Wake vortex turbulence is the rotating pattern of disturbed air created by an aircraft as it moves through the atmosphere.
Wake vortex turbulence leads to increased drag, reduced lift, and generation of rolling and pitching moments.
CFD simulation provides insight into the complex flow pattern of the wake vortex turbulence and helps designers come up with a mitigation strategy to reduce its impact on aircraft performance.
Wake vortex turbulence is a common phenomenon observed in aircraft during flight. When the aircraft moves forward, the wings push the air downwards and create a rotating pattern of the air—wake vortex, which trails behind the aircraft. The vortex created is strong and can cause a turbulent effect on other aircraft flying in close proximity. The study of wake vortex turbulence is, thus, essential for developing strategies to ensure aircraft safety and efficiency.
In this article, we will discuss wake vortex turbulence in aircraft and the role of computation fluid dynamics (CFD) in providing insight into the physics behind such turbulent effects.
Wake Vortex Turbulence and Its Effects
In aircraft, wake vortex turbulence is generated when the wings create lift. During flight, there is a pressure difference between the upper and lower surface of the wings, which allows the aircraft to stay airborne. The air flows faster at the upper wing surface, creating a lower pressure compared to the lower surface of the wing. To balance the difference in pressure, the air along the lower wing surface is drawn upwards, which flows around the wingtip to create a circulating pattern of air masses. This is the vortex that trails behind the aircraft.
These vortices, or primary wake, sink and move away from the flight path due to wind and gravity. The sinking motion causes the vortices to interact with the surrounding air, leading to turbulence. The turbulence associated with the wake vortex may be a major safety concern for other aircraft that follow the same flight path or fly at a close distance. This is especially true in the case of smaller aircraft coming closer to the path of heavier aircraft because the intensity of the vortex produced is much stronger by wings producing more lift.
Effects of Wake Vortex Turbulence
Effects of Wake Vortex Turbulence on Aircraft Performance
Wake vortex turbulence mixing with air
Loss of lift, increased thrust, increased fuel consumption
Separation of boundary layer due to turbulence
Loss of airspeed, increased thrust, reduced fuel efficienc
Roll and pitch moments
Rolling and pitching effects caused by vortices
Changes in altitude or vertical speed
The wake vortex turbulence leaves a trail of disturbed air during the flight, disrupting the airflow for the following aircraft and leading to reduced lift generation.
The turbulent air disrupts the smoothness of the airflow over the wings and increases drag. This induced drag reduces the net lift generated by the wing.
The sinking vortex induces a downward flow of air, affecting the airflow over the wings of the following aircraft. This is called downwash, and it reduces the effective angle of attack of the wings – less lift is produced.
When the wingtip vortices of the leading aircraft interact with the wings of the following aircraft – wingtip vortex interaction – the disruption in smooth flow will lead to flow separation, increased drag, and reduced lift.
Wake vortex turbulence causes the addition of aerodynamic forces to increase drag in the following aircraft.
Induced drag is directly related to the lift generated by the wings. As the aircraft comes in the path of a wake vortex generated by the leading aircraft, the disturbance in airflow causes a reduction in lift and an increase in induced drag.
When the aircraft encounters a wake vortex, a frictional resistance is caused between the moving aircraft and the turbulent airflow. The turbulent airflow causes disruption in the boundary layer and may lead to flow separation. The flow separation creates an area of low pressure and the turbulence increases the shear stress between the air and the aircraft surface, leading to higher drag.
Roll and Pitch Moments
Aircraft may experience rolling and pitching moments when it comes in contact with the disturbed airflow from wake vortex turbulence, affecting the stability and control of the aircraft.
When the aircraft passes through the wake vortex, the disturbed airflow may cause an imbalance in the lift distribution between the wings. The difference in lift may generate a rolling moment in the aircraft.
The interaction between the disturbed airflow and the tail surface, such as the horizontal stabilizer, can cause changes in the aerodynamic forces experienced by the aircraft. These changes in forces alter the balance of moments around the aircraft’s center of gravity, resulting in a pitching moment – causing the aircraft’s nose to pitch up or down.
These various effects of wake vortex turbulence can be analyzed using CFD simulation. A deeper insight into the behavior of wake vortex turbulence can help engineers develop risk mitigation strategies for better aircraft performance.
Mitigating Wake Vortex Turbulence Effects With CFD Simulation
CFD simulation takes the mathematical modeling approach to accurately predict and analyze the behavior of wake vortex turbulence. CFD simulation allows solving the governing fluid flow equation to simulate the airflow around the aircraft and predict the formation and behavior of the wake vortex. This allows engineers to gain deeper insight into the complex flow patterns of the vortex. Engineers can also calculate various flow parameters such as velocity, pressure, vorticity, turbulence intensity, etc., to understand the impact of the wake vortex turbulence on the surrounding flow field.
The analysis of vortices and their influence on the aerodynamic forces acting on the aircraft is essential for coming up with appropriate design modifications necessary for reducing the impact of wake vortex turbulence. By simulating different scenarios, CFD simulation can test and validate these different design optimization strategies. Through a detailed assessment of wake vortex turbulence and its impact on aircraft performance, the safety and efficiency of the flight can be ensured.