Lift in an airplane can be explained with an understanding of Bernoulli’s principle and the Venturi effect.
The Venturi effect explains how the air pressure decreases and velocity increases at the constriction.
CFD simulation can explain how Venturi tubes can function at different flow velocities.
Among the many features desired in airplane components, the provision for maximum lift is the primary one. However, many factors come into play during the design–velocity and pressure being the primary ones. The behavior of how air flows across the top and bottom surface and at different regions of the wing affects the airplane's aerodynamics. One way to understand this influence is through a detailed understanding of the impact of the Venturi effect in airplane design. In this article, we will learn how this effect, as well as Bernoulli’s principle, can impact CFD simulation during airplane design.
Explaining Bernoulli’s Principle and the Venturi Effect in Airplanes
What is the impact of the Venturi effect on airplanes? Is it the reason an airplane lifts off? To answer that, we must start by exploring Bernoulli’s principle.
Bernoulli’s principle can be applied in aerodynamics to explain lift and is simply the reiteration of the principle of conservation of energy. According to Bernoulli, when the velocity of the fluid increases, there is a decrease in pressure and vice versa. This principle is most commonly expressed as:
Here, v is velocity, ρ is density, and p is pressure. The term gz is the gravitational potential energy.
The above statement is an important explanation for lift generation in aircraft. When the air flows over the airfoil, it is shaped so that the air on top of the airfoil flows faster than at the bottom. The high speed of air decreases the pressure around the top airfoil surface. The pressure at the bottom of the airfoil becomes greater than at the top. This difference (net pressure is positive in the upward direction) creates lift.
A common assumption made when establishing a relation between the Venturi effect and Bernoulli's principle is that when the air flows over the top surface of the airfoil, it acts like half the Venturi tube. The implication of increased speed and low pressure is then applied, which is true for Bernoulli's principle as well as the Venturi effect. However, the problem is that airfoil is not a Venturi tube and the above assumption doesn’t correctly represent the velocity and flow behavior in the Venturi tube. It also entirely ignores the shape as well as velocity and pressure at the bottom surface of the airfoil.
Thus, in its entirety, the Venturi effect can not completely explain lift. However, the one situation where it could work is if all the parameters in the flow field are known.
Utilizing the Venturi Effect in Airplanes
The above explanation of Bernoulli’s principle can, however, come in favor of light aircraft where the Venturi tube is installed on the side of the fuselage. The tube inlet creates a suction, allowing wind to pass through the tube. As the air passes through the central constriction, the air molecules speed up to facilitate the passing of the same amount of air molecules from the outlet as entered through the inlet. This ensures the conservation of mass for incompressible, inviscid fluid. Once passed through the constriction, the air molecules slow down again during the exit.
The suction created by the Venturi tube in airplanes can play an important role in operating gyroscopic instruments. In simple early aircraft, the Venturi tube provides a low-cost option for creating suction. However, there has been a decline in the use of these tubes commercially, given that they require substantial height and airflow speed to take effect.
CFD Analysis of the Venturi Effect in an Airplane
When it comes to the study of lift in an airplane or the suction through the Venturi tube, CFD analysis is an effective method. With CFD tools, multiple simulations can be run to analyze the function of the Venturi tube at different speed ranges. The laminarity or turbulence in the flow behavior can be visualized, while the different velocities and pressures can be calculated for compressible or incompressible flow analysis. With ideal airflow analysis, airfoil design and Venturi devices can be ideally optimized.