The Effect of Varied Length to Diameter Ratio for Cold-Wire Thermal Wake Measurements in Turbulent Flow

In the field of fluid mechanics, flow can be characterized as laminar, transitional or turbulent. . Laminar flow consists of flow in laminae with little cross-stream mixing.  In contrast, turbulent flow is characterized by significant cross-stream mixing while transitional flow consists of a mixture of laminar and turbulent flow regions.  One example is the mixing of hot and ambient air in a mixer.  This type study requires the measurement of time-resolved temperature and velocity which can be obtained, using, respectively, cold- and hot-wires.  For a cold-wire, the time-resolved temperature is directly proportional to the change in resistance of the cold-wire. The fluctuating resistance is converted to a fluctuating voltage by passing a very small, constant current through the cold wire.  In order to provide the electrical connection to the cold-wire, the cold wire is attached to two stainless steel prongs. Since there is thermal conduction between the cold wire and the prongs, there is a temperature variation along the cold wire and the average temperature of the cold wire is less than the true temperature of the fluid.  For a cold wire of fixed diameter (0.625in the current study), as the cold-wire becomes longer the effect of the prong on the measure temperature is reduced.  The goal of the present study is to determine the effect of cold-wire length on the measured temperature statistics and to determine the minimum length to diameter ratio for which the end effects have an error of less than 2% on the temperature statistics.