Hot-film Anemometry Research Articles

Reynolds Number Invariance of the Structure Inclination Angle in Wall Turbulence

Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.

Development of two-phase flow downstream of a horizontal sudden expansion

Experiments were performed to characterize the development of air–oil two-phase flow downstream of horizontal sudden expansions with area ratios of 0.0625 and 0.25. The tests were performed at oil and air superficial velocities in the range 0.02–0.756 and 0.136–3.75 m/s. In addition to the axial variation of the pressure and area-averaged void fraction, time-averaged local profiles of the void fraction, mean liquid velocity and turbulence intensity were measured downstream of the expansion using hot-film anemometry. The fully developed downstream void fraction can either increase or decrease from its upstream value, and depends on the flow pattern change across the expansion. The turbulence intensity is higher in the immediate vicinity of the sudden expansion. It then decreases with axial distance and asymptotes to an approximately constant value. The phase redistributions immediately downstream of the expansion and the developing length was found to be strongly dependent on the upstream flow pattern and the sudden expansion area ratio.

Experimental and numerical studies on the starting effect on the secondary flow in a bend

A numerical and experimental modelling study was carried out in a curved tube to analyse the behaviour of unsteady flows in a bend. Based on a test bench, with no mechanical disturbances, the flow behaviour was observed using fluorescein injection. Velocity measurements were performed using hot-film anemometry. In addition, a finite volume method was used to perform three-dimensional unsteady numerical simulations. Womersley parameter values between 8 and 21 and Dean number values between 110 and 420 were used to assess the parameters affecting the flow behaviour. Secondary motions were observed, experimentally and numerically, showing the complexity of the flow patterns. The initiation and subsequent development are explained quantitatively. Based on our analysis of the starting effect, the secondary patterns were found to be highly dependent on both the initial conditions and the flow waveforms.

On the liquid turbulence energy spectra in two-phase bubbly flow in a large diameter vertical pipe

The liquid turbulence structure of air–water bubbly flow in a 200 mm diameter vertical pipe was experimentally investigated. A dual optical probe was used to measure the bubble characteristics, while the liquid turbulence was measured using hot-film anemometry. Experiments were performed at two liquid superficial velocities of 0.2 and 0.68 m/s for gas superficial velocities in the range of 0–0.18 m/s, corresponding to an area averaged void fraction up to 13.6%. In general, there is an increase in the liquid turbulence energy when the bubbles are introduced into the liquid flow. The increase in the energy mainly occurs over a range of length scales that are on the order of the bubble diameter. A suppression of the turbulence was observed close to the wall at very low void fraction flows. Initially, the suppression occurs in the low wave number range and then extends to higher wave numbers as the suppression is increased.

Stall in Yawed Flow Conditions: A Correlation of Blade Element Momentum Predictions With Experiments

Yawed flow conditions introduce unsteady loads in a wind turbine that affect generated power quality and fatigue life. An unsteady phenomenon of special concern is dynamic stall, due to the significant load fluctuations associated with it. Although the assumptions underlying blade element momentum (BEM) models are totally inadequate in yawed flow conditions, these models, modified with engineering models, are still widely used in industry. It is therefore relevant to assess the capabilities of BEM models in predicting the location of dynamic stall on the blade for a rotor in yawed flow conditions. A rotor model is placed in an open jet wind tunnel and tested in yawed flow conditions. The locations of dynamic stall on the blade of a rotor model as a function of the blade position are observed. Two experimental techniques are used; tufts glued to the blade and hot-film anemometry in the near wake. The results from the two techniques are compared and possible causes for differences are identified. Furthermore, the rotor model in yaw is modeled with a simple BEM model, utilizing a Gormont dynamic stall model. The regions of dynamic stall on the blades predicted by the BEM model are compared with the experimental results. The BEM model seems capable of a crude prediction of the dynamic stall locations found for the rotor model in yawed flow conditions.

2616 熱膜流量計を有するY字型バルブレスマイクロポンプの開発(S44-2 マイクロ・ナノ流れの実験・計測,S44 マイクロ・ナノフルイディクス)

2616 熱膜流量計を有するY字型バルブレスマイクロポンプの開発(S44-2 マイクロ・ナノ流れの実験・計測,S44 マイクロ・ナノフルイディクス)

Flow Visualization and Characteristics of a Coaxial Jet with a Tabbed Annular Nozzle

Flow visualization and measurements of mean and fluctuating velocities were performed on a coaxial jet with a velocity ratio of 0.6 at a Reynolds number of 3 000 in an open water tank using hot-film anemometry, particle image velocimetry (2D and stereoscopic PIV) and laser-induced fluorescence (LIF). Axisymmetric and streamwise vortical structures were revealed in the near-field of the coaxial jet. The annular nozzle has six vortex generators in order to enhance the streamwise vortices generated in the mixing layer. Furthermore, the annular jet was excited by a shaker in order to enhance the axisymmetric vortices. For the tabbed coaxial jet, jet spreading downstream was greater than for the jet without tabs. The cause of the entrainment increment is the development of axisymmetric and streamwise vortex structures. In the case of excited jets, significant axisymmetric and streamwise vortical structures develop, and the jet width expands from the exit nozzle. Consequently, the flow rate of the excited jet with tabs is larger than that of the unexcited jet without tabs.

Turbulent bubbly flow

The effect of bubbles on fully developed turbulent flow is investigated numerically and experimentally, summarizing the results of our previous papers (Mazzitelli et al., 2003, Physics of Fluids 15, L5. and Journal of Fluid Mechanics 488, 283; Rensen, J. et al. 2005, Journal of Fluid Mechanics 538, 153). On the numerical side, we simulate Navier–Stokes turbulence with a Taylor–Reynolds number of Re λ≈60, a large large-scale forcing, and periodic boundary conditions. The point-like bubbles follow their Lagrangian paths and act as point forces on the flow. As a consequence, the spectral slope is less steep as compared to the Kolmogorov case. The slope decrease is identified as a lift force effect. On the experimental side, we do hot-film anemometry in a turbulent water channel with Re λ ≈ 200 in which we have injected small bubbles up to a volume percentage of 3%. Here the challenge is to disentangle the bubble spikes from the hot-film velocity signal. To achieve this goal, we have developed a pattern recognition scheme. Furthermore, we injected microbubbles up to a volume percentage of 0.3%. Both in the counter flowing situation with small bubbles and in the co-flow situation with microbubbles, we obtain a less spectral slope, in agreement with the numerical result.

The effect of bubbles on developed turbulence

Hot-film anemometry measurements are performed in a fully developed turbulent bubbly flow. For the bubble detection in the signal, both a threshold method and a new pattern recognition algorithm are employed. The measurements are carried out with gas fractions up to 3% and a mean water velocity of 0.20 m s$^{-1}$, corresponding to a Reynolds number of about $9\,{\times}\,10^4$. The typical bubble radius is 1?2 mm, corresponding to 10?20 Kolmogorov length scales. In this regime, a `bubblance? parameter $b$ which compares the kinetic energy originating from the rising bubbles with that of the turbulence fluctuations is smaller than 1. Probability distribution functions, structure functions (with and without the extended self-similarity (ESS) method), and spectra of the water velocity time series are calculated. Both our results for the turbulent energy spectra and the second-order structure functions show qualitative agreement with numerical results by Massitelli, Lohse & Toschi (Phys. Fluids, vol. 15 (2003), p. L5), i.e. a more pronounced energy enhancement on small scales than on large scales owing to the presence of bubbles, leading to a less steep slope in the spectrum as compared to the Kolmogorov $-5/3$ law. These results are robust, i.e. do not depend on details of the bubble detection scheme.

Data analysis for hot-film anemometry in turbulent bubbly flow

We report on a new method for the analysis of hot-film anemometry in turbulent bubbly flows based on stochastic pattern recognition summarizing our results of [J.M. Rensen, S. Luther, J. de Vries, D. Lohse, Hot-film anemometry in bubbly flow I: bubble–probe interaction, in press; S. Luther, J. Rensen, Hot-film anemometry in bubbly flow II: local phase discrimination, in press; J.M. Rensen, S. Luther, D. Lohse, Velocity structure functions in turbulent two-phase flows, J. Fluid Mech., in press]. It consists of an optimal signal decomposition using an adaptive wavelet transform and neural network based classification. We discuss the application of autoregressive models to obtain energy spectra for gapped time series.

USING EXPERIMENTAL FLOW MEASUREMENT TECHNIQUES TO IMPROVE CARDIOVASCULAR DEVICES: THE PENNSYLVANIA STATE EXPERIENCE

We discuss the role of experimental fluid mechanics in improving cardiovascular devices from the perspective of our roughly 30 year experience at The Pennsylvania State University. Our approach to experimental design, including the choice and expectations for measurement techniques, is examined. Our experience is mainly with artificial heart pumps and mechanical heart valves. We have studied these devices using flow visualization, hot film anemometry, ultrasound, laser Doppler anemometry and particle image velocimetry. Our interest has been in evaluating regions of high and low mean and turbulent stress and their relationship to thromboembolisms and hemolysis. We have also contributed to the understanding of mechanical heart valve cavitation. Comparison of in-vitro and in vivo measurements and the issue of clinical relevance are highlighted through example. Finally, in consideration of the ever increasing speed and detail available from experimentation (and computation), we discuss the future of experimental fluid mechanics as an integral design tool.

Experiments on the flow past long circular cylinders in a shear flow

This paper describes an experimental investigation of the flow past circular cylinders, with the mean flow perpendicular to the cylinder axis, at conditions that yield a strong three-dimensional behaviour. The experiments were carried out in the subcritical regime. Long cylinders with end plates were subjected to shear flow with a linear velocity profile in the spanwise direction generated by means of a curved gauze. It was concluded that spanwise cellular structures of vortex shedding emerged in the wake, more clearly for some boundary conditions than others. These structures are characterised by a portion of spanwise length, a cell, having constant shedding frequency over a time average, which implies that there were no vortex dislocations inside that cell during that time. These features were studied using flow visualisation and hot-film anemometry. Spectra of the local shedding frequency are shown, revealing the effect of the shear parameter β (=0.02 and 0.04) and aspect ratio L/D (=20.6 and 8) on the stability and geometry of the cells at several Reynolds numbers in the range of 3.13×103≤Rem≤1.25×104.

Hot-film anemometry in bubbly flow I: bubble–probe interaction

The interaction between a bubble, which is rising in a descending water flow, and a hot-film anemometer was experimentally investigated using stereoscopic high-speed imaging. The mean downward water velocity varied from 0 up to 0.15 m/s, i.e., relatively low, allowing for an extended bubble–probe interaction. Moreover, the direction of the water causes the wake of the probe to play a role before the bubble touches the probe. The equivalent bubble radii were 0.4–2.8 mm and the bubble velocities relative to the probe ranged from 0.04 to 0.38 m/s. Image processing techniques were applied to reconstruct the bubbles’ path, shape, and orientation during the interaction process. As a result, three types of interactions were found, namely penetrating, bouncing, and splitting interactions. The image sequences were compared with the corresponding time series of the hot-film anemometer. From the time series the type of interaction cannot be deduced, at least not for the analyzed flow situation. Furthermore, we demonstrated that the residence time estimate from the hot-film data is systematically biased in our type of experiments. Finally, it was found that the velocity of a bubble may be altered considerably due to the interaction.

A Numerical Investigation of Heat Transfer Cardiac Output Measurements

Measurement of cardiac output is often investigated using a technique based on hot‐film anemometry. Here, we discuss a modification to hot‐film anemometry, which involves a cylindrical heating element mounted flush on the surface of a typical Swan‐Ganz catheter. In contrast to traditional thermodilution, the method discussed here has the potential to allow continuous monitoring of cardiac output.This paper demonstrates that there is a simple approximate relationship between the power input to the device to maintain a temperature of one degree above blood heat and cardiac output. Since, the heat transfer and the fluid flow decouple, a numerical model of the heat transfer of a cylindrical catheter (with heating element) sitting concentrically within a rigid cylindrical artery is developed. Numerical results were obtained for a wide selection of flow profiles, including experimental data. The results indicate that the cardiac output/power input relationship is extremely robust with respect to flow profile and system parameter variation.

Local modification of benthic flow environments by suspension-feeding stream insects.

Larval black flies often exhibit spatially aggregated distributions, and individuals within patches can potentially reduce the supply of suspended food particles to downstream neighbors by modifying local flow characteristics. We used hot-film anemometry to quantify the magnitude and spatial extent of flow modifications downstream from feeding Simulium vittatum larvae in a laboratory flume, and to determine whether temporal patterns of flow variation are related to movements of the larval feeding appendages. Mean velocity 1 mm downstream from feeding larvae was reduced by 75%, and the percent reduction in velocity diminished asymptotically with downstream distance. Reduced velocities were evident as much as 60 mm downstream from, and 3 mm to either side of, larvae. Turbulence intensity (i.e., the SD of the velocity time series) was generally higher in this region relative to control flow conditions. Three results demonstrate the major contribution of the larval feeding appendages (i.e., labral fans) to such flow modification. First, there was a minimal reduction in mean velocity 5 mm downstream from non-feeding larvae (i.e., with closed labral fans), whereas mean velocity at the same location was reduced markedly when larvae were feeding. Second, the power spectrum of the velocity time series exhibited greatest power at frequencies that corresponded to the frequency of labral fan motions. Third, fan flick times accounted for most of the variance in the velocity power spectrum. The large local flow modifications that we documented have potentially important consequences for the feeding performance and growth of individuals located within larval aggregations, and are likely to influence behavioral interactions and spacing patterns.

Direct measurements of bed stress under swash in the field

Utilizing flush mounted hot film anemometry, the bed stress under swash was measured directly in a field experiment conducted on Barret Beach, Fire Island, New York. The theory, development, and calibration of the instrument package are discussed, and results from the field experiment are presented. Examples of bed stress time series throughout a swash cycle are presented, and an ensemble averaged swash bed stress cycle is calculated. Strong asymmetry is observed between the uprush and backwash phases of the swash flow. The maximum bed shear stress exerted by the uprush is approximately double that of the backwash, while the duration of the backwash is 135% greater than that of the uprush. Friction coefficients in the swash zone are observed to be similar in magnitude to those from steady flow, with the mean observed friction coefficient equal to 0.0037. Swash friction coefficients derived from the current measurements exhibit a Reynolds number dependence similar to that observed for other flows. A systematic difference between coefficients for uprush and backwash is suggested.

SHEAR STRESS PARTITIONING FOR IDEALIZED VEGETATED SURFACES

Vegetation and other surface roughness materials partition the shear force of flowing water into a portion actingon the vegetation (vegetal shear) and the remainder acting on the intervening soil surface (particle shear). The fraction actingon the soil surface is directly involved in subsequent particle detachment. The purpose of this study was to directly measurethe components of shear stress and to quantify the shear partition for various densities of idealized elements representativeof non-submerged rigid vegetation in overland flow. Insight into the magnitude of particle shear and vegetal shear isnecessary for understanding the role of vegetation in reducing particle shear and, consequently, reducing potential erosion.Circular cylinders and idealized elements with differences in the rate of change in upstream frontal area with flow depth wereused to model vegetation. Detailed spatial and temporal particle shear measurements were made using a unique hydraulicflume and hot-film anemometry. Drag force was measured on individual elements within test arrays. This combination ofmeasurements allowed for direct determination of the shear partition. The tests were conducted on three uniform elementdensities at discharges of 0.005 and 0.01 m3/s. Element width-to-spacing ratios ranged from 0.04 to 0.20. Over the rangeof densities studied, particle shear accounted for 13% to 89% of the total shear, indicating that complete surface coverageis not required to significantly reduce the shear stress acting on soil particles. Existing shear partitioning theory, in whichthe partition is a function of the ratio of element to surface drag coefficients and the roughness density, was found to representthe observed partition reasonably well (mean squared error = 0.036). The results from this study are important for selectingappropriate plant species and densities for erosion control systems.

Hot-film measurements in a two-layer system of a magnetic liquid and a siliconeoil

Constant-temperature hot-film anemometry measurements in a two-layer system of asilicone fluid layer over a magnetic fluid layer has been investigated. Complicating factorspotentially affecting the measurements were very slow velocities, the unusually highthermal conductivity of the magnetic liquid, the vicinity of solid boundaries, the presenceof the interface, and the possibility of magneto-convection in addition to naturalconvection. All these factors required a calibration procedure combining both experimentalcalibration and a set of numerical models of the system.The findings obtained during the calibration were that neither natural convection normagneto-convection were adversely affecting the measurements, probably due to theeffects of the solid boundaries and the interface. It was furthermore observedthat the fluid interface resulted in an effect analogous to the near-wall effect,but deformations of the interface due to the sensor or its mounting preventedmeasurements close to the boundary. Spatial variation of the fluid velocities affected thecalibration of the sensor noticeably. While a simple calibration, either by movingthe probe or by assuming a uniform velocity, can give a qualitatively reasonablevelocity profile, it will quantitatively deviate significantly from the true velocities.

Turbulent Flow: Analysis, Measurement, and Prediction

Preface. Acknowledgments. 1. Preliminaries. 2. Overview of Turbulent Flow Physics and Equations. 3. Experimental and Numerical Methods. 4. Properties of Bounded Turbulent Flows. 5. Properties of Turbulent Free Shear Flows. 6. Turbulent Transport. 7. Theory of Idealized Turbulent Flows. 8. Turbulence Modeling. 9. Applications of Turbulence Modeling. 10. Large Eddy Simulations. 11. Analysis of Turbulent Scalar Fields. 12. Turbulence Theory. Author Index. Subject Index.

3축 HOT-FILM 풍속계에 의한 연장된 앞전을 갖는 삼각날개 속도장의 측정

연장된 앞전을 갖는 삼각날개 위에서 시위방향의 여러 위치에서 3축 hot 필름 풍속계를 사용하여 속도장 데이터를 획득하였다. 속도장 데이터는 받음각 24도와 32도에 대하여 중심 시위선을 기준으로 한 레이놀즈수 <TEX>$1.76{\times}10^6$</TEX>에서 날개면에 수직한 유동단면에서 측정하였다. 3축 hot 필름 프로브에 의한 세 가지 속도 성분을 측정함으로써 연장된 앞전을 갖는 삼각날개에서의 와류장을 정량적으로 분석할 수 있었다. 날개 와류와 LEX 와류는 평균 축방향 속도가 국소 최고 속도를 갖는 영역에 존재하였다. 아울러, 앞전 근처의 날개면 위에서 주와류와 반대 방향으로 회전하는 이차와류의 생성을 관찰할 수 있었다. 측정 프로브의 유동장에의 삽입은 와류 중심 위치에 크게 영향을 주지 않음을 알 수 있었다. Velocity data were acquired at a series of stations in the chordwise direction above a delta wing with leading edge extension, using a triple axis hot film anemometry. Surveys normal to planform yield velocity field data at incidence angle of 24<TEX>$^{\circ}$</TEX>and 32<TEX>$^{\circ}$</TEX>at a centerline chord Reynolds number of <TEX>$1.76{\times}10^6$</TEX>. Experimental results of velocity measurements of mean velocity of three components gave a confidence to quantitative investigate the vortical flow field over a LEX-delta wing with this probe. The present experiments indicated the existence of both wing and LEX vortex where the local mean axial velocity is maximum. It also shown the development of secondary vortex of opposite sign of rotating above the wing surface near the leading edge. The insertion of probe across the flow field was found to have little influence on the position of the vortex core.