#flow viz

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The most interesting thing about modern F1; cars covered in Flow Viz paint.

Credits racecar-engineering.com/WRi2/F1

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Week 6, Day 1:

Today we presented our findings to the class on our attempts to visualize and positively impact flow around the miniature supermileage car. After further analysis of the footage we obtained, we ended up concluding that we were able to delay separation by 6-8% of the length of the car using the larger trip strip configuration, which is more successful than I had expected. The smaller strip had little effect on most of the boundary layer due to the weakness of the disturbance it provided.

Overall, this was an enlightening experience and it was fun to have the opportunity to design an experiment for the water tunnel.

Of the presentations from the other groups, the one that interested me the most related to flow around a matchbox car in the water tunnel, both with and without the windows and rear window removed. They were able to show that, after dye had been left in the bed, it would circulate through the rear window and then out the other windows before entering the freestream rather than becoming entrained in the flow separated off of the roof of the car.

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Week 5, Day 2:

Today we collected data on our attempt to push back the separation location near the trailing edge of the supermileage car. While we think we have plenty of footage to analyse, insufficient analysis has taken place for us to draw conclusions as to whether or not we actually had a positive (or negative) influence on the flow features in the area of greatest interest to us from a drag reduction perspective.

After a preliminary review of the footage obtained after the installation of the 1mm trip strip, we were advised to attempt to have a greater impact on the flow by introducing a larger obstacle. The goal was to put up a wall which would energize the boundary layer to a greater depth in order to produce the turbulence necessary to keep the flow attached more consistently further downstream.

Our efforts to produce an appropriately sized obstruction resulted in a half-attached strip of tape added to the downstream side of our existing strip, extending about 4mm into the flow, with the detached portion angled about 45 degrees off of the surface. Here’s a clip from the run completed in this configuration:

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Week 5, Day 1:

Today was primarily spent poring over the footage we collected last week in the water tunnel in an effort to develop the best understanding of what’s going on before we decide what to change or investigate further next class.

Once we had the video up on a big screen, the point at which laminar recirculation began became much more clear. Watching carefully, there were several occasions in the videos in which inconsistencies in the concentration of the dye could be tracked as they made their way back upstream after having been entrained in the flow in the laminar recirculation area.

Using these observations in particular, we concluded that laminar separation occured at about 5″ forward of the trailing edge of the body. The next step was to attempt to improve upon this by tripping the flow just upstream of this point with a trip strip.

(photo of trip strip)

We layered up some thin strips of aluminum tape, oriented spanwise, to about 1mm thickness at a location 5.5″ forward of the trailing edge in an effort to accomplish this. Hopefully our test session later in the week will yield positive results!

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Week 4, day 2:

We tested the miniature supermileage car today!

As always seems to be the case, we ran into some unexpected complications as we set things up. For example, we hadn’t expected the model to float in the water channel, and some quick modifications our mounting rig had to be made to accommodate this.

Also, since we’re most interested in separation off of the central 2D cross section of the model, it was crucial that the dye be introduced to the boundary layer of the model in the right place. This proved to be problematic because, in order to position the region of interest near the center of the transparent-walled test section, the front of the car had to be well in front of the beginning of the transparency. Essentially, we were flying blind when maneuvering the dye injector.

Fortunately, we got some useful footage out of the session. Here’s a still:

The grid behind grid behind the test section has 1″ resolution. In this frame, it looks like the laminar separation point is about 5″ forward of the trailing end of the model. Once the laminar boundary layer has separated, it appears to transition to turbulent flow around 1-2″ after the initial separation point, and then indications of the turbulent BL “attempting” to reattach are visible about 1″ further downstream. The turbulent reattachment seems to be weakened by the fact that, by this point, the curvature of the model seems to have created a relatively strong adverse pressure gradient.

Using this photo, we think we’ve been able to identify the laminar separation point for the cross section we’re interested in. With this information in hand, we’re going to come back next week and attempt to trip the flow upstream of this location to see if we can delay separation.

We experimented with a few different lighting set ups as the session went on. Unfortunately, the most illustrative dye positions seemed to occur while we were using the worst lighting set up. The gif at the begging of the post is from earlier in the session when the lighting was better.

Although it isn’t clear from any of this still, since it is not time-averaged, we did observe some turbulent reattachment downstream of the laminar separation. Since this process is very unsteady, time averaging is required to capture it in any quantifiable detail. 

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Week 4, day 1:

Initially, we had planned on testing the supermileage car model in the tunnel today, but instead we decided to iron out the kinks in our experimental set up and test later in the week.

The plan we settled on for securing the model in the tunnel involves suspending it from some long bolts from crossmembers across the top of the water channel. Fortunately, all of the hardware was available in the lab to make this happen.

Since the mounting points on the model are located on the bottom of the wheel fairings, the model will be upside down when we test.

We also spent some time today prepping the model by retaping over some the pressure taps in the vicinity of the region we’ll be most interested in.

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Week 3, day 2:

Today was spent processing our wake data generating a test plan for our next experiment, which we’ve now decided will involve flow visualization on a model of Cal Poly’s current competition supermileage car.

We’ll be running a dye test in the water tunnel. Since the test section is fairly small (only about 6” by 8” in cross section), the blockage will be fairly high, but hopefully this won’t interfere substantially with the features we’re looking to observe.

I think it’ll be most interesting to use the dye to determine the location of flow separation off the back spine of the car (without any side slip). Since the water tunnel operates at relatively low speeds (capping out at about 5 inches per second), the Reynolds numbers will be quite low compared to the ones we’re used to seeing in the wind tunnel. As a result, there’s a good chance that we’ll get to observe a laminar separation bubble followed by turbulent reattachment further downstream.

If this is the case, our stretch goal is to add some trip tape just upstream of the observed laminar separation point to see if we can delay separation. Exciting stuff!

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Final Thoughts: Flow Visualization

Aside from some hiccups,  this lab was a though exercise  in how I can get frustrated and upset when things aren't going perfectly. It one of the  things that I believe makes me thoughtful in the design process or planning process but extremely liable to anger when things aren’t going well.  This happened a lot when I played sports in high school and even in college. My hot head would get in the way of performing well and actually making good decisions on the court.  I think that this is probably one of my failings and although it give me room to get better, it is also the reason why we work together in a lab. Our combined strengths make us better as scientists and experimenters. (don’t know if that's a real word). One of the things I am glad about is the fact that we didn’t try to make this test overly complicated or difficult, the difficulties came out of things that we didn’t have control over and that is something that we dealt with and I am proud of that.  

I learned a lot from this lab and it showed me that sometimes when things don't go your way, all is not lost, you can still produce meaningful results. Our experiment showed this, that the even when we didn’t get all that we hope for, all was not lost and we still got what we came for. I am looking forward to the lab hack challenge next week as the quarter comes to a close, it seems that this lab hack will give us a greater degree of freedom, something that I've been looking for in this lab.