Constant Temperature Anemometry Research Articles

The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow

Cavity flows occur in a wide range of low-speed applications (Mach number ≤0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 kg/ms2, 0.44 kg/m·s2, and 0.96 kg/m·s2). The investigation was performed for an open cavity with length to depth ratio of 4 at the Reynolds number based on a cavity depth of approximately 50,000. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed in this investigation. The aim of the jet blowing is to separate the cavity separated shear layer from the recirculation zone to reduce the cavity return flow, and hence stabilise the cavity separated shear layer. It was found that increasing the jet momentum flux causes an increase in the cavity return flow due to the increase in the thickness of the cavity separated shear layer. The study also found that the jet populates the separated shear layer with a large number of small-scale disturbances. These disturbances increase the broad band level of the pressure power spectra and Reynolds shear stress in the cavity separated shear layer. On the other hand, the jet disturbances make the shedding of the large vortical structures more intermittent.

Particle image velocimetry and constant temperature anemometer measurements of the jet produced by a centrifugal fan

This article presents a detailed investigation of the three-dimensional flow field produced by a small centrifugal fan used for cooling of electronic components, for instance, in the automotive industry. A particle image velocimetry (PIV) system captured numerous cross sections of the fan jet for analysis of velocity distribution and jet development. General parameters, such as the spreading rate, and flow specifics, such as the full width at half maximum contours in planes parallel to the fan outlet and the jet's rotation rate, were thus evaluated. In addition, constant temperature anemometry enabled a thorough investigation of the flow field at the fan's outflow port. Hot-wire measurements complement the PIV results by providing the power spectral density of the turbulent kinetic energy at several locations. Our results demonstrate that the cross-flow profile at the fan outlet consists of two counter-rotating vortices. This leads to a jet with a nearly elliptical, rotating cross section, which does not propagate in a direction perpendicular to the outlet plane. Several aspects of these distinctive flow features are evaluated and presented to provide both fluid mechanics and heat transfer engineers with (i) data on the air jet produced by a small centrifugal fan and (ii) reference data for computational fluid dynamics simulations. Despite complex flow development in the near field of the jet and the nonaxisymmetric jet contour in the intermediate field, our results are in good agreement with published data on axisymmetric jets in terms of spreading rate and the development of turbulence spectra observed in the intermediate field.

Influence of length and effective stiffness of an attached flexible foil for flow over a square cylinder

The effect of length and stiffness of an attached flexible foil on the flow field of a square cylinder is studied. The intermediate Reynolds numbers are considered in the study. For flow diagnosis, particle image velocimetry, constant temperature anemometry, and flow visualization techniques are used. The increase in the length of the foil modifies the Strouhal frequency and mean drag coefficient non-monotonically, which owes to the different wake regimes. Three different stiffness ranges are selected to see its effects on the wake characteristics of the bluff body and the dynamic response of the foil. The dynamic response of the foil depends on the wake frequency and the structural natural frequency of the foil. With an increase in the flexibility of the foil, the bending mode excitation increases, and the fluid–structure interaction becomes complex. The lock-in between the wake frequency and the natural flapping frequency adversely affects the drag coefficient of the bluff body.

Investigation of the atmospheric surface layer using a novel high-resolution sensor array

Representing land-atmosphere exchange processes as lower boundary conditions remains a challenge in numerical weather predictions. One important reason is the lack of understanding of heterogeneities in topography, land cover, stability, and their effects on all aspects of the flow field and scalar transport. Well-resolved flow measurements can shed light on these near-surface processes, yielding improved modeling approaches. Yet, it is precisely the heterogeneous characteristics in question—along with the large separation of scales—that make field measurements notoriously challenging. To address some of the difficulties encountered in probing the atmospheric surface layer, a unique and economically scalable field measurement platform was designed around the nanoscale thermal anemometry probe technology, which has previously been used successfully at high Reynolds numbers in laboratory settings. The small size of the nanoscale sensors not only provides a high spatial resolution but also allows for velocity and temperature measurements with the same constant current operating circuit. This operating mode is more economical and straightforward to construct than conventional constant temperature anemometry systems, providing a scalable platform for multi-point measurements. The measurement platform was deployed at the Surface Layer Turbulence and Environmental Science Test site in Utah’s West Desert as part of the Idealised horizontal Planar Array study for Quantifying Surface heterogeneity. Streamwise velocity and temperature data were acquired within the first meter above ground with good agreement in spectral behavior to well-known scaling laws in wall-bounded flows.

Spatial and temporal resolution of a fast-response aerodynamic pressure probe in grid-generated turbulence

The spatial and temporal resolution of a fast-response aerodynamic pressure probe (FRAP) is investigated in a benchmark flow of grid-generated turbulence. A grid with a mesh size of M=6.4 mm is tested for two different free-stream velocities, hence, resulting in Reynolds numbers of Re_M= {4300,12800}. A thorough analysis of the applicability of the underlying assumptions with regard to turbulence isotropy and homogeneity is carried out. Taylor’s frozen turbulence hypothesis is assumed for the calculation of deducible flow quantities, like the turbulent kinetic energy or the dissipation rate. Furthermore, besides the examination of statistical quantities, velocity spectra of measurements downstream of the grid are quantified. Results of a small fast-response five-hole pressure probe equipped with piezo-resistive differential pressure sensors are compared to single-wire hot-wire constant temperature anemometry data for two different wire lengths. Estimates of temporal and spatial turbulent scales (e.g., Taylor micro scale and Kolmogorov length scale) show good agreement to data in the literature but are affected by filtering effects. Especially in the energy spectra, very high bandwidth content cannot be resolved by the FRAP, which is mainly due to bandwidth limits in the temporal calibration of the FRAP and the minimal resolution of the integrated sensors.Graphic abstract

Numerical and Experimental Analysis of Flow Pattern, Pressure Drop and Collection Efficiency in a Cyclone with a Square Inlet and Different Dimensions of a Vortex Finder

The paper presents results of numerical simulations and experimental investigations of a cyclone with a square inlet and different dimensions of a vortex finder. Investigations were conducted for five, different cyclone models. The main goal of the research was to determine the influence of cyclone vortex finder geometry (diameter, length) on key parameters for a cyclone operation, such as: gas flow pattern, gas velocity and pressure distribution, pressure drop and collection efficiency. The analysis of flow pattern inside the cyclone was carried out with the use of CFD (Computational Fluid Dynamics) simulations, verified using CTA (Constant Temperature Anemometry). Similarly, pressure distributions, pressure drop and collection efficiency for cyclones were determined numerically and with measurements. The study demonstrated that dimensions of the vortex finder have a significant effect on a cyclone performance. The numerical visualisations of flow showed some unfavorable and beneficial effects and phenomena that may occur in cyclones. Moreover, the smallest pressure drop (305 Pa) was predicted for the cyclone with a maximum diameter of the vortex finder (De = 0.105 m), the largest (358 Pa) when this diameter was the smallest (De = 0.075 m). The tests did not show any significant influence of the vortex finder length on the pressure drop in a cyclone. A different tendency was observed in the case of collection efficiency. The maximum value of this parameter (89.5%) was predicted for the cyclone with the shortest vortex finder (s = 0.060 m), minimum (85.5%) when the vortex finder was the longest (s = 0.220 m). In this case, however, tests did not show a significant impact of an outlet diameter on the collection efficiency. Results and conclusions presented in this paper can be practically used for design optimization of cyclone separators, and also to select their operating parameters.

Investigation into the turbulence statistics of installed jets using hot-wire anemometry

This work presents a detailed study of the turbulence flow statistics of a jet mounted with its axis parallel to a rigid flat plate. Hot-wire constant temperature anemometry has been used to measure the single-point and two-point statistics of the axial velocity component at several locations within the jet flow field. Results show that the jet mean flow near the plate surface is subjected to a local acceleration and redirection due to a Coandă-type effect. The propagation of these effects downstream of the plate trailing edge is strongly dependent on the plate position. Regarding the velocity fluctuations, the mean turbulence intensity levels are seen to decrease as the radial distance between the jet and surface decreases. Analysis of the single-point power spectral density data on the shear layer close to the plate shows that the reduction in magnitude of the low-frequency content of the energy spectrum is responsible for the decrease in turbulence intensity. Additionally, the characteristic time and length scales computed from two-point measurements reduce as the plate is mounted closer to the jet centre-line. The axial eddy convection velocity is seen to increase in the region of high turbulent kinetic energy in the shear layer adjacent to the surface. Empirical models for turbulence characteristic scales and eddy convection velocity are presented. These findings suggest that both the amplitude and distribution of the jet mixing noise sources are affected when closely installed next to a surface. This paper is a continuation of a recent investigation on the turbulence statistics of isolated jets presented in Proença (Exp Fluids 60(4):63, 2019).Graphic abstract

Effect of the Oscillator Length on the Characteristics of a Feedback Type Fluidic Oscillator

This work characterizes the various length fluidic oscillators with constant depth and having the same main geometric dimensions. A change in fluidic oscillator's length affects the frequency and sweeping characteristics of the fluidic oscillators. These characteristics were extracted by means of constant temperature anemometry hot-wire measurements and water flow visualizations. A total number of ten fluidic oscillators were compared to the baseline fluidic oscillator design. It was observed that shortening the fluidic oscillator ceases the oscillations after a threshold oscillator length whereas elongating the fluidic oscillator decreases the frequency and sweep angle gradually. Furthermore, the frequency maps obtained from hot-wire measurements for each considered fluidic oscillator design provided overall detail about the frequency of the oscillations for a wide range of flow rate. For a constant supply flow rate where the frequency and the sweep angle of an oscillator is constant, changing the length of a fluidic oscillator will allow varying these characteristics of the oscillator. Thus the oscillator can be tailored for a specific need of an application.

Development of scale-up enabling technique using constant temperature anemometry for turbulence measurement in flotation cells

The success of flotation scale-up depends on understanding of the hydrodynamic condition of flotation cells at different scales and design specifications. In order to achieve this, it is very important to measure hydrodynamic parameters such as fluid velocity, turbulent kinetic energy (TKE) and turbulent kinetic energy dissipation rate (TKEDR) in flotation cells. There are many methods that are available to measure velocity and velocity fluctuations in turbulent fluids, especially for single-phase flows or two-phase flows, however very few of them can be used for flotation cells. In this study, constant temperature anemometry (CTA) and power measurement methods were used to measure the parameters in two scales of flotation cells. Acceptable correlation between the CTA measurements and the TKEDR has been found under different hydrodynamic conditions in both cells. It shows that this measurement technique enables the scale-up analysis for turbulence in flotation cells.

Structure turbulent flow behind a square cylinder with an angle of incidence

This article shows the results of a study of the structure of turbulent flow behind a square profile ALUTEC 45×45mm with T-slots. The angle of rotation of the profile relative to its axis varied α=0°, 15°, 30° and 45°. During the experiment, the flow velocity was 5m⋅s−1, Reynolds number was 7.7⋅104. The Constant Temperature Anemometry technique was used for experimental studies. To avoid backflow, the measuring plane was positioned at the rear of the profile at a distance of x⋅d−1≈2.2. As a result of the studies, it was found that the highest Taylor microscale Reynolds number and standard deviation for turbulent flow was observed in the area behind the cylinder. The width of this area is 3.5 times the width of the cylinder. With the distance from the center of the cylinder in the spanwise direction to flow the Taylor microscale Reynolds number and standard deviation sharply decreases. The maximum values of the Taylor microscale Reynolds number are observed at α=45° is 426 and 398. It has also been found that behind the cylinder there is some area in which the some parameters of the turbulent flow vary greatly with the change angle α. The lowest energy dissipation rate in this range is observed for α=15°−68m2⋅s−3 and the largest for α=0°−138m2⋅s−3. We also found that the minimum value of the Kolmogorov scale and the Kolmogorov time is observed at α=0°. The minimum values of the Kolmogorov scale η=71μm and the Kolmogorov time τη=0.33ms. The maximum values for previous parameters observed at α=15° is η=84μm and τη=0.47ms. We also found that the flow rate and standard deviation distributions between the ALUTEC profile and the ordinary square cylinder different. This can be observed when the upper part of the profile is tightly closed and the lower part is open. In this case, along with the profile inside of the T-slot, there is a generation of internal flow. This reduces the total backflow area behind the ALUTEC profile by 20% compared to an ordinary square profile.

Investigation of Flow Around and in Wake of a Heated Circular Cylinder at Moderate Reynolds Numbers

Abstract The flow around a heated circular cylinder and the wake behind it were studied in wind tunnel flow using two methods of anemometry, i.e., particle image velocimetry (PIV) and constant temperature anemometry (CTA) with a special technique using a rotating slanted hot-wire probe. The Reynolds number ranged from 2000 to 20,000 and the cylinder wall temperatures varied between 27 °C and 177 °C. The wake is characterized by mean wind and fluctuation contour maps. Significant changes in wake patterns were observed while the cylinder was being heated, thus increasing its wall temperatures at low Reynolds numbers. At higher Reynolds numbers, the effects of cylinder heating on wake properties were negligible. The research fills a gap observed in the literature for a certain combination of velocity, cylinder aspect ratio, and cylinder surface temperature.

Identification of structures and mechanisms in a flow field by POD analysis for input data obtained from visualization and PIV

This paper investigates the application of proper orthogonal decomposition (POD) for data obtained from visualizations. Using the POD method, the flow field behind one and two cylinders in a staggered configuration was analyzed. The data processed by this method were obtained from experimental measurements of flow fields using the particle image velocimetry (PIV) method and visualization. The dominant frequencies of the flow pattern from these data were compared using constant temperature anemometry (CTA) measurements. Attention was mainly focused on the flow at three Reynolds numbers: 500, 1200, and 2500. Velocity and vortex fields were created from PIV measurements in the wind tunnel for Re = 500, and video images of flow fields were obtained from dye visualizations in the hydrodynamic tunnel. The components of velocity, vorticity (both of PIV), and change in grayscale (from visualization) were used as input data for POD analysis. A methodology for data processing from visualizations was developed for subsequent analysis using the POD method. A new technique has been found to identify structures in the wake of the cylinders in a staggered configuration by analyzing POD based on various types of input data. The measured fields of dominant frequencies from the CTA and a thorough analysis of the POD modes and their relative energy values for each type of data made it possible to identify the structures and mechanisms that occur in the wake of cylinders. This analysis facilitated a better understanding of the importance of these structures and mechanisms, which can then be used to control the flow behind the cylinders.Graphic abstract

A Comparison of Turbulence Levels From Particle Image Velocimetry and Constant Temperature Anemometry Downstream of a Low-Pressure Turbine Cascade at High-Speed Flow Conditions

AbstractThe development and verification of new turbulence models for Reynolds-averaged Navier–Stokes (RANS) equation-based numerical methods require reliable experimental data with a deep understanding of the underlying turbulence mechanisms. High accurate turbulence measurements are normally limited to simplified test cases under optimal experimental conditions. This work presents comprehensive three-dimensional data of turbulent flow quantities, comparing advanced constant temperature anemometry (CTA) and stereoscopic particle image velocimetry (PIV) methods under realistic test conditions. The experiments are conducted downstream of a linear, low-pressure turbine cascade at engine relevant high-speed operating conditions. The special combination of high subsonic Mach and low Reynolds number results in a low density test environment, challenging for all applied measurement techniques. Detailed discussions about influences affecting the measured result for each specific measuring technique are given. The presented time mean fields as well as total turbulence data demonstrate with an average deviation of ΔTu<0.4% and ΔC/Cref<0.9% an extraordinary good agreement between the results from the triple sensor hot-wire probe and the 2D3C-PIV setup. Most differences between PIV and CTA can be explained by the finite probe size and individual geometry.

Experimental evaluation of cylinder vortex shedding noise reduction using porous material

The tonal noise generation of a circular cylinder in a uniform flow is an important source of aerodynamic noise. It can be found at parts of the landing gear of airplanes, at pantographs of trains, at antennas and basically all other protruding parts of vehicles. This noise is due to the periodic shedding of vortices along the cylinder span. One method to reduce this noise is the use of flow permeable covers around the cylinders. In the present study, measurements were performed in an aeroacoustic wind tunnel on a large set of porous covered cylinders. In addition to varying the porous material, which is characterized by its air flow resistivity and its porosity, the thickness of the porous layer was varied as well. The measurements were performed at Reynolds numbers between 14,000 and 103,000 using microphones located in the acoustic far field. It was found that the porous covers lead to a notable narrowing of the vortex shedding tonal peak in the sound pressure level spectra, an effect that increases with increasing porosity and thickness and decreasing air flow resistivity of the porous layer. Based on the large set of experimental data, basic trends were derived for the estimation of the vortex shedding Strouhal number and the reduction in the energy in the vortex shedding peak using the method of linear regression. Constant temperature anemometry measurements in the wake of selected cylinders basically showed a similar narrowing of the vortex shedding peak in the spectra of the turbulent velocity fluctuations. In addition, the measurement of wake profiles showed a reduction in the mean velocity and the turbulence in the wake as well as a widening of the wake region, while an analysis of the spanwise coherence revealed that the cause of the overall noise reduction is not a breakup of spanwise turbulent structures. Rather, the results imply that viscous damping of turbulent flow pressure amplitudes by the porous material strongly contributes to the noise reduction.Graphic abstract

Investigating channel flow using wall shear stress signals at transitional Reynolds numbers

Time-resolved wall shear stress measurements are conducted to investigate channel flow at transitional Reynolds numbers. Constant temperature anemometry (CTA) is employed to measure the instantaneous wall shear stress using glue-on hot films as the sensing probes. Pressure-drop measurements are conducted to calibrate the mean hot-film voltage signals and to ensure that the pressure drop is measured in the so-called “fully-developed” region of the channel, a study of effect of entrance length on the pressure-drop measurements is carried out. Time history and higher order statistics of wall shear stress fluctuations reveal that the flow remains laminar until Reτ(=uτh/ν)≈43 in our channel flow facility, where uτ, h and ν are the friction velocity, channel half-height and kinematic viscosity, respectively. Third and fourth order moments of wall shear stress jump at the onset of transition and increase significantly until they reach maxima at about Reτ ≈ 48. After this Reynolds number, these two higher order moments start to decrease gradually with increasing Reynolds number and after Reτ≈73−79, any significant dependence of these two moments on Reynolds number disappears. Multiple hot-film measurements, which are located at different spatial locations, are conducted to characterize the large-scale turbulent structures. It is observed that there are structures, at least 7h wide, for Reτ between 46.8 and 53.9. Two-point spatial correlations reveal that on average these large structures are angled at approximately 17o for Reτ=46.8 and roughly between 32o and 37o for 48.7 < Reτ < 53.9 relative to the streamwise direction.

Flow control over a square cylinder using attached rigid and flexible splitter plate at intermediate flow regime

A detailed flow field behind a stationary square cylinder with attached rigid and flexible splitter plates has been studied using particle image velocimetry, constant temperature anemometry, and flow visualization techniques. A wide range of lengths of the splitter plate (L/B = 0–8) are considered, and their respective wake interference is reported. The investigation is carried out at an intermediate flow regime at three Reynolds numbers 600, 1000, and 2000 (based on blocking width “B” of the cylinder). The literature seriously lacks the information on a passive flow control of bluff body wakes in this flow regime. This study shows that the wake frequency and mean drag coefficient vary nonmonotonically to splitter plate lengths. The length of the splitter plate is a critical parameter, which, apart from flow control, can also bring a significant wake transition. At L/B &amp;gt; 3 to L/B = 4, strong secondary vortices are shed from the trailing edge. The shedding of the secondary vortex leads to a sudden shrinkage in the recirculation bubble and an increase in the periodicity of the unsteady flow. The onset of high amplitude flapping occurs in a flexible splitter plate (L/B = 3) at Re = 2000. The vortex shedding frequency becomes higher than the first mode natural frequency of the flexible splitter plate for this length and remains in the same regime for L/B &amp;gt; 3. The amplitude of flapping increases up to L/B = 5 and then again recedes. The high amplitude flapping of the flexible splitter plate adversely affects the mean drag coefficient of the bluff body.

Monte-Carlo simulation study for uncertainty evaluation in turbulent statistics obtained by constant-temperature anemometry

We evaluate the uncertainty of a recalibration scheme correcting for measurements of constant-temperature anemometry (CTA), where an observed velocity by CTA would be affected by variations in ambient temperature in an experiment. The present study elucidates the characteristics of the uncertainty quantitatively. A temporally evolving calibration curve is used to examine the uncertainty of the calibration curve. In the present study, we consider the temporally evolving curve to be a reference curve. The calibration curve predicted by the scheme is first examined. Here, the uncertainty of the gradient of the calibration curve is also considered. A Monte-Carlo simulation investigates the uncertainty in intensity and higherorder statistics of output-voltage fluctuation. Using the gradient of the calibration curve, this study derives a model for the statistics of the output voltage fluctuation, which is validated by the numerical simulation based on the Monte-Carlo simulation.

Simple mean-error estimation of a recalibration scheme for constant-temperature anemometry measurements

The present study has addressed the error of a recalibration scheme for constant-temperature anemometry (CTA) measurement proposed in a recent study. This recalibration scheme could reduce the effects of the ambient temperature change, which should be negligible in a CTA measurement. The present study has investigated a temporal variation of the error due to the use of the recalibration scheme. In the present study, a calibration curve produced by the recalibration scheme is validated using a temporally varying calibration curve, which is based on the general form of a calibration curve used in CTA measurement. The range of velocity U considered in the present study is U = 1 − 40 m/s. As a result, the relative error of the calibration curve is significant in the range of small velocity. The relative error of the gradient of the calibration curve, which is equivalent to the error of the velocity fluctuation, is also significant in the range of both small velocity and free-stream velocity. A simple cubic function could accurately approximate the temporal variation of these relative errors. Moreover, a simple form for calculating the mean value of the relative error could be derived and validated using the present numerical results.

Correlation between single-wire and multi-wire tripping of the flow past a circular cylinder

An experimental study is carried out to explore if the collective influence of multiple tripwires applied spanwise on a circular cylinder can be correlated to the well-known effects of single-wire tripping from previous studies. Constant Temperature Anemometry and Particle Image Velocimetry tests were conducted on single-wire- and two-wire-fitted cylinders in subcritical flow. Noting that a single tripwire can only influence the shedding frequency when it is within a certain range of locations on the cylinder surface, two scenarios for two-wire-fitted cylinders are noteworthy: (1) One of the two wires can fall within this range of locations. This is the case where flow features for the two-wire-fitted cylinder are identical to the single-wire-fitted one. In this category, the Karman vortex shedding loses its coherence and a bistable separation behavior sets in when the influential wire is at the first critical location (+/−θc1) of the single-wire application. (2) Both tripwires can be in the range of locations where the single tripwire application influences the shedding frequency. This is the scenario where flow is controlled by both tripwires. The most dominant influence comes from the tripwire that is closer to the θc location (another critical location identified by previous studies for single-wire applications). Furthermore, in this category, the drastic attenuation of the Karman instability and the onset of the bistable shear-layer separation occur when one of the wires is at a new θc1 location that is about 5°–7° larger than the θc1 reported for single-wire tripping by previous studies. This change in θc1 comes together with a large degree of near-wake contraction to satisfy the simultaneous entrainment demand needed for the early transition of both shear layers in the two-wire-fitted case. A shortage of fluid in satisfying such an increased entrainment demand is speculated to be the cause of the observed shift in θc1.

Wind tunnel experiments on cross-ventilation flow of a generic sheltered building in urban areas

Results of an extensive wind tunnel experimental measurements for cross-ventilation in a sheltered building are presented against different wind angles. Buildings were placed in a regular arrangement with a constant planar area ratio of 0.25. Instantaneous velocity components were measured inside and around the cross-ventilated building using a split fiber probe (SFP) and constant temperature anemometry (CTA) module. Moreover, the surface wind pressure coefficients over the building surfaces and the crossing airflow rate through the building openings were measured utilizing pressure tap and tracer gas systems, respectively. Results of the experimental measurements provide a very useful database for validation of numerical models based on computational fluid dynamics (CFD). It was shown that the channeling effect caused by surrounding buildings has significant effects on the airflow distribution inside and around a building. The very complex behavior of cross-ventilation flow and its interaction with the outdoor airfield was observed while highly-transient nature of cross-ventilation was conformed against different wind angles.