Introduction
Formula 1 race cars turn at lightning speed, producing lateral
forces up to 4.5 times that of gravity. By wind power alone, yachts could
round the globe. The lifting force created by an airfoil is all the
reason for these incredible features. But everything comes at a cost, so do the
lifting and it is called drag.
It is frequently important for fluid flow simulation to
determine the forces which the fluid exerts on the body — for example, lifting
and dragging forces on an airfoil or a vehicle. Engineers may use these
strengths to measure design efficiency and aerodynamic performance.
Lift and
Drag
If the flow of fluids is passing through a body, the force on
the surface would be exerted. As known
in the motor racing world, Lift or downforce is a force produced
perpendicular to the direction of a moving object (gas or liquid). The same
effect occurs in a stationary object while fluid is moving, such as the wind
tunnel airfoil. Airfoils are the most efficient forms to date that
allow for lifting while reducing the drag at the same time.
The lift a coefficient is a number that aerodynamicists use to model all of the
complex dependencies of shape, inclination, and some flow
conditions on the lift. This equation is simply a rearrangement of
the lift equation where
we solve for the lift coefficient in terms of the other variables. The lift
coefficient Cl is equal to the lift L divided
by the quantity: density p times half the velocity V squared
times the wing area A.
The quantity one half the
density times the velocity squared is called the dynamic
pressure q.
Drag depends on the density of
the air, the square of the velocity, the
air's viscosity and
compressibility, the size, and shape of
the body, and the body's inclination to
the flow. In general, the dependence on body shape, inclination, air viscosity,
and compressibility is very complex.
One way to deal with
complex dependencies is to characterize the dependence by a single variable.
For drag, this variable is called the drag
coefficient, designated "Cd." This allows us
to collect all the effects, simple and complex, into a single equation. The
drag equation states that drag D is equal to the drag
coefficient Cd times the density p times half
of the velocity V squared times the reference area A.s
Drag is an inevitable result of a fluid moving by an entity. Drag
is the force generated by an object moving through a fluid in parallel and in
opposition to its direction. Drag can be divided into two components followed
as:
· Form Drag
- based on the shape of a fluid entity.
· Skin
friction - The wall shear stress depends on the viscous friction between a
moving surface and a liquid.
Pressure force and viscous force are two distinct contributors
to lift and drag forces. The pressure force, also known as
pressure-gradient force, is the force caused by a pressure difference over a
surface. The viscous force is the frictional force that acts in the
opposite direction of the flow. Depending upon the type of flow,
the magnitudes of pressure force and viscous force can differ significantly.
For example, the movement around a moving vehicle is often dominated by pressure
force.
Induced drag is induced at wingtips when high-pressure air
from the lower wing surface is driven by a desirable pressure gradient (high to
low) around to the low-pressure air on the upper surface, culminating in
wing-tip vortices.
Wing-Tip
Vortex Evident in Wake for Panel Method Calculation
On humid days, these vortices can be seen as water-vapor
trails, particularly during high-lift conditions when jetliners take off and
land. End plates on racing car wings and winglets on airplanes help minimize wing-tip vortices' impact, lowering induced drag.
Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the tunnel's test section, and the air is made to flow past the model. Various types of instrumentation are used to determine the forces on the model. The most basic type of instrument is force balance.
Force balances are used to directly measure the aerodynamic forces and moments on the model. On this page, we will discuss the measurement of the drag force to demonstrate the basic principles involved with force balances. This configuration is called a one-component balance since it only measures one force. There are more sophisticated, three-component balances that can simultaneously measure lift. drag and pitching moment. A six-component balance is required to measure all three forces (lift, drag, and side) and three moments (pitch, roll, and yaw) that determine an aircraft's motion through the air.
This method of determining drag requires an electronic strain gauge and a laptop computer to convert the electronic output into units of force. To accurately determine the aerodynamic forces and moments on an aircraft model in a "real" wind tunnel requires even more sophisticated instrumentation and larger computer systems for data reduction and display. Multiple electronic strain gauges are often placed inside the model, or on a measuring platform outside the tunnel. Multiple gages permit the determination of multiple forces and moments during the same test.
Angle
of Attack
Geometry is usually not fully compatible with
the direction of the flow. The angle between the principal line of geometry and
the input flow is known as the Angle of Attack. It is denoted by the
Greek letter alpha. The angle of attack is used in aerospace
engineering since it is the angle between the airfoil chord line and free
stream direction. The figure below shows the link between the
lift, drag, and angle of attack of an airfoil.
Lift and
Drag coefficient
The angle an airfoil makes with its direction of travel
through a fluid determines lift and drag. This is referred to as the angle
of attack, angle of incidence, or alpha. It is standard practice while testing
an airfoil to perform an alpha sweep, which records the lift and drag of an
airfoil at different angles. Lift and drag are usually depicted as
dimensionless quantities.
Graph
comparing simulation and experimental data of the lift coefficient on an
airfoil at different angles of attack
As seen in the graph above, there is no discernible difference
between the computational and experimental results within the range of angle of
attack values used in this simulation. The experimental results appear
to show that the airfoil stalls at high angles of attack.
Airfoil NACA 0010-35
It is, as the asymmetrical section was chosen from the UIUC Airfoil Database(n.d.) to simulate.
For this, we would consider
an airfoil (NACA 0010-35) with different angels of airflows based on ANSYS with
2D CFD (Computational Fluid Dynamics) simulation and study the design
process of various aerofoils and their flow simulation to understand how they
work.
A C-shape domain has been sketched around the airfoil, and the size was 12.5 m for both the semicircle and the rectangle.
The geometry setup of Airfoil present in the center of the figure
C-Mesh was applied to both airfoils to guarantee the accuracy of the model by getting more refined mesh over the edge of the trail and the surface of the airfoils.
The figure shows the C-mesh of the simulation
Input and Boundary conditions
It involves inlet, outlet, and wall boundary the velocity
components are calculated for each angle of attack case as follows. All the
outermost boundaries are considered as the “Pressure Far Field” boundary
conditions in Ansys-Fluent.
|
Input |
Value |
|
Velocity of Flow |
50.0 m/s |
|
Operating temperature |
300 k |
|
Operating pressure |
101325 Pa |
|
Turbulence Model transition |
K-e |
|
Density of fluid |
1.0 kg/m3 |
|
Kinematic viscosity |
1.4607 x 10-5 |
|
Reynolds number |
3.62 x 106 |
|
Angle of attack |
-5⁰ to
+20⁰ |
|
Fluid |
Air as an ideal |
Stalling point
The table shows that the angle of attack was
increased to reach the maximum CL (Stalling point). Referring to the
table and the figure is given below, it can be seen that as the angle of attack
increases, the CL increases as well until the angle of attack is set
to 14 then the CL starts to decrease, so the angle of attack 14 is
called the stalling point where it has the maximum CL of the
airfoil.
Maximum Efficiency of the airfoil
The efficiency is defined
as the ratio between CL and CD at each angle of attack,
so referring to the below figure, it can be seen that the maximum efficiency is
reached at the angle of attack 4 for the airfoil.
By
Shubham Handibag, Harsh Mehta, Harshvardhan Kolekar, Harshwardhan Thakare, Shridhnyan Haval
