In Most CFD project, we design geometry, create a static mesh for this geometry and simulate this in by fix mesh. But sometimes we want to move some boundary or we have deforming shape during times. We must use dynamic mesh method. For example in mots aerodynamic problem we use fix mesh for our simulation but you can imagine there are two airplanes that are getting closer and you want to know effect of fluid flow behavior around airplanes and determine interaction between them during the time. So the relative position them are changing during time and should be modeled using dynamic mesh method. By using this method, mesh size and shape will be changed. If we use re-meshing or layering method, number of element also will be changed. Dynamic mesh method is entirely different from moving and sliding mesh also moving reference frame. In moving mesh whole zone rotate or translate in some direction but in dynamic mesh method boundary start to translate or transform. We can apply a predefined velocity by UDF or profile for boundaries or velocity of an object can be predicted based on fluid flow and gravity force balance. If we want to know the velocity of object based on this balance we should use SIX DOF dynamic mesh method.