Computational Aero-Acoustics

Cost effective noise prediction

Accurate simulation of flow-driven acoustics requires detailed resolution of the small, noise-generating turbulent eddies and the full spectrum of transmitted acoustic waves. CAA++ provides a suite of tools that simplifies these tasks, greatly reducing the cost in comparison to traditional CFD methods.

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OVERVIEW
FEATURES
APPLICATIONS
TECHNOLOGY
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OVERVIEW
FEATURES
APPLICATIONS
TECHNOLOGY
PUBLICATIONS
OVERVIEW
FEATURES
APPLICATIONS
TECHNOLOGY
PUBLICATIONS
OVERVIEW

What is CAA®?

Beyond traditional aeroacoustics


The CAA++® suite contains algorithms that allow acoustic simulations to focus on component parts. Acoustic-damping boundary conditions, automatic turbulent eddy-seeding at inlets, specialized acoustic solvers and a unique sub-domain capability allow computational efforts to be focused on the design feature of interest.  

CFD++®, like many other commercial CFD solvers, allows for direct prediction of aeroacoustic phenomena using LES or hybrid RANS/LES. However, such methods are often prohibitively expensive, particularly for near-wall flows, far-field propagation or scattering of high-frequency acoustic waves.  CAA++® allows you to go beyond these traditional methods with a variety of analytic tools for far-field propagation and more efficient, specialized numerical solvers for modeling sound generation.

Accurate simulation of flow-driven acoustics requires detailed resolution of the small, noise-generating turbulent eddies and the full spectrum of transmitted acoustic waves.  CAA++® provides a suite of tools that simplifies these tasks, greatly reducing the cost in comparison to traditional CFD methods.

CAA++® enhances CFD++® noise-prediction capabilities


✓ Waveprop tools – fast, analytic wave propagation
✓ Hybrid RANS/LES – 2nd gen. methods with LEST
✓ NLAS – Non-Linear Acoustics Solver for coarse meshes and/or flows with weak shear instabilities
✓ Sub-domain and re-interpolation tools to reduce cost
✓ State-of-the-art far-field propagation/directivity tools
✓ Advanced signal processing tools

A comprehensive suite of tools


CAA++® is a comprehensive suite of tools for simulating aeroacoustics problems. Designed to synergistically complement CFD++®, CAA++® provides the user with an extended set of analytical and numerical methods to simulate the generation and transmission of sound waves through fluids. Many of the CAA++® methods are designed to operate on a reduced set of data. For example, by exploiting established mean and statistical data on modest meshes covering small sub-domains, NLAS offers a cost-effective noise prediction approach for boundary-layer or weak-shear problems in which hybrid RANS/LES would only give rise to quasi-steady solutions. The CAA++® suite also includes sub-domain extraction, mapping and re-interpolation tools as well as a wide variety of post processing tools

FEATURES

Many CFD codes offer unsteady flow modeling capabilities, so what sets CAA++® apart?

Advanced 2nd generation hybrid RANS/LESImproved accuracy
More reliable switching from RANS to LES modes
Automatic eddy seeding from RANS statistics
Sub-domain modeHuge reduction in memory and CPU requirements
Automatic prescription of realistic outer boundary conditions
Self-tuning acoustic wave absorbing layers
Smart Sub-Grid-Scale LES modelsMILES-like accuracy away from walls
RANS behavior maintained near walls in hybrid RANS/LES
Non-Linear Acoustics SolverReduced cost for simulations of weak-shear instabilities
Reduced cost for coarse-mesh simulations
ImExAutomatic implicit/explicit time-stepping options
Far-field CAA++ toolsMassive cost-reduction for long-range acoustic wave propagation

The CAA++® suite of Tools

  • Acoustic surface creation tools
  • Acoustic reinterpolation tools
  • Time-domain volume source integration
  • Frequency-domain volume source integration
  • Surface source integration tool
  • Wavepath integration tool
  • Far-field directivity tool
  • Surface data maps

See CAA++ in action

Product Overview
APPLICATIONS

Nozzle Flow Acoustics

Acoustics simulation of the flow-induced noise from a transonic jet. This example considers a transonic cold air jet issuing from a converging nozzle section. The initial statistics for the acoustics solver were computed using cubic k-epsilon RANS model and the acoustics data was subsequently interpolated on an acoustics mesh with reduced outer domain and relaxed near-wall stretching.

Car side mirror noise

This example considers the flow over a full car geometry with attached side mirror. The initial set of statistics for the acoustics solver were established on a standard RANS mesh and the NLAS solver was then run on a reduced mesh with far-field absorbing layers.

High Lift Trap Wing

A low Mach number flow over a high-lift, slat-wing-flap configuration. The acoustic pressure from the NLAS is shown radiating from the turbulent shear layers and, in particular, the separated flow in the slat cove.

Landing Gear Acoustics

Using the new ImEx scheme, CAA++® automatically determines which cells are treated explicitly and which are treated implicitly. Cells operating at CFL=1 are updated explicitly (no iteration, no uncertainties over inner-residual convergence criteria), uniformly second-order accurate and perfect conservation over all cells.

TECHNOLOGY

CAA++® technology in summary

Extensive use of modeling inevitably leads, at some point, to situations that deviate too far from the model calibration range to provide meaningful solutions. Consequently, the usefulness of simplified wave-equation solvers are typically limited to a restricted class of problem. The CAA++® Non-Linear Acoustics Solver (NLAS) allows for much greater generality, by numerically modeling both acoustic disturbances and some of the larger scale fluctuations.The method is based on the solution of disturbance equations, which describe perturbations around a mean set of data, which is provided (along with relevant statistics) by CFD++®. The CAA++® solvers, including NLAS, can be used with the same arbitrary geometries and meshes as CFD++®.

NLAS allows important large-scale generation effects to be captured directly on the mesh and provides a means of modeling reflection, refraction and blocking effects caused by the presence of complex surface geometries. NLAS offers a number of interesting capabilities. Calculations can be performed on separate acoustics meshes, which can require less near-wall resolution and a reduced far-field extent, due to specialized boundary-condition treatments. The benefits of this are that the acoustics solver can operate on more isotropic cells (particularly in the near-wall region, where a grid converged RANS solution is already available), resulting in a reduction in the overall number of mesh points from the relaxed near-wall requirements and a suitably truncated outer domain. Truncated outer boundaries in NLAS are assigned self-tuning absorbing layer boundary conditions, with far-field (and damping layer) data provided by the (a priori) RANS solution. This provides a good description of the outer boundaries and minimizes spurious wave reflections back into the simulation domain, even for boundaries located close to the source region of interest.

Compared with direct numerical simulation (DNS), the reduced grid requirements of a traditional LES are rather minimal, particularly in the near-wall region. Hybrid RANS/LES methods can achieve a reduction in mesh size by eliminating the mesh requirements in planes parallel to the wall (the normal-to-wall resolution is still required for the near-wall RANS modeling). NLAS further relaxes these meshing requirements, since a priori RANS statistics are always available, even on coarser regions of the NLAS mesh.

PUBLICATIONS
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