Single Hot-wire Probe Research Articles

Reynolds number dependence of azimuthal and streamwise pipe flow structures

This paper revisits the study by Baileyet al.(J. Fluid Mech., vol. 615, 2008, pp. 121–138), adopting a higher-fidelity calibration approach to reveal subtle flow variations with Reynolds numbers that were not discernible previously. The paper aims therefore to provide insights into the characteristics of azimuthal and streamwise pipe flow structures adopting two-point joint statistics and spectral analysis for shear Reynolds numbers in the range$2\times {10^3}\le {Re_\tau }\le {16\times {10^3}}$, where${Re_\tau }$is based on the wall friction velocity$u_{\tau }$, the pipe radius$R$, and the fluid kinematic viscosity$\nu$. The streamwise velocity fluctuations were measured at four wall-normal locations,$0.1\le {x_{{2}}/R}\le {0.7}$, covering the logarithmic and core regions of fully developed turbulent pipe flow based on 35–41 azimuthal probe separations using, simultaneously, two single hot-wire probes. A uniquein situcalibration approach for both probes was adopted where a potential flow was insured, resulting in consistent and precise pipe flow data. The azimuthal velocity correlation, the cross-power spectral density and the coherence function of the streamwise velocity fluctuations are discussed, revealing a clear dependence of the azimuthal scales of the large and very large flow motions on the wall-normal location, the azimuthal separation, the streamwise wavenumber and the Reynolds number. Along the logarithmic region, a linear growth of the azimuthal scales of the large- and very-large-scale structures was observed; however, they do scale nonlinearly and reach their maximum sizes in the core region, i.e. near the centreline of the pipe. Additionally, the streamwise very-large- and large-scale motions were evaluated using the premultiplied energy spectra, showing wavelengths${\approx }{18R}$and${\approx }{3R}$for${Re_{\tau }}\approx {16\times {10^3}}$at half of the pipe radius, respectively.

Reconstruction of the deterministic turbulent boundary layer for the study of aerofoil self-noise mechanisms

In the experimental aeroacoustics, it is always a challenge to study the far-field radiation and near field hydrodynamics simultaneously, and be able to firmly establish the causality between them. The main objective of this paper is to present an experimental technique that can exploit the deterministic turbulent boundary layer generated under a base flow of either mildly separated or laminar boundary layer to either disrupt an existing acoustic scattering mechanism, or reconstruct a new acoustic scattering scenario to enable the ensemble-averaging and wavelet analysis to study the aerofoil trailing edge noise source mechanisms in the spatial, temporal and frequency domains. One of the main attractions of this technique is that the experimental tool does not need to be extremely high fidelity as a priori in order to fully capture the pseudo time-resolved boundary layer instability or turbulent structures. In one of the case studies presented here, roll-up vortices of the order of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} kHz can be captured accurately by a 15-Hz PIV. A single hot-wire probe is also demonstrated to be capable of reconstructing the turbulent/coherent structures in a spatio−temporal domain. The proposed experimental technique can be extended to other self-noise scenarios when the aerofoil trailing edge is subjected to different flow control treatments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present.Graphical abstract

Numerical Investigation of Unsteady Flow and Aerodynamic Noise Characteristics of an Automotive Axial Cooling Fan

Low-speed axial cooling fans are frequently used to manage engine temperature by ensuring that adequate quantities of air pass through heat exchangers, even at low vehicle speeds or in the idle condition. This study aims to provide a better understanding of the unsteady flow behavior around an automotive axial cooling fan with seven blades and its impact on the aerodynamic noise generation. Large Eddy Simulation (LES) near the near-field region and the Ffowcs-Williams and Hawkinbygs (FW-H) method were performed to analyze the flow characteristics around the fan and predict the aerodynamic noise emitted from the fan under a constant rotational speed of 2100 rpm. The simulation results for the velocity distributions and aerodynamic noise were compared with the experimental data measured by single hot-wire probe and in a dead-sound room. The results showed a comparatively good agreement upstream and downstream from the fan and at two different receivers of 0.5 m and 1.0 m. When the fan was rotating, a strong tonal noise numerically existed near the leading edge of the blades at the tip and amounted to 110 dB sound pressure level (SPL) caused by the increasing angles of attack with the increasing radial velocity near the ring, which caused the entire air foil to emit a low-frequency noise. Furthermore, the different SPL decay characteristics of approximately 5 dB in the near-field region and 6 dB in the far-field region were observed each time the distance from the fan doubles. The findings of this research can provide important insights into the design of axial fans with low noise and high performance.

Experimental study of the aerodynamic perturbations induced by the single normal hot-wire probe positioned normally in the vicinity of a wall

The conducted research, though still at the preliminary stage, is a step toward a better understanding of the flow phenomena accompanying hot-wire probes. For the flow in the vicinity of a wall, a potentially non-optimal case was analyzed – a single normal hot-wire probe oriented normally to the surface. According to the results, undisturbed flow, as measured in the absence of a hot-wire probe, was significantly altered in its presence. Differences in the vertical profiles of the velocity magnitude in the upstream direction of up to 40% were observed 0.43 mm from the probe’s longitudinal axis of symmetry, and up to 8% at a distance of 3.73 mm. The lower Reynolds numbers were considered the more complex velocity profile perturbations were present. The highest differences in undisturbed vs. disturbed flow were observed at the level of the wire and below, regardless of the distance from the hot-wire probe.

External intermittency compensation of dissipation scale distributions in a turbulent boundary layer

We investigate the effect of external intermittency on dissipation scales in turbulent boundary layer flow at $R\phantom{\rule{0}{0ex}}{e}_{\ensuremath{\tau}}\ensuremath{\approx}1000$ using a single hot wire probe for one point longitudinal velocity measurements. We find improved scaling universality by accounting for external intermittency throughout the flow.

Phase-averaged flow statistics in compressors using a rotated hot-wire technique

A technique based on a rotated hot wire has been developed to characterise the unsteady, three-dimensional flow field between compressor blade rows. Data are acquired from a slanted hot wire rotated through a number of orientations at each measurement point. Phase-averaged velocity statistics are obtained by solving a set of sensor response equations using a weighted, non-linear regression algorithm. The accuracy and robustness of the method were verified a priori by conducting a series of tests using synthetic data. The method is demonstrated by acquiring a full set of phase-averaged flow statistics in the wake of a compressor stator blade row. The technique allows three components of phase-averaged velocity, six components of phase-averaged deterministic stress, and six components of phase-averaged Reynolds stress to be recovered using a single rotated hot-wire probe.

Combining time-resolved multi-point and spatially-resolved measurements for the recovering of very-large-scale motions in high Reynolds number turbulent boundary layer

A methodology to recover very-large-scale motions based on the combination of time-resolved multi-point measurements and spatially-resolved full field information is here presented and applied to a high Reynolds number turbulent boundary layer flow developing over a flat plate. These very-large-scale motions are characterised by a large degree of persistence, both in time and space, and are believed to play an active role in the development of turbulence in the near-wall region (Bandyopadhyay and Hussain, 1984; Chung and McKeon, 2010; Hutchins and Marusic, 2007; Marusic et al., 2010). Due to the very large streamwise extent of these structures and of their meandering nature, point-measurement techniques or low time-resolved full field techniques such as Particle Image Velocimetry (PIV) offer a limited view when used alone. In the present paper, a time-resolved estimate of the full velocity vector field in a spanswise/wall-normal plane is obtained by combining low-repetition two-dimensional stereo-PIV measurements and a two-dimensional rake of 143 single hot-wire probes sampled at 30kHz. The latter is used to animate the PIV data at a high sampling rate by implementing an extended version of the complementary technique (Bonnet et al., 1994) which combines Proper Orthogonal Decomposition (POD) and multi-time Linear Stochastic Estimation (mLSE) (Kerhervé, 2012). The results show the potential of the methodology to extract large-scale motions in turbulent boundary layer. This paves the way for further advanced time-resolved description of the spatially developing very-large-scale structures populating the logarithmic region. In addition, the main characteristics of the structures observed are found to be well consistent with that described in literature, validating the methodology and the time-resolved data-set built.

Uncertainty Quantification of Turbulence Measurements in a Fully Pulsed Axisymmetric Subsonic Air Jet

This paper reports the statistical accuracy evaluation of the implementation of ensemble-averaged equation used for the measurement of turbulence quantities in a fully pulsed, axisymmetric-subsonic air jet issuing into the still surrounding air. A constant mass flow rate and frequency of pulsation were used to generate the intermittent free jet, which physically contained a period of no flow between pulses due to a mechanical excitation. Time-history records of the random process were gained from a single normal hot-wire probe by a digital sampling technique, where the probe was operated in a constant temperature mode and placed at a fixed location in the jet field with the probe stems parallel to the mean flow direction. Within the framework of amplitude-domain analysis, the accuracy assessment was performed to which the uncertainties of mean value and higher-order moments were evaluated from the given values of sampling parameters, where the estimate of integral-time scale of the random process was the basis for determining the given sampling parameters. The present results suggest appropriate values of number of samples, sampling frequency, and sampling time of the hot-wire anemometer (HWA) technique for a repeatable turbulence measurement in the unsteady jet within a high degree of accuracy.

Combined hot-wire and PIV measurements of a swirling turbulent flow at the exit of a 90° pipe bend

Measurements of turbulent swirling flow by means of hot-wire anemometry and stereoscopic particle image velocimetry were performed, 0.67 pipe diameters downstream a 90° pipe bend. The flow for a wide range of swirl numbers up to $$S=1.2$$ , based on the angular velocity of the pipe wall and bulk velocity, was investigated and compared to the non-swirling case. The limitations and advantages of using a single hot-wire probe in a highly complex flow field are investigated and discussed. The present paper makes available a unique database for a kind of flow that has been neglected in literature and which is believed to be useful for validation purposes for the computational fluid dynamics community.

Evaluation of hot-wire spatial filtering corrections for wall turbulence and correction for end-conduction effects

Several recent spatial filtering corrections for Reynolds stress measured by single component hot-wire probes were assessed using turbulent channel flow data measured over a moderate Reynolds number range. Using measurements with a variety of hot-wire lengths and aspect ratios, the current work determines the impact of these corrections on the actual magnitude and Reynolds number dependence of the near-wall turbulent peak in Reynolds stress and compares it to results from prior direct numerical simulations of turbulent channel flow. Comparison of the results following application of previously published correction schemes were found to produce similar results, with some limitations observed for each technique. Comparison to direct numerical simulation results suggested that additional corrections were needed to correct for end conduction effects. An additional modification for these effects was devised which improved agreement between probes of different lengths and aspect ratios and improved agreement between the measured and direct numerical simulation results .

0514 正方形噴流に対する周期的撹乱の影響

The effect of periodic disturbances introduced with oscillating plates has been investigated on a jet issuing from a rectangular nozzle. The oscillating plate consists of a laminated piezoelectric element and stainless plate. The experiment was performed at the outlet velocity of U_0=5(m/s). The cross sectional area of the rectangular nozzle has a size of 10mm×10mm. Velocity components were measured with single hot-wire probe and CTA. Oscillating frequency was set at 1/4, 1/3, 1/2 and 1 times of a natural occurrence frequency. From the streamwise variation of turbulent intensity on the jet centerline, disturbance was classified into three non-dimensional oscillating frequency of 0, 1/2 and 1/3. In an early stage of the jet, stable large-scale vortical structures were observed at the non-dimensional oscillating frequency of 1/2 and 1/3. The decay of the streamwise mean velocity on the centerline and growth rate of the half width increase at the non-dimensional oscillating frequency of 1/2.

Comparison of experiments and simulations for zero pressure gradient turbulent boundary layers at moderate Reynolds numbers

A detailed comparison between recent direct numerical simulation (DNS) and experiments of a turbulent boundary layer under zero pressure gradient at Re θ = 2,500 and 4,000 (based on the free-stream velocity and momentum-loss thickness) is presented. The well-resolved DNS is computed in a long spatial domain (Schlatter and Orlu in J Fluid Mech 659:116, 2010a), including the disturbance strip, while the experiments consist of single hot-wire probe and oil-film interferometry measurements. Remarkably, good agreement is obtained for integral quantities such as skin friction and shape factor, as well as mean and fluctuating streamwise velocity profiles, higher-order moments and probability density distributions. The agreement also extends to spectral/structural quantities such as the amplitude modulation of the small scales by the large-scale motion and temporal spectral maps throughout the boundary layer. Differences within the inner layer observed for statistical and spectral quantities could entirely be removed by spatially averaging the DNS to match the viscous-scaled length of the hot-wire sensor, thereby explaining observed differences solely by insufficient spatial resolution of the hot-wire sensor. For the highest Reynolds number, Re θ = 4,000, the experimental data exhibit a more pronounced secondary spectral peak in the outer region (y/δ 99 = 0.1) related to structures with length on the order of 5–7 boundary layer thicknesses, which is weaker and slightly moved towards lower temporal periods in the DNS. The cause is thought to be related to the limited spanwise box size which constrains the growth of the very large structures. In the light of the difficulty to obtain “canonical” flow conditions, both in DNS and the wind tunnel where effects such as boundary treatment, pressure gradient and turbulence tripping need to be considered, the present cross-validation of the data sets, at least for the present Re θ -range, provides important reference data for future studies and highlights the importance of taking spatial resolution effects into account when comparing experiment and DNS. For the considered flow, the present data also provide quantitative guidelines on what level of accuracy can be expected for the agreement between DNS and experiments.

Effects of Freestream Turbulence on the Characteristics in the Boundary Layer Near the Transition Onset Location

Experimental data is presented for a flat plate test facility with augmented levels of freestream turbulence (FST). The turbulence decay downstream of two turbulence generating grids, in addition to the integral length scales, is provided and good comparison with established correlations is presented. Boundary layer measurements using a single normal hotwire probe were obtained at FST intensities of 7%, 6%, 5.5%, 1.55%, and 1.45%, and the results presented include the fifth and 95th percentile of the velocity fluctuations and the root mean squared (RMS) velocity profiles near the transition onset region. The transition onset Reynolds number for each of the turbulence levels considered is consistent with theoretical findings. In all cases analyzed, the maximum fifth and 95th percentile far exceeded the maximum RMS values, with the location of the maximum 95th percentile closer to the wall compared to the maximum fifth percentile. Using probability density function (PDF) analysis, it is demonstrated that there is a dominating positive velocity fluctuation in the near-wall region and a dominating negative velocity fluctuation further out in the boundary layer and that the fluctuations in the boundary layer are greater compared to the freestream. The effect of the FST on the boundary layer is discussed with comparison to the Blasius solution and the influence of the fluctuations on the deviation from the Blasius profile is presented and discussed. Through investigation of the energy spectrum of the fluctuating velocity component within the boundary layer, it is shown that there is a higher energy content at lower frequency in the boundary layer when compared to that of the freestream.

Multi-time delay, multi-point linear stochastic estimation of a cavity shear layer velocity from wall-pressure measurements

In this paper, the recently introduced multi-time-delay linear stochastic estimation technique is thoroughly described, focusing on its fundamental properties and potentialities. In the multi-time-delay approach, the estimate of the temporal evolution of the velocity at a given location in the flow field is obtained from multiple past samples of the unconditional sources. The technique is applied to estimate the velocity in a cavity shear layer flow, based on wall-pressure measurements from multiple sensors. The cavity flow was investigated by performing simultaneous measurements of a single hot-wire probe, traversed on a fine grid in the shear layer, and of multiple wall-mounted condenser microphones in the cavity region. The paper compares classical high-order single-time-delay estimation approaches with the multi-time-delay technique, which is significantly more accurate as it produces a much lower mean-square estimation error, thus providing a faithful reconstruction of the time-evolution of the velocity field. This improved accuracy is strongly dependent on the number n of past wall-pressure samples used in the estimate. In this paper, we also demonstrate that the estimated velocity field only contains the signature of the relevant flow mechanisms which correlate well with the wall-pressure, while incoherent components are filtered out. The multi-time-delay approach successfully captures the spatio-temporal dynamics of the velocity fluctuation distribution in the shear layer, as it clearly resolves the dynamics of the relevant flow structures. The effect of the number of sensors used in the estimate was also considered. In general, it was evident that use of more sensors leads to better accuracy, but as the number n of past values increases, the gain becomes marginal.

0120 壁面粗さの急変化に対する円管内流れの応答に関する実験的研究(OS1 噴流,後流および剥離流れ現象の探求と技術革新,オーガナイズドセッション)

Response of the fully developed turbulent pipe flow has been investigated to sudden change of wall condition from smooth to rough. Experimental study of mean and turbulent quantities focuses on recovery process and distance required to reach to new equilibrium state. The inner diameter and total length of the pipe flow equipment are D=70[mm] and L=12[m] (=17 ID), respectively. Ak-type roughness composed of array of square bars is employed as rough wall and velocity measurement was made by a constant temperature anemometer and a single-hot wire probe. Derivative of wall static pressure with respect to the longitudinal direction takes the minimum value at x/D=15 and then approaches asymptotically to constant for jc/D=40〜50. The mean velocity and turbulent intensity profiles suggest that the internal boundary layer reaches to the pipe center at around x/D=15.

Effect of Inlet Geometry on Fan Performance and Flow Field in a Half-Ducted Propeller Fan

In order to clarify the effect of rotor inlet geometry of half-ducted propeller fan on performance and velocity fields at rotor outlet, the experimental investigation was carried out using a hotwire anemometer. Three types of inlet geometry were tested. The first type is the one that the rotor blade tip is fully covered by a casing. The second is that the front one-third part of blade tip is opened and the rest is covered. The third is that the front two-thirds are opened and the rest is covered. Fan test and internal flow measurement at rotor outlet were conducted about three types of inlet geometry. At the internal flow measurement, a single slant hotwire probe was used and a periodical multisampling technique was adopted to obtain the three-dimensional velocity distributions. From the results of fan test, the pressure-rise characteristic drops at high flowrate region and the stall point shifts to high flowrate region, when the opened area of blade tip increases. From the results of velocity distributions at rotor outlet, the region with high axial velocity moves to radial inwards, the circumferential velocity near blade tip becomes high, and the flow field turns to radial outward, when the opened area increases.

Unsteady flow at low flow-rate region in a semi-open propeller fan (velocity fluctuation outside of blade tip)

In order to clarify the unsteady flow fields at low flow-rate region with positive gradient on pressure — flow-rate curve, the experimental investigation was carried out at rotor inlet and outside of rotor blade tip without casing in a semi-opened propeller fan using a hot-wire anemometer. A single I-type hot-wire probe was used, and the data obtained were processed by the use of phase-locked averaging, ensemble averaging and FFT analyzing. The flow fields at rotor inlet and outside of rotor blade tip were discussed mainly using the results from distributions of velocity fluctuations and power spectrum density. It was found from these results that there are the two types of different periodical fluctuations and both of those frequencies were not the same of rotor rotating frequency (RRF; 15Hz). One was observed at relatively high flow-rate region at relatively downstream area in measurement and its frequency was approximately 7Hz (47% of RRF). The other was observed at relatively low flow-rate region at relatively upstream area in measurement and its frequency was approximately 10Hz (67% of RRF)”. As the velocity fluctuations with the flow fields are rapidly increased by the former fluctuation, it is thought that its fluctuation is the trigger of blade stall.

Parameterizable constant temperature anemometer: a new method for the analysis of velocity–temperature coupling in turbulent heat transfer

A novel anemometer was designed and implemented for simultaneous measurement of velocity and temperature in air flows with a single hot wire probe. The principle of periodically varying the overheat ratio of the wire was selected and applied through a parameterizable electronic chain. The specific methods developed for the calibration procedure and the signal processing are explained. The accuracy of the measurements was assessed by means of Monte Carlo simulations. Examples of the results are given for a non-isothermal low speed mixing layer. Finally, the ability to extract velocity–temperature coupling from a turbulent non-isothermal flow is highlighted through a conditional probability density function (PDF) analysis.

0137 2次元粗面に付加した微細な縦溝に対する乱流境界層の応答(OS1-7 噴流,後流および剥離流れ現象の解明と制御,オーガナイズドセッション)

Mean velocity and turbulent intensity measurements have been made for the turbulent boundary layer developed over two-dimensional square rib rough surface with longitudinal micro-grooves. The measurement was performed under a zero pressure gradient and momentum thickness Reynolds number of 8200. The velocity components were measured with a single hot wire probe and a constant temperature anemometer. The mean velocity and turbulent intensity close to the rough surface increase in comparison with those of two-dimensional rough surface. Then, the integral length is about 1.2 times of that the two-dimensional rough surface. As the result, it is expected that an error in origin has a large value compared with that of the two-dimensional rough surface.

Flow fields with vortex in a small semi-open axial fan

In order to clarify the effect of tip clearance (TC) size on fan performance and the flow field at rotor outlet in a small semi-open axial fan, the experimental investigation was carried out. The tip diameter of test fan rotor was 180mm and test TC sizes were 1mm (TC=1mm) and 4mm (TC=4mm). Fan characteristics tests were carried out for two cases of TC size and three-dimensional velocity fields at rotor outlet were measured using a single slant hot-wire probe at four flow-rate conditions. As a result, it was found that the pressure — flow-rate characteristics curves for two cases showed almost the same tendency. However, the ensemble averaged velocity profiles along radial measurement stations of TC=4mm largely changed compared with that of TC=1mm in cases of small flow-rate condition. From the phase-locked averaging results, it was also found that the vortex existed in the rotor outlet flow field of high flow-rate condition for each TC case. Compared with the vortices for TC=1mm and TC=4mm, the vortex for TC=4mm was stronger than that for TC=1mm.