M.Sc Thesis

M.Sc StudentValeriya Komkov
SubjectDesign of a Circular to Rectangular Transition Duct for 2D
Exhaust Tail Pipe
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus Levy Yeshayahou
Professor Arieli Rimon
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


A numerical parametric investigation for a circular to rectangular transition duct is presented hereby for a 2D exhaust tail pipe. This research is focused on the influence of geometric parameters along the duct on the flow characteristics. The numerical solution is validated by its comparison to experimental data available on a specific duct geometry. The solution’s sensitivity to different turbulence models was examined, and a parametric study of the transition duct shape (length of the duct and aspect ratio of the rectangular section) is shown. Also the impact the various geometries on the drag and pressure losses is displayed. The nine following geometry types were checked in this research: three different aspect ratio of exit section (2, 3 and 4) for three different duct lengths (1.5, 2.5 and 3.5 inlet diameters).

All the geometries with an exit section of aspect ratio 2 demonstrated smooth attached flow with no separation (i.e. the flow was able to follow the geometry changes). In these cases the dominant parameter leading to the flow losses is the skin friction force, whose magnitude directly caused a drop in the stagnation pressure.

As for the geometries with aspect ratio of 3, flow separation was found only at the two shortest tube lengths. All geometries with aspect ratio of 4 displayed flow separation. In all these cases the flow separation bubbles are open and their shape remained almost unchanged. Right at the beginning of transition area, 4 bubble shaped separation regions were observed near the wall at the point the change in the cross section shape starts (symmetrically around cross section axis - Y and Z). These separation bubbles do not reattach to the duct’s wall. Thus, 4 limited separation zones continue to the end of the computation domain.

Cases with the exit section having an aspect ratio of 3 displayed a higher drag in geometries with flow separations compared to the one with the longest duct length and without the flow separations. Regarding the aspect ratio 4 cases, the minimum drag was detected in the medium length case. Shorter tube length showed the highest drag caused by the relative large flow separation.

Generally, it is clear that increase in the tube surface area (as a result of an increase in the tube's length) causes only small increase in the overall drag and flow losses. Significant increase in the drag is caused by pressure drag due to flow separations.