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It's a long time since I had fluid mechanics, but I think this is because hot water is less viscous than cold water. The viscosity, along with flow rate, determines what type of flow you get: laminar or turbulent.
You can observe this if you pour both hot and cold water from the same kettle. Hot water looks lumpy and chaotic (turbulent) while cold water flows in a nice stream. Google for Reynold's number if you want the mathematical theory.
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