Hot-wire Anemometer Measurements Research Articles

Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle

The velocity fields and impinging pressure characteristics of confined jet impinges on a flat surface subject to the influence of various axial stages of a flat plate were experimentally examined. The instantaneous flow patterns, time-averaged flow patterns and velocity characteristics, velocity histories, and impingement pressure distributions were obtained using flow visualisation, particle image velocimeter (PIV), hot-wire anemometer measurement, and pressure detection techniques, respectively. From the analysis of the flow patterns, three characteristics of flow regimes are identified – laminar vortical wake, transition, and turbulent vortical wake – in the domain of Reynolds number and jet-to-wall distance. In laminar vortical wake mode, a ‘fat' pair of counter-rotating vortices appears between the confining surface of the nozzle and the flat plate. This results in a large region of turbulent intensities due to the entrainment of the jet flow. In transition mode, shear-layer vortices appear on the shear layer of the bifurcated jet. In turbulent vortical wake mode, the large momentum of jet fluids impinges the flat plate and deflects radially. The counter-rotating vortices and shear-layer vortices no longer exist in the wake of the bifurcated jet. The large eddies quickly break up into small eddies as the momentum of the jet-dominated flow. No obvious peak value of oscillating frequency was detected in this flow mode along the shear layers of the bifurcated impinging jet. The momentum of the fluid structures is converted into stagnation pressure on the flat plate when a strong jet impinges on it; therefore, the impingement pressure is large, consequently. The impingement pressure in the mode of the turbulent vortical wake was significantly greater than that in the modes of laminar vortical wake and transition.

Comparative Testing the Uncertainty of A/D Converters in A Hot-Wire Anemometer Measurement Using Grid-Generated Turbulence

This study investigates the uncertainty of the A/D converter when measuring multi-scale turbulence fluctuations using a hot-wire anemometer. The flow is generated in a low-speed wind tunnel, the turbulence is generated by a turbulence grid and the fluctuations are measured. A compact hot-wire anemometer was used and a high accuracy A/D converter was introduced for the measurements. Methods to minimise the influence of noise were also incorporated and measurements were carried out in two situations, one with and one without turbulence. The experimental results showed that measurements under a specific condition (Case 1 in this study) had the sufficienty reproducibility of turbulence fluctuations and the least effect of noise. The results showed that the high frequency characteristics of the turbulence could also be accurately measured under this condition. In conclusion, it is clear that the choice of a suitable A/D converter and the optimisation of the measurement environment are critical for turbulence measurements with a hot-wire anemometer.

Tethered Balloon-Borne Turbulence Measurements in Winter and Spring during the MOSAiC Expedition

During the Multidisciplinary Drifting Observatory for the Study of Arctic Climate expedition, a tethered balloon system was operated with a turbulence probe attached to study the lower troposphere in the high Arctic. Overall, measurements were conducted on 34 days between December 2019 and May 2020, resulting in 47 quality-assured sampling records consisting of vertical profiles and constant-altitude measurements. The continuous profiles extend from the surface, i.e., the sea ice floe, to a height of several hundred meters typically. The high-resolution wind velocity measurements using a hot-wire anemometer and temperature measurements using a thermocouple provide a comprehensive basis for examining the dynamical processes and thermodynamic stratification in the Arctic atmospheric boundary layer under cloudless and cloudy conditions. This paper provides a detailed technical description of the turbulence payload, including calibration and quality assurance, and a general overview of the data. A particular focus of this work is the estimation of local energy dissipation rates. The data are freely available from the World Data Center PANGAEA.

Row of jets in cross-flow as wind barrier

Studies on jet-in-crossflow have been extensively conducted in the last century. In our paper, we present the results obtained from the measurements of four jets-in-crossflow model developed at a wind tunnel testing facility. The interaction between four jets in a row and a boundary layer crossflow was visualized using a propylene glycol-based fog, and hot-wire anemometer measurements were conducted for quantitative analysis. Vertical profiles of the mean wind speed, relative wind speed, and turbulence intensity are presented for distances of 3.37 and 6.84 diameters from the central plane of the row. Such analysis was also performed for distances between the jets of 1.5, 2, and 3 diameters and velocity ratios of 1.6, 2, and 3.5. A comparison with the literature showed the model performance is comparable to currently reported wind barriers.

Evolution of instability modes and spreading characteristics of a planar jet flow under long-wave excitations

Switching phenomenon and parametric resonance have been identified in near field flows subjected to high-frequency controlled perturbations. This study reports the correlation between switching phenomenon, parametric resonance, and spreading characteristics caused by the instability modes after the end of the potential core of a planar jet (far field). Two periodically oscillating strips were used to control perturbations in free shear layers and three different long-wave excitations (i.e., symmetric, antisymmetric, and one-sided) were imposed at the excitation frequency (fe). The excitation frequency was fixed relative to the preferred mode frequency (fp) at fe = fp/4. To investigate the evolution of coherent structures, frequency spectral analysis and phase correlation were applied to hot-wire anemometer measurements. Under symmetric excitation, the instability mode of vortical structures is observed twice at mode switching, retaining the in-phase pattern. Jet puffing occurs when the cross correlation R12(0) is positive at the far field. These features cause vortex shedding to remain symmetric and suppresses the flapping motion of the jet. However, out-of-phase behavior reoccurs under antisymmetric and one-sided excitations and the cross correlation R12(0) becomes a negative after the end of the potential core. As a result, flapping motion is enhanced and the jet spreads wider after the end of the potential core.

Investigation of heat transfer from thin wires in air and a new method for temperature correction of hot-wire anemometers

Variations in fluid temperature are the most important error source in hot-wire anemometer measurements and must sometimes be compensated for. Although many temperature compensation schemes have been proposed over the years, no consensus seems to be reached regarding correction methods. In this paper new precision measurements on heat transfer from heated wires have confirmed that, provided the fluid properties are evaluated at the mean temperature between the air and sensor temperatures, the Nusselt number is a consistent function of the Reynolds number without any further corrections. Based on this knowledge a new calibration function which accounts for temperature dependencies in fluid properties is proposed. The aim has been to come up with a calibration function that is useful in practical anemometry. The only parameters in the function are air- and sensor temperatures. Velocity calibration is only necessary at one air temperature because temperature dependencies on conductivity and viscosity are incorporated in the calibration function.

Экспериментальные исследования влияния периодической модуляции течения на развитие возмущений в пограничном слое скользящего крыла при числе Маха 2,5

The results of experimental studies on the impact of periodic flow modulation on the development of natural disturbances and artificial localized wave packets in the boundary layer of a swept wing with a leading edge swept angle of 40 degrees at a Mach number M = 2.5 are presented. The boundary layer was modulated using periodic roughness on the model surface. Artificial wave packets were generated by a pulsed glow discharge. Hot-wire anemometer measurements showed that, according to the experimental conditions, periodic modulation of the boundary layer leads to the stabilization of the development of controlled localized disturbances and to an increase in the growth of natural disturbances.

Burning Velocity of Turbulent Methane/Air Premixed Flames in Subatmospheric Environments.

The aim of our work was to study turbulent premixed flames in subatmospheric conditions. For this purpose, turbulent premixed flames of lean methane/air mixtures were stabilized in a nozzle-type Bunsen burner and analyzed using Schlieren visualization and image processing to calculate turbulent burning velocities by the mean-angle method. Moreover, hot-wire anemometer measurements were performed to characterize the turbulent aspects of the flow. The environmental conditions were 0.85 atm, 0.98 atm, and 295 ± 2 K. The turbulence–flame interaction was analyzed based on the geometric parameters combined with laminar flame properties (which were experimentally and numerically determined), integral length scale, and Kolmogorov length scale. Our results show that the effects of subatmospheric pressure on turbulent burning velocity are significant. The ratio between turbulent and laminar burning velocities increases with turbulence intensity, but this effect tends to decrease as the atmospheric pressure is reduced. We propose a general empirical correlation as a function between ST/SL and u′/SL based on the experimental results obtained in this study and the equivalence ratio and pressure we established.

Experimental investigation of axisymmetric, turbulent, annular jets discharged through the nozzle of the SPP1980 SpraySyn burner under isothermal and reacting conditions

The present study contributes to the characterization of the complex flow fields that arise from annular turbulent non-reacting and reacting jets dispersed through the SPP1980 SpraySyn-burner. The SpraySyn-burner is a new lab-scale standard burner to study the production of functional materials by flame spray pyrolysis. Velocity measurements of axisymmetric turbulent annular jets issued through the nozzle of the SpraySyn-burner are performed with 2D-2C particle image velocimetry. Investigated cases include a turbulent annular free jet and turbulent annular jets weakly confined by co-flowing low-momentum streams under non-reacting and reacting conditions. The results of an isothermal case are validated with hot-wire anemometer measurements and compared with large eddy simulations. Fields of mean velocity and its standard deviation, as well as their centreline profiles are presented for all cases and compared to previous research studies of round, annular and co-axial jets. Significant changes in the flow velocity dynamics between non-reacting and reacting cases are discussed. Additionally, normalized radial profiles of axial mean velocity and Reynolds stresses are related to self-similarity theories. The data reveal that the results of self-similarity profiles are in good agreement with those found in the literature. Meanwhile, the self-similar profiles of Reynolds stresses from the studied annular jets asymptotically converge to unique self-similar profiles with differences in magnitudes due to variations in the initial conditions, boundaries and jet history. The present results are primarily useful for providing validation data for computational simulations that model the intricate droplet-particle formation using the SpraySyn-burner.

К взаимосвязи возмущений свободного сверхзвукового потока с возмущениями пограничного слоя при числе Маха 2

An experimental study of the effect of pulsations of free supersonic flow on the pulsations of the boundary layer of a flat plate with a sharp edge using a new three-channel hot-wire anemometer measurement system is carried out. The spatial-temporal distributions of the correlation coefficient are obtained for the boundary layer cross sections and for the laminar-turbulent transition region.

Optimisation of a multi-duct cleaning device for rice combine harvesters utilising CFD and experiments

As the grain-cleaning process in a combine harvester relies on pneumatic separation of the grain and chaff, the aerodynamic forces created by the airflow have a large impact on the cleaning performance. Therefore, to optimise a multi-duct cleaning device in a rice combine harvester, First, the equivalent resistances for the different fan ducts were quantified with a resistance model of fluidised grain and an airflow resistance model for the cleaning sieve. Perforated plates with different opening ratios were designed to simulate working loads. A computational fluid dynamics (CFD) model was validated by comparing simulation results with hot-wire anemometer measurements. The effects of working loads on airflow velocity and volume distribution at the ducts, and total pressure distribution inside the fan were studied in simulation and experiments. After integration of the designed multi-duct fan into the cleaning system, airflow measurements inside the cleaning shoe were made and cleaning performance tests were carried out on a test bench. The ideal airflow velocity within the cleaning shoe was investigated by correlating airflow velocities at different measuring points and analysing the cleaning performance. An evaluation of this multi-duct cleaning system in the field showed large improvements in terms of the grain sieve losses ratio and grain impurity ratio compared to a combine harvester with the single-duct cleaning system. The corresponding grain sieve loss decreased from 2.46% to 0.08%, while the grain impurity ratio decreased from 4.78% to 0.511%.

The function of the alula on engineered wings: a detailed experimental investigation of a bioinspired leading-edge device

Birds fly in dynamic flight conditions while maintaining aerodynamic efficiency. This agility is in part due to specialized feather systems that function as flow control devices during adverse conditions such as high-angle of attack maneuvers. In this paper, we present an engineered three-dimensional leading-edge device inspired by one of these specialized groups of feathers known as the alula. Wind tunnel results show that, similar to the biological alula, the leading-edge alula-inspired device (LEAD) increases the wing’s ability to maintain higher pressure gradients by modifying the near-wall flow. It also generates tip vortices that modify the turbulence on the upper-surface of the wing, delaying flow separation. The effect of the LEAD location and morphology on lift production and wake velocity profile are investigated using force and hot-wire anemometer measurements, respectively. Results show lift improvements up to 32% and 37% under post and deep stall conditions, respectively. Despite the observed drag penalties of up to 39%, the aerodynamic efficiency, defined as the lift-to-drag ratio, is maintained and sometimes improved with the addition of the LEAD to a wing. This is to be expected as the LEAD is a post-stall device, suitable for high angles of attack maneuvers, where maintaining lift production is more critical than drag reduction. The LEAD also accelerates the flow over the wing and reduces the wake velocity deficit, indicating attenuated flow separation. This work presents a detailed experimental investigation of an engineered three dimensional leading-edge device that combines the functionality of traditional two dimensional slats and vortex generators. Such a device can be used to not only extend the flight envelope of unmanned aerial vehicles (UAVs), but to also study the role and function of the biological alula.

Wind Tunnel Investigation of the Wind Patterns in the Launching Pad Area of the Brazilian Alcântara Launch Center

This paper presents a wind tunnel study of the flow patterns in the main Brazilian Space Port, the Alcântara Launch Center (ALC), located in the Northwest region, over an irregular coastal cliff of 40 m height. In the ALC region, constant winds of strong incidence are very common, which can change the characteristics of the atmospheric boundary layer and affect safety conditions during launching operations. This study was conducted using a scaled model of the Launching Pad Area (LPA) in ALC complex with the purpose of getting insights about the wind flow patterns, as zones of strong vorticity, in that region when the wind incidence angle and coastal cliff characteristics change. The experiments were carried out in an aerodynamic wind tunnel, and for this reason it was necessary to use some techniques described in the literature to simulate an atmospheric boundary layer in a short test section wind tunnel. Hot-wire anemometer measurements were carried out for turbulence level evaluation with empty wind tunnel test section, and a two-dimensional Particle Image Velocimetry (PIV) was used for flow field velocity measurements in different confi gurations of wind incidence angle (α), Reynolds number and coastal cliff slope angle (β).

Hot-wire Anemometer and PIV Measurement of Fractal Grid Turbulence

Hot-wire Anemometer and PIV Measurement of Fractal Grid Turbulence

Investigation of Submerged Jets with an Extended Initial Laminar Region

The method of producing laminar submerged jets using a device, whose length is comparable with the jet diameter, is described. A submerged air jet, 0.12 m in diameter, produced by means of this technique is experimentally investigated in the Reynolds number range from 2000 to 13 000. Hot-wire anemometer measurements of the flow parameters and laser visualization of the flow are performed. It is shown that the device developed makes it possible to produce submerged jets with the laminar regions as long as 5.5 jet diameters. The initial regions of such jets can be used to study the development of disturbances in submerged jets, as well as used in medicine and engineering in organizing various gasdynamic curtains which produce zones with given properties with respect to purity and composition inside another gas media.

Effect of chord-to-diameter ratio on vertical-axis wind turbine wake development

The wake structure of a vertical-axis wind turbine (VAWT) is strongly dependent on the tip-speed ratio, $$\lambda$$ , or the tangential speed of the turbine blade relative to the incoming wind speed. The geometry of a turbine can influence $$\lambda$$ , but the precise relationship among VAWT geometric parameters and VAWT wake characteristics remains unknown. To investigate this relationship, we present the results of an experiment to characterize the wakes of three VAWTs that are geometrically similar except for the ratio of the turbine diameter (D), to blade chord (c), which was chosen to be $$D/c =$$ 3, 6, and 9. For a fixed freestream Reynolds number based on the blade chord of $$Re_c = 1.6\times 10^3$$ , both two-component particle image velocimetry (PIV) and single-component hot-wire anemometer measurements are taken at the horizontal mid-plane in the wake of each turbine. PIV measurements are ensemble averaged in time and phase averaged with each rotation of the turbine. Hot-wire measurement points are selected to coincide with the edge of the shear layer of each turbine wake, as deduced from the PIV data, which allows for an analysis of the frequency content of the wake due to vortex shedding by the turbine.

Experimental study of shock-wave structure of supersonic underexpanded microjets

The structure of supersonic underexpanded room temperature air microjets is experimentaly studied. Microjets escaping from the nozzles with characteristic heights of 22.3 and 83.3 μm and widths of 2410 and 3900 μm respactacly with the Reynolds numbers of 640 and 2390. At these Reynolds numbers hot-wire anemometer measurements of mass flow pulsations are performed. It is shown that low value of the mass flow pulsations are corresponding with the high supersonic core length.

An experimental study of an airfoil with a bio-inspired leading edge device at high angles of attack

Robust and predictable aerodynamic performance of unmanned aerial vehicles at the limits of their design envelope is critical for safety and mission adaptability. Deployable aerodynamic surfaces from the wing leading or trailing edges are often used to extend the aerodynamic envelope (e.g. slats and flaps). Birds have also evolved feathers at the leading edge (LE) of their wings, known as the alula, which enables them to perform high angles of attack maneuvers. In this study, a series of wind tunnel experiments are performed to quantify the effect of various deployment parameters of an alula-like LE device on the aerodynamic performance of a cambered airfoil (S1223) at stall and post stall conditions. The alula relative angle of attack, measured from the mean chord of the airfoil, is varied to modulate tip-vortex strength, while the alula deflection angle is varied to modulate the distance between the tip vortex and the wing surface. Integrated lift force measurements were collected at various alula-inspired device configurations. The effect of the alula-inspired device on the boundary layer velocity profile and turbulence intensity were investigated through hot-wire anemometer measurements. Results show that as alula deflection angle increases, the lift coefficient also increase especially at lower alula relative angles of attack. Moreover, at post stall wing angles of attack, the wake velocity deficit is reduced in the presence of alula device, confirming the mitigation of the wing adverse pressure gradient. The results are in strong agreement with measurements taken on bird wings showing delayed flow reversal and extended range of operational angles of attack. An engineered alula-inspired device has the potential to improve mission adaptability in small unmanned air vehicles during low Reynolds number flight.

Flow and Turbulence Control in a Boundary Layer Wind Tunnel Using Passive Hardware Devices

The Climatic Wind Tunnel (CWT) is a new experimental facility of the Institute of Theoretical and Applied Mechanics (ITAM) in Prague, Czech Republic. It is uniquely able to replicate precipitation and freezing effects on structural models in the climatic test section, and simultaneously perform aerodynamic and aeroelastic tests on structural models in the aerodynamic test section. Extensive measurements are carried out to investigate airflow and turbulence characteristics in the empty aerodynamic test section. Capability to experimentally simulate the atmospheric boundary layer (ABL) is studied using castellated barrier wall, Counihan vortex generators, and surface roughness elements. An individual influence of each of these passive hardware devices is separately tested as well, as they are commonly used combined, and their individual contributions to the ABL model characteristics are not well known. The analysis is based on hot-wire anemometer measurements carried out at various cross sections in the aerodynamic test section of the CWT. Experimental results are reported as mean velocity and turbulence intensity fields for the empty aerodynamic test section without passive hardware devices. Vertical profiles of mean velocity, turbulence intensity, and turbulence length scales, as well as power spectral density of velocity fluctuations, are reported for the ABL small-scale model. The barrier is observed to cause a decrease in flow velocity and an increase in turbulence close to surface. Vortex generators enhance turbulence throughout the entire height range under scope. Increased turbulence and decreased flow velocity are particularly exhibited close to surface due to surface roughness. Influence of each of the studied passive hardware devices on flow and turbulence is quantified in a dimensionless form to allow for separately accounting for their effects.

Multivariate and conditioned statistics of velocity and wall pressure fluctuations induced by a jet interacting with a flat plate

The increasing size of aircraft engines is leading to reconsideration of their conventional integration in the under-wing configuration due to the strong interaction between the jet and the airframe components. As a consequence, more insight is needed into the complex mechanisms underlying the interaction phenomenon between the jet flow and a surface. The objective of this paper is to carry out a series of experimental tests on a simplified laboratory-scale model to approach/deepen the problem. This analysis is the continuation of a previous study (Di Marco et al., J. Fluid Mech., vol. 770, 2015, pp. 247–272) on a rigid flat plate installed tangentially to the axis of an incompressible jet. In the present work, the velocity and wall pressure fields were simultaneously measured for different radial distances of the plate from the nozzle axis. Pointwise hot-wire anemometer measurements were carried out to characterize the effect of the plate on the velocity field statistics up to the fourth-order moment. The analysis revealed that the presence of the plate brings about a deflection of the mean aerodynamic field over the surface and a reduction of the turbulence intensity. Wall pressure fluctuations induced by the jet flow over the plate were measured by a linear array of cavity-mounted microphones placed along the streamwise direction. The velocity/pressure cross-statistics are achieved in the time and frequency domains using cross-correlations and Fourier analysis. A wavelet-based conditional sampling procedure is applied as well to characterize the flow signatures related to the velocity and wall pressure fluctuations, revealing that the surface induces the breakdown of the large-scale turbulent structures. The dependence of the multivariate and conditioned statistics on the plate distance from the jet as well as on the streamwise and crosswise probe positions is extensively discussed. Different organized flow motions over the surface are found based on the wall pressure events detected. A scaling criterion for velocity signatures is also presented addressing the governing parameters of the jet–plate interaction phenomenon.