The Boat Design & Boatbuilding Site
lift without downwash?
- Thread starter lunatic
- Start date
jehardiman
Senior Member
- Joined
- Aug 5, 2004
- Posts
- 4,206
- Reaction score
- 1,581
- Location
- Port Orchard, Washington, USA
It depends on what you mean and define as "downwash". In limited aerodynamic wing lift terms, yes. A helium balloon has lift without downwash...in fact it has "upwash".
In hydrodynamic terms, it is more complicated in the fact that we don't consider "lift" in such simplistic terms as they are used by aerodynamics. In order to generate a force on an object in a fluid you must have or cause a pressure gradient in the fluid. An example of this is the force of static buoyancy (i.e. the balloon above) where the body is "lifted" by the fluid density against gravity. If the object is moving, the fluid must flow to get out of the way, and it is this motion that generates a dynamic pressure gradient. The Work done by the body advancing along it path imparts Energy in the fluid. Because the fluid has mass and viscosity, it continues to disperse the Energy according to the laws of physics (2nd Law of Thermodynamics) after the body. This generates a measurable flow field/pressure gradent in the fluid behind the body which can be termed a "wake" or "downwash".
Wether the pressure gradient generated by the moving body is helpful (i.e. propeller thrust) or hurtful (i.e. hull drag) depends on what you need it for. FWIW, in my opinion there is no "lift"...there is only drag in the direction you want to go. And it is important where your point of reference is to the flow...i..e. the fluid in the relative "wake" never slows down, it speeds up because it wasn't moving to begin with.
In hydrodynamic terms, it is more complicated in the fact that we don't consider "lift" in such simplistic terms as they are used by aerodynamics. In order to generate a force on an object in a fluid you must have or cause a pressure gradient in the fluid. An example of this is the force of static buoyancy (i.e. the balloon above) where the body is "lifted" by the fluid density against gravity. If the object is moving, the fluid must flow to get out of the way, and it is this motion that generates a dynamic pressure gradient. The Work done by the body advancing along it path imparts Energy in the fluid. Because the fluid has mass and viscosity, it continues to disperse the Energy according to the laws of physics (2nd Law of Thermodynamics) after the body. This generates a measurable flow field/pressure gradent in the fluid behind the body which can be termed a "wake" or "downwash".
Wether the pressure gradient generated by the moving body is helpful (i.e. propeller thrust) or hurtful (i.e. hull drag) depends on what you need it for. FWIW, in my opinion there is no "lift"...there is only drag in the direction you want to go. And it is important where your point of reference is to the flow...i..e. the fluid in the relative "wake" never slows down, it speeds up because it wasn't moving to begin with.
PI Design
Senior Member
- Joined
- Oct 18, 2006
- Posts
- 673
- Reaction score
- 21
- Location
- England
Nope.
The question refers to a sail's TE but the hurtful cost of the form of the wake is what I am after. I have sailed leading edge vortex rigs which seem to leave a single trailing vortex with no conventional downwash. I know the penalty of backside lift of these rigs, but if the wake were less hurtful, I would try to put the helpful, negative drag on a forward facing surface.
Submarine Tom
Previous Member
- Joined
- Aug 10, 2008
- Posts
- 0
- Reaction score
- 81
- Location
- North America
Yes, but it will be minuscule.
upchurchmr
Senior Member
- Joined
- Feb 13, 2011
- Posts
- 3,287
- Reaction score
- 260
- Location
- Ft. Worth, Tx, USA
My understanding of downwash is that it is a redirection of air caused by an airfoil.
If that is accepted, you have to have downwash to get lift, unless you are talking about the trivial example of a helium balloon.
Please provide an image or sketch of the "leading edge vortex" rig.
You need to talk about the whole airfoil to make sense, not one part.
Where are the experts to take me apart!!! Or set me straight, often needed.
If that is accepted, you have to have downwash to get lift, unless you are talking about the trivial example of a helium balloon.
Please provide an image or sketch of the "leading edge vortex" rig.
You need to talk about the whole airfoil to make sense, not one part.
Where are the experts to take me apart!!! Or set me straight, often needed.
Have a look at potential flow theory. Some info around on the web, it's the basis of all 2D and 3D panel methods. That should give you some background of the fluid mechanics. Then you can answer your own question.
Gonzo - I hope you mis-read the question - downwash is very definitely real (and measurable). The jib/main combination works as a multi-element foil, so more detailed analysis (minimum 3D vortex-lattice method) is required for the flow in the slot.
Tim B.
Gonzo - I hope you mis-read the question - downwash is very definitely real (and measurable). The jib/main combination works as a multi-element foil, so more detailed analysis (minimum 3D vortex-lattice method) is required for the flow in the slot.
Tim B.
jehardiman
Senior Member
- Joined
- Aug 5, 2004
- Posts
- 4,206
- Reaction score
- 1,581
- Location
- Port Orchard, Washington, USA
For a sail, where the fluid is transfering work to the object, there is no "lift" ...it is all just drag (i.e. work) in the direction you want to go. What you wish to call "downdraft" is just the reduced energy wake caused by the extraction of work from the flow. As Tom and upchurch point out, if the energy in the wake is unchanged, you could not have generated any work (i.e. "lift"). The "vortex rings" you you may have seen are just artifacts of the pressure gradinet/flow field and are not entities unto themselves. One problem many people have with understand flow is that early theorists seized upon the vortices as a way to solve the energy potential problem. Vorticies do not have to be and are not always present in wakes, but modeling flow with them makes the math easier.
N.B.: Remember that energy and work contain vector components and aero/hydro-dynamics is all about manipulating the direction and transfer between the two.
Last edited:
He didn't mis-read the question. He obviously doesn't know what downwash is. See his reply to you.
Paul, yes, I noticed.
This is confused by the fact the the "downwash" from sailing boat foils and sails is actually manifested sideways, not vertically. The reason we call it downwash is because it stems from the aerospace world. It is actually the position of the wake, and it can be seen in a plane normal to the spanwise vector. ie. normal to the mast.
Hard to ask for better illustrations than these. Note the inflow and outflow are at significantly different angles.
http://www.anderswallin.net/category/model-yachting/iom-design/
This is confused by the fact the the "downwash" from sailing boat foils and sails is actually manifested sideways, not vertically. The reason we call it downwash is because it stems from the aerospace world. It is actually the position of the wake, and it can be seen in a plane normal to the spanwise vector. ie. normal to the mast.
Hard to ask for better illustrations than these. Note the inflow and outflow are at significantly different angles.
http://www.anderswallin.net/category/model-yachting/iom-design/
You may find these excellent papers of interest.
Telltales at leading and trailing edges of a well trimed sail, show upwash-downwash phenomena.
(modified image stoled from archifamous Arvel Gentry's articles)
No matter how strange a foil can be, I can not imagine how, when Downwash is parallel to external flow (ie no Downwash), you can have Lift
(modified image stoled from archifamous Arvel Gentry's articles)
No matter how strange a foil can be, I can not imagine how, when Downwash is parallel to external flow (ie no Downwash), you can have Lift
Attachments
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making any potentially dangerous or financial decision, always employ and consult appropriate professionals. Your circumstances or experience may be different.
When making any potentially dangerous or financial decision, always employ and consult appropriate professionals. Your circumstances or experience may be different.
Advertisement:
Similar threads
