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The volume of fluid ( VOF ) and porosity enthalpy methods are employed to track the molten pool free surface and solidification front , respectively .
 concept or principle: volume of fluid; concept or principle: VOF; machanical property: porosity; concept or principle: molten pool free surface; concept or principle: solidification
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 concept or principle: volume of fluid; concept or principle: VOF; machanical property: porosity; concept or principle: molten pool free surface; concept or principle: solidification