· ∞ · condition and surface pressure, c¥ is the wherefreestreamdp is the velocity, pressure and differenceADrivAer is between the frontal quiescent area of the condition model. It and was found surface that pressure, the pressurec∞ is the recovery at the rear window and on the upper trunk deck gradually decreases with increasing yaw freestream velocity, and ADrivAer is the frontal area of the model. It was found that the pressure angles (marked with 4 in Figure2). Moreover, the local high-pressure area, which is caused by the recovery at the rear window and on the upper trunk deck gradually decreases with increasing yaw C-pillar vortex and depicted by the point 3 , decreases under crosswind conditions compared with the angles (marked with 4 in Figure2). Moreover, the local high-pressure area, which is caused by the one at 0 . In addition, a crosswind sensitive location was identified at the upper edge of the windward C-pillar vortex◦ and depicted by the point 3 , decreases under crosswind conditions compared with the C-pillar ( 1 ).