Vibroacoustic Simulation. Alexander Peiffer

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Vibroacoustic Simulation - Alexander Peiffer


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Start 1 By 1 Matrix 1st Row upper F EndMatrix equals m Start 1 By 1 Matrix 1st Row ModifyingAbove u With two-dots EndMatrix (1.91)

      Using the complex amplitude notation of harmonic motion this leads to:

       bold-italic upper F Subscript i j Baseline equals minus omega squared m Subscript i Baseline bold-italic q Subscript i j Baseline with j equals x comma y comma z (1.92)

      For every mass mi at node i the local mass matrix is depending on the available degrees of freedom for each mass node. For a two-dimensional system with {q}i={ui,vi}T we get:

       minus omega squared Start 2 By 2 Matrix 1st Row 1st Column m Subscript i Baseline 2nd Column 0 2nd Row 1st Column 0 2nd Column m Subscript i Baseline EndMatrix StartBinomialOrMatrix bold-italic u Subscript i Baseline Choose bold-italic v Subscript i Baseline EndBinomialOrMatrix equals StartBinomialOrMatrix bold-italic upper F Subscript x i Baseline Choose bold-italic upper F Subscript y i EndBinomialOrMatrix (1.93)

       Start 1 By 1 Matrix 1st Row upper M EndMatrix equals Start 8 By 8 Matrix 1st Row 1st Column m 1 2nd Column 0 3rd Column ellipsis 4th Column Blank 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 2nd Row 1st Column 0 2nd Column m 1 3rd Column 0 4th Column ellipsis 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 3rd Row 1st Column Blank 2nd Column 0 3rd Column m 2 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 4th Row 1st Column Blank 2nd Column Blank 3rd Column 0 4th Column m 2 5th Column 0 6th Column Blank 7th Column Blank 8th Column Blank 5th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column 0 5th Column m 3 6th Column 0 7th Column Blank 8th Column Blank 6th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column 0 6th Column m 3 7th Column 0 8th Column Blank 7th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column 0 7th Column m 4 8th Column 0 8th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column Blank 7th Column 0 8th Column m 4 EndMatrix (1.94)

      1.4.2 Assembling the Stiffness Matrix

      A similar procedure is applied to the springs and the related stiffness matrix. Every spring ki connects different points in space, so there are at least two nodes involved, unless the spring is connected to a rigid wall. Inspecting the spring in Figure 1.18 the force balance is:

      Figure 1.18 Spring and damper connection of two nodes. Source: Alexander Peiffer.

       StartBinomialOrMatrix bold-italic upper F Subscript x Baseline 1 Baseline Choose bold-italic upper F Subscript x Baseline 2 Baseline EndBinomialOrMatrix equals Start 2 By 2 Matrix 1st Row 1st Column k Subscript s Baseline 2nd Column minus k Subscript s Baseline 2nd Row 1st Column k Subscript s Baseline 2nd Column k Subscript s Baseline EndMatrix StartBinomialOrMatrix 0 Choose bold-italic u 2 EndBinomialOrMatrix equals StartBinomialOrMatrix minus k Subscript s Baseline bold-italic u 2 Choose k Subscript s Baseline bold-italic u 2 EndBinomialOrMatrix right double arrow bold-italic upper F Subscript x Baseline Subscript 1 Baseline equals minus bold-italic upper F Subscript x Baseline Subscript 2 Baseline equals minus k Subscript s Baseline bold-italic u 2 (1.96)

      If the spring is connected to the wall, only u2 changes with applied forces, and the reaction force in the constraint is −ksu2. Practically that is the reaction force that must be provided by the wall to keep the node in place. In the finite element terminology these constraints are called single point constraints (SPC).

      The local matrices must be distributed among the global stiffness matrix [K]. For better visibility this is done for springs 2 and 4. The place of the local matrix of spring 2 in the global stiffness matrix follows from the u1 and u2, the position of spring 4 from v1 and v3.

       Start 1 By 1 Matrix 1st Row upper K 2 EndMatrix equals Start 8 By 8 Matrix 1st Row 1st Column k Subscript s Baseline 2 Baseline 2nd Column 0 3rd Column minus k Subscript s Baseline 2 Baseline 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 2nd Row 1st Column 0 2nd Column 0 3rd Column 0 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 3rd Row 1st Column minus k Subscript s Baseline 2 Baseline 2nd Column 0 3rd Column k Subscript s Baseline 2 Baseline 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 4th Row 1st Column 0 2nd Column 0 3rd Column 0 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 5th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column 0 6th Column Blank 7th Column Blank 8th Column Blank 6th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column 0 7th Column Blank 8th Column Blank 7th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column Blank 7th Column 0 8th Column Blank 8th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column Blank 7th Column Blank 8th Column 0 EndMatrix Start 1 By 1 Matrix 1st Row upper K 4 EndMatrix equals Start 8 By 8 Matrix 1st Row 1st Column 0 2nd Column 0 3rd Column Blank 4th Column Blank 5th Column 0 6th Column 0 7th Column Blank 8th Column Blank 2nd Row 1st Column 0 2nd Column k Subscript s Baseline 4 Baseline 3rd Column Blank 4th Column Blank 5th Column 0 6th Column minus k Subscript s Baseline 4 Baseline 7th Column Blank 8th Column Blank 3rd Row 1st Column Blank 2nd Column Blank 3rd Column 0 4th Column Blank 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 4th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 5th Row 1st Column 0 2nd Column 0 3rd Column Blank 4th Column Blank 5th Column 0 6th Column 0 7th Column Blank 8th Column Blank 6th Row 1st Column 0 2nd Column minus k Subscript s Baseline 4 Baseline 3rd Column Blank 4th Column Blank 5th Column 0 6th Column k Subscript s Baseline 4 Baseline 7th Column Blank 8th Column Blank 7th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column Blank 7th Column 0 8th Column Blank 8th Row 1st Column Blank 2nd Column Blank 3rd Column Blank 4th Column Blank 5th Column Blank 6th Column Blank 7th Column Blank 8th Column 0 EndMatrix (1.97)

      For the system from Figure 1.17 and putting all free and constraint springs together we get:

       Start 1 By 1 Matrix 1st Row upper K EndMatrix equals Start 8 By 8 Matrix 1st Row 1st Column k Subscript s Baseline 1 Baseline plus k Subscript s Baseline 2 Baseline 2nd Column 0 3rd Column minus k Baseline 2 4th Column 0 5th Column Blank 6th Column Blank 7th Column Blank 8th Column Blank 2nd Row 1st Column 0 2nd Column k <hr><noindex><a href=Скачать книгу