2
$\begingroup$

So as the title suggests, I was wondering if the wake vortices formed by a plane interfere with the flow over/under a wing. (My assumption is that they don't, but I'm not sure)

Also, how would speed and angle of attack affect this?

Picture to show what I mean by wake vortices enter image description here

$\endgroup$

3 Answers 3

4
$\begingroup$

Wake vortices start over and under the wing.

Air moves towards low pressure and away from high pressure. A lift-creating, straight wing has its lowest pressure right in the middle while below it pressure is relatively high. Therefore, the streamlines converge towards the middle on the upper and away from it on the lower side. When that air leaves the wing at the trailing edge, it has an inward component above and an outward component below it. All what is needed for the vortices to form is to be free of that separating layer called "wing".

A different and less intuitive explanation uses bound vortices to explain lift. Since those vortices have no end, but the wing has, they leave the wing in backward direction (and are then called free vortices). At every point where this vortex strength changes, bound vortices bend backwards and leave the wing. This vortex sheet then combines into the two vortices visualized by water droplets in the picture of your question at some distance behind the wing.

Both explanations are equivalent and mean the same. Lift creation over a limited wingspan will cause those vortices, and the pressure distribution over the wing (which produces lift) produces those vortices as an inseparable byproduct as well. To call this "interference" would be inappropriate.

Vortex strength is inversely proportional to the square of flight speed and proportional to the square of span loading (mass per unit of wingspan). When doubling the lift by doubling angle of attack (counted from the zero-lift angle), vortex strength will quadruple.

$\endgroup$
5
  • $\begingroup$ I see, thanks for your answer. One question I have though: if there is an outward component of flow on the bottom of the wing and an inward component on the top, that would create a vortex along the wing, right? Like a vortex along the span somewhere, not the tip vortex. $\endgroup$
    – Wyatt
    Dec 8, 2023 at 17:29
  • $\begingroup$ @Wyatt No, since the overwhelming speed is still backwards. The streamlines will only bend slightly and gain a small sideways component over the wing. Only after they leave the wing does the full rotation develop. $\endgroup$ Dec 8, 2023 at 18:12
  • $\begingroup$ ah, I see, thanks. I was curious about how swept wings would fair with the airflow bending inward (on the top of the wing) due to them having a spanwise outward flow component already. Would they just mathematically add, meaning the spanwise flow would be a little less due to the inward bending air? Also, I don't want to ask tons of questions, but how come the pressure is lowest in the middle of a wing? $\endgroup$
    – Wyatt
    Dec 8, 2023 at 20:29
  • $\begingroup$ @Wyatt Good question about swept wings! Short answer: Poorly! The center of swept wings has much less lift per area than the inner wing because streamlines cannot bend inwards there ("Mitteneffekt"). Streamlines bending outward is a different, boundary-layer-related phenomenon. Why lowest pressure in the middle of a straight wing? Because there the distance to the tips is greatest there and flow is more 2D than farther out. $\endgroup$ Dec 9, 2023 at 7:18
  • $\begingroup$ That makes sense, thanks! I think I had misinterpreted what you had said about the lowest pressure in the middle of a wing, but now that makes total sense. Thanks again Peter, you always answer my questions! $\endgroup$
    – Wyatt
    Dec 9, 2023 at 18:01
2
$\begingroup$

(Generalisations/simplifications do apply here...)

They do - or to be more precise are more of less an effect of an interfered airflow on the wing - but it depends where on the wingspan.

Vortexes are more or less created by a (partially) spanwise flow of air near the wingtips to equalize the pressure diffential below and above the wing.

See https://en.wikipedia.org/wiki/File:Tip_vortex_rollup.png for a graphic on that subject. Vortex rollup

So yes, there is a section on the wing where the airflow is disturbed by the generation of the vortices.

$\endgroup$
3
  • 1
    $\begingroup$ "Vortexes are more or less created by a (partially) spanwise flow of air near the wingtips" - more precise: They are shed by the wing wherever the local lift changes over span. This effect is strongest at the tips, but happens over the whole span. $\endgroup$ Dec 8, 2023 at 13:31
  • $\begingroup$ @tsg oh okay thanks. So basically the vortices I’m talking about pull some air spanwise? How would that happen because those vortices are formed below the wing, correct? $\endgroup$
    – Wyatt
    Dec 8, 2023 at 14:42
  • $\begingroup$ To be clear because I think there possibly could be some confusion: I’m talking about the vortices in the wake of the plane, not the tip vortices. $\endgroup$
    – Wyatt
    Dec 8, 2023 at 15:40
1
$\begingroup$

The question doesn't really make sense in terms of real physics. There is only one real flow field created by the passing of the plane. There aren't multiple features that can interfere with with each other via action at a distance.

When modelling flow fields with potential flow models, we do use imaginary vortex points or lines or sheets; or imaginary dipole points or lines or sheets. And these do affect the flow over the entire domain - action at a distance. But real flows don't have any action at a distance.

To get the best results using potential models, some of these imaginary entities have to be sprinkled in the wake to get the wake to develop properly. In this sense, within the potential model, wakes can interfere with the flow over the wing. But this is basically a kludge or exploit of the model to get better agreement with real behavior.

The real vortices are inextricably linked to the production of lift; and their creation requires energy that manifests as a drag component of the plane. If you try to actively mess with the vortices in an attempt to reduce its associated drag component, you can't - the energy has already been spent. For instance, installing a counter-rotating prop to try to counter the vortex with propwash swirl doesn't reduce drag.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .