טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZvi Nadler
SubjectEffects of Low-Profile Vortex Generators on a Turbulent
Boundary Layer
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Iosilevskii Gil
Professor Arieli Rimon


Abstract

Boundary layer separation limits aircraft performance. A practical method to alleviate separation is by using vortex generators. Recent studies suggest that the size of the generators can be made considerably less than the thickness of the boundary layer. The purpose of the current research is to understand the effect of these minuscule generators on the flow field. Investigation methods included CFD, wind tunnel measurements and analysis of the findings.

The nominal test case was based on a flat-plate turbulent boundary layer flow. A single generator was put in the flow field. It had a slender trapezoidal planform, and its height was 30% of the boundary layer thickness. It was observed that the generator induces a complex array of vortices, similar to those behind a slender delta wing. The strongest vortex separates from the leading edge; it remained observable at distances of more than fifty generator’s heights downstream. The vortices direct high-energy flow towards the wall and takes low-energy flow away from the wall. The balance of this transport process was assessed using the Lamb vector (the cross product of velocity and vorticity) formulation. It revealed an overall acceleration of the flow over the distances of a few tens of generator heights downwind of the generator, akin to the effect of a positive pressure gradient. 

For the next case, six generators were placed abreast. The distance between neighboring generators was twice their height. It was shown that the vortices generated by the array merged downwind creating a vortex sheet akin to a flattened large vortex. Its effect on the boundary layer was essentially the same as that of a single vortex, but the scales were significantly different.

Two additional cases repeated the preceding two with flow over a bump, rather than over a plate. An adverse pressure gradient and a closed separation bubble appear in downwind of the bump. Vortex generators were shown to reduce considerably the extent of flow separation.  

Empirical correlation was developed between the circulation of the leading edge vortex and the ratio between the generator height and the boundary layer thickness. It allowed replacing the actual vortex generator in CFD simulation with an equivalent vortex. It yielded practically the same flow field, but only at a fraction of the computational cost.