Characteristic lengths for Reynolds, Grashof, Nusselt, and Biot numbers vary based on the geometry of the object, such as a plane wall, cylinder, or sphere. For instance, the characteristic length for Reynolds number in a pipe is its diameter, while for the Biot number, it is calculated as volume divided by external surface area. The choice of characteristic length often aligns with the correlation being used in specific problems. Resources like Wikipedia and the Incropera textbook provide foundational information on these concepts. Understanding these lengths is crucial for accurate fluid dynamics and heat transfer calculations.
