In cities, many companies are now considering urban air mobility vehicles, from drones for parcel delivery to air taxis for travel. However, gusts of air, especially around buildings, can create pockets of wind with different properties, which could cause areas of turbulence and affect the stability of the aircraft. A research team led by Dr Abdulghani Mohamed, senior lecturer at the Royal Melbourne Institute of Technology (RMIT University) in Australia, studied the flow fields that form in urban environments. They discuss how regulations could be put in place to make sure that these drones are safe for future use.
Within urban environments like cities, companies are increasingly looking to operate flying vehicles such as drones, which can make deliveries, or even air taxis to transport passengers. These uncrewed air vehicles, or UAVs, offer the chance to improve travel infrastructures and provide efficient services. However, with these developments comes the requirement to ensure that they are safe to use in an urban environment.
These crafts will need to fly between buildings, around various parts of the infrastructure, and perhaps, even through windows to make deliveries. As gusts of wind blow around these different obstacles, they can create flow fields and cause vehicles to be affected by turbulence. Many of us will have experienced turbulence on a larger aeroplane before. Because UAVs land and take off at a lower speed, they are even more affected by turbulence, and within cities, UAVs risk collisions.
To overcome these risks and ensure these drones and air taxis can be safely regulated, Dr Abdulghani Mohamed, senior lecturer at the Royal Melbourne Institute of Technology (RMIT University) in Australia, and his team create simulations of the flow field around buildings. They consider how UAVs experience this and characterise how this varies with different design choices in the UAVs. This leads the researchers to outline how regulations could help ensure a safer future for urban air mobility vehicles – and how this might impact future designs for UAVs in urban environments.
Modelling turbulence in urban environments
Turbulence occurs when there are chaotic and very nonlinear air flows. When considering how UAVs might fly in urban locations, the researchers look at the atmospheric boundary layer, or ABL. This is the layer from the surface of the Earth up to where the wind flow is no longer altered by the roughness of the ground or by any buildings or other structures on the surface. Using three-dimensional computational fluid dynamics simulations, the team modelled how the ABL interacts with buildings. These show that there are regions of coherent turbulence generated by a building, with the turbulent region having similar dimensions to the structure itself. These dissipate as they travel through the environment, creating a mixed turbulent wake downstream. Therefore, as the UAV travels through the ABL, it experiences regions of different turbulence.To ensure drones and air taxis can be safely regulated, Mohamed and his team create simulations of the flow field around buildings.
Previous models of gusts have often been one-dimensional, meaning that they don’t fully account for gradients in the gusts or the variation of wind velocity over the horizontal or vertical distance, known as wind shear. However, wind shear can be produced within cities because they have many buildings of different sizes. This creates gusts that can generate large forces in the plane in which the UAV is flying, which affect the flight axes (yaw, pitch, and roll). Near these buildings, the UAVs can also experience gusts that are much larger than the ambient airflow and can very quickly lead to catastrophic effects. To understand this further, the researchers look to characterising three-dimensional gust fields in urban environments, allowing them to consider the flights of these urban air mobility vehicles.
How does turbulence affect UAVs?
To model the gusts around buildings, the researchers create a computational fluid dynamic (CFD) simulation of a cuboidal building, like a medium-rise building or office block frequently seen in urban settings. The simulation uses boundary conditions to replicate the conditions of the ABL. Different gusts and the various effects the UAV will experience based on its flight path and the direction of the wind are also considered. For example, gusts may blow around buildings, creating a streamwise gust that could cause the aircraft to roll, or there could be a gust of wind over the top of a building, which – depending on the trajectory of the UAV – could induce pitch or roll.
The team’s simulation looks at two flight paths, both where the UAV is flying over the roof of a tall building. They consider one path where the aircraft is travelling directly into the gust head-on, or at a 0o flight path angle, and another at a 45o flight path angle, which will experience greater roll. These simulations showed that, in the flow of wind around the building, there was a large increase in the wind velocity closest to the leading edge of the top of the building or the second edge of the building as the UAV flies over. The change in wind velocity occurs very sharply, so the UAV experiences a large change as it approaches the building edge. The angle of the flow in the wind field has a smaller change at the leading edge of the building; however, this has a more significant increase at the trailing edge of the building or the first edge that the UAV passes over. Therefore, when the UAV flies over a building, it experiences different areas of wind, which can influence its flight trajectory.
The researchers also considered how the flight paths differ for different types of UAVs – fixed-wing UAVs, which have wing structures similar to that seen on commercial aeroplanes, or rotor UAVs, which use small propellors to control the flight of the drone. The two different types of UAVs will perceive the gusts differently. For the fixed-wing UAVs, the team calculated that the conventional sensing and control systems may potentially take too long to react to mitigate the gusts of wind it experiences, as the team shows that the gusts can affect the craft’s flight path in as little as 0.52 seconds. However, there are potential new designs that may improve the steadiness of a UAV.
Rotary-wing UAVs operate differently to fixed-wing UAVs, since the rotors create lift to get the aircraft off the ground and then tilt to move the craft in different directions. Therefore, as the rotor angle varies, the angle at which it interacts with the gust of wind also alters during the flight. The team found that a rotorcraft has a more variable reaction to turbulence. This is because the rotor disk is affected by turbulence from all directions and has to compensate for a larger variation in the flight path due to gusts compared to fixed-wing designs.The team shows that the gusts can affect the craft’s flight path in as little as 0.52 seconds.
A safer future
Drawing from the results of the team’s CFD simulations of the flow around the building and their research on the different design parameters for the UAV, the researchers propose specific recommendations on how to regulate UAVs. As the design choices of rotary or fixed wings respond differently to the disturbances, the possibility of a hybrid model designed with both types could allow drones to fly close to buildings. Further research is required to establish how this would interact with varying weather conditions. UAV manufacturers also need to minimise the surface area of designs so that the drones can fly carefully around buildings without experiencing large turbulent forces.
The researchers also suggest how this could be incorporated into safety certifications. For urban air mobility vehicles to be viable, they must be able to overcome the turbulence in different seasonal conditions, so that they can operate all year round. The UAVs require a large power reserve, as well as a large open space for take-off and landing, to allow room for the propellors to respond to wind conditions and correct the drone’s flight path. Also, as the drone travels to and from its power hub, known as a vertiport, the vertiport itself needs to be designed to minimise turbulence. Currently, there is a lack of published research on the best vertiport design, or which building shapes result in minimal turbulent flows. More research is required before UAVs become a feature in our cities.
Overall, Mohamed and his team’s simulations show how urban environments offer a unique set of challenges for UAVs due to the turbulent flows generated by infrastructure. Current designs are susceptible to these gusts in specific scenarios and may not have the response times necessary to overcome these flows. They also discuss how regulations should reflect these challenges, both in the structure of the device itself and the vertiports it uses. While there are exciting possibilities for the development of these technologies, further work must be done to ensure the steadiness of the UAVs and the safety of deliveries and future passengers. The researchers’ work has made an important step in that direction.
Personal ResponseWill we see urban air mobility vehicles in the near future? If so, how much future work and regulation must be done to make this exciting possibility a reality?
There is scepticism; however, there is a business case that supports this new technology and numerous companies are racing towards establishing themselves in this market. There are many challenges that will have to be overcome for a reliable and safe ‘air taxi’ service, and success of the technology will depend on public acceptance and uptake of the service.