Carl Beck Posted May 31, 2007 Share Posted May 31, 2007 Hi Guys: I'm not certain I understand what some of the numbers actualy indicate. For example, in the columns on the data sheets from the wind tunnel test # 1 in the CL (total lift) what dose the number .473 indicate? If the car is sitting on pressure pads - would that .473 number indicate that 0.473% of the starting weight, has been removed from the pressure pad via Lift ? For the column marked CD - how was that measured for the different cars tested. Does the wind tunnel have some way to scan the front profile of the car and then compute the total area involved? On the data sheet there is a "REQ'D INPUT" note that lists mph=120. Was the wind speed in the tunnel actually 120mph or something less, then using 120mph as an input to a computer program, were the numbers calculated at what they would have been if the wind speed was 120 mph? What do the numbers in the columns marked "Rad Top", "Rad Ctr." and "Rad Bot" mean? Is there a booklet that the wind tunnel gives out - that explains all the numbers, how they are measured or calculated, and how they are reported? Just trying to gain a better understanding of these numbers... thanks, Carl B. Quote Link to comment Share on other sites More sharing options...
jt1 Posted May 31, 2007 Share Posted May 31, 2007 CD and CL are coefficent of drag and lift, respectively. These are unitless numbers which are used in aero equations to calculate the forces at a certain speed. Basically the smaller the number, the less lift or drag is generated, so for our purposes smaller is better. Bob Smith and I measured the gnose car to come up with the frontal area. There are some conventions used, but it's basically the silhouette of the car, with the area under the car included. Using a jig he had, we measured the height at the roof, the height at the drip rails, the height of the rolls on the doors, and the widths at each of these points, then he has a program that gives the frontal area. The wind speed was actually 80 mph. As I understand it, the reynolds curve has a knee point at 80mph, so if you test at that speed you can interpolate up or down. The "REQD INPUT" is a value you can change on the actual spreadsheet to calculate forces at different speeds. In the case of the sheet Mike posted, all the forces are at 120mph. If you change the speed, the forces change, but the coefficents do not, they stay the same. So for the purposes of discussion, it best to talk about CD, CLF, and CLR, the coefficent of drag, lift front, and lift rear. The "RAD" numbers have no meaning for this test. As far as I know, the tunnel doesn't put out anything explaining the data, although it would be cool if they did. John Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 1, 2007 Share Posted June 1, 2007 The lift coefficient CL relates the Lift to the available momentum of the fluid in a unitless fashion. CL=L/(1/2 x rho x V^2 x A) where L is lift rho is the density of air V is the free stream velocity of the air A is the frontal area As an example from our data, Run 1 lists the following values: L = 323.528 lbf rho at STP = 1.225 kg/m^3 (0.076 lbm/ft^3) V = 120 mph A = 18.5 ft^2 From these values and proper conversion of units, you can solve for CL CL = 323.528 lbf / (1/2 x 0.076 lbm/ft^3 x (120 mph)^2 x 18.5 ft ^2) The numerator of the equation has units of force and CL is a unitless value, therefore the denominator must be manipulated to get units of force. We can do this as follows: (1/2 x 0.076 lbm/ft^3 x (120 mph x 5280/3600)^2 x 18.5 ft ^2 / 32.174 = 678.5 lbf Therefore CL = 323.528 / 678.5 = 0.476 The important thing to realize is that CL is constant, so once determined it can be used to extrapolate values of lift for other velocities. L=CL x (1/2 x rho x V^2 x A) So say we want to know what the lift will be at 150 mph L = .476 x (1/2 x 0.076 lbm/ft^3 x (150 mph x 5280/3600)^2 x 18.5 ft ^2 / 32.174 = 505 lbf The lift at 150 mph for the vehicle in test set-up #1 will be 505 lbs and at 200 mph would be 899 lbs. The other unitless ratios Cd, Cy relate the drag force and yaw force to the same denominator used in the equation for CL. Quote Link to comment Share on other sites More sharing options...
Carl Beck Posted June 2, 2007 Author Share Posted June 2, 2007 Hi John / 74_5.0L_Z : Thanks for taking the time to respond. I hope you guys bear with me .... I think I have an inkling of what the coefficient of drag and lift represent, as well as how they are calculated. Now I know where the "area" for the profile of the G-Nose came from - and would I be safe to believe that the other three cars involved were also measured in the same manor? At this point however - I am still left wondering exactly what was measured in the wind tunnel. From our above discussion - once you knew the area of the profile of the car - everything from there was calculated as it related to Drag and Lift.... With the fans putting out a constant 80 mph - was it the actual velocity of the free stream of air - at the front and rear wheels that was actually measured in and by the wind tunnel - so that the Drag and Lift calculations could be made for each end of the car? My questions are somewhat aimed at the thought, that someone might be able to take some of the formulas, factors and now known input values surrounding all this - into a visual or graphical Mathematica Demonstrations Model.... that with the new free Mathematica Viewer - anyone could play with a graphic simulation model of the 240-Z - and thus gain a better conceptual understanding of cause and effects brought about by specific changes to the car. Looking through Mathematica's current library - I see no aerodynamics visual demonstrations models... http://demonstrations.wolfram.com/about.html Interesting to look at some of the existing demonstration models already there... Since I retired... I no longer have access to Mathematica, nor do I believe I will paying the $6K license fee for home use. Nonetheless I thought perhaps someone still working in an engineering field, or at a university - and with a younger better working mind than mine, - - could build a demonstration model or two - - - if they could be supplied with all the necessary data/information to build the graphical model. Sliders for Velocity, increased/decrease profile area (by raising or lowering the car - or by extending and air damn downward etc, wind speed over the front or rear of the car to recalculate front and rear lift ... and all the calculations would be made in real time, so anyone could more easily visualize all this. FWIW, Carl B. Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 2, 2007 Share Posted June 2, 2007 With the velocity and the frontal area known, lift and drag were measured. Then they were able to calculate the lift and drag coefficients from the measured lift and drag as well as the known velocity, air density, and frontal area. The coefficients, frontal area, and how they were affected by the various vehicle configurations is the information that was gathered. With some relatively minor changes, the crew took the Z car from a stock configuration that produced 288 lbs of lift and 309 lbs of drag at 120 mph (set-up 19) to a configuration that produced 108 lbs of downforce with only 275 lbs of drag (set-up 28). And they did all of this with minor add-ons. Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 2, 2007 Share Posted June 2, 2007 Now I know where the "area" for the profile of the G-Nose came from - and would I be safe to believe that the other three cars involved were also measured in the same manor? The same frontal area was used throughout the tests, for all cars. Bob gave me a quickie run thru of the conventions aerodynamicists use to determine frontal area while we were measuring the car. As I understand it, the silhouette of the car at the largest point, plus the area under the car, is the area used by all tunnel operators to keep it consistent between tunnels. So, if you were looking at the car from the front and could see the whaletail protruding around the cabin area, that doesn't affect the frontal area because it's not at the largest point on the car. This is like many of the conventions and definitions used in all engineering tests and calculations. Also remember that we were not looking for razor sharp absolute values, we were more interested in trends of the data. This let us get a lot more tests done in the time we had available. With the velocity and the frontal area known, lift and drag were measured. Then they were able to calculate the lift and drag coefficients from the measured lift and drag as well as the known velocity, air density, and frontal area. That is correct. The tunnel has four pads in the floor that the tires are strapped to, and the pads have sensors attached to them that give XYZ force vectors. Then, knowing the force exerted, wind speed and frontal area, the respective coefficents are calculated, then used to calculate forces at speeds other than the test speed. The tunnel has some sensors inside which monitor wind speed, pressure, direction, and more, but these are used to determine the performance of the tunnel itself. Any data related to the car comes from the force pads in the floor. It's possible to directly measure aero pressure at different points on the car, but the setup takes up a lot of tunnel time, at 490 $/hr. We didn't do any of the point pressure testing. The "rad" numbers you asked about were sensors used on the radiator in a previous group's test. Apparently they were picking up some breeze in the basement under the tunnel, which is where the sensors are located. John Quote Link to comment Share on other sites More sharing options...
blairjj Posted June 4, 2007 Share Posted June 4, 2007 Mikelly and others, Is the actual spreadsheet available? I thought I'd ask before I retype it. (Jpeg is hard on my peepers). Thanks! Jay Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 5, 2007 Share Posted June 5, 2007 Blairjj: Look for the sticky "windtunnel test data" in this forum... Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 6, 2007 Share Posted June 6, 2007 I have nearly finished a spread sheet that will allow the user to vary the velocity, air density, and such and see how each of the test configurations respond. I have completed all of the columns except CYM, CRM, and Q(psf). At first glance, I did not think that I had enough info to complete those columns. Then something occurred to me. So my question is this: Do the numbers in the Pt column correspond to the left and right wheel, and which is which? If that is the case, I will have enough info to complete the spreadsheet. Mikelly, Can I post it here when its complete? I don't think we want it on the downloads page for everone, do we? Would you like to review it first? Oh yeah, for Q(psf), I assume this represents the entire lift force over some reference area. What was the value of the reference area and what is its significance? Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 6, 2007 Share Posted June 6, 2007 Do the numbers in the Pt column correspond to the left and right wheel, and which is which? No. When the tunnel comes up to speed and stabilizes, one set of data is recorded, which is Pt1, then after a few more seconds a second set is recorded and becomes Pt2. Then these are averaged to produce the "avg" line. Oh yeah, for Q(psf), I assume this represents the entire lift force over some reference area. What was the value of the reference area and what is its significance? I recall Bob saying something about Q(psf), but I don't recall what is was. Tom probably knows. jt Quote Link to comment Share on other sites More sharing options...
Mikelly Posted June 6, 2007 Share Posted June 6, 2007 Mikelly, Can I post it here when its complete? I don't think we want it on the downloads page for everone, do we? Would you like to review it first? Oh yeah, for Q(psf), I assume this represents the entire lift force over some reference area. What was the value of the reference area and what is its significance? In short, no. The data format was so unstable, we had a hard time locking it in without the tables doing some very strange things on us. Every member of the team had been given copies and tried to play with the data, and each of them can atest to the trouble and decision made to lock the data. I can't release the raw data because of this. Mike Quote Link to comment Share on other sites More sharing options...
Carl Beck Posted June 8, 2007 Author Share Posted June 8, 2007 In short, no. The data format was so unstable, we had a hard time locking it in without the tables doing some very strange things on us. Every member of the team had been given copies and tried to play with the data, and each of them can atest to the trouble and decision made to lock the data. I can't release the raw data because of this. Mike Hi Mike: I hope you'll forgive me if I'm asking questions that should go without saying, but your response above has left me puzzled to say the least. What is an unstable data format? Are you saying that 74_5.0L_Z is not allowed to build and publish a useful spreadsheet based on the released numbers in the tables; or are you saying that it is not possible to do, using the numbers published in the tables? Your reply above seems to be a way of saying that the numbers published in the tables aren't accurate and therefore not repeatable; nor could they be used to re-calculate the different variables, if plugged into a spread-sheet with the proper formulas. Sorry, but you lost me there - right when I thought I was beginning to gain a grasp of the process. Can you elaborate more on this? I really would like to understand what these results mean, and how they interrelate. thanks, Carl Quote Link to comment Share on other sites More sharing options...
Mikelly Posted June 8, 2007 Share Posted June 8, 2007 Carl, where to start... The formula used by the software at the windtunnel is one I've not seen before. We couldn't do simple things as merge the two separate sheets of data into one. When we tried, the fields would change. numbers would change, sometimes values would change. The data we collected is accurate, since we did a snapshop of the screens, and created JPEGs. That's the only way we could do anything to publish the data. Any number of other normal ways you would think to work in Excel wouldn't seem to work. The other guys can attest to the level of stress caused by doing anything other than opening the file on the CD and VIEWING the data. The data in the JPEG files I attached are absolutely accurate and are the raw data that was copied from the workstation onto the CD I have in my posession. Can anyone recreate the spreadsheet? Sure, knock yourself out. Will you be able to duplicate the formulas? I do not believe so. When I had one of my MS Excel gurus look at the formula, he found that the base formula from the program used at the wind tunnel does some funky conversion to the excel formula tables. It would be difficult to duplicate, was his comment to me. What do these results mean? To me, having been there, they mean a lot of things. I learned what needs to be done to make the car more stable, and why I've spent some coin and taken steps to change the basic design of my project. I'm sure the other members are doing the same thing. Will everyone be "satesfied" that the data is accurate and worth the effort? Who knows. We tried to please a large number of folks. I believe we did our best, and better than most had expected. Mike Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 10, 2007 Share Posted June 10, 2007 From the snapshot and some basic aerodynamic relationships I was able to reverse engineer all of the lift, and horsepower numbers from the coefficients and vice versa. After I was sure that I had established relationships based in physics that work for all the data, I built a spreadsheet that allows the user to vary the velocity, air density and frontal area and see how it affect all of the test cases. I will post this here, and if you inspect it and determine that it does not truly represent the data, I will not be upset if you delete it. The relationships that I used to go back and forth between the coefficients and the forces are as follows: CL=L / (1/2 x rho x V^2 x A) CD=D / (1/2 x rho x V^2 x A) Cs=S / (1/2 x rho x V^2 x A) CPM=M / (1/2 x rho x V^2 x A x WB) where M = (Lf - Lr) x WB/2 HP = D x V From the data given, I don't have enough information to calculate CYM, CRM and Q(psf). These bits of data would be nice, but they are almost inconsequential to the tests that were performed. Wind Tunnel Results.rar Quote Link to comment Share on other sites More sharing options...
Carl Beck Posted June 10, 2007 Author Share Posted June 10, 2007 Carl, where to start... The formula used by the software at the windtunnel is one I've not seen before. We couldn't do simple things as merge the two separate sheets of data into one. When we tried, the fields would change. numbers would change, sometimes values would change. The data we collected is accurate, since we did a snapshop of the screens, and created JPEGs. That's the only way we could do anything to publish the data. Any number of other normal ways you would think to work in Excel wouldn't seem to work. ....snipped.... Mike Hi Mike: Thanks for taking the time and expending the effort to bring me along. I fully understand what you are saying. You guys did a great job with a very difficult task at hand. Data Information Knowledge Wisdom I'm just trying to make the transition between Information and Knowledge here... Your Project adds a lot to the Knowledge Base on these 30+ year old Z's. Everything past the data input from the force pads on the floor and the known wind speed - would "seem" to be a calculation based on formulas - I would not be surprised to find that the formulas have been twick'ed and fine tuned with correction factors, developed by feedback loops from the real world over the years, by the guys at the Wind Tunnel. Design, model, simulate, build, test. Perform enough Design Verification Loops to fully dial in the models and simulations .... and somewhere along the way we wind up with a patchwork of code, incorporating undocumented correction factors and bug fixes... aka - Job Security for the old timers responsible for the software mess -vbg- and hence the phrase.... "if it ain't broke don't fix it". If the final output data consistently matches the real world - you are in good shape. Oh.. BTW - I don't often drive at 120mph - did you do a screen shot of the data tables at 80mph? That's about the speed at which I spend most of my Freeway time. thanks again, Carl Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 10, 2007 Share Posted June 10, 2007 OK guys, I have attached a .rar file with the spreadsheet in my post above. If anyone finds any errors with it, let me know and I will fix it. Thanks, Dan McGrath Quote Link to comment Share on other sites More sharing options...
Carl Beck Posted June 12, 2007 Author Share Posted June 12, 2007 Hi Dan: The spreadsheet "seems" to work fine - Great Job. At test #19 - With Speed adjusted to 70mph - on the Stock 240-Z the CD seems to be way off - 0.227, as well as Drag HP and Drag lbs. There is only one line of data from one run - so it looks like the avg. line is divided in half... Could be a download error... thanks, Carl Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 12, 2007 Share Posted June 12, 2007 I'll fix that. Also, I am working on an improved version that features interactive graphing. I'll post it tonight. Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 12, 2007 Share Posted June 12, 2007 What opens a .rar file? I know, I'm clueless. John Quote Link to comment Share on other sites More sharing options...
jdarkme Posted June 12, 2007 Share Posted June 12, 2007 What opens a .rar file? I know, I'm clueless. John Download winrar http://www.download.com/WinRAR/3000-2250_4-10690736.html?tag=lst-0-2 Quote Link to comment Share on other sites More sharing options...
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