fluids Article Some Observations on Shape Factors Influencing Aerodynamic Lift on Passenger Cars Jeff Howell 1,*, Steve Windsor 2 and Martin Passmore 1 1 Aeronautical and Automotive Engineering, Loughborough University, Loughborough LE11 3TU, UK; [email protected] 2 Aerodynamics Department, Jaguar Land Rover, Gaydon CV35 0RR, UK; [email protected] * Correspondence: [email protected] Abstract: The car aerodynamicist developing passenger cars is primarily interested in reducing aerodynamic drag. Considerably less attention is paid to the lift characteristics except in the case of high-performance cars. Lift, however, can have an effect on both performance and stability, even at moderate speeds. In this paper, the basic shape features which affect lift and the lift distribution, as determined from the axle loads, are examined from wind tunnel tests on various small-scale bodies representing passenger cars. In most cases, the effects of yaw are also considered. The front-end shape is found to have very little effect on overall lift, although it can influence the lift distribution. The shape of the rear end of the car, however, is shown to be highly influential on the lift. The add-on components and other features can have a significant effect on the lift characteristics of real passenger cars and are briefly discussed. The increase in lift at yaw is, surprisingly, almost independent of shape, as shown for the simple bodies. This characteristic is less pronounced on real passenger cars but lift increase at yaw is shown to rise with vehicle length. Keywords: car aerodynamics; lift; lift distribution; wind tunnel; shape effects; yaw angle Citation: Howell, J.; Windsor, S.; Passmore, M. Some Observations on 1. Introduction Shape Factors Influencing The primary role of the aerodynamics development engineer working on passenger Aerodynamic Lift on Passenger Cars. car design is typically to reduce drag without compromising stability or refinement. All Fluids 2021 6 , , 44. https://doi.org/ aerodynamic characteristics are affected by the shape of the car and cannot be developed 10.3390/fluids6010044 in isolation. For example, the shape factors which influence drag will also influence the parameters which have an effect on stability, including lift. Received: 29 November 2020 Aerodynamic lift has a strong effect on car stability at high vehicle speeds. In particular, Accepted: 14 January 2021 Published: 18 January 2021 the lift coefficient and lift balance (the difference between front and rear axle lift coefficients) has a pronounced effect on high-speed cornering, lane change manoeuvrability, and straight Publisher’s Note: MDPI stays neutral line stability [1]. Lift also has an influence on crosswind sensitivity and high-speed braking with regard to jurisdictional claims in performance. In addition, high lift characteristics are associated with increased drag, published maps and institutional affil- through the vortex drag component [2], and yawing moments [3], which affect drivability iations. in windy conditions. During the car development process, the targets for the lift coefficient are set accord- ing to the stability requirements of the vehicle. These targets usually take the form of a maximum permissible lift coefficient for each axle or combinations of them. The magnitude of the limiting lift coefficients is reduced as the performance of the car increases. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Passenger car shapes are effectively imposed by the design department (styling), and This article is an open access article the car aerodynamicist must manipulate the shape to achieve the desired aerodynamic distributed under the terms and performance within the constraints set by design. The shape evolves through the devel- conditions of the Creative Commons opment process but is essentially fixed before the build process of prototypes begins. The Attribution (CC BY) license (https:// aerodynamic development prior to style sign-off, for most manufacturers, consists of a creativecommons.org/licenses/by/ mix of wind tunnel tests and computational fluid dynamics (CFD) simulations of the 4.0/). evolving shape to monitor changes and suggest modifications. Most original equipment Fluids 2021, 6, 44. https://doi.org/10.3390/fluids6010044 https://www.mdpi.com/journal/fluids Fluids 2021, 6, x FOR PEER REVIEW 2 of 17 Fluids 2021, 6, 44 2 of 16 evolving shape to monitor changes and suggest modifications. Most original equipment manufacturers (OEMs)(OEMs) useuse small-scalesmall-scale modelsmodels forfor the initial phase of wind tunnel testing, withwith scalesscales thatthat rangerange fromfrom 20%20% toto 40%,40%, whilewhile full-sizefull-size aerodynamicaerodynamic bucksbucks areare typicallytypically used for the laterlater phase,phase, beforebefore prototypesprototypes becomebecome available.available. Various studiesstudies havehave beenbeen conductedconducted onon thethe influenceinfluence ofof shapeshape onon thethe aerodynamicaerodynamic characteristics of passenger passenger cars. cars. Carr Carr [4], [4], in in an an early early extensive extensive study, study, developed developed a simpli- a sim- plifiedfied quarter-scale quarter-scale saloon saloon car car shape shape from from a rect a rectangularangular box box and and then then explored explored the the effect effect of ofmodifying modifying most most surfaces surfaces on onthe the car car model. model. As Aswell well as asthe the configuration configuration changes, changes, the the ef- effectsfects of of sharpening sharpening individual individual edges edges of of the the car car model model were were also investigated. Carr mademade thethe observationobservation thatthat all all the the changes changes in in lift lift were were identifiable identifiable from from the the changes changes in thein the camber- cam- line—theber-line—the line line passing passing through through the centroidthe centroid of each of each cross-section, cross-section, as shown as shown in Figure in Figure1.A positive1. A positive camber camber or incidence or incidence change change generated generated an increase an increase in lift while in lift negative while negative changes hadchanges the oppositehad the opposite effect. effect. Figure 1. Camber and incidence for car shapes. Another extensive study of shape changes was conducted by Gilhaus and Renn [[5],5], using a 3/8th3/8th scale scale model model of of a generic hatchback car, a vehiclevehicle typetype whichwhich diddid notnot existexist whenwhen Carr’sCarr’s investigationinvestigation waswas made.made. An assessmentassessment ofof squaringsquaring individualindividual edgesedges waswas also included. The results of this study figurefigure stronglystrongly inin thethe reviewreview ofof liftlift toto bebe foundfound inin both Hucho [[6]6] and SchultzSchultz [[7].7]. Before thethe timetime whenwhen CFDCFD waswas available,available, anan attemptattempt atat liftlift andand pitchingpitching momentmoment predictionprediction forfor carcar shapesshapes waswas mademade byby MorelliMorelli [[8],8], usingusing anan elegantelegant adaptationadaptation ofof slenderslender wingwing theory.theory. InIn thisthis model,model, thethe locallocal liftlift isis aa functionfunction ofof thethe curvaturecurvature of thethe camber-line,camber-line, span, and groundground clearance.clearance. Later, butbut beforebefore CFDCFD waswas aa reliablereliable technique,technique, aa predictionprediction method forfor liftlift waswas developeddeveloped by by Carr Carr et et al. al. [9 [9],], based based on on the the geometric geometric features features of of a simple a sim- carple model.car model. This paper doesdoes notnot attempt toto predictpredict thethe liftlift characteristicscharacteristics ofof passengerpassenger cars.cars. AsAs allall aerodynamic development engineers areare aware,aware, veryvery smallsmall changes,changes, especiallyespecially inin criticalcritical regions,regions, cancan havehave aa significantsignificant effecteffect onon lift,lift, andand thisthis isis exploitedexploited byby usingusing smallsmall add-onadd-on components to tunetune thethe liftlift characteristics.characteristics. The main aim of the paperpaper isis toto makemake somesome general observations on thethe effecteffect ofof large-scalelarge-scale shapeshape changeschanges onon lift.lift. TheseThese havehave beenbeen noted over many yearsyears ofof wind tunnel testingtesting ofof a wide range of small-scale simple bodiesbodies representingrepresenting carcar shapes.shapes. InIn manymany cases,cases, thesethese effectseffects havehave notnot beenbeen previouslypreviously reported.reported. 2. Experimental Details 2. Experimental Details The data presented in this paper were acquired over a considerable period of time. This isThe not, data therefore, presented a systematic in this paper investigation, were acquired although over individuala considerable sections period may of havetime. beenThis carriedis not, therefore, out in a systematic a systematic manner. investigation, A range of although different modelsindividual have sections been used may for have the variousbeen carried investigations, out in a systematic but most ofmanner. the studies A range involve of different very simple models models have with been extensive used for flatthe surfacesvarious investigations, to make configuration but most changes of the studies simpler involve and easier very to simple understand. models The with different exten- modelssive flat are surfaces described