Random VATV Forced Response

You can use the Random VATV Forced Response command to compute vibro-acoustic responses of Vibro-Acoustic Transfer Vectors (VATV) with an applied Turbulent Boundary Layer (TBL) excitation. The VATVs let you efficiently compute vibro-acoustic pressure due to aerodynamic or acoustic loads on flexible structures in the context of two-way (strong) coupled vibro-acoustic problems. You use this method, for example, to compute vibro-acoustic pressure near an automotive window that is excited by a turbulent external loading, or the sound pressure level inside an aircraft.

In the figure below on the left a vibro-acoustic FEM model has a structure mesh imbedded inside an acoustic mesh. A microphone mesh will capture radiated sound. The figure on the right is the VATV alternate representation of the FEM, shown as the superelement representation.

Vibro-acoustic model 1) Structure mesh, 2) Acoustic fluid mesh (shown cutaway), 3) Microphone mesh	Vibro-acoustic model with VATV superelement representation

Using intermediary mesh

The software computes intermediary mesh to map from the FEM model to the VATV. When you apply the TBL model, you can select all the fluid faces, or only selected ones. When you load the intermediary mesh you can select all the mesh faces or only those on which you wish to apply the TBL. If the model has many fluid faces, loading the intermediary mesh may take some time.

Turbulent Boundary Layer models

You can apply the TBL loading to all the acoustic fluid VATV faces or use only selected faces. You then set parameters to control the TBL model. The available TBL models are:

• Corcos

• Efimtsov

• Chase

• User Defined

You set parameter values for your combination of correlation and auto-spectrum models. Depending on the selected TBL models, only specific parameters are available.

Note:

• You can find valid parameter values in the literature, or obtain them from experiments or previous simulations.

• The software does not check the parameter values; therefore you must enter valid values. For example, fluid density, displacement thickness, decay rates, and so on cannot be zero or negative.

• You must specify convective velocity parameters for all TBL models, even if you use a User Defined correlation spectrum model that may contain information about convective velocity in a user defined .dll file.

• In some cases, even though convective flow in some directions may be zero, you may have to apply small but non-zero velocities to attain a valid solution.

To obtain the appropriate solution output, you specify the following output requests: Auto Power Spectral Density, Cross Power Spectral Density, or both. The Auto Power Spectral Density output request computes only the diagonal matrix terms. The Cross Power Spectral Density output request computes the entire matrix.

Defining the correlation spectrum

You select a correlation spectrum model in the Turbulent Boundary Layer dialog box. In the Parameters group you set the spectrum-related parameters after you select one of the following TBL models:

Corcos

This model is especially useful for accurately predicting the spectrum convective ridge for high-speed flows, and for its simple formulation. You need to adjust only the Correlation Decay Rate (alpha) and Correlation Decay Rate (beta).

Efimtsov

This is the only model in Simcenter 3D that was developed from aircraft instead of laboratory measurements. This model was validated with measurements on an aircraft traveling at speeds between Mach 0.41 and Mach 2.1.

Chase

This model provides a direct formulation for the wall pressure spectrum because it is not defined as a product of a correlation and auto-spectrum model.

User Defined

This model allows you to define your own model with a field.

Defining the auto-spectrum

You select auto-spectrum models that are one-sided in Simcenter 3D. That is, the software considers only positive frequencies. Therefore, all input and output spectra are one-sided quantities.

You enter TBL spectrum-related parameters in the Parameters group, after you select one of the following models:

Robert (Corcos)

This model is especially useful for accurately predicting the spectrum convective ridge for high-speed flows, and for its simple formulation. You need to adjust only the Correlation Decay Rate (alpha) and Correlation Decay Rate (beta).

Efimtsov

This is the only model in Simcenter 3D that was developed from aircraft instead of laboratory measurements. It has been calibrated from measurements of TBL wall pressure fluctuations on an aircraft. It is valid over a wide range of Mach numbers above 0.015 and below 4, Reynolds numbers above 6E2 and below 1.5E5, and Strouhal numbers above 0.2 and below 1E4.

Goody

This model is limited to zero pressure gradient flows but considers the effect of the Reynolds number variations through the time scale ratio. This model shows good agreement with experimental data over Reynolds numbers above 1400 and below 23400.

Smolyakov

This model is a semi-empirical model and mostly applicable to flat plate surfaces.

Cockburn-Robertson

This model is used for flows at transonic and supersonic speeds. The model was calibrated when investigating the vibration response of spacecraft shrouds to in-flight fluctuating pressures. The effects of the Mach number affect only the mean square pressure, and become negligible at transonic and lower speeds.

Smolyakov-Tkachenko

This model is a semi-empirical model and mostly applicable to flat plate surfaces.

Chase-Howe

This model is limited to near or below the universal range of a quantity.

User Defined

This model allows you to define your own model with a field.

Using a post-processing scenario

You use a post-processing scenario to plot the computed PSD pressure:

• Versus frequency as XY plots, color bars, or bar charts.

• As contour plots on microphone mesh

PSD response versus frequency for a VATV model under TBL excitation

Contour plot of PSD pressure on microphone mesh

Additional information

For more information on correlation spectrum and auto-spectrum models that you can specify in the Turbulent Boundary Layer dialog box, see Turbulent Boundary Layer Models.

How do I

Create a Random VATV Forced Response solution

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Random VATV Forced Response, Simcenter 3D 2021.1 Series

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