Underwater Shock
Underwater Shock
(OP)
Need some help understanding the reality of underwater shock. Have done some Modal analysis on the subject above and come up with a number of conclusions/assumptions can someone set me straight.
I am aware that when an explosive device goes off underwater a pulsing gas bubble is created which subjects any vessel within proximity to a shock wave of certain frequency - Normally modelled as a damped sinusoid.
The vessel and any equipment within will then respond to this energy input but what is actually occuring does :
a) The energy input primarily affect only the equipment of similar natural frequency (or harmonics of that frequency), with subsequent effect on other parts of the vessels strucuture
b) Is the energy input to every item onboard and set up their own responses e.g. ringing.
c) Some other explanation
I am aware that when an explosive device goes off underwater a pulsing gas bubble is created which subjects any vessel within proximity to a shock wave of certain frequency - Normally modelled as a damped sinusoid.
The vessel and any equipment within will then respond to this energy input but what is actually occuring does :
a) The energy input primarily affect only the equipment of similar natural frequency (or harmonics of that frequency), with subsequent effect on other parts of the vessels strucuture
b) Is the energy input to every item onboard and set up their own responses e.g. ringing.
c) Some other explanation





RE: Underwater Shock
A shock wave produces many different frequencies - the spectrum looks like shaped broadband noise, typically with a fairly gentle high frequency roll off at a frequency related to the shock duration.
It is directly analagous to an impact test in experimental modal analysis.Think about it - a shock wave sounds like a thump, not a pure tone.
Simple linear systems tend to respond at their first natural frequency, as a damped sinusoid. Perhaps that is what you meant.
If I am right then (b) is the nearest approximation to the right answer.
Cheers
Greg Locock
RE: Underwater Shock
I agree with your view on the above, however my initial statement was purely based upon my experience (which isn't a lot) of working with Military standards. When performing a dynamic analysis naval vessels,a shock input, normally a specific frequency damped sinusoid, is used as an input and the response calculated from this. Again not knowing for certain, I have assumed that this would primarily affect items of a conincident natural frequency.
Stuart
RE: Underwater Shock
If the sinusoid is very heavily damped then it approximates to an impulse anyway.
Cheers
Greg Locock
RE: Underwater Shock
RE: Underwater Shock
By an impulse I meant a positive going spike of arbitrarily short time span. The bandwidth of the impulse is strongly related to the inverse of its length (ie a 1ms impulse will have a bandwidth of /about/1 kHz irrespective of the precise shape of the pulse). A heavily damped sinusoid can be considered to be an impulse of time t/2, followed by a weaker negative impulse t/2 later. The first one dominates, because the sine wave is exponentially decaying, rapidly. The point is, even though you 'think' it is a sine wave, it still has energy content at almost all frequencies. Bear in mind that this is a very heavy level of damping, with a time constant of the order of 1/f.
Cheers
Greg Locock
RE: Underwater Shock
The heavyweight test involves the "barge" test, where actual explosives are detonated around a barge upon which the UUT is mounted. I don't have the Navy paper in front of me, but the measured stimulus is also not a damped sinusoid.
In all cases, there is no official definition of the input disturbance, so I'm not sure where the damped sinusoid is coming from, unless it's the response of your test fixture to the input shock.
TTFN
RE: Underwater Shock
Cheers
Greg Locock
RE: Underwater Shock