Experimental determination of probe-length requirements for studies of the turbulent wake behind a cylinder

The attenuation of turbulence and mean velocity signals due to the line averaging imposed by hot wires when used in the wake of an isolated circular cylinder has been investigated in a wind tunnel by measurements using several choices of hot-wire length, cylinder diameter, and freestream mean velocity. The results are presented graphically in order to provide a practical method for determining attenuation of the turbulence and mean velocity signals obtained in a wake. The length scale of the wake can be defined as L=0.6[(x-x(o)) d](1/2), where x is the downstream distance from the cylinder, d is the cylinder diameter, and x(o)=25d. For all the wires tested, the attenuation of the measured turbulence signal is limited to within 5% only if the wire length is smaller than 0.1 L. For a wire normal to the cylinder and cross wind, the attenuation of the signal of the mean velocity-defect factor, expressed as (1-u/u(infinity)), where u and u(infinity) are local and free-stream velocities, respectively, is less than 5% only if the wire is less than 0.5 L in length.