Yaris hatchback grill block aerodynamics
 

Hello everyone! I've found cool areodynamic simulation of my car but can't post link yet. You can find it on youtube just type yaris totalsim. Air flow over top grill is good and bottom one is bad. I've tried block top grill at winter time and radiator fan was running more often at traffic idling and battery life decreased much quicker. When I cover whole bottom grill transmission warmed up much quicker and less wind noise on highway, radiator fan was running more often as well at idle. Is it better to block whole or partial top or bottom grill in my Toyota Yaris Hatchback at winter time?

I wouldn't say airflow over the top grill is "good" vs. "bad" over the lower. Let me explain.

What we're seeing in the first part of that video is a map of the velocity of the particles close to the vehicle surface. The CFD model does not have open grill areas (no airflow through them); instead, those surfaces are closed in order to generate a velocity or pressure map (they're related) over an idealized solid surface.

The purple area on the lower grill illustrates the stagnation point, where the energy of the airflow is almost entirely converted to pressure (velocity near zero), while the green over the upper grill area shows higher velocity/lower pressure. You'll get more airflow through the radiator (by having a greater pressure differential in front of/behind it) by leaving the lower grill open, while blocking the upper grill won't affect radiator efficiency as much but should improve drag by giving the airflow there a surface to flow over.

Radiator efficiency can be further improved by building a sealed duct from the grill to the radiator and, if you're really ambitious, a duct behind as well--but usually there's no room to do that without moving or angling the radiator itself.

Thank you for detailed explanation Vman455! Does building sealed radiator duct would require to cut vents in the hood to redirect incoming air out?

Well, that's certainly an option. You could also choose to vent to a wheel housing, under the car, or out the side behind the wheel arch. Manufacturers have used all of these approaches.

For example, the Dodge Viper vented engine bay air out the sides since its first generation:

https://ecomodder.com/forum/member-v...-viper-063.jpg

The BMW i8 has a nicely ducted radiator with a hood outlet:

https://ecomodder.com/forum/member-v...101-161848.jpg

The 2019 Honda Insight vents one of its radiators (inverter cooling system, maybe?) to the driver's side wheel housing:

https://ecomodder.com/forum/member-v...219-163413.jpg

Maybe you should investigate an openable grille block, like the one I had in my previous Peugeot:
http://ecomodder.com/forum/attachmen...1&d=1345062885
Closed for faster warm up, open only above a certain temperature.

Those are very nice solutions. I like one with wheel housing vent because is the easiest to do and hidden.

Is your radiator fan running more often when you sit in traffic? Do you keep this set up whole year long?

The pic is of the lower grille. It is on year round.

The upper grille gets blocked only for the winter. The block is set a few cm in front of the upper grille opening, so that it diverts direct airflow, but still allows air to be sucked in should the fan turn on.

I used the lower grille block permanently, and the upper block seasonally, for 8-9 years. A few times I took both down for maintenance, so I had an idea of what the "baseline" is. Compared to this baseline, closing the lower grille block improves warm up time, blocking the upper grille improves further. Opening the lower block noticeably lowers the coolant temperature. Knowing how my engine behaves, ie how load and air temp effect coolant temp, allowed me to open and close the grille block early enough to avoid overheating. Yes in traffic the fan would turn on, but I doubt that this happened very often.

On the other hand, I drove so efficiently that with both grilles unblocked the engine would be a bit cooler than it should.

I decided to block half top and to get still some air flow to battery and half bottom grill for quicker warm ups.