Constant-temperature Hot-wire Anemometer Research Articles

Experimental study of nonlinear processes in a swept-wing boundary layer at the mach number M=2

Results of experiments aimed at studying the linear and nonlinear stages of the development of natural disturbances in the boundary layer on a swept wing at supersonic velocities are presented. The experiments are performed on a swept wing model with a lens-shaped airfoil, leading-edge sweep angle of 45°, and relative thickness of 3%. The disturbances in the flow are recorded by a constant-temperature hot-wire anemometer. For determining the nonlinear interaction of disturbances, the kurtosis and skewness are estimated for experimentally obtained distributions of the oscillating signal over the streamwise coordinate or along the normal to the surface. The disturbances are found to increase in the frequency range from 8 to 35 kHz in the region of their linear development, whereas enhancement of high-frequency disturbances is observed in the region of their nonlinear evolution. It is demonstrated that the growth of disturbances in the high-frequency spectral range (f > 35 kHz) is caused by the secondary instability.

English

The pulsating heated flows are traditionally a difficult subject to treat with conventional hot wire or film methods. Special factors that complicate matters are flow reversal and non linear flow effects of vortices and wire probe wake disturbances on the heat transfer to the hot film or wire sensor in heated pulsating flows. The presence of these strongly nonlinear and unknown terms leads to great difficulties in calibration of hot film probes in this particular regime. The paper analyses the current state of matters in the field and reports a series of solutions that have been practically tested in a case of a high speed pulsated heated flow. Normally such measurements are made in a non-contact fashion using a LDV system or various visualization techniques but there have been recent attempts to use a constant temperature hot wire anemometer system (CTA).To obtain meaningful calibration for hot wire films in hot pulsating flows, a comparison system on other principles (LDV) was used, as well as a specially designed nozzle to replace the calibrator unit that could not be operated with heated fluid due to structural integrity reasons. The method as described below works well for the expected speed range that could be generated using the special nozzle.

The Hot-Wire Anemometer/Anemometr Z Grzanym Włóknem

Abstract This study analyses the behaviour of a hot-wire anemometer incorporated into a bridge circuit in the function of the voltage Uz supplying the bridge circuit and hence the current Iw flowing through the hot wire. The dependence of differential voltage ΔU on Iw and the value of derivative d(ΔU)/dIw as a function of the current supplying the hot-wire element were determined. These data enable the determination of working conditions of the constant-resistance (i.e. the constant-temperature) hot-wire anemometer.

Performance and Flow Field Evaluation of a Savonius Rotor Tested in a Wind Tunnel

A renewed interest on Vertical Axis Wind Turbines (VAWTs) arose from their great capacity for integration within urban areas and for applications of distributed generation. In order to be able to highly improve their performance, making them competitive with respect to the more consolidated Horizontal Axis Wind Turbines (HAWTs), it is fundamental to have a deeper comprehension of their fluid dynamic behavior. In order to reach this goal, a two-bucket Savonius rotor has been designed, built from a PVC pipe with a nominal diameter of 200mm, and tested in the wind tunnel of the Department of Mechanics, Mathematics and Managment (DMMM) of the Politecnico di Bari. The Savonius rotor is connected to an AC brushless servo motor, able to control either the braking torque or the rotational speed. This paper describes the experimental evaluation of the unsteady flow field downstream the rotor by means of a Constant Temperature hot wire Anemometer (CTA). Whilst, for performance analysis, the torque measurements have been obtained directly from the Servo Amplifier torque monitor.

An Adaptive Response Compensation Technique for the Constant-Current Hot-Wire Anemometer

An adaptive response compensation technique has been proposed to compensate for the response lag of the constant-current hot-wire anemometer (CCA) by taking advantage of digital signal processing technology. First, we have developed a simple response compensation scheme based on a precise theoretical expression for the frequency response of the CCA (Kaifuku et al. 2010, 2011), and verified its effectiveness experimentally for hot-wires of 5 μm, 10 μm and 20 μm in diameter. Then, another novel technique based on a two-sensor probe technique—originally developed for the response compensation of fine-wire thermocouples (Tagawa and Ohta 1997; Tagawa et al. 1998)—has been proposed for estimating thermal time-constants of hot-wires to realize the in-situ response compensation of the CCA. To demonstrate the usefulness of the CCA, we have applied the response compensation schemes to multipoint velocity measure- ment of a turbulent wake flow formed behind a circular cylinder by using a CCA probe consisting of 16 hot-wires, which were driven simultaneously by a very simple constant-current circuit. As a result, the proposed response compensation techniques for the CCA work quite successfully and are capable of improving the response speed of the CCA to obtain reliable measurements comparable to those by the commercially-available constant-temperature hot-wire anemometer (CTA).

An analysis of flow and heat transfer in separated flows over blocked surface

The flow and heat transfer characteristics of flat and blocked surfaces were experimentally examined under the influence of the free stream velocity of 3, 5, 10 and 15 m/s encompassing laminar, transitional and turbulent flows. A constant-temperature hot wire anemometer was used for the velocity and turbulent intensity measurements, and copper-constant thermocouples and a micro-manometer for temperature and static pressures measurements, respectively. The flow over blocked surface separated in front of the first block and attached on it, then circulated between blocks, and then reattached behind the last block. The results showed that the flow separation before the first block occurred earlier in laminar–laminar separated–reattached flow than the transitional and turbulent flows and turbulent–turbulent separated–reattached flow leading to a shorter reattachment region with high free-stream turbulence. The presence of the separation and reattachment caused the heat transfer enhancement, which was more pronounced in the laminar flow and new empirical equations were developed for the local Stanton numbers.

Development of the Wave Packets-Forerunners at the Straight Wing Boundary Layer

This work is devoted to the study of wave packets (forerunners) formed in straight wing boundary layers in the regions preceding a drastic change in the flow velocity inside the boundary layer, near the fronts of streaks. The investigations were carried out in the subsonic low turbulence wind tunnel. Measurements of the flow fields were carried out using a single-wire probe of a constant-temperature hot-wire anemometer. It was shown that the localized structures, which modeled artificially from incoming flow, generate streaks in a boundary layer. The wave packets (forerunners) are appears in the regions preceding a drastic change of flow velocity inside the boundary layer at the streaks fronts. The characteristics of the forerunners, affected by the external-flow pressure gradient, intensity of streak, and local velocity gradients near the front of the streak were investigated

The Experimental Investigation of Energy Transfer in Wall Turbulence Based on Wavelet Transform and FFT and HHT

Using constant temperature hot-wire anemometer IFA300, we measured the wall turbulence on different normal location of the boundary layer in the wind tunnel experiment section. The wind tunnel is of low turbulence intensity. We have applied Fourier transform, Wavelet transform and Hilbert-Huang transform to analyze the turbulent fluctuating velocity obtained by measurement and have compared turbulent energy transfer law in different wall distance. This article quantitatively analyzed on the eddy motion process for different wall distance in wall turbulence from the energy point of view. It also discussed the energy transfer law based on eddy combination theory and the inertial sub-range.

Wavelet packet analysis of particle response to turbulent fluctuation

The fluid–particle synchronous measurements in a boundary layer wind tunnel were conducted to determine the particle concentration response to turbulent velocity fluctuation. Three groups of natural sand samples (diameter of 300–500, 100–125 and 63–80μm) were employed in the experiments. Consecutive instants of saltating particles were recorded by using a high-speed digital camera at 2000 frames per second and a constant-temperature hot-wire anemometer was used to measure the turbulent fluctuation simultaneously. The particle concentration in the saltation layer was calculated by the dynamic-threshold binarization algorithm. The results confirm that the concentration fluctuation is a fairly typical stochastic process, and the low-frequency variation of particle concentration is closely related to the turbulent fluctuation. Moreover, a method was developed to apply wavelet packet transform to two-phase data analysis from the viewpoint of frequency-domain energy structure. Further analysis shows that the concentration fluctuation is predominant in the low frequency band less than 250Hz. In addition, the particle concentration response to the turbulent fluctuation is significantly correlated with the particle diameter. For the fine particles (63–80μm), medium particles (100–125μm) and coarse particles (300–500μm), the highest response frequencies of particle concentration variation to the turbulent fluctuation are 60, 40 and 30Hz, respectively, which demonstrates that an appropriate sampling rate is crucial in saltation measurement. These qualitative and quantitative results are beneficial to understand the fluid–particle interaction mechanism.

Coal dust/air explosions in a large-scale tube

Coal dust/air mixture explosions under weak ignition conditions have been studied in a horizontal experimental tube of diameter 199 mm and length 29.6 m. The experimental tube is closed at one end and open at the downstream end. An array of 40 equally spaced dust dispersion units was used to disperse coal dust particles into the experimental tube. The coal dust/air mixture was ignited by an electric spark. A constant-temperature hot-wire anemometer was used to measure the gas velocity during the dispersion process. Kistler piezoelectric pressure sensors were used to measure the propagation of the pressure wave during the explosion process. The maximum overpressure of the coal dust explosion under the weak ignition conditions in the tube was 70 kPa and the propagation velocity of the pressure wave along the tube was approximately 370 m/s. The minimum concentration for obtaining a coal dust explosion that propagated along the tube was 120 g/m 3. The suppressing effects on the coal dust explosion of two different kinds of suppressing agents have been studied.

Construction and experimental testing of the constant-bandwidth constant-temperature anemometer

A classical constant-temperature hot-wire anemometer enables the measurement of fast-changing flow velocity fluctuations, although its transmission bandwidth is a function of measured velocity. This may be a source of significant dynamic errors. Incorporation of an adaptive controller into the constant-temperature system results in hot-wire anemometer operating with a constant transmission bandwidth. The construction together with the results of experimental testing of a constant-bandwidth hot-wire anemometer prototype are presented in this article. During the testing, an approximately constant transmission bandwidth of the anemometer was achieved. The constant-bandwidth hot-wire anemometer can be used in measurements of high-frequency variable flows characterized by a wide range of velocity changes.

Fluctuation measurements in a supersonic boundary layer implying in-situ calibration of the hot-wire anemometer

A calibration technique for the constant-temperature hot-wire anemometer is presented, which is based on traversing the probe through the boundary layer of a flat plate while simultaneously performing fluctuation measurements. The free stream Mach number was M = 2.54, and the Reynolds number Red, based on wire diameter, ranged from 9 to 23. A comparison of the sensitivity values obtained with the aid of such a calibration procedure — under the condition of neglecting low temperature loadings (t<0.6) — agrees well with sensitivities determined with free-stream data-The use of a modified transfer function for correcting the power spectra of flow perturbations revealed a conformity of wide parts of the corrected spectra with the Kolmogorov decay. The fluctuation levels of total temperature and mass flux were computed for the boundary layer of a flat plate.

Normal flat plates in tandem: An experimental investigation

This paper reports an experimental study on the flow over two tandem flat plates perpendicular to the flow. The study was carried out by means of a constant-temperature hot wire anemometer and smoke visualizations at Re=8340. Accurate measurements of the Strouhal number were obtained for plate separations ranging from 0.25× chord to 7.5× chord. Two different flow regimes are described, depending on the separation between the plates, by exploiting evidence from the Strouhal number behaviour and the flow visualizations. A narrow interval of separation values is found where both regimes alternatively appear.

Liner dome shape effect on the annulus flow characteristics with and without swirl for a can-combustor model

The annulus flow characteristics of a can-combustor model for different liner dome shapes have been experimentally established under isothermal flow conditions for both nonswirling and swirling flows. Pressure taps were used to measure the wall pressure along the air casing and liner, whereas, velocity and pressure profiles were measured with the help of a threehole pressure probe in null mode. Turbulence intensity has also been measured with a constant temperature hot-wire anemometer. The effects of dome shapes on the recirculation zone, as well as on flow uniformity in the annulus region, for fixed dump gap, have been specially looked into. It was observed that the liner dome shape affects the size of recirculation zone and flow uniformity in the annulus region. The performance of the system was also assessed in terms of total pressure loss and static pressure recovery between the first and last measurement stations. It was found that swirling flow with a hemispherical dome liner gives better flow characteristics in the annulus region when compared with the other two dome shapes (namely, ellipsoidal, and vertical ellipsoidal).

Experimental investigations of flow through conical diffusers with and without wake type velocity distortions at inlet

Diffusers play a vital role in many fluid machines to convert kinetic energy into pressure energy. The field of application of diffusers is very wide and diffusers are used in gas turbines, pumps, fans, wind tunnels, water tunnels and many other fluid flow systems. This paper discusses the results of flow through straight conical diffusers of half-cone angles 5° and 7° with steady uniform velocity of flow and wake type distorted flow at inlet. The wake type distortion at inlet was produced by a streamlined body and a bluff body. A low speed wind tunnel was used for the experiments and the diffusers were fabricated from cast aluminium blocks. The mean velocity and the turbulence parameters were measured using a constant temperature hot-wire anemometer.

Statistically steady measurements of Rayleigh-Taylor mixing in a gas channel

A novel gas channel experiment is described to study the development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing a helium-air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, has short transients, and is statistically steady. The facility was designed to be capable of large Atwood number studies (At∼0.75). We describe initial validation work to measure the self similar evolution of mixing at density differences (0.035&amp;lt;At&amp;lt;0.1). The purpose of this paper is to describe the new high Atwood number gas channel facility and present validation results for experimental runs at Atwood numbers up to 0.1. Diagnostics include a constant temperature hot wire anemometer, and high resolution digital image analysis. The hot-wire probe gives velocity statistics of the mixing layer. A multiposition single-wire technique was used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the mixing as it develops to a self-similar regime in this flow. A digital image analysis procedure was used to characterize various properties of the flow and also to validate the hot wire measurements.

Fluid flow and heat transfer in transitional boundary layers: effects of surface curvature and free stream velocity

Velocity and wall temperature measurements, over flat plate, concave and convex walls, were experimentally investigated in a low-speed wind tunnel with inlet velocities of 4 and 12 m/s encompassing the transitional region with streamwise distance Reynolds numbers from 3.15×105 to 1.04×106. As the velocity profiles, recorded by a semi-circular pitot tube and a digital constant-temperature hot-wire anemometer, were compared to exact Blasius profile and (1/7)th power law, experimental local Stanton numbers to analytical flat plate solution and turbulent correlation formula. Intermittency factors, derived from velocities and local Stanton numbers, were presented both in streamwise and pitchwise directions. It was found that the convex curvature delayed transition up to Re x =1.04×106, with a mean intermittency value of 0.61 and a shape factor of 1.81, where the similar intermittency and shape factors were determined at Re x of 8.33×105 and 4.25×105 for the flat plate and concave wall, indicating the enhancing role of concave curvature on the transition mechanism. The thinner boundary layers of the concave surface resulted in higher intermittency values, corresponding to higher skin friction and Stanton numbers; moreover the lowest gap between the measured and derived Stanton numbers were also obtained over the concave surface. Destabilising role of the concave wall caused Stanton numbers to increase up to 22%, whereas the convex wall, due to its stabilising character, produced lower Stanton numbers by 12% with respect to those of the flat plate.

Turbulence modification in vertical upward annular flow passing through a throat section

Experimental studies on the turbulence modification in annular two-phase flow passing through a throat section were carried out. The turbulence modification in multi-phase flow due to the interactions between two-phases is one of the most interesting scientific issues and has attracted research attention. In this study, the gas-phase turbulence modification in annular flow due to the gas–liquid phase interaction is experimentally investigated. The annular flow passing through a throat section is under the transient state due to the changing cross sectional area of the channel and resultantly the superficial velocities of both phases are changed compared with a fully developed flow in a straight pipe. The measurements for the gas-phase turbulence were precisely performed by using a constant temperature hot-wire anemometer, and made clear the turbulence structure such as velocity profiles, fluctuation velocity profiles. The behavior of the interfacial waves in the liquid film flow such as the ripple or disturbance waves was also observed. The measurements for the liquid film thickness by the electrode needle method were also performed to measure the base film thickness, mean film thickness, maximum film thickness and wave height of the ripple or the disturbance waves.

The CREC Fluidized Riser Simulator. Characterization of Mixing Patterns

The CREC Riser Simulator Reactor is a new laboratory scale unit that was invented at CREC-UWO (de Lasa, 1991). It was specially designed for catalyst testing and kinetic modeling under the conditions of riser and downer reactors. Preliminary work on mixing patterns in the Riser Simulator was already effected by our group using a helium tracer and a constant temperature hot wire anemometer (Pekediz et al, 1993) with both of these devices placed in the outer reactor annulus. However and given the possible ambiguity of the mixing patterns established with these data, helium tracer was injected in the present study into the central catalyst basket, while velocity measurements were effected in the outer annulus. This methodology allowed the definition of effective gas mixing time and gas superficial velocities at various reactor pressures and impeller speeds with increased confidence. This study found that gas mixing times corresponding to high gas recirculation rates in a CREC Riser Simulator occur over impeller rotational speeds ranging from 3000-6000 rpm and total reactor pressures ranging from 1-5 atm. With this end, gas mixing patterns in a Riser Simulator represent a well mixed unit, and thus, the importance of the Riser Simulator can be realized in the context of its potential use for gas-solid reaction kinetics.

D203 SEPARATED VORTICES GENERATED BY SPIRAL UP FLOW AND THIN-PLATE AIRFOILS

Measurements of the vorticity field flowing around the front platen are firstly made with the IFA300 constant temperature hot wire anemometer and the six-sensor hot-wire probe, which is located on a 3-D movable frame for coordinate setting, in the tangentially fired furnace. Separated vortices excited appear at tail of multi-platen for compound function being composed of spiral up flow and thin-plate airfoils. Six-sensor hot-wire probe data demonstrate many interesting features of the flow. Equicontours of mean longitudinal vortex show that separated vortices are distributed at tail of multi-platen in the upper furnace. It is firstly discovered that there are evidently separated vortices at tail of the front platen on the right half of the furnace exit. Also some vortices appear accompanied by another opposite vortex. Mean velocity and vorticity data show that the velocity is higher but also the vorticity is larger nearby the right wall.