On-the-Fly FW-H Model Reference

The On-the-Fly FW-H acoustic model provides the aerodynamic noise prediction in parallel with the aerodynamic computation. The On-the-Fly FW-H node becomes available when the Ffowcs Williams-Hawkings Unsteady model is active.

A transient simulation using the On-the-Fly FW-H model does the following:

It computes the time-accurate data on the emission surfaces (sources of noise), which are the integration surfaces for the FW-H solver. Time-accurate solutions can be obtained from an unsteady turbulent or laminar simulation (DES/LES simulations are suggested).
Time variation of the sound pressure at receiver locations is computed based on the FW-H solver.
It can calculate sound pressure from quadrupole noise sources for all regions selected in all point receivers when Quadrupole Noise is On.


Theory	See Ffowcs Williams-Hawkings Model.
Provided By	[physics continuum] > Models > FW-H Unsteady Models
Example Node Path	Continua > Physics 1 > Models > On-the-Fly FW-H
Requires	Material: one of Gas, Liquid, Eulerian Multiphase, Multi-Component Gas, Multi-Component Liquid (For Eulerian Multiphase, Multi-Component Gas or Multi-Component Liquid, further models are required to expose the Flow models.) Flow: Segregated Flow or Coupled Flow Time: Implicit Unsteady Viscous Regime: Turbulent or Laminar Optional Models: Aeroacoustics Aeroacoustic Models: Ffowcs Williams-Hawkings Unsteady
Properties	Key property is: Quadrupole Noise. See On-the-Fly FW-H Model Properties.
Activates	Solvers	FW-H Unsteady Solver. See FW-H Unsteady Solver Properties.
	Field Functions	FW-H Density, FW-H Pressure, FW-H Velocity. See Field Functions.

On-the-Fly FW-H Model Properties

Quadrupole Noise: Determines if the volumetric quadrupole radiated noise is calculated.; When On, the model calculates the quadrupole noise. The Quadrupole Volume Term field function is activated. An option to gather quadrupole noise data at each FW-H receiver is activated.; When Off (which is the default), the quadrupole noise is not calculated. This property is common to both stationary simulations and simulations with rigid body motion.; The On-the-Fly FW-H model calculates quadrupole noise using the collapsing sphere formulation. This formulation is independent of the formulations used to compute the sound pressure for the selected FW-H surfaces, such as Farassat's Formulation 1A or Dunn-Farassat-Padula Formulation 1A.
The FW-H quadrupole calculations and integration on regions are performed independently from the calculations on surfaces.

FW-H Unsteady Solver Properties

This solver implements the advanced time approach for subsonic and transonic flows, to account for the time delay between the emission time and reception time.

 The advanced time approach is also known in the literature as a “source-time-dominant” approach. It was developed by Brentner [45], based on Formulation 1A of the FW-H analogy by Farassat [57]. It was extended to permeable (or penetrable) integration surfaces by di Francescantonio [54] and by Brentner and Farassat [46].

This approach looks forward in time to see when the observer receives the currently generated sound waves. In the source-time-dominant algorithm, it fixes the source time and determines when the sound from each point on the FW-H Surface reached the observer. The arrival time of the acoustic signal is different for each point on the FW-H Surface. Then the sound pressure at the receiver is obtained by accumulating the arriving signals in time slots. The overall observer acoustic signal is obtained from the summation of the acoustic signal that is radiated from each source element of the FW-H Surface during the same source time.

Note	It is important to collect a time history of the sound pressure at the receiver location long enough to do spectral analysis using Fourier Transforms.

Start Time: The elapsed simulation time before the solver begins calculating.
Solver Frozen: When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Field Functions

FW-H Density: Represents the density for the FW-H model, $ρ_{f}$ or $P_{a} / c^{2} + ρ_{f}$ , as determined by the Acoustic Data Source setting.
FW-H Pressure: Represents the pressure for the FW-H model, $P_{f}$ , $P_{a} + P_{f}$ , or $P_{a}$ , as determined by the Acoustic Data Source setting.
FW-H Velocity: Represents the velocity for the FW-H model, $v_{f}$ , $v_{a} + v_{f}$ , or $v_{a}$ , as determined by the Acoustic Data Source setting.
Loading Surface Term [Receiver Name]: ${p′}_{L} (x, t)$ in Eqn. (4761) for general flows and in Eqn. (4763) for flows with rigid body motion or moving reference frames. In each equation, the field function is normalized with face-area. The unsteady motion of the force distribution on the body surface generates the Eqn. (4752) noise.
Quadrupole Volume Term [Receiver Name]: ${p′}_{Q} (x, t)$ in Eqn. (4766), normalized with cell volume. Non-linearities in the flow generate quadrupole noise. The field function is only enabled when you activate the Quadrupole Noise expert property.
Thickness Surface Term [Receiver Name]: ${p′}_{T} (x, t)$ in Eqn. (4760) for general flows and in Eqn. (4762) for flows with rigid body motion or moving reference frames. In each equation, the field function is normalized with face-area. The displacement of fluid as the body passes generates the Eqn. (4751) noise. For stationary bodies (such as side view mirrors and circular cylinders) the thickness surface term is zero.
Total Surface Term [Receiver Name]: ${p′}_{S} (x, t)$ in Eqn. (4759). The summation of the Loading Surface Term and the Thickness Surface Term for a particular source surface generates the Total Surface Term.