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Enthalpy

Vortex Generator

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Hello nice people!

Some objects moving against a gas or liquid, especially wind turbines, aeroplanes and water craft, include a vortex generator at their wing, fuselage or hull, usually before the body recesses. The vortices avoid or ease the "flow separation", so a wing lifts at higher angles of attack, and at fuselages, the drag decreases despite the vortices.
wikipedia
Heat exchangers can use them too.

The common explanations are as undetailed as "turbulent flows separate later" or "introduce energy in the boundary layer". Consistently, usual designs of vortex generators are quite crude, as depicted below or with small variations.

Introduction.png.d93ba0b4dd7af477f9512e7c75de5ef9.png

While quite a bit of experimenting was done in half a century, the generator's shape varied in small amounts, and the performance gain is limited. But maybe the items can be re-thought. I mean, re-thinking is always possible, but sometimes it improves something.

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Posted (edited)

The dimples on a golf ball are essentially vortex generators. By increasing the viscous drag (shear drag) and growth of the boundary layer, they delay separation, and reduce form drag (pressure drag) by substantially more than the increase in viscous drag, so the total drag is reduced and the ball goes further.

Heat exchange benefits are similar to the effects of convection vs conduction. In the orthogonal direction a pure laminar flow would essentially only heat exchange due to conduction (radiation effects notwithstanding)

Edited by J.C.MacSwell

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The vortex generator has a more detailed explanation, less common than the vague ones, but existing already.

  • Friction slows down the fluid near the wing's, body's... surface, creating a boundary layer there.
  • Where the wing, body... recedes, the fluid must regain pressure from its speed.
  • But the fluid has less speed in the boundary layer. Fluid from other origin, with more speed+pressure, can dislodge it. Usual explanation of flow separation and stalling.
  • The intentional vortex replaces slow fluid near the surface by fresh fast fluid or a mix of both. This avoids or eases the flow separation.

If this explanation holds, vortex generator designs can improve. We need movement to brings fresh fluid near the surface, but no short-range turbulence. And creating a side fluid movement is the same as creating lift (...at least far from other surfaces), for which wings have been optimized for decades.

BadBetter.png.332348d201d20f8b1ba7e65aee52dc98.png

  1. The vortex' side speed must swap slow and fresh air over the useful distance. Faster is wasted power. Also, the usual excessive angle of attack of the generator's elements lets the flow separate from their extrados, which reduces their lift. Some designs visibly integrate that, but most don't.
  2. The elements should have a curved profile, not straight. Same reason and optimum as a wing profile. Where a big angle of attack is needed, use a Fowler design or similar.
  3. Lift is unwanted at the tip of the elements. The short-range vortex there creates drag but mixes nothing. The chord and the local angle of attack can decrease towards the tip. The best distribution differs from a wing's ellipse: it would be linear for a uniform vortex angular speed if the movement were uniform upstream the element, but speed is smaller near the surface. Designs that don't resemble blades can improve that too.
  4. Wings improve their lift-to-drag with shorter chords, but I presently believe this holds at constant area, not at constant width. More reasons favour shorter chords, which several elements per vortex can enable. They would help an other like turbine blades do.

If algae, fluid compressibility... need it, the elements can be swept back or have a delta shape, like a wing.

Marc Schaefer, aka Enthalpy

==========

Hi MacSwell, thanks for your interest!

I also read (but have no opinion) that dimples also make the path of a golf ball better predictable, by spoiling the Coanda effect.

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The vortex generator elements can have new shapes.

TShape.png.69e09eec07d4e7ce8700700179f2b906.png

The T-shaped element has its deflecting surface outside the boundary layer to catch fresh fluid and steer it towards the surface of the wing, body etc. This faster and less turbulent fluid takes less power to steer than the boundary layer fluid it dislodges or mixes with. Where the surface recedes, the converging speed of the fresh fluid contributes to prevent flow separation.

UJShape.png.85e9a7de0c7f64883b3dfe995c346e77.png

The inverted-U-shaped element is sturdier and nearly all its area serves to deflect fluid, even if in the boundary layer. The continuous shape minimizes the unnecessary turbulence. For high speed, the top can be swept back or one foot can stay forward, but this shape can still catch stray objects in the fluid.

The inverted-J-shaped element can be swept back, with the tip well behind the foot, and then it should release most stray objects it catches.

Marc Schaefer, aka Enthalpy

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Do you realize that you can download and install software simulating flow of fluids around geometry that you will make to check out how much you will gain (if anything)?

e.g.

https://www.autodesk.com/products/cfd/free-trial

This way your posts would look much more professional if they will include recorded animation of simulation uploaded to YouTube with comparison of various designs..

 

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If a vortex generator serves to replace the boundary layer with fresh fast fluid, as I believe, then a vortex that extends for many turns wastes power and is less efficient. I propose instead to straighten the flow after half a turn or after a semi-integer number of turns.

No more power is wasted in the fluid's lateral speed. Or equivalently, the straightener regains as a thrust a part of the drag occurring at the diffuser. The complete vortex generator shall then add a smaller drag and possibly work better.

Straightened.png.b76896817b80d00dd17481b305d8364a.png

Here the chosen elements are perpendicular to the surface, but pretty much any shape fits, and the diffuser and straightener can use different elements and orientations. It's an opportunity to illustrate the aforementioned several elements per vortex and the flaps and Fowlers.

Marc Schaefer, aka Enthalpy

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Only familiar with vortex generators on military planes.
These usually take the form of sharply swept leading edge root extensions on the wings, where hi-pressure underLERX flow is allowed to circulate to the top of the LERX, setting up the spiral circulation for the over-wing vortex flow. This effect re-energizes 'stagnant' flow, and allows an aircraft to remain controllable at extreme angles of attack without experiencing 'stall'.
Notable examples being the F-5E, F-16 and the F-18 with its massive LERX.

Other systems include 'canard' foreplanes as originally used on the SAAB AJ-37 Viggen, to generate the vortex.
Small strakes above the delta wing of the Dassault Mirage 2000.
Nose, or forebody, 'chines', originally seen on the Lockheed SR-71, but now very common on the F-22 and F-35
 

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On 7/19/2020 at 4:19 PM, Sensei said:

[...] download and install software simulating flow of fluids [...] your posts would look much more professional [...]

Hi Sensei, thanks for your interest!

Before putting a first bit of confidence in a simulation software, I'd need to know exactly what it does and how, and compare by myself its results with measurements. If for instance a software for PC computes a boundary layer thickness based on an abscissa, rather than computing paths of air elements, then its predictions about a vortex generator have zero value. But if the software tries to compute turbulence in detail (a hard task for a PC!) then the result use to be random and depend too much on the upstream turbulence.

My posts don't look professional, and I don't care. When I see research papers that publish only simulation, I think "cheap bullsh*t"" and "is there any science there?". Burt Rutan designed his planes with a pocket calculator or with a pencil and a sheet of paper

20 hours ago, MigL said:

[...] vortex generators on military planes [...]

Hi MigL, nice to read you here!

Several vortex generators you cite have the advantage of acting at high angle of attack but stay harmless in normal flight to add less drag. They are often meant for angles of attack never considered on commercial planes.

I hope the designs I propose create less drag than the existing ones. Beyond qualitative thinking, verification needs trials in a wind tunnel.

Several proposals I made would also be efficient just before the position where the wing or body begins to shrink. This is a need for low-drag aeroplanes, and gliders put their turbulators there, far to the aft. They also have their maximum section much at the rear where possible, to keep a quiet flow for as long as possible.

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Not sure if you'll be able to improve vortex generator aerodynamics using Autodesk software.
( although I've never used it )

Most working aerospace engineers will use CATIA software by Dassault Digital Systems.

https://en.wikipedia.org/wiki/CATIA

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