Acoustic CFD Simulation in a Turbojet (Intake Fan), ANSYS Fluent Training

Project Description

The present problem simulates the air flow inside a turbojet and examines the acoustic wave and the sound produced inside this turbojet by ANSYS Fluent software. The model includes a turbojet that has a fan at its inlet. This fan is rotating at 2000 rpm and around the X-axis in the current model. Therefore, an area of airflow is defined around the fan, which is modeled using frame motion. This turbojet is moving in the air with a Mach number of 0.5, which indicates that the flow can be considered compressible; Because the value of Mach number is more than 0.3.

Mach number is equal to the ratio of the velocity of the object in the fluid to the velocity of sound in the same fluid. Therefore, in this model, a density-based solution is used and for the air flow in the model, the density is defined as ideal gas. Acoustic model is used in the software to study sound or acoustic waves. In general, mechanical waves within a fluid are caused by the vibrations and reciprocating motions of the fluid layers. For example, when a layer of air moves forward in a certain direction, the next fluid layer pushes itself forward and the layer itself returns to its original position.

Project Description

These reciprocating movements of the air layers must continue until the energy within the air flow is depleted. Now, if this number of reciprocating movements reaches more than 16 times per second, sound is produced. In fact, when we hit the surface of a solid object with our hand, the layers of air between our hand and the surface begin to move back and forth, if the number of these round trips exceeds 16 times per second, the sound is produced by our hand hitting a solid surface.

In the present simulation, the defined air flow around the turbojet has a pressure far field boundary condition of Mach number of 0.5. Also, the pressure and temperature of the air flow are 85416.92 pascal and 283.9524 K, respectively, which are obtained according to the following equations. The Broadband Noise Sources model is also used to define the acoustic model. Definitive density is equivalent to air density, i.e. 1.225 kg.m-3, and sound speed is equivalent to sound speed in air, i.e. 340 m.s-1, and reference acoustic power is equal to 1e-12.

Geometry & Mesh

The current model is designed in three dimensions using Design Modeler software. The model consists of two parts, which include the body of a turbojet with a fan inside it, which is located inside a computational domain for air flow in the form of a cylinder. The area around the fan is defined as an independent computational area so that the area under rotation of the fluid due to the fan rotation can be defined using the frame motion method. Also, the entire defined cylindrical space around the turbojet body is defined as a pressure far-field boundary condition. The following figure shows a view of the geometry.

The meshing is done in three dimensions using the ANSYS Meshing software. The mesh type is unstructured and the element number is 3723166. The following figure shows an overview of the mesh.

Acoustic CFD Simulation

To simulate the present model, several assumptions are considered:

• We perform a density-based solver; Because Mach number in the model is more than 0.3 and the fluid can be assumed to be compressible.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

A summary of the defining steps of the problem and its solution is given in the following table:

Models
Viscous		k-omega
	k-omega model	standard
Acoustics Model		Broadband Noise Sources
	far-field density	1.225 kg.m-3
	far-field sound speed	340 m.s-1
	reference acoustic power	1e-12 W
	number of realizations	200
	number of Fourier modes	50
Boundary conditions
Far Field		Pressure Far Field
	Mach number	0.5
	gauge pressure	85416.92 pascal
	temperature	283.9524 K
Body		Wall
	wall motion	stationary wall
	heat flux	0 W.m-2
Fan		Wall
	wall motion	stationary wall
	heat flux	0 W.m-2
Methods
Formulation		Implicit
	flow	second order upwind
	turbulent kinetic energy	second order upwind
	specific dissipation rate	second order upwind
Initialization
Initialization methods		Standard
	gauge pressure	85416.92 pascal
	x-velocity	168.8385 m.s-1
	y-velocity & z-velocity	0 m.s-1
	temperature	283.9524 K

Acoustic Results

At the end of the solving process, two-dimensional contours related to velocity, Mach number, temperature, pressure and density, as well as velocity vectors are obtained.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.