The drag coefficient is a key indicator of automobile performance, playing a crucial role in enhancing driving stability and reducing energy consumption. As the global society’s concern for resources continues to heat up, more and more attention has been paid to the development of means of automobile drag reduction, and the Ahmed body has become the object of extensive research because of its simple and representative model structure. This paper provides an overview of the structure of the three-dimensional flow field around the Ahmed body and reviews the research done by researchers in recent years on drag reduction in this body. We classify drag reduction methods into passive and active categories based on the presence or absence of energy inputs. Passive drag reduction is further subdivided into aerodynamic accessories, shape optimization, and platoon. Meanwhile, active drag reduction is categorized into open-loop and closed-loop control, with each further divided into conventional control and artificial intelligence control. The article reviews and summarizes each category, and concludes by addressing the challenges in drag reduction using the Ahmed body and expressing anticipation for future research directions.
KimCH.A streamlined design of a high-speed coach for fuel savings and reduction of carbon dioxide. Int J Automot Eng2011; 2(4): 101–107.
2.
SchoonRE. On-road evaluation of devices to reduce heavy truck aerodynamic drag. SAE technical paper 2007-01-4294, 2007.
3.
HuchoWSovranG.Aerodynamics of road vehicles. Annu Rev Fluid Mech1993; 25(1): 485–537.
4.
AhmedSRRammGFaltinG.Some salient features of the time -averaged ground vehicle wake. SAE Trans1984; 93: 473–503.
5.
AhmedSRBaumertW. The structure of wake flow behind road vehicles. In: Aerodynamics of transportation, ASME-CSME conference, Niagara Falls, 1979, pp.93–103.
6.
MinguezMPasquettiRSerreE.High-order large-eddy simulation of flow over the “Ahmed body” car model. Phys Fluids2008; 20(9): 095101.
7.
WangXWZhouYPinYF, et al. Turbulent near wake of an Ahmed vehicle model. Exp Fluids2013; 54(4): 1490.
8.
LiuKZhangBFZhangYC, et al. Flow structure around a low-drag Ahmed body. J Fluid Mech2021; 913: A21.
9.
ChoiHLeeJParkH.Aerodynamics of heavy vehicles. Annu Rev Fluid Mech2014; 46(1): 441–468.
10.
YuZBingfuZ.Recent advances in wake dynamics and active drag reduction of simple automotive bodies. Appl Mech Rev2021; 73(6): 060801.
11.
KrajnovićSDavidsonL.Flow around a simplified car, part 1: large Eddy simulation. J Fluids Eng2005; 127(5): 907–918.
12.
KrajnovićSDavidsonL.Flow around a simplified car, part 2: understanding the flow. J Fluids Eng2005; 127(5): 919–928.
13.
SpohnAGillieronP. Flow separations generated by a simplified geometry of an automotive vehicle. IUTAM Symposium: unsteady separated flows. Vol. 1. Kluwer Academic, 2002.
14.
ZhangBFZhouYToS.Unsteady flow structures around a high-drag Ahmed body. J Fluid Mech2015; 777: 291–326.
15.
LienhartHBeckerS.Flow and turbulence structure in the wake of a simplified car model. SAE Trans2003; 112: 785–796.
16.
LienhartHStootsCBeckerS. Flow and turbulence structures in the wake of a simplified car model (ahmed modell). In: WagnerSRistUHeinemannHJ, et al. (eds) New results in numerical and experimental fluid mechanics III: Contributions to the 12th STAB/DGLR symposium, Stuttgart, Germany, 2000. Berlin, Heidelberg: Springer, 2002, pp.323–330.
17.
RouméasMGilliéronPKourtaA.Analysis and control of the near-wake flow over a square-back geometry. Comput Fluids2009; 38(1): 60–70.
18.
DuellEGGeorgeAR.Experimental study of a ground vehicle body unsteady near wake. SAE Trans1999; 108: 1589–1602.
19.
BarrosDRuizTBoréeJ, et al. Control of a three-dimensional blunt body wake using low and high frequency pulsed jets. Int J Flow Control2014; 6(1): 61–74.
20.
ÖsthJNoackBRKrajnovićS, et al. On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body. J Fluid Mech2014; 747: 518–544.
21.
GrandemangeMGohlkeMCadotO.Turbulent wake past a three-dimensional blunt body. Part 1. Global modes and bi-stability. J Fluid Mech2013; 722: 51–84.
22.
PaviaGPassmoreMSarduC.Evolution of the bi-stable wake of a square-back automotive shape. Exp Fluids2018; 59(1): 20.
23.
FourriéGKeirsbulckLLabragaL, et al. Bluff-body drag reduction using a deflector. Exp Fluids2011; 50(2): 385–395.
24.
LucasJMCadotOHerbertV, et al. A numerical investigation of the asymmetric wake mode of a squareback Ahmed body – effect of a base cavity. J Fluid Mech2017; 831: 675–697.
25.
EvrardACadotOHerbertV, et al. Fluid force and symmetry breaking modes of a 3D bluff body with a base cavity. J Fluids Struct2016; 61: 99–114.
26.
GrandemangeMCadotOCourboisA, et al. A study of wake effects on the drag of Ahmed’s squareback model at the industrial scale. J Wind Eng Ind Aerodyn2015; 145: 282–291.
PujalsGDepardonSCossuC.Drag reduction of a 3D bluff body using coherent streamwise streaks. Exp Fluids2010; 49(5): 1085–1094.
29.
Hung TranTHijikuroMAnyojiM, et al. Surface flow and aerodynamic drag of Ahmed body with deflectors. Exp Therm Fluid Sci2023; 145: 110887.
30.
MaineMOumamiMEBouksourO.Study of the effect of some deflector’s geometry factors on the reduction of the aerodynamic drag of the car model. Jordan J Mech Ind Eng2021; 15(3): 265–272.
31.
TranTHHijikuroMAnyojiM, et al. Effect of a short, bio-mimetic control device on aerodynamic drag of Ahmed body. J Fluids Eng2023; 145(3): 031206.
32.
KimDLeeHYiW, et al. A bio-inspired device for drag reduction on a three-dimensional model vehicle. Bioinspir Biomim2016; 11(2): 026004.
33.
SiddiquiNAChaabMA.A simple passive device for the drag reduction of an Ahmed body. J Appl Fluid Mech2020; 14(1): 147–164.
34.
KeirsbulckLCadotOBasleyJ, et al. Base suction, entrainment flux, and wake modes in the vortex formation region at the rear of a three-dimensional blunt bluff body. Phys Rev E2023; 108(1): 015101.
35.
Lorite-DíezMJiménez-GonzálezJIPasturL, et al. Drag reduction on a three-dimensional blunt body with different rear cavities under cross-wind conditions. J Wind Eng Ind Aerodyn2020; 200: 104145.
36.
Koppa ShivannaNRanjanPClementS. The effect of rear cavity modifications on the drag and flow field topology of a square back Ahmed body. Proc IMechE, Part D: J Automobile Engineering2021; 235(7): 1849–1863.
37.
ConnollyMGO’RourkeMJIvankovicA.Reducing aerodynamic drag on flatbed trailers for passenger vehicles using novel appendable devices. Fluids2023; 8(11): 289.
38.
VerziccoRFaticaMIaccarinoG, et al. Large Eddy simulation of a road vehicle with drag-reduction devices. AIAA J2002; 40(12): 2447–2455.
39.
KhalighiBChenKHIaccarinoG.Unsteady Aerodynamic flow investigation around a simplified square-back road vehicle with drag reduction devices. J Fluids Eng2012; 134(6): 061101.
40.
ChengSYChinKYMansorS, et al. Experimental study of yaw angle effect on the aerodynamic characteristics of a road vehicle fitted with a rear spoiler. J Wind Eng Ind Aerodyn2019; 184: 305–312.
41.
Bello-MillánFJMäkeläTParrasL, et al. Experimental study on Ahmed’s body drag coefficient for different yaw angles. J Wind Eng Ind Aerodyn2016; 157: 140–144.
42.
MeileWLadinekTBrennG, et al. Non-symmetric bi-stable flow around the Ahmed body. Int J Heat Fluid Flow2016; 57: 34–47.
43.
TunayTFiratESahinB.Experimental investigation of the flow around a simplified ground vehicle under effects of the steady crosswind. Int J Heat Fluid Flow2018; 71: 137–152.
44.
HungTTHijikuroMAnyojiM, et al. Deflector effect on flow behavior and drag of an Ahmed body under crosswind conditions. J Wind Eng Ind Aerodyn2022; 231: 105238.
45.
TianJZhangYZhuH, et al. Aerodynamic drag reduction and flow control of Ahmed body with flaps. Adv Mech Eng2017; 9(7): 168781401771139.
46.
BeaudoinJFAiderJL.Drag and lift reduction of a 3D bluff body using flaps. Exp Fluids2008; 44(4): 491–501.
47.
DarabaszTBonnavionGCadotO, et al. Drag reduction using longitudinal vortices on a flat-back Ahmed body. Exp Fluids2023; 64(1): 20.
48.
YangXHuYGongZ, et al. Numerical study of combined drag reduction bases on vortex generators and riblets for the Ahmed body using IDDES methodology. J Appl Fluid Mech2021; 15(1): 193–207.
49.
SiddiquiNAAgelin-ChaabM.Experimental investigation of the flow features around an elliptical Ahmed body. Phys Fluids2022; 34(10): 105119.
50.
AbdolmalekiMMashhadianAAmiriS, et al. Numerical-experimental geometric optimization of the Ahmed body and analyzing boundary layer profiles. J Optim Theory Appl2022; 192(1): 1–35.
51.
HulukaAWKimCH.A numerical analysis on ducted ahzmed model as a new approach to improve aerodynamic performance of electric vehicle. Int J Automot Technol2021; 22(2): 291–299.
52.
BonnetCFritzH.Fuel consumption reduction in a platoon: experimental results with two electronically coupled trucks at close spacing. SAE technical paper 2000-01-3056, 2000.
53.
PagliarellaRMWatkinsSTempiaA.Aerodynamic performance of vehicles in platoons: the influence of backlight angles. SAE technical paper 2007-01-1547, 2007.
54.
LuoJMiKTanD, et al. Investigation of the aerodynamic characteristics of platoon vehicles based on Ahmed body. Shock Vib2022; 2022: 1–19.
55.
WatkinsSVinoG.The effect of vehicle spacing on the aerodynamics of a representative car shape. J Wind Eng Ind Aerodyn2008; 96(6–7): 1232–1239.
56.
MirzaeiMKrajnovićS. Large Eddy simulations of flow around two generic vehicles in a platoon. In: SegaliniA (ed.) Proceedings of the 5th international conference on jets, wakes and separated flows (ICJWS F2015). Cham: Springer International Publishing, 2016, pp.283–288.
57.
MirzaeiMKrajnovićS. Numerical study of aerodynamic interactions in a homogeneous multi-vehicle formation. In: SegaliniA (ed.) Proceedings of the 5th international conference on jets, wakes and separated flows (ICJWSF2015). Cham: Springer International Publishing, 2016, pp.289–294.
58.
MirzaeiMPavlenkoAKrajnovicS, et al. Experimental and numerical study of flow structures between two Ahmed bodies with various inter-body distances. In THMT-15 Proceedings of the Eighth International Symposium On Turbulence Heat and Mass Transfer. Begel House Inc., 2015, pp.835–838.
59.
EbrahimHMDominyRGLeungPS. Evaluation of vehicle platooning aerodynamics using bluff body wake generators and CFD. In: 2016 international conference for students on applied engineering (ISCAE), Newcastle Upon Tyne, UK, 20–21 October 2016, pp.218–223. New York: IEEE.
60.
RobertsonFHBourriezFHeM, et al. An experimental investigation of the aerodynamic flows created by lorries travelling in a long platoon. J Wind Eng Ind Aerodyn2019; 193: 103966.
61.
TörnellJSebbenSSöderblomD.Influence of inter-vehicle distance on the aerodynamics of a two-truck platoon. Int J Automot Technol2021; 22(3): 747–760.
62.
PiDXuePWangW, et al. Automotive platoon energy-saving: a review. Renew Sustain Energy Rev2023; 179: 113268.
63.
BruneauCHKhadraKMortazaviI.Flow analysis of square-back simplified vehicles in platoon. Int J Heat Fluid Flow2017; 66: 43–59.
64.
AubrunSMcNallyJAlviF, et al. Separation flow control on a generic ground vehicle using steady microjet arrays. Exp Fluids2011; 51(5): 1177–1187.
65.
BrunnAWassenESperberD, et al. Active drag control for a generic car model. In: KingR (ed.) Active flow control. Berlin, Heidelberg: Springer, 2007, pp.247–259.
66.
BruneauCHCreuseEDepeyrasD, et al. Active procedures to control the flow past the Ahmed body with a 25° rear window. Int J Aerodyn2011; 1(3/4): 299.
67.
JahanmiriMAbbaspourM.Experimental investigation of drag reduction on Ahmed car model using a combination of active flow control methods. Int J Eng2011; 24(4): 403–410.
68.
LiuKZhangBZhouY.Correlation between drag variation and rear surface pressure of an Ahmed body. Exp Fluids2021; 62(6): 124.
69.
HuchoWH.Aerodynamik der stumpfen Körper: Physikalische Grundlagen und Anwendungen in der Praxis. Vieweg, 2002.
70.
ZhangBFLiuKZhouY, et al. Active drag reduction of a high-drag Ahmed body based on steady blowing. J Fluid Mech2018; 856: 351–396.
71.
KumarVAlviFS.Use of high-speed microjets for active separation control in diffusers. AIAA J2006; 44(2): 273–781.
72.
RouméasMGilliéronPKourtaA.Drag reduction by flow separation control on a car after body. Int J Numer Methods Fluids2009; 60(11): 1222–1240.
73.
ParkHChoJHLeeJ, et al. Experimental study on synthetic jet array for aerodynamic drag reduction of a simplified car. J Mech Sci Technol2013; 27(12): 3721–3731.
74.
JosephPAmandoleseXEdouardC, et al. Flow control using MEMS pulsed micro-jets on the Ahmed body. Exp Fluids2013; 54(1): 1442.
75.
LehugeurBGilliéronP. Active control of vortex breakdown phenomenon in the wake of a simplified car geometry. In: ASME 2006 2nd joint U.S.-European fluids engineering summer meeting collocated with the 14th international conference on nuclear engineering, Miami, FL, USA, 17–20 July 2006, pp.709–714. New York: ASME.
76.
GilliéronPKourtaA.Aerodynamic drag control by pulsed jets on simplified car geometry. Exp Fluids2013; 54(2): 1457.
77.
KrentelDMuminovicRBrunnA, et al. Application of active flow control on generic 3D car models. In: KingR (ed.) Active flow control II. Berlin, Heidelberg: Springer, 2010, pp.223–239.
78.
McNallyJMazellierNAlviF, et al. Control of salient flow features in the wake of a 25° Ahmed model using microjets. Exp Fluids2019; 60(1): 7.
79.
WangBYangZZhuH.Drag reduction on the 25° Ahmed body using a new zero-net-mass-flux flow control method. Theor Comput Fluid Dyn2019; 33(5): 411–431.
80.
WassenEThieleF. Simulation of active separation control on a generic vehicle. In: 5th flow control conference, Chicago, IL, USA, 28 June–1 July 2010, p.4702. Reston, VA: American Institute of Aeronautics and Astronautics.
81.
ChoiHJeonWPKimJ.Control of flow over a bluff body. Annu Rev Fluid Mech2008; 40(1): 113–139 .
82.
WassenEThieleF. Drag reduction for a generic car model using steady blowing. In: 4th flow control conference, Seattle, WA, USA, 23–26 June 2008, p.3771. Reston, VA: American Institute of Aeronautics and Astronautics.
83.
WassenEThieleF.Road vehicle drag reduction by combined steady blowing and suction. In: 39th AIAA fluid dynamics conference, San Antonio, TX, USA, 22–25 June 2009, p.4174. Reston, VA: American Institute of Aeronautics and Astronautics.
84.
MetkaM. Flow control on the Ahmed body vehicle model using fluidic oscillators. In: ASME 2013 international mechanical engineering congress and exposition, San Diego, CA, USA, 15–21 November 2013, p.V07BT08A077. New York: American Society of Mechanical Engineers.
85.
EdwigeSEulalieYGilotteP, et al. Wake flow analysis and control on a 47° slant angle Ahmed body. Int J Numer Methods Heat Fluid Flow2018; 28(5): 1061–1079.
86.
Lorite-DíezMJiménez-GonzálezJIMartínez-BazánC, et al. Perimetric blowing at the rear of a bluff body: consequences on the wake dynamics and drag reduction. PAMM2019; 19(1): e201900044.
87.
BruneauCHCreuséEDepeyrasD, et al. Coupling active and passive techniques to control the flow past the square back Ahmed body. Comput Fluids2010; 39(10): 1875–1892.
88.
LiRBoréeJNoackBR, et al. Drag reduction mechanisms of a car model at moderate yaw by bi-frequency forcing. Phys Rev Fluids2019; 4(3): 034604.
89.
BourqueCNewmanBG.Reattachment of a two-dimensional, incompressible jet to an adjacent flat plate. Aeronaut Q1960; 11(3): 201–232.
90.
DjojodihardjoH.Overview of Coănda jet and vortex cell circulation control. Appl Mech Mater2014; 564: 118–125.
91.
BarrosDBoréeJNoackBR, et al. Bluff body drag manipulation using pulsed jets and Coanda effect. J Fluid Mech2016; 805: 422–459.
92.
HaffnerYBoréeJSpohnA, et al. Unsteady Coanda effect and drag reduction for a turbulent wake. J Fluid Mech2020; 899: A36.
93.
Lorite-DíezMJiménez-GonzálezJIPasturL, et al. Drag reduction of three-dimensional bodies by base blowing with various gas densities. Phys Rev E2020; 102(1): 011101.
94.
Lorite-DíezMJiménez-GonzálezJIPasturL, et al. Experimental analysis of the effect of local base blowing on three-dimensional wake modes. J Fluid Mech2020; 883: A53.
95.
LiYCuiWJiaQ, et al. Explorative gradient method for active drag reduction of the fluidic pinball and slanted Ahmed body. J Fluid Mech2022; 932: A7.
96.
ZhangBFFanDWZhouY.Artificial intelligence control of a low-drag Ahmed body using distributed jet arrays. J Fluid Mech2023; 963: A3.
97.
LiRNoackBRCordierL, et al. Drag reduction of a car model by linear genetic programming control. Exp Fluids2017; 58(8): 103.
98.
FanDZhangBZhouY, et al. Optimization and sensitivity analysis of active drag reduction of a square-back Ahmed body using machine learning control. Phys Fluids2020; 32(12): 125117.
99.
DengGMFanDWZhangBF, et al. Sensitivity analysis of large body of control parameters in machine learning control of a square-back Ahmed body. Proc R Soc Math Phys Eng Sci2023; 479(2269): 20220280.
100.
LeeCKimJChoiH.Suboptimal control of turbulent channel flow for drag reduction. J Fluid Mech1998; 358: 245–258.
101.
BushnellDMMcGinleyCB.Turbulence control in wall flows. Annu Rev Fluid Mech1989; 21: 1–20.
102.
KingRAleksicKGelbertG, et al. Model predictive flow control-invited paper. In: 38th fluid dynamics conference and exhibit, Seattle, WA, USA, 23–26 June 2008, p.3975. Reston, VA: American Institute of Aeronautics and Astronautics.
103.
MuminovicRHenningLKingR, et al. Robust and model predictive drag control for a generic car model. In: 4th flow control conference, Seattle, WA, USA, 23–26 June 2008, p.3859. Reston, VA: American Institute of Aeronautics and Astronautics.
104.
BrunnAHenningLNitscheW, et al. Application of slope-seeking to a generic car model for active drag control. In: 26th AIAA applied aerodynamics conference, Honolulu, HI, USA, 18–21 August 2008, p.6734. Reston, VA: American Institute of Aeronautics and Astronautics.
105.
LiRBarrosDBoréeJ, et al. Feedback control of bimodal wake dynamics. Exp Fluids2016; 57(10): 158.
106.
EvstafyevaOgansASDalla LongaL.Simulation and feedback control of the Ahmed body flow exhibiting symmetry breaking behaviour. J Fluid Mech2017; 817: R2.
107.
VaronEAiderJLEulalieY, et al. Adaptive control of the dynamics of a fully turbulent bimodal wake using real-time PIV. Exp Fluids2019; 60(8): 124.
108.
BrackstonRDGarcía De La CruzJMWynnA, et al. Stochastic modelling and feedback control of bistability in a turbulent bluff body wake. J Fluid Mech2016; 802: 726–749.
109.
RigasGMorgansASBrackstonRD, et al. Diffusive dynamics and stochastic models of turbulent axisymmetric wakes. J Fluid Mech2015; 778: R2.
110.
ChovetCFeingesichtMPlumjeauB, et al. Sliding mode control applied to a square-back Ahmed body. Eur J Mech B Fluids2020; 81: 151–164.
111.
FeingesichtMPolyakovAKerhervéF, et al. SISO model-based control of separated flows: sliding mode and optimal control approaches. Int J Robust Nonlinear Control2017; 27(18): 5008–5027.
112.
GautierNAiderJL.Feed-forward control of a perturbed backward-facing step flow. J Fluid Mech2014; 759: 181–196.
