Читать книгу Vibroacoustic Simulation - Alexander Peiffer - Страница 15

Without external excitation as shown in Figure 1.1 a) the motion depends on the initial conditions at time t = 0 with the displacement u(0)=u0 and velocity vx(0)=vx0. The damping is supposed to be viscous, thus proportional to the velocity Fxv=−cvu˙. The equation of motion

(1.1)

is a homogeneous second order equation with a solution of the form u=Aest. Entering this into Equation (1.1) leads to the characteristic equation

(1.2)

with the two solutions

(1.3)

Hence,

(1.4)

with B₁ and B₂ depending on the initial conditions. The root in Equation (1.3) is zero when c_v equals 4mks. This specific value is called the critical viscous damping damping

(1.5)

We use the following definitions:

(1.6)

ω₀ is the natural angular frequency, ζ is ratio of the viscous-damping to the critical viscous-damping. There are additional expressions for the period and frequency

(1.7)

where f₀ is the natural frequency and T₀ the oscillation period. Equations (1.1)–(1.3) can now be written as

(1.8)

(1.9)

(1.10)

The problem falls into three cases:

 ζ > 1 overdamped

 ζ < 1 underdamped

 ζ = 1 critically damped.

The first case leads to two real roots, and no oscillation is possible. The second case gives two complex roots, which means that (damped) oscillation occurs. The third case is a transition case between the two other. Subsections 1.1.2–1.1.4 deal with each case in detail.