Here's a piece we just did on the new GM Reduced Scale Wind Tunnel at the Tech Center in Warren, Michigan.
Not terribly in-depth, but if anyone has further questions I can possibly answer them.
http://www.macsmotor...le-wind-tunnel/
Posted 19 November 2015 - 07:20
Here's a piece we just did on the new GM Reduced Scale Wind Tunnel at the Tech Center in Warren, Michigan.
Not terribly in-depth, but if anyone has further questions I can possibly answer them.
http://www.macsmotor...le-wind-tunnel/
Advertisement
Posted 19 November 2015 - 22:29
One can use increased airflow velocity to accurately correct for Reynolds Number/scaling issues using models? Is the required airspeed multiple a direct function of the model's scale?
Posted 19 November 2015 - 22:38
My first thought was Why build a scale wind tunnel when you have a full-size? Thanks for going through all the reasons - not hard to understand the drivers.
Posted 19 November 2015 - 22:50
No you can't. Yes Re is the proportional to the product of free steam velocity and characteristic length
Reduced scale models are particularly susceptible to problems with boundary layer growth, which is a non linear function of Re, and that function is different for laminar and turbulent BLs.. In particular this means the blocking characteristic between road and floor is wrong, but it also affects drag and lift estimates of the upper surface
Having said that lot of useful work is done on scale models, and of course, your CFD can be run at the same scale for calibration purposes.
Posted 20 November 2015 - 05:08
So I guess the idea would be that designs flow from CFD to 40% to full scale, thus narrowing down the iterations that are produced and tested in full scale?
Posted 20 November 2015 - 05:31
I bet they still start with 1/5 models, but yes
Here's some pretty pictures by our lot, page 7 is of interest to me, that's where we talk to them a lot, simulating cross wind stability. We also do high speed stability stuff but that's a bit tame in comparison, although we frequently find one end or the other lifting off early in the program. http://www.cd-adapco...ustralia_TM.pdf
Posted 20 November 2015 - 06:31
Just the cost of electrical power required to operate the full scale wind tunnel's 4500hp fan is huge. Operating a 40% scale tunnel using an 1100hp fan would likely save a couple million dollars per year just in electrical power costs. Then there is the cost difference between constructing dozens of detailed test models each year at full scale versus 40% scale. Just the foam blocks used to mill a 100% scale model probably cost a couple thousand dollars.
Posted 20 November 2015 - 07:21
We have to book time on the Monash full size tunnel with the electricity generating people, indicating either the enormous power of the Monash tunnel, or the general shonkiness of electrical infrastructure in Melbourne.
Posted 21 November 2015 - 13:34
With a scale model of an object -- say, oh, 1:3 -- the model will confront only one-third the air molecules as the original 1:1 object. Increasing the airspeed 3X will compensate for this, but not for other scaling factors. That said, this does not prevent reduced scale tunnels from providing useful and valuable simulation --- which is all any wind tunnel does.
All types of wind tunnels have their pros and cons, including full-scale. The giant 4,500 hp electric motor in the full-scale GMAL tunnel at the GM Tech Center generates far more heat than the installation can manage on hot summer days. At these times the tunnel must simply be shut down and run only at night. This is one of several items GM hopes to improve when the tunnel is shut down next year for renovations. Rolling ground planes have also been difficult in full-scale facilities, but a lot of money can fix a lot of troubles. When GMAL reopens in 2017, it will have a moving road plane.
GM uses not just two but a number of wind tunnels -- its own and outside facilities, reduced-scale and full-scale--- along with CFD and field methods. By collating and comparing all these data sources, GM can develop a rather fine idea of how they all relate to each other.
Edited by Magoo, 21 November 2015 - 13:34.
Posted 21 November 2015 - 22:29
Incidentally there's been a lot of work on the different types of ground planes, each has its own compromises.
Posted 22 November 2015 - 06:04
A bit OT, but if you've ever seen one of the sub-scale blow-down tunnels used for high speed aero testing, they have these massive tanks that are pressurized by electrical compressors at night when demand for power is low. For the test, the compressed air is released into the tunnel and it provides a few seconds of high speed flow in the test section. The old Microcraft transonic tunnel in El Segundo, California used two 10,000hp compressors to pressurize its eight 214,000 cubic foot spherical tanks.
Posted 22 November 2015 - 23:05
These days most places in Oz have ample generating capacity in the middle of the day - due to the proliferation of roof-top solar PV. Recently there was a time when the wholesale electricity price dipped below zero due to a massive surplus.
Posted 23 November 2015 - 02:17
Front suspension on a 40-percent Chevy pickup with my hand in the frame as a size reference.
Rapid prototyping methods including 3D printing now enable realistic scale models with reasonable cost and time factors. Functional suspension for the rolling ground plane and accurate undercarriage details are critical in generating useful wind tunnel data with reduced scale models.
Is this cool or what?
Posted 23 November 2015 - 02:50
That is terrific stuff. Widespread 3d printing will allow manufacturing engineers to play with assembling the cars in the round, rather than on screen(Actually we'll need 40% scale humans as well, we'll be providing daycare).
Posted 25 November 2015 - 04:16
Need stuff indeed! Do those 40% models also include airflow thru the engine compartment?
Posted 25 November 2015 - 06:23
You would have to think so, having seen the effort put into the scaled suspension.
Posted 25 November 2015 - 23:21
Yes, models include functional grilles and realistic engine compartment. Through-car and undercar airflow have become more critical as they look for more drag reduction.
Aero is by far the cheapest way reduce carbon and fuel consumption, according to GM, but on the body exterior, all the low-hanging fruit is long gone. One item they would love to get rid of is door mirrors, which are worth 20 counts. They can be replaced with video, but there are regulatory obstacles at the moment. Another large potential gain is in rear wheel skirts aka spats, but buyer resistance is keeping that advance in check.
On the models, they are quickly amassing an inventory of reduced scale suspension and undercarriage components, both in digital and physical form -- which speeds the process in a way they hadn't totally anticipated.
Posted 26 November 2015 - 02:33
That is terrific stuff. Widespread 3d printing will allow manufacturing engineers to play with assembling the cars in the round, rather than on screen(Actually we'll need 40% scale humans as well, we'll be providing daycare).
Do they only get a 40% paycheck???