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Equations and discussions related to the analysis of lightly damped nonlinear systems under broad band excitations using an approximate method. Topics such as fpk equation, backward kolmogorov equation, first passage times, and safe regions.
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Markov Vector Approach-
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Solution ofFPK equationsTransient & Steadystate solutions Steady statesolutions
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Moment equations form an infinite hierarchy of equations which at no stage provide sufficient
CL
number of equations
to solve for the moments:
This is the characteristic feature of
OSURE PROBLEM
Remarks
moment equations
of non linear systems driven by random excitations
Closure approximations
Assume that the higher order moments, beyond a given order,are related to the lower order on
es as though they
obey an adhocly specified pdf (Ex: Gaussian closure)Neglect cumulants beyond a specified order
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Example
specified.
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m Question: Are the steady state mom
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Steady state
ents realizable?
That is. are these moments stable?
perturb the solutions and see if
Strategy : perturbations die out.
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Consider the FPK equation
,
It follows that
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Let
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we have
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Backward Kolmogorov equation and reliabilityfunction
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BCS :
1
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Vector version of Backward Kolmogorov equation
0
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, 2,
,
, lim
; |
;
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j x
n
p x t
x
t
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If we are interested in pdf of
for a given initial condition
we use forward equation.
If we are interested in the pdf of time for first passage across a threshold, we use backward equation.
X
t
Remarks
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