Introduction

The use of compact vehicles has become increasingly attractive in large urban centers. In this context, CFD simulation analysis contributes to the development of solutions focused on urban mobility. Furthermore, the greater ease of maneuvering and parking smaller vehicles represents an opportunity for companies in the sector.

In addition to their ease of maneuvering in traffic, the type of propulsion used in these vehicles is also a distinguishing feature. Some models use hybrid propulsion via pedals with electric assistance, similar to e-bikes. However, these vehicles feature fairings and offer greater comfort to the user.

One of the main obstacles to the wider use of bicycles in large cities is, above all, exposure to the elements. Rain, wind, pollution, and even mud puddles can sometimes make the daily commute to work impractical.

In this regard, streamlined vehicles have been gaining prominence, especially in more developed countries such as Germany. The TWIKE, for example, is a well-established initiative in this segment.

Figure 1 – TWIKE hybrid vehicle powered by human and electric propulsion.

In Brazil, a company based in São Paulo has developed a vehicle called BOTO, with organic shapes inspired by the mammal of the same name, which is very common in the tropics.

To develop the BOTO prototype, Kot conducted simulations using CFD to evaluate improvements aimed at reducing aerodynamic drag. Given the limited power of human propulsion supplemented by an electric motor, any reduction in drag has a positive impact on the vehicle’s performance, both in terms of speed and range.

Modeling

Since the vehicle is in the early stages of development, a scaled-down model was used to create the finite-volume mesh. The scaled-down model was then scanned, allowing it to be quickly imported into the CFD software. As a result, time-consuming steps in the process, such as detailed modeling, were significantly reduced.

The use of scanned models has some limitations in terms of accuracy. However, in analyses conducted during the early stages of prototyping, the speed of iterations is often more important than the absolute accuracy of the results.

This is because the prototype undergoes constant changes during development. As a result, testing new solutions and improvements becomes a frequent occurrence, as does the need to quantify small performance gains.

Figure 2 – Modeling using the digital scanning technique for scaled-down models.

Subsequently, the model underwent minor adjustments before being integrated directly into the CFD software. Scale adjustments were made to reflect the actual dimensions, and corrections were applied to areas with imperfections resulting from the scanning process.

Results

The simulation results therefore indicated the need for improvements to the upper rear section of the prototype. The goal was to shift the point of flow separation while keeping the flow closer to the surface. As a result, total drag was reduced.

Figure 3 – Results – Pressure distribution on the fairing of the BOTO prototype.

Since this is a boxy body without aerodynamic profiling, the drag coefficient (Cd) calculated by CFD was on the order of 0.20. For comparison, several traditional compact vehicles sold in Brazil can be considered.

The Fiat 500, for example, has a drag coefficient between 0.33 and 0.36. The GM Kadett, on the other hand, known for its low aerodynamic drag, achieved a drag coefficient of approximately 0.30 in its best versions.

In contrast, bicycles without any type of fairing have drag coefficients ranging from 1.1 to 0.88. Therefore, there is a significant difference compared to the proposed vehicle.

Figure 4 – Results – addition of a fairing near the rear wheel of the prototype.

Final considerations

The proposed improvements were quickly implemented using the scanning technique. This made it possible to quickly evaluate the changes made to the scale model.

Based on CFD flow analyses, it became possible to propose improvements aimed at reducing aerodynamic drag. Consequently, vehicle performance improved, contributing to the development of alternatives for urban mobility.

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