Fluid mass accounting in structural analysis

[elogesh]

Dear Experts,

We are interested in calculating the modal frequency automotive oil sump filled with oil for fixed boundary condition at the bolted face with engine block.Our concern is about the representation of oil.

How oil mass needs to be accounted.

1) whether we can represent oil as NSM (Non-structural mass) at the wetted faces
2) lumped mass (conm2) with RBE3 connections
3) modeling oil as solid cavity and node to node merging with oil sump structure. will specify lower almost zero young modulus for the oil cavity.

Kindly let us the know modeling methodology of oil to account in modal analysis

Thanks
ELogesh

 

[ThomasH]

Hi

I can't say that I have any experience for this particular application. But since the oil adds mass and no stiffness I would use a non-structural mass to get the correct mass in the system.

I also know that in some softwares you can use fluids as virtual masses to add the mass. I'm not sure if it is exactly the same as the NSM approach but it should be similair.

Regarding the option with solids. I think a solid with near zero Young modulus would create a lot of numerical issues. But that is just a guess.

Good Luck

Thomas

 

[bkal]

It depends also on which FE program are you using and how detailed you need your analysis to be. I have come across a paper which may explain some of the issues and possible solutions:

Sumulation of Fuel Sloshing - Comparative Study, M Vesnjak, H Mullerschon, A Humme, Z Ren, 3 LS-DYNA Anwenderforum, Bamberg 2004.

In civil/structural engineering similar problems are encountered with tanks subject to seismic effects.

 

[BlasMolero]

Dear Elogesh,
An interesting option exist with NX NASTRAN to consider the fluid-structure interaction: virtual fluid mass (MFLUID Method). I think the MFLUID method would be much more reliable than the RBE3 method. A virtual fluid volume produces a mass matrix which represents the fluid coupled to a boundary consisting of structural elements and other effects, such as free surfaces, planes of symmetry, and infinite fluids. The incompressible fluid produces a mass matrix defined with full coupling between accelerations and pressures on the flexible structural interfaces.

Although free surfaces are allowed, no gravity effects are included directly. Because the fluid is represented by a coupled mass matrix attached directly to the structural points, this capability is allowed in all dynamic solution sequences. This capability may be used to model a wide variety of fluid-structure interaction problems. Some examples are fuel tanks, nuclear fluid containers, drilling platforms, and underwater devices.

A free surface is defined as an x-y plane in any local rectangular coordinate system. The user simply specifies a value of z as the upper limit of the fluid volume. Different fluid volumes may have different levels and orientation. It is not required that the surface coincide with the element properties since partially wetted elements are allowed.

The virtual mass fluid option may be used in all NX Nastran dynamics solutions, including the following special approaches:

- Superelements (Residual Only)
- Nonlinear Analysis (SOL 129)
- Optimization (does not create sensitivity matrices)

The virtual mass method (MFLUID) is well suited for the following problem types:

- An infinite fluid surrounding part of a structure.
- A fluid with a free surface contained within a flexible structure.
- Multiple fluid volumes with combinations of the options above such as a half of a ship floating in water with several internal tanks, each with free surfaces and symmetric boundaries.

Note that compressibility and surface gravity effects are neglected. It is assumed that the important frequency range for the structural modes is above the gravity sloshing frequencies and below the compressible acoustic frequencies. It is further assumed that the density within a volume is constant and no viscous (rotational flow) or aerodynamic (high velocity) effects are present. In other words, a ship traveling at a high rate through a thick oil patch would require some extra modeling effort.

The MFLUID method is supported since years in FEMAP, and the Fluid Region command is very similar to the Connection Region command. The difference is that instead of creating regions for Contact purposes, this command creates individual segments representing incompressible fluid volume regions used for the purpose of generating a virtual mass matrix (MFLUID entry in Nastran input files). This capability is available in FEMAP supported Nastran Solution Sequences 103 (Modal Analysis), 107 through 112 (Complex Modal Analysis and Dynamic Analyses), 129 (Nonlinear Transient Analy­sis), and 200 (Optimization).

When a Fluid Region is present in your model, FEMAP provides an Output Request (Nastran only) called “Fluid Pressure” which will return an elemental “fluid-structure pressure” along with any other requested results.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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