Design and Construction of a Dynamometer for the Union College Water Tunnel

In order to design any boat or submarine the designers must understand how that vehicle moves through water. This means that the designers are looking to understand how much drag (the force opposing motion) or lift (the force perpendicular to motion) is acting on that vehicle as it moves though the water at different speeds. Often to understand how these vehicles react to moving though the water designers will make small models and place them within water tunnels and measure the drag and lift forces acting on the vehicle using a dynamometer. This dynamometer is a device that measure forces acting on the object that allows scholars to understand how these objects react to the flow field around them. Through my research on force measurement devices and water tunnels I have designed a dynamometer that can be used in the Union College water tunnel for educational development. The dynamometer uses two commercially available thin beam load cells, an Arduino load cell amplifier and Red Board, and a data acquisition system built into Excel. The load cells can measure a range of forces between 0.1-20 lbs with 90% accuracy up to 0.05 lbs of force. The structural design of the dynamometer is made to allow for multiple different types of objects to be tested in the water tunnel. The dynamometer structure is constructed from 1/8^th inch 6061 aluminum. A stab is used to connect the object to the load cells and can be raised and lowered within the water tunnel to vary the depth of the object. The stab can also be moved laterally between the walls of the water tunnel. With this system the lift and drag forces acting on an object placed within the water tunnel can be measured. To use the system, the object is suspended in the water tunnel by the stab and an overhanging mount that rests on top of the water tunnel. To determine the accuracy of the dynamometer, drag measurements were made on a 1.5 inch diameter smooth sphere for a range of fluid velocities from 0.1 m/s to 3 m/s (Reynolds numbers of 1.12 E3 to 3.36 E4) and compared to existing data. The main purpose for the dynamometer is to provide capability for students and faculty to perform experiments and learn about fluid mechanics.