TheNeedForZ Posted May 30, 2006 Share Posted May 30, 2006 Maybe you really should use water + lexan plates for this project. Quote Link to comment Share on other sites More sharing options...
nope Posted May 30, 2006 Share Posted May 30, 2006 for reynolds number confusion try thinking of it this way. you have a 10 foot long plate with air traveling over the top at some given speed and you observe boundary layer separation (transition stage to fully turbulent flow) half way down the plate, so 5ft. now you set up a 1 foot long plate (1:10 scale model) in the same fashion. how fast do you think you need the air to flow across the top to separate half way down this scaled down plate? air needs to flow 10 times as fast to get boundary layer separation 10 times sooner. ive done some wind/water tunnel testing in some of my labs and honestly i dont think its too practical to try this test at home using atmospheric air at such a small scale. your best bet to make this experiment succesful is using another fluid like water and feeding a stream of food coloring in through a very small tube upstream of the car. but then thats more complex.. and more money. or get a pirated version of solidworks and get really good at modeling cars then test in flowworks. Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted June 3, 2006 Share Posted June 3, 2006 Olderthanme, You can approach the complexity and Reynold number problem by moving the model in the fluid, i.e. pulling the model with a string/pulley in a long, transparent water trough. You won't have to make the fluid circulate in such system, and the higher density of fluid will result in better a Reynold number. Quote Link to comment Share on other sites More sharing options...
TimZ Posted June 3, 2006 Share Posted June 3, 2006 yes exactly. Except I am using a 1/25 scale model. My $12 fan gives about 4.2 mph which is 105.9 mph at scale speed on the low fan speed. On high I get like 139 scale mph. Sorry to rain on your parade because I think it's a cool idea, but this isn't right. Because of the physics of the air moving over the surface (fluid dynamics), the smaller the model, the faster the wind tunnel airspeed needs to be to simulate the same effect. So, for a 1/18 scale model to simulate 100mph, you would need to have a wind tunnel speed of 1800 mph:shock: , which of course is almost mach 2.5, so that probably wouldn't work so well. http://www.aerodyn.org/WindTunnel/wind_tunnel.html#scale_effects Quote Link to comment Share on other sites More sharing options...
nope Posted June 3, 2006 Share Posted June 3, 2006 Olderthanme' date=' You can approach the complexity and Reynold number problem by moving the model in the fluid, i.e. pulling the model with a string/pulley in a long, transparent water trough. You won't have to make the fluid circulate in such system, and the higher density of fluid will result in better a Reynold number.[/quote'] If he did this he wouldnt be able to collect any data (even just quantitative) very easily. I guess he could hook up a really sensitive load gauge to the string to the pull string to measure drag but then wouldnt be able to pull it by hand for sake of repeatability. It would also be difficult to make improvements since you still cant see what the water is doing (no dye). Best bet is circulating water with a stream of dye running over the car. One more thing to think about for the circulatingtest tunnel.. youll have to come up with some way to return the water's flow to laminar before it runs over the car again. Something like a big negative venturi should work, so the water can expand into a lage chamber and essentially slow to a stop then smoothly enter a straight pipe section to the car model. Laminar entrance flow is the only way youre gonna see anything useful otherwise the water will hit the dye and just mix it everywhere. I dont know how easy its going to be to set up an accurate continuous testing rig without spending some cash, i think thats just how it goes. Hrm.. maybe you could get a big clear plastic tube several feet long and place it vertically on the ground and fill it with a transparent oil (if you can find something thick thats cheap). Im suggesting oil to further lower the required velocity factor for an equivalent reynolds number. So char. length is set, and you can find the oil's properties online or something, all you need to play with now is speed. You drop the model into the tube nose first and watch it fall. Im assuming the model is really light so you can play with adding weights in the nose section to help it drop faster. Im not sure this will put you in the range of 100 scale mph but it will definitely help get you there. Youll have to play with ideas to get dye in there though. Either rig something to the front of the model that lets out dye or.. insert some long pipette to the bottom, let the oil come to a stand-still again and slowly pull the pipette up the center while releasing a line of dye (thin it out with oil first so it doesnt cloud up your test oil after like one run).. hopefully the oil will be thick enough to keep the dye in a line long enough for you to dump the car in there. Cool thing about oil flow tests is everything goes really slow so it gives you time to observe everything and the flow lines last after the test is done. Also no circulation needed, but then that makes this test non-continuous. Always use gravity if you can, i think we always forget. Oh and dont forget some thread tied to the back of the model so you can pull it back up. I really hope this helps your project, good luck man. Quote Link to comment Share on other sites More sharing options...
TimZ Posted June 3, 2006 Share Posted June 3, 2006 Sorry to rain on your parade because I think it's a cool idea' date=' but this isn't right. Because of the physics of the air moving over the surface (fluid dynamics), the smaller the model, the faster the wind tunnel airspeed needs to be to simulate the same effect. So, for a 1/18 scale model to simulate 100mph, you would need to have a wind tunnel speed of 1800 mph:shock: , which of course is almost mach 2.5, so that probably wouldn't work so well. http://www.aerodyn.org/WindTunnel/wind_tunnel.html#scale_effects[/quote'] Oops - crap! It looks like a missed the second and third pages of this thread - sorry guys. :redface: Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted June 3, 2006 Share Posted June 3, 2006 If he did this he wouldnt be able to collect any data (even just quantitative) very easily. I guess he could hook up a really sensitive load gauge to the string to the pull string to measure drag but then wouldnt be able to pull it by hand for sake of repeatability. It would also be difficult to make improvements since you still cant see what the water is doing (no dye). It can be easy or not easy depending on how resourceful one is at solving the problems. You wind the string with an electric motor that is reduced by gearing, one from a toy car will do since the gear ratio limit the max rpm, that takes care of repeatability. The winding pulley can be made with various diameter, that will give you different speed. Part of the string includes a spring/elastic band, the extended length can be measured once it reaches steady speed. The string will be moving but a digital camera or video can be used to make a record. The speed and stretch can be measured on the video. If you put a ruler next to the string when you make the video, then it is even easier to measure. To be able to see the fluid streams, can add solid particles that has density similar to water and then add salt or alcohol to the water to match the density. Then the particles will confront to the fluid. There you go, nothing complicated. Quote Link to comment Share on other sites More sharing options...
nope Posted June 4, 2006 Share Posted June 4, 2006 eh, i say oil is still best. no moving parts, perfectly reapeatable, and most importantly could easily stay under $40. tube oil thread pipette food coloring model car lead weights clay Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 4, 2006 Author Share Posted June 4, 2006 cool guys...Time to get a waterproof tube I guess... Just a big PVC one should do with a plexiglass window...I am going to go through with the current wind tunnel and use incense then move on to the water system. For determining lift I was thinking of runing a string front the front bumper foreward, over a teflon pully to a 50gram (or whatever would work 10g to 100g) weight sitting on a digital scale and seeing how much weight is removed from the scale at wind speed. Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted June 4, 2006 Share Posted June 4, 2006 eh, i say oil is still best. no moving parts, perfectly reapeatable, and most importantly could easily stay under $40. In the test you mentioned, the centerline of the car has to be vertical at all times. Just by dropping it, it is not going to stay vertical since the car is asymmetric in terms of drag, even if you put a majority of weight in the front. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted June 4, 2006 Share Posted June 4, 2006 This is making my idea of on-road testing look very simple. But I think its great you are having a crack. Keep us informed of the results. Dave Quote Link to comment Share on other sites More sharing options...
nope Posted June 5, 2006 Share Posted June 5, 2006 In the test you mentioned, the centerline of the car has to be vertical at all times. Just by dropping it, it is not going to stay vertical since the car is asymmetric in terms of drag, even if you put a majority of weight in the front. yea thats totally true. hey an even better idea.. install a non-metallic "ground" plate vertically inside the tube (just like you have now in your wind tunnel setup) and glue a magnet to the bottom of the car. on the underside of the ground plate you can place a matching poled larger magnet with wieght of choice. this way the plate runs between the magnets and the bulk of the weighted item is removed from the car and out of its flow path. any other variety of testing rig components can be linked to the car this way and kept completely out of the test section. this also provides a ground for the car and keeps it straight as you mentioned. good idea with the weight and pulley btw. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted June 5, 2006 Share Posted June 5, 2006 I think the original Japanese engineers already tested the S30 in a cardboard tunnel with household fans... That's how it got to be as "aerodynamic" as it is already. <(my sarcastic humor) It is a really cool project you are undertaking and definitely sounds like alot of fun. Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted June 6, 2006 Share Posted June 6, 2006 yea thats totally true. hey an even better idea.. install a non-metallic "ground" plate vertically inside the tube (just like you have now in your wind tunnel setup) and glue a magnet to the bottom of the car. on the underside of the ground plate you can place a matching poled larger magnet with wieght of choice. this way the plate runs between the magnets and the bulk of the weighted item is removed from the car and out of its flow path. any other variety of testing rig components can be linked to the car this way and kept completely out of the test section. this also provides a ground for the car and keeps it straight as you mentioned. The "ground plate" will exacerbate the problem because a flat plate dropped into water never travels straight down. It falls like a leaf does. No offence, but the model dropping idea leads to too many problems. You have to provide a "ground" for the car; you have to make sure the car travels vertically; you have to make sure the heading is straight down; you have to control the speed of the car, and that's all in a standing, giant long transparent tube/chute filled with liquid. If you can find round tube of transparent material with one end already sealed that's fine, if you have to build it from plates the rig won't hold pressure for such a long, standing chute. Remember the tube/chute must be much larger than the model so the walls of the tube/chute won't affect results. Do it horizontally. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted June 6, 2006 Share Posted June 6, 2006 I think that the experiment could provide some usefull information regarding to the airflow movements and characteristics regarding the s30 chassis. This information, while not being truly scaled in comparison is not irrelevant compared to the real world, just not an apples to apples comparison at high speeds. The low speed airflow characteristics may change in relation to high speed airflow, but that has to be seen. Case in point would be the flow over the top of the car and the effects of spoilers. I'd be interested to see what the experiment reveals. Video would be really helpfull or at least pictures. Quote Link to comment Share on other sites More sharing options...
bjhines Posted June 6, 2006 Share Posted June 6, 2006 hmm... flow straightening would be a useful idea in your design... take a look at what the Wright Bros used for their tunnel.. .. Anyone ever consider that modern cheap cameras can be had for relatively little money.. they are small and fit into interesting locations.... Little tabs of soft yarn taped across the entire surface of areas you want to test can be videotaped in motion at various speeds... even in the engine compartment and under the car... this won't give you any hard data on overall drag... but it might help more than the homemade wind tunnel.... I am not trying to discourage.... the wind tunnnel is a NEAT project... I never thought I could attain the desired data without spending more time and money on the test apparatus than the subject of the test... Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted June 6, 2006 Author Share Posted June 6, 2006 so far the 5 ft long tube was $6 and the fan was $12... I am going to use incense and use different front ends to see what will flow the most air over the car. I have given up on the idea of getting accurate downforce results just because of the fact tht teh model is soo small and no scale could measure accurately to what I want it to. I am just going to be playing with different air dams made out of card paper for the models. I just want to test out ways of keeping air out of the engine bay and under the car. I just cut out a splitter out of my aluminium that goes back to the first crossmember and will be mounting that soon. I am also taking efforts to close the huge mouth of the Z... Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted June 7, 2006 Share Posted June 7, 2006 While you are at it, please make a "belly pan" to seal the bottom of engine bay. I always wondered about the effect of belly pan for the S30. Keep us updated. Quote Link to comment Share on other sites More sharing options...
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