Wind over Tide - Page 3

Rex Delay · 04-11-2009, 13:58

Thank you. That was a very clear explanation and I should be able to get it across to them with as much clarity.

LakeSuperior · 04-11-2009, 07:35

Technically, viscosity is to fluids (water and air) as friction is to solids.

Viscosity determines the boundary layer thickness(es) and shear forces for a given velocity differential between two fluids or a fluid and a solid. Maybe think of thick grease versus thin lubricating oil.

Lodesman · 04-11-2009, 08:48

Quote:

Originally Posted by LakeSuperior

Technically, viscosity is to fluids (water and air) as friction is to solids.

Viscosity is to fluid, as malleability is to solids. If I chuck a glob of grease on the side of my brick house and another glob on the window, the thickness of the grease, or its viscosity will hold the glob together, but the difference in friction means the one glob will stay put on the brick, but the other will slide down the glass.

LakeSuperior · 04-11-2009, 10:31

Quote:

Originally Posted by Lodesman

Viscosity is to fluid, as malleability is to solids. If I chuck a glob of grease on the side of my brick house and another glob on the window, the thickness of the grease, or its viscosity will hold the glob together, but the difference in friction means the one glob will stay put on the brick, but the other will slide down the glass.

Lodesman, you are mixing apples, oranges, and peaches together here. Words like viscosity and friction have a specific technical meanings.

Dynamic fluid mechanics and mechanical wave theory needed to technically answer the post is saved for graduate school and it is a extremely difficult subject for many students. There is little chance of a definitive answer regarding the initial post given this forum. In fact, I am sure there are still many unanswered questions with regard to ocean wave propagation particularly in the surf zone.

However, I think it is a great problem and it is interesting to speculate on the phenomenology.

chris_gee · 04-11-2009, 15:36

The difficulty is that by experiment the water in a wave does move albeit in a circular pattern so it ends in much the same place while the wave form moves on. The source and experiments to back it up are in conflict with Lodesman's view that a left to right wave has an anti clockwise pattern.
While it is true that friction of wind on water or the fluid dynamics equivalent produces waves some of the explanations don't seem right.
Take 20 knots of wind against say a 3 knot current. The contrary current peaks say at the mid ebb. So not only is there usually not great fetch but there is only a limited time for the sea to build in these conditions.
It is difficult to estimate wavelength but in the conditions I frequently encounter it would be say 10m although it seems less. That gives a wavespeed of around 16 knots roughly. Wave height is said to be wavelength/7 ie 1.4m.
According to my source the circulatory motion is halved that is the radius of the circular movement and therefore the angular velocity at a depth of wavelength/9 so around a metre.
I calculate the velocity due to circular motion as about 6 knots directed back to the front of the wave from the preceding trough. That velocity in the top metre or so plus the current gives 9 knots going back, versus 3 deeper.so the top part is moving faster and piling up on the front of the approaching wave.
Considering the crest as one goes up the back the forward velocity due to circular motion gradually increases as it becomes more forward until at the crest you get 6-3 going forward so the apparent wind decreases to 20-3 =17. This is versus a relative wind with no current of 20 so it is not due to the wind effect which is less at the crest with a contrary current.
At the trough though the relative wind over water speed, assuming that the wave did not shelter it, would be 20 +9 29 giving some pushing off water forward steepening both trough and crest.
Long time since I did any physics so just my reading of it.

newt · 05-11-2009, 10:51

I think Paradix got it right. The water moves up and down in a typical wave. With the current moving the base, a sinusoidal wave becomes lopsided. It is kinda like sweeping the bottom out from underneath the wave, thus shorter, steeper waves. The gulf stream threw us around like a toy between Miami and Biminni with a Northwesterner. I should have taken alot of pictures, because it will be the only time I will be in waves that I could stand at the wheel, reach out and touch the middle of the wave. I will always have respect for ocean currents!

04-11-2009, 07:35	#2
LakeSuperior Registered User Join Date: Jan 2008 Boat: Siskawit - Mason 44 Posts: 80	Friction Technically, viscosity is to fluids (water and air) as friction is to solids. Viscosity determines the boundary layer thickness(es) and shear forces for a given velocity differential between two fluids or a fluid and a solid. Maybe think of thick grease versus thin lubricating oil.

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04-11-2009, 13:58	#1
Rex Delay Registered User Join Date: Oct 2006 Location: Banff, AB; Nanaimo, BC Boat: Sceptre 36 Posts: 50	Thank you. That was a very clear explanation and I should be able to get it across to them with as much clarity.

04-11-2009, 15:36	#5
chris_gee Registered User Join Date: Dec 2006 Posts: 95	The difficulty is that by experiment the water in a wave does move albeit in a circular pattern so it ends in much the same place while the wave form moves on. The source and experiments to back it up are in conflict with Lodesman's view that a left to right wave has an anti clockwise pattern. While it is true that friction of wind on water or the fluid dynamics equivalent produces waves some of the explanations don't seem right. Take 20 knots of wind against say a 3 knot current. The contrary current peaks say at the mid ebb. So not only is there usually not great fetch but there is only a limited time for the sea to build in these conditions. It is difficult to estimate wavelength but in the conditions I frequently encounter it would be say 10m although it seems less. That gives a wavespeed of around 16 knots roughly. Wave height is said to be wavelength/7 ie 1.4m. According to my source the circulatory motion is halved that is the radius of the circular movement and therefore the angular velocity at a depth of wavelength/9 so around a metre. I calculate the velocity due to circular motion as about 6 knots directed back to the front of the wave from the preceding trough. That velocity in the top metre or so plus the current gives 9 knots going back, versus 3 deeper.so the top part is moving faster and piling up on the front of the approaching wave. Considering the crest as one goes up the back the forward velocity due to circular motion gradually increases as it becomes more forward until at the crest you get 6-3 going forward so the apparent wind decreases to 20-3 =17. This is versus a relative wind with no current of 20 so it is not due to the wind effect which is less at the crest with a contrary current. At the trough though the relative wind over water speed, assuming that the wave did not shelter it, would be 20 +9 29 giving some pushing off water forward steepening both trough and crest. Long time since I did any physics so just my reading of it.

05-11-2009, 10:51	#6
newt Registered User Join Date: Oct 2008 Location: Layton, Utah Boat: Valiant 40, Compac 23 Posts: 265	I think Paradix got it right. The water moves up and down in a typical wave. With the current moving the base, a sinusoidal wave becomes lopsided. It is kinda like sweeping the bottom out from underneath the wave, thus shorter, steeper waves. The gulf stream threw us around like a toy between Miami and Biminni with a Northwesterner. I should have taken alot of pictures, because it will be the only time I will be in waves that I could stand at the wheel, reach out and touch the middle of the wave. I will always have respect for ocean currents!