The Open Jet Facility is a spectacular and popular laboratory. The large wind tunnel of 13 metres wide and eight metres high with smooth airflow is very popular with sports teams. Models of professional cyclists, bicycle helmets and skating outfits have already been tested on aerodynamics here. The 500 kilowatt fan can generate wind speeds of up to 35 metres per second (over 120 km/hour).
After six months on a waiting list, PhD student Jelle Poland (Faculty of Aerospace Engineering, AE) was recently able to use the facility for a fortnight to take unique measurements. Unique because, as far as we know, no one ever put a kite in the wind tunnel.
The largest kite just fits against the wall of a sports hall
In the world of kite power generation (Airborne Wind Energy systems), there is an understandable desire for ever larger kites. After all, the larger the kite, the more power. The Delft KitePower company, for instance, is working with kites of 25 and 60 square metres (to generate 30 and 100 kilowatts of electrical power respectively). The smallest kite has twice the surface area of an average student room, while the largest kite just fits against the wall of a sports hall. That’s about the limit of what craftsmanship, experience and intuition can deliver. For even larger kites, researchers and wind energy pioneers would like to know the exact level of performance they can expect in advance.
To this end, Jelle Poland is developing a simulation programme that supports the design of extremely large kites. It calculates the forces on kites from passing airstreams. But then again, it is desirable if the results of the simulation matches reality. […]
This is a quote from the article that I do not take personally because NASA did it, and published: Low-speed wind-tunnel investigation of a series of twin-keel all-flexible parawings on
Model wind-tunnel and flight investigation of a parawing-lifting-body landing system on
And also, about “Flexible-wing (parawing)”, to start with Figure 1 (author Francis M. Rogallo, who is the designer and director of the Parawing), on
An excerpt page 3:
In the pre-NASA years the wing was called a kite because of the three experimental methods used: testing in a home made wind tunnel, free flying as hand-launched gliders, and tethered outdoor flying as kites.
The well-known NPW (Nasa Para Wing) power kites were therefore tested in wind tunnels decades ago, in various forms including the flexible kite.
I have had the pleasure of working with Jelle Poland, and I’m sure he is [still] doing great work with these wind tunnel tests. The information discovered here will likely become important for scaling of kites to larger sizes. I also talked to him during AWEC, and he has some great ideas, so I’m sure we have not hear the last of him.
I think this is a little bit nitpicky. Also not a discussion leading to any valuable insight. The paper is about testing a kite in the wind tunnel, not about claiming to be a world first. Of course people have put many things in wind tunnels. But the exact work done at Delft has quite surely not been done before.
Unique because, as far as we know, no one ever put a kite in the wind tunnel.
I agree this claim is exagerated though. I would chalk that up to journalistic language.
Still, not all the measurements have been processed yet. PIV (Particle Image Velocimetry) is a technique using helium bubbles, lasers and multiple cameras that captures the airflow around the model in unprecedented detail. But this requires so much computing power that its results are still some time away.
This maybe, better describes what is new?
The truth is sometimes unpleasant.
On the contrary, the NASA documents that I cited are very interesting on the much earlier use of wind tunnels to test kites.
The paper claims to be a world first for testing a kite in a wind tunnel, what is wrong.
I was quite careful to qualify this statement which is contradicted by the facts and the sources I cited.
I think that this would merit a correction of the article on this point, even if the qualities of the research can possibly be identified.
I did not see a paper, rather just an article…
Something true, although written by “Delta, independent journalistic platform Tu Delft”, and therefore in relation to the scientific aspect of the academic work of Tu Delft, which accentuates the quality of writing which seems well beyond that of a simple press article, and which has also allowed you to identify interesting technical aspects of this research.
Hi Pierre, I think it is said in “the” wind tunnel, not in a wind tunnel.
Just making a google search, we can see that many wind tunnel (big or small) experiments on kite where already done.
I had myself the chance to visit the experiment of Emmanuel Du Pontavice during his thesis preparation https://robokite.blogspot.com/2015/01/portrait-emmanuel-du-pontavice.html, just as a less known example among others, but which has inspired development of Liberty kites for example.
Hi Baptiste, You are right, and I am wrong! I misinterpreted “the” as being “this” specific tunnel, and not tunnels in general.
This invalidates my comments on this quote. Sorry.
Finally, the experiences told on kites tested in wind tunnels are enriched throughout the topic thread.
Anyhow extremely misleading for the average reader.
Love the liberty kites!
From the article:
For that reason, the Airborne Wind Energy research group (AE) took measurements in the Open Jet Facility wind tunnel last month. To do so, a scale model (1:6.5) had to be made of a 25 m2 kite. The Curve Works company manufactured the 1.3 metre wide model from carbon composite. Once set up in the wind tunnel, sensors recorded the forces on the kite from the strong airflow. Poland worked with graduate student Mark van Spronsen to take the measurements under the supervision of Dr Roland Schmehl. Many KitePower employees were also present.
“The measurements went very well,” Poland explains afterwards over the phone. “The forces on the model matched the calculations of the simulation programme well.”
The “1.3 metre wide model from carbon composite” seems to be an elegant way to figure the structure of leading edge inflatable (LEI) @Kitepower 's kites such as it appears in the illustration on the description below:
One note: such a tunnel, and such a team, that of Jelle Poland, constitute an asset for current or future research. Let’s imagine that we could test the scalability, including for the lift and drag coefficients and lift-to-drag ratios, the behavior in flight, in turns, by different speeds, during the depower phases if there are any…, of the AWE designs listed ( I have a few in mind). All this in an orchestrated test campaign…
To match the Reynolds numbers of very large kites in air, one might consider using a water-tunnel, which would allow modeling an incompressible flow at higher Reynolds numbers, using smaller-sized models and tunnels. 
A 1000 m2 kite matched for Re number under water would be 5 m2 according to chatgpt. That sounds very right though there are no wind/water tunnels, right?
The lift may become enormous though
For a static lifter Kites, all we need is to tow the kite with a truck at various velocities. The important factors are kiteline angle and tether tension as a function of wind velocity. (truck speed)
Do you mean a combined wind and water tunnel? I would think the entire wing would be submerged? There are water tunnels with flowing water.
If this video, which seems to show the scale of the water tunnel: MARIN Breakin Wave impact loading on offshore structures and this video MARIN - FOIL JIP, which seems to show flowing water, were shot in the same water tunnel, then this water tunnel seems like it could be used for this.
Or the first one is maybe a wave tank, and the second one already gives you some indication of scale, but not of depth.
See also the rest of the videos from the channel. And a previous mention: Slow Chat - #631 by Windy_Skies
Cavitation tunnels are used to experiment marine turbines, like Minesto does:
Page 448:
5 EXPERIMENTAL TEST […] We carried out two rounds of model scale test in the SSPA cavitation tunnel with the turbines developed by SSPA.
Page 447:
Since the SSPA’s turbine performed better than base-line turbine for kite and had less drag,
there was room to scale up the new design. Therefore, we investigated the up-scaled turbine
with the 14% larger diameter and compared its result with the model size tested in cavitation
tunnel and the quarter scale model for Strangford Lough.
In the abstract:
The operational condition for this turbine is very different from stationary hydrokinetic turbines due to correlation between the kite’s maneuvering speed, turbine drag and power production.
How do you find all these!
This paper is interesting because it hilights a turbine onboard a kite looks quite different to a turbine mounted on a tower.
Back when I was caught up in the hype of “officially legitimizing” ideas I already knew were valid, like many AWE would-be manufacturers, I would spend too much time running around looking for CFD studies, wind tunnel demos, etc. At a major university in Southern California, I was introduced to the “water tunnel” which was just a wind tunnel at a smaller scale, at lower speeds, that nonetheless performed a similar function as a wind tunnel, with less effort, and at lower cost.
I would have liked to find what I was looking for: water-tunnels for marine kites.
And I came across Minesto, and the cavitation tunnels that one uses to test marine turbines, including Minesto which doesn’t seem to use these tunnels to test their marine kites themselves.
Are the results sufficiently reliable, knowing that air is compressible unlike water?