As I said before, I’m not interested too much in the throttle body thread anymore because I wrote it a while ago, in a few hours, and only to give some basic info to the members who kept PMing me about what size they needed. I hope you read through the ensuing posts as well as several questions were raised and answered.
However, I just want to have my say on the topic of textured surfaces and flow characteristics. I know you guys want to argue with me over all this, so please take the time and find your references as well and we can have a good debate over all this, I haven’t had a good one in a while.
Why is a turbulent boundary layer better for overall flow characteristics?
Here I am going to talk specifically about the boundary layer, not the overall stream flow characteristic, but it will tie in at the end, so don’t think I’m talking about creating a turbulent flow in the main stream. When you look at a typical flow boundary layer you see a nice gradual increase in the fluid velocity from zero at the surface’s edge. What you see below is a typical subsonic boundary layer.
Specifically referring to subsonic flows, when you look at this boundary layer, you will find that the boundary layer’s terminal velocity (the velocity that matches the main flow’s max velocity) is the same for both smooth and rough surfaces, so maximum flow velocity is not changed, however, the rough surface actually decreases the distance from the wall that the maximum velocity is attained, so the mean velocity is increased.
Now, I’m hesitant to bring up the whole golf ball theory simply because everyone focusses on the fact that it’s an external spherical surface, however, the surface theory is the same if you factor in the rear curve of the cross section. What matters is the flow over a surface, not an object.
So we can have a quick look at the cross section of the sphere, but I want you to focus on the upper trailing quarter section of the 10<Re<10^5 and Re>10^5 pictures because no transition in our intake or intake manifold maintains a true laminar or bound vortex flow pattern.
What you see is a smooth surface when subjected to a laminar flow is the flow will separate more readily than when subjected to a turbulent flow because the laminar flow doesn’t have the inertia to wrap around the surface. Now, by creating a turbulent boundary layer, you can take the flow and move the transition point forward on the surface but effectively move the separation point further downstream and even curve the flow to a degree.
By moving the separation point downstream you increase
the skin friction drag, but you drastically decrease
the pressure drag on the flow, so when making changes in direction or compressing the flow you will see a decrease in the boundary layer and an increase in the mean flow velocity.
What the dimpled surface is trying to mimic, is the reintegration of the streams, similar to a streamlined body. In the case of enclosed pipe flow, it’s preventing separation of the inner curve flow from the wall and crowding into the outer wall flow and impacting the outer wall which would create a very drastic change in direction and induce turbulence into the mainstream flow pattern.
What this all means is that by having a rough surface rather than a smooth one will increase the Reynolds value and turbulence of the boundary layer, but decrease the effect of the boundary layer on the overall flow velocity. What the rough surface does is create a ‘lubricating’ layer that is held to the outer surface of the tube by the lower pressure pockets created by the surface variations. Now, there are limiting factors to this effect, so taking a dremel and destroying the inner surface of a tube will not be ideal, but an even surface abrasion makes a big difference.
So now I’ll pull some important quotes from my references below:
Whilst separation occurs in both laminar and turbulent flows, it has been studied to a greater extent in turbulent flows. This is because
a) Turbulent flows are more commonly encountered than laminar flows.
b) Separation is more likely to occur when the flow is turbulent.
c) Due to inertial effects, separation has a much greater influence in turbulent flows. It accounts for the majority of the drag on a bluff body. Delaying separation until near the end of a bluff body can greatly reduce drag.
What parts of the engine will benefit from a textured surface?
Prevention of Separation
1) Move the boundary with the stream. eg for a rotating cylinder there may be no boundary layer on the side of the cylinder rotating with the flow.
2) Suction of fluid to the wall (eg porous diffuser). This removes decelerated fluid from the wall region.
3) Acceleration of the boundary layer (blowing), eg slotted wing. Gives a high kinetic energy to fluid in the boundary layer to overcome the adverse .
Any subsonic flow in the engine that is affected by surface friction and the boundary layer. The exhaust ports and headers actually handle a supersonic flow so it is not affected by surface texture and can benefit from a mirror-like finish. However, any surface that sees subsonic flow is affected by the boundary layer and by reducing the boundary layer’s effect you can create a more efficient flow.
If you were to talk to most head porters and asked for a polished intake and exhaust they would basically tell you to pound sand or try to talk you out of it. Polishing intake ports has shown zero gains over the past 40 years of performance parts production. Some head porting companies are going so far as to pass abrasive materials through the intake ports to rough up the surface, so there’s obviously a reason for it to be done. Here’s a quote from an unverified source on a LS forum:
I'm a tech inspector for two racing associations. Here’s the scenario. end of season, big money race using spec engines. One car is obviously faster than everyone else, while all year he was slow. Upon inspection it is revealed that the entire floor and walls of the intake are dimpled, much like a golf ball. We confiscated the intake, ran back to back tests on two different engines on a Superflow Dyno swapping intakes between tests. Exact same intake, one dimpled, one not. Both intakes port matched to the heads. On these two engines the dimpled intake made 23.9 and 23.2 Hp more, all else being equal. Take it for what it’s worth. This was performed on engines in the 650 Hp range.
The reason is actually two-fold; now that modern CNC machines are able to do some truly spectacular work you’re starting to see fully textured inner surfaces, and the results aren’t earth shattering but can make a difference.
One reason is on pre-valve fuel injection systems you want to maintain suspension of the fuel in the intake flow and with smooth surfaces you’ll find pooling and runnels collecting on the surface, but with rough surfaces the suspended fuel droplets are pushed back into the flow by the turbulence and carried into the combustion chamber. That however is only the last 1” of the flow pattern in the hemi’s intake runners. This is important to know because you have to pay attention to the limiting port velocity if you inject further upstream such as with nitrous, but it’s less an issue in typical N/A or boosted engines considering the short distance that the fluid travels and the velocity and parallel direction that it is injected into the flow.
The other reason is to decrease the boundary layer effect and increase mean flow velocity.
Now, I hope we can have a productive discussion on this. As to why you see so many ‘smooth’ intakes is because of production costs and nothing more. The average joe knows that dragging objects across a smooth floor is easier than dragging them across a rough one, so they look at an intake the same way, and almost every volume CAI manufacturer out there plays on this since it’s cheaper to manufacture an intake tube with a smooth interior than it would be to maximize flow with a textured surface. Molded plastic is also cheap compared to seamless tubing and less labor intensive than fiberglass or carbon fiber. Not too many people I know outside of my dedicated race group are willing to spend $600 on a carbon fiber inner textured true CAI system with properly tapered connections.