Marcus Hultmark

Turbulence measurements using a nanoscale thermal anemometry probe

A nanoscale thermal anemometry probe (NSTAP) has been developed to measure velocity fluctuations at ultra-small scales. The sensing element is a free-standing platinum nanoscale wire, 100 nm × 2 µm × 60 µm, suspended between two currentcarrying contacts and the sensor is an order of magnitude smaller than presently available commercial hot wires. The probe is constructed using standard semiconductor and MEMS manufacturing methods, which enables many probes to be manufactured simultaneously. Measurements were performed in grid-generated turbulence and compared to conventional hot-wire probes with a range of sensor lengths. The results demonstrate that the NSTAP behaves similarly to conventional hot-wire probes but with better spatial resolution and faster temporal response. The results are used to investigate spatial filtering effects, including the impact of spatial filtering on the probability density of velocity and velocity increment statistics.

Scaling of near-wall turbulence in pipe flow

New measurements of the streamwise component of the turbulence intensity in a fully developed pipe flow at Reynolds numbers up to 145 000 indicate that the magnitude of the near-wall peak is invariant with Reynolds number in location and magnitude. The results agree with previous pipe flow data that have sufficient spatial resolution to avoid spatial filtering effects, but stand in contrast to similar results obtained in boundary layers, where the magnitude of the peak displays a prominent Reynolds number dependence, although its position is fixed at the same location as in pipe flow. This indicates that the interaction between the inner and outer regions is different in pipe flows and boundary layers.

Azimuthal structure of turbulence in high Reynolds number pipe flow

Two-point hot-wire measurements of streamwise velocity were performed in the logarithmic and wake regions of turbulent pipe flow for Reynolds numbers, based on pipe diameter, ranging from 7.6 × 10 4 to 8.3 × 10 6 at four wall-normal positions with azimuthal probe separation. The azimuthal correlations were found to be consistent with the presence of very large-scale coherent regions of low-wavenumber, low-momentum fluid observed in previous studies of wall-bounded flows and were found to be independent of changing Reynolds number and surface roughness effects. At the edge of the logarithmic layer the azimuthal scale determined from the correlations was found to be similar to that observed for channel flows but larger than that observed for boundary layers, inconsistent with the concept of a universal logarithmic region. As the wall-normal position increased outside the logarithmic layer, there was a decrease in azimuthal scale relative to that of channel flow. Using cross-spectral analysis, high-wavenumber motion was found to grow azimuthally with wall-normal distance at a faster rate than the low-wavenumber motions.

Temperature corrections for constant temperature and constant current hot-wire anemometers

Changes in the ambient fluid temperature change the calibration curve for velocity measurements taken using hot-wire anemometry. New correction methods are proposed to account for the effects of relatively large temperature changes in the heat-transfer process and on the fluid properties. The corrections do not assume any particular heat-transfer correlation, and they do not require multiple calibrations over a range of temperatures. The corrections are derived for the constant temperature and constant current modes of operation.

The intermediate wake of a body of revolution at high Reynolds numbers

Results are presented on the flow field downstream of a body of revolution for Reynolds numbers based on a model length ranging from 1.1 × 10 6 to 67 × 10 6 . The maximum Reynolds number is more than an order of magnitude larger than that obtained in previous laboratory wake studies. Measurements are taken in the intermediate wake at locations 3, 6, 9, 12 and 15 diameters downstream from the stern in the midline plane. The model is based on an idealized submarine shape (DARPA SUBOFF), and it is mounted in a wind tunnel on a support shaped like a semi-infinite sail. The mean velocity distributions on the side opposite the support demonstrate self-similarity at all locations and Reynolds numbers, whereas the mean velocity distribution on the side of the support displays significant effects of the support wake. None of the Reynolds stress distributions of the flow attain self-similarity, and for all except the lowest Reynolds number, the support introduces a significant asymmetry into the wake which results in a decrease in the radial and streamwise turbulence intensities on the support side. The distributions continue to evolve with downstream position and Reynolds number, although a slow approach to the expected asymptotic behaviour is observed with increasing distance downstream.

Measurement of local dissipation scales in turbulent pipe flow.

Local dissipation scales are a manifestation of the intermittent small-scale nature of turbulence. We report the first experimental evaluation of the distribution of local dissipation scales in turbulent pipe flows for a range of Reynolds numbers: 2.4x10(4)<or=ReD<or=7.0x10(4). Our measurements at the nearly isotropic pipe center line and within the anisotropic logarithmic layer show excellent agreement with distributions that were previously calculated from numerical simulations of homogeneous isotropic box turbulence and with those predicted by theory. The reported results suggest a universality of the smallest-scale fluctuations around the classical Kolmogorov dissipation length.

A new criterion for end-conduction effects in hot-wire anemometry

The effect of end conduction on constant temperature hot-wire anemometry was studied. A new parameter, , is proposed to describe the significance of end conduction more comprehensively than the commonly used length-to-diameter ratio l/d, in that it allows for material property variations, resistance ratio and Reynolds number effects. Numerical and experimental data are used to show that Γ improves the correlation of the attenuation of measured turbulence fluctuations, and it is found that Γ > 14 is required to avoid such effects.

Dynamic calibration and modeling of a cold wire for temperature measurement

The dynamical behavior of cold wires and their supporting structure is investigated. It is shown that conventional cold wires have a much more limited frequency response than previously believed, which can cause substantial inaccuracies in temperature data. Here, a lumped parameter model that accounts for the effects of end conduction and wire response is developed. The model is verified by comparing to the experimental data, where the frequency response of the wire is investigated under different heating conditions, with convincing agreement. All the parameters in the model can be found from the geometry of the sensor and the properties of the materials used in its construction. The new model can either be used as a sensor design and optimization tool, in order to design better sensors, or it can be used to correct the data acquired with conventional cold wire sensors, with non-negligible end-conduction effects, so that accurate measurements can be obtained.

Errors in parallel-plate and cone-plate rheometer measurements due to sample underfill

The effect of sample underfill on parallel-plate and cone-plate rheometers is examined. Sample underfill can be caused by incomplete filling of a sample or loss of fluid during a test by, for example, evaporation. It is shown that even a small degree of sample underfill can lead to significant errors in measuring viscosity. A method is proposed to reduce these errors by directly monitoring the sample radius over the full course of the test. It is shown that the accuracy of the rheometer even while testing simple fluids like water is greatly improved.

Stratified thin-film flow in a rheometer

When two immiscible layered fluids are present in a rheometer, interfacial distortions driven by the centripetal pressure gradient can modify torque measurements and induce dewetting. In particular, we examine the steady-state interface shape of a thin film coating a stationary substrate beneath a second immiscible fluid that is driven by a rotating parallel-plate or cone. An asymptotic analysis of the interfacial distortion for the parallel-plate flow is compared with numerical solutions for both the parallel-plate and cone and plate configurations. We develop asymptotic criteria for dewetting of the thin film as a function of fluid and flow properties, and show that significant interfacial distortion and dewetting can occur due to secondary flow effects even at low Reynolds numbers. The distortion of the interface can result in increased or decreased torque measurements depending on the viscosity and density ratios between the two fluid layers. We relate these effects to recent experimental studies on l...