Air drag reduction on vehicle bodies is an ever-improving area of the auto industry. Minimizing drag forces on semi-trucks is especially important as they are not the most fuel efficient vehicles to begin with, and yet a single truck can log well over a hundred thousand miles annually. Because of this, there are more add-on drag-reducing accessories, and more varieties of them, on the road than ever before. This blog will use SOLIDWORKS Flow Simulation to look at the effect that a couple of the most common fairing panels.
Trucks differ in size and shape quite a bit, so first we’ll establish our baseline model as a fairly standard looking (at least in my mind) semi with a 40′ trailer. Operating conditions also differ across semis, but drag-reduction devices are best-suited for highways, and we’ll assume a highway speed of 65 mph. Creating the flow study is pretty simple (though not necessarily our computation, as we’ll soon see) with the Wizard: an external study using air at 65 mph in otherwise standard conditions.
Setting up the computational domain is mostly just giving the boundaries of our model sufficient breathing room, save for two: the bottom boundary will serve as our road and is coincident with the tire bottoms, and we can also take advantage of the truck’s symmetry in one plane and bisect it (though we’re assuming then no flow across this plane, which is pretty idealized). It becomes more feasible then to implement a mesh control in the areas of high flow and near model surfaces to return a study with a just short of a million cells that takes a few hours to run to ensure accurate CFD results.
There are a few results that may be of interest to us, but we’ll focus our attention on pressure around the vehicle, and on the forces that that pressure creates. Above is a cut plot showing our pressure gradients about halfway up the vehicle – and note that because this was done in SOLIDWORKS 2016, we have the ability to mirror the plot across our symmetrical plane. However I just chose to show a qualitative plot, as the power of Flow Simulation here is that we can extract forces on our semi. Any Flow Sim value is best used when set up beforehand as a goal the analysis uses to converge, and so we’ll specify a global x-direction (ie, driving-direction) force goal.
A trailer skirt seems to be the most common add-on body on the market, aimed at reducing the large drag forces underneath the truck’s trailer. Our model has a configuration that uses a thin skirt that spans about 16′ and extends 9″ above the road to cover the middle of the trailer. The cut plot below shows the velocity distribution on the trailer’s underside (plotting on a plane just below it), with a clearly defined boundary at the skirt (dotted pink for clarity).
Maybe the most noticeable fairing is the trailer tail fin, a foldable body extending a few feet from a trailer’s end with a slight taper. These have the effect of streamlining the back end, shown below with a sort of vortex effect on the “inside” of the tail.
A combination case with skirts and a tail fin was also ran, and all force values for the cases are summarized. Turning this into fuel efficiency isn’t exactly straightforward, but we can make the assumption that maybe half of the total energy loss on a 65 mph semi is due to highway drag. Applying that assumption to our difference in force values gives the following approximated savings:
Not a bad result, in fact right in line with some real world reports. I was a bit surprised that the tail fin outperformed the skirts, however the skirt would have more of an effect when the truck isn’t travelling straight ahead. We would expect to see even more of a difference in all results if drag on the semi-truck was optimized first – it was really just the trailer being modified here. Looking through some other plots (eg, the first pressure cut plot), perhaps the next opportunity would be adding fairings between truck cab and the front edges of the trailer to reduce the pressure buildup in the gap. In any case, don’t expect to see these drag-reducers going anywhere, and think about how Flow Simulation could optimize some of your own products!
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