Part 3: From Flow Simulation to Acoustic Insight with Cradle CFD and Actran

In Part 1, we introduced aeroacoustics as the sound generated by moving air. In Part 2, we compared the main simulation strategies and looked at why the right method depends on the design question. In this final part, we focus on what happens next: how Cradle CFD and Actran turn those methods into a practical workflow for engineering teams.

Turning Multiphysics into Guided Workflows: Cradle CFD and Actran

Aeroacoustic simulation is not just about selecting a solver. It is about connecting the flow physics, the acoustic source definition, and the propagation environment in a way that gives engineers usable answers. In a hybrid strategy, the workflow is intentionally split: Cradle CFD resolves the flow behavior, while Actran serves as the core acoustic solution for aeroacoustics investigations.

In a typical Cradle CFD–Actran workflow, scFLOW provides the unsteady flow field, and Actran focuses on the acoustic side of the problem. This separation is especially useful when the flow creates the noise but the acoustic field does not significantly alter the flow, which is often the case in practical aeroacoustic design studies.

From CFD Results to Acoustic Sources

The workflow begins with source extraction. Aeroacoustic sources are computed from the computational fluid dynamics (CFD) mesh, mapped and integrated onto an acoustic mesh, and then transformed from the time domain to the frequency domain through a Fourier transform when frequency-domain propagation is required. The resulting source terms are typically Lighthill or Möhring sources, which are then propagated in Actran.

This division of responsibility is important. The CFD model can concentrate resolution where the unsteady flow generates noise, while Actran can focus on how that noise travels through the surrounding medium. If the propagation setup changes, for example, because the receiver position, acoustic treatment, or surrounding geometry changes, engineers can often reuse the CFD-derived sources instead of restarting the full flow simulation.

Choosing the Right Actran Propagation Technology

The right Actran propagation technology depends on the characteristics of the fluid medium, particularly the presence of mean flow, flow heterogeneities, obstacles, boundary layers, and ducts. Standard Actran finite element technology is well suited when only a mean flow is considered and when heterogeneities remain smooth, even in the presence of obstacles. Actran DGM is better adapted to complex flow patterns, such as rotational mean flow, strong heterogeneities, obstacles, and boundary layers. Actran TM is dedicated to high-order mode propagation in ducts, especially where high flow speeds and heterogeneities are present.

Where the Workflow Applies

For fan noise in a free field, with no flow or fluid heterogeneity in the propagation medium, the Lighthill approach can be used with inputs from an unsteady scFLOW run. This workflow can capture both tonal and broadband noise, making it useful for evaluating the acoustic signature of rotating machinery such as fans.

When the propagation medium becomes more complex, Actran DGM is the stronger choice. One example is open-rotor engine noise around an aircraft fuselage, where acoustic waves interact with the aircraft environment and refraction effects become visible near the fuselage boundary layer. Jet-engine exhaust noise is even more demanding because the jet introduces high-temperature gradients and complex flow dynamics that can strongly refract acoustic waves.

For ducted propagation, Actran TM is well-suited to applications such as short nacelle inlets, where flow velocities can easily reach Mach 0.3 and higher-order acoustic modes become important. In these situations, engineers need a propagation approach that captures the interaction among the flow, internal acoustic pressure, and the acoustic near field around the inlet.

For vehicle wind noise, Actran SNGR offers a faster route because it can work from simplified steady-state RANS CFD runs, making large-object investigations more affordable.

A full-vehicle wind-noise study, for example, can reveal how aerodynamic noise varies across the vehicle, including higher levels around the rear wheel than the front wheel in representative operating conditions such as 100 km/h. The same approach can also support high-frequency side-mirror noise studies, where mean-flow results can be used to predict acoustic behavior with strong experimental correlation while reducing CFD runtime by an order of magnitude.

The Practical Takeaway

The strength of the Cradle CFD–Actran workflow is that engineers can match the acoustic strategy to the physics of the problem. scFLOW captures the unsteady or steady flow information needed to define the aeroacoustic sources. Actran then propagates those sources using the approach that best fits the environment, whether that is free-field fan noise, propagation around a fuselage, strongly refracted jet-exhaust noise, high-speed duct acoustics in a nacelle, or vehicle wind-noise and side-mirror investigations using SNGR.

If you are starting with the basics, read Part 1 to understand how air becomes sound, and if you want to compare simulation strategies, read Part 2 to choose the right method. 

Written by Vicky Tsianika, Product Management Director, and co-authored by Nicolas Driot, Senior Principal Product Manager.