Carlos Pimentel's blog

Go to the first part:  On innovation and other hoaxes: a true story at university (Part 1) After numerically understanding the effects of adding lateral wakes to flat plates, especially for low aspect ratio (LAR) configurations, a more complex steady-state scheme was proposed by my advisor. It consists of including internal detached wakes to account for flow separation, similar to Gersten's vortex model (see Fig. 1, up). Therefore, I first explored a simpler model, the lifting line method (based on the Lifting Line Theory or LLT), by including detached horseshoe vortices instead of bounded vortex rings as in the VLM. Such a scheme is not new, in fact some authors have proposed similar approaches in the past to account for flow separation in the context of Potential Flow Theory (PFT), improving the obtained results by far. Furthermore, I found that in the 90s, Prof. D.A. Durston of NASA published a similar vortex model ( LinAir code) to account for flow separation by including trai...

Go to the first part:  On innovation and other hoaxes: a true story at university (Part 1) After the development, verification, and validation (v&v) of the last code for massive flow separation, the time has come to explore a three-dimensional low-order panel method. The vortex lattice method (VLM) was chosen because of its simplicity in implementation compared to more complex potential-based schemes, such as the doublet lattice method (DLM). In addition, the VLM has historically provided good agreement with the expected results at the lowest computational cost, although it only allows calculations for zero-thickness bodies or plates, similar to a fabric (e.g., a parachute canopy). Originally, the standard (or single wake) VLM provides a linear solution (for the lift coefficient), since it allows flow separation only along the trailing edge. Historically, such a method has been applied from medium-high (AR>4) to high aspect ratio configurations, such as wings or airplanes, ...

This is a true story told in the first person. Real names are omitted because my goal is not to demonize anyone; fight ideas, not people. However, this situation may soon change as all of those mentioned continue to affect my interests. It was 2017 when I crossed the Atlantic Ocean for the third time to attend a conference series about computational fluid dynamics (CFD) software that we distribute in my country. The last two times it was to take summer courses at Moscow and Kharkiv/Kharkov aviation institutes, in 2011 (MAI) and 2014 (KhAI), during my bachelor and master programs, respectively. This last time, I took the opportunity to meet my future PhD advisor and co-advisor, whose I contacted some months ago via e-mail to ask for a chance to develop a project related to parachute aerodynamics, a field in which I have some experience, designing, manufacturing and testing small-scale ones for different applications ( www.chuteshiut.com ). Everything goes fine, as I presented some C...

I remember when some years ago I found an interesting and nice visualization on YouTube called "Inviscid Flow ovrer an airfoil", where some kind of green flow (or fluid) passes an aerodynamic/hydrodynamic shape with incidence (at a high angle of attack; AoA). Most aeronautical engineers know that under normal operating conditions (at a large Reynolds number; Re), fluid detaches near the leading edge, forming a chaotic wake downstream, leading to stall since for this particular case, the AoA is about 22 degrees (I made the measurement by a print screen in Paint and based on pixels; sorry for the lack of scientific rigor). Now the question is: is that title correct? And the answer is: Absolutely NOT! Video 1: Viscous fluid over an airfoil ( Re~1 ). Source:  Inviscid Flow ovrer an airfoil Despite the flow in such a visualization, or even better, the fluid seems to follow more or less perfectly defined streamlines as it flows around the airfoil, similar to the shown by the Pote...

Although the title of this blog article sounds too formal, as if it were a scientific publication, it follows the same idea as most of the previous ones, maintaining more or less a simple explanation of some topics of interest in fluid dynamics, within an acceptable extension. Therefore, I will logically explain why the rotationality remains from the incompressible Navier-Stokes equations (i-NSE) to incompressible Euler (i-Euler) ones after its simplification (from viscous fluid to inviscid flow) and how this concept leads to a better understanding of fluid motion from an alternative vorticity-based perspective. The i-NSE are a set of non-linear partial differential equations (PDEs) that allow approximating the numerical solution for a viscous fluid since they can include all the acting forces such as gravity, pressure, viscous diffusion, and advection (sometimes called convection ) terms. Such equations are mainly described in their velocity-pressure (v-p) formulation, however, they ...

Yes, it is a bug! This can be easily demonstrated by visualizing the velocity field directly in Fluent instead of the Results module, where no kind of boundary layer (BL) appears: Video: A visualization bug in the Results module for an inviscid simulation (version: 2024 R2). However, this bug is only present for simulations performed directly by the native inviscid model. Note that not only the lower limit for visualization is affected, but also the maximum (by almost 2 m/s), as if they were two different simulations. At this point, I will not go deeper into this subject, as trying to find the error in the error is nonsense. Even for an AoA case (3 degrees; not converged solution) such a BL evolution remains (see Fig. 4), while in Fluent it does not. I will probably contact someone who can help me understand what is happening with such results...who cares, nobody performs inviscid simulations nowadays! Fig. 4 Velocity contours for the AoA=3 deg. case; a modeled BL is also present. As...

In my opinion, one of the worst ways to justify a physical hypothesis is to perform simulations, especially when they are subject to modeling and approximation rather than the direct solution of equations, which could also be limited by simplifications and assumptions. However, this time I will make use of a commercial tool that is accepted with enough credibility in the field of Computational Fluid Dynamics (CFD) to justify that vorticity can be generated between the interaction of a solid body and an inviscid medium , following the line of previous articles in this blog. To accomplish this task, I have tried to reduce all the steps involved to a minimum, keeping only the essential ones (e.g. I avoided using a viscous model, put the viscosity to zero, and manually set a free-slip wall)*. A two-dimensional simulation in one of the most popular CFD software (based on the finite volume method; FVM), consisting of an inviscid flow past a NACA 0006 airfoil at zero degrees of angle of attac...

The pressure field seems to be the quick answer to almost any question in aerodynamics (and fluid dynamics), from the cause of the lift "force" on an airfoil and wing tip vortices to more specific aspects such as fluid separation or the generation of vorticity on a surface. For example, the term "adverse pressure gradient" (it sounds so smart) is often used by those trying to justify the cause of fluid separation behind an aerodynamic body. According to a recent survey conducted in a LinkedIn CFD group (see Fig. 5), it is clear that two-thirds (36/53) of the participants believe that there is no direct cause-effect between velocity and pressure fields since both are coupled, while the remaining one-third (17/53, including me) mark a causal in one or the other direction (velocity causes pressure or pressure causes velocity). With such disagreement, this uncertainty must be considered an open question. Therefore, in this short blog post, I will try to justify, from b...