Separation processes exploit differences in the physical or chemical properties of the particles to separate them from each other. These processes are classified according to the different properties of particles they exploit and the method that they use for separation. Among available techniques, acoustophoresis is considered as a method that can separate particles and cells regardless of being metal or non-metal. There is no need to use additive substances in this technique and being a non-contact method, acoustophoresis is able to manipulate living cells without any malicious effects on them. Miniaturizing is another advantage of this method. Acoustophoresis is made of two parts, "acousto" as a method which uses sound waves to cause the movement and levitation of particles and cells and "phoresis" meaning migration. Particles that are exposed to an acoustic standing wave experience acoustic radiation forces due to their differences in their acoustic properties with the host fluid. Acoustic streaming is a vortical time-averaged phenomenon in acoustofluidic devices that arises due to the acoustic waves in a viscous fluid near solid boundaries, e.g. walls. Particles which are present in a streaming field experience drag forces due to this flow. Numerical modeling of acoustophoresis problems has been very challenging in the last decades. Numerical modeling of the 3D acoustophoresis problem with direct solving of the governing equations uses high computational costs. In this research, acoustophoresis problem is numerically modeled and 1st order acoustic fields and 2nd order streaming fields are solved and analyzed. Forces acting on the particles consisting of radiation force and drag force due to the streaming field are calculated and particles are traced fin the microchannel. Reynolds Stress Method (RSM) and Limiting Velocity Method (LVM) are separately used to model the 2D acoustophoresis problem. The results obtained from these methods are compared with each other and verified with the results from other researchers. LVM is used to model the 3D problem due to its low computational costs compared to the RSM. Results of numerical solution of 3D fixed microchannel model are compared with 2D results and their validation is confirmed. Finally, the effect of rotation of the microchannel on the displacement of the particles is studied. Centrifugal and drag forces stemmed from the equilibrium state of the fluid are added to the radiation and drag forces stemmed from the acoustic streaming. By using rotation, one can separate particles with larger radiuses from the ones with smaller radiuses. This separation can be achieved as the particles with a small radius experience smaller radiation forces. Large particles move towards the pressure node, whereas small particles are affected by centrifugal and drag forces stemmed from acoustic streaming and equilibrium state of the fluid and move towards the sidewalls of the microchannel. Keywords: Acoustophoresis, Centrifugal, Separator, Acoustic, Standing Wave, Radiation Force, Viscous Fluid