The lift force is the reason for the flight of the aircraft and the rotation of the turbines. In this article, we will examine the lift force, its importance, how it is created, and its effect on airfoils. We will compare experimental, analytical, and simulation methods for calculating this force. We will describe the average lift force and its components and discuss the capabilities of ANSYS Fluent in calculating this force.
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What is Aerodynamics Lift?
The lift force is produced by the fluid flow when passing over the surface of the objects, perpendicular to the ground and contrary to the force of weight, this force is the reason the plane stays in the air, so in the design of the planes, efforts have been made to maximize the lift force. On the other hand, engineers try to minimize the impact of lift force on cars because an increase in lift force decreases the friction between the car and the ground, which in turn reduces the car’s driving force.s
How is Lift Generated?
If the fluid pressure at the bottom of the body is greater than at the top of the body, an upward force, lift, acts on the body. According to Newton’s third law, when the fluid flow creates an upward force on the body, an equal and opposite downward force is exerted on the fluid. This force manifests in the fluid flow after interacting with the body.
In addition to the lift force, when the fluid passes over the object, it also exerts a drag force, which is in the direction of the fluid’s collision with the object, for more information, refer to “What is Drag in Aerodynamics?“.
When fluid flows over the surface of an object, it applies both lift and drag forces to it. As a result of these forces and their reactions, the fluid velocity decreases slightly in the horizontal direction, and the fluid gains a downward component of motion in the vertical direction.
How Do Airfoils Generate Lift?
Engineers have developed airfoil designs to create objects that produce maximum lift force and minimum drag force in interaction with fluid flow. The shape of an airfoil can be observed in nature, resembling the cross-section of birds’ wings.
Airfoil design has been associated with many challenges. Experimental methods such as wind tunnels have helped significantly in the design process. In a wind tunnel, fluid flow is generated by fans, and smoke is introduced at several points to visualize the flow. By observing the smoke around the airfoil, researchers gain a proper understanding of the fluid flow. Additionally, by installing sensors on the airfoil, various parameters can be measured.
With the increase in computing power, airfoil design using CFD (Computational Fluid Dynamics) simulations has shown good results. Engineers have been able to measure lift and drag forces with CFD simulations and obtain results similar to experimental findings.
Potential flow methods, in which the fluid is assumed to have zero viscosity, have historically been widely used. In these methods, variables are defined to simplify the solution of the Navier-Stokes and continuity equations. On the other hand, mathematical transformations are used to convert complex airfoil shapes into simpler geometries, such as circles, to facilitate easier equation-solving.
Another explanation of the airfoil function uses Bernoulli’s equation, again assuming inviscid flow. For more information about Bernoulli’s equation, refer to “What is Static Pressure?”. This method states that the fluid flow is divided into two parts, above and below the airfoil, which then reunites after the airfoil. It claims that the path the fluid takes above the airfoil is longer, so its speed is higher and, according to Bernoulli’s equation, its pressure is lower. Consequently, the pressure below is greater than above, producing lift force. While this explanation can be found on many sites, it is incorrect and should not be relied upon.
The cross-section of an airfoil and the flow of fluid over it are shown. The figure is illustrative, and the actual fluid flow can be much more complicated than this.
Apart from airplane wings, airfoils are widely used in turbomachines. Unlike airplane wings, some turbomachines operate with liquid fluids, which require their own specific airfoil designs. In reality, fluid velocity distribution and pressure distribution around objects are complex and require advanced CFD simulations or experimental tests. In modern times, analytical methods for airfoil calculations are rarely used.
The equation for Lift Force
A standard equation is defined for lift force:
Where Fl is the lift force [N], Cl is the lift coefficient [dimensionless], ρ is the density of the fluid [kg.m-3], V is the freestream velocity [m.s-1] and A is the reference area [m2].
The reference area depends on the shape of the object and its orientation relative to the fluid flow. There are many tables and graphs showing the lift coefficient as a function of Reynolds number. To use this formula, knowing the shape of the desired object and the direction of the fluid flow, one should refer to reference books to find the appropriate reference area and lift coefficient. If your specific shape is not in the references, you can resort to experimental methods or CFD simulations.
Aerodynamics Lift Force Simulations by ANSYS Fluent
CFD simulation of fluid flow around objects and the calculation of drag force present many challenges. The flow can be compressible or incompressible, smooth or turbulent, and ultrasonic or infrasonic. Additionally, convergence problems usually arise in such simulations. ANSYS Fluent has demonstrated its ability to calculate lift and drag forces in many projects carried out by major aircraft companies. This software offers numerous capabilities for simulating turbulent flows, which is one of the most important problems in aerodynamics. It includes both RANS (Reynolds-Averaged Navier-Stokes) and LES (Large Eddy Simulation) models, and it is possible to adjust the parameters of these methods to work effectively.
At CFDLAND, we have completed many CFD projects with ANSYS Fluent. Maybe one of these projects will work for you. You can order your CFD projects in ORDER CFD PROJECT. Trust the ability of our experts.
Velocity contour and streamlines of fluid flow around the airfoil, adopted from “Suction Slot On 3D Airfoil CFD Simulation”
Click to access the Aerodynamics CFD Projects.
Conclusion
In conclusion, lift force is very important in the design of airplanes and turbomachines. This force can be calculated using experimental, analytical, and CFD methods. Analytical methods are based on many simplifying assumptions and do not capture the details of fluid movement around the object well. Experimental methods are very accurate, but they are expensive. The CFD method has its own complications, but with a computer and suitable experts, it is possible to design, optimize, and study objects and airfoils relatively quickly and at a lower cost.