Slides for ME 115 Laboratory Wind tunnels Purposes To provide a uniform velocity field (e.g. aeronautic, automotive testing) and low turbulence intensity To provide suction or blowing and measure flow rate (e.g. airflow impedance testing, AC experiment, heat exchanger testing) Week 7 notes Windtunnels

Slides for ME 115 Laboratory Wind Tunnel Design Proper shape of the inlet is important – several journal papers offer guidance. Flow straighteners (honeycomb) and screens used to straighten flow and provide minimal turbulence intensity. Test section should not be too far downstream from inlet – otherwise boundary layers will be large for slow flows. If used, orifice plate or venturi tube must be design per strict ASME standards.

Wind tunnel and test section model at NASA-Ames Slides for ME 115 Laboratory Wind tunnel Examples blower Test section: Uniform velocity Here are some examples of wind tunnels designed to produce test sections with uniform velocity. Features: Inlet: Curvy bell-shaped inlet is designed to produce uniform flow and minimal boundary layer formation. Design guidelines are specified by ASME. What would the velocity profile look like in the test section? Test section: Often clear (plexiglas or glass) for visualization. Blower: Provides pressure drop needed for air movement. NASA-Ames windtunnel: Largest wind tunnel in world. Housed in bldg shown in photo top right; Screened wall is intake. Picture, bottom right: Model of Space shuttle installed in test section. Inlet: Designed for uniform inlet flow and minimal boundary layer formation Wind tunnel and test section model at NASA-Ames Source: Electronic Laboratory Design

More Wind tunnels Slides for ME 115 Laboratory Example of wind tunnel designed to provide suction/blowing and measure flow rate (with nozzle plate, see schematic). Used in electronics cooling industry. Picture from E114.

Velocity Measurement Pitot-static tube Hot-wire anemometer Slides for ME 115 Laboratory Velocity Measurement Pitot-static tube Hot-wire anemometer Senses stagnation pressure (at tip) and static pressure (at holes on periphery); Po – P can be measured. Velocity calculated from Bernoulli’s equation. Uses electricity to heat wire connecting two prongs to a constant temperature. Required current and/or voltage is calibrated to velocity. Review two velocity measurement techniques. Both probes must be aligned with flow. (I.e. Pitot-static tube must face incoming flow, hot-wire must be perpendicular to flow. Ask class: What is Bernoulli’s equation? What variables do you need to calculate velocity from dP? A: Po – P = ½ rho V^2, need to know rho of air. How would you find rho? A. Look up in air property table as a function of temperature, pressure. What are the mechanisms behind hot wire, I.e. why does this device measure velocity? A: Power supplied to the hot wire is V/I = V^2/R = I^2 R, which is equal to convective heat transfer rate from wire at steady state. If temperature of wire is constant, Twire – Tambient = constant, and thus Power/(Twire – Tambient) is proportional to heat transfer coefficient, h, which is a function of velocity. More specifically, we are looking at heat transfer rate from a cylinder in cross-flow. Will the hot-wire work for heated flows? A: No, Tambient is no longer constant, making Twire – Tambient no longer constant. Some probes compensate for this by measuring temperature in addition, and factoring it into their calculations. Will both sensors work at high altitudes? A. Yes, pitot-static tube must be used with correct density, and hot-wire must be recalibrated. (No, Reynolds number is a function of density, so Nusselt number and thus, h, will be different. Think Reynolds-Nusselt-Prandtl number correlations). How do these sensors compare to the devices used to measure mass flow rate? (Mass flow rate device similar to Pitot-static tube, I.e. dP is correlated to dV or dMdot.) Orifice plates and venturi tubes are often used as well (discussed during AC experiment).

Slides for ME 115 Laboratory Hot Dog Anemometer Using a cylinder of known properties, a thermocouple, and stopwatch, fluid velocity can be determined. If a warm cylinder is placed in a wind tunnel, the faster the air flows around the cylinder the faster the temperature of the cylinder changes. You will use the concepts of transient heat conduction and convection around a cylinder in this lab to calculate the air temperature. In the past, a hot dog was used as the warm cylinder. However, a thermocouple must be placed right in the center, and it’s tough to make sure of that with a hot dog, so a plastic cylinder is now used.