Aerodynamic investigations of a bus under high side wind conditions conditions
Typ
Projektarbete, avancerad nivå
Program
Publicerad
2020
Författare
Hellsten, Oskar
Pettersson, Oskar
Minár, Matús
Gefors, Hugo
Forsström, Birk
Ravindra, Nithin Bharadwaj
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
The Norwegian Public Roads Administration (NPRA) is looking for different solutions on how to
shorten the travel time for road vehicles between Bergen and Trondheim. The main reason for
the long traveling time is due to the amount of fjords that needs to be crossed by ferries when
traveling by the coast. Because of this several alternatives to ferries are explored and one of these
alternatives is using floating bridges. Since side winds affect the stability of the floating bridge
coverage of the sides will be minimal and therefore all vehicles will be fully exposed to side winds.
Larger vehicles like buses and trucks are the main concern in regards to sudden instability or a
rollover. To prevent accidents it has been decided that a vehicle dynamic model will be made to
evaluate the wind speed threshold for when the bridge should be closed or the vehicle speed should
be limited. The magnitude of the side forces on long vehicles during high wind conditions is needed
as an input to create this vehicle dynamic model. By using CFD together with wind tunnel testing
with a scale model an investigation of these forces has been performed.
A scale model of the Volvo 9700 series coach bus was 3D printed to be used for wind tunnel
testing. Because of a predetermined available mounting area as well as with regards to wind tunnel
blockage ratio a scale of 1:18 was chosen. The model was then printed in 9 different parts
excluding the wheels because of print volume limitations. To be able to try different design configurations
the model was made modular. The front and rear were attached using neodymium
magnets while the other parts were either permanently attached using epoxy based glue or screws
together with threaded inserts that were melted in to fuse with the plastic. To get a good surface
finish for added accuracy in the wind tunnel the model was covered with body filler, sanded, then
covered in spray filler and lastly painted black.
The wind tunnel testing of yaw angles covered a complete 360 sweep with increments of 5 up to
90 and then increments of 10 for the rest. The results were mainly used as reference to the CFD
results since the wind tunnel setup did not include rotating wheels and boundary layer suction.
Visualization of the airflow around the bus was achieved by using tufts, smoke and thermal camera.
A steady state incompressible RANS equations(Reynolds Averaged Navier Stokes) are numerically
solved in StarCCM+ to determine forces and moments. These forces were determined by running
simulations for a changing yaw angle from 0 to 90 in 5 steps. Non-dimensional co-efficients of
drag force, lift force, side force, yaw moment, roll moment and pitch moment are calculated to
compare the data from wind tunnel. This is done as the wind tunnel testing is obtained for a
scaled model and in CFD the actual size of the bus is used. Results show that the trends observed
in CFD follow the trends from wind tunnel test data.
It was observed that coefficient of side force and roll moment increases with the increasing yaw
angle and reaches a maximum value at 90 yaw. This shows the importance of the effect of cross
winds on vehicles with large side area. This data can be further used in a vehicle dynamic model
to evaluate the impact on vehicle stability.