Flettner Rotors - Calculating Rotating Skin Friction

Discussion in 'Hydrodynamics and Aerodynamics' started by rwatson, Jun 16, 2018.

  1. rwatson
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    rwatson Senior Member

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  2. Dolfiman
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    Dolfiman Senior Member

    Thanks for this new paper, I read it rapidly, some comments :
    - It is about numerical experiences with various turbulence models and paying careful attention to grid meshing and numerical stability,
    - the great thing is that they summarized their results in formulations for Cl and Cd ( formulations (8) in the paper) with as input just 3 nondimensional data : Spin ratio (SR), the aspect ratio (AR) and diameter ratio of the upper plate (de/d), and giving all the needed coefficients in Table 6. From this, one can build a helpful spreasheet application for early stage project.
    - the frustrating thing is that nothing new about experimental results : they compare their results with experimental ones said given by Pearson (2014), but actually this paper refers itself to experimental works done by Jakob Ackeret 1932 ! (Fig. 3 of Pearson paper). Cascade of references in the recent papers usually fall to same old experimental (public) data.
    - And also nothing about the energy input needed to rotate the cylinders, not even mentioned.
    >>> Again, playing with sophisticated CFD tools is great but there is also a crucial need of new experiments aiming a full coverage of the issue (inc. the resistant torque) and with the aim to publish the full results, an opportunity for a University.
     
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  3. rwatson
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    rwatson Senior Member

    And yet they said they referenced data from recent installations like the Enercon Ship ?
     
  4. Dolfiman
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    Dolfiman Senior Member

    To my knowledge, inthe Enercon document they referenced or others similar, there is no info on CL, CD in relation with spin ratio, aspect ratio, de/d, .... than can be compared with the numerical tests of Di Marco and al., just global info on the installation and energy saved. I suppose all these data are part of their their know how and they don't want to make that public. It is why I advocate a University investigation on that issue.
     
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  5. Dolfiman
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    Dolfiman Senior Member

    I looked again to this paper and developed an application (here attached) with using their formulations for Cl and Cd ( formulation (8) and coefficients aijk and bijk in Table 6 in the paper), with :
    - as input 2 nondimensional data : the aspect ratio (AR) and diameter ratio of the upper plate (De/D)
    - computation with the spin ratio SR = 1 to 3
    - as output : CL, CD
    Some comments :
    - I first checked the formulation (8) with their numerical results : when De/D is 2 and over, there is a lot of discrepancies, so I suggest a use limited to :
    --- when De/D = 1 (no end plate ) >> AR = 2 to 8
    --- when De/D = 1 to 1,5 >>> AR = 2 to 6
    --- numerically hasardous to use it for De/D > 1,5
    - I then checked the CL, CD results with some available test results : Ackeret with AR = 5 and De/D = 1,5 (as reported by Pearson 2014) , Naca Reid tests (approached with AR = 8 and De/D =1) >>> the good news is that the CL and CD curves have similar shape and that introduction of same correction factors (1/1,45 for CL; 1/2 for CD) can fit with the results for both cases. So I adopt these corrections in the application >>> comparison attached with using the corrections.

    To make complete the application, I also compute CP cyl. and CP plate, the adimensional coefficients for the power to rotate (aero part) the cylinder and the end plate if any, as investigated previoulsy in this thread (pdf attached as a remind), by introducing these definitions :
    CP cyl = Power /(Cf ½ Rho Swcyl U^3)
    CP plate= Power/(Cf ½ Rho Swplate U^3)
    And idem, I introduce a correction factor (x 1,20) for CP cyl to fit with the (only one) test results available (comparison attached).
     

    Attached Files:

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  6. Kai Rabenstein
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    Kai Rabenstein Junior Member

    Fascinating discussion - I'm planning to commission a motor variant of a 10m/34ft sailing catamaran (5 tons lightweight) shortly and am looking at propulsion options that utilise wind without the need to retain its classic sailing rig and 15m tall mast. For running downwind I am minded to use a kite sail (e.g. the French LibertySail product available in 10 and 20 m2) while for sailing on a beam reach I am looking at a Flettner rotor (no commercially available products as yet). As the motor propulsion will be at least partially electric I am keen to use hydrogeneration to keep the lithium batteries topped up whenever the ship is being sailed under wind power (or anchored in a strong current). Naturally an extensive solar panel estate will be needed as well, alongside some wind turbines.

    In the context of your Flettner rotor discussions I am thinking that a modern version of the rotor could of course incorporate several useful features not seen in previous iterations, such as solar panels wrapped around the rotor surface to increase the solar estate (especially for lower sun angles as the remainder of the estate typically faces skyward from the cabin and cockpit roofs) and also a joint that allows the rotor to be lowered into a horizontal position when not in active use - this should radically reduce the bridge height, and windage in the marina. The former would naturally increase both the weight of the rotor (hence power consumption of the drive) and require a smart solution for transmission of solar panel generated electricity out of the rotor onward to the ship's batteries. The latter increases mechanical complexity but this should not be insurmountable as long as the overall size and weight of the rotor is moderate. A potential third adaptation might be to provide attachment points for blades (and have a suitably engineered drive motor) so as to turn the rotor into a wind turbine generator when the ship is stationary ...

    I have often wondered why no-one has yet thought to develop a compact and lightweight Flettner rotor system for dinghies which could help to generate much more data in order to refine our assumptions about the impact of aspect ratios, end-plate dimensions, and even Thom disks. Lutz Fiesser of course has gone some way towards this with his Uni-Kat proa design in Flensburg but like many other laudable efforts it seems not to have led to any follow-on work. His proa had a 71cm diameter rotor that seems to have been about 4.5m tall and on the available videos appears to run at about 60rpm. I am currently trying to contact Professor Fiesser but have not succeeded in this effort yet.

    Any comments on the idea of creating an experimental Flettner rig on my future boat, and also on the idea of a mini-Flettner rig for dinghies, would be highly welcome. I am a physician not an engineer, so limited in my ability to follow higher maths and engineering calculations, but very happy to hear experts discuss both theory and practice of this fascinating alternative wind power rig!
     
  7. Dolfiman
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    Dolfiman Senior Member

    Hum…, it is a real complication, even with flexible solar panels. I suggest you to envisage that later on, after a first successful demonstrator.

    This is much more interesting to explore in my opinion, we debated on that idea in the thread herebelow, from quote#462 and the sketch proposed by Rawtson. To turn the rotor into a wind generator when the ship is stationary and also, by
    nearly rear wind when the rotor efficiency falls, to test it i​
    nto a "spi" configuration with blades just deployed and still.
    And if you are intesrested by experiences with careful measurements, to measure the power to rotate in various conditions, we need experimental data on this issue !
    Everything Old is new again - Flettner Rotor Ship is launched https://www.boatdesign.net/threads/everything-old-is-new-again-flettner-rotor-ship-is-launched.24081/page-31
     
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  8. rwatson
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    rwatson Senior Member

  9. Kai Rabenstein
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    Kai Rabenstein Junior Member

    Happy to stand corrected @rwatson - was aware of the Uni-Kat Flensburg (in fact I'm in email contact with its creator Prof. Fiesser) but not of Stephen Thorpe's Sussex rotorboat - sadly I could not find any further info after his 2006 club publication. I might try to see if he can be tracked down in Seaford or thereabouts as I live not far away. I agree the solar panel installation would be challenging but should be possible although it might take a considerable amount of time, expertise and ingenuity. The folding flaps or vanes, on the other hand, could be made cheaply from plastic or plexiglass for a small size rotor. As has been pointed out, data is sadly lacking on the behaviour of Flettner rotor columns rotating passively when facing unfavourable wind directions (i.e. into the wind or running before the wind), and the energy a rotor with flaps/vanes extended could generate under such conditions. I would hope to be able to incorporate a properly resourced engineering data collection work stream into my project once it is underway. Input from anyone with connections in the marine engineering/academic communities would be highly appreciated as I envisage putting together a bid for institutional funding.
     

  10. rwatson
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    rwatson Senior Member

    Historic Update Question - you ever make any progress ?
     
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