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In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.
In mechanics, friction is one such damping effect. For many purposes the frictional force F_{f} can be modeled as being proportional to the velocity v of the object:
where c is the viscous damping coefficient, given in units of newtonseconds per meter.
Generally, damped harmonic oscillators satisfy the secondorder differential equation:
where ω_{0} is the undamped angular frequency of the oscillator and ζ is a constant called the damping ratio. For a mass on a spring having a spring constant k and a damping coefficient c, ω_{0} = √k/m and ζ = c/2mω_{0}.
The value of the damping ratio ζ determines the behavior of the system. A damped harmonic oscillator can be:
The frequency of the underdamped harmonic oscillator is given by
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In physics and engineering, damping may be mathematically modelled as a force synchronous with the velocity of the object but opposite in direction to it. If such force is also proportional to the velocity, as for a simple mechanical viscous damper (dashpot), the force F may be related to the velocity v by
Where c is the viscous damping coefficient, given in units of newtonseconds per meter.
This relationship is perfectly analogous to electrical resistance. See Ohm's law.
This force is an approximation to the friction caused by drag.
An ideal massspringdamper system with mass m (in kilograms), spring constant k (in newtons per meter) and viscous damper of damping coefficient c (in newtonseconds per meter or kilograms per second) is subject to an oscillatory force
and a damping force
Treating the mass as a free body and applying Newton's second law, the total force F_{tot} on the body is
where a is the acceleration (in meters per second squared) of the mass and x is the displacement (in meters) of the mass relative to a fixed point of reference.
Since F_{tot} = F_{s} + F_{d},
This differential equation may be rearranged into
The following parameters are then defined:
The first parameter, ω_{0}, is called the (undamped) natural frequency of the system . The second parameter, ζ, is called the damping ratio. The natural frequency represents an angular frequency, expressed in radians per second. The damping ratio is a dimensionless quantity.
The differential equation now becomes
Continuing, we can solve the equation by assuming a solution x such that:
where the parameter γ (gamma) is, in general, a complex number.
Substituting this assumed solution back into the differential equation gives
which is the characteristic equation.
Solving the characteristic equation will give two roots, γ_{+} and γ_{−}. The solution to the differential equation is thus^{[1]}
where A and B are determined by the initial conditions of the system:
The behavior of the system depends on the relative values of the two fundamental parameters, the natural frequency ω_{0} and the damping ratio ζ. In particular, the qualitative behavior of the system depends crucially on whether the quadratic equation for γ has one real solution, two real solutions, or two complex conjugate solutions.
When ζ = 1, there is a double root γ (defined above), which is real. The system is said to be critically damped. A critically damped system converges to zero faster than any other, and without oscillating. An example of critical damping is the door closer seen on many hinged doors in public buildings. The recoil mechanisms in most guns are also critically damped so that they return to their original position, after the recoil due to firing, in the least possible time.
In this case, with only one root γ, there is in addition to the solution x(t) = e^{γt} a solution x(t) = te^{γt}:^{[2]}
where A and B are determined by the initial conditions of the system (usually the initial position and velocity of the mass):
When ζ > 1, the system is overdamped and there are two different real roots. An overdamped doorcloser will take longer to close than a critically damped door would.
The solution to the motion equation is^{[3]}:
where A and B are determined by the initial conditions of the system:
Finally, when 0 ≤ ζ < 1, γ is complex, and the system is underdamped. In this situation, the system will oscillate at the natural damped frequency ω_{d}, which is a function of the natural frequency and the damping ratio. To continue the analogy, an underdamped door closer would close quickly, but would hit the door frame with significant velocity, or would oscillate in the case of a swinging door.
In this case, the solution can be generally written as^{[4]}:
where
represents the natural damped frequency of the system, and A and B are again determined by the initial conditions of the system:
For an underdamped system, the value of ζ can be found by examining the logarithm of the ratio of succeeding amplitudes of a system. This is called the logarithmic decrement.
Viscous damping models, although widely used, are not the only damping models. A wide range of models can be found in specialized literature, but one of them should be referred here: the so called "hysteretic damping model" or "structural damping model".
When a metal beam is vibrating, the internal damping can be better described by a force proportional to the displacement but in phase with the velocity. In such case, the differential equation that describes the free movement of a singledegreeoffreedom system becomes:
where h is the hysteretic damping coefficient and i denotes the imaginary unit; the presence of i is required to synchronize the damping force to the velocity (xi being in phase with the velocity). This equation is more often written as:
where η is the hysteretic damping ratio, that is, the fraction of energy lost in each cycle of the vibration.
Although requiring complex analysis to solve the equation, this model reproduces the real behaviour of many vibrating structures more closely than the viscous model.
A more general model that also requires complex analysis, the fractional model not only includes both the viscous and hysteretic models, but also allows for intermediate cases (useful for some polymers):
where r is any number, usually between 0 (for hysteretic) and 1 (for viscous), and A is a general damping (h for hysteretic and c for viscous) coefficient.
Komkov, Vadim (1972) Optimal control theory for the damping of vibrations of simple elastic systems. Lecture Notes in Mathematics, Vol. 253. SpringerVerlag, BerlinNew York.
