Patreon Update - Gravitational Potential Energy Transfer
Added 2019-05-02 16:04:37 +0000 UTC
Comments
Would you like to work for a company that produces satellite Engines It Is going to be the leading technology Worldwide
James mitchell
2019-05-22 09:57:19 +0000 UTCHello Tom, I like your Methods And designs. When are you In Birmingham again.
James mitchell
2019-05-22 09:51:35 +0000 UTCAlso, if you were to use the initial, theoretical KE on release, not the measured by camera KE, that would put you close to or over the point were it becomes more beneficial to have a smooth-surface sphere. In general, 200,000 to 300,000 are good initial estimates of release Reynolds number (Re) based on: mass = 0.11 kg; Dia = 1.8*Dia_golf = 0.0768 m; nu_{dry-air @ 18 deg C} = 14.9 E-6; V_{0,measured} = 41 m/s or V_{0,PE-theory} = 57 m/s. Drag force goes like V^2. In a ballistic trajectory the (negative) acceleration is proportional to gravitational force plus drag force. Therefore the deceleration is proportional to gravitational force plus coefficient of drag times V^2. The coefficient of drag here is a function of Reynolds number, but if we assume constant air density and temperature we can assume constant kinematic viscosity. So, given balls with the same diameter the Re is a linear function of velocity. You can setup a system of ODEs and then solve the optimization problem with both the C_d for a rough and smooth sphere. This is non-trivial math, but can be done with adaptive RK ODE integrators relatively easily. Since the ball is being shot upwards, the velocity (and hence Reynolds number) will steadily decrease on the way up throughout the flight, so there will always be a time of the flight where the roughness is beneficial. But once the ball becomes sufficiently large and/or the release becomes fast enough, there will be a regime where both spheres have turbulent BLs for a long enough period of time that the lower drag coefficient of the smooth sphere in this regime means that the rough sphere wastes more energy while it's traveling fast than it saves when traveling slower. (The region of the C_d plot where rough sphere has a lower C_d than the smooth one does.)
Zaak Beekman
2019-05-17 22:29:12 +0000 UTCAs an aerodynamicist, my biggest pet-peev is "put golf ball dimples on it." I saw your vehicle, and I was about to RAGE, but then I did the math. So, two things to note about the dimples: 1. Whether they may help depends on the geometry. What's really happening is you're "tripping" the boundary layer on the surface of the thing to become turbulent. Any appropriately sized roughness elements will do this, and the tennis ball fuzz may also act to lower drag in this same way. It's important to note that adding surface roughness *INCREASES* the viscous drag. (AKA skin friction, wetted surface drag... people use different names for it in different fields.) However, in some *limited* circumstances this can delay boundary layer separation, thereby decreasing your pressure drag, even though your skin friction drag went up! So this dimple effect is specific to spheres. An airfoil shape or a more streamlined shape would behave differently; in some cases you may be able to prevent flow separation with roughness/tripping. 2. It is Reynolds number dependent. By making the sphere larger than a golf ball, you are driving the Reynold's number up since the diameter of the sphere appears in the numerator. Fortunately, it seems you are still at a modest enough Reynolds number (211,000 at release, by my back of the envelope math) that you are still in the region where the coefficient of drag on a rough sphere is lower than a smooth one. See https://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html I will note, however, that the scaling of the roughness elements is a much more complicated topic, and is not a simple function of the sphere Reynolds number, but more of the viscous length scales in the boundary layer of the ball. Because of this you probably don't want to just scale the dimples with the diameter. You may need dimples a bit larger than those of a golf ball, but much less than 1.8 times larger.
Zaak Beekman
2019-05-17 21:52:57 +0000 UTCooh that juicy slo-mo in that intro tho :O
Gavin Remme
2019-05-04 02:18:26 +0000 UTCLooks like a fun project! I'm looking forward to the results! I also saw your cameo over on Andre's channel, hoping you're planning on doing another paramotor video in the future!
Luke McNinch
2019-05-03 17:08:39 +0000 UTCIf you're interested in an altimeter for these kinds of projects, I would like to plug my servo deploy "LaunchPad AlTImeter" project. <a href="http://www.uswaterrockets.com/documents/LaunchPad_AlTImeter/manual.htm" rel="nofollow noopener" target="_blank">http://www.uswaterrockets.com/documents/LaunchPad_AlTImeter/manual.htm</a> I have designed a small PCB that compacts the DIY design down into a size approximately 1"x1". I have extras and could send one to you if you think you could make use of it.
U.S. Water Rockets
2019-05-02 22:17:50 +0000 UTC
