The aging of the reactor pressure vessel (RPV) can be a limiting factor in life time extension of pressurized water reactors. Important thermal stresses can be generated in the RPV in the case of loss of coolant accidents. Cold emergency core cooling (ECC) water will be injected under high pressure conditions into the cold leg (horizontal pipe) and can come in contact with the hot RPV wall. This process is called pressurized thermal shock (PTS). The efficiency of the PTS is dependent on the mixing between cold ECC water and the hot coolant inventory in the cold leg and the upper downcomer (annular space). Several PTS scenarios have been analyzed experimentally in the TOPFLOW-PTS facility, situated at Helmholtz Zentrum Dresden Rossendorf (HZDR), Germany. The experimental objective was reproducing whole scenarios of PTS events under realistic small break loss of coolant accident thermal hydraulic conditions. A single phase TOPFLOW experiment is analyzed numerically in order to get access to detailed information on the temporal development of both thermal stratification and flow behavior in the cold leg and the upper downcomer. Large Eddy Simulations (LES) are applied together with two approaches to account for thermal effects: incompressible fluid hypothesis with Boussinesq approximation and dilatable fluid hypothesis. For an ECC injection transient of 400 seconds, the calculated flow field in the downcomer is discussed and calculated temperature profiles in the cold leg are compared to detailed temperature measurements. The dilatable approach reproduces well the experimental results.