Using you CAD program of choice, create the model you wish to investigate. This example uses an ellipsoid for use in the 1/6th scale 7x5 tunnel (wooden tunnel). The following considerations need to be made:

1. Make sure you have checked the test section size so your model doesn't exceed any critical dimensions. In the case of this 2D ellipsoid, the height between the two mounting points of the test section cannot be more than 255 mm (top image). You may also wish to determine the maximum solid blockage and to see if you exceed 5%. Blockage will be dependent on your chord and span lengths.

2. Ensure that your mounting locations are feasible and don't prevent access to key parts. Keep in mind the pneumatic tubes will need to exit the model somewhere and a hole large enough to allow this is required (middle image).

3. Pick a tap layout which gives you sufficient coverage of the surface as well as regions of interest, while being practical (e.g. is there enough space for tubes?). Taps should be designed so they are perpendicular to the surface of the test article and are often staggered when considering 2D flows. This is to reduce the impact of upstream taps interfering with the flow at the downstream tap (bottom image). Note that the main reason taps are included in the 3D printed part are for positioning purposes. It is impossible to accurately place taps by hand, especially if staggered. Also remember the total number of taps will be limited to the equipment you are using to take measurements. Remember to have sufficient wall thickness for the brass to be inserted into and be held firmly. The wall thickness here is 5 mm.

4. It may be most practical for access reasons to split the test article somehow, e.g. along the centreline, along the span etc. But careful consideration needs to be given to the impact of having a seam in the model, as this may change the flow behaviour. This model has been split in thirds along the span as it allows easy access to taps and the seams are in locations which will not affect the flow. 3mm dowel connectors are used to align the pieces.

Typically when a model piece comes out of the printer, there will be some support material and the finish will be poor (top image). Smaller layer heights will prevent large ridges from forming, but layer height is a trade-off with printing time. Choose the printer settings which suit your needs best. The following steps should be followed to prepare for pressure tapping.

1. Remove large support material by hand or with plyers. Note that printed plastic is sharp and can cause cuts, so please wear gloved. Using coarse sand paper (80 grit) sand the surface so that the ridges are less visible. Follow this using less coarse sandpapers (up to about 400 grit) until the surface is relatively smooth (second image).

2. Prepare the resin and filler of choice. I have used Zap finishing resin mixed with a microballoon filler. This makes the resin very easy to sand and will fill the ridges. Mix your resin as per the instructions and add in the microballoons before mixing thoroughly.

3. Using a disposable paintbrush, paint the surface of the test article, applying the resin evenly with smooth strokes (third image).

4. Once the complete test article has been covered, ensure you partially or completely clear the printed tap holes using anything with a smaller diameter than your tap tubes. Partial clearing is sufficient as the holes will be drilled out and only require enough to guide the drill bit to the correct location (fourth image).

5. Allow plenty of time to dry (resin and temperature dependent). Sand smooth (start with a 180 grit and then move to 800+) and paint if desired. If one coat is not enough for smoothness, repeat more coats until you are satisfied. Only paint once the desired surface finish has been achieved.

The reason the surface finishing is completed prior to tapping is because paint and resin will clog the taps. You can tap the article first and apply the resin and paint afterwards, but this will give an inconsistent surface finish.

This example uses brass tubing with a 1 mm outer diameter and a 0.55 mm inner diameter. This is then connected to 0.8 mm internal diameter tubing which we connect directly to a Scanivalve MPS4264 miniature pressure scanner. Please select tubing which is appropriate for your own needs.

1. Cut the brass tubes to the appropriate length, taking into consideration the wall thickness and the space available around the tap. Remember, it is possible to bend the brass tubes slightly to avoid collisions with other tubing, but care must be taken not to pinch the tube closed. Depending on the tools used for this job and the size of the tube, the brass might become pinched closed. In this case, it should be re-opened by applying pressure in the appropriate direction with some pliers or similar. The hole can then be further cleaned out by drilling the opening.

2. Apply glue to the tubes (below the tap orifice) and slowly insert them one by one into the designated hole. Do this from the internal cavity pushing the tubes up to the surface rather than from the front down. This is to prevent a "glue -ring" around the tap on the smooth surface of the test article. Alternatively you can push the brass tubes through from the external surface without glue, then dab glue around the base of the brass tube. In this example I have used a 5 minute epoxy.

3. Prepare the flexible tubing by cutting lengths for each tap. If you are not concerned with the response time of the tubes or have already determined correction factors, calculate the longest tube you would need and multiply it by 1.3. If you are concerned with response time, keep the length as short as possible.

4. Once all the brass tubes have been inserted and the epoxy has dried, begin attaching the flexible tubing. As you attach the tubes, be sure to label them with a sensible designation so you can easily identify the tubes later. To fit all the flexible tubing, some of the brass tubes may need to be slightly bent to make room. Hard to reach places may require the use of long nose pliers, be be careful to not to compromise the flexilbe tubing.

It is critical that all of the pressure ports be checked prior to finalising the model and assembling it. This is because it is often difficult to gain access to the individual taps once the model is put together. This can of course be overcome by integrating mechanical fixtures to allow easy assembly/disassembly of the model. But this increases model complexity and potentially has a negative impact on flow quality. This example uses a simple design with limited ability to reach taps once the model is completed. Therefore, it is extremely important that the model is puff tested.

1. Puff testing involves connecting the pressure lines to a digital pressure scanner or manometer and providing a small "puff" of air to the individual pressure ports. If a signal is recorded (pressure increases), then the port is not blocked. If any ports are found to be blocked, the flexible tube or brass should be checked and the part showing a blockage replaced.

2. Once all ports are found to be clear, the remaining parts need to be prepared. In this case, the two end pieces need to be sanded and resined as was done for the tapped section. Here rivnuts have been used as the attachment points and are installed with glue into two pre-printed holes at each end.

3. The parts are then attached and aligned using 3 mm diameter pins which fit into printed holes. Glue should be using on the pins and the flat surfaces for best bonding. If the region where the two parts butt together has a significant ridge or gap, this should be filled using the same resin/microballoon mixture as for the surface finish. Gap filling is best performed with a very thick mixture of the resin. This is achieved by letting it almost set and prevents dripping.

4. Make sure that the pressure taps are covered with tape or plastic when completing the final sand over the filled junction gaps. Take care not to use tape which leaves a sticky residue which can clog the taps.

5. The final paint job is completed (if desired) once the finish over the whole test article is satisfactory. As the central tapped section is already painted, tape off this section appropriately allowing some overlap of paint at the section junctions. Keep the tape covering the taps in place just in case. Complete the final paint of the top and bottom sections

6. Install in the wind tunnel and perform testing!

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