# Efficiency of Pulsating Base Bleeding to Control Trailing Edge Flow Configurations
## 3 Results and Discussion
### Trailing Edge Pulsating Actuation
#### 3.2.3 Base Region _Coanda_ Effect  
The evolution of non-symmetrical perturbations at the end of the base bleed injection channel develops the interaction of the upper and lower recirculation regions, eventually causing a deflection of the trailing edge jet following a so called \"weak _Coanda_\" effect [18]. This asymmetry in the base region is evaluated herein by means of two interrelated variables: the injection jet angle, \(\beta\), and the averaged value of the lift coefficient, \(\overline{C_{l}}=\bar{I}/q_{\infty}c\), \(l\) being the transversal aerodynamic force.  
The base region flow properties are modified over time as a combination of the shear layer oscillations, vortex shedding, and the behavior of the base bleed jet. The jet angle, \(\beta\), indicates the direction of the injected base bleed flow with respect to the symmetry plane. In this regard, several probes are located at the end of the injection channel, monitoring the different velocity components. An oscillation of \(\beta\) is expected when vortices are shed from the base region, while steady deviations of the injection flow, with respect to the horizontal, are caused by the onset of the aforementioned _Coanda_ effect. Averaged values of the absolute value of the jet angle, \(|\beta|\), are depicted in Figure 10, indicating that a _Coanda_ effect appears for mid and low values of \(C_{b}\), in a similar behavior to that obtained for a steady blowing, and its magnitude increases with the frequency of the pulsating injection. However, as opposed to steady base bleed configurations, where the deviation in the jet remains constant, now an oscillation in the direction of the jet linked to the intermittent flow injection appears, causing a further increment of the absolute value of the average jet angle. It can be argued that this phenomenon is related to the ramping-down effect of the jet intensity when the _duty cycle_ is on its low stage, as the ejected mass flow decreases and acquires values for which a _Coanda_ effect might develop. Finally, in a similar way to the observed trends during a steady actuation, at higher values of the injection rate, the _Coanda_ effect is negligible.  
Figure 8: Main dimensions of the model and location of numerical probes for flow monitoring. Regions for jet flow analysis are highlighted in blue, and the location of the wake analysis probe is named \(F_{w}\).  
Figure 9: Frequency spectrograms taken at location \(F_{w}\) (see Figure 8) during four actuation cycles, for a pulsating base bleed of \(f=100\) Hz and different non-dimensional base bleed rates. (**a**) \(C_{b}=0.00\). (**b**) \(C_{b}=0.13\). (**c**) \(C_{b}=0.90\).  
In absence of trailing edge injection, the mean value of the lift force is zero despite the flow oscillations. However, the _Coanda_ effect induced by certain base bleed intensities introduces a non-symmetric flow topology, causing transversal loads with a non-zero average value. This effect is reflected in the results gathered in Figure 10, where the averaged lift force as a function of the injected mass flow rate is depicted. For illustration, lift forces are compared with those obtained with a non-blowing configuration. As expected, low values of the jet angle are linked to near zero averaged values of the lift force. Lift values increase with low \(C_{b}\) values, this effect being more pronounced for higher injection rates. Without contradicting the aforementioned results of the jet angle analysis, it appears that, despite the low jet angle for high \(C_{b}\) values, the large mass flow injected at the trailing edge has an non-negligible effect on the aerodynamics forces.  
It may be argued that the global effect on the aerodynamic loads over a symmetric airfoil as the one analyzed here is relatively low. However, this effect could gain relevance on cambered airfoils with different trailing edge shapes, possibly causing circulation alterations and thus lift variations.