## Power Spectral Density

Heya Everyone! I was going through topics on Atmospheric turbulence and their effects on stability of aircrafts...and I came across Power spectral density. I could'nt understand it properly. Could any one please give me an idea of what it is an refer me some literatures / books on it or topics related with atmospheric turbulence and its effect of aircraft stability?
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 Power spectral density basically tells you how much power is in certain frequencies. For example consider measuring the air velocity at a single point in a turbulent flow for an arbitrary amount of time. You have measured the velocity in the time domain so you can plot velocity versus time. Using fast fourier transforms you can convert this data to the frequency domain where the x-axis will be frequency and the y-axis will now be a measure of how much power is contained in a each frequency. So in a turbulent flow, a spike in the power spectral density at a certain frequency would likely indicate some kind of flow phenomena occurring at that frequency, for example vortex shedding from a cylinder which for certain Reynolds numbers occurs at a specific frequency. I don't know too much about aircraft stability but I would imagine this would be important because flow phenomena at a particular frequencies could excite certain unstable modes of the aircraft.
 RandomGuy is correct. I'll elaborate more on the uses in aircraft control. The power spectral density is used to create a gust model to simulate random motion and/or certain maneuvers, such as pitch, yaw, roll, etc... Also it can be used to excite short period and long period modes. These models are used and applied as a disturbance model to simulate the desired gust frequency, shape or even strength to analyze the robustness of the model.

## Power Spectral Density

@Random Guy, Viscous low: Thanks alot both of you! its been of great help!

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