Unsteady aerodynamic effects on the driving stability of passenger vehicles
Doktorsavhandling, 2023

The passenger car is a vital part of modern society, giving people the freedom of flexible travel. As technology advances, customers increase their demand for future products. The automotive industry must therefore adapt to society's requirements for energy-efficient travel, where developing low-drag vehicles is key. However, if not designed with care, streamlined bodies of low drag might impair the driving stability. In addition, raised customer demands of perceived control and stability elevate the research needs on driving stability.

Vehicles travelling on open roads are continuously exposed to changing crosswind conditions. Most road vehicles have the aerodynamic centre of pressure located at the front, making them sensitive to these unsteady crosswinds. Strong winds and sensitive vehicle designs degrade the driving stability perceived by drivers and passengers. Furthermore, as aerodynamic loads increase with flow velocity, the sensitivity becomes greater at high speeds. High speeds affect stability performance even without variations in crosswind. The balance of the time-averaged lift forces between the front and rear axles influences understeering and, consequently, vehicle handling. However, the averaged forces cannot always predict the stability performance, which increases the need to explore the unsteady aerodynamic effects on vehicle handling.

The assessment of driving stability for a vehicle in development is often done on test tracks in late design phases when prototype vehicles are available. However, the current demands of faster development times require robust virtual tools for earlier assessment. This thesis aims to develop virtual tools for assessing straight-line driving stability and to gain insights into the interdisciplinary physics between aerodynamics and vehicle dynamics.

By conducting on-track measurements, it was demonstrated that crosswinds deteriorate driving stability and that the vehicle motions of lateral acceleration and yaw velocity correlate with the drivers' subjective assessment. A driving simulator study confirmed these lateral motions, and the path curvature, as significant measures. To reduce the lateral vehicle response to crosswinds, the centre of gravity should move forward, while the aerodynamic yaw moment should be reduced (moving the centre of pressure rearward). For high speed stability, without varying crosswinds, it was demonstrated that the unsteady base wake also plays an important role. Stability issues on the test track correlated with bi-stable wake dynamics, primarily affecting the fluctuating rear lift force. Configurations that stabilised the wake led to subjective improvements on the test track, highlighting the importance of unsteady wake aerodynamics.

aerodynamics

wake dynamics

high speed

Vehicle dynamics

driving stability

crosswinds

Chalmers campus Johanneberg , room HA4, Hörsalsvägen 4
Opponent: Prof. Dr.-Ing. Andreas Wagner, University of Stuttgart, Germany

Författare

Adam Brandt

Chalmers, Mekanik och maritima vetenskaper, Fordonsteknik och autonoma system

Quantitative High Speed Stability Assessment of a Sports Utility Vehicle and Classification of Wind Gust Profiles

SAE Technical Papers,;Vol. 2020-April(2020)

Artikel i vetenskaplig tidskrift

Base wake dynamics and its influence on driving stability of passenger vehicles in crosswind

Journal of Wind Engineering and Industrial Aerodynamics,;Vol. 230(2022)

Artikel i vetenskaplig tidskrift

Brandt, A. Sebben, S. Jacobson, B. Wake dynamics of passenger vehicles and its influence on high speed stability

Brandt, A. Jacobson, B. Sebben, S. Drivers' perceived sensitivity to crosswinds and to low-frequency aerodynamic lift fluctuations

Bilen har under det senaste århundradet blivit en central del i vårt moderna samhälle och ses som en frihet av många. I vår omställning till ett mer hållbart samhälle spelar därför fordonsbranschen en viktig roll i att minska bilens energiförbrukning.  En stor del av energiförbrukningen hos en personbil orsakas av det aerodynamiska luftmotståndet. När en bil utvecklas för att ha lågt luftmotstånd kan dessvärre bilens stabilitet försämras. Bilen får då ett nervöst beteende vid höga hastigheter och föraren måste kontinuerligt korrigera bilen för att hålla kursen.

Idag kan bilens stabilitet endast utvärderas sent i utvecklingsfasen, då prototypbilar finns tillgängliga som testas i höga hastigheter. Om stabilitetproblem upptäcks så sent innebär det höga kostnader för att åtgärda problemet. Dessutom är merparten av bilens konstruktion redan bestämd, vilket ger litet utrymme för förbättringar.

Detta arbete avhandlar strategier för att flytta utvärderingen av bilens stabilitet från testbanorna till den virtuella miljön. Virtuella datorsimuleringar kan användas tidigt i utvecklingsfasen och möjliggör kostnadseffektiva förbättringar innan prototypbilarna byggs. Arbetet visar bland annat hur en bil kan designas mindre känslig för kastvindar, genom att flytta bilens tyngdpunkt framåt samt genom förbättrad aerodynamisk form. De metoder och resultat som presenteras i denna avhandling kan hjälpa ingenjörer att lösa stabilitetsproblem utan att öka luftmotståndet.

Aerodynamisk och fordonsdynamisk körstabilitet av personbilar vid hög fart

China-Euro Vehicle Technology (CEVT) AB, 2018-08-01 -- 2022-12-31.

Styrkeområden

Transport

Ämneskategorier

Farkostteknik

Strömningsmekanik och akustik

ISBN

978-91-7905-805-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5271

Utgivare

Chalmers

Chalmers campus Johanneberg , room HA4, Hörsalsvägen 4

Online

Opponent: Prof. Dr.-Ing. Andreas Wagner, University of Stuttgart, Germany

Mer information

Senast uppdaterat

2023-03-10