Does the Bernouli Effect Work On Cars?

[MikeMass]

Hi Geniuses...
I joined this group because I have a question I can't answer myself. This is about cars. We're all familiar with classic cars having a grill up front and a space right after it, and then usually a radiator to cool the water/coolant in the engine. The discussion we're having in a car forum is about the path of least resistance and whether any improvements can be made.

So let's assume that a car traveling at 50 mph has a certain volume of air that is passing through the grill and into that space in front of the radiator. Most of today's cars also have a air conditioning condenser in front of the radiator. Both the condenser and the radiator have a grid of cooling tubes with thin fins through which air will pass and as it does, it cools the fluid. Many modern cars have sealed off this area top and bottom so a minimum (if any) air can "leak" out through holes and openings rather than going through the condenser and radiator fins. The radiator and condenser represent a sort of "obstacle" in that they are not wide open, and the fin grid limits some of the air flow. The picture below shows a typical classic car with a condenser in front of a radiator.

In this case, the bottom section between the front valance panel and the core support to which the condenser and radiator are mounted... is wide open. My contention is that air moving at 50 mph will choose the path of least resistance and a good deal of the air will flow out the bottom opening, but some air will flow through the condenser and radiator.

In the above picture, we've added a boxed out lower closeout baffle that pretty much closes out the large opening just below and in front of the condenser & radiator. We're not talking 100% closeout... there's still some opportunity for spillage to the sides, but there are plenty of large obstacles to the sides.

So the question(s) of the hour deal with whether focusing the airflow by eliminating the biggest opening will help the flow of air through the condenser and radiator, or whether some sort of back pressure is created and inadvertently hinder airflow through the condenser & radiator? I'm a 79 year old senior LOL, and I vaguely remember the Bernouli Equation from high school physics but I'm unsure of whether it applies here or some other equation applies? I sincerely appreciate your comments and opinions.-Mike/Mass

 

[russ_watters]

In this case, the bottom section between the front valance panel and the core support to which the condenser and radiator are mounted... is wide open. My contention is that air moving at 50 mph will choose the path of least resistance and a good deal of the air will flow out the bottom opening, but some air will flow through the condenser and radiator.

In the above picture, we've added a boxed out lower closeout baffle that pretty much closes out the large opening just below and in front of the condenser & radiator. We're not talking 100% closeout... there's still some opportunity for spillage to the sides, but there are plenty of large obstacles to the sides.

So the question(s) of the hour deal with whether focusing the airflow by eliminating the biggest opening will help the flow of air through the condenser and radiator, or whether some sort of back pressure is created and inadvertently hinder airflow through the condenser & radiator? I'm a 79 year old senior LOL, and I vaguely remember the Bernouli Equation from high school physics but I'm unsure of whether it applies here or some other equation applies? I sincerely appreciate your comments and opinions.-Mike/Mass

Your instincts are leading you in the right direction: yes, closing off around the condenser will force more air through it. It will also increase the pressure drop, causing more air to deflect around the front grille. Another effect to consider is how this will change the airflow under the car.

 

[Arjan82]

Bernoulli applies perfectly well here, lower velocity means higher pressure.

Look from the perspective of the moving car. Air that is passing by undisturbed will have pressure 0 (gauge pressure, or pressure minus atmospheric pressure). If you put something in front like a radiator the air's velocity is lowered and thus the pressure is raised. This raised pressure causes a pressure differences over the radiator and thus a flow of air through the radiator. Note that the amount of air through a grill is determined by the pressure difference over that grill (thus the pressure in front of the grill minus the pressure at the back side of the grill, which we assume equal in all cases here).

So, the balance that is struck is the following: a given airflow through the radiator needs a certain pressure difference to obtain it. Furthermore, there are two paths: one through the radiator and one past the radiator. The latter also needs a certain amount of pressure difference to obtain a certain amount of airflow, but you need less pressure difference for the same amount of flow. So now you can draw a parallel with an electric circuit with a voltage difference over two parallel resistors. The lower resistance draws the higher current, and in this case the path past the radiator draws the higher flow.

If you add extra resistance on the path past the radiator by closing it off, you increase the resistance, increase the pressure difference over the radiator and thus increase the flow through the radiator.

Ps: isn't there usually also a fan attached to the radiator which forces a flow through it as well (I hardly ever open a bonnet myself...)? That changes the picture since a fan at a certain constant rpm enforces a flow rate (rather than a pressure difference) so a fan with a constant rpm (usually regulated by the engine?) will 'prevent' a higher flowrate by the closing to some extent, however, the torque on the fan will be less so you draw less electrical power :).

 

[MikeMass]

Arjan... Thank you so much for the comments and explanation. And yes, you are correct, all radiators have either a mechanical or electric fan usually pulling air through the condenser & radiator. Most older cars had mechanical belt driven fans, some had "clutch" fans that disengaged if a coil in the clutch reached a certain temperature so as not to tax the engine with a load. Today, eFans are used and are usually electronically controlled. So if the engine thermostat is a 180° one, usually the eFan sensor will trigger the fans at 185°-195° and remain on until the engine coolant temp drops below the "ON" threshold.

Your comment in the 3rd paragraph says what I thought was the case.
"If you add extra resistance on the path past the radiator by closing it off, you increase the resistance, increase the pressure difference over the radiator and thus increase the flow through the radiator."

So in Layman's terms, closing off the lower section increases the pressure of air inside that area and since the radiator is porous, the air flow through the fins will increase? Is that measured in CFMs?

TIA-Mike

 

[Arjan82]

So in Layman's terms, closing off the lower section increases the pressure of air inside that area and since the radiator is porous, the air flow through the fins will increase? Is that measured in CFMs?

Correct, and I think the effect is strong enough to be measured in CFMs yes. How many I couln't tell you. Also note that with a running fan this effect is much less strong, since a fan at constant rpm enforces a flowrate rather than a pressure (within reasonable operating conditions of the fan). So changes in the pressure are nullified by the running fan (but the fan torque is altered). If the fan is not powered and left to rotate freely, then the effect of closing off the spills does have an effect.

 

[jrmichler]

If you want to learn more about making and measuring the effect of aerodynamic changes to your vehicle, an excellent book is Modifying the Aerodynamics of Your Road Car, by Julian Edgar: https://www.amazon.com/dp/1787112837/?tag=pfamazon01-20.

Chapter 8 is titled Improving airflow through heat exchangers.

Disclaimer: He devotes two pages to discussing the aerodynamic topper I built for my truck.

 

[jack action]

You imagine the air pressure in the lower open area is low, thus an easier path to travel compared with the radiator; but is it?

Air going under a car tends to increase the pressure and contributes to aerodynamic lift, a problem with typical passenger cars.

The radiator has a fan that forces air in the engine bay and that air may exit underneath the car, right behind the air openings you show in the pictures. So it is possible that the air underneath the car, behind the radiator, causes enough restriction to divert the air coming under the front bumper to be diverted upward and through the radiator, especially with the help of the fan. In such a case, by blocking these openings, it would be the equivalent of blocking the front grill.

I'm not saying this is the case, but if a car could be improved by simply putting a piece of plastic to direct the flow, car manufacturers would probably have done so by now (and some do on certain models). That is, considering the usual vehicle speed range conditions as well as the most extreme ones.

That being said, one car I know that could come optionally with such panels is the Pontiac Firebird '69. Here is what a standard '69 Firebird engine bay looks like:

Here is what a '69 Firebird with the Heavy Duty cooling option (with a bigger engine or A/C) looks like (baffle panels added):

There is also a lower baffle panel added (similar to what you did):

Note also the vertical lip on the lower baffle panel. This may look insignificant, but it does increase the restriction on the airflow underneath the car and helps redirecting it upward, including through the front grill.

 

[Chestermiller]

In my judgment, the Bernoulli equation is not adequate for this application. Air drag across the banks of finned tubes also needs to be considered in the pressure and flow distribution.

 

[MikeMass]

Jack...
Excellent response! And yes, GM engineers tried various things to limit air "spillage" with the hope that it would improve cooling efficiency and performance. On the 69 Camaros with big blocks and air conditioning they installed a rubber seal at the core support that sealed off the core support to the hood. Now in my case, I went with something similar to what you show on the Firebird with AC. I installed an upper closeout panel as seen here, and yes, its also serving a cosmetic effect. I also added a molded hood pad which pretty much kisses the polished SS upper closeout panel.

And interestingly enough, there was another GM lower closeout design used on some of the F-Body cars with AC. Looked like this:

There aren't many pictures of this one mounted (that I have found yet) but it seems to extend beyond the core support between the engine and the radiator. And always consider that in that era, all those F-Body cars had mechanical, belt driven fans.

Generally speaking, I'm going to go out on a limb here and suggest that GM engineers knew there was an issue of airflow/cooling efficiency and tried various approaches... perhaps even in a wind tunnel before they started tooling these parts. That leads me to believe I'm not on the wrong track with my simple approach of this design.

Although I don't have the scientific ability to test Chester's theory, I would note that the original radiator in these cars are not as efficient as the newer aftermarket aluminum radiators. The old radiators were 4-core designs with very small 1/2" tubes. The newer ones use two (2) much larger tubes that are 1.250" which most agree promotes better cooling.