Turbojet Intake Fan: Acoustic

Acoustic in a Turbojet Intake Fan CFD Simulation

Description

This project simulates airflow inside a turbojet, examining the acoustic waves and sound generated within it using ANSYS Fluent. The incoming airflow is defined at a pressure of 85,416.92 Pa and a temperature of 283.9524 K, derived from the relevant governing equations.

The model includes a turbojet fitted with a fan at its inlet, rotating at 2000 rpm about the X-axis. A dedicated airflow region surrounding the fan was defined and modeled using the Moving Reference Frame (MRF) approach to capture this rotational motion. The turbojet itself moves through the air at Mach 0.5 — since this exceeds the commonly used Mach 0.3 threshold for treating flow as compressible, the simulation accounts for compressibility accordingly, with a density-based solver applied and air density defined via the ideal gas law. The surrounding airflow domain was assigned a pressure far-field boundary condition at Mach 0.5.

The 3D geometry was built in Design Modeler, consisting of the turbojet body with its internal fan positioned within a cylindrical computational domain representing the surrounding airflow. The region immediately around the fan was defined as an independent computational zone, allowing the fluid rotation induced by the fan to be captured through the frame motion method, while the full surrounding cylindrical domain carried the pressure far-field boundary condition. The domain was meshed in ANSYS Meshing using an unstructured grid totaling 3,723,166 elements.

Methodology

Acoustic behavior was modeled using the Broadband Noise Sources model, with reference values set to match standard air properties: a reference density of 1.225 kg/m³, a reference sound speed of 340 m/s, and a reference acoustic power of 1×10⁻¹² W.

Conclusion

Results include contours and vector fields for velocity, pressure, temperature, Acoustic Power Level (dB), and Surface Acoustic Power Level (dB) throughout the domain, offering detailed insight into the turbojet's acoustic behavior. As air strikes the fan and its surrounding wall, the resulting acoustic parameters become most clearly defined in the region immediately downstream of the fan, with the Surface Acoustic Power Level concentrated along the fan surface itself — identifying it as the dominant noise source in this system.

Plots of Acoustic Power Level and Surface Acoustic Power Level taken along the domain's centerline further clarify the precise magnitude and distribution of acoustic activity downstream of the fan, providing a clear quantitative picture of how the fan's rotation drives the turbojet's overall acoustic signature.