Carlos Pimentel's blog

This list is a summary of misinterpretations that I have noticed during the last years as an aeronautical engineer, which from my perspective (and probably in the opinion of the editors and reviewers who have understood and published my research) do not allow to know the fundamental physics of real aerodynamics (and fluid dynamics). As usual in this blog, I try to be concise by explaining each of them in a single paragraph. No more blah, blah, blah, and let's get to the point. Fig. 1 Continuity exists everywhere. Sky and Water (M.C. Escher, 1938). 1. "Ideal flows perfectly attach to the surfaces". Although it is not yet possible to confirm or deny this from an experimental point of view ( the superfluid fountain is not a flow past an object experiment), the Potential Flow Theory (PFT) assumes that an ideal (i.e. incompressible, irrotational, and inviscid; i-i-i) flow will turn around, even with sharp leading edges, to maintain a perfectly attached flow pattern to the bo

In the context of high Reynolds number aerodynamics, what does it mean to have a "laminar boundary layer" (BL)? How long is it on a wing under normal operating conditions? According to Reynolds number calculations, at best it can be no longer than a few centimeters from the leading edge! (see Fig. 1). So can we talk about a laminar BL in both the physical transition and turbulent regimes, including their instabilities and vortices ? What about so-called "laminar airfoils" ? Perhaps most of the aerodynamicists (experimentalists, theorists, and computationalists) are abusing on oversimplifying things to make everything fit with their own interpretations? Fig. 1 Surface oil flow visualization of a wing upper surface (AR=3, AoA=0 deg., Re=1E6). The fluid separation region (transition to turbulent BL; ignore the red rectangle) is clearly marked around the top of the wing.  Source: "Variable Camber Compliant Wing-Wind Tunnel Testing", C.R. Marks et al. (2015)

Eddies, vortices, whirlpools, tornadoes, and hurricanes; all of these fluidic structures have the same thing in common: they all rotate around a point by a vectorial quantity (which may vary with time). This is where the term "vorticity" comes in. Mathematically, vorticity is defined as the curl of the velocity : 𝜔 ( r , t)=∇× 𝑢 ( r , t). Since the velocity field depends on its position ( r ) and time (t), vorticity also does. Then, vorticity within a vortex can be explained as a measure of the rotation of a flow element as it moves along a closed (circular) streamline about an axis, maintaining a circumferential translation. In an ideal (incompressible, irrotational, and inviscid) vortex, all flow elements remain without local rotation, where they are still irrotational   ( 𝜔 =0 everywhere, except along the axis). Fig. 1 Spiral vorticity. Unlike solid-body rotation, the tangential velocity decreases with increasing distance from the vortex center. Source: https://64.media