Hot-wire Measurements Research Articles

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.

Экспериментальное исследование влияния единичного числа Рейнольдса на положение ламинарно-турбулентного перехода на крыле с дозвуковой передней кромкой при числе Маха 2

Experimental studies of the influence of unit Reynolds number on the laminar-turbulent transition in a supersonic boundary layer of a swept wing with a subsonic leading edge at Mach number 2 are performed. The experiments were performed on a model of a swept wing with a swept angle of the leading edge of 72 degrees and with a 3% profile with a variable chord length in span. The hot-wire measurements showed that a laminar-turbulent transition in a supersonic boundary layer of a swept wing with a subsonic leading edge occurs earlier (~25-30%) than on a model with a supersonic leading edge with the same oncoming flow parameters. It is shown that a change unit Reynolds number insignificant influence the laminar-turbulent transition in the boundary layer of a swept wing with a subsonic leading edge.

Hot-Wire Measurement of Asymptotic Characteristics of Lobed Jet Flow

Hot-Wire Measurement of Asymptotic Characteristics of Lobed Jet Flow

Design, validation, and benchmark tests of the aeroacoustic wind tunnel in SUSTech

The aeroacoustic wind tunnel at Southern University of Science and Technology (SUSTech) is a new facility commissioned for studies of sound generation mechanisms, noise reduction methods, and aerodynamic designs. The wind tunnel is operated at low Mach numbers and low-to-moderate Reynolds numbers. This study details the design methodology for wind-tunnel major components. The validation tests of the flow quality are performed by pitot-tube and hot-wire measurements, and those of the aeroacoustic performance are conducted by far-field microphone measurements. The aerodynamic and acoustic performance is compared with established data, and the results show that the wind tunnel has a high quality of flow field and low background noise levels. A benchmark test of airfoil tonal noise is conducted by microphone and Particle Image Velocimetry (PIV) measurements. The observed tones exhibit consistent features with existing experimental observations and the mechanisms are explained with the help of flow field near the trailing edge.

Impact of Topographic Steps in the Wake and Power of a Wind Turbine: Part A—Statistics

We experimentally explored the modulation of various forward- and backward-facing topographic steps on the wake and power output of a wind turbine model. The sharp surface changes located in the vicinity of the turbine tower consisted of steps Δz0/dT=−0.64, −0.42, −0.21, 0, 0.21, and 0.42, where Δz0 is the level difference between the upwind and downwind sides of the step and dT is the turbine diameter. Particle image velocimetry was used to obtain the wake statistics in the wake within the streamwise distance x/dT∈[2, 5] and vertical span z/dT∈[−0.7, 0.7], where the origin is set at the rotor hub. Complementary single-point hotwire measurements were obtained in the wake along the rotor axis every Δx/dT=1 within x/dT∈[1, 8]. Mean power output and its fluctuations were obtained for each of the six scenarios. The results indicate strong modulation of the steps in the wake statistics and some effect on the power output. Remarkably, the backward-facing steps induced a larger velocity deficit in the wake with respect to the base case with substantial wake deflection. In contrast, the forward-facing steps exhibited a much lower velocity deficit and negligible wake deflection. The mean flow and velocity gradients’ changes promoted distinct turbulence dynamics and, consequently, associated levels. In particular, turbulence intensity and kinematic Reynolds shear stress were enhanced and reduced with the backward- and forward-facing steps, respectively. It is worth pointing out the particular effect of the steps on the transport of the turbulence kinetic energy TKE. Ejections were predominant around the top tip, whereas sweeps dominated around the turbine hub height. The magnitude of these quantities was sensitive to the step height. In particular, a much weaker sweep occurred in the forward-facing steps; in addition, the flat terrain and the backward-facing step cases shared strong sweeps.

Computational and experimental study of 3D flow near the cube immersed in the turbulent free convection boundary layer on a vertical heated plate

Results of a coordinated computational and experimental study of time-averaged velocity and temperature fields in the vicinity of an adiabatic cube that is inserted into the turbulent free convection vertical-plate boundary layer are presented. The numerical simulation was based on the steady-state RANS approach using the k-ω SST turbulence model and a version of the differential Reynolds stress model (DRSM). The hot-wire technique was used for velocity measurements, simultaneously with temperature measurements by an accompanied “cold” wire. Two turbulence models produced similar vortex flow structures near the obstacle; however some important quantitative distinctions were observed. The velocity value profiles predicted for the middle vertical plane showed generally a good accordance with the hot-wire measurements carried out in this plane, especially in the DRSM case.

Whittle maximum likelihood estimate of spectral properties of Rayleigh-Taylor interfacial mixing using hot-wire anemometry experimental data.

Investigating the power density spectrum of fluctuations in Rayleigh-Taylor (RT) interfacial mixing is a means of studying characteristic length, timescales, anisotropies, and anomalous processes. Guided by group theory, analyzing the invariance-based properties of the fluctuations, our paper examines raw time series from hot-wire anemometry measurements in the experiment by Akula etal. [J. Fluid Mech. 816, 619 (2017)JFLSA70022-112010.1017/jfm.2017.95]. The results suggest that the power density spectrum can be modeled as a compound function presented as the product of a power law and an exponential. The data analysis is based on Whittle's approximation of the power density spectrum for independent zero-mean near-Gaussian signals to construct a maximum likelihood estimator of the parameters. Those that maximize the log-likelihood are computed numerically through Newton-Raphson iteration. The Hessian of the log-likelihood is used to evaluate the Fisher information matrix and provide an estimate of the statistical error on the obtained parameters. The Kolmogorov-Smirnov test is applied to analyze the goodness of fit, by verifying the hypothesis that the ratio between the observed periodogram and the estimated power density spectrum follows a χ^{2} probability distribution. The dependence of the parameters of the compound function is investigated on the range of mode numbers over which the fit is performed. In the domain where the relative errors of the power-law exponent and the exponential decay rate are small and the goodness of fit is excellent, the parameters of the compound function are clearly defined, in agreement with the theory developed in the paper. The study of the power-law spectra in RT mixing data suggests that rigorous physics-based statistical methods can help researchers to see beyond visual inspection.

Experimental investigation of the angle of attack dependence of the flow past a cactus-shaped cylinder with four ribs

The aerodynamic properties of a two-dimensional cactus-shaped cylinder with four ribs are studied experimentally. The cross section of the cylinder corresponds to a modified square cylinder with rounded corners and concave sides inspired by tall succulents with four ribs. The mean aerodynamic coefficients, fluctuating lift coefficient, Strouhal number, and mean surface pressure distribution are measured as a function of the angular orientation for Reynolds numbers ranging from 50,000 to 150,000. Hot-wire measurements are conducted at two locations in the wake for several angles of attack to provide insight into the vortex shedding frequencies. The results show that the studied shape exhibits a strong angle of attack dependence while no Reynolds number dependence is found within the tested range. As for the square cylinder, a critical angle of attack of approximately 13° is observed at which drag and fluctuating lift forces are minimised while mean lift and Strouhal number reach their highest values. Reattachment of the shear layer at and above the critical angle of attack is confirmed using PIV. Overall, succulents with four ribs retain some of the aerodynamic benefits that have been observed for cactus-shaped cylinders with many ribs, albeit over a limited range of angles of attack.

Investigation of the influence of the three-dimensional roughness element on the flow over trapezoidal flying wing

The paper is devoted to the investigation of the flow and effects taken place when three-dimensional roughness element is placed on the leading edge of the trapezoidal flying wing. The experimental study was carried out in the working park of the subsonic wind tunnel. Low Reynolds numbers allowed running experiments at in-flight-like conditions. Pictures of the flow structure were obtained by liquid crystal thermography. It was found that longitudinal disturbances on the leading edge and behind three-dimensional roughness element form differently. Inner configuration of the longitudinal structure was shown by hot-wire measurements.

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.

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.

Free-stream turbulence interaction with a wing-tip vortex

The interaction of a wing-tip vortex of a rectangular, square-tipped wing having a NACA 0012 airfoil section with a grid-generated turbulent flow was investigated in this paper. The experiments were conducted in the near and mid-wake regions at three free stream turbulence (FST) intensities of 0.5%, 3% and 6%, and at two Reynolds numbers, Re, based on the wing chord length, cw, of 2×105 and 3×105. Stereoscopic Particle Image Velocimetry (SPIV) and hot-wire measurements were carried out at four downstream positions, namely x/cw=1.25,3.25,6.25 and 7.75. Streamwise velocity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. In the vortex core region, the streamwise velocity decelerated while the vortex adopted a wake-like profile. FST was found to decrease the vortex circulation and to increase the vortex radius and vortex meandering amplitude. When increasing Reynolds number, the grid cases showed little variation of the vortex radius and peak vorticity levels, particularly at larger downstream positions, suggesting that the effects of FST and Re number on the vortex development are nearly independent. The measured total turbulent kinetic energy (TKE) was found to be mostly due to vortex meandering. In that, total TKE levels devoid of meandering showed a virtually turbulence-free vortex core.

Simulation of spectators’ aerodynamic drag using porous models approximation

Evaluation of the thermal comfort is essential for complex ventilation systems design. Assessment of thermal indices requires representative velocity and pressure fields' values. When simulating the air flow in large facilities such as stadium, the effect of crowds' geometrical features needs to be captured. Using porous models approximations to simulate the aerodynamic effect of detailed spectators' geometry reduces the required mesh size and associated processing time. This paper investigates the use of different porous media models approximations for capturing the effect of large crowds inside complex building systems, such as stadiums. Their efficiency of capturing the effect of spectators on the air flow were compared to the simulation of the exact spectators' geometry. The exact spectators' geometrical model was of a stadium tiers section with 28 spectators. Using a wind tunnel, the exact spectator's model results were validated against a 1:10 scaled physical model. The experiments included PIV and hot-wire velocity measurements. The results of the pressure drop were used to obtain the coefficients needed to utilize the porous models. Compared to the exact spectators' case, the three-dimensional porous volume model approximation yielded an average absolute error of 24.5% in velocity, while the two-dimensional porous jump model yielded results with an average error of 1.5%. In comparison to the exact model cooling load, the results yielded a difference of 6% for the 2D porous jump and 6.5% for the 3D porous volume. Nevertheless, both models yielded more representative results than the case of simulation of empty bleachers.

EXPERIMENTAL STUDY OF THE INFLUENCE OF ATMOSPHERIC TURBULENCE ON THE BOUNDARY LAYER FLOW ON THE GLIDER WING

Flight experiments aimed at studying the transition from the laminar to turbulent flow in the boundary layer on the glider wing at different levels of atmospheric turbulence performed in the 1980s are reviewed. The results are obtained by means of hot-wire measurements and visualization of the flow on the wing surface by the method of sublimating coatings. The transition is found to proceed in several stages: emergence, evolution, and failure of a discrete packet of instability waves in the region of the adverse pressure gradient. The data obtained in the study are compared with the results of similar investigations carried out in a large wind tunnel on the same real glider wing at realistic Reynolds numbers.

Some Similarities and Differences Between the Observed Alfvénic Fluctuations in the Fast Solar Wind and Navier–Stokes Turbulence

Abstract: Using two long data sets analyzed on equal footing, the properties of Alfvenic fluctuations in the fast (coronal-hole-origin) solar wind and Navier-stokes turbulence are compared. A 26.4-s-long interval of hot-wire measurements in the ONERA wind tunnel is used and a 71-hr-long interval of unperturbed coronal-hole plasma measured by the WIND spacecraft at 1 AU is used. Similarities and differences between a Navier-Stokes fluid and the collisionless magnetized solar-wind plasma are discussed, as are differences between the physical natures of the advecting evolving turbulent fluctuations and the propagating non-evolving Alfvenic fluctuations. The details of the power spectral densities of the turbulence and the Alfvenic fluctuations are compared. Statistics of first and second time derivatives are examined for the wind-tunnel and solar-wind time series and the statistics are compared with the statistics of time derivatives of phase-randomized time series. Using running medians, the statistics of flat spots in the time series of Alfvenic fluctuations is examined, which is evidence of a cellular structure to the magnetic field and velocity field. A call for a campaign of expanded coordinated future work is made.

Aeroacoustic Characteristics of a Synchronized Fluidic Oscillator

A new opposite facing oscillator pair is presented where shared feedback channels enable synchronized sweeping of the exiting jets. The design has no moving parts and the oscillator pair is composed in a back-to-back configuration. The synchronized operation generates a near-field acoustic tonality and the objective is to determine the emitted directivity. The acoustic generation mechanisms were determined using compressible large eddy simulations, which was validated with hot-wire and microphone measurements. Different Reynolds number (Re) conditions up to 21,250 were analyzed for a synchronized oscillation with Strouhal number (St) of order 0.01. Dominant acoustic sources emerging during synchronized sweep oscillation were classified by the interpretation of directivity patterns for a selected flow rate. The near-field directivity pattern could be decomposed as a periodic signal consisting of one acoustic mode with a dipole-like pattern, showing two major lobes associated with the jet sweeping oscillation frequency: one acoustic monopole-like directivity for the first overtone; and dipole-like directivity with two lobes for the second harmonic. It is shown that the acoustic sources are generated by the synchronized pressure oscillation of the exiting jets. The vortical structures inside the fluidic oscillator interact with the non-slip walls. These manipulate the curvature of the central jet and causes an unsteady loading towards the discharge. The new fluidic oscillator design gives synchronized exiting sweeping jets and a large flow length scale near-field directivity pattern. These features give feedback type fluidic oscillators a wider application range.

Aerodynamic Performance Improvement of a Wing Model Using an Array of Slotted Synthetic Jets

Abstract This work deals with the improvement in aerodynamic performance of a NACA0025 wing model using an array of slotted synthetic jets (SJs). A novel SJ actuator was designed and located at 30% of the chord from the leading edge (LE). Time-resolved particle image velocimetry (TR-PIV), force balance, static pressure distribution, and hotwire measurements were carried out in a subsonic wind tunnel to assess the performance enhancement due to the slotted SJ array. Initially, the SJ velocity was measured in quiescent flow condition at different actuation frequencies and amplifier voltages. Actuation at 1000 Hz and 200 V resulted in the highest blowing velocity of 10.5 m/s. Experiments were performed at various actuation frequencies, namely, 200, 600, and 1000 Hz. It was observed that actuation at 1000 Hz led to the highest increase in lift coefficient by 35.6% and reduction in average drag coefficient by 33%. TR-PIV measurements showed flow separation with flow reversal in the baseline case. After switching on the SJ array at 1000 Hz, the flow separation was completely eliminated. The momentum transfer from the highenergy primary flow to the retarding boundarylayer flow and actuation of SJ in a particular frequency range was observed to be the mechanisms for the flow separation control. Subsequently, fast Fourier transform (FFT) power spectra of hotwire data were computed from 40% to 80% of the chord. The FFT power spectra showed the successful stabilization of the flow field at the actuation of 1000 Hz.

Flow characterization of an array of finite-span synthetic jets in quiescent ambient

An experimental study on the flow characteristics of an array of finite-span Synthetic Jets (SJs) issuing at different conditions in a quiescent ambient was carried out by means of hot-wire and phase-locked planar Particle Image Velocimetry measurements. The actuator comprises twelve slots with aspect ratio 15 fed by a common resonant cavity and off-centered from the axis of the cavity. The frequency response indicates that the SJs are formed in the range of Stokes numbers between 4 and 12.5, in which two operating conditions were inspected yielding the SJs issuing either at Reynolds (Re) and Strouhal (St) numbers of Re = 40 and St = 0.84 or Re = 213 and St = 0.50. It is found that, for the case at Re = 40 and St = 0.84, the flow field undergoes the asymmetries of the actuator. The axis switching persists in the very-near field of the SJ array, while further downstream the spreading of each SJ becomes more pronounced promoting the jet interaction. For the case at Re = 213 and St = 0.50, the main flow direction along the cross-sectional plane tilts of 14.8° towards the side where the cavity extends. The formation of a recirculation region promotes the enhancement of turbulent dissipation mechanisms. The phase-average flow fields reveal that, similarly for both cases, the imprint of the vortical structures exhibits symmetric organization during the ejection phase.

250 kHz three-component Doppler velocimetry at 32 simultaneous points: a new capability for high speed flows

Time-resolved Doppler global velocimetry (TRDGV) is used to demonstrate three-component velocity measurements of 32 planar points, acquired simultaneously at 250 kHz. Photomultiplier tube arrays are used to detect the flow signal, with each array having eight rows of four pixels. The system utilizes frequency scanning of a continuous wave laser, allowing for simple calibration of the signals used to determine the incident and Doppler-shifted frequency. The TRDGV system is used to measure an unheated, Mach 0.91 free jet downstream of the potential core collapse. Axial mean velocities were measured within a root-mean-square error of 7 m s−1 compared to Pitot probe validation measurements. Additionally, axial turbulence intensities showed agreement to hot wire validation measurements and exhibited typical magnitudes reported in the literature. Velocity spectra were obtained for the three components of velocity, revealing a − 5/3 decay in the axial velocity spectra in the inertial subrange frequencies, and generally less decay present in the transverse component spectra. Hot wire measurements and spectra from literature were used for validation of the TRDGV spectral results, showing broadband spectral agreement. The instrument described in this work shows promising capability for multi-point, time-resolved velocity vector measurement in high speed compressible flows with acceptable levels of uncertainty.

Synchronization of a Pair of Opposed Facing Oscillators in a Side-by-Side Configuration

This study considers two opposed facing fluidic oscillators in a side-by-side layout. Each fluidic oscillator shares one of its feedback channels with the other oscillator thus this configuration provides a mean for communication between fluidic oscillators. As a result of this communication through the shared feedback channel, the exiting jets of the fluidic oscillator pair are synchronized. The details of this synchronization are revealed by means of hot-wire measurements, flow visualizations and validated numerical simulations. The oscillation frequencies for both jets were the same and varied from 123 Hz to 476 Hz for a flow rate range from 5 SLPM to 25 SLPM per inlet corresponding to Reynolds number range of 2,125 to 10,625. The calculated cross-correlation coefficients were over 0.88 for the considered flow rate range. The flow visualizations exposed that the exiting jet of each fluidic oscillator move away from the shared feedback channel and move toward the shared feedback channel simultaneously indicating motion in the same direction. Numerical results indicated that the shared feedback channel plays a pressure balancing role while allowing inter-oscillator flow through each side of the shared feedback channel thus controlling the flow direction of each exiting jet yielding a synchronized output from the fluidic oscillator nozzles.