with additional contributions by John H. Huckans, Nathan A. Kurz, and Carla Zembal-Saul

AAPT Apparatus Competition, Entry Description

Introductory Laboratory Category

Apparatus Title: The Bernoulli Box

Abstract (50-75 words):

The Bernoulli Box is a wind tunnel suitable for use in a high-school science classroom. The wind tunnel can be used both as an experimental apparatus and as a visual aid (especially when used with tracers) to teach such concepts as Bernoulli’s principle, lift, drag, aerodynamic geometries and more. The wind tunnel was designed to be built by anyone with access to a high school wood shop.

Description:

The Bernoulli Box is a wind tunnel similar to the one used by the Wright Brothers during their airfoil tests in the early 1900’s. The air travels first through a duct fan, which expels air into a sheet-metal box, and then it flows onward through a number of flexible PVC tubes into a long, clear Plexiglas tunnel. The array of tubes was designed to reduce turbulence, which disturbs the flow out of the fan. Just imagine how many lesson plans you can create with a clear box of gentle and smoothly flowing air! A wing section placed within the box creates air-flow patterns that are fundamentally identical to those near an airplane wing in flight. This similarity, of course, is just the reason that wind tunnels are used in airplane design. Our apparatus lends itself well to teaching concepts such as drag, lift, velocity, streamlines, friction and the Bernoulli principle. It can also be used for more integrated projects, such as creating and analyzing airfoils, using force sensors to measure drag or wind velocity, or designing aerodynamic shapes and surfaces.

While misconceptions about the mechanics of flight abound (in people of all ages), the power of visualization that the box provides enables even young students to grasp the basic principles, in a vivid and dramatic way. The Bernoulli box can be used in conjunction with tracers, such as dry ice mist or the cloud of a fog machine, which reveal the flow pattern of air over a wing section. The wing sections themselves can be adjusted to manipulate their aerodynamic properties. After presenting students with several wings of varying thickness, weight and shape, ask them “Which of these wings will fly, and why?” Better yet- have them create their own “perfect” wing. With this challenge, students can see how small changes in the geometry of a wing affect its behavior while it is moving through the air.

Lessons can be tailored to suit the needs of the teacher and the level of the students’ prior knowledge.

The Bernoulli box can be built with about $200 worth of materials and 20 hours of unskilled labor.

Teachers will realize the great investment they’ve made when they see the extent to which the box

allows students to develop correct and imaginative understanding of such interesting, but difficult-to teach,

topics. Best of all, many of the concepts are covered in most states’ science standards! For example, the newly adopted Pennsylvania Academic Standards for Science and Technology require that fourth grade students be able to “Identify transportation technologies of propelling, structuring, suspending, guiding, controlling and supporting.”

Construction Notes:

The Bernoulli box consists of a segmented plexiglass tunnel which is fed air indirectly from a high static fan. The fan first discharges into a sheet metal box. The box is fitted with flexible tubing which leads to the plexiglass tunnel. The purpose of the box and tubing is to create a straight, smooth air flow within the plexiglass tunnel.

Plywood Base and Plenum Seat

Cut a piece of 1/2 in. thick plywood into a 16 in. x 40 in. rectangle. Cut another piece of the 1/2 in.

plywood into a 8 in. x 16 in. rectangle. The large piece is the overall base for the wind tunnel. The

smaller piece is the seat for the sheet metal box. Glue the seat onto the base 5 in. from one end using

wood glue and clamps. Let it dry overnight.

Sheet Metal Box

Construct a sheet metal box to receive the wind tunnel fan. We suggest that the box be built by a local sheet metal shop. We had our box fabricated by Duck's Sheet Metal Shop of State College, PA (814-

237-3493). They were very fast (two day turnaround), inexpensive (approximately $35 per box) and

reliable.

The box should be constructed using 24 gauge galvanized sheet metal. The dimensions are 15 in. x 15 in. x 8 in. deep. One of the 15 in. x 15 in. ends is completely open with a 1/2 in. metal flange going all

the way around. The flange is formed by starting out with a 15 in. x 15 in. x 8.5 in. deep box, making

1/2 in. cuts at the open corners and then bending the metal back. The other 15 in. x 15 in. end of the box is closed except for a 2 in. deep, 10 in. diameter metal inlet collar to receive the fan. The collar is centered in the middle of the 15 in. x 15 in. box face. The remaining four smaller sides of the metal box are completely closed. Screw the box down onto the seat using No. 8 x 1/2 in. sheet metal drill screws, letting the flange hang over the edge of the seat.

Plywood Face Plate and Drain Grate Strainers

Cut a piece of 1/4 in. thick plywood into a 16 in. x 16 in. square. Using a jigsaw, cut four 6 in. diameter lobed circles out of the center of the plywood as shown on Figure 2. Obtain four 6-3/4 in. diameter polypropylene drain grate replacement strainers (PHP Item No. 30032) from Prairie Home Products of Peculiar, MO (800-367-1568). Note that you may not be able to purchase the drain grate strainers directly from PHP. Often, manufacturers will only sell through licensed distributors or contractors with previously established accounts. We bought through the plumbing department of Lowe's in State College, PA (814-237-2100). Alternatively, you may try Home Depot (800-553-3199).

Drill eight 1/4 in. diameter holes in the lobes of the plywood face plate using the drain grates as guides

in positioning the holes. Secure the four drain grates to the plywood face plate using 1/4-20 x 1 in.

machine screws with washers and nuts. The screws should have counter-sunk style (as opposed to

bugle) heads to mate with the drain grates. Attach the face plate to the sheet metal box flanges with the

drain grates facing out using No. 8 x 1/2 in. sheet metal drill screws.

Plexiglass Tunnel and Flexible Tubing

Note that one of the tunnel sections uses four plexiglass pieces. This section is movable on the plywood base. The other tunnel section uses only three plexiglass pieces - it has no bottom. It is screwed down on the plywood base 6 in. from the drain grates. Both tunnel sections employ four wood dowels. The wood dowels on the fixed tunnel section are used to secure the tunnel to the base. We suggest using No. 6 x 1 in. wood screws and finishing washers.

Using No. 6 x 1 in. wood screws and No. 6 x 9/16 in. fender washers, attach an 8 in. x 8 in. square of

hardware cloth with a 1/2 in. x 1/2 in. mesh to the end of the fixed tunnel facing the drain grates.

Hardware cloth is like heavy duty chicken wire. It is available from most building supply stores.

The most time-consuming part of the construction and also the part requiring the most artistry is the

fitting of the flexible tubing between the drain grates and the fixed tunnel. Obtain approximately 180 ft

of 7/16 in. diameter flexible PVC tubing on a roll from a local hardware or plumbing supply shop.

Make sure the fixed tunnel is screwed down tightly onto the plywood base. Begin cutting 8 in. long

tubes from the roll. Insert one tube end into the lowest row of hardware cloth openings and the other

end into the low openings of the drain grates. Gradually work up higher, building row upon row. The

tubes will have to get longer and longer with the highest tubes approximately 11 in. long. It is important to keep the tubes pointing straight within the tunnel. Do not insert the fan into the sheet metal box until all the tubes are in place. This is because you will want to get your hand inside the box to help position the tubes.

Fan and Electronic Speed Control

The most important component of the Bernoulli box is the fan. We suggest the model DB410 from

Suncourt Inc. of Durant, IA (800-999-3267 ; www.suncourt.com). This is a 10 in. diameter propellerstyle fan with high static capability and sufficient airflow. Insert the fan into the receiving inlet collar of the sheet metal box and fasten with the sheet metal drill screws. Attach a safety grill at the fan inlet using sheet metal screws or construction adhesive.

Purchase a minimum 3 amp/120 VAC electronic speed control and cover plate such as by Broan-

NuTone of Hartford, WI (800-558-1711 ; www.broan.com). Also purchase a heavy duty grounded

(three-prong) minimum 6 ft long power cord. Wire the control between the power cord and the fan

using standard electrical wire nuts. Install the control in an electrical box with suitable strain relief.

Again, you may not be able to purchase directly from Suncourt and Broan-NuTone. We purchased the

fans and controls from Keystone Refrigeration & Heating Supply of State College, PA (800-281-1558). Bolt the electrical box down onto the plywood base using a No. 10 x 1 in. wood screw and 3/16 in. x 1 in. fender washer.