3MEMS3: Launcher Funnell Optimisation
As shown in my previous blog I’ve made a gas cylinder to launch ping pong balls for the robots obstacle course. In order to translate the pressure being released from the canister to the ping pong ball and launch it efficiently, I had to design a sort of funnel.
Due to the nature of the course, the launcher will have to launch multiple ping pong balls predictably and consistently. The funnel should allow as similar propulsion as possible with the gradual pressure drop that will occur in the canister.
To try and achieve this, my objective with the design was to achieve a balanced spread of pressure across the ping pong ball area with minimal pressure drop and turbulence.
To try and develop a further understanding of the design of the part beyond my intuition, I decided to make use of computational fluid dynamics (CFD) tools. I used Ansys Fluent. CFD is a branch of fluid mechanics that uses numerical analysis methods to model and help solve fluid flows.
Given the complexity of the tool and the nature of fluid mechanics, my CFD is likely not within 20% accuracy quantitatively. As is the case with just about all simulation tools, “All models are wrong, but some are useful”.
As far as my knowledge indicates to me the models I built have some uses from a qualitative standpoint and can indicate weak spots in the design, visualisation of the airflow will reveal areas of inefficiency and be used to form conclusions as to how the device can be more effective.
I set up my model using a simple straight funnel-shaped geometry as can be seen below.
As can be expected from quite a blunt geometry there is a lot of separation and turbulence along the internal walls large vortices immediately form as the part funnels and almost all gauge pressure is lost by the outlet. This design is not very efficient and would rely on high inlet pressure to propel the ping pong adequately.
Also worth noting that the vortex shown above is one continuous doughnut-shaped vortex as the streamlines are shown in a section view.
Adding a constricted section at the entry to induce the Venturi effect. The Venturi effect occurs when a fluid (air in this case) flows through a constricted section of pipe (orifice), the pressure of this fluid reduced leading to an increase in velocity.
I reran the simulation with the updated geometry and obtained these results.
As can be seen here there are still big vortices however the initial laminar push from the entry is sustained deeper into the part. There is a significant rise in peak pressure despite the same boundary conditions and mesh methods being applied for both models. A similar large vortex is still forming from the separation however there is marginally less than before.
The pressure plot above also validates the Venturi effect occurring with the blue region indicating a pressure drop plus a higher pressure zone, marked in orange, where the ping pong ball will be — aiding propulsion.
On the surface, it may seem counter-intuitive to reduce pressure n entry to the funnel to the funnel but this is a worthwhile compromise as the velocity is increased, therefore the pressure further into the funnel and launch tube is increased also. There is a concentrated higher pressure region that can be identified above by the orange circle type shape but also the average pressure in the entire region in which the ball sits is higher.
This is a result of the stagnation that occurs after the fluid has been accelerated through the orifice
A more exaggerated choke should be more favourable again — vortices form further down the tube, greater pressure acting on the ball. exact vorticity formation not accurate as the ball is not in place for this simulation. Considering the ball takes up most of this space these vortices will only form in this manner after the ball has fully cleared the launcher.
These predictions appear to have been validated when this sim was run with a more pronounced geometry, as can be seen below.
Just to state again, due to the lack of accuracy of these simulations the exact fluid flows and figures shouldn't be taken as truth but the general trends shown should correlate to reality. The purpose of these sims is to be an optimisation tool for the geometry of the funnel entry.
The final design was then added to the CAD assembly of the full launcher!
The final design shown was 3d printed and features a hole for the ping pong ball hopper to drop the next ball in place. This simply fulfils the no manual reloading requirement.
This is a key element of the entire launcher which is now being assembled and will be fully revealed in the next blog!
