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Dispersion in Cylinders

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As you may know wave propagation in circular cylinders is dispersive (its response depends on the frequency). As shown by Pochhammer, there could be several different modes propagating in a cylinder with different wave speeds. Im modeling a finite 3D circular cylinder is LS-DYNA with solid elements and Im applying a sinusoidal pressure along the cylinder axis at one end while the other end is free. Based on Pochhammer solution at a certain frequency there could be different modes propagating in the cylinders with different speeds. Ive observed this in ultrasonic experiment. I apply sinusoidal pressure wave at one end of a circular cylinder and receive the signal at the other end and I can see different modes propagating with different speeds at different frequencies. My question is why I dont see this effect in my 3D FE model? All I see in my FE model is the 1st mode propagating in the cylinder in all different frequencies.