# Efficiency of Pulsating Base Bleeding to Control Trailing Edge Flow Configurations
## 1 Introduction  
In pursuing higher performance, the architecture of aircraft engines evolves towards lighter and more compact designs. Compactness can be achieved using counter-rotating subsonic turbines [1], transonic turbines [2], or supersonic turbines with pressure gain combustion [3]; but those configurations normally increase the unsteadiness and flow detachment of the boundary layer downstream of the trailing edge, which significantly contributes to the profile losses [4]. To illustrate the problem, Figure 1 depicts a representative trailing edge flow topology. As the flow approaches the trailing edge, the boundary layers from the pressure and suction sides of the turbine airfoils separate into two alternate shear layers, rolling into vortical structures. Eventually, the two periodic shear layers merge downstream at a confluence point, forming a confined region of low pressure and momentum, known as the base region [5]. The flow structures that form downstream of the turbine blade and their behavior are strongly affected by the base region properties, in particular the intermittent shear layer detachment that generates one or more pairs of vortexes that roll downstream. Both the rear suction side of the airfoil and the trailing edge must be carefully designed as they are prime contributors to aerodynamic drag and a source of forcing to downstream blade rows. One way of mitigating both the losses and the source of unsteady forcing is to add momentum to this region, disrupting the base pressure, wake, and the downstream vortical pattern [6]. As high-pressure-turbines work in extreme temperatures environments, they require continuous airfoil cooling. In particular, the trailing edge of the airfoils is refrigerated by the redirection of coolant flow constantly ejected through holes or slots. When ejected at the trailing edge, this mass flow may be exploited to control the flow topology of the base region [6; 7]. In recent years, the use of trailing edge or base bleed coolant ejection has proven to be effective in the mitigation and modulation of wake flow structures [8; 9; 10], or, when high velocities are involved, the trailing edge shock wave [11; 12; 13; 14]. Available results in the literature mostly consider a constant mass flow ejection. Towards a more efficient use of this flow actuation, the modulation of the base bleed flow by means of intermittent trailing edge injection instead of a constant mass flow can be considered. This topic was numerically and experimentally covered in the low supersonic regime [15; 16; 17], with interesting results. While part of the published numerical data suggests that employing intermittent base bleed actuation would not offer significant gains when compared to constant actuation [16], empirical results indicate the contrary. It has been shown that forcing base bleed to actuate by pulses may enhance blade cooling and reduce associated losses, as the modulation of the trailing edge flow could alter the impingement of the trailing edge shock wave on the suction side of downstream blades [17]. However, trailing edge coolant injection may lead to the onset of unintuitive flow configurations. [PERSON] et al. [14] reported for the first time the onset of non-symmetric trailing edge flows when the flow is ejected in symmetric supersonic turbine blade configurations. A hydrodynamic stability analysis of the former configuration performed by [PERSON] et al. [18] demonstrated that a global instability may become dominant for specific blowing intensities, eventually breaking the symmetry of the base bleed jet. The flow topology at the base region may yield flow perturbations, whose properties depend on the ejected flow intensity and may tend to grow, eventually introducing a source of asymmetry, developing a _Coanda_ effect produced by local pressure gradients. While this effect was present only at certain base bleed intensities, its link with the vortex detachment could affect the downstream flow structures and the aerodynamic forces developed on the airfoil [19].