This paper presents unsteady simulations of an established laminar compressible boundary layer suddenly exposed to an adverse pressure gradient. The mean flow deceleration is imposed by the mean of a suction at the upper side of the computational domain. In the first part, the suction boundary condition is validated for the two dimensional laminar solution: results of Pauley et al. (1990) are recovered, and especially the threshold value of the pressure gradient for the steady separation to evolve towards an unsteady one, with periodic vortex shedding at the right frequency. In the second part, a large eddy simulation of the three dimensional case is presented and compared to the results of a quasi-equivalent direct numerical simulation performed by Spalart and Strelets (1997). Mean characteristics of the flow are recovered, and some statistics are produced. In the last part, we simulate the same case with the Spalart-Allmaras one equation turbulent model, which gives fairly good results, even though it evolves towards a steady state.