VM154

VM154
Vibration of a Fluid Coupling

Overview

Reference:

R. J. Fritz, "The Effect of Liquids on the Dynamic Motions of Immersed Solids", ASME, J. of Engr. for Industry, Vol. 94, Feb. 1972, pp. 167-173.

Analysis Type(s):

Mode-frequency Analysis (ANTYPE = 2)

Element Type(s):

Dynamic Fluid Coupling Element (FLUID38)

Spring-Damper Elements (COMBIN14)

Input Listing:

vm154.dat

Test Case

A long cylinder is immersed in a circular hole as shown. The cylinder is separated from the containment surface by a frictionless, incompressible liquid annulus. A spring restraint is attached to the cylinder from ground. Determine the natural frequency f of the system based upon the hydrodynamic mass of the liquid annulus.

Figure 216: Fluid Coupling Problem Sketch

Material Properties

Geometric Properties

ρ = 0.0000934 lb-sec²/in⁴

k = 10 lb/in

β = 30 x 10⁴ psi

a = 7 in

b = 8 in

Analysis Assumptions and Modeling Notes

The problem is solved using the fluid coupling element (FLUID38).

The total length of the assembly is assumed to be long in comparison to its radius. The solution is based on radial motion of a unit length of the assembly. The cylinder is assumed to be massless so that all mass effects are from liquid annulus. The nodes are defined as coincident, but they are shown apart for clarity. An effective harmonic spring constant, k=10 lb/in is used for each spring element to produce equivalent spring force.

Results Comparison

	Target	Mechanical APDL	Ratio
f, Hz (FLUID38)	1.5293	1.5293	1.000