OK, there is no way to say this without sounding arrogant so please accept that I'm not trying to come off that way.
I am a professional aerodynamicist; I deal in physical reality and proven science. In the area of subsonic flow applicable to LSR from 100 to 500 mph there is no theory, only well known aerodynamic principles that have been proven for over half a century. It is regretable that this long-proven science is not taught in any easily understandable form for racers. Since it isn't a lot of folklore rules the pits, especially outside of the big money series. Colleagues of mine, some of whom are the finest aero scientists in the world, work on the aerodynamics in IRL, NASCAR, and other series that can pay for their work. All of this said, some fundamental principles would help the people posting here make better aero decisions and more stable and faster cars.
There are three terms that get bandied about a lot: laminar, turbulence, and separation. To understand the basics of subsonic flow, let's start with the correct pairing and definitions:
The boundary layer
is the thin layer between the skin of a vehicle and the free stream
, the flow that is going around the vehicle. In subsonic flow, everything important happens in the boundary layer.Laminar
flow refers to the flow within the boundary layer all going one direction and in an organized fashion.Turbulent
flow refers to the flow within the boundary layer mixing both vertically and laterally in a not-so-organized fashion. Turbulence
is a generic term for disorganized air flow. It is not used by aerodynamicists since it can mean anything from wind shear, to vortex flow, to separation. These effects should be referred to by their proper terms.Attached
flow refers to a boundary layer that follows the contour of the vehicle.Separated
flow refers to a boundary layer that has detached, or separated from the vehicle surface and is no longer influenced by the shape of the vehicle. Separated flow includes both organized
flow. Organized flow includes vortexes that are stable such as flow from a vortex generator
Now for the hard part: Laminar
flow can separate
without becoming turbulent
, and turbulent
flow that is attached
has less drag than laminar
flow that has separated
So to describe the condition of the boundary layer at any given point on the vehicle, flow is always laminar or
attached. This is not semantics, using these words incorrectly will guarantee poor performance.
Most streamlined LSR vehicles have laminar flow over the first few percent of their noses. Then the flow hits some surface imperfection and trips
to turbulent flow. Turbulent flow has more energy in the boundary layer than laminar flow and therefore tends to stay attached. Unless it encounters something blunt or sharp, the flow will accelerate as it flows around the widest point of the vehicle. As it does, it loses pressure and this sucks the flow to the surface. So it stays attached and has low drag. Once the flow reaches the area of the vehicle where it has to contract, it must slow down to free stream velocity and recover
the lost pressure from the high velocity. As the pressure in the boundary layer increases, it can separate. If this pressure recovery
happens too quickly or is combined with pressure recovery from another area (like a parachute fairing or wheel strut) the flow will separate
. Separation causes high drag. In most cases, a small amount of separation creates more drag than the skin friction of the entire rest of the vehicle.
If the flow hits something blunt or sharp, it can separate. Once separated, the flow is almost impossible to re-attach. Separation is the largest component of drag on every LSR vehicle I have seen with the sole exception of BUB 7.Supercavitation
refers to the use of air or ionized gas to reduce the shear forces in the boundary layer of a WATER borne vehicle. Hypersonic air vehicles ionize the boundary layer due to aerodynamic heating and this reduces drag at velocities that we will not see in lsr in our lifetimes.Sucking
the boundary layer into the vehicle and venting it out the back reduces drag and takes power. Suction is especially effective when it reduces or eliminates separation
. In particularly good designs, suction can create negative pressure drag, or thrust. Unless the suction
is part of the rules-legal engine system, I would protest it as a second power source and win the protest.
Laminar flow is something you see in the wind tunnel under controlled circumstances. The air you are piercing is rarely just sitting there. If there is any wind at all you are piercing a turbulent environment.
Take a model of your vehicle and plunge it into the pool at a high rate of speed. What? A big splash? Dang, you'd think it would go into it like sliced butter.
Theory, theory, theory. Just go out and run. Once you break the existing record then maybe you can go to the wind tunnel and fine tune it for better.
Dean LA is right about the most important thing: TEST
. The one thing I can add to that is to put yarn tufts on your vehicle. Run it and photograph it; video is good too. The yarn tufts will make the boundary layer turbulent, but as I pointed out before: it already is. Separated flow will show up like a sore thumb with yarn at as little as 60 mph, so we can (and SHOULD) do this long before we ever get near Bonneville or Elmo.