Oscillating Middle Vehicle in a Platoon

TL;DR

This study investigates how oscillations of the middle vehicle in a three-car platoon affect drag forces, emphasizing the importance of transient analysis over steady-state methods for accurate predictions in oscillating scenarios.

Contribution

It introduces a detailed CFD analysis of oscillating middle vehicles in a platoon, highlighting the significance of transient forces and dynamic effects on drag.

Findings

Oscillations cause cyclically repeatable drag variations.

Transient analysis reveals 75% difference from steady-state results.

Higher reduced frequency increases drag amplitude for the oscillating vehicle.

Abstract

A three-member platoon of Ahmed bodies was studied to investigate the effects of drag variation when the middle member goes out of synchronization and starts to oscillate longitudinally. Oscillations were carried out at two different reduced frequencies corresponding to two different drafting scenarios in NASCAR. Computational Fluid Dynamics simulations were performed to calculate the transient drag force, which was compared with the steady averaged drag results obtained with the middle member in different locations with respect to the lead and trailing bodies. Oscillations create dynamic forces in a platoon, such as suction and compression, which are taken into account by transient analysis. Since the steady cases failed to reproduce these forces on the oscillating vehicle, steady and transient drag comparison for the middle vehicle showed about 75\% difference at normalized time of 1.5 for reduced frequency of 0.4. Drag oscillates for every platoon member and remains cyclically repeatable after the first cycle. Higher reduced frequency increases the drag amplitude for the oscillating member. For the rest of the platoon, changing frequencies does not show significant effects. At very small separation gaps, pressure loss occurs resulting in sudden change in the drag. Drag peaks are mostly governed by the dynamic forces, therefore transient methods should be used for studying the oscillating effects in a platoon.