This thesis presents
an innovative approach to investigating automotive underbody aerodynamics
through the development and application of an on-site 3D Lagrangian Particle
Tracking (LPT) system. Automotive performance, particularly in high-speed
racing applications, is significantly influenced by the aerodynamic efficiency of
vehicle diffusers. The work addresses the challenges associated with accurately
capturing complex three dimensional flow structures beneath a moving vehicle,
where traditional flow measurement techniques struggle to capture underbody
flows, especially in an experimental setting.
The research builds
upon a previous study on a diffuser equipped radio-controlled car, which uses a
measurement technique known as the Ring of Fire. By improving the camera setup,
as well as creating better seeding and illumination, the setup allowed for
successful particle tracking of neutrally buoyant Helium Filled Soap Bubbles
underneath a car model, driving at around 7.5 m/s. The particle tracks captured
in a measurement domain the size of (300 x 150 x 200) mm3 allow for the
reconstruction of a velocity field around three tested car geometries, by
combining data from multiple runs of the car driving through the measurement
domain. These geometries are a flat floor car model, a car model fitted with a
15◦ planar diffuser, and a car model with the same diffuser, but also an
additional strip of vortex generator fins placed ahead of the diffuser leading
edge.
Using a pressure
gradient integration method, a pressure field around the car models was
obtained. Looking at both the velocity and pressure distribution around the
models, the setup was able to capture the difference in peak velocity
underneath the car, where the diffuser equipped model showed a maximum velocity
of around 1.4 times the freestream velocity. Velocity and pressure coefficient
profiles measured along the car’s centerline closely match those reported in
the literature, confirming that the diffuser primarily impacts the rear region
of the vehicle.
Streamwise vortices
introduced into the diffuser by the vortex generator strip showed to be
primarily moving high momentum flow closer to the diffuser surface, while
potentially resolving a laminar separation bubble near the diffuser leading
edge, observed for the plain diffuser case. Difference in local velocity
magnitude and pressure coefficient measured at the diffuser leading edge
between the flat floor and diffuser equipped models proved to be large enough
to be statistically significant. An estimated 25 runs was needed to reach a
velocity convergence inside the diffuser within 1% of the mean car velocity,
where only 4 or 5 runs would be enough for the flat floor regions upstream of
the diffuser.
This work shows the
improvement made to the Ring of Fire setup developed to measure on-site
automotive underbody aerodynamics. It proves the capabilities of applying 3D
LPT to quantify underbody flows, and the potential to apply this setup on
larger and faster vehicles.
