# Numerical Assessment of Side-Wind Effects on a Bus in Urban Conditions
## 1 Introduction  
Fuel consumption of vehicles is dependent on many factors, so generally, it is highly unpredictable. This fact is attributed to the properties of the vehicle such as losses from the drivetrain, rolling, drag, and external factors such as the quality and slope of the road, traffic, and wind. By specifying the scenario and the vehicle, a more precise estimation can be made to give a consumer a more accurate view of the fuel consumption. By solely focusing on the relationship between the wind and aerodynamic losses of vehicles in general, a connection can be found, which could give a more precise prediction for fuel usage.  
Drag loss calculation can be performed using multiple approaches. [PERSON] et al. [1] made on-road measurements. They have attained a route-specific drag coefficient that corresponds to the average wind direction; thus, an average yaw angle for the location was concluded. It was shown that the wind tunnel underpredicts the drag losses by 33%, compared with an on-road scenario [1].  
Side-wind studies are notably carried out on long ground vehicles such as trains, in which the high-speed stability is examined [2; 3; 4; 5]. These high-speed trains due to safety concerns need to be examined when critical conditions occur; by contrast, at lower speeds, stability is not an issue. The abovementioned papers confirm that both the Reynolds averaged numerical simulation and large-eddy simulation give accurate predictions when forces are assessed. [PERSON] et al. [6] showcased that the presence of side wind at a travelling speed of 27 m/s could increase the drag loss on vehicles.  
In urban areas, the estimation of the actual flows is challenging, due to complex building shapes, trees, and rough terrain [7]. The numerical models in street canyons show that the wind speed is often negligible at near-ground level [8]. Due to the blockage effect of the buildings, a considerable amount of the mesoscale wind speed dropped to street level [9] even in extreme cases; nevertheless, some studies [10; 11] indicated that when the blockage effect is negligible at street level, high velocities can be obtained. In a study by [PERSON] et al. [12], it can be seen that at around two-metre height, the peaking velocityis around 1.4 m/s. For cars, it was indicated that even with a slight side wind, the flow structure and drag could change considerably [13].  
Wind tunnel measurements are considered to be a baseline method and can be used for automotive measurements. However, necessary corrections have to be made such as the application of a belt system and rotating wheel [14; 15]. [PERSON] et al. [16] pointed out that detailing the tyre surface and adding rotation in numerical simulation could reduce drag by 4-5%, compared with wind tunnel measurement.  
The movement of city buses is usually restricted by urban traffic and their schedule. During their operation, they often stop and cannot even reach the speed limit of the area. Using a driving cycle dataset, the average cruising speed can be found, which represents the closest steady state of that operation and can be applied for drag loss calculations. [PERSON] et al. [17; 18] mentioned that in the city of Debrecen, the average cruising speed is 19.197 km/h.  
The scope of this assessment was to examine how much could a perpendicular wind load alter the drag losses of a city bus. The research was limited to constant travelling and wind speeds; thus, decelerations and accelerations, as well as wind gusts, were not examined. For low-Reynolds number flows, not many drag assessments have been carried out. This paper highlights what occurs with the aerodynamic loss when a bus is affected by a steady side wind.  
Based on the literature review [17; 18; 19; 20], common cruising and wind speeds in Debrecen were chosen as the input parameters. The model of the bus was created based on the schematics provided by the Public Transportation Company of Debrecen (DKV ltd.). Then, large-eddy simulations were made to examine the effect of the side wind on the bus.