Grille Shutter Flushness?

[Blue Angel]

I've seen several references to a "bubble effect" that apparently happens on the leading face of a vehicle where a recessed feature can cause air to build up in front of it, hindering airflow. Examples would be:

-Recessed grille openings
-Recessed fog light bezels
-Recessed headlights

Generally, it seems, placing a smooth sheet over these features will streamline airflow. If I am headed in the wrong direction, please point me straight!

My question is this: How "flush" behind a grille does a grille block need to be for maximum effectiveness?

My Cruze Eco has an upper grille that's mostly blocked. It has a hexagonal grille pattern that is blocked, leaving the pattern exposed at the front. The center section of the grille has the same hexagonal pattern, but the holes go through. See this pic:

Chevrolet Cruze Eco 2011 2011 Chevrolet Cruze Eco - Grille View

If I was to design a variable shutter grille block, the simplest thing I could do would be to build a box structure behind the grille with shutters in the back of the box. This, however, would leave a large air "pocket" between the grille and the shutters when the shutters are closed.

Would it be worth the extra effort to build something that sat closer to the grille to minimise the amount of air between the grille and shutters?

[COcyclist]

Grill blocks help mpg intwo ways; better aerodynamics and quicker warm-up times. If you pulse and glide with the engine off, you can keep engine heat in with a blocked grill. IMHO an adjustable grill block behind the hexagons would benefit you much more than a slightly more aerodynamic but fixed grill block in front of the grill.

[Jicafold]

I was also pondering the presence of recessed grill openings and fog light or faux fog light pockets. Why do new automakers still design vehicles this way? It can't be as aerodynamic as a flush surface. Is it worth it to block these off and make the surface flush with the rest of my bumper and front air dam?

[turbovr41991]

I am having this same problem on my car. Trying to make it as flush and smooth as possible but still having it look normal / stockish. I think the 96-98 civics really struggle in the lower grill block area on pulling this off.

[ChazInMT]

It has been discussed many times in the forums that smooth is only a slight benefit on the front of the car. A recessed block will give you 95% of the benefit of a smooth one. Obsess away if you wish.

It has to do with the air stacking up and sort of forming its own shape. By diverting the air around or over your car, you are sending it on a less resistive path than going through your radiator and engine bay.

[Shepherd777]

Originally Posted by Blue Angel

I've seen several references to a "bubble effect"

Like they say, one picture is worth a thousand words...

[RedDevil]

Originally Posted by ChazInMT

It has been discussed many times in the forums that smooth is only a slight benefit on the front of the car. A recessed block will give you 95% of the benefit of a smooth one. Obsess away if you wish.

It has to do with the air stacking up and sort of forming its own shape. By diverting the air around or over your car, you are sending it on a less resistive path than going through your radiator and engine bay.

Yes, and since the air just in front of the grill is moving at almost the same speed as that grill, the friction is negligible.
The higher pressure ahead is pushing the surrounding air aside and over the car, and does so almost completely frictionless.

On the other hand, recesses that are placed in the air stream like at the very edge of the bumper or the headlight area, they do have an impact and would gain from flushing.

[Blue Angel]

Originally Posted by Shepherd777

Like they say, one picture is worth a thousand words...

Beautiful! Now what I need to see is what the feature looks like that is causing that bubble, and how much of a negative affect it has on the desired airflow results... got any more pics you can share regarding that sim?

[Blue Angel]

Originally Posted by ChazInMT

A recessed block will give you 95% of the benefit of a smooth one...

...By diverting the air around or over your car, you are sending it on a less resistive path than going through your radiator and engine bay.

I'm assuming this to be somewhat true since the factory grille shutters GM put in the lower grille opening are recessed quite a bit. Although on a mass production platform like the Cruze I bet the Engineers had to work within a lengthy set of compromises to get the idea into production, the largest of which could have been the requirement to work with the same facia as all the other models...

[aerohead]

Originally Posted by Blue Angel

I've seen several references to a "bubble effect" that apparently happens on the leading face of a vehicle where a recessed feature can cause air to build up in front of it, hindering airflow. Examples would be:

-Recessed grille openings
-Recessed fog light bezels
-Recessed headlights

Generally, it seems, placing a smooth sheet over these features will streamline airflow. If I am headed in the wrong direction, please point me straight!

My question is this: How "flush" behind a grille does a grille block need to be for maximum effectiveness?

My Cruze Eco has an upper grille that's mostly blocked. It has a hexagonal grille pattern that is blocked, leaving the pattern exposed at the front. The center section of the grille has the same hexagonal pattern, but the holes go through. See this pic:

Chevrolet Cruze Eco 2011 2011 Chevrolet Cruze Eco - Grille View

If I was to design a variable shutter grille block, the simplest thing I could do would be to build a box structure behind the grille with shutters in the back of the box. This, however, would leave a large air "pocket" between the grille and the shutters when the shutters are closed.

Would it be worth the extra effort to build something that sat closer to the grille to minimise the amount of air between the grille and shutters?

A recessed depression isn't necessarily a liability.If it's non-aspirated,like a foglight,as long as there is ample radius around it,the surrounding airflow will go past the stagnant pool of air trapped in the bucket as if it was already faired in flush.
In a crosswind,there might be premature separation,but that might be viewed by some as a positive when yawing moments are concerned.
As far as the grille block itself,you'll want enough radius where the air is passing through or you'll have turbulence right there.
To visualize bad flow,check out 'vena contracta,submerged vena contracta,or Borda mouthpiece.' Bad! Really bad!