Steady flow separation patterns in a 45 degree junction

Numerical and experimental techniques were used to study the physics of flo w separation for steady internal flow in a 45 degrees junction geometry, su ch as that observed between two pipes or between the downstream end of a by pass graft and an artery. The three-dimensional Navier-Stokes equations wer e solved using a validated finite element code, and complementary experimen ts were performed using the photochromic dye tracer technique. Inlet Reynol ds numbers in the range 250 to 1650 were considered. An adaptive mesh refin ement approach was adopted to ensure grid-independent solutions. Good agree ment was observed between the numerical results and the experimentally meas ured velocity fields; however, the wall shear stress agreement was less sat isfactory. Just distal to the 'toe' of the junction, axial flow separation was observed for all Reynolds numbers greater than 250. Further downstream (approximately 1.3 diameters from the toe), the axial how again separated f or Re greater than or equal to 450. The location and structure of axial flo w separation in this geometry is controlled by secondary flows, which at su fficiently high Re create free stagnation points on the model symmetry plan e. In fact, separation in this flow is best explained by a secondary flow b oundary layer collision model, analogous to that proposed for flow in the e ntry region of a curved tube. Novel features of this flow include axial flo w separation at modest Re (as compared to flow in a curved tube, where sepa ration occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.