Method Development for CFD Analysis of Heavy Truck Aerodynamics: Investigations of Effects of Partitioning for Parallel Simulations, Bi-stable Flow States for Steady Simulations, and Effects of Split-line Variation on Drag
| dc.contributor.author | Larsson, Henrik | |
| dc.contributor.department | Chalmers tekniska högskola / Institutionen för mekanik och maritima vetenskaper | sv |
| dc.contributor.examiner | Davidson, Lars | |
| dc.contributor.supervisor | Forsgren, Johan | |
| dc.date.accessioned | 2020-06-23T15:54:50Z | |
| dc.date.available | 2020-06-23T15:54:50Z | |
| dc.date.issued | 2020 | sv |
| dc.date.submitted | 2020 | |
| dc.description.abstract | According to the European Commission, heavy trucks are responsible for about 4% of total European CO2 emissions. One way to lower emissions is to reduce aerodynamic drag. In order to effectively design low drag trucks, computational fluid dynamics (CFD) simulations are used for the analysis of aerodynamic performance. This thesis consists of three parts that investigate CFD simulations for truck aerodynamics in three areas. Firstly, differences appearing between parallel simulations are investigated. As truck aerodynamics is very computationally expensive, simulations are run on parallel computer clusters. It has been seen that running simulations on different clusters may cause differences in simulation results, which should not should not occur according to theory. In the second part two bi-stable ow states that appear in steady simulations are investigated. Small geometrical variations cause the solution to enter one of two modes, with no clear pattern. Lastly, real trucks will deviate from the nominal geometry to some degree, due to natural variation. Simulations are however mostly performed on nominal geometry, which leaves some uncertainty regarding the performance of real vehicles. Therefore the effects of split-line variation on drag are investigated. A design of experiment (DOE) approach was used to identify which parallel parameters were affecting results, both in parallel meshing and parallel simulations. It was found that parallel effects on the meshing was negligible. The differences were found to appear when running simulations on different clusters and different number of processors. The cause for the difference was an underlying instability in the form of the bi-stable ow states. Differences in the algebraic multigrid (AMG) linear solver or in floating point errors caused sensitive simulations to change mode. Differences between clusters could be eliminated using the command -mppflags "-e MPI_COLL_FORCE_CONSTANTORDER=1" in Star-CCM+ for clusters with Platform MPI (Message Passing Interface). The bi-stable modes were investigated for some different cases. The flow fields, solution development, local convergence and streamwise pressure gradient were investigated. A mesh study was performed, and a turbulence model study with both Reynolds-averaged Navier-Stokes (RANS) models and a detached eddy simulation (DES) model. The modes were partially explained by interaction with the front door split-line. One of the modes, Mode 1, was found to be most probably physical. The physicality of the other mode could not be determined with certainty. It was however found not to be the dominant mode. A possible solution to the problem is to use the realizable k " model, which was most consistent with the modes predicted by the DES. Alternatively a more accurate transient model such as DES can be used, if it can be afforded. Split-lines in four different areas of the truck cab were investigated using a design of experiment approach. Gap and ush at minimum and maximum tolerance values were considered for the split-lines of the selected areas. It was found that the impact on drag is within 5 DC (drag counts), and that most trucks will be within 3 DC of nominal CD. The most important of the investigated areas are the front corners corners of the cab. It was furthermore found that gap has a larger effect on drag than flush, for the split-lines and tolerances considered. | sv |
| dc.identifier.coursecode | MMSX60 | sv |
| dc.identifier.uri | https://hdl.handle.net/20.500.12380/300983 | |
| dc.language.iso | eng | sv |
| dc.relation.ispartofseries | 2020:46 | sv |
| dc.setspec.uppsok | Technology | |
| dc.subject | CFD | sv |
| dc.subject | Commercial Vehicles | sv |
| dc.subject | Aerodynamics | sv |
| dc.title | Method Development for CFD Analysis of Heavy Truck Aerodynamics: Investigations of Effects of Partitioning for Parallel Simulations, Bi-stable Flow States for Steady Simulations, and Effects of Split-line Variation on Drag | sv |
| dc.type.degree | Examensarbete för masterexamen | sv |
| dc.type.uppsok | H | |
| local.programme | Applied mechanics (MPAME), MSc |