What phenomenon occurs that reduces drag when a swim cap is applied

[jeeloong]

Dear Experts,

I am 4th year undergraduate who did experiments for drag reduction in swim cap. I had applied hydrophobic and hydrophilic coatings on my cap. My final results are shown in attachment below.

The no cap has more drag than body with swim cap without coatings. There is not much different in drag coefficient for body with swim cap, hydrophobic coating and hydrophilic coating.

The body with goggle has most drag which is noticeable in experiment as the body has most obstruction which creates turbulence.

However, anyone knows what is happening for swim cap profile? why there is a sudden rise in acc for split second. Is it the turbulent Boundary layer has been tripped and therefore delay the flow separation and reduces the wake??

I found it hard to explain. It appears that a streamline swim cap reduces the drag of a no cap polystyrene sphere.

 

[jeeloong]

The sudden rise in acceleration at 1.2s is due to the fact that the body has escape out of water surface! and moving into less dense medium that why suddenly accelerate again until being overcome by gravity and decelerate downward!

 

[jeeloong]

By the way terminal velocity is only obtained for body with goggle and body with large cap. as u can see the acceleration is approaches zero.

 

[boneh3ad]

It is difficult to say what is going on without more information about your experiment. Are you dropping this apparatus and letting it sink to the bottom of the tank or are you detaching it an letting it float to the top? What is the character of the surface under the cap? Are there pictures of each configuration for comparison?

I'd hesitate to call your results with goggles valid. Given the size and shape of your test article here, the results for the goggles are not going to be relevant or comparable to similar results on a swimmer with goggles.

 

[jeeloong]

I am letting the object float to the top using buoyant force. Sorry there is some error in my calc the C_d for swim cap with goggle is 0.5++ as I forgeet to include v to the square.

The surface under the cap is normal polystyrene foam with some grain boudnaries as u can see. THe swim cap is silicon speed swim cap. And I also applied hydrophobic and hydrophilic to it. However there the profile is very distinctive for the one with swim cap with no coatings with hydrophobic and hydrophilic the same. My weight is 2.5 kg W = mg and buoyant force is approximately 35 N

 

[jeeloong]

You are right boneh3ad, the Cd for goggle is out of context given that Cd of 0.8!

 

[boneh3ad]

Is the sudden spike in acceleration coincident with the test apparatus breaking the surface? It appears near the end of the measurement period so it may just be the spike being the brief period where the buoyant force is still large but the top of the apparatus is out of the water and therefore experiencing significantly less drag.

 

[jeeloong]

Thanks boneh3ad, I completely agree with you the sudden spike in acceleration at the end of experiment is due to test apparatus breaking surface. However, the one with small streamlined swim cap with sudden rise in acceleration at 0.6 s is really puzzling. I believed it has something to do with tripping of BL. However there is no trip wire or any sort of thing except for smooth silicon surface of swim cap.

 

[boneh3ad]

What is the difference between the "large" and "small" caps that you used? There very well may be some sort of boundary-layer transition effect going on. There need not be any trips to cause transition. The thing is, in all likelihood the styrofoam surface would transition before the cap surface would given its surface character and that seems to show up in your data if that is the true explanation for your data. The "no cap" version has a small jump there earlier than the capped versions do. The real issue is that the phenomenon occurs much more quickly on your "no cap" and "large cap" cases, whereas it seems to take longer for the other cases. I don't know about the "large cap" case, but it seems like if it is a transition issue, the transition likely occurs quickly for the bare styrofoam on account of the roughness while the smooth caps may have a more smooth movement of its own transition front from front to back as the speed (and thus Reynolds number) increases. That still doesn't necessarily explain everything though, and I am not sure you have enough data to actually answer these questions.

 

[Baluncore]

Any rational analysis of the real world must allow for the irrational component.
Much of the effect observed, when the cap is worn by a swimmer, may be placebo effect.

 

[hpkido404]

Hi, I am a student looking to do a similar project. Can you send me your paper and notes so I can get an idea of what your procedure was. Thanks.