Pimentel's blog

Recently, my second journal article in the series ( The Unsteady Full Multi-wake Vortex Lattice Method: a full rolled-up detached vorticity approach [1]) was declined for publication four months after its acceptance (already peer-reviewed) in a second quartile (Q2) journal. This is not due to a lack of consistency in methodology or results, but due to lack of payment of the Article Processing Charge (APC)!, which neither the university nor the Science Council 'can' afford, although both of them are included in the remaining two articles, published without APC in a Q1 journal. Fortunately, I have never been interested in becoming a  paper-publishing researcher  to try to play the game imposed by the current academic system; I have only published my research as a requirement to obtain my PhD degree. However, this does not mean that I believe that knowledge should be restricted; ideally, the knowledge should be open and universal, especially when it concerns the foundations of a ...

There is a classic figure shown in theory of flight  (for pilots) and aerodynamics and flight mechanics subjects (for aeronautical engineering students) that shows a free body diagram with four "forces" acting on an airplane: weight (W), thrust (T), lift (L), and drag (D). Even the NASA Glenn Research Center website shows it: Fig. 1 Retrieved from:  What is Lift? | Glenn Research Center | NASA NASA begins by answering the question, "What is lift?" :  "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air...Lift is a  mechanical aerodynamic force  produced by the motion of the airplane through the air" . But are lift and drag really forces? Or are they just the perpendicular and parallel projections of a resulting force vector? I am pretty sure that all aerodynamicists at NASA know the correct answer. Presenting it in this way is indeed easier to explain to kids and aeronautics novices, however, such express...

Is vorticity generation at surfaces a viscous mechanism? This is one of the fundamental questions in fluid mechanics, and it has a big impact on how fluid dynamics (FD) and aerodynamics are understood today. I posted such a question a few days ago in a LinkedIn group called "Computational Fluid Dynamics" with about 28K members. Surprisingly, it has been replied to by 34 people so far (about 0.12%), which is actually more than expected, considering that most of the publications there do not get a single "click" (most of them for "likes"). Do not be cruel, click on them! Fig. 1 Results for the question after 34 votes (there were 35, but someone removed his participation). I suspect that this "huge result" is due to the controversial nature of the question, which has been simplified to reach a wider audience. According to public information, the participants come from universities to research centers, including both industrial and academic profiles...

Yes, it can (but not in an optimal way)*. That is why aerodynamics has very little to do with curvatures, rounded leading edges, smooth surfaces, sharp trailing edges, and other embellishments ; these are all just makeup! In fact, the word "aerodynamics" etymologically means "air in motion," so a turbulent fluid is more aerodynamic than a laminar one from a kinematic point of view. So, strictly speaking, a car or plane designed in the 50's or 60's is more aerodynamic than a newer one. This is not just a word game, a philosophical question, or to sound woke . To try to understand or explain a thing, it should be named as precisely as possible, avoiding the use of veiled terms, most of which focus on oversimplifying a complex phenomenon by trying to give only quick answers, leaving aside formal justifications. Fig. 1 An artistic description made in Paintbrush (now simply Paint). In this order of ideas, anything can fly/glide with sufficient velocity! Exactly...

Finally, two years after the first rejection, my first paper was accepted until showing the results obtained by the corresponding unsteady vortex element ( "balls and sticks" ) method, based on the same original steady concepts. By the way, who is the lord of the cats ? And now, the most difficult reasoning test ever: 1. Viscosity is related to attached fluids as well as inviscidity is related to: a) Also, attached fluids 🙈 b) Neither attached nor detached but all the opposite 🤡 c) Detached flows 🙉 d) Only God knows 😇 "Learn what is to be taken seriously and laugh at the rest".  —Hermann Hesse

Oversimplifying things risks losing details that could affect the correct interpretation of a physical phenomenon. However, using common sense and logic based on premises derived from physical observations can lead to a better interpretation of veiled concepts that cannot be understood more abstractly. This test consists of four questions, of which at least the first three seem so obvious that they could fall into the concept of  silly  questions; however, it is necessary to ask them to understand the last one and the whole meaning of this publication.  All pictures are retrieved from  'An Album of Fluid Motion'  (van Dyke, 1982), in which a viscous flow (fluid: water) past a circular cylinder under different Reynolds number (Re) conditions. 1.   Which of the following two images represents a fluid past at an  extremely low Reynolds number ?: a)   b) a)  This option is a  completely opposite condition , where inertial forces dominate ov...

Since the Coandă effect (CE) is precisely an 'effect', it cannot be by itself a cause of any force, the same as the pressure field in the velocity-vorticity formulation of the Navier-Stokes equations (vortex methods), in which such a field is decoupled from the flow solution (it can be obtained by reconstruction of the stream function). The real cause of lift (and drag!) generation is vorticity, detached from the entire surface [1,2,3], in addition to viscosity (under certain operating conditions; low AoA range). Thus, the correct question should be: Is the "Coanda effect"* related to lift on an airfoil? The short answer is: yes, and no. I mean, in first instance, it depends on the orientation of the flow when such an airfoil is operating. For the low AoA range, the CE seems to be present due to the effect of the viscosity and the curvature in contact with the fluid; however, if such an airfoil (at the same Reynolds number; Re) is stalled, the CE is absent since th...

Potential Flow Theory (PFT) has always had a bad reputation among most fluidynamicist, among other things, because it is quite abstract and has only been successfully applied to explain, more or less, basic concepts of fluid dynamics, more specifically in aerodynamics, mainly through low-order numerical methods, such as panel ones, including the "well-known" vortex lattice method (VLM). Most lecturers teach during the fluid dynamics or computational aerodynamics courses that the PFT defines an irrotational (vorticity-free), incompressible (divergence-free) and inviscid (viscous-free) flow, which they call "ideal". However, its supposed inviscid characteristic is not theoretically defined and its current numerical implementation (including its attached circulation) is only a crude assumption! Such a misunderstanding can be demonstrated with a single image, from which it can be concluded that in fact, the current interpretation of the PFT has been used until now to...

The Potential Flow Theory (PFT) states that a 'potential flow' must be incompressible (divergence-free) and irrotational (vorticity-free). However, compressible potential flow also exists, but it is not defined within the classical PFT. Then, strictly and in a broader sense, a potential flow must only fulfill the irrotationality condition. At this point, the term 'viscosity' is absent from such a definition, however, for the same reason that such a term does not appear, the fluid-flow viscosity value can be considered zero. Then, an 'inviscid potential flow' is now called an 'ideal flow' to be more specific. However, from the current interpretation, a viscous-forced potential flow (Kutta-Zhukovski-type pseudo- real flow) can also be defined, hence, 'potential flow' is not a synonym of ideal flow, as most authors misunderstand. In other words, the ideal flow is a potential flow, but a potential flow is not necessarily the ideal flow (see the ...