A flying hot wire study of the turbulent near wake of a circular cylinder at a Reynolds number of 140,000

An experiment was performed in the GALCIT 10-foot wind tunnel to study the flow in the near wake of a circular cylinder at a Reynolds number of 140,000. The main objective of this investigation was to study the phenomenology of the processes of vortex formation and transport in the near wake, at a Reynolds number sufficiently high to insure a fully turbulent wake, but low enough to insure a laminar separation. The latter requirement anticipates the eventual use of the results as a test case for advanced calculation codes. Much current experimental work on turbulent flows is concerned with large, coherent, organized vortex structures which have a relatively long lifetime and which account for much of the transport of mass, momentum and heat in turbulent shear flows. High Reynolds number flow past a cylinder is one case where such structures dominate. The apparatus developed for measuring this flow consists of x-array hot wire probes mounted on the ends of a pair of whirling arms. In such a flow, where large changes in flow direction occur, a fixed hot wire would rectify the velocity signal and give ambiguous results. However, by applying a large enough bias velocity to the wires, the relative velocity vector can be maintained within the [+/-]30 degree range of sensitivity of the x-array. One useful property of this technique is that a rotation of the arms in a uniform flow applies a wide range of relative flow angles to the x-arrays, making them inherently self-calibrating in pitch. The most important element of the instrumentation concomitant to the flying hot wire is a computer controlled data acquisition system which is slaved to the position of the rotating arm and which manages, monitors, edits and records the vast profusion of data which is continuously poured out by the device. A fast sensor responding to model surface pressure was used to generate a signal synchronized with the vortex-shedding process. This signal was recorded along with the hot wire data and used later to sort the data into populations having the same phase. Ensemble averages conditioned this way yield an average picture of the instantaneous flow field in which the vortices are frozen as they would be in a photograph. In addition to the conventional velocity, pressure and stress data, results are presented which show the instantaneous (in the sense of an average at constant phase) velocity, intermittency, vorticity and stress fields as a function of phase for the first six diameters of the near wake. In the present study, the Reynolds stresses are broken up into the contribution from large scale periodic motions and that from background or random turbulence, and, when dissected in this way, permit an enlightening look at the anatomy of this turbulent flow. Laid against the background of the instantaneous velocity, vorticity and intermittency, the stresses in the near wake emerge as a concatenation of peaks and valleys, some the result of strong induced motions in the outer flow which cause free stream fluid to move rapidly inward toward the center of the wake, others the result of the random motions of the background turbulence.