Jabulani is the name of the official ball of the World Cup in South Africa made by Adidas (like all balls since 1970.) Jabulani means ‘to celebrate’ in Zulu. The ball was unveiled last December in Cape Town.
Adidas posted a few of videos about the ball. In the videos they mention that the design of the ball took 6 years (!!) and that is made of eight thermally bonded 3D panels and is perfectly round. In one of the videos they show the whole manufacturing process which is pretty amazing for a ball.
In a couple of other videos they talk about the aerodynamic design of the ball.
In one of them they initially show flow past a smooth sphere in a wind tunnel.
In the video frame shown here the streamlines are clearly visible as the air flows from left to right. Because of viscous effects (friction, thermal conduction, etc) the flow separates over the back of the ball creating a recirculating flow (i.e. turbulent wake) downstream of body. This separated flow is responsible for the pressure (or form) drag which contributes greatly to the total drag of the ball thus affecting its flight performance characteristics.
In the video, the separation points seem to be either ahead of the maximum thickness of the sphere or very close to it. That indicates a Reynolds number (Re) that is less than 5e5 (for a smooth sphere.) For such an Re the separation points form at about 82 degrees measured from the stagnation point (front center of the sphere.)
The dimensionless Reynolds number (named after its discoverer, Osborne Reynolds) is a measure of the ratio of inertia forces to viscous forces in a flow. It is given by Re = rVL/m, where V is the free stream velocity, r is the free stream density, L is the characteristic length of the body immersed in the fluid (in this case the diameter), and m is the free stream viscosity coefficient.
Assuming that a standard size 5 smooth ball (about 0.23 m in diameter) would fly in standard air (constant r = 1.2 kgr/m3) at 90 km/hr (25 m/sec) after a decent kick the Re would equal:
Re = 1.2 * 25 *0.23 /1002e-6 = 6.9e+3
This is well within the regime of Re < 5e+5. Above that the separation points would move further back and thus pressure drag would actually reduce. That is because the boundary layer (a thin layer of fluid very close to the surface of the body that the viscous effects matter) would transition from laminar to turbulent which separates at about 125 degrees (measured from the forward stagnation point.) This results in a thinner turbulent wake with higher pressures and a lower drag coefficient for the transition point (i.e Re ~ 5e+5 for the sphere.)
However, it would be impossible to achieve something like in football (i.e. the velocity of the ball in flight would have to be a few thousands km/hr!) Therefore another technique is needed to push the stagnation points further back and reduce the turbulent wake and thus the pressure drag.
Enter the ridges!
Later in the video the ball replaces the sphere in the wind tunnel but unfortunately they do not show the flow pattern around the ball, presumably to protect proprietary information. The engineering manager for Adidas, Dr. Tim Lucas, explains that the designers spent a lot of time on the design of the ridges on the ball and ‘they play a very important role so that the ball is stable in flight.’
Presumably, the ridges break up the laminar flow within the boundary layer and transition it to turbulent at much lower Re thereby reducing the form drag and improving flight performance. The ‘rough’ surface of Jabulani should lower the critical Reynolds number and possibly make sudden drop in drag coefficient attainable, which would be a pain for goalkeepers!
Finally, these effects would affect the bending of the ball. If the critical Re is low enough to be in the region relevant to football then you could have Re larger than the critical one. In that case a kick that adds backspin to the ball would cause separation on the bottom side to occur earlier than on the top side. Therefore the bottom side will incur higher pressure and thus generate a higher upward force (lift.) Higher lift would result in longer balls and possibly flatter trajectories. More bad news for the goalkeepers!!
Never heard of this blog before but was reading about the Jabulani and how it had ridges and I also naturally thought it must be to induce turbulence (like a golf ball). And so I agree with your statements. But I also read that the ball tends to be very unpredictable in a knuckle-kick (no spin). I'm wondering if the asymmetry of the ridges could contribute here? I know in baseball the unpredictability in a knuckleball is supposed to be due the the presence of the seams. Would be interested to hear your thoughts!
ReplyDeleteHi there,
ReplyDeleteThank you for you comment and for sharing your observation. I have also read some of the negative comments made by goalkeepers about
Jabulani and I have to admit that I have some doubts about the nature of the complaints.
Some players have called the ball 'lightweight' and 'small' that 'bounces off the gloves; ' It has also been compared to a volley ball and a supermarket ball! That hardly seems like an accurate description since FIFA regulates the weight (about 400-450 gr) and the size (around 25 cm) of the ball. Therefore, I cannot believe that the weight and size of the ball is problem.
Which brings us to your point: Is it possible that the ridges cause trouble for certain types of kicks? As you mentioned, it could be that
a kick that induces no spin on the ball could have consequences similar to a knuckleball.
Though I am not a baseball expert, my understanding is that a pitcher throws the knuckleball without any spin. In that case the orientation of the seam would play a significantly role in the ball trajectory
because the baseball seam is not symmetric. For example, if the ball is thrown without any spin and the seam on the top of the ball is ahead of the seam on the bottom then an asymmetric (top versus bottom) separation would occur. That would cause an asymmetric velocity and
pressure profile along the surface of the ball and thus bend (or lift) and at the same time rotate slightly. The new position of the seam
would generate new asymmetric flow profiles and thus the ball will again change direction and position slightly with yet another brand
new flow profile. I would expect that during flight the knuckleball spins very slowly continuously changing lift characteristics which
would result in an erratic flight.
Could it be the same action at play for Jabulani?
I think that the key word here is asymmetric:
a) If the ridges all around the ball are symmetric and uniform, no matter its orientation, then the flight path should be very predictable. In that case the reason(s) of the keeper's complaints is something other than the flight path (e.g. it is faster than other balls, or it is very round, etc.)
If the ridges on the ball are not symmetric around the ball, and they are orientation-dependent then you are right; If you do not induce spin during the kick then the ball could have an unpredictable flight path.
Unfortunately I have not seen an official Jabulani except in pictures. (At US $150 it is rather expensive!) In the pictures the ridges on the ball look symmetric but I cannot be absolutely sure.
Hello...I stopped in a sporting goods store to consider buying one of these balls, and I looked at four or five. They all had ridges running around them that were pushing up from under the surface and were not part of the actual surface texture. I asked about them and the salesmen suggested inflating it more, and when he did they started to disappear. (And when he let air out, the ridges became much more apparent.) Unfortunately he didn't have a gauge, and we hesitated inflating it enough for the ridges to go away completely, and I passed on it, thinking maybe it was a bad batch. There has been so much written about the ball being a perfect sphere, but these ridges compromised that description. Can you provide any insight? Thanks! David
ReplyDeleteHi David
ReplyDeleteThank you for comment.
The official ball is supposed to be a perfect sphere with the ridges on the surface to improve the aerodynamic performance of the ball (as I discussed for the original post.) Based on pictures and videos the ridges are on the outer surface layer.
Any chance that the ball you saw was a replica or made by a different manufacturer? Based on the images I have seen, the various versions of Jabulani replicas made by Adidas are smooth.
gk.
Hi and thanks, gk. It was the official $150 adidas ball. I agree it looks smooth in photos... That's what's making me wonder, in addition to the spherical claims. The balls I saw actually looked like a traditional soccer ball with a skin over it. The ridges run under the surface like mountain ranges, and they don't match up with the patterning on the surface. (I acknowledge the many bumps and ridges that are molded on the actual surface.) someone I called at a soccer store said they were there to help grip the ball and part of the aerodynamics, but I'm still confused and looking for independent confirmation, or a chance to check it out elsewhere in person...it's just not that easy to find around here. If you get your hands on one, I'd love to hear your thoughts. Thanks! David
ReplyDeleteHello,
ReplyDeleteTake this into consideration - Jabulani has airflow ridges, while an ordinary ball(earlier addidas or nike with wich they play in Premier League) has mostly ridges and small bumps on stiches.
Point is that viskosity of the air at one point(speed/spin relation) in trajectory starts to make air have friction to surface of the ball which makes leaf, drop and all kinds of curved shots possible. Structure of the surface of the spinning ball takes a great part in a final calculation of point and rate of change of trajectory.
So, different surface means different curvature of path and that is where sudden change of direction is coming from. Also, airflow ridges are basicly helping the ball to mantain it's spin, so it spins more then usual at the end of the path so it can "jump out" of the unprepared goalkeeper's hands.
Hi Rastko,
ReplyDeleteI agree with you that Jabulani behaves differently than a standard ball by design. And if you add other factors that affect the trajectory of the ball (e.g. altitude of venue, pressure of air inside the ball, etc) then the goalkeepers that are not familiar with the behavior of the ball at that environment could easily be fooled and mishandle it.
gk.
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ReplyDelete