Hi, I was wondering if anybody was an "expert" in vortices. In general, I have a few questions:
1. How are vortices formed? By a pressure difference?
2. What are vortex generators, and how do they increase downforce?
From my point of view, a vortex energizes a flow such that it speeds up any slow or disturbed air, therefore reducing the pressure within the air. Kinda like a vacuum sucking in the dirty air. Like in the toyota GTOne, supposedly creating a vortex under the wing will speed up the flow underneath it. But then I read reports that vortices will prevent air from spilling to a low pressure side too. I am confused...
Bring back wider rear wings, V10s, and tobacco advertisements
I dont profess to know the techical reasons but on Aircraft you will notice on the top of the wings at about the SPAR position very small pieces of bent Alloy aligned with the fuse but only maybe 1 inch high. Well when I was working on aircraft ( large jets I am talking about ) these were called vortex generators, and their impact on the aircraft, as I understand it. was to FENCE in the air between each other thus adding to the lift of the wing with the necessary very small amount of drag associated with them. It was imperative for us to check if any of these were missing and raise a repair order to be carried out !
Why does a Golf ball sport those dimples?
These create small vortices which enegize the flow around the flying ball ,reducing the buildup of a boundry layer(Air speed on the surface of the ball is almost zero whereas the ball travels at speed thru the air ,the ball builds up some air cushion around it ,wich increases it´s aero resistance .The vortex generators reduce boundry layer buildup and make the ball smaller /more slippery in the air ..
same goes for Vortex generators at the entrance of the Underfloor :
reduces boundry layer buildup.
Placed at strategical positions these generators effectively seal the underfloor from Air trying to enter the underfloor from the sides of the car,
so these divices form a skirt of energized air.
Indeed, a ball without dimples would have the same effect as a bigger ball. That's why smooth surfcases aren't always the 'smoothest' thruw air. I once experimented with some sailing boat hulls (scale models) and tried them with different surfcases. When I went over the paint with sand paper the friction thruw water became lower. So instead of waxing and polishing your race cars paint you might wanna try sand-paper instead
Vortices are created where different pressure regions meet, as the high pressure air meets a lower pressure area, it diverts and turns into a tumbling action.
They can be formed at a point as “Tip vortices” which appear like expanding spirals, or along an edge “shed vortices” creating a sheet of reversed flow.
In motorsport and F1 vortex generators are devices that create a tip vortex, Tip vortices are also created at the interface of the rearwing and the endplates.
The vortices shed from the rear wing are currently an area for development as the spiralling flow structure inhibits the (downforce producing) wake from the rear wing forming and add drag. By equalising the pressure difference at the point the vortex is created the size and direction of the vortex is minimised. Hence the Renault, BAR and Toyota endplates.
Vortex generators are used not create downforce per se, but “condition” the flow into a direction that may make another aerodynamic device produce less drag or more downforce.
The Shark tooth added behind the large bargeboards on the Ferrari\Jaguar act to excite the airflow running along the step under the floor, this energised flow improves the filling of the diffuser and hence actually creates more downforce.
The pointed gurneys and front wing endplate shaping all aimed at directing flow around the inside of the front wheel and keeping it clear from reattaching along the sides of the car, this doesn’t produce downforce, but the cleaner air running along the centre of the car makes for more efficient aerodynamics.
The tip vortex produced by the winglet on the sidepods is used not for downforce drag reduction either, the vortex pulls air from the chimney (Renault for example) and throws it clear of the rear wing, again increasing another parts efficiency.
Other devices can be used to condition flow without inducing tip vortices, the shaping of the inboard ends of wishbones, bargeboards and flip ups for example….
Scarbs post is exhaustive so I want just to make a few comments/corrections on the previous posts.
The purpose of the vortex generators on the wing of big aircrafts described by rodders is to increase the energy of the boundary layer when flaps are extracted (hence during takeoff and particularly landing). Because of low speed/high angle of attack, the boundary layer in the area of the flap is relatively weak and there’s the risk of separation, hence of stall exactly when you need the maximum coefficient of lift, during the landing; the vortex generators energize the boundary layer allowing it to follow the curvature. Obviously it’s important to immediately replace the missing ones.
About the golf ball I want to explain it in a more precise way, there are apparently minor differences with the previous explanation but these differences are important.
The aim of the dimples isn’t to stop the build up of the boundary layer, it’s to force the transition of the boundary layer from laminar to turbulent. The laminar b.l. has a lower friction drag (*) but it’s also weaker so it’s very prone to separate generating a big wake hence a lot of form drag. A turbulent b.l. on the contrary has an higher friction drag because of an higher shear stress, but exactly because of that it’s also lot more stable, more attached to the body, it still separates but a lot later (in term of space on the surface) so the form drag reduced. Since form drag for bluff bodies is lot more important than friction drag there’s a clear advantage.
The transition laminar -> turbulent happens naturally above a certain Reynolds number (= density * reference dimension * velocity / viscosity) depending by the shape, but speed and size of the golf ball are too small so the Re is too low and on a smooth surface there’s no transition and the laminar boundary layer separates. There are several methods to force the transition, the more simple is a wire perpendicular to the flow. Anyway these methods are mainly patches to be used when the Reynolds is low and if you can’t change too much the shape of the body, examples are the cyclists helmets or the skaters suits or golf balls... no help on race cars.
The reason Plato’s boat was faster with a rough surface on the contrary is different and it has to do only with the friction drag, not with the form drag; the hull of a boat is a streamlined body and the wake is already small so the friction drag is a big percentage of the total drag.
As I’ve said the turbulent b.l. generates an higher friction drag, because of the small (very small) scale turbulence very close to the surface increasing the shear stress. The rough surface destroys these vortices, the b.l. is still turbulent, but the friction drag is reduced. The same concept of sharkskin and riblets. Anyway it’s important to understand that these methods are very helpful only when the friction drag is a big % of the total drag, in case of streamlined bodies (an example are the solar cars) and particularly with bodies in water, because of the high viscosity.
On race cars form drag and induced drag are way more important so the advantage would be negligible.
At the end, the guest also mentioned the sealing effect of the vortex alongside the sidepods; surely you have noticed that on the FW26 there was is Spain a diagonal bar limiting the deflection of the horizontal fin just in front of the sidepods, that’s probably because there was a misalignment between the vortex generated by the fin and of the edge of the sidepod with the result that the sealing effect was reduced.
(*) drag is the sum of basically 3 parts : friction drag (caused by the shear stress on the surface), form or pressure drag (caused by the wake generated by the body) and induced drag (caused by the lift/downforce generation)
Micky, yes the the wings chords and cambers are reduced at the endplate to reduce the strength of the vortex, the slots and other work on the endplate act in a similar way.
Reca wrote:The purpose of the vortex generators on the wing of big aircrafts described by rodders is to increase the energy of the boundary layer when flaps are extracted (hence during takeoff and particularly landing). Because of low speed/high angle of attack, the boundary layer in the area of the flap is relatively weak and there’s the risk of separation, hence of stall exactly when you need the maximum coefficient of lift, during the landing; the vortex generators energize the boundary layer allowing it to follow the curvature. Obviously it’s important to immediately replace the missing ones.
ln addition to the low-speed, high-lift application, another common application is for transonic flows, where the design intent of the v.g. arrays are to delay buffet onset and/or penetration by adding some passive control to the shock-boundary layer interaction such that the flow development during initial separation is controlled whilst maintaining acceptable stability and control characteristics and thus increasing useable lift in the cruise (or manoeuvre for military applications), but with an associated drag penalty. However, if the v.g. array otherwise prevents gross separation and shock travel at the operating condition then a net benefit is considered achieved.
