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Hydrofoil during CFD analysis

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zfennell
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Re: Hydrofoil during CFD analysis

Postby zfennell » Mon Mar 02, 2015 8:22 pm

BWD wrote:Intuitively my take is accelerated flow from over top of the front wing approximately matches the velocity across the top (high pressure side in this design) of the stab in a useful velocity range, reducing stab lift and drag in that range..... Naturally as an unschooled Neanderthal in the area I may be overlooking a few details...

But variations in fuse length over the last while of developing current racing foils may support this idea, or not, what do I know?

yes,
except for the 1st and 3rd plots (3 & 8 m/s) which have comparatively high peak velocities in the rear fin (high negative lift values and velocities higher than the suction side of the front wing)

thus suggesting that pitch control could be on/off/on as a function of velocity while pitch angle is held constant.
at the moment, it hard to come up with a story I believe.
-bill

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Re: Hydrofoil during CFD analysis

Postby shawn13 » Mon Mar 02, 2015 8:30 pm

zfennell wrote: yes,
except for the 1st and 3rd plots (3 & 8 m/s) which have comparatively high peak velocities in the rear fin (high negative lift values and velocities higher than the suction side of the front wing)
-bill
I'm going to check the AoA on the rear to confirm its at zero as per the front.

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Re: Hydrofoil during CFD analysis

Postby shawn13 » Mon Mar 02, 2015 8:56 pm

davesails7 wrote:What are you trying to get out of the analysis?
I have a background in Off gas systems, we use CFD in hood collection and duct design. I'm playing with the software to expand my knowledge into a fluid scenario. Over the past few months, my hobby of board building has expanded my knowledge base in software I use since most of my work designs are repetitive.

I'm trying to understand the foil design, and hopefully expand to different wing shapes/sizes and positions. In reality, it would be almost impossible to run every scenario that a foil encounters, since there's numerous variables and almost no constants. When starting on the foil it is never in one position in direction to flow, tide, currents, wave cavitation, board angle under feet, etc. But, if I can start from a base line and add different factors to the equation, I may be able to pull data that could result in important design changes for wing efficiency.

Hopefully, by discussions and feedback, this thread can generate important info to progressing the foil, or push further limits to light wind kiting and result in good open source info for the home builders. As a hobby, i don't mind doing 'work' stuff in my spare time since it challenges and stimulates me.


Plus, its Winter and I'm bored...

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Re: Hydrofoil during CFD analysis

Postby zfennell » Mon Mar 02, 2015 10:38 pm

shawn13 wrote:
zfennell wrote: yes,
except for the 1st and 3rd plots (3 & 8 m/s) which have comparatively high peak velocities in the rear fin (high negative lift values and velocities higher than the suction side of the front wing)
-bill
I'm going to check the AoA on the rear to confirm its at zero as per the front.
I'm sure the geometry / mesh has not changed from one run to the next
( you would certainly know the answer to that)

But, regarding your comment about interference from the front wing:
are the velocity vectors near the stagnation point of the back wing much different from one run to the next?....direction and/or magnitude?
particularly the 3 & 8 m/s runs

normalize any velocity number to freestream velocity ( Vinf )
pressure becomes a synonym to velocity : Cp = 1- (V/Vinf)^2

-bill

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Re: Hydrofoil during CFD analysis

Postby shawn13 » Mon Mar 02, 2015 11:02 pm

zfennell wrote:
shawn13 wrote:
zfennell wrote: yes,
except for the 1st and 3rd plots (3 & 8 m/s) which have comparatively high peak velocities in the rear fin (high negative lift values and velocities higher than the suction side of the front wing)
-bill
I'm going to check the AoA on the rear to confirm its at zero as per the front.
I'm sure the geometry / mesh has not changed from one run to the next
( you would certainly know the answer to that)

-bill
No the geometry has not change,d but Im questioning if the whole series has the rear in zero in the first place. (Need to verify model)



I've had a discussion with our guy here, and he said that the turbulent flow off the front wing shown in the 6 m/s graph is absolutely true. There will be flow disruption to the rear, and is shown in the light blue low velocity behind the rear. However, if the fuselage was included, this would most likely result in more laminar flow over the rear as the fuselage would be directing the flow.

Once I can achieve this, I can move on to adding shear stress boundary on free flow surface to simulate the water surface/wave action. He also mentioned that the initial flow shouldn't be much of a concern, more of the flow patterns in average riding velocity for wing position.

Ill try to mesh the wing/fuselage sometime this week to see the difference.

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Re: Hydrofoil during CFD analysis

Postby zfennell » Mon Mar 02, 2015 11:16 pm

I agree,
but by specifically omitting his opinion on the 3 m/s and 8 m/s runs it sounds like he may also think they are possibly BS.

w/o knowing why, its tough you just toss (or believe) the ones you don't understand.

your data files should provide you enough info to compute lift for each wing at each speed.
find that output variable or do the integration.
plot the answers

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Re: Hydrofoil during CFD analysis

Postby shawn13 » Tue Mar 03, 2015 12:57 am

zfennell wrote:I agree,
but by specifically omitting his opinion on the 3 m/s and 8 m/s runs it sounds like he may also think they are possibly BS.

w/o knowing why, its tough you just toss (or believe) the ones you don't understand.
I was not omitting his opinion, i thought previously an issue with the rear wing losing lift was raised.

He had no worry with any of the runs, they all looked fine and stated that some investigation needs to be made with the fuselage included to fully analysis the flow. The fuselage is going to direct flow much differently than with only two wings.
zfennell wrote: your data files should provide you enough info to compute lift for each wing at each speed.
find that output variable or do the integration.
plot the answers
I will look into this for you.

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Re: Hydrofoil during CFD analysis

Postby zfennell » Tue Mar 03, 2015 2:50 pm

my apologies shawn.
poor choice of words on my part.

my point was simply that the more you are able to validate the inconsistencies in your results the better footing you will have for moving forward .

I believe you are most correct to get the 'baseline' right before adding xtra complexities.
the more your 'math' is able to predict simulations with known results, the better

at the moment you have results that appear to be speed dependent.
imo, finding the reason is important.
for instance.
increasing speed certainly increases Reynolds number, that translates to increase in shear stress, reduction in BL stability, increase in lift and drag, potential for BL transition from laminar to turbulent, potential for BL separation, potential for cavitation.

how does your code deal with all of these?
can you see the any of them happening, ?
are they predicted by your code or just input by the operator at the beginning.

all of the above may be a function of speed but its typically treated as steady state.
which doesn't do much to explain the oscillations in lift of the rear wing.
does the software have the ability to deal with time-variable flows?
like large scale vortices off the wing tip
or vortex shedding from the trailing edge? are one of your test speeds close to a struhall shedding frequency?

I guess if you were really paranoid, you'd want to confirm simple things like conservation of mass and energy. do all of the things going into your control volume come out somewhere?
does the pressure and skin friction drag predicted by the simulation equate to change in momentum in the fluid ?

sorry for the rambling as my point is no longer simple.
but you have accomplished a lot that no one has offered so far.
and I believe the vast majority of what your are predicting is correct.
But you may need to back up a bit, to even simpler models with known behaviors so that you can get a better grip on how your CFD code deals with different variables.

regardless.
it's all pretty cool.
thanks,
-bill

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Re: Hydrofoil during CFD analysis

Postby ZigZag » Tue Mar 03, 2015 7:09 pm

I would re-analyze the simulation that gave the anomalous result. I would run a ladder series of simulations. I would keep the velocity constant, but would incrementally change the location of the rear wing, from some distance below the plane of the front wing, to some distance above the plane of the front wing. If your theory is correct (loss of negative lift due to turbulence coming off the front wing), then re-positioning the rear wing should result in a change. If there is no change, there may be an error in your model.

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Re: Hydrofoil during CFD analysis

Postby jb92563 » Sun Mar 08, 2015 5:56 am

Has anyone tried to design a canard layout, with the small stabilizing wing ahead of the bigger main wing.

There would be less flow disruption if you stagger the main wing and the stabilizing wing would be in clear flow and result in an easier more stable ride as speed increases.

It would require more weight on the rear leg vs the front which is more like what kite surfers are used to instead of the opposite as current designs.

Also if you hit something, the smaller cheaper wing takes the damage potentially saving the main wing from damage.

Seems like the current designs are backwards.

Just pondering improvements/evolution.


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