A set that is not convex is called a non-convex set. A polygon that is not a convex polygon is sometimes called a concave polygon, and some sources more generally use the term concave set to mean a non-convex set, but most authorities prohibit this usage.
In either case you don't need to deal with the absolute value. A rectified sine wave is a periodic signal with a period equal to half of the full sinusoid, I would write the sine in exponential form and integrate over one period to find the series coefficients.
The reflection of the beam creates some comet-tail reverberation artifacts, called B-lines or ultrasound lung comets. A B-line is a discrete, laser-like, vertical, hyperechoic image, that arises from the pleural line, extends to the bottom of the screen without fading, and moves synchronously with respiration.