An improved method for measuring velocity and concentration by thermo-anemometry in turbulent helium-air mixtures

 A hot wire/film probe is used with new operating conditions for measurements of velocity and concentration in helium/air mixtures. With the upstream wire much more overheated than the downstream film, the sensing elements behave almost independently and the calibration map is composed of clearly distinct iso-velocity and iso-concentration curves. This behaviour is largely explained by the difference in the heated elements’ diameters. Tests of the probe in a turbulent boundary layer with large density variations indicate very satisfactory measurements. The domain of use for the probe is considerably extended with these new operating conditions. Received: 8 January 1996/Accepted: 29 May 1996     

A four-sensor hot-wire probe system for three-component velocity measurement

 A velocity measurement system based on a miniature four-sensor hot-wire probe capable of simultaneous three-component measurements throughout a wide range of flow angles has been developed. The calibration technique allows measurements to be made throughout the acceptance cone of the probe without being restricted by the errors associated with analytic angle response equations. This technique is based upon look-up tables with values which tend to vary slowly, allowing a simple interpolation scheme to be used. Measurements made in a turbulent pipe flow verify the accuracy of the technique. Received: 27 September 1996/Accepted: 20 October 1997     

Application of rainbow schlieren deflectometry for concentration measurements in an axisymmetric helium jet

 The rainbow schlieren deflectometry technique was used to measure oxygen concentrations in a laminar, isothermal helium jet discharged vertically into ambient air. The concentration distributions were inferred from the color schlieren image by taking into consideration the sampling interval and noise in measurements, especially near the jet center. Excellent quantitative agreement was reached between measurements from schlieren and a continuous sampling probe. This work demonstrates the capability of the schlieren technique for providing accurate, spatially-resolved, nonintrusive, full-field of view measurements of species concentration in an isothermal binary system. Because the basic quantity measured is the refractive index, the present schlieren technique can be extended for quantitative measurements of other scalar flow properties related to the refractive index. Received: 21 April 1997 / Accepted: 14 November 1997     

Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble

 Measurements with a directional sensitive hot-wire probe have been carried out in a two-dimensional laminar separation bubble caused by an adverse pressure gradient. The probe has three parallel, in plane wires and can be traversed in the boundary layer in all spatial directions. The central wire, operated as a conventional hot-wire in CTA mode, and two surrounding resistance wires measure the instantaneous magnitude and direction of the flow, respectively. The probe is calibrated and operated in a similar way as a single hot-wire probe for boundary layer measurements. The frequency response is high enough for measurements of naturally occurring instability waves in the bubble. The flow direction intermittency was measured inside the bubble and regions with reversed flow were mapped out. Prior to reattachment periodical oscillations of the flow direction are found associated with shedding of vortical structures from the bubble. Received: 13 March 1998/Accepted: 22 April 1998     

PLIF imaging of mean temperature and pressure in a supersonic bluff wake

 Planar laser-induced fluorescence (PLIF) of seeded nitric oxide was used to obtain mean 2-D temperature and pressure fields in the near-wake region of a thick flat plate in a Mach 3 flow. A two-line ratio technique was used to obtain the temperature field, while an image obtained at the limit of low quenching rate was used to infer the pressure field. An analysis shows that these time-average measurements can suffer from significant weighted averaging bias errors in regions where there are large temperature fluctuations; however, these bias errors can be minimized by judicious selection of the absorption lines used. The resulting temperature field reveals the warm upstream boundary layer, the temperature jump across the recompression shocks and the expected minimum and maximum temperatures in the expansion and recirculation regions, respectively. The pressure measurements indicate a uniform low pressure in the base region, a rapid increase near reattachment, followed by a gradual approach to the free stream value farther downstream. Received: 25 July 1996 / Accepted: 11 September 1997     

Accuracy limits for planar measurements of flow field velocity, temperature and pressure using Filtered Rayleigh Scattering

 We present experimental results using Filtered Rayleigh Scattering to make planar measurements of velocity, temperature and pressure in ambient air and in a Mach 2 free jet. The ambient air measurements are used to identify and calibrate experimental uncertainties. The Mach 2 measurements demonstrate the usefulness of the technique for making accurate planar measurements in a high speed flow. The measured values for velocity, temperature and pressure in the Mach 2 jet ranged, through a shock system, from 205 to 235 m/s, 150 to 170 K and 700 to 1000 torr, with estimated uncertainties of ±5.4 m/s, ±3.2 K and ±38 torr (±2 to 3%, ±2% and ±4–5%, respectively). Received: 10 December 1996/Accepted: 14 July 1997     

A directionally sensitive hot- wire anemometer

 A new directionally sensitive velocity measurement system based upon a single oscillating hot-wire probe has been developed. The vibration of the hot-wire was effected by a piezoelectric oscillator that could be driven at different amplitudes and frequencies. The sign of the local flow velocity was determined by the phase shift between the motion and the output from the hot-wire. The system was used to measure mean and fluctuating velocities profiles of the separated flow behind a backward facing step. Received: 29 November 1995/Accepted: 26 July 1996     

A high-resolution laser Doppler anemometer for three-dimensional turbulent boundary layers

 A new laser Doppler anemometer optimized for high spatial resolution near a wall is described. The instrument uses short focal length optics, a mirror probe in the flow, and side-scatter collection to produce a measuring volume 35 μm in diameter by 66 μm long. Data are presented for a two-dimensional boundary layer, demonstrating the instrument’s ability to measure Reynolds shear stresses as close to the wall as 0.1 mm, or y ⁺≈3. Received: 6 October 1995/Accepted: 9 April 1996     

Understanding biases in the near-field region of LDA two-point correlation measurements

 Two-point LDA measurements are shown to be strongly affected by both geometric and coincidence timing parameters, some effects of which are akin to those previously documented for nonorthogonal three-component LDAs. High spatial resolution is shown to be a necessity for small scale two-point measurements. It is suggested that a maximum effective probe volume dimension of one Kolmogorov length be considered as an upper bound for lateral and spanwise correlation measurements if microscale estimates are intended. However, if the effective probe volume shape is roughly spherical and a proper coincidence window is chosen, the spatial resolution of longitudinal correlation measurements appears to be independent of probe dimension. Received: 24 September 1996/Accepted: 4 June 1998     

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     

Transitional intermittency detection by neural network

 A neural network has been used to predict the flow intermittency from velocity signals in the transition zone in a boundary layer. Unlike many of the available intermittency detection methods requiring a proper threshold choice in order to distinguish between the turbulent and non-turbulent parts of a signal, a trained neural network does not involve any threshold decision. The intermittency prediction based on the neural network has been found to be very satisfactory. Received: 15 December 1997/Accepted: 30 December 1998     

Direct measurements of in-cylinder integral length scales of a transparent engine

 This paper presents a hardware technology and its accompanying software developed to measure the in-cylinder integral length scales of the flow inside a transparent four-stroke engine directly, using a fiber laser Doppler velocimeter (LDV). A traverse table was designed to combine two 1-component fiber optical probes to form an LDV system with the capability of both 2-component single-point and 1-component two-point simultaneous measurements for velocities. This paper demonstrates the evaluation of radially separated lateral and longitudinal integral length scales. The location was at a mid-plane of the top dead center (TDC) clearance height, and the engine was motored at a speed of 500 rpm. Data analysis was processed using statistical techniques and the physical meaning of the results was explained. Received: 27 August 1996/Accepted: 6 December 1996     

Turbulent flow around a rotating cylinder in a quiescent fluid

 Hot-wire measurements have been carried out in the turbulent flow around a rotating circular cylinder in still air for Reynolds numbers Re=∣U _w ∣D/ν=1.5×10⁴ to 10⁵. The experimental results confirm the analysis derived by asymptotic theory for high Reynolds numbers. Two different ways of deriving the friction law from the experiments (via shear stress and via velocity distribution) resulted practically in the same law. It is shown, that in spite of the curvature of the streamlines the universal logarithmic velocity distribution is still valid near the wall. Received: 8 August 1996/Accepted: 24 April 1998     

Nonintrusive measurements of the boundary layer developing on a single and two circular cylinders

 The boundary layers developing on a single and two tandem circular cylinders were examined using multiple hot-film sensor arrays for Re=2.4–5.1×10⁴. Hot-wire and surface pressure measurements, and smoke-wire flow visualization were also made to better understand the flow pattern and the evolution of the vortex street. The results show that, by use of the sensor arrays in conjunction with a bank of constant-temperature anemometers, (i) the effects of the upstream cylinder on the boundary layer developing on the downstream cylinder, (ii) the frequency of the vortex shedding, and (iii) the locations of flow separation and reattachment can be determined non-intrusively and simultaneously. These measurement capabilities will provide a practical means for the characterization and manipulation of unsteady flow phenomena. Received: 27 October 1996 / Accepted: 13 February 1997     

Influence of humidity on the convective heat transfer from small cylinders

 The convective heat transfer from a cylinder to a humid air stream flowing normal to the cylinder was investigated experimentally at atmospheric pressure over a range of variables which is relevant to the use of hot‐wire anemometry: air temperatures between 30 °C and 70 °C and velocities between 12 and 37 m/s. For molar fractions of water vapour up to 0.27, the heat transfer increased with increasing humidity. The ratio of heat transfer rates in humid air and dry air is a unique function of the molar fraction of water vapour, independent of the air temperature and flow velocity. Received: 28 November 1996/Accepted: 5 July 1997     

Further considerations of the astigmatism error associated with off-axis alignment of an LDA-probe

 Off-axis alignment of an LDA-probe in internal flow measurement is always associated with the appearance of astigmatism and this may have a large effect on the measurement accuracy. In addition to the earlier investigations reported by Zhang and Eisele (1995a, 1996) it is shown that the improper off-axis alignment of the probe leads to separation of laser beams in the formation of the measurement volume. Quantitative calculations made in this paper give a sound basis for estimating this separation and the results imply that the error arising from beam separation is very serious both for LDA- and for PDA-measurements. In addition, the analysis in Sect. 3 reveals that the reduction of the data rate from astigmatism due to off-axis alignment of the probe depends on the focal length of the receiving optics. This relationship has been validated by experiment. Received: 23 December 1996/Accepted: 24 July 1997     

A miniature triple hot-wire probe for wall bounded flows

 Four orthogonal and one non-orthogonal miniature triple hot-wire probes have been developed and tested in a two-dimensional turbulent boundary layer. The influence of the different wire configurations on measurements of the Reynolds stresses has been studied. A directional calibration with an analytical solution for the wire response equations was used for the measurements of the non-orthogonal probe and was tested for the orthogonal probes in order to correct their possible geometrical imperfections. It is shown that a directional calibration does not significantly improve the quality of the measurements for precisely manufactured orthogonal probes and that measuring errors are related rather to the measuring volume, the size of the domain of unique solutions for the instantaneous velocity vector and interference effects, i.e. the wire configuration itself. Received: 7 February 1997/Accepted: 18 November 1997     

Vortex stretching in a laminar boundary layer flow

 A new technique to produce controlled stretched vortices is presented. The initial vorticity comes from a laminar boundary layer flow and the stretching is parallel to the initial vorticity. This low velocity flow enables direct observations of the formation and destabilization of vortices. Visualizations are combined with quasi-instantaneous measurements of a full velocity profile obtained with an ultra-sonic pulsed Doppler velocimeter. Several modes of destabilization are observed and include pairing of two vortices, hairpin deformation, and vortex breakdown into a coil shape. Received: 3 April 1996/ Accepted: 4 October 1996     

Simultaneous wall shear rate measurements by a three segmented electrodiffusion probe and laser-Doppler-anemometry

 In a fully developed turbulent channel flow, the instantaneous wall shear gradient was measured simultaneously by an electrodiffusion (ED) probe and by laser-Doppler anemometry (LDA). The LDA measurements were done in the viscous sublayer and the linear velocity profile was used to calculate the instantaneous shear gradient at the wall. The electrodiffusion probe requires presence of a suitable electrolyte (e.g. an iodine and potassium-iodine aqueous solution) for reduction of the species on the working electrode (cathode). This results in a concentration gradient of the ions due to the convective diffusion of the [I₃]^- to the cathode. Due to the limited molecular diffusion rate and due to the dimension of the electrodiffusion probe its frequency resolution is limited. It is shown in which limits the correction of the signal on the concentration boundary layer inertia is not necessary and the probe can be used for measuring instantaneous wall shear gradients in turbulent flows. The results are compared with those obtained by LDA and elucidate the capability of the ED probe for measuring instantaneous shear rates at low Reynolds numbers. Received: 25 October 1995/Accepted: 16 July 1996     

Measurement of droplet size,velocity, and temperature distributions in rayleigh flow

 Experiments were performed on the droplets formed from a heated, unforced, cylindrical column of water (Rayleigh flow). Temperature distributions, velocity distributions and size distributions were obtained. A thermocouple was used for the temperature measurements. Two time constants were determined – one for the drop impinging on the thermocouple and another for the drop leaving the thermocouple. Received: 21 September 1995 / Accepted: 10 May 1996