[ joel - 6 - 09:01:41 ]
Are you going to use dry ice in the wind tunnel? Or maybe I should ask what kind of wind tunnel are you going to use? Hopefully I'm on the right track.

[ Moderator - 7 - 09:04:36 ]
Jani; Hello Joel ... Our chat isn't starting until 10 AM - Will you be able to join us then?

[ joel/FruitaMonumentHigh - 8 - 09:05:53 ]
What are you trying to accomplish in your experiment? It seems as though there are many variables but can't wait to see the outcome.

[ joel/FruitaMonumentHigh - 9 - 09:07:28 ]
Hi Jani, Joel here- the time here is 10:05. Sorry I forgot about the time difference.

[ Moderator - 10 - 09:24:01 ]
RE: [joel/FruitaMonumentHigh] Hi Jani, Joel here- the time here is 10:05. Sorry I forgot about the time difference.
Dave: Joel, we are going to clear the chat soon so you may want to save your questions so that you don't have to type them in again.

[ Moderator - 12 - 09:57:47 ]
Good Morning Everyone - This is Jani Pallis. And I an the Principal Investigator for the Aerodynamics in Sports project. Not all of our experts are here yet. NASA is helping us get set up. BUT I know that John Yandell our project manager is here. Good MOrning John!

[ Moderator - 13 - 09:59:22 ]
John Yandell, Project Manager: Hi Jani. How is everyone feeling about the science of tennis today??

[ Moderator - 14 - 10:01:10 ]
Well we are ready to take our first question.

[ Moderator - 15 - 10:02:43 ]
Joel from Fruita Monument High asked about our wind tunnel test. Rabi will be here shortly and can explain in detail - but we will be using smoke not ice.

[ Moderator - 16 - 10:04:07 ]
RE: [joel/FruitaMonumentHigh] What are you trying to accomplish in your experiment? It seems as though there are many variables but can't wait to see the outcome.
Hi Joel, which experiment is that you are asking about?

[ Moderator - 17 - 10:05:20 ]
John since you are here can you tell us about what we are finding out about ball spin from the US Tennis Association testing

[ Moderator - 18 - 10:05:34 ]
John Yandell:

[ Moderator - 19 - 10:05:39 ]
Rabi Mehta --- Hi everybody

[ Moderator - 20 - 10:06:18 ]
Shishir Pandya, computer simulation specialist: Good Morning everyone.

[ Moderator - 21 - 10:06:52 ]
Nasif: Good morning!

[ Moderator - 22 - 10:06:59 ]
John Yandell: Well, many amazing things. Our camera allowed us to "see" the ball bounce on the court, and actually count the number of revolutions before and after the bounce.

[ Moderator - 23 - 10:07:01 ]
HI Rabi - Joel from Fruits Monument high asked

[ Kevin/WatertownHighShool - 24 - 10:07:14 ]
What effect does the coure surface have on the spin of the ball?

[ Moderator - 25 - 10:07:14 ]
Rabi -- What was the question re. w/t

[ Moderator - 26 - 10:07:54 ]
What kind of wind tunnel we are using and if we were using dry ice. Can you answer that for him?

[ Moderator - 27 - 10:07:58 ]
John Yandell: Hi guys! Nas are you in the physics lab? Any star UHS students around?

[ Moderator - 28 - 10:08:35 ]
Nasif: Sorry John, all of our students are at an assembly right now. I'm sitting in my office.

[ Moderator - 29 - 10:09:00 ]
RE: [Moderator] What kind of wind tunnel we are using and if we were using dry ice. Can you answer that for him?
Rabi mehta --- we are using a suck-down wind tunnel with a test section of 3 X 4 feet -- we will be injecting "smoke" -- the type used on stage and in movies

[ Moderator - 30 - 10:10:48 ]
RE: [Kevin/WatertownHighShool] What effect does the coure surface have on the spin of the ball?
John Yandell: THAT is a good question. The friction of the bounce actually generates topspin. The bounce in fact often causes a level of topspin in excess of what the players themselves impart. To give one example, a flat shot on a hard court thrown by a ball machine will have 2000-2500rpm after the bounce!

[ Kevin/WatertownHighShool - 31 - 10:11:22 ]
What do you hope to accomplish with the wind tunnel tests?

[ Moderator - 32 - 10:11:41 ]
Jani: just to introduce everyone. Dr. Rabi Mehta is our wind tunnel expert, Shishir Pandya is our Computational Fluid Mechanics expert, Nasif Iskander is our co-investigator/teacher/physicist, JOhn Yandell is our Project Manager and I'm Dr. Jani Macari Pallis the prinicipal investigator. And all our experts are now here and we are eager to answer your questions.

[ Moderator - 33 - 10:12:14 ]
Jani: Rabi can you answr Kevin's question. Good Morning Kevin!

[ BoGriffinCreekSchool - 34 - 10:12:46 ]
Good morning everyone. What are your ideas about how to mount the ball? Also, why so big? 14 inches!?

[ Kevin/WatertownHighShool - 35 - 10:13:11 ]
What if that same flat shot were hit on a grass court?

[ Moderator - 36 - 10:13:16 ]
RE: [Kevin/WatertownHighShool] What do you hope to accomplish with the wind tunnel tests?
Rabi Mehta -- In this first phase of the experiments, the main goal will to study to photograph the flow over a model of the ball. We will show how the flow changes with flowspeed, surface roughness (fuzz vs. no fuzz) and also the effects of the spin.

[ Moderator - 37 - 10:14:27 ]
RE: [BoGriffinCreekSchool] Good morning everyone. What are your ideas about how to mount the ball? Also, why so big? 14 inches!?
Jani: Good Morning Bo - Bo wrote me yesterday with his idea on how to mount the tennis ball. I'll explain about the mount and then I'll let Rabi explain about "why so big?' Okay Rabi

[ Moderator - 38 - 10:15:53 ]
RE: [Kevin/WatertownHighShool] What if that same flat shot were hit on a grass court?
John Yandell: That's exactly what we wondered! The high speed film shows that the flat ball still leaves with topspin BUT less, between 2000 and 2200rpm. Also, we could see that the bounce was substantially lower.

[ Moderator - 39 - 10:15:54 ]
RE: [BoGriffinCreekSchool] Good morning everyone. What are your ideas about how to mount the ball? Also, why so big? 14 inches!?
Rabi -- The ball will be mounted on a sting protruding into the test section from one side. The reason for using a 14" ball is that the tests can be run at a lower speed which makes the flow visualization easier. There is a non-dimensional number (Reynolds number) which has to be matched between the real situation and the test --- one of the terms in this number is the ratio of flowspeed to ball sixe.

[ Hans/McNeeseState - 40 - 10:16:42 ]
Jani: I talked to you on e-mail the other day. I suggested that you might take readings on humidity levels as they might affect ball rotations. Any comment?

[ Moderator - 41 - 10:17:02 ]
John Yandell: Nasif- was it your opinion that this indicates the ball is skidding more on grass, as opposed to rolling on a hard court?

[ Moderator - 42 - 10:17:25 ]
RE: [BoGriffinCreekSchool] Good morning everyone. What are your ideas about how to mount the ball? Also, why so big? 14 inches!?
We now have a little sketch of the wind tunnel out on the Web At http://wings.ucdavis.edu/Tennis/Project/wind-01.html We have two options we can mount it from the side of the tunnel or bolt it into the bottom. Bo has suggested filling the ball with metal or concrete and placing a rod in it to mount it. We are afraid that the rod will move too much so we are trying a rod with a wedge in it - it looks like the end on arrow.

[ Moderator - 43 - 10:18:50 ]
RE: [Moderator] John Yandell: Nasif- was it your opinion that this indicates the ball is skidding more on grass, as opposed to rolling on a hard court?
Nasif: Yes John. If the grass is more slippery, then the ball will actually pick up less spin during the bounce. If you imagined a ball bouncing off of a completely frictionless surface, it would gain no spin at all.

[ Moderator - 44 - 10:18:53 ]
RE: [Hans/McNeeseState] Jani: I talked to you on e-mail the other day. I suggested that you might take readings on humidity levels as they might affect ball rotations. Any comment?
Rabi -- I am not sure that the ball rotation will be affected by humidity unless the racket strings or ball material is affected. But, the Reynolds number (discussed above) also has a viscosity term in it and this can be affected by humidity.

[ Moderator - 45 - 10:19:03 ]
RE: [Hans/McNeeseState] Jani: I talked to you on e-mail the other day. I suggested that you might take readings on humidity levels as they might affect ball rotations. Any comment?
Hello Hans - we did take those measurements and it was humid. As a matter of fact we were rained out several times and had to run from the Lipton court and cover the equipment. I'll let JOhn answer this question.

[ firstflightschool - 46 - 10:20:25 ]
we are here

[ Moderator - 47 - 10:21:01 ]
RE: [Hans/McNeeseState] Jani: I talked to you on e-mail the other day. I suggested that you might take readings on humidity levels as they might affect ball rotations. Any comment?
John Yandell: Hi Hans! Jani asked me to comment on the ball wear. In using the ball machine to fire the test balls, we estimate that no ball was fired from the machine more than 30 times, and generally it was much less. To the naked eye the ball wear was minimal to non-existent--just an occasional "hair" starting to fluff. Much less than say a match warm up or even a couple of games.

[ Moderator - 48 - 10:21:09 ]
RE: [firstflightschool] we are here
Jani: Good Morning First Flight Middle School from North Carolina!

[ Kevin/WatertownHighShool - 49 - 10:21:42 ]
how does each surface effect the spin? not just hard courts and grass.

[ Moderator - 50 - 10:22:38 ]
RE: [Moderator] Nasif: Yes John. If the grass is more slippery, then the ball will actually pick up less spin during the bounce. If you imagined a ball bouncing off of a completely frictionless surface, it would gain no spin at all.
John Yandell: Nasif--now that's the kind of court surface I could serve effectively on!

[ Moderator - 51 - 10:22:54 ]
RE: [Kevin/WatertownHighShool] how does each surface effect the spin? not just hard courts and grass.
Jani: John can you answer Kevin's question?

[ Moderator - 52 - 10:23:30 ]
RE: [Moderator] John Yandell: Nasif--now that's the kind of court surface I could serve effectively on!
Nasif: Of course it would have some drawbacks. You wouldn't be able to walk or run on that surface either!

[ Moderator - 53 - 10:25:45 ]
RE: [Kevin/WatertownHighShool] how does each surface effect the spin? not just hard courts and grass.
John Yandell: Kevin, I like the way you think. Nasif, maybe you could comment on this as well since you have graphed this so clearly. What we find is that the effect of all surfaces is to create topspin--we tested green clay and red clay as well. The clay courts seemed to generate a bit more off the bounce, but we are not convinced that it was statistically significant.

[ Moderator - 54 - 10:25:53 ]
RE: [Hans/McNeeseState] Jani: I talked to you on e-mail the other day. I suggested that you might take readings on humidity levels as they might affect ball rotations. Any comment?
Jani: Hans what is your feeling on the humidity and its effect on the ball. Am I correct - you are located in Louisianna?

[ Moderator - 55 - 10:26:39 ]
RE: [Moderator] Nasif: Of course it would have some drawbacks. You wouldn't be able to walk or run on that surface either!
JY: Nas--a boring match then--an endless series of ace serves...

[ Moderator - 56 - 10:27:39 ]
Jani: First Flight - Do we have any tennis players or sports scientists there?

[ Moderator - 57 - 10:27:47 ]
RE: [Moderator] John Yandell: Kevin, I like the way you think. Nasif, maybe you could comment on this as well since you have graphed this so clearly. What we find is that the effect of all surfaces is to create topspin--we tested green clay and red clay as well. The clay courts seemed to generate a bit more off the bounce, but we are not convinced that it was statistically significant.
Nasif: That sounds like what I saw in the data. The only court that had a significantly different effect on spin was grass. An interesting part of this is that when the ball gains spin it loses speed. So a grass court is going slow the ball down less than the other courts that we tested. This fits with the general observation that grass courts are the "fastest".

[ Moderator - 58 - 10:28:01 ]
JY: Nas could you comment on my response to Kevin?

[ Moderator - 59 - 10:28:25 ]
RE: [Moderator] JY: Nas--a boring match then--an endless series of ace serves...
Rabi ---assuming you got the serve off-- ever try serving while standing on ice??

[ Moderator - 60 - 10:30:31 ]
Jani: So I'd like to ask our students - how do they think we should mount the tennis ball in the wind tunnel? It can not vibrate around and if it falls off its mount it will damage the wind tunnel. If we poke a hole in the ball - we have to do something so that it will retain its shape.

[ Moderator - 61 - 10:30:55 ]
RE: [Moderator] JY: Nas could you comment on my response to Kevin?
JY: Nas. Thanks. Is it possible that the spin could remain fiarly constant over the courts, but they could differ in the trajectory after the bounce or the height? It seems that either there is some factor that accounts for players' real sense of court differences, or possibly, our experiment did not reach the threshold difference?

[ Moderator - 62 - 10:32:05 ]
rabi -- Another for the students: What makes the tennis ball swerve in flight?

[ Moderator - 63 - 10:32:39 ]
RE: [Moderator] Rabi ---assuming you got the serve off-- ever try serving while standing on ice??
A test of my balance--something that separates levels of play. I propose we have McEnroe, Sampras and myself all serve on ice and see who falls first.

[ Kevin/WatertownHighShool - 64 - 10:33:00 ]
Doesn't spin effect the swerve.

[ Moderator - 65 - 10:33:52 ]
RE: [Moderator] JY: Nas. Thanks. Is it possible that the spin could remain fiarly constant over the courts, but they could differ in the trajectory after the bounce or the height? It seems that either there is some factor that accounts for players' real sense of court differences, or possibly, our experiment did not reach the threshold difference?
Nasif: It's possible that the way teh court deforms during the bounce could affect the path teh ball takes without affecting its spin. Rabi, do you have any ideas about this?

[ Moderator - 66 - 10:34:11 ]
RE: [Kevin/WatertownHighShool] Doesn't spin effect the swerve.
Rabi -- great -- what do you think the spin does to the airflow?

[ Hanh - 67 - 10:34:17 ]
Hi!

[ Moderator - 68 - 10:35:04 ]
RE: [Kevin/WatertownHighShool] Doesn't spin effect the swerve.
John Yandell: Kevin--you got it. Topspin will cause the ball to drop, underspin to rise. But don't forget the wind--the worst part is it's unpredictable compared to the spin players hit--that you can get used to...

[ Hans/McNeeseState - 69 - 10:35:08 ]
Jani, My playing experience down here in Louisiana is that humidity greatly effects the flight of the ball. The fibers absorb moisture and on really humid days the fibers are pulled out and the ball looks like it had its "hair" teased. On our few low humid days(20% at the least) the fibers mat and are easily torn off with playing wear. The effort required to hit the ball the same distance "seems" to vary greatly.

[ Moderator - 70 - 10:35:15 ]
RE: [Moderator] A test of my balance--something that separates levels of play. I propose we have McEnroe, Sampras and myself all serve on ice and see who falls first.
Rabi -- John, at least McEnroe has the shortest distance to fall!!!!

[ HanhEmily - 71 - 10:35:57 ]
What does wing shape do to flight path?

[ Moderator - 72 - 10:36:00 ]
RE: [Hanh] Hi!
John Yandell: Hi Hahn. Have you got a question?

[ Kevin/WatertownHighShool - 73 - 10:36:03 ]
It forces the air to a certain side of the ball depending on the spin.

[ Moderator - 74 - 10:36:14 ]
RE: [Hanh] Hi!
Jani: Hello Hanh -

[ Moderator - 75 - 10:37:00 ]
RE: [HanhEmily] What does wing shape do to flight path?
Shishir can you answer Hanh Emily's question?

[ Moderator - 76 - 10:37:25 ]
RE: [Hans/McNeeseState] Jani, My playing experience down here in Louisiana is that humidity greatly effects the flight of the ball. The fibers absorb moisture and on really humid days the fibers are pulled out and the ball looks like it had its "hair" teased. On our few low humid days(20% at the least) the fibers mat and are easily torn off with playing wear. The effort required to hit the ball the same distance "seems" to vary greatly.
Rabi -- Hans, sounds like the tennis ball may also absorb moisture, thus making it heavier -- you may want to weigh the ball on days with greatly varying humidity -- please let us know what you find.

[ Moderator - 78 - 10:38:02 ]
RE: [Moderator] Rabi -- John, at least McEnroe has the shortest distance to fall!!!!
JY: Rabi. McEnroe claims to have excellent balance on a snowboard, so I would like his chances in the contest.

[ HanhEmily - 79 - 10:39:38 ]
Please hurry~! We have to switch classes

[ Moderator - 80 - 10:39:40 ]
RE: [Kevin/WatertownHighShool] It forces the air to a certain side of the ball depending on the spin.
Rabi --- Good job --- that is a good way to think of it -- the spin produces an asymmetric flow around the ball -- you can certainly think of the spinning ball wrapping it to one side -- the asymmetry then produces a pressure difference which causes the ball to swerve.

[ Moderator - 81 - 10:39:45 ]
RE: [Moderator] Rabi -- Hans, sounds like the tennis ball may also absorb moisture, thus making it heavier -- you may want to weigh the ball on days with greatly varying humidity -- please let us know what you find.
Nasif: Hans, I'm curious as to which way it goes. On humid days is it harder or easier to hit the ball farther?

[ HanhEmily - 82 - 10:40:41 ]
please!!!!!!

[ HanhEmily - 83 - 10:41:14 ]
we're impatient science students!

[ Moderator - 84 - 10:41:56 ]
RE: [Hans/McNeeseState] Jani, My playing experience down here in Louisiana is that humidity greatly effects the flight of the ball. The fibers absorb moisture and on really humid days the fibers are pulled out and the ball looks like it had its "hair" teased. On our few low humid days(20% at the least) the fibers mat and are easily torn off with playing wear. The effort required to hit the ball the same distance "seems" to vary greatly.
JY: Hi Hans. Jani asked me to respond. As I grew up playing tennis in Oklahoma and Texas, I know what you are talking about. Forget about the effect on the balls, I remember the effect on myself (heat stroke). But be that as it may, I feel confident that the humidity was a constant and that its effect was minimal to non-existent over time in our experiments. By summer standards, it wasn't bad at all--maybe 60%. You must have played in 90% plus and 100 degrees, right?

[ MarciStJosephsSchoolfortheDeaf - 85 - 10:42:39 ]
Hello Everyone St. Joseph's School for the deaf is here now. We hav 6 students from our 5th grade. There names are Joel, Likemma, Jazmine, Cenaida, Direlka, Erica. Trisha and myself are here along with two of our assistants- Dorothy and Valerie.

[ Moderator - 86 - 10:43:21 ]
RE: [MarciStJosephsSchoolfortheDeaf] Hello Everyone St. Joseph's School for the deaf is here now. We hav 6 students from our 5th grade. There names are Joel, Likemma, Jazmine, Cenaida, Direlka, Erica. Trisha and myself are here along with two of our assistants- Dorothy and Valerie.
Jani: Good Morning St. Joseph's School for the Deaf. How is everyone there?

[ Moderator - 87 - 10:43:46 ]
Shishir Pandya: If the wing is thick, there is more drag and the paper plane will not go as far. What do you think of that Hanh Emily?

[ Moderator - 88 - 10:45:12 ]
JY to Nas--Back to the previous question--I can't believe that there was no difference (speed, spin, angle, height of bounce) in the way the ball came off the court. No doubt the ball seemed much higher on the clay courts.

[ MarciStJosephsSchoolfortheDeaf - 89 - 10:45:21 ]
We are fine We are trying to explain the wind tunnel to the children Is it possible to get a really simple explaination Why you are using it and what does it do?

[ Hans/McNeeseState - 90 - 10:45:36 ]
Rabi--It requires more effort on humid days. Perhaps, now that I think about it the real noticeable difference is felt during a topspins stroke rather than a flat stroke. The fibers would grab the strings more readily and the player would be imparting more spin than anticpated. This would reduce speed and the player would be hitting the ball a shorter distance. Does this make sense??

[ Moderator - 91 - 10:47:21 ]
RE: [Moderator] JY to Nas--Back to the previous question--I can't believe that there was no difference (speed, spin, angle, height of bounce) in the way the ball came off the court. No doubt the ball seemed much higher on the clay courts.
Jani: At this time that's actually what we are looking at specifically from the footage that ws taken at the training center. We're looking at the angles in and out, velocity and rebound height.

[ Moderator - 92 - 10:47:41 ]
John Yandell to Kevin: Here is a question for you. Based on what we have said, what would happen to an underspin shot as it bounced, in terms of the spin?

[ Moderator - 93 - 10:48:11 ]
RE: [Moderator] JY to Nas--Back to the previous question--I can't believe that there was no difference (speed, spin, angle, height of bounce) in the way the ball came off the court. No doubt the ball seemed much higher on the clay courts.
Nasif: You clearly saw a difference in the way the ball bounced, but it didn't really show up in your spin measurements. So it must not be the spin changes that cause the difference. I don't know what does.

[ Moderator - 94 - 10:48:42 ]
RE: [Hans/McNeeseState] Rabi--It requires more effort on humid days. Perhaps, now that I think about it the real noticeable difference is felt during a topspins stroke rather than a flat stroke. The fibers would grab the strings more readily and the player would be imparting more spin than anticpated. This would reduce speed and the player would be hitting the ball a shorter distance. Does this make sense??
Rabi -- Kind of -- I think the main effect on the spin would come through the strings becoming more elastic or the ball becoming softer -- the inceased contact area would then generate more spin. Everything else being equal, if the ball becomes heavier, then obviously it will take more effort to hit it the same distance.

[ Moderator - 95 - 10:49:05 ]
RE: [Moderator] Jani: At this time that's actually what we are looking at specifically from the footage that ws taken at the training center. We're looking at the angles in and out, velocity and rebound height.
Jy to Jani: Right! I guess what I'm dying to see is if the spin truly is constant--what else could change???

[ Hans/McNeeseState - 96 - 10:49:05 ]
Jani -- 90% plus humidity levels. That's why they sent soldiers to Ft. Polk in Louisiana to train during the Vietnam war.

[ Moderator - 97 - 10:50:06 ]
RE: [MarciStJosephsSchoolfortheDeaf] We are fine We are trying to explain the wind tunnel to the children Is it possible to get a really simple explaination Why you are using it and what does it do?
Jani: A wind tunnel looks like a large pipe or duct. The wind tunnel at NASA takes up a large room. We blow air over a model - like an airplane wing or the tennis ball and special instruments measure forces on the ball. In our wind tunnel test we are going to blow smoke over the tennis ball and you will be able to see the smooth and then the rough (turbulent) flow. We will show you a video of our test in the next Internet Video Conference.

[ Moderator - 98 - 10:51:21 ]
RE: [Moderator] Nasif: You clearly saw a difference in the way the ball bounced, but it didn't really show up in your spin measurements. So it must not be the spin changes that cause the difference. I don't know what does.
JY to Nas: That's what experiments are for. Going back to the spin, on average some of the increases in topspin where high on the clay courts, but what you're saying is that 200-300rpm is probably not significantly different??

[ Moderator - 99 - 10:51:30 ]
RE: [Hans/McNeeseState] Jani -- 90% plus humidity levels. That's why they sent soldiers to Ft. Polk in Louisiana to train during the Vietnam war.
Jani; Please - my hair frizzed the entire time I was in Florida!!!!!!!!

[ BoGriffinCreekSchool - 100 - 10:52:15 ]
I seem to have gotten 2 different answers about how the ball will be mounted. On a string or on a rod?

[ Moderator - 101 - 10:52:19 ]
RE: [Moderator] Jani: A wind tunnel looks like a large pipe or duct. The wind tunnel at NASA takes up a large room. We blow air over a model - like an airplane wing or the tennis ball and special instruments measure forces on the ball. In our wind tunnel test we are going to blow smoke over the tennis ball and you will be able to see the smooth and then the rough (turbulent) flow. We will show you a video of our test in the next Internet Video Conference.
Nasif: Since we are trying to understand what happens to the ball as it flies through the air, we can simulate that by making the air fly past the ball. The ball won't know the difference.

[ Moderator - 102 - 10:52:38 ]
RE: [Hans/McNeeseState] Jani -- 90% plus humidity levels. That's why they sent soldiers to Ft. Polk in Louisiana to train during the Vietnam war.
JY: Yow. We used to pour a couple of cans of water over our heads on the change overs.

[ Moderator - 103 - 10:53:44 ]
RE: [BoGriffinCreekSchool] I seem to have gotten 2 different answers about how the ball will be mounted. On a string or on a rod?
Rabi --- Definitely on a rod -- you cannot have a flexible mount. The rod has to be rigib and firmly mounted within the ball -- NASA would not be too happy if they found a tennis ball stuck in their compressor!!!

[ Moderator - 104 - 10:54:26 ]
RE: [Moderator] JY to Nas: That's what experiments are for. Going back to the spin, on average some of the increases in topspin where high on the clay courts, but what you're saying is that 200-300rpm is probably not significantly different??
Nasif: Yes. 300 rpm is about 10% of some of the measurements, and is hard to distinguish from the error in some cases. We will have to do more sophisticated error analysis to see if that means anything.

[ Moderator - 106 - 10:54:49 ]
RE: [Moderator] Rabi --- Definitely on a rod -- you cannot have a flexible mount. The rod has to be rigib and firmly mounted within the ball -- NASA would not be too happy if they found a tennis ball stuck in their compressor!!!
That would set back the science of tennis a few years.

[ Moderator - 107 - 10:55:02 ]
RE: [Moderator] Rabi --- Definitely on a rod -- you cannot have a flexible mount. The rod has to be rigib and firmly mounted within the ball -- NASA would not be too happy if they found a tennis ball stuck in their compressor!!!
Rabi -- What is rigib, you are all thinking! I obviously meant RIGID.

[ BoGriffinCreekSchool - 108 - 10:56:12 ]
Will the ball be able to spin in the tunnel? Why or why not?

[ Moderator - 109 - 10:56:36 ]
Why are the flights of the different balls so different? Do they have varying weights?

[ Moderator - 110 - 10:57:40 ]
RE: [BoGriffinCreekSchool] Will the ball be able to spin in the tunnel? Why or why not?
Jani: What we have designed is " a hand crank" - the ball will be in the wind tunnel and the rod will go through the side wall of the tunnel - then one of the engineers will use the hand crank to "spin the ball" - we don't know if this will work or not yet - its what we are trying now.

[ Moderator - 111 - 10:58:16 ]
RE: [Moderator] Nasif: Yes. 300 rpm is about 10% of some of the measurements, and is hard to distinguish from the error in some cases. We will have to do more sophisticated error analysis to see if that means anything.
JY: That would be great. 10% is something you can really "feel" on a court. You can "feel" the difference in every brand of ball. Say a 10% higher bounce? That's huge. Hypothetically, if we saw corresponding changes in the 10% range on the other factors--angle, etc, would that validate the significance of spin differences.

[ Moderator - 112 - 10:58:22 ]
RE: [BoGriffinCreekSchool] Will the ball be able to spin in the tunnel? Why or why not?
Rabi -- we are hoping to spin the ball in the wind tunnel -- fortunately since it is a big ball being tested at low speeds, we will only have to spin it at about 2 revs/sec. A tennis ball is always spinning during flight so it would be nive if we can simulate that. John, ever see a tennis ball "knuckle"??

[ Moderator - 113 - 10:59:18 ]
Rabi: Someone asked earlier about "why the tennis ball will be so big?" - can you explain that?

[ Hans/McNeeseState - 114 - 11:00:39 ]
JY--Did you like the presentation by the person from Germany regarding stroke velocity at the USTA Biltmore conference? The modern tennis game is looking more and more like table tennis on the tennis court and this will increase with the new titanium rackets (8ozs).

[ Moderator - 115 - 11:01:18 ]
RE: [Moderator] Rabi: Someone asked earlier about "why the tennis ball will be so big?" - can you explain that?
Rabi -- for the newcomers, the big ball (about 6 times the real size) lets us test at lower speeds -- basically the ratio of the flowspeed to ball diameter has to be matched. So a real ball flying at 120 mph can be simulated by a ball that is 6 times bigger at 20 mph. The lower speeds make the flow visualization easier and the experiment safer!!

[ Moderator - 116 - 11:01:34 ]
RE: [Moderator] Rabi -- we are hoping to spin the ball in the wind tunnel -- fortunately since it is a big ball being tested at low speeds, we will only have to spin it at about 2 revs/sec. A tennis ball is always spinning during flight so it would be nive if we can simulate that. John, ever see a tennis ball "knuckle"??
I guess it depends what you mean--a flat ball out of the ball machine or a misshit seems to "float" or wobble but whether that is enough to effect the flight path, I doubt it--it seems to travel straight.

[ BoGriffinCreekSchool - 117 - 11:01:49 ]
Thanks, Jani. So the ball will be able to rotate on only one axis? Can you move it any other way, like a ball changing spin during flight?

[ Moderator - 118 - 11:02:15 ]
RE: [Moderator] Why are the flights of the different balls so different? Do they have varying weights?
Jani: The balls did have different weights - but also they probably have different internal air pressures and different types of fuzz. The Slazenger ball used on the grass court "appearedto me" to fly differently ---- and it had the thickest fuzz of any of the balls we used? So everyone out there .... WHAT DOES THE FUZZ THICKEST HAVE TO DO WITH AERODYNAMICS?

[ Moderator - 119 - 11:03:55 ]
RE: [Moderator] Rabi -- for the newcomers, the big ball (about 6 times the real size) lets us test at lower speeds -- basically the ratio of the flowspeed to ball diameter has to be matched. So a real ball flying at 120 mph can be simulated by a ball that is 6 times bigger at 20 mph. The lower speeds make the flow visualization easier and the experiment safer!!
Rabi -- this may confuse some students. let me spell it out --- the relevant parameter we try and match is a so-called "Reynolds number" which is defined as Ud/v -- U is the flowspeed, d is the ball diamter and v is the air viscosity. So the goal is to keep U X d constant.

[ Moderator - 120 - 11:04:32 ]
RE: [Hans/McNeeseState] JY--Did you like the presentation by the person from Germany regarding stroke velocity at the USTA Biltmore conference? The modern tennis game is looking more and more like table tennis on the tennis court and this will increase with the new titanium rackets (8ozs).
Hans--Thought it was fabulous. The fact that Bruguera's racket was going faster than Pete's but not hitting the ball at maximum velocity was interesting. Pete's swings slower, flatter, more efficiently, with a heavier racket, possibly. I haven't hit with the new rackets yet--what are they like? the last ultra light I used crushed the ball and also my shoulder.

[ Moderator - 121 - 11:04:36 ]
RE: [BoGriffinCreekSchool] Thanks, Jani. So the ball will be able to rotate on only one axis? Can you move it any other way, like a ball changing spin during flight?
Jani: We'll do backspin and topspin - actually its what we did in Florida with the ball machine as well - Straight back or top spin. Also we are going to compare the wind tunnel data to the computer simulations that Shishir and the engineers at Cislunar have done

[ BoGriffinCreekSchool - 122 - 11:04:51 ]
The thicker the fuzz, the more topspin on contact with the ground. What else?

[ Moderator - 123 - 11:05:54 ]
RE: [BoGriffinCreekSchool] The thicker the fuzz, the more topspin on contact with the ground. What else?
Jani: How do you think the fuzz will effect the drag on the ball?

[ Kevin/WatertownHighSchool - 124 - 11:06:12 ]
How much does string tension effect the spin?

[ Moderator - 125 - 11:06:13 ]
Rabi -- John, actually I am juts wondering if you have ever seen a non-spinning tennis ball fly over the court -- this can occur in theory, but i doubt that you ever see it.

[ Moderator - 126 - 11:06:50 ]
Jani: Everyone out there - we tried regular balls and balls with NO FUZZ on them at all - what do you think the difference was??????

[ Moderator - 127 - 11:08:00 ]
RE: [Moderator] Rabi -- John, actually I am juts wondering if you have ever seen a non-spinning tennis ball fly over the court -- this can occur in theory, but i doubt that you ever see it.
Out of the ball machine you can see Penn 7. I mean I have seen it. The ball seems to float and wobble.

[ TrishaSJSD - 129 - 11:08:37 ]
Is the temperature in the wind tunnel at a certain degree? Is it really hot or really cold ? or Is it room temperature inside?

[ Moderator - 130 - 11:09:11 ]
RE: [Moderator] Out of the ball machine you can see Penn 7. I mean I have seen it. The ball seems to float and wobble.
Rabi, No, I meant in a game, a ball hit with a racket.

[ Moderator - 131 - 11:10:47 ]
RE: [BoGriffinCreekSchool] Last time someone said more turbulence caused by fuzz would lower air pressure around the surface of the ball and allow the ball to go faster. Others said it would cause drag and slow the ball.
Jani: I'll tell you what we saw and we are going to try and put the video of this up. John was at the end of the court catching the tennis balls as they were fired out of the machine and he is a tall person ---- the fuzzless balls acted like handballs --- they kicked up way over John's head and into the tennis court stands !!!!!

[ Moderator - 132 - 11:11:13 ]
RE: [Kevin/WatertownHighSchool] How much does string tension effect the spin?
JY to Kevin: Borg hit heavy topspin and strung his rackets at 80lbs. The strings would wake him up at night when they broke in hotel rooms from the pressure. McEnroe strung hsi rackets 20-30lbs less and hit flatter. But the stroke pattern can be a factor. Sampras strings his rackets at 80lbs too, but hits much less spin. For a given racket for a given player, however, the tighter strings will probably give you the ability to generate more spin.

[ Moderator - 133 - 11:11:29 ]
Shishir: I think the first of those choices is the right one. The fuzz causes turbulence and keeps the air from separating from the ball. This causes the drag to reduce and so the ball would fly faster.

[ Moderator - 134 - 11:11:34 ]
RE: [TrishaSJSD] Is the temperature in the wind tunnel at a certain degree? Is it really hot or really cold ? or Is it room temperature inside?
Rabi - Can you answer Trisha's question?

[ Moderator - 135 - 11:12:15 ]
RE: [Moderator] Jani: I'll tell you what we saw and we are going to try and put the video of this up. John was at the end of the court catching the tennis balls as they were fired out of the machine and he is a tall person ---- the fuzzless balls acted like handballs --- they kicked up way over John's head and into the tennis court stands !!!!!
JY to Bo: my serve was HUGE with the fuzzless balls.

[ Hans/McNeeseState - 136 - 11:12:35 ]
John--The titanium I used was great for me. I have the world's shortest backswing and coupled with slow footspeed, I can face and and hit top on a lob or dip at volley's feet with no backswing. Hitting the "can opener" is a breeze too. Downside is loss of feel on the serve(so light) and elbow strain. I haven't significantly separated my strings on top like this since college.

[ BoGriffinCreekSchool - 137 - 11:12:38 ]
Jani, I think the balls with no fuzz had less spin, for one thing.

[ Moderator - 138 - 11:12:44 ]
RE: [Moderator] Jani: I'll tell you what we saw and we are going to try and put the video of this up. John was at the end of the court catching the tennis balls as they were fired out of the machine and he is a tall person ---- the fuzzless balls acted like handballs --- they kicked up way over John's head and into the tennis court stands !!!!!
Nasif -- Does this mean that the ball was going faster, or did it come out of the machine in a different direction?

[ Moderator - 139 - 11:13:33 ]
RE: [BoGriffinCreekSchool] Last time someone said more turbulence caused by fuzz would lower air pressure around the surface of the ball and allow the ball to go faster. Others said it would cause drag and slow the ball.
Rabi -- Yes, I know there has been some confucion over this. The reason is that it depends on the ball speed -- at lower speeds, a smooth ball will have less drag, but at some point the flow over the fuzzed ball will change (become turbulent) and the drag on it will drop significantly. This will also happen on the smooth ball, but at a higher speed since it does not have the roughness (fuzz) to make the flow turbulent.

[ Kevin/WatertownHighSchool - 140 - 11:13:35 ]
What have you learned about stroke patterns and full body motion?

[ Moderator - 142 - 11:15:15 ]
RE: [Moderator] Rabi, No, I meant in a game, a ball hit with a racket.
JY to Rabi: yeah it happens. I think i can reproduce it. Sometimes my grounstroke drives appear "flat" and sometimes they have topspin. You can vary it too. But I mean the ball isn't totally flat--its not like that Penn 7 doesn't twist and turn a bit.

[ Moderator - 143 - 11:15:56 ]
RE: [Moderator] Rabi - Can you answer Trisha's question?
RAbi: Hi Trisha. The temperature in the wind tuneel will be the same as the room temp. at the time -- the tunnel sucks-in air from the room and feeds into a big compressor. So fortunately, we shouls not have to worry about temp. effects in this experiment.

[ BoGriffinCreekSchool - 144 - 11:16:34 ]
Jan', Oops. More spin then. Maybe because more ball surface contacted the court surface?

[ Moderator - 145 - 11:16:58 ]
RE: [BoGriffinCreekSchool] Jani, I think the balls with no fuzz had less spin, for one thing.
Jani: We haven't analyzed the footage on those cases yet - the fuzzless balls are grey and it is difficult to see the spin. We have 60 tapes from the Open, 15 from the Sybase and 20 from Florida to go through and analyze. BUt we will look at that.

[ Moderator - 146 - 11:18:01 ]
Shishir: I think a tennis ball being served over a 100mph falls in the same flow regime and so the golfball logic should work here as well. Rabi is right, the speed of the ball(and therefore Reynolds number) has a lot to do with it.

[ BoGriffinCreekSchool - 147 - 11:18:15 ]
A new question: Is the fuzz length on the 14 inch ball in correct proportion to the diameter of the ball?

[ Moderator - 148 - 11:19:00 ]
RE: [Kevin/WatertownHighSchool] What have you learned about stroke patterns and full body motion?
JY; Kevin. It's incredible to see the strokes in slow motion. We are seeing similarities and big difference among the players. In common immediate unit turns with feet and shoulders. Differences are grips and length and shape of swings. The camera allows us to really see the position of the hitting arm as it moves through contact, and the all important start of the followthrough, prior to the wrap--all the players really move the racket through the line of the shot.

[ JoelSJSD - 149 - 11:19:09 ]
Do you ever use people in the wind tunnel for experiments?

[ Moderator - 150 - 11:19:17 ]
RE: [BoGriffinCreekSchool] Jan', Oops. More spin then. Maybe because more ball surface contacted the court surface?
Jani: I suspect that the thick fuzz "damps" out the ability to "rebound" off the court - it affects the coeffiecients of restitution and friction between the ball and the court. Without the fuzz is like a little rubber ball.

[ Moderator - 151 - 11:19:57 ]
RE: [BoGriffinCreekSchool] A new question: Is the fuzz length on the 14 inch ball in correct proportion to the diameter of the ball?
Rabi -- great question. i don't think it is -- right Jani?

[ Moderator - 152 - 11:21:15 ]
Rabi and Bo: I agree great question. Have we measured this?

[ Moderator - 153 - 11:22:13 ]
That was JY to Rabi and Bo.

[ Moderator - 154 - 11:22:30 ]
RE: [JoelSJSD] Do you ever use people in the wind tunnel for experiments?
Rabi: No, we do not here at NASA. However, a few years ago, I saw a presentation where downhill skiers were "installed" in wind tunnels with full gear on -- they could see a "drag meter" through a window in the floor. The whole idea was to figure out the best position for minimum drag -- a super experiment (as long as the skier remains mounted in the test section!!!)

[ Moderator - 155 - 11:22:34 ]
RE: [BoGriffinCreekSchool] A new question: Is the fuzz length on the 14 inch ball in correct proportion to the diameter of the ball?
Jani: Bo - that is a great question. The fuzz is "standard" Wilson tennis ball fuzz. So it is not "scaled" with the tennis ball as the size increased. The team all agreed that the wind tunnel test will give us a good feel for what is going on - in spite of that. We are looking in particular for flow separation and transition from smooth to rough (laminar to turbulent flow). We want to check that it matches what the NASA computer codes "predicted" the flow would look like in the CFD simulations.

[ Hans/McNeeseState - 156 - 11:22:54 ]
Did the team film serves? If so, were height of contact points measured? What about height of contact for groundstrokes?

[ Moderator - 157 - 11:23:51 ]
RE: [Moderator] Rabi: No, we do not here at NASA. However, a few years ago, I saw a presentation where downhill skiers were "installed" in wind tunnels with full gear on -- they could see a "drag meter" through a window in the floor. The whole idea was to figure out the best position for minimum drag -- a super experiment (as long as the skier remains mounted in the test section!!!)
JY to Rabi: Which would cause the most problem for NASA a skier or a giant tennis ball clogging the tunnel??

[ Moderator - 158 - 11:24:07 ]
RE: [Moderator] Rabi -- great question. i don't think it is -- right Jani?
Rabi: I am pretty sure it is not scaled. Ideally, we would want it scaled, but for the experiments (mainly qualitative) it should not matter that much.

[ Moderator - 159 - 11:25:56 ]
RE: [Hans/McNeeseState] Did the team film serves? If so, were height of contact points measured? What about height of contact for groundstrokes?
Nasif -- In a set of experiments designed to measure ball speed and path during competitive matches, we were able to measure the ball height to within a few inches at all points during its flight. We are still analyzing footage from the last two years' Sybase Open tournaments, and we will be tying that data into the high speed footage that was measured at the US Open and in Florida.

[ Moderator - 160 - 11:26:36 ]
RE: [Moderator] JY to Rabi: Which would cause the most problem for NASA a skier or a giant tennis ball clogging the tunnel??
Rabi: The skier, I think or may be not -- we have enough problems with animal activists here, but I have a feeling they may not feel the same about humans!!! THIS IS A JOKE -- PLEASE DO NOT GET UPSET

[ Moderator - 161 - 11:27:22 ]
RE: [Hans/McNeeseState] Did the team film serves? If so, were height of contact points measured? What about height of contact for groundstrokes?
JY to Hans: Yeah we filmed Pete, Venus, Phillipousis, Rafter. Lindsay Davenport, among others. As yet we haven't done quantitative measurements like that. I will tell you that virtually every server coils on the front leg and propels himself into the air. Also surprising was the extreme height many groundstrokes are played at--up there around the shoulder.

[ Moderator - 162 - 11:27:50 ]
Jani: OKAY ..... On that note --- we are approaching the end of our time - we can take one more question?

[ BoGriffinCreekSchool - 163 - 11:29:57 ]
When is the next chat?

[ Moderator - 164 - 11:30:05 ]
RE: [Moderator] Rabi: The skier, I think or may be not -- we have enough problems with animal activists here, but I have a feeling they may not feel the same about humans!!! THIS IS A JOKE -- PLEASE DO NOT GET UPSET
JY to Rabi: on a related note, we can't light the courts in Golden Gate Park due to avid animal activist defense of racoon nocturnal behavior--a threat to the species, apparently.

[ Moderator - 165 - 11:31:30 ]
Jani: I want to thank everyone in todays chat. Our Experts: Nasif, Rabi, Shishir, and John. And we want to thank all of you as well. A reminder that we are adding new information almost daily now - so visit our Web pages at http://wings.ucdavis.edu/Tennis The next video conference/chat is planned for early June - we do not have a date yet.

[ Moderator - 166 - 11:32:32 ]
Nasif : THanks everybody! Good bye.

[ BoGriffinCreekSchool - 167 - 11:33:10 ]
Thank YOU, experts. Bye.

[ Moderator - 168 - 11:33:14 ]
Rabi: Bye everyone, hope you all had as much fun as I did -- whenever I do a chat, I just wish that I had this kind of interactive oppurtunity as a kid.

[ Moderator - 169 - 11:33:40 ]
Shishir: Good Bye.

[ Hans/McNeeseState - 170 - 11:33:51 ]
Hans to JY: I've always felt that with a volleyball serve arm position, low toss and Micheal Jordan calf muscles that a "jump serve would be very difficult to return - The margin of error window would be much larger and the receiver would have less time to react to the ball if the serve contact point is literally just a few yards from the net.

[ TrishaandMarciSJSD - 171 - 11:34:11 ]
Thank you for an interesting chat We all really enjoyed the many questions and answers.We look forward to the experiment

[ Hans/McNeeseState - 172 - 11:36:18 ]
Thank you for the chat. It was great!!

[ Moderator - 173 - 11:38:39 ]
RE: [Hans/McNeeseState] Hans to JY: I've always felt that with a volleyball serve arm position, low toss and Micheal Jordan calf muscles that a "jump serve would be very difficult to return - The margin of error window would be much larger and the receiver would have less time to react to the ball if the serve contact point is literally just a few yards from the net.
JY to Hans: Yeah I've been waiting for someone 6'8" to develop that and blanket the net. I believe the only rule is that you must hit the ball before you land.