Investigation of separation on a forward facing step

The study of a flow sepaiating from the surface of a solid body, and the deterrnination of globalchanges in the flow field that develop as a result of the Separation,are among the most fundamental and difficu.lt problcms of fluid dvnamics. The phenomenon of Separation is offen encountered in industrial and engineering applicat.io. ns, such as in heat exchangers and flows over buildings and in gcophysical problems, e.g. flows over dunes and grooves. In order to facilitate a better understandingof the Separation process as a wholc, the Separation on a forward facing step in a straight Channel was investigatedby cxperimental and computationalmethods. In the expcrimental part of the study, differeilt visualisation techniques were uscd to reveal the basic structure and dynamics of Separation and to illustrate the flow states and flow behaviour for low and moderate Reynolds numbers (Re-numbers). The visualisation indicates that there exist two critical Re-numbers which dctermine the flow behaviour. Below the first critical Re-number Re/,. C7. ,U ? s 65 (normalised by the step height h and the mean bulk velocitv ü), the Separation is purely twodimensional (2D) if minor three-dimensional (3D) effecrs in the limited region of the side walls are neglected. Above RehyCria the Separation devclops a steady 3D structure over the whole span. T'his structure co.nsi. sts of Separation cells with a wiclth of about 5 step heights. The fluid is entraincd into the cell and relcased at the ends of the cell in streaks over the step. Beyond the second critical Re-numberReh „? f2 ~ 135 these cells. or the relcased streaks start to move in the span-wise direction. This transition process is not altered by the presence of side walls and the streaky pattern can also be observed in turbulent separaten! flow fields. although the movement of the streaks is more irregulär. In order to obtain quantitativemeasurements.the 3D PTV technique was applied for Re-numbers well beyond Reh C! ll2, to reveal the lopology of the 3D Separation in the laminar and in the turbulent ca.se and the mechamsm of entrainrnentand relea.se of the fluid. The Separation oecurring in front of a forward facing step at moderate Re-numbers is an open 3D bubble such that the fluid is entrained between the two Separation surfaces over the whole Separation cell wiclth while insi.de the cell the fluid is transported in a helical fashion until it is continuouslv relcased in longitudinalvortices over the step. Measurements of the velocitv component in span-wise direction show that its maximum is about 25c/c of the mean upstream velocitv. Invariant Charts of the velocity graclient fcensor in the Separation zone show that the 3D Separation is to pologically not limited, althoughthe stähle stretching focus prevails. The existence of all topological possibilities in the Separation zone is also reflected in the results obtained for the second invariant of the velocity gradient tensor calculated along a Single partiele trajeetorv. The structure extraction rnethod which is based on the eigenvalucs of the pressure Hessian (X, enteriou). allowed an unambiguousdefinition of the Separation vortex structure and was for the first time successfully applied to cxperimental data. No pronounced difference between turbulent and laminar separ-