Carlos Pimentel's blog

Go to the first part:  On innovation and other hoaxes: a true story at university (Part 1) Most real aerodynamicists, even computationalists, know the typical pressure coefficient (cp) distribution for an airfoil (with positive AoA) in the pre-stall region,  cp tends to increase from the suction side (top) towards the leading edge (LE), typically with a negative peak value. On the other hand, at the frontal stagnation point, such a value must be equal to 1 (for the incompressible case), which no longer coincides with the geometric frontal point of the airfoil, since it is naturally shifted backwards due to its inclination. Obviously, a zero value of cp must be located in such a curved region, i.e. close to the geometric frontal point (see Fig. 1). But what happens if instead of an airfoil we use an "infinitely thin" flat plate? Yes, in the limit of thickness equal to zero, cp must be equal to the difference between the upper and lower side cp's , which have a different...

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 ...

Go to the first part:  An 'inviscid' boundary layer! Is this a bug?! (Part 1) 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 within 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 no...