Prijon Kodiak

[Pai Mei]

Ότι η στιλπνή επιφάνεια είναι υδροδυναμικά αποδοτικότερη είναι αδιάψευστη αλήθεια,  δεν διανοήθηκε να την αμφισβητήσει ποτέ κανείς.
Ο γρίφος είναι,  ποια επιφάνεια αποδείχνεται στην πράξη περισσότερο στιλπνή,  η λεία που όμως εγκλωβίζει έμμονες φυσαλίδες αέρα ή η τραχειά,  που τις ξεκολλάει και τις διώχνει ;

Γιώργο, εμένα ως "καχύποπτο", μόνο το πείραμα μπορεί να με πείσει. 8)

Θα θέσω όμως χάριν της συζήτησης ορισμένα ερωτήματα, τροφή για σκέψη:

1. Αν η τραχύτερη επιφάνεια έχει την ιδιότητα απομάκρυνσης των φυσαλίδων και άρα καθιστά τη γάστρα πιο αποδοτική υδροδυναμικά σε σύγκριση με μία λειασμένη, τότε για ποιόν λόγο μία μουραβιασμένη γάστρα ζημιώνεται σε κόμβους και κατανάλωση, δηλ. σε απόδοση; Και τανάπαλιν η αφαίρεση της μουράβιας και η λείανση της επιφάνειας αποκαθιστά τη χαμένη απόδοση;

2. Αν ισχύει ο παραπάνω ισχυρισμός σου, γιατί να μην διαθέτουν και οι γάστρες των πολυεστερικών τραχειά επιφάνεια -ένα πέρασμα με χοντρό γυαλόχαρτο π.χ.- ώστε να γίνουν πιο αποδοτικές αφ' ενός, αφ' ετέρου πιστεύεις ειλικρινά ότι η τραχειά επιφάνεια των χαμηλότατου κόστους παραγωγής PE σκαφών (κάποιων εξ αυτών) προήλθε από πειραματισμό των κατασκευαστών και αποτελεί μέσο για βελτίωση της υδροδυναμικής αποδοτικότητας και όχι ένα απλό κατασκευαστικό χαρακτηριστικό, μια απλή ιδιότητα του υλικού και της εφαρμογής του;

3. Αν αυτό ίσχυε, γιατί να μην έχουν όλα τα σκάφη γάστρες με τραχύτερη επιφάνεια; Είναι δυνατόν αυτό να έχει προβλεφθεί για τα πάμφθηνα καγιάκ πολυαιθυλαινίου και να μην έχει εφαρμοστεί σε άλλου τύπου σκάφη, μηχανοκίνητα π.χ. όπου και το κέρδος σε απόδοση θα ήταν ανάλογο;

Να σημειώσω σε αυτό το σημείο ότι το wet-sanding, η υδροβολή δηλ. εφαρμόζεται ως τεχνική σε αγωνιστικά ιστιοπλοϊκά σκάφη, με σκοπό η γάστρα να αποκτήσει τραχύτερη επιφάνεια, πρακτική εμπνευσμένη από το παράδειγμα που ανέφερε και ο Random, με το μπαλάκι του γκολφ, η οποία θεωρείται ότι μπορεί να έχει αντίκρυσμα και στις γάστρες. Ας δούμε τι αναφέρει όμως ένας ειδικός επί του θέματος:

Perhaps it would help to debunk some of the psuedo-science myths that people use when defending wet-sanding. I’ve heard the golf ball theory used. They say that a dimpled golf ball travels farther than a smooth one. They’re right. They then relate this to the rougher surface caused by the sandpaper, as compared to the smoothness of the wax/polished surface and claim this is faster. They’re wrong. A golf ball is spinning in an airstream caused by its forward motion. A perfectly smooth ball would suffer flow separation very early around its surface, a large wake and subsequently large pressure drag. Remember that Parasitic Drag = skin friction drag + pressure drag. A smooth ball has low skin friction drag, but really high pressure drag, because even though the flow is laminar, it separates from the ball very early. Now, if you put dimples on the ball to roughen the surface, the turns turbulent and the resulting higher energy flow can stay attached to the ball longer, delaying separation, making a smaller wake and reducing the pressure drag. You have traded off the increased skin friction drag against an order of magnitude drop in pressure drag. Thus, the total drag drops and your drive goes farther. Spheres (golf balls) are very special cases, from an aerodynamic point of view. See Figure 1 & Figure 2, below.


Incidentally, tennis balls are fuzzy for the same reasons golf balls are dimpled. I can also explain how the stitches on a baseball are necessary to make them curve, some other time, if you wish. Don’t even get me started on a Whiffle™ ball’s aerodynamics. That’s really scary. Great. Now, here’s the difference between the golf ball and your boat and it’s subtle, so try to stay with me. You’re not sailing a spinning golf ball through air at 120 mph. You’re dragging a cigar shaped thing through water at 10-20. If you guess that this makes a difference in the total drag picture, give yourself a cigar. You’d need some pretty big dimples ?. At the speeds your hull(s) is(are) traveling combined with the “roughness” caused by 600-1000 grit sandpaper, you aren’t getting squat for lift and even if you did it’d be in the vertical plane and wouldn’t help forward velocity unless it caused a planing situation, which it won’t. (No this lift would not help you to windward!). All you’re getting is the parasitic and induced drag components. The pressure drag component of the equation is small enough compared to the other components as to be ignored. Airplane wings would be completely smooth, waxed and all, in an ideal world. However, engineers put turbulators, vortex generators and other devices on them to "roughen the surface" of the wing. Why? Not to reduce drag and make them faster. It’s to increase lift, or improve low-speed stall characteristics and/or to try to reattach turbulent flow before it completely detaches. Any roughening of the surface increases parasitic drag and decreases top speed potential in a foil type surface. The engineers just put up with this cause they have to. Keep in mind all of this theory is in reference to a particular body's Reynolds Number. A Hobie Cat will be operating at a very low average Reynolds Number, so allot of this talk is rendered moot. Just because one has a low Re doesn’t mean one experiences laminar flow. Thus the importance of determining the critical Re, which is no small feat. And all talk of fluid dynamics pivot on the viscosity of the fluid involved and water and air aren’t even close in viscosity. The bottom line is whatever one has to do to improve the flight of spinning golf ball really has no bearing on a Hobie hull(s) being dragged through the water by its sails.

[...]

The difference in drag, along a 16-20 foot hull, between a wet sanded hull and a waxed hull has never been experimentally confirmed, to my knowledge. That's how minute is the difference. Until someone drags a 600 grit sanded hull through a tank of water, with transducers attached to measure drag, then drags the same hull after waxing and proves a significant drag increase, I must insist that the lower drag will be attained with the waxed (smooth) hull.

Frank Bethwaite, on page 263 of his brilliant book “HIGH PERFORMANCE SAILING”, states "...at practical yacht or dinghy speeds, only the bow area of the hull can hope to run with a laminar boundary layer. Under this area the surface should certainly be highly polished. But beyond this zone the flow will become turbulent (remember the Reynolds Number equation and the relation to distance behind the leading edge/bow) and under turbulent flow a highly polished surface will not be any faster than some rougher surface, provided always that the roughness is less than some small fraction of the boundary layer thickness." Central to this is that roughness. I believe the boundary layer thickness to be extremely thin at the hull water interface and while Mr. Bethwaite does not concretely recommend waxing/polishing the entire hull, he doesn’t preclude its success and distinctly promotes leading edge treatment. The key is to have your hull as aerodynamically smooth as possible to keep the flow attached for as long as possible, keeping the transition from laminar to turbulent flow as far downstream from the leading edge as possible.

Christopher H. VanEpps
Aeronautical Systems Engineering
Lockheed Martin.
[email protected]

Για όποιον έχει όρεξη για διάβασμα υπάρχει και αυτή η διατριβή, η οποία εμπεριέχει και πείραμα:

http://my.fit.edu/~swood/Reduction%20of%...Layers.pdf

Δεν μπόρεσε όμως να αποδείξει κάποια επίδραση, ούτε αυτή:

Based on these results, it cannot be determined if the reduction of ship resistance through induced turbulent boundary layers was a success for this hull form; however, it can be concluded that it was not a failure. The hypothesis was neither proved nor disproved providing motivation for further investigation. If this topic is to be reinvestigated, it is the suggestion of the author that a more systematic approach be taken.