Sentence:  A crude analogy is to liken this to water draining noisily from a bathtub. As the liquid falls down into the <WSD>drainpipe</WSD>,  some of its swirling kinetic energy is converted into sound waves, water bashing against molecules of air. 
Meaning of the word marked with <WSD> tags:  A pipe that carries wastewater from a bathtub, shower, sink, etc.