Pulsed-wire anemometry near walls

 This experimental investigation compares two configurations of pulsed-wire probe for measurements in the very near wall region, as introduced by Castro and Dianat (1990) and Devenport et al. (1990). Diffusion of the thermal wake causes significant errors in regions where the velocity is low and the velocity gradient high. These errors, which are about the same for either configuration, are large in the viscous sub-layer. An analysis of diffusional effects is made, and a method of correction is given that applies equally in laminar and turbulent flow. It is necessary to calibrate a probe for the effects of shear in a known flow. Received: 11 July 1997 / Accepted: 5 January 1998     

Spatial correlation measurement in turbulent flows using pulsed wire anemometry

Details of a new technique in pulsed wire anemometry, developed to allow measurement of two-point spatial velocity correlations in highly turbulent flows, are described. An outline of the interface devices necessary for linking two anemometers to a microcomputer is given and examples of the use of the technique are presented. Firstly, measurements of spatial correlations with and without time delay in the near wake of a nominally two-dimensional cylinder normal to a uniform stream confirmed the viability of the technique. Secondly, measurements in the highly turbulent, separated region behind a normal flat plate fitted with a downstream splitter plate are presented as a demonstration of the effectiveness of the technique in such difficult flows. We believe that these are the first direct measurement of spatial correlation functions within a separated flow.     

Combined laser-doppler and cold wire anemometry for turbulent heat flux measurement

 combined laser-doppler and cold wire anemometry technique for determining turbulent heat flux is described. The system can be used in flows of arbitrarily high turbulent intensity and large temperature variations. Its potential is demonstrated via measurements in a simulated stable atmospheric boundary layer, for which the Monin-Obukhov length scale was about 70% of the boundary layer depth. Mean and turbulence properties were obtained throughout the boundary layer and the results are shown to be both internally consistent and similar to corresponding field data. Measurements in the highly turbulent, separated flow behind a bluff body mounted in the stable boundary layer are also presented. Received: 9 May 1997 / Accepted: 2 September 1997     

Use of hot-wire anemometry for measuring the nanopowder flow velocity

A new technique for measuring the flow velocity of nano-scale powders is used. The hot-wire anemometry method widely used in gas flows is employed for investigating nanopowder flows. By way of illustration, the flows of nanopowders of aluminum oxide C and silicon dioxide aerosil A-90 and A-380 in a vertical channel are studied. The results obtained show that nanoscale powder flow investigation by means of the hot-wire anemometry is promising.     

Laser and avalanche diodes for velocity measurement by laser Doppler anemometry

The paper describes the possibilities of using semiconductor components for laser Doppler anemometers. This includes laser diodes D at the transmitting optics and PIN and avalanche diodes at the receiver. The advantages and disadvantages of various laser diode types are described, reference is made to problems such as focussing and wavelength stabilization and practical hints are given for the use of LDA. Extensive measurements of the attainable signal-to-noise ratios (SNR) of photomultipliers and of a number of photodiodes of different make have shown that some photodiode models are now superior to photomultipliers. Laser diodes should be used together with photodiodes to construct miniaturized laser Doppler anemometers, as such semiconductor LDA are much more efficient than conventional anemometers with gas lasers and photo-multipliers. Moreover, miniaturized LDA designs can now also be used for battery operation and field application.     

Measurement of velocity in rotational flows using ultrasonic anemometry: the flowmeter

In this paper a previously developed theoretical model of the measurement process performed by a transit-time ultrasonic anemometer is applied to a fluid flowing through a circular section pipe. This model considers the influence of the shift of the acoustic pulse trajectory from straight propagation due to the flow on the measured speed. The aim of this work is to estimate the errors induced in the measured velocity by the shift of the acoustic pulse trajectory. Using different duct’s flow models, laminar and turbulent regimes have been analyzed. The results show that neglecting the effect of shift of the acoustic pulse trajectory leads to flow rate measurement underestimation.     

Towards a theoretical model of heat transfer for hot-wire anemometry close to solid walls

Hot-wire anemometry readings where the sensor is close to a solid wall become erroneous due to additional heat losses to the wall. Here we examine this effect by means of experiments and numerical simulations. Measurements in both quiescent air as well as laminar and turbulent boundary layers confirmed the influences of parameters such as wall conductivity, overheat ratio and probe dimensions on the hot-wire output voltage. Compared to previous studies, the focus lies not only on the streamwise mean velocity, but also on its fluctuations. The accompanying two-dimensional steady numerical simulation allowed a qualitative discussion of the problem and furthermore mapped the temperature field around the wire for different wall materials. Based on these experimental and numerical results, a theoretical model of the heat transfer from a heated wire close to a solid wall is proposed that accounts for the contributions from both convection and conduction.     

Pulsed-wire anemometry

Pulsed-wire anemometry was first developed as a tool for making velocity and turbulence measurements in the late 1960s. It has been continuously refined and its potential exploited by a number of groups who have obtained reliable data in situations where the use of other techniques would have been extremely difficult. The technique is particularly useful in flows of high turbulence intensity and has therefore been used to greatest effect in separated flows. Although its range of applicability is much more restricted than that of laser-Doppler anemometry, it is an order of magnitude cheaper, and it does not require seeding of the flow, with all the attendant uncertainties.

First the physical basis of pulsed-wire anemometry is briefly described, and the major sources of experimental error are outlined for cases in which the technique is used both remote from and close to walls. Progress in the design of probes, which have been improved in a number of ways since the early days to reduce errors, is also outlined. The author then reviews the kinds of measurements that have been successfully made and the consequent improvements in the understanding of the physics of complex flows. Examples are drawn from a wide range of work, including some unusual applications like measurements in very low velocity gas mixtures. The paper closes with a summary of the limitations of the technique and an overall assessment of the likely potential for its use in future turbulence research.     

An economical droplet fog generator suitable for laser Doppler anemometry and particle imaging velocity seeding

An inexpensive device for generating fogs of micron-sized droplets is described. The liquid does not pass through small orifices. It is easily constructed and suitable for liquids containing particulates, seeding high-pressure environments and laser Doppler anemometry, and particle imaging velocimetry. Phase Doppler anemometry measurements show size distributions as a function of pressure for three working fluids. The device consistently produced a narrow size range with a mean diameter of a few microns.     

Detonation attenuation by foams and wire meshes lining the walls

In the present study systematic photographic investigations were performed of detonation interactions with foams and wire meshes lining the channel walls. An initial cellular detonation wave was propagating along a damping section (acoustic absorbing walls) which removed the transverse waves associated with its cellular structure. In some cases the wave had failed and a fast deflagration wave (a shock followed by a decoupled flame) was obtained and propagated at about half the C-J detonation speed. The events were studied photographically using a high speed framing camera and smoked foils.     

Flying hot-wire anemometry

A flying hot-wire arrangement has been developed for the measurement of the velocity characteristics of the flow around airfoils, and particularly in regions where negative values of instantaneous velocity occur. The mechanism and signal processing system are described and appraised by comparing stationary and flying wire measurements obtained in the trailing edge region of a flap at an angle of attack which leads to upper-surface separation.List of symbols time averaged quantity - E voltage from hot wire anemometer - Q, magnitude and direction of cooling velocity viewed from a frame of reference on the probe: Q=U+V _p - q ₁, q ₂, q ₃ cooling velocity fluctuations oriented with respect to the -direction - Q _eff magnitude of effective cooling velocity measured by the hot wire: Q _eff = (Q _N1 ² + h ² Q _N2 ² + k ² Q _T ^{2 1/2} - t time - q _eff fluctuations of the effective cooling velocity - Q _N1, Q _N2, Q _T axial, normal and tangential components of the cooling velocity relative to the hot wire - Q _eff (= 10°) magnitude of the effective cooling velocity with - Q _eff ( = 0°) Q _eff ( = 30°) magnitude of the effective cooling velocity with the wire pitched at 10 ° and 0° to the flow velocity - Q _eff ( = 45°) Q _eff ( = 30°) magnitude of the effective colling velocity with the wire yawed at 45° and 30° to the flow velocity - U, magnitude and direction of flow velocity - u, v, w flow velocity fluctuations (x, y, z) - u ₁, u ₂, u ₃ normalised fluctuations of cooling velocity: u _i=q _iQ for i=1,2,3 - V _p, magnitude and direction of probe velocity - v _p probe velocity fluctuations along the -direction - yaw angle of hot wire relative to the probe axis - angle of mean flow velocity to the probe axis - angle of mean axial cooling component to mean cooling velocity viewed from the wire - pitch angle of probe axis relative to tunnel coordinates (x, y, z) - x, y, z orthogonal coordinate system with the x-direction aligned with the wall (boundary layer) or tunnel centre-line (wake) - x _w, y _w, z _w orthogonal coordinate system with the z _w-direction aligned with the wire and the probe pintels in the x _w- z _w plane     

On the uncertainty of digital PIV and PTV near walls

The reliable measurement of mean flow properties near walls and interfaces between different fluids or fluid and gas phases is a very important task, as well as a challenging problem, in many fields of science and technology. Due to the decreasing concentration of tracer particles and the strong flow gradients, these velocity measurements are usually biased. To investigate the reason and the effect of the bias errors systematically, a detailed theoretical analysis was performed using window-correlation, singe-pixel ensemble-correlation and particle tracking evaluation methods. The different findings were validated experimentally for microscopic, long-range microscopic and large field imaging conditions. It is shown that for constant flow gradients and homogeneous particle image density, the bias errors are usually averaged out. This legitimates the use of these techniques far away from walls or interfaces. However, for inhomogeneous seeding and/or nonconstant flow gradients, only PTV image analysis techniques give reliable results. This implies that for wall distances below half an interrogation window dimension, the singe-pixel ensemble-correlation or PTV evaluation should always be applied. For distances smaller than the particle image diameter, only PTV yields reliable results.     

Hydrodynamic disruption of human erythrocytes near a transversely oscillating wire

Summary A novel ultrasonic device is presented which generates controlled hydrodynamic shear forces in small volumes of liquid. This device has been used to measure the mechanical fragilities of human blood cells under a variety of experimental conditions. A value of 1–2.000 dynes cm^–2 has been obtained for the critical shear stress required to disrupt the weaker erythrocytes in saline. Shear stresses of this order cause sub-lethal damage to surviving cells, which significantly reduces theirin vivo andin vitro life span.

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Zusammenfassung Menschliche Erythrozyten wurden in einer physiologischen Salzlösung hydrodynamischen Scherkräften ausgesetzt; diese wurden in einem Strömungsfeld um einen teilweise eingetauchten, dünnen Metalldraht erzeugt, der eine Querschwingungsfrequenz von 20 kHz hatte. Für eine Vielzahl von Bedingungen wurden Werte für die kritischen Scherbelastungen erhalten, bei der Erythrozyten platzen. Bei Scherbelastungen nahe dem kritischen Wert hatten einige Zellen einen Teil ihrer Zellmembrane ausgestülpt, um einen röhrenförmigen Schutz zu bilden. Zahlreiche hämoglobingefüllte kugelförmige Zellen (Mikrozellen) mit einem Durchmesser von 1–2 Mikron wurden bei den gleichen Belastungswerten beobachtet; diese haben sich wahrscheinlich beim Einklappen und Zusammenwachsen von dieser röhrenförmigen Schutzhülle gebildet. Einige physikalische und biochemische Parameter von intakten Erythrozytenzellen wurden untersucht, um zu sehen, ob wiederholter, kurzzeitiger Einfluß von hydrodynamischen Kräften eine vorübergehende oder dauerhafte Veränderung in den behandelten Zellen hervorgerufen hatte.

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Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970.     

Effect of wire diameter and overheat ratio near a conducting wall

The influence on several hot wire measurements of the diameter d and overheat ratio a of a wire placed near a conducting wall in a turbulent boundary layer is investigated. These measurements include the mean velocity and second, third and fourth order moments of the longitudinal velocity fluctuation u. A decrease in either d or a reduces the departure of the measured mean velocity from the expected linear sublayer value. The measured influence of d and a precludes the use of a universal correction for the mean velocity. There is no influence of a on the rms, skewness and flatness factors of u. Only a small increase is found in the skewness and flatness factors of u when d is decreased.     

Mixing in a stably stratified medium by horizontal shear near vertical walls

Stratified environmental flows near boundaries can have a horizontal mean shear component, orthogonal to the vertical mean density gradient. Vertical transport, against the stabilizing force of gravity, is possible in such situations if three-dimensional turbulence is sustained by the mean shear. A model problem, water with a constant mean density gradient flowing in a channel between parallel vertical walls, is examined here using the technique of large eddy simulation (LES). It is found that, although the mean shear is horizontal, the fluctuating velocity field has significant vertical shear and horizontal vorticity, thereby causing small-scale vertical mixing of the density field. The vertical stirring is especially effective near the boundaries where the mean shear is large and, consequently, the gradient Richardson number is small. The mean stratification is systematically increased between cases in our study and, as expected, the buoyancy flux correspondingly decreases. Even so, horizontal mean shear is found to be more effective than the well-studied case of mean vertical shear in inducing vertical buoyancy transport as indicated by generally larger values of vertical eddy diffusivity and mixing efficiency.