Report on Coanda Effect: Experimental and CFD analysis

The Coanda effect on the lower surface of the Anuloid model (scale 1:22) has been investigated by means of CFD analysis and experiments on a simplified configuration. The motivations of this investigation are essentially related to the crucial aspect concerning the original maneuvering solution of the aircraft based on the realization of the Coanda effect properly generated on a specific sector of the external annular body on the lower surface. This work was not planned in the submitted proposal and was activated on the light of the suggestions pointed out from the project evaluators focused just on a possible critical behavior of the Coanda effect. On the respect of these suggestions the priority was given in this direction cancelling the work related to the WP 3 " Power means, flight control means and remote (or tethered) control system" and related deliverable. From this point of view a CFD analysis has been performed by FESA evaluating the effects of the modification of the geometry of the circumferential vanes on the flow evolution around the lower surface of the aircraft. Moreover also the ground effect has been analyzed considering several distances between the model and the ground. The main result of this analysis reports that Coanda effect in the original design is metastable. The new geometry shows instead that a very stable Coanda effect. The experimental investigation performed by POLITO was focused on the measurements of the pressure distributions around the lower curved surface in the case of the original geometry. Also some modifications of the base of the central body aimed at promoting the Coanda effect were considered mounting different disks on the base that changed the geometry of the annular exit section of the wall jet. A comparison between the experimental data and a model present in the literature representing the Coanda effect in terms of angle of separation as a function of a geometric parameter was also made. Simple flow visualizations were performed using tufts. Furthermore also the effects of asymmetric disks mounting were tested in order to verify the asymmetry of the Coanda effect devoted to the generation of appropriate load aerodynamic distribution on the lower surface aimed at the maneuverability of the aircraft. Finally the effects of the Reynolds number were evaluated. From this analysis is evidenced that the original geometry does not show any evidence of Coanda effect for all the Reynolds number tested. Introducing the base modifications the flow is more attached to the rounded lower base highlighting the promotion of the Coanda effect that in any case does not involve the whole rounded lower wall of the model