Monday, December 3, 2012

From the ground...down (part 2)



Last post we talked about the idea of a “penetration depth” to describe how deeply a fluctuating surface temperature might affect the temperature of the ground below.  

 Before we go on to talk more about the ground, we should point out that that kind of calculation isn’t limited to just temperature distribution in the ground.  It can be used to approximate the penetration depth of many situations where the surface temperature fluctuates periodically.  For example, if you wanted to estimate how deeply the temperature fluctuations penetrate into the wall of an engine cylinder, you might use properties for steel (α =17.7 mm^2/s) and the frequency of the periodic temperature variation of the cylinder (if the engine was running at 2000 rpm, you’d have 1000 temperature cycles per minute, or a frequency of 105 rad/s).  With those numbers, and using our definition of 2.2% fluctuation for the penetration from the last post, you could calculate a penetration depth of 2.2 mm.