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A CFD Study Of The Aerodynamic Eects OfPlatooning Trucks
KTH, School of Engineering Sciences (SCI), Mechanics.
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In the present work the aerodynamic forces on trucks driving in so-called platoon are

investigated in a numerical fashion. Driving in platoon, or convoy, refers to in an orderly

manner driving in a line, one truck after the other, taking advantage of the unrecovered

_ow behind each truck. The phenomenon is called slipstreaming or drafting. The Compu-

tational Fluid Dynamics (CFD) software STAR-CCM+ is used to calculate the flow field

around a platoon consisting of two and three trucks at different distances, ranging from 5

to 70 m. Two numerical approaches are used, one is the Reynolds Averaged Navier Stokes

based (RANS) two-equation turbulence model k 􀀀 " realizable model with a two-layer

treatment. The second one is the Menter's Shear Stress Transport (SST) k 􀀀 ! Detached

Eddy Simulation (DES) model. The first one is time independent, so-called steady-state,

where platoons consisting of two and three trucks are used in the simulations. However,

the nature of the flow field around vehicles is inherently time-dependent, which makes

it difficult to receive a steady-state solution and thus, the reliability of the result is neg-

atively affected. The second model is time dependent and much more computationally

expensive, where only a platoon consisting of two trucks is simulated. Addition to this,

simulations with an isolated truck will be conducted in order to make a relative study for

both turbulence models. Since numerous of errors are introduced when approaching the

problem numerically, it is important to have a reference case to compare with, set under

the same conditions. Also, comparisons with other studies are done. A mesh independent

study is conducted with the function of investigating how the mesh density influences the

result, together with a mesh quality study, both helpful when assessing the credibility of

the results.

For the RANS approach, it is shown that for the 2-truck platoon, drag reductions

are the greatest at the closest distance, 5 m, with 26:9 and 28:1 % reductions in drag

for the leading and trailing vehicle, respectively, compared to the isolated case. There is

an increase in drag for both vehicles with increasing distance, however, the trend turns

around at 10 m for the trailing vehicle, where it also reaches its maximum, 5:5 % larger

drag than that of the reference case's. Then a reduction is seen for all distances greater

than 11 m. For the leading truck, the drag coeffcient CD is equal to the reference case's

around 18 m, with an overshoot of 2 􀀀 3 % afterward, which may be a result of numerical

errors. The same trend is seen for the 3-truck platoon, with largest reductions at the

closest distance 5 m, with the reductions 31:5 %, 48:5 % and 33:2 % for truck one, two

and three, respectively. At 10 m, there is also an abrupt increase in drag for the trailing

trucks, however, the drag never reaches over the drag of an isolated truck. An overshoot

is also seen for the first vehicle in the 3-truck platoon and it stops benefiting from platoon

driving around 22 m. It was found that at 10, thick low-velocity boundary layers were

formed on the leading trucks, which may be one of the reasons for the increase in drag.

For the time-dependent approach, the drag behavior is similar to the RANS cases for

the leading vehicle, but no overshoot is seen, instead the drag is always smaller than the

reference case's. The maximum reduction is also found at 5 m, with the value 31:7 %. A

completely different trend is found for the second vehicle, where the drag decreases with

increasing distance, where there is a minimum reduction at 5 m (4:0 %) and a maximum

reduction at the largest investigated distance 50 m (24:3 %). This kind of trend is also

seen for the RANS-based simulation in the interval 10 􀀀 50 m, but the reductions are not

as large. After 12 m, the trailing truck benefits the most.

It was found that the vortices and the time dependence of the flow field are important

features. The RANS-based model produced poor results in region of strong swirl and

therefore it is not a suitable model for the flows of this type. Also, based on the good

agreement with PowerFlow VLES (Very Large Eddy Simulation) simulations with the

DES ones even further puts great distrust on the RANS simulations. The k 􀀀 " realizable

model with a two-layer treatment has also shown deficiencies in predicting the downstream

effects (over predicts) and the size and intensity of recirculation areas (for instance, the

wake) as shown in the work of P.L. Davis, A.T. Rinehimer and M. Uddin, 20th Annual

Conference of the CFD Society of Canada.

Place, publisher, year, edition, pages
2014. , p. 135
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-203931OAI: oai:DiVA.org:kth-203931DiVA, id: diva2:1083070
Supervisors
Examiners
Available from: 2017-03-20 Created: 2017-03-20 Last updated: 2017-03-20Bibliographically approved

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