dear sir:

SIMPLE or SIMPLEC are not good for unsteady analysis so you have to select PISO algorithm to couple the flow field characters of velocity and pressure depend on your investigations.

good luck

Dear Abhay,

Simple family of algorithms rely on iterative correction of the velocity field at each time step to satisfy the divergence free condition. Therefore this relatively crude approximation only can effect the rate of convergence of the outer iteration. 

SIMPEL algorithm actually has a poor rate of convergence for highly time dependant flows. There are methods to improve this, most straightforward improvement is to use SIMPLEC where you assume that the neighbour velocities are approximately equal to the face velocity u_e. There are several improvements to SIMPLE type algorithms (SIMPLEST,SIMPLER) to increase the rate of convergence. However, these methods are NOT the method of choise for highly time dependant problems where accurate time history of the solution is requred to calculate flow statistics eg turbulent, reactive etc. You can instead consider using projection methods such as the celebrated fractional step methods (see the seminal paper by Kim and Moin)

Good luck

Sina 

Patankar's text book is a must-read textbok for understanding SIMPLE-like scheme. He has expleined the question like this in the book.

http://www.amazon.com/Numerical-Transfer-Hemisphere-Computational-Mechanics/dp/0891165223

I also suggest the book by Patankar, Numerical heat transfer and fluid flow, there is a very clear explanation of Simple as well Simpler algorithm, i.e. how to couple pressure and velocities and the consequences of it (one of the issues is that we do not have a special equation for pressure) 

dear sir:

SIMPLE or SIMPLEC are not good for unsteady analysis so you have to select PISO algorithm to couple the flow field characters of velocity and pressure depend on your investigations.

good luck