Citation: Bergadà, J.M.; Bugeda, G. Flow Control, Active and Passive Applications. Appl. Sci. 2023, 13, 9228. https://doi.org/10.3390/ app13169228 Received: 1 August 2023 Accepted: 7 August 2023 Published: 14 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Editorial Flow Control, Active and Passive Applications Josep M. Bergadà 1, * and Gabriel Bugeda 2,3, * 1 Department of Fluid Mechanics, Universitat Politecnica de Catalunya, 08034 Barcelona, Spain 2 Department of Civil and Environmental Engineering, Universitat Politécnica de Catalunya, 08034 Barcelona, Spain 3 International Center for Numerical Methods in Engineering (CIMNE), 08034 Barcelona, Spain * Correspondence: josep.m.bergada@upc.edu (J.M.B.); gabriel.bugeda@upc.edu (G.B.); Tel.: +34-937398771 (J.M.B.); +34-934016494 (G.B.) 1. Introduction The Boundary Layer (BL) dynamic performance greatly affects the forces acting on any Bluff body. Ideally, the boundary layer should be attached to the surface but when separation occurs, the vortical structures and the dynamic forces’ amplitude rapidly in- crease. In many aerodynamic applications, the Bluff bodies are shaped in such a way that the boundary separation is delayed as much as possible. Nevertheless, using novel tech- nologies, it is possible to reattach the previously separated BL, or at least further delay its separation. One of the novel techniques which allows for the modification of the separation point of the BL is Active Flow Control (AFC). This consists of injecting/sucking fluid in pre-defined locations. In the vast majority of AFC applications, it is essential to perform an energy assessment in order to make sure that the energy saved by the reduction/increase of the forces due to the modification of the BL separation point is much larger than the energy employed for the actuation. In order to achieve this goal, it is essential to properly tune the five parameters associated to any AFC implementation, groove position, groove width, momentum coefficient, jet inclination angle and jet frequency. Such tuning can be carried out via a parametric optimization or using any optimizer. In other words, AFC is always associated with optimization methodologies; otherwise, the energy assessment cannot be successfully accomplished. The present book is based on a set of published articles that highlight some novel applications of flow control. 2. Book Contents The articles presented in this book are related to novel flow control technologies and are divided into four main categories. The first one presents several Passive Flow Control (PFC) applications, which highlights the current relevance of passive methodologies. PFC is used to improve the performance of axial compressors by reducing the generation of shedding vortices at the trailing edge of a blade. This was initially investigated by Gao et al. [1]. Flow field improvements in highly loaded compressors and aeroengines are studied numerically by Xu et al. [2] and Lei et al. [3], respectively. In both cases, it was observed that appropriate three-dimensional blading resulted in an increase in static pressure and a reduction in the influence of secondary flows. A flapping bionic wavy leading edge wing is studied by Bai et al. [4], and they realized that it generated a higher lift than conventional airfoils. The passive flow-control application section is closed by the transient analysis undertaken on cruise missiles’ submerged inlet by Zhang and Mi [5]. The use of a distributed submerged inlet proved to have clear advantages compared to the conventional inlet. Closely related to passive flow control applications is the section related to surface micro-machining, where micro-texturing is employed by Shang et al. [6], to reduce the drag and modify the cavity area in hydrostatic bearings. Micro-groves are used by Cacciatori et al. [7], to reduce the drag in an Unmanned Aerial Vehicle’s (UAV’s) fixed wing. The next section presents several Active Flow Control (AFC) investigations. In the Appl. Sci. 2023, 13, 9228. https://doi.org/10.3390/app13169228 https://www.mdpi.com/journal/applsci