Constant Temperature Hot-wire Anemometry Research Articles

A wind tunnel study of adverse pressure gradient impact on wind turbine wake dynamics

Wind tunnel experiments were conducted to investigate the impact of an adverse streamwise pressure gradient on the wake dynamics of a small-scale horizontal-axis wind turbine. The study used high-frequency constant temperature hotwire anemometry to measure wake velocities up to 7 rotor diameters (dT) downstream. Two cases were studied: zero pressure gradient (ZPG) and adverse pressure gradient (APG). Experimental conditions included a Reynolds number based on the rotor diameter of Re≈6.7×104 and an inflow velocity of 7 m/s. The results show significant differences between the ZPG and APG cases as the wake progresses downstream. The APG case exhibits a higher maximum velocity deficit in the far wake and a more distorted, oval-shaped wake than the ZPG case. Turbulence intensity trends also differ, with a decreasing trend in the ZPG case and an increasing trend in the APG case. Energy spectra analysis at specific wake locations offers insights into flow structure development. In both cases, small-scale (high-frequency) energy decreases as the wake advances, while large-scale (low-frequency) energy increases. Specifically, the adverse pressure gradient attenuates certain large-scale flow structures λx/dT∈[0.05,0.2], corresponding to f∈[20,100]Hz near the rotor tip height and narrows downstream. Additionally, in the APG case, energy increases in the lower half of the wake for flow structures ranging from λx/dT∈[0.04,0.5], or f∈[10,100]Hz near the rotor, narrowing further downstream. The results suggest that an adverse pressure gradient enhances turbulence and hinders velocity recovery toward freestream conditions in the wake of a wind turbine.

Roughness sublayer over vegetation canopy: A wind tunnel study

Atmospheric flows in the inertial sublayer (ISL) and the roughness sublayer (RSL) are complicated by vegetation canopies. They subsequently modify the exchange of momentum and tracer fluxes between the plants and the overlying atmospheric surface layer (ASL). In this study, the flows over idealized vegetation canopies are modeled by wind tunnel experiments. Using fast-response, constant-temperature (CT) hot-wire anemometry (HWA), mean winds and turbulence are measured to test the sensitivity of the dynamics to the configurations of tree models. The drag coefficient measured is in the range of 4.0 × 10-3 ≤ Cd ≤ 7.6 × 10−3. The dimensionless profiles of mean and fluctuating velocities agree well with previous results obtained by wind tunnel experiments and large-eddy simulation (LES) over various vegetation canopies. Incorporating a correction function, a new analytical solution to the mean-wind-speed profile, which is applicable to both RSL and ISL continuously, is developed that is verified favorably by the current wind tunnel data. One of the key parameters, the RSL constant, is found in the range of 2.1 ≤ μ ≤ 2.6 that is larger than its counterpart over urban canopies. Unlike ISL, the skewness and kurtosis of flows show that RSL momentum transport is mainly governed by rare, high-speed, downdrafts (u’ > 0 and w’ < 0; sweep Q4) and massive, low-speed, updrafts (u’ < 0 and w’ > 0; ejection Q2). This finding concurs the dominant events unveiled by quadrant-hole analysis. The results contrast the dissimilarity between ISL and RSL that improve our understandings of their dynamics over vegetation canopies.

Roughness sublayer flows over real urban morphology: A wind tunnel study

Atmospheric surface layer (ASL) consists of the inertial sublayer (ISL) and the roughness sublayer (RSL) with different dynamics. This study characterizes the ASL flows over real urban morphology using wind tunnel experiments. The building models of downtown Kowloon Peninsula, Hong Kong are fabricated by 3D printing. The vertical profiles (3 × 45 numbers) on three vertical planes in the streamwise direction are selected for velocity measurements by constant-temperature (CT) hot-wire anemometry (HWA). There exist noticeable variations of mean wind speed u‾, fluctuating velocities u’’ and w’’ together with vertical momentum flux u’’w’’ in the ISL and RSL over urban areas. On the plane with maximum drag coefficient Cd (25% higher than that on the other two), the RSL is thicker by 10% in which the winds evolve in response to individual buildings. A new analytical solution is derived to improve the prediction of ASL mean wind speeds whose root-mean-square error (RMSE) is reduced by 10 times. Its key variable, the RSL constant, converges asymptotically μ = 1.7 for urban setting. Unlike the ISL, turbulence statistics show that the RSL velocities are positively skewed (Su > 0) with high kurtosis (Ku > 3). The occurrence of ejection Q2 (detrainment; 32%–37%) is more frequent than sweep Q4 (entrainment; 30%–32%). Whereas, high-speed Q4 contributes most (0.2u’’w’’ to 0.4u’’w’’) to the RSL vertical momentum flux among the four quadrants. It is thus suggested that fresh air entry is driven by spurlike downdrafts while street-level aged air is removed by massive uprising current.

Preflight Ground Test Analyses of the Boundary Layer Transition (BOLT) Flight Geometry

Wind-tunnel experiments were performed to quantify boundary-layer instabilities both on and off the surface of a 33% scale model of the U.S. Air Force Research Laboratory/U.S. Air Force Office of Scientific Research Boundary Layer Transition (BOLT) flight geometry. Measurements were obtained in the Mach 6 Quiet Tunnel and Actively Controlled Expansion (ACE) tunnel located at the Texas A&M University National Aerothermochemistry and Hypersonics Laboratory. Surface heating was quantified using infrared thermography. Under quiet conditions, heat flux was characterized by a streak structure with no transition observed. Transition was observed within the conventional noise ACE facility in two regions: 1) in the vicinity of a mixed-mode structure and 2) in an outboard traveling crossflow region. The flow structure within a coherent vortex rollup located off centerline was further characterized under quiet flow conditions with constant-temperature hot-wire anemometry and high-frequency surface-pressure spectra. The measured mean and disturbance flow structure were in qualitative agreement with the simulations. The hot-wire spectral content showed growth in the 25–40 kHz range, which also agreed with the simulations. Surface-pressure spectral results showed similar growth in the 25–50 kHz range observed under both quiet and conventional freestream conditions.

Experimental Investigation into the Turbulence Flowfield of In-Flight Round Jets

In this paper, insight is provided into the modeling of single-point and two-point statistics of a subsonic round jet discharged into a moving ambient medium. An experimental campaign has been performed comprising two round, unheated air flows: a central jet at Mach number equal to 0.6 surrounded by a slower flight flow. Constant-temperature hot-wire anemometry was used to measure both the axial and radial velocity fluctuations within the turbulent jet flowfield. A Mach 0.6 jet was discharged into the flight flow, which ranged from zero up to Mach 0.3. The data show that the degree to which the jet stretches with increasing flight velocity can be discerned with the knowledge of the decay of the mean velocity field downstream of the end of the jet’s potential core. This stretching factor can then be used to predict the changes in the static jet turbulence statistics for the in-flight case. Additionally, in the region of high-turbulence kinetic energy, the two-point statistics can be estimated using information about the single-point statistics and the local mean velocity. Empirical models for the in-flight jet’s shear stresses, cross-correlations, and power spectral density functions are presented and compared with those derived for the static jet case.

Machine learning strategies applied to the control of a fluidic pinball

The wake stabilization of a triangular cluster of three rotating cylinders is investigated. Experiments are performed at Reynolds number Re ∼ 2200. Flow control is realized using rotating cylinders spanning the wind-tunnel height. The cylinders are individually connected to identical brushless DC motors. Two-component planar particle image velocimetry measurements and constant temperature hot-wire anemometry were used to characterize the flow without and with actuation. Main open-loop configurations are studied and different controlled flow topologies are identified. Machine learning control is then implemented for the optimization of the flow control performance. Linear genetic algorithms are used here as the optimization technique for the open-loop constant speed-actuators. Two different cost functions J are considered targeting either drag reduction or wake symmetrization. The functions are estimated based on the velocity from three hot-wire sensors in the wake. It is shown that the machine learning approach is an effective strategy for controlling the wake characteristics. More significantly, the results show that machine learning strategies can reveal unanticipated solutions or parameter relations, in addition to being a tool for optimizing searches in large parameter spaces.

EXPERIMENTAL INVESTIGATION OF FLOW RESPONSE TO STATIONARY DISTURBANCE IN ROTATING-DISK FLOW

Three-dimensional boundary-layer flow is well known for its abrupt and sharp transition from laminar to turbulent regime. The presented study is a first attempt to achieve the target of delaying the natural transition to turbulence. The behaviour of two different shaped and sized stationary disturbances (in the laboratory frame) on the rotating-disk boundary layer flow is investigated. These disturbances are placed at dimensionless radial location (Rf = 340) which lies within the convectively unstable zone over a rotating-disk. Mean velocity profiles were measured using constant-temperature hot-wire anemometry. By careful analysis of experimental data, the instability of these disturbance wakes and its estimated orientation within the boundary-layer were investigated.

Revisiting end conduction effects in constant temperature hot-wire anemometry

The effect of end conduction losses in constant temperature hot-wire anemometry is studied theoretically and experimentally. A perturbation analysis is employed to determine the dynamic response of a hot-wire together with its constant temperature anemometer (CTA), with unetched portions of the wire (stubs) and prongs taken into account. Results of the model reveal that when the end conduction losses are significant, dynamic sensitivity of the hot-wire-CTA system can be either more or less than the static sensitivity based on the specifications of the hot-wire probe and the CTA. This means that contrary to the commonly accepted scenario, a hot-wire with insufficient length-to-diameter ratio could either amplify or attenuate turbulence energy depending on probe and CTA parameters. Experiments performed in a turbulent boundary layer flow are used to confirm this result. Examination of variance of streamwise velocity fluctuations and energy spectra from the experiments reveal that interaction of the wire filament with the supports through axial conduction results in one or two low frequency steps in the transfer function of the hot-wire-CTA system. These steps can be either positive or negative depending on the hot-wire geometry and damping of the system.

Modelling and operation of sub-miniature constant temperature hot-wire anemometry

High-Reynolds number flows are very common in technological applications and in nature, and hot-wire anemometry is the preferred method for measuring the time-series of fluctuating velocity in such flows. However, measurement of very high-Reynolds number flows requires hot-wires with higher temporal and spatial resolution than is available with conventional probes. Much effort has therefore been devoted to decreasing the size of the hot-wire probes and this has led to associated challenges with operation. It is this latter operation problem which is the focus of this paper. To this end, an existing theoretical model of constant-temperature hot-wire anemometers (Perry 1982 Hot-Wire Anemometry (New York: Oxford University Press), Watmuff 1995 Exp. Therm. Fluid Sci. 11 117–34) is applied, and its accuracy is tested for the first time by comparison to measurements using an in-house constant temperature anemometer (CTA) for both conventional m-diameter wires and sub-miniature hot-wires. With the aid of this model, we propose modifications to the CTA design and demonstrate successful operation of the CTA with the Princeton nano-scale thermal anemometry probe (NSTAP) (Bailey et al 2010 J. Fluid Mech. 663 160–79). It is also shown that the transfer function obtained from the model can be utilized to estimate the true frequency response and cut-off frequency of a hot-wire-CTA system to the velocity fluctuations, which is essential in accurate measurements of energy spectrum and higher order statistics of turbulent flows.

The Effect of Nitrogen Cross-Over on Water Balance Measurements in Proton Exchange Membrane Fuel Cell Using Constant Temperature Anemometry

R&D on Proton exchange membrane fuel cells (PEMFC’s) technology has been accelerated in the last few years to reduce the dependency on fossil fuel. PEMFC’s operate by internally combining oxygen from air with hydrogen to form water and generate electricity and waste heat. The most common hurdle for enhanced PEMFC durability and performance is still the water management: the proton exchange membrane in the center of these fuel cells has to be hydrated in order to keep its ability to conduct protons from anode to cathode side while on the other hand excessive liquid water can lead to cell flooding and increased degradation rates of the cell. Thus, a detailed understanding of all aspects of water management in PEMFC is important. This includes the fuel cell water balance, i.e. the question which fraction of the product water leaves the fuel cell via the anode channels versus the cathode channel. Our research group is currently developing a state of art technology to obtain an ad-hoc and real time electrical signal of the fuel cell water balance by employing a constant temperature hot wire anemometry [1]. The hot wire sensor is placed in the anode outlet of PEMFC, and the voltage signal received gives valuable insight into heat and mass transfer phenomena which can be interpreted directly to real time PEMFC water balance. So far, ex-situ experiments have been conducted to measure the voltage signal of the hot wire anemometer for a known gas stream that contains a binary mixture of hydrogen and water vapour as would ideally leave the fuel cell anode. However, in an operating fuel cell there are two main uncertainties in this method: (i) there is some internal hydrogen crossover that does not load to an external current, thus the exact amount of hydrogen leaving the fuel cell anode is not known, and (ii) that there is nitrogen crossing over from the cathode side to the anode side, and this nitrogen does slighty falsify the voltage signal that the hot wire yields. The effects of nitrogen-cross over on the hot wire voltage signal at the anode outlet and consequently on the measured water balance (rd ) is studied in this work. In our previous work it was shown that the only unknown in the determination of the hot wire voltage signal is the equation that determines the heat transfer around the hot wire, and we have shown that it is necessary to employ a power-law equation as suggested by Hilpert: Nu = C Pr0.33 Rem where Re is the Reynolds number and Pr is the Prandtl number are calculated at film temperature. The constants C and m usually depend on the Re, and they will be determined out of the experimental data for the gas mixture, by plotting the measured (Nu/ Pr0.333 ) versus the Re number of the gas mixture stream. The missing C and m from Hilpert equation can then be easily obtained from a power-fit of the general form Y = CXm . Also, it was shown somewhere else that in fact only the exponent to the Reynolds number, of the gas mixture stream is important when determining the fuel cell water balance out of the hot wire signal [1]. The nitrogen-cross over is experimentally demonstrated by introducing 1% of nitrogen to the dry hydrogen molar flow. The 99%H2 +1%N2 of the dry mixture is humidified with water vapor by controlling the relative humidity (RH) of the dry mixture with in the range of (0-100)%RH, simulating the PEMFC anode outlet. The hot wire voltage is measured with and without nitrogen and it was slightly lower with the presence of 1 % nitrogen in the flow. The effect of the voltage reduction on the measured water balance can be neglected. This because the effect of 1% nitrogen on power law constant’s m which is used in determining the water balance as explained somewhere else is extremely low. It might be concluded that, the hot wire technique for measuring the water balance is still accurate and can be used for ad-hoc and real time water balance measurements and the nitrogen cross-over affect can be neglected, knowing that the 1% N2 is one order of magnitude higher than the actual concertation of 0.02 m2 active area cell used for lab testing. [1] Berning, T., & Al Shakhshir, S., Applying hot wire anemometry to directly measure the water balance in a proton exchange membrane fuel cell–Part 1: Theory. International Journal of Hydrogen Energy, 40(36) (2015), 12400-12412.

The Effect of Nitrogen Cross-Over on Water Balance Measurements in Proton Exchange Membrane Fuel Cell Using Constant Temperature Anemometry

A novel method to obtain an ad-hoc and real time electrical signal of the PEMFC water balance by employing a constant temperature hot wire anemometry has been developed by our fuel cell research group. In this work, the effect of nitrogen-cross over on this method is experimentally demonstrated by introducing 1% of nitrogen concentration to the dry and humidified hydrogen flow simulating the PEMFC anode outlet. The hot wire voltage is measured with and without nitrogen and it was slightly lower with the presence of nitrogen. The effect of the voltage reduction on the measured water balance is small. This is due to the effect of 1% nitrogen on power law constant’s “m” which can be used in determining the water balance as explained in previous work was extremely low. Thus, the hot wire technique for measuring the PEMFC water balance is still accurate with the nitrogen cross-over.

Fibre behaviour in the spunbonding process. Part I: Characterisation of air flow and fibre motion in the diffuser

The random fibre oscillatory behaviour induced by turbulence in the diffuser of an industrial spunbonding rig is measured experimentally. The turbulent air flow is firstly characterised by constant temperature hot-wire anemometry: averaged flow quantities, such as the mean velocity and the turbulent kinetic energy, as well as time dependent quantities, such as the integral time and the energy spectrum, are measured. The influence of the turbulent flow on the dynamics of a nylon fibre of diameter 200 µm and a spunbonding fibre of diameter 18 µm in the diffuser is then investigated by extracting the transverse displacement from images acquired by a digital camera.

Crossflow Instability on a Yawed Cone at Mach 6

Boundary-layer measurements were performed in a Mach 6, low-disturbance wind tunnel on a 7∘ cone at 5.6∘ angle of incidence such that the model was primarily subject to crossflow instability. Constant-temperature hot-wire anemometry was used to measure the streamwise mass flux at a series of planes normal to the cone axis. A dominant stationary wave is observed to achieve nonlinear saturation at approximately 23% with maximum rms fluctuations of approximately 8%. Additionally, traveling crossflow waves were observed in a broad frequency band centered at f = 40kHz and secondary instabilities were observed in a broad band centered at f = 100kHz. Measurements show excellent agreement with previously published computational results and are qualitatively similar to studies performed in subsonic flows. Transition to turbulence was not observed to occur under these conditions.

Velocity Deficit and Swirl in the Turbulent Wake of a Wind Turbine

AbstractEnergy production data from several of the existing large offshore wind farms indicate that turbine arrays can suffer from a significant overall energy production shortfall, due to wakes generated by turbines upstream interacting with turbines downstream. An experimental investigation of the axial and azimuthal (swirl) velocity field in the wake of a single three-bladed wind turbine with rotor diameter of 0.91 m was conducted. The turbine was positioned in the free stream, near the entrance of the 6 m×2.7 m cross section of the UNH Flow Physics Facility, a 72-m-long boundary layer wind tunnel. The turbine model was tested at various rotor loading conditions with blade tip-speed ratios up to 2.8. A Pitot-static tube and constant temperature hot-wire anemometry with a multiwire sensor were used to obtain velocity field measurements in the wake of the model turbine up to 20 diameters downstream. The results of an equilibrium similarity theory for the axisymmetric wake with rotation are presented. The measurements obtained were used to examine the validity of the derived scaling functions for streamwise and azimuthal velocity, wake growth, and turbulence.

Finite mixture model applied in the analysis of a turbulent bistable flow on two parallel circular cylinders

This paper presents a study of the bistable phenomenon which occurs in the turbulent flow impinging on circular cylinders placed side-by-side. Time series of axial and transversal velocity obtained with the constant temperature hot wire anemometry technique in an aerodynamic channel are used as input data in a finite mixture model, to classify the observed data according to a family of probability density functions. Wavelet transforms are applied to analyze the unsteady turbulent signals. Results of flow visualization show that the flow is predominantly two-dimensional. A double-well energy model is suggested to describe the behavior of the bistable phenomenon in this case.

Experimental characterization of transition region in rotating-disk boundary layer

The three-dimensional boundary layer due to a disk rotating in otherwise still fluid is well known for its sudden transition from a laminar to a turbulent regime, the location of which closely coincides with the onset of local absolute instability. The present experimental investigation focuses on the region around transition and analyses in detail the features that lead from the unperturbed boundary layer to a fully turbulent flow. Mean velocity profiles and high-resolution spectra are obtained by constant-temperature hot-wire anemometry. By carefully analysing these measurements, regions in the flow are identified that correspond to linear, weakly nonlinear, or turbulent dynamics. The frequency that dominates the flow prior to transition is explained in terms of spatial growth rates, derived from the exact linear dispersion relation. In the weakly nonlinear region, up to six clearly identifiable harmonic peaks are found. High-resolution spectra reveal the existence of discrete frequency components that are deemed to correspond to fluctuations stationary with respect to the disk surface. These discrete components are only found in the weakly nonlinear region. By systematically acquiring low- and high-resolution spectra over a range of narrowly spaced radial and axial positions, it is shown that while the transition from laminar to turbulent regimes occurs sharply at some distance from the disk surface, a complex weakly nonlinear region of considerable radial extent continues to prevail close to the disk surface.

Boundary-layer instability and transition on a flared cone in a Mach 6 quiet wind tunnel

Measurements of boundary-layer transition location and boundary layer profiles on a sharp-tipped 5o-half-angle flared cone were made in a low-disturbance Mach 6 wind tunnel. Uncalibrated boundary-layer profiles of mean and fluctuating voltage representative of mass flux are obtained using constant temperature hot-wire anemometry at several axial locations, and are notionally compared with preliminary simulations. Spectral energy content is observed between 250 and 310 kHz – the first measurements of frequencies typical of the second mode instability at Texas A&M. Growth of this high-frequency content is compared with N-factor results from linear parabolised stability equation (LPSE) computations. Possible sources of disagreement between the experimental and computed frequencies for second-mode growth are discussed, as are future improvements to the hotwire anemometry technique. Nevertheless, the successful measurement of high-frequency content highlighted here constitutes an important step toward acquisition of calibrated measurements of hypersonic boundary-layer instabilities to be used as code validation.

1226 THE COLAPIPE - A HIGH REYNOLDS NUMBER PIPE TEST FACILITY

A new pipe test facility for investigations on high Reynolds number pipe flow turbulence (Rem≦ 1.5 X 106) was setup, namely the CoLaPipe. The facility is designed to be closed-return with two available test sections providing a length-to-diameter ratio of L/D = 148 and L/D = 79. We present intensive measurements on the development of the flow from laminar to turbulent flow state with respect to natural transition as well as artificial transition. Our focus is on the detection of the fully developed turbulent flow state due to the characteristic behavior of the mean flow and the higher order statistics, e.g. turbulence intensity, skewness and flatness. The influence of tripping devices, i.e. orifices with different blockage ratios, on this evolution is of great interest, respectively. Here the question arises whether the turbulent flow state can be induced earlier in streamwise direction or not, which is important for further experiments, like CICLoPE. As the turbulent flow state is important due to structural observations and the deduced scaling behavior, we present additional experimental results to shed light on some open questions, i.e. inner peak behavior. All presented measurements are conducted on a hydraulically smooth surface by using single-wire probes working in constant-temperature hot-wire anemometry mode.

A study on the influence of triggering pipe flow regarding mean and higher order statistics

The evolution of statistical pipe flow quantities, such as the turbulence intensity, the skewness and the flatness, are investigated to clarify, hich development length is needed, until the state of fully developed turbulence is achieved. This observations take place in a relatively large pipe test facility with an inner pipe diameter of Di = 0.19 m and a total length of L = 27 m. The reached Reynolds number range is 1.5 · 105 ≤ Rem ≤ 8.5 · 105. To quantify the mean and fluctuating velocity as well as the depending statistical quantities, the constant temperature hot-wire anemometry is applied. Through these intensive centerline measurements we observe a development length of L = 70 D, to ensure a fully developed turbulent flow state.

A new criterion for end-conduction effects in hot-wire anemometry

The effect of end conduction on constant temperature hot-wire anemometry was studied. A new parameter, , is proposed to describe the significance of end conduction more comprehensively than the commonly used length-to-diameter ratio ℓ/d, in that it allows for material property variations, resistance ratio and Reynolds number effects. Numerical and experimental data are used to show that Γ improves the correlation of the attenuation of measured turbulence fluctuations, and it is found that Γ > 14 is required to avoid such effects.