Hot-film Measurements Research Articles

Extraction and characterisation of underlying velocity waveforms in poststenotic flow

The problem of extracting and characterising underlying velocity waveforms in poststenotic flow is examined using a set of hot-film anemometer measurements in the descending thoracic aorta of a dog. The question of separating turbulence from the underlying waveform is studied by applying ensemble averaging and lowpass filtering methods. Results conclusively demonstrate that lowpass filtering is inadequate for defining the underlying waveform, and thus ensemble averaging is required. The effect of the number of beats taken in forming the ensemble averaging is required.The effect of the number of beats taken in forming the ensemble is examined with the resultant recommendation that at least 20 cycles be employed. Finally, the use of autoregressive methods of spectral analysis are introduced to determine the dominant frequencies in the underlying waveform; and these are shown to be superior to Fourier transform methods for this purpose, provided note is taken of appropriate relationships between the sampling frequency and the autoregressive model order.

Hot-wire and hot-film anemometry and conditional measurements: A report on Euromech 132

The European Mechanics Colloquium, Euromech 132, was held at the Ecole Centrale de Lyon from 2 to 4 July 1980. Specific areas of hot-wire or hot-film anemometry were presented and discussed, more especially the effect of the finite time constant of the wire supports, the use of yawed hot wires in supersonic flows, the possible improvement of vorticity meters, and multi-point measurements of wall-shear-stress fluctuations. Other subjects described during the meeting included a new technique for concentration measurements in flames, developments and new uses of digitization and conditional sampling, pattern recognition analysis of fluid flow from multi-point, multi-time velocity measurements, and new turbulence measurements in complex flows and in fluid-flow machinery.An exhibition of hot-wire and hot-film anemometers and associated equipment was held during the colloquium.

Ｔｈｉｎ Ｌａｙｅｒｅｄ Ｍｉｌｋ法による往復流境界層の可視化について

THe process of generation of turbulence in oscillatory boundary layer was elucidated by flow visualization technique and compared with the results of hot-film measurement. For the flow visualization the bed of water tunnel was covered by a thin layer of milk with specific gravity 1.015(Thin layered milk method). The thickness of the layer was about 1mm. The piston-stoke was started from rest to a sinusoidal movement. The movement of milk was recorded photographically with a motor-driven camera set stationary above the tunnel. Photographs show turbulence bursts occur in each cycle of oscillatory flow which bursts are followed by the relaminarization during the same cycle.

Measurements of Unsteady Vortex Flowfields

A combined surface hot film and laser velocimeter measurement technique has been developed for the study of time-dependent turbulent flows. Data obtained in a compressible cylinder wake (M(sub infinity) = 0.6) are presented, and its structure in both the Eulerian and Lagrangian frames is discussed. Turbulence data obtained by conventional and phase averaging of the velocity fluctuations provide details of the small- and large-scale contributions to the total turbulent field.

Characteristics of Tornado-Like Vortices as a Function of Swirl Ratio: A Laboratory Investigation

Abstract The investigation of tornado vortex dynamics by means of a laboratory simulation is described. Based on observations from nature and an examination of the Navier-Stokes equations, a laboratory simulator of the Ward type has been constructed. This simulator generates various vortex configurations as a function of swirl ratio, radial Reynolds number and aspect ratio. Configurations which are described are 1) a single laminar vortex; 2) a single vortex with breakdown bubble separating the upper turbulent region from the lower laminar region; 3) a fully developed turbulent core, where the breakdown bubble penetrates to the bottom of the experimental chamber; 4) vortex transition to two intertwined helical vortices; and 5) examples of higher order multiple-vortex configurations that form in the core region. Hot-film anemometry measurements of the magnitude of the velocity vector and inflow (swirl) angle have been obtained in a sequence of flows characterized by progressively increasing values of swirl...

Features of a separating turbulent boundary layer in the vicinity of separation

Measurements of a separating two-dimensional incompressible boundary layer with an airfoil-type pressure distribution are reported. Unique mean and fluctuation velocity measurements and the distribution of the fraction of the time γp during which the flow moves downstream were obtained in the separated region using a directionally sensitive laser anemometer. Linearized hot-film anemometer measurements of mean velocities, turbulent shearing stress and intensities, eddy speeds, spectra and dissipation were made for γp > 0·8. The wall shearing stress, bursting frequencies, wall speed and spanwise structure were obtained using flush-surface hot-film sensors. The turbulent/non-turbulent interfacial intermittency γ and the frequency of passage of turbulent bulges were determined using smoke as a turbulence marker and the laser anemometer system for illumination and signal detection.Upstream of separation the velocity profile correlations of Perry & Schofield (1973) are supported within the uncertainty of the data. Normal-stress effects are very important, influencing $-\overline{uv}/\overline{q^2} $ and the dissipation length correlations, and directly providing sizable terms in the momentum and turbulence energy equations. The criteria of Sandborn for turbulent separation and fully developed separation are found to hold. Downstream of separation there is apparent similarity of $\overline{u^2}$, U and γp throughout the shear flow. The passive low velocity backflow near the wall apparently just serves to satisfy continuity requirements after the energetic outer-region flow has deflected away from the wall upon separation.The wall bursting frequency nA scales on outer velocity and length scales, with U∞/δnA ≈ 10, or about twice the value observed for zero-pressure-gradient flows. The non-dimensional spanwise spacing of wall eddies is given approximately by the relation λzUM/v ≈ 100 upstream of separation, where $U_M = (- \overline{uv}_{\max})^{\frac{1}{2}}$. The speed of wall eddies is found to be about 14Uτ.

Airborne Hot-Film Measurements of the Small-Scale Structure of Atmospheric Turbulence During GATE

Abstract Fluctuations of temperature, horizontal velocity and vertical velocity were measured at scales from 50 m to 5 cm with airborne hot-film anemometers at altitudes of 150 and 900 m in clear air, and in subcloud air with and without rainfall. Although nearly inertial subrange spectral behavior was often present at scales smaller than 20 m, significant regions existed where inertial behavior did not appear until scales smaller than a few meters were reached. The energy dissipation rate varied intermittently by two orders of magnitude or more over scales ranging from 100 m to several kilometers. High Reynolds number intermittency effects were observed in the temperature spectra. In an anomalous region, here called a “dry hole,” the microstructure of the velocity and temperature fields was radically different from that of the surrounding environment. Spectral intensity decreased by an order of magnitude and spectral shape was definitely non-inertial. Despite these changes, the probability distribution o...

Nonasymptotic behavior of developing turbulent pipe flow

Detailed measurements of mean velocity U profiles, in the inlet 70 diameters of a pipe, show that the development of turbulent pipe flow is nonasymptotic. Experiments were done at seven Reynolds numbers in the range 56 000–15 3000. Contours of U and V fields are presented for two representative Reynolds numbers. A U component peak exceeding the fully developed values has been found to occur along the pipe centerline. The Reynolds number behavior of the peak position has been determined. Hot film measurements of the mean wall shear stresses in the inlet region also show a nonasymptotic development consistent with the mean velocity results.

Flow Regimes in Two-Dimensional Ribbed Diffusers

Water table flow visualization tests of a straight wall diffuser and corresponding ribbed diffuser were conducted over a wide range of total divergence angles and diffuser length to throat width ratios. Rib parameters such as inter-rib cavity depth to width ratio, inter-rib cavity width, rib thickness, and number of ribs were varied, as well as rib contour. Motion pictures were obtained of the secondary flow patterns occurring within the inter-rib cavities, and hot film measurements were made to determine the extent of velocity fluctuations induced by the cavities. In general, the addition of ribs increased the no-appreciable stall regime.

Pipe flow measurements of turbulence and ambiguity using laser-Doppler velocimetry

The laser-Doppler ambiguities predicted by George & Lumley have been verified experimentally for turbulent pipe flows. Experiments were performed at Reynolds numbers from 5000 to 15000 at the centre-line and near the wall. Ambiguity levels were measured from power spectral densities of FM demodulated laser signals and compared with calculations based on the theory. The turbulent spectra for these water flows after accounting for the ambiguity were equivalent to hot-film measurements at similar Reynolds numbers. The feasibility of laser-Doppler measurements very close to the wall in shear flows is demonstrated.

Flow characteristics in models of arterial stenoses — II. Unsteady flow

In Part I of this paper the effect of a stenosis on the steady flow through a tube was considered. In Part II the effect of unsteadiness is investigated experimentally using the same axisymmetric and nonsymmetric models. Oscillating flow, with and without a steady component, was utilized and the pressure drop was determined as a function of several parameters. An approximate equation for predicting the pressure drop across a stenosis is developed, and verified experimentally. It is shown that three dimensionless parameters can be used to characterize the unsteady flow in a stenosis. Hot-film measurements were used to investigate the development of turbulence and it was found that the oscillating flow was more stable than the corresponding steady flow for mild constrictions. For the severely constricted models the oscillating flow was slightly less stable than for steady flow.

Flow characteristics in models of arterial stenoses. I. Steady flow.

Abstract In Part I of this paper the effect of a stenosis on the steady flow through a tube was considered. In Part II the effect of unsteadiness is investigated experimentally using the same axisymmetric and nonsymmetric models. Oscillating flow, with and without a steady component, was utilized and the pressure drop was determined as a function of several parameters. An approximate equation for predicting the pressure drop across a stenosis is developed, and verified experimentally. It is shown that three dimensionless parameters can be used to characterize the unsteady flow in a stenosis. Hot-film measurements were used to investigate the development of turbulence and it was found that the oscillating flow was more stable than the corresponding steady flow for mild constrictions. For the severely constricted models the oscillating flow was slightly less stable than for steady flow.

Sequential velocity development in the ascending and descending aorta of the dog.

A conical hot-film probe was used to measure instantaneous velocities in the ascending and descending aorta of anesthetized open-chest dogs. From these point measurements, the radial distributions of velocity were obtained over one cardiac cycle. In general, the hot-film measurements confirmed the observation that the velocity profiles tend to be flat with the highest rates of shear confined to the region of the wall. There were, however, significant variations in the detail from one dog to another. These variations in the shape of the profiles probably are a consequence of many geometrical factors, which include valve inlet geometry, configuration and orientation of the valve plane, and aortic curvature and branching in the descending aorta.

The wall region in turbulent shear flow

Hot-film measurements in a fully developed channel flow have been made in an attempt to gain more insight into the process of Reynolds stress production. The background for this effort is the observation of a certain sequence of events (deceleration, ejection and sweep) in the wall region of turbulent flows by Corino (1965) and Corino & Brodkey (1969). The instantaneous product signal uv was classified according to the sign of its components u and v, and these classified portions were then averaged to obtain their contributions to the Reynolds stress $-\rho\overline{uv} $. The signal was classified into four categories; the two main ones were that with u negative and v positive, which can be associated with the ejection-type motion of Corino & Brodkey (1969), and that with u positive and v negative, associated with the sweep-type motion. It was found that over the wall region investigated, 3·5 [les ] y [les ] 100, these two types of motion give rise to a stress considerably greater than the total Reynolds stress. Two other types of motion, (i) u negative, v negative, corresponding to low-speed fluid deflected towards the wall, and (ii) u positive, v positive, corresponding to high-speed fluid reflected outwards from the wall, were found to account for the ‘excess’ stress produced by the first two categories, which give contributions of opposite sign.The autocorrelations of the classified portions of uv were obtained to determine the relative time scales of these four types of motion. The positive stress producing motions (u < 0, v > 0 and u > 0, v < 0) were found to have significantly larger time scales than the negative stress producing motions (u < 0, v < 0 and u > 0, v > 0). It was further surmised that turbulent energy dissipation is associated with the Reynolds stress producing motions, since these result in localized shear regions in which the dissipation is several orders of magnitude greater than the average dissipation at the wall.

Mechanics of Sheet Flow under Simulated Rainfall

Friction factors are presented for spatially varied sheet flow with rainfall on a hydraulically smooth surface as obtained from both surface profile and hot film surface probe measurements. The results are presented in the form f = C /R, in which C increases with rainfall intensity and surface slope. A two-part velocity profile mathematical model based on von Karman's similarity hypothesis is presented and correlated with hot film measurements. Relative longitudinal turbulence intensity profiles with rainfall are characterized by a sharp increase in relative intensity near the surface and a general increase throughout the depth with increasing rainfall. Turbulent energy spectral analyses indicate that the rainfall causes a shift of the energy content to higher frequencies than for the case of uniform flow at the same Reynolds number.

Boundary-layer transition on sharp cones at hypersonic Mach numbers.

An experimental investigation to locate the beginning of transition from laminar to turbulent boundary-layer flow has been conducted at zero angle of attack on sharp, smooth cones having semiapex angles of 2.87°, 5°, and 10° in the contoured nozzle of the Langley 22-in. helium tunnel at a freestream Mach number of about 20. Local Mach number at the boundarylayer edge was thus varied from 7.4 to 16.6. The data indicate that local transition Reynolds number increases rapidly with local Mach number. Techniques used to detect onset of transition included surface pilot tube, drag force, boundary-layer pitot-pressure surveys, schlieren photographs, and hot-film anemometer measurements. Skin-friction coefficient, displacement thickness, momentum thickness, and velocity ratio profiles were determined for laminar, transitional, and turbulent hypersonic boundary layers. A hot-film anemometer survey of the model boundary layers showed disturbances originating within the boundary layer at much lower Reynolds numbers than the Reynolds number for which transition is felt at the model surface. The maximum disturbance level occurred at a location corresponding to about 0.845 (boundary-layer thickness) with the disturbance speed being subsonic relative to the local edge velocity. In addition, source flow effects on transition Reynolds number were examined at a local Mach number of 15.8.

Measurement of Turbulence in Water

A study of hot-film anemometer measurements in water resulted in a hypothesis that: Dirt and air bubbles accumulating on the hot-film sensor change the mean voltage for a given velocity, but have only a minor, outside the frequency domain in water, effect on the frequency response of the sensor to velocity fluctuations. And for a given sensor there is a unique family of voltage/velocity relations that can be defined by calibration with different overheat ratios. The hypothesis was experimentally verified by comparing turbulence measurements made in the same flow field with well filtered and with very dirty water. The method was used to measure the characteristics of turbulence measurements made in ordinary laboratory water for flow over hydraulically-rough and hydraulically-smooth boundaries. Reynolds numbers ranged from 8,000 to 20,000 and mean velocities from 0.3 fps to 2.8 fps. Relative turbulent intensities for flow over the smooth boundary were in good agreement with Laufer's measurements in air. Power spectrum analysis of the turbulence showed very little energy in frequencies larger than 100 Hz.

Anomalous Pitot tube and hot film measurements in dilute polymer solutions

The experiments reported show that Pitot tubes and hot film cylinders behave anomalously in the turbulent flow of dilute polymer solutions. The discrepancy in the Pitot tube readings increases with the absolute velocity, the molecular weight and concentration of polymer, and with decreasing Pitot tube diameter. This discrepancy is also a function, as yet undetermined, of the ambient strain rate. Hot film measurements indicate that for cylinders in cross flow the heat transfer is poorer in polymer solutions than in the solvent and exhibits abrupt transitions over which the heat transfer coefficient may vary by threefold. The heat transfer improves with increasing ambient strain rate and with decreasing polymer molecular weight.