**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

**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/).

research by Carbajosa et al. [8], the effectiveness of a pulsating jet is compared with a steady blowing jet, aiming to control the flow on a turbine airfoil trailing edge. Several injection frequencies were evaluated, and they demonstrated that pulsating forcing is more effective than steady actuation. A parametric optimization considering three AFC parameters on an NACA-8412 airfoil at an Angle of Attack (AoA) pf 15 degrees and Reynolds number of 68.5\*10<sup>3</sup> is undertaken by Couto and Bergadà [9]. The effectiveness of using plasma actuators on an NACA 0015 airfoil at pre-stall AoA at a Reynolds number of 63000 is analyzed by Ogawa et al. [10]. An optimized design of a slat channel configuration, aiming to increase the lift of a given airfoil profile, is presented by Yu and Mi [11]. In fact, this final paper opens the door to the final chapter, which is **Optimization Techniques** (OT). At present, in many applications and especially those involving AFC technology, it is necessary to optimize the parameters to minimize the energy used for the application while maximizing the outcome. In AFC applications, five parameters need to be optimized, and despite the fact that parametric optimizations can be quite useful, the use of optimizers based on Genetic Algorithms (GA) or gradient-based methods appear to be a much more precise way to accurately tune the required parameters. This is the direction of the work conducted by Coma et al. [12]. In this research, the performance of GA and gradient-based methods is compared when these methods applied to an SD7003 airfoil to tune the AFC parameters. A hybrid evolutionary optimization method (HCGA) in combination with a CFD solver, is presented in the work of Zhao et al. [13], as a decision-maker design tool for aerodynamic shape design. The final contribution of this book was made by Li and Qin [14], who present a review of the different flow control techniques used for gust load alleviation.

### **3. Concluding Remarks**

The reader should consider that the present book aims to simply introduce some of the many existing flow control applications. In fact, the goal of the editors is to open a door to this rather novel technology and hopefully highlight the importance of optimization techniques in AFC applications. We sincerely hope that the reader will enjoy the different research works published here while noticing the variety of applications of flow control technologies.

As a final remark, we would like to reproduce one of Albert Einstein's quotes: "**Life is like riding a bicycle. To keep your balance, you must keep moving**". In this way, may this book help the reader to keep moving.

**Author Contributions:** Both authors have contributed equally to this book. All authors have read and agreed to the published version of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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