Dave Willis on some Scientific Challenges... ============================= This is, verbatim, an email reply to a student's question about her final project. Question pieces are scattered throughout the reply and preceeded by >'s Enjoy! ========================================= ========================================= ========================================= > My goal is to design a bat simulator to allow an evolutionary biologist > to vary the shape and size of a bat wing and observe the changes in > lift, manueverability, and energy cost in a virtual wind tunnel. The > final design will give the user the ability to toggle on/off anatomical > layers and flow/vortices, flip through a simulated animation, and modify > the wing shape using a "stretch" feature. Gauges will show percentages > of lift, maneuverability, and energy cost. The user will be able to > create his/her own wing shapes by stretching the bones, muscles, and > membrane. Just to be clear, is this a design thing, or, is this something you plan to implement as a prototype/demo or is it a planned "fully operational system"? This will help me with understanding the depth you want for answers. If fully operational, the project is significant... ==================== the next couple of paragraphs are a scientific aside which goes into some challenges ================================= this may seem scary, and if it is, ignore it I think is the best advice...as these are scientific problems the bat research group must deal with eventually and your vis. environment may provide us a nice way to do this.... ======================= Not to scare you, but to make you aware, for "real" implementation of a system like this there are some major challenges which remain ....some of what you are trying is unexplored territory from a high fidelity/accurate scientific viewpoint(I'll highlight them here quickly, just so you are aware, please don't get concerned on this front...these are scientific quesitons/concerns which need addressing either in the future to carry this project to fruition). Again, these are only questions which in the future need to be addressed...for your info: a) When changing the wing shape and size, are the bone motions modified? Prescribed? Interpolated from other bats? or are new "optimal" bone motions solved for? Wing motions can drastically affect all your desired outputs and can wash out the effect of the changes in wing shape if the motions aren't appropriately set. I don't know the best way to do this...and am not sure if anyone else does at this point. b) When the wings change size, the aerodynamic forces also change (there is a heavy force dependenc on wing area, and a slightly smaller dependenthce on aspect ratio in fixed wing flight) . If this is the case and lift == weight, then we need to adjust the wing beat motions and wing shape in order to make sure there is a balance of forces -- if the wing size and lift increases, then the bat will climb unless we trim out the bat wing motions -- hope this makes sense (we must make sure make sure the vehicle is trimmed if we are comparing things OR...we could look at dimensionless coefficients....I can talk more about this...but this would erase the effects of area changes but would not erase the effects of aspect ratio changes (shape changes)--as an analogy, consider and aircraft...if we change the wingarea (or in some cases the shape), then there may be some adjustment in the trim (angle the aircraft makes with the relative air velocity) of the aircraft inorder for it to fly level without limbing/sinking). Also as the wing sizes change, do we require a different bat weight to be "lifted" into the air? On a similar note, the thrust has to equal the drag in non-accelerating forward flight (This allows you to compare bats from an apples to apples viewpoint). One must adjust the flight parameters to make the bat fly so that the resulting flow structure from the "newly" shaped bat makes sense and is an apples to apples comparison. This could be averted by somehow interpolating the results from data collexted from different bat species and that way you can get something close to a decent representation. Although this would be a possible solution, it isn't likely to be the most thorough one. c) One of the biggest shallenges is the different wing sizes need to have a specified motion. The shape of the batwing is crucial to the lift generation. A change in the curvature of the wing of 5% can drastically alter the lift generation and give you a completely different result. The question is, when we vary the wing size, what are we using to determine the motions. Finally on this note, membrane deflections are heavily dependent on the distances between the supporting skeleton....this means a change in size of the skeleton, may mean the membrane deflections are vastly different that the baseline bat...again altering the force properties. That said, and if I didn't scare you too much....As you can see there are MANY unanswered problems....It is a challenge to simulate a bat given the actual wing shapes, let alone figure how a scaled bat will change the flight properties (since we need to ensure the scaled bat is something representative of a bat)....The thing I would tend to do from a scientific viewpoint is try to boil the problem down a little. I would scientifically look at how shape shanges affect stationary wings (a much better understood problem) and then try to build on that to a simple flapping wing, then to a more refined multi-jointed wing.... One of the things I am working on is a model which does not require the bat shape details. Again, if interested I can talk more about this. ================= ANSWERS TO YOUR QUESTIONS ================= > If you were to use/design a simulator like this, what features would you > want to control? I'm not clear I am sure what you mean by features....I'll make some assumptiosn and list some thoughts (if you have something else in mind, lemme know, I am sure there are plenty of things i'd like to see): --- *Some* Things I'd like to mess around with to do with the bat (the list would grow given more time): o The effect of forwrd bat velocity on the bat (each bat may have a different forwrd speed and perhaps a best forward speed for each of the lift, energy and manouvrability metrics) o The effects of flexibility: A more flexible large span wing will have significant bending... o the effect of flapping frequency o The effect of the wing shape o the effects of camber variations (either through passive membrane deflections of active skeletal wing shaping). --- Visually: Features I'd (personally) like to see (again the list may grow with more time): o What is the vortex structure behind the flapping wing o What is the flow structure(s) areound the wing. Is there a leading edge vortex? o How does the fluid structure interaction affect the problem? o General streamline and flow traces (body fixed and flow fixed reference frames). o Velocities, pressures, etc in the flow field. ======================= Q2 ======================= > What tools would be useful to you in exploring the > three variables? Current list: By three variables, I assume you mean velocity, energy consumption and manourvability. Another thing is flight efficiency or propulsive efficiency....the effectivenss of the energy conversion for work in-->work out. Obviously one of the nice things to do is have an optimization capability in order to minimize certain things. For example it would be nice to minimize the overal power input required for level flight. Or minimize the drag of the bat. A grapical output of the timevarying and avergae values of the variables/properties (as the wing beats the thrust, lift, etc change). ========================= Q3 ================== > How might one go about calculating the variables? This is tough to do and one of the big challenges I face daily :-) ..... There is some hope :-).... Lift, energy and drag: Computational fluid dynamics (CFD) for "accurate" computations...again a poorly defined bat shape in....means a poor result out (aerodynamics is heavily dependent on the shape, hence, this is one of the big difficulties). Panel method/vortex lattice models for modelling and approximate answers. These are all relatively costly to run (timewise: CFD ==> a month or two, Modeling methods: 10 minutes to an hour per bat flight per couple of flapping cycles) and hard to develop. The other approach is to calculate very approximate values from near-realtime simulations...something like the ASWING or AVL (Lookup Drela at MIT for these codes....though they aren't really setup for flapping wings in their released form....not sure if it is freeware/something else)...If you are interested further in this, talking in person would be best (I'll be down at Brown on Tuesday next week). A very approximate way ti to use quasi steady 2-D theory and extrapolate it to 3-d....this will be ok for some simple computations, but not to be heavily trusted as a fully complete "theory". If you wanted to actually compute the values, let me know and we can look into ways to do this. there may be a way to use a "batless" model to do some approximations.... Manouvrability Capability: I'm not sure how we calculate this...I think the first step is to understand the feature(s) that make bats so manouvrable....and then figure how this can be computed based on that. The motions are heavily dependent.... ======================= Q4 =========================== > Are there other vital elements that would affect these variables? Again list may grow : Shape of the wing (profile shape, something like camber) Flexibility of the membrane and bones Frequency and amplitude of flapping Mass of wings and bones (gives and indication of the moment of inertia or the resistance to acceleration). ====================== Q5 =============================== > Would you > be interested in varying the density of the bones? fitness/strength of > the muscles? Denstity of bones: Yes Strength fo the muscels: Yes ====================== That's all folks ===============