# Efficiency of Pulsating Base Bleeding to Control Trailing Edge Flow Configurations
## 4 Conclusions  
In this work, a thorough numerical analysis on the effects and gains caused by pulsating flow actuation applied at an airfoil trailing edge has been conducted and compared with previous results obtained for constant base bleed and configurations without flow control at subsonic inflow speeds. In particular, actuation frequencies linked to the periodicity of the flow that may be caused by the blade passing on high pressure turbines is investigated. The potential capabilities of pulsating bleeding at the trailing edge as an active flow control methodology are notable, with proven effectiveness on the modulation of downstream flow structures, reducing the overall drag and controlling the frequency of the vortex detachment and transversal forces. Considering the injected mass flow and the frequency of the injection as critical parameters, the results show a drag reduction of almost 85% for a non-dimensional injected mass flow of \(C_{b}=0.13\). While this gain is comparable to that obtained with steady blowing, a pulsed injection results in a more efficient use of the energy compared with constant bleeding, with this effect being more pronounced for higher injection powers. The use of base bleed reduces the amplitude of the force oscillations when compared to the non-blowing configuration, but the effect of the pulsating injection on the lift force associated frequencies is less evident. While relatively low (\(C_{b}\approx 0.13\)) and high (\(C_{b}>0.8\)) values of the non-dimensional base bleed mass flow are still related to a drastic reduction of the wake perturbations, indicating a vortex shedding neutralization, intermediate values  
Figure 11: Dissipated power, \(P_{D}\), against injection power, \(P_{I}\), for constant (\(\square\)) [19] and pulsating base bleeds at 100 Hz (\(\triangle\)), 150 Hz (\(\times\)), and 200H z (\(\bigtriangledown\)). behave more erratically and seem to be more dependent on the pulsating frequency than expected. Further investigation is needed in this regard. However, the obtained frequency spectrograms for the trailing edge downstream flow show a clear modulation of the perturbations and a coupling with the frequency of actuation. Interestingly, 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 (measured through the angle of the base bleed jet angle) increases with the frequency of the pulsating injection. Contrary to the cases with constant base bleed, 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 an increment of the absolute value of the average jet angle. As coolant flow ejected at the trailing edge is currently employed on the enhancement of airfoil performance through base pressure modulation, the results shown herein present the use of pulsating flow actuation as a more efficient flow modulating tool to be considered on cooling flow systems. Additionally, intermittent flow actuation allows for modulate the airfoil wake structures, coupling it to the employed actuating frequencies and allowing for further exploitation of these methodologies. Conceptualization, A.M.-C. and G.P. and E.V. ; Methodology, C.C. and A.M.-C.; Validation, C.C. and A.M.-C.;Writting, C.C., A.M.-C., E.V. and G.P. ; Funding Acquisition, E.V. and G.P. All authors have read and agreed to the published version of the manuscript. This research has received funding from he Ministerio de Ciencia, Innovation e Universidades of Spain under the project SIMOPAIR (Ref: RTI2018-480 09/075-B-I00). This work has received funding from the European Union's Horizon 2020 research and innovation programme under the project FLOWCID, grant agreement No 101019137. Not Applicable. The authors gratefully acknowledge the Universidad Politecnica de Madrid for providing computing resources on Magerit Supercomputer. The authors declare no conflict of interest.