Study of turbulent structure with hot wires smaller than the viscous length

Abstract

The small-scale structure of the streamwise velocity fluctuations in the wall region of a turbulent boundary layer is examined in a new wind-tunnel facility using hot-wires smaller than any previously constructed (typical dimensions: l = 25 μm, d = 0.5 μm). In the boundary layer in which the measurements were made, the ratio of the hot-wire length to the viscous length is 0.3. The turbulent intensity measured with the small hot wires is larger than that measured with longer wires owing to the better spatial resolution of the small wires. The velocity fluctuations measured by the small hot wires are also analysed to determine the burst frequency at two Reynolds numbers and at various distances from the wall. The dimensionless burst frequency does not depend on the Reynolds number when scaled with wall parameters. However, it increases with Reynolds number when scaled with outer variables. Velocity fluctuations measured by two hot wires, less than two viscous lengths apart, are analysed to reveal the small-scale features present during a burst and in the absence of a burst. The main conclusions are: (1) intermittent small-scale shear layers occur most frequently when bursts are present, less frequently just after a burst, and even less frequently just before a burst; and (2) on occasion the velocity gradient of the small-scale shear layers is as large as the mean-velocity gradient at the wall.

Hot-wire height gauge using a laser and photodiodes

“…The present data are 6% higher than Johansson and Alfredsson's RMS values at y + = 10.3 and y + = 15.8. This is consistent with the data of Willmarth and Sharma (1984) who showed that a hot wire with an l + = 14 should yield RMS levels approximately 8% lower than the true RMS value a t y + = 13.3.No evidence of mean velocity broadening were seen in any of the RMS results due to the size of the probe volume normal to the wall. This was expected since the broadening correction scheme of Karpuk and Tiederman (1976) predicts corrected values of u' less than 0.3% different than the uncorrected value in the worst case for the present measurements.…”

An experimental investigation of wall pressure fluctuations beneath nonequilibrium turbulent flows

“…In order to minimize these errors a very small x-wire is needed or, alternately, measurements in a thick boundary layer must be made. Examples of these two approaches are give by the studies of Willmarth and Sharma (1984), who used small x-wires, and by Eckelmann…”

Flow and swell noise in marine seismic data

“…They investigated wall pressure fluctuations in conjunction with the mean velocity profiles. Willmarth and Sharma (1984) and Snarski and Lueptow (1995), performed similar studies where also the turbulent flow properties were investigated. In the latter study two different groups of fluctuations in the turbulent boundary layer surrounding a cylinder were identified.…”

“…All these works investigate wall pressure fluctuations in conjunction with the mean velocity profiles. In Willmarth and Sharma (1984) and later also in Snarski and Lueptow (1995), similar studies were performed where the turbulent flow properties also were investigated. However, despite decades of research on the subject, there is still uncertainty over many of the fundamental relations for turbulent, axisymmetric boundary layers.…”