Effect of Roughness Location

To study the effect of the location of the roughness elements on the loss reduction, the straight step S-2 was placed at three different streamwise lo­cations between the velocity peak and the separation onset location, i. e. at
46%S0, 50%So and 54%S0. The measured loss coefficients under steady and unsteady fbw conditions are presented in Figure 15(a) and 15(b), respectively.

Figure 15. Loss coefficients with s-2 at different streamwise locations

As the roughness elements were moved upstream to 46%S0, just after the velocity peak, the roughness elements induce transition earlier due to the higher local velocity and result in smaller separation bubbles. In the case of the steady fbw conditions, the loss is reduced at all the Reynolds numbers. In unsteady fbw conditions the losses are further reduced at lowest Reynolds numbers. However, at high Reynolds numbers, the earlier induced transition results a larger area covered by the turbulent flow. This results in a higher loss than that when the roughness was place at 50%So. When the roughness element S-2 was moved downstream to 53%S0, the loss coefficients increase significantly under steady and unsteady conditions. This is not only because the roughness element induces transition a little later and thus results in a larger separation bubble but also because that the separation after the roughness element merges with the separation bubble due to the pressure gradient. The combined bubble gets larger, especially in height. Once the roughness element is placed inside of the separation bubble, there will be little effect from roughness on the boundary layer transition. Roman et al. (2002) made the same observation. Therefore, the optimum location of the surface roughness is at the halfway between the velocity peak and the separation onset location.

2. Conclusions

The parametric study of roughness elements including roughness size, type and location under steady and unsteady flow conditions was performed on a fht plate. The boundary layer was subjected to the pressure distribution repre­sentative of that on the suction surface of an ultra high lift LP turbine blade.

The combined effects of surface roughness and unsteady wakes can further reduce the profile losses of ultra highly loaded LP turbine blades, for which the separation bubble is large due to the high adverse pressure gradient and low wake passing frequency. The loss reduction due to unsteady ft>w and surface roughness is a compromise between the positive effects from the separation bubble reduction due to the surface roughness and the calmed region and the negative effects from the larger turbulent wetted area, the latter two being a direct consequence of the wake-induced transition.

There is an optimum height of roughness element for each flow condition. The optimum roughness height is only 0.15% of the surface length or about 62% of the local displacement thickness. The roughness element does not induce transition immediately after itself but only hastens the transition in the separated shear layer.

The step-type roughness elements are more effective at inducing boundary layer transition than the wire-type roughness due to the higher disturbance level generated after the sharp edges. The wavy roughness elements are more effec­tive at inducing transition and further reduce the profile losses under steady ft>w conditions due to the strong vortices generated by the tails. However, the strong vortices may weaken the effects of the calmed region and result in a negative effect in the loss reduction in unsteady conditions.

The optimum location of the roughness element is at the halfway between the velocity peak and the separation onset location.

Acknowledgments

The work reported in this paper was conduced as a part of the research project UTAT – "Unsteady Transitional Flows in Axial Turbomachineries", funded by the European Commission under contract number G4RD-CT-2001- 00628. The authors wish to thank ITP for the design work of the contour walls. The help provided by Mr. Maciek Opoka is also gratefully acknowledged.

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