In the design of devices that are in contact with fluids, engineers usually need to consider turbulent flow in their designs. In this article, we will examine turbulence, explore why it is crucial in airplane design, discuss its types and how it is formed, and finally, explain how we can study the turbulence phenomenon using CFD methods and ANSYS Fluent to design suitable airplanes.
What is Turbulence?
Turbulence represents a complex fluid flow state, or regime, characterized by unpredictable and irregular patterns. This description of turbulence may vary based on individual perspectives, and a universally agreed-upon definition remains elusive.
The study and understanding of many fluid phenomena depend on the flow regime, turbulence, or laminar flow. The turbulence regime of the flow is determined by the Reynolds number. For more information, refer to the articles “What is Reynolds Number (Re)?” and “The Differences Between Laminar and Turbulent Flow”.
The Effects of Turbulence on an Airplane
The phenomenon of turbulence and its effect on airflow has a significant impact on the design and performance of airplanes. Some of the effects of turbulence are:
- Drag and lift forces: Turbulence affects all the forces on an airplane, with drag and lift forces being particularly influenced by turbulence. The wing of the airplane is designed to minimize drag and maximize lift, aiming to prevent flow separation, which can lead to a sharp increase in drag force.
- Structural stress: A high force applied to the body and wings of the aircraft may damage them. Additionally, turbulence in the fluid flow causes fluctuations in fluid speed, leading to vibrations in the structure of the aircraft. These vibrations can increase to the point of resonance, potentially causing parts of the aircraft structure to break. Moreover, these vibrations may lead to structural failure due to fatigue. In the fatigue phenomenon, very small cracks in the aircraft structure grow over time due to vibrations.
- Control challenges: Due to turbulence, there are frequent small changes in the forces and speed of the aircraft. This problem affects the moving blades and the control state of the plane.
- Comfort of passengers and crew: By now, you have probably experienced the vibrations in an aircraft. These vibrations are caused by turbulence in the airflow, which can cause tension among passengers. Additionally, due to the more challenging control of the aircraft in such situations, the pilot’s workload increases.
Turbulent flow can be observed clearly on the planet Jupiter. The size of the vortices is sometimes very large; for example, the diameter of the Great Red Spot, as shown in the figure, is larger than the diameter of the Earth.
Turbulence Types
Turbulence can be classified into several types based on different criteria such as the nature of the flow, the geometry of the system, and the mechanisms driving the turbulence. Here are some common classifications:
Clear air turbulence
It is usually caused by the collision of jet streams with the plane and occurs at high altitudes, in areas where there are no clouds and no visible trace of turbulence for the pilot.
Frontal turbulence
When two fronts of warm and cold air collide, the warm air rises and creates turbulence at the boundary between the two fronts. Thunderstorms can indeed form from this turbulence, particularly in the presence of significant moisture and instability in the atmosphere.
Mechanical turbulence
It is formed due to the impact of airflow with obstacles on the ground, such as buildings. This happens at low altitudes and is usually experienced during the landing or takeoff of the plane.
Mountain wave turbulence
It is caused by the impact of the wind on mountains. The wind flow goes up when it hits the mountain and then goes down.
Thermal (convective) turbulence
Due to the impact of sunlight on the earth, the surface of the earth heats up, warming the air at the surface. This air is warmer and less dense than the air at higher altitudes, so it rises, creating turbulent airflow in some areas.
Wake turbulence
Due to the way the air flows over the wing of the aircraft, a large vortex is created that moves towards the tip of the wing. This vortex increases the drag force on the wing and causes severe turbulence behind the aircraft. When a large aircraft passes, smaller aircraft that follow the same route can be strongly affected by this turbulent flow. At some airports, small aircraft are not allowed to fly immediately after the passage of a large aircraft.
To reduce wake turbulence, a blade, known as a winglet, is used almost perpendicular to the wingtip. This blade helps prevent the formation and movement of the vortex.
Wake turbulence produced by an airplane wing can sometimes be seen in the clouds.
Turbulence Simulations by ANSYS Fluent
The way turbulence currents are formed and their understanding and recognition determine from which direction the aircraft will come into contact with the turbulence current. Numerical simulation of these phenomena has many challenges because a large scope is needed to investigate weather phenomena. In addition, phenomena that occur at small scales affect much larger phenomena. For example, water vapor and differences in air humidity affect the direction of storms.
By determining the direction and intensity of turbulence, the effect of vortices and fluid flow on the aircraft can be studied. This study includes the drag and lift forces and their effect on the movement of the aircraft, as well as the stress and strain on the aircraft structure.
ANSYS Fluent software has so far shown excellent results for turbulent flow simulation, and engineers have used this software in many projects for aircraft design. There are many methods in CFD for turbulent flow simulation, including RANS and LES methods. Fluent enables all these simulation methods with details; the user can adjust the parameters of each of these methods. Also, in FSI simulations, you can calculate the effect of fluid flow in ANSYS Fluent, transfer it to ANSYS Mechanical, and observe the resulting stress and strain in the aircraft structure. For more information, refer to the article “What is Fluid-Structure Interaction?”.
We at CFDLAND have conducted numerous CFD simulations using ANSYS Fluent. one of them might be useful for you. Trust our experts and order your turbulent flow simulation projects at ORDER CFD PROJECT.
Conclusion
In conclusion, the study of turbulence is important for aircraft design for various reasons, such as reducing drag force, increasing lift force, reducing fuel consumption, and improving passenger comfort. There are different types of turbulence, many of which are caused by weather phenomena. It is possible to understand these phenomena and their impact on aircraft through CFD simulations. Engineers have used ANSYS Fluent for CFD simulations to design and study aircraft performance, and the results show that this software has significant capabilities in this field.
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