**6. Conclusion**

In the beginning of the chapter we have achieved an analytic solution that approximates a two-dimensional Coanda flow. The validity of the results is limited to the cases given by *b R*/ 1, since in the tangential component of momentum equation the curvature was neglected \* *y* 1 .

The validity of the laminar solution is for Reynolds numbers smaller than the critical value which is of the order <sup>4</sup> 3 10 . For the turbulent flow the similarity assumption has been reduced primarily to the general development along the flow, leaving the transverse distribution of shear stress.

In this model the edge of the intermittent flow has been neglected, so that assuming constant turbulent viscosity in cross section has lead to similar velocity profiles for both the laminar and turbulent flow. However, the general configuration of the flow development was different. In order to specify the development of this self-modeled flow only one empirical constant was need.

In many applications that use boundary layer control by tangential blowing, the solid surface downstream of blowing slot is strongly curved and, in this case, the prediction of jet involves separation and a more accurate knowledge of the flow (radial and tangential pressure - velocity profiles) which has been done by CFD methods.

The compressible Reynolds-averaged Navier–Stokes (RANS) equations have been solved for circulation control (CC) airfoil flows. Different turbulence models have been considered for closure, including the Spalart–Allmaras model with and without a curvature correction and the shear stress transport (SST) model of Menter. Numerical solutions have been computed with a structured grid solver. The effect of mesh density on the solutions has been examined.

We have investigated the characteristics of various Coanda surfaces, involving smooth curved surfaces and a polygonal curved surface with flap. Using the FLUENT code we have analyzed the distribution pressure and separation on the considered surfaces.

Further, we have taken interest in the detailed behavior of an existing Coanda ejector model, used in propulsion systems. For numerical investigations we have used an implicit formulation of RANS equations for axisymmetric flow with a shear stress transport *k* (SST model) turbulence model. The numerical results have been obtained for a total pressure range of 1-5 bars, imposed at the reservoir inlet. The goal was to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector performance. The effect of various factors, such as the pressure ratio, primary nozzle and ejector configurations on the system performance has been evaluated based on the performance parameters. The mixing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio of secondary mass flow rate to primary mass flow rate and the mixing length.

Because single jet flows or multi-jet flows are extensively applied in conjunction with the Coanda surface, as confined or free jet flows, in the last part of the chapter we have provided further insight into complexities involving issues such as the variety of flow structure and the related bifurcation and flow instabilities.

We have considered two cases: i) the flow bifurcation in the symmetric planar contraction channel for different contraction ratio and Reynolds number (single jet) and ii) the flow structure as bifurcation phenomena involved in the confined twin-jet flow field, related to the parameters of jet momentum (Re), side-wall confinement and jet proximity effects.

Also was presented a simple exemplification for bifurcation in transonic flow over an particular airfoil.

Thus, we have determined the conditions and the limits within which one can benefit from the advantages of Coanda-type flows.
