Hot-film Anemometer Research Articles

Dynamics of constant temperature anemometers for the Martian Atmosphere

The objective of this paper is to understand the limits on the response time generated by the thermal dynamics of the Martian wind sensor of the MEDA instrument, working under Constant Temperature Operation (CTO). This technology has been flown aboard the Mars Science Laboratory, InSight, and Mars 2020. It will be shown that active control thermal anemometry enables a time response to wind fluctuations faster than the time constants defined by the thermal inertia of the sensor components. It will also be shown that the temperature redistributions on the temperature-uncontrolled parts of the supporting structure impose a limit on the bandwidth of the sensor. We present a model of the MEDA hot film anemometers describing what parameters control their dynamical response, why wind tunnel testing confirmed them to be faster than 1 s, and criteria to select materials that reduce the response time to approach 0.2 s.

The Role of Coherent Airflow Structures on the Incipient Aeolian Entrainment of Coarse Particles

AbstractThe role of coherent airflow structures capable of setting gravel‐sized particles in motion is studied theoretically and experimentally. Specifically, a micromechanical model based on energy conservation is proposed to describe the incipient motion of large‐particles ranging from rocking (incomplete entrainment) to incipient rolling (full entrainment). Wind tunnel experiments were conducted on an aerodynamically rough bed surface under near‐threshold airflow conditions. Synchronous signals of airflow velocities upwind of the test particles and particle displacement are measured using a hot film anemometer and a laser distance sensor, respectively, from which coherent airflow structures (extracted via quadrant analysis) and particle movements are interlinked. It is suggested that the incipient motion of gravel‐sized particles (rocking and rolling) may result from sufficiently energetic sweep events corresponding to aerodynamic drag forces in excess of the local micro‐topography resistance. However, full entrainment in rolling mode should satisfy the presented work‐based criterion. Furthermore, using an appropriate probabilistic frame, the proposed criterion may be suitable for describing processes of energy transfer from the wind to the granular soil surface, ranging from the creep transport of gravels to the “mechanical sieving” of mega‐ripples, as well as the transport of light anthropogenic debris (such as plastics).

Multi-dimensional characteristics of upward bubbly flows in a vertical large-size square channel

Gas-liquid two-phase flows frequently occur in various flow channels in engineering systems with heat and mass transport processes. The channel geometry affects thermo-fluid behavior generally. Extensive research is done on gas-liquid two-phase flows in circular and annular channels with a wealth of local two-phase flow measurement data. Square channels are used in various engineering disciplines, but there has only been limited experimental research on two-phase flows in square channels with local flow parameter measurements. For two-phase flow analyses in square channels, knowledge of the flow characteristics and model validation against some key parameters, such as the drift-flux parameters and the interfacial area concentration, is crucial. An experiment for upward bubbly flows was conducted in a large square channel with an inside cross-section of 0.136 × 0.136 m. A four-sensor optical probe and a hot-film anemometer were used to measure local two-phase flow parameters at 66 points in an octant symmetric triangular cross-section at three axial locations. The measured two-phase flow parameters were the void fraction, axial gas velocity, axial liquid velocity, interfacial area concentration, and bubble Sauter mean diameter. The flow characteristics through flow development were investigated based on the local measurement data. The local measurement data directly determined the drift-flux parameters through their definitions. The distribution parameters determined through experimentation could be reproduced by the existing correlation for large circular pipes indicating that the flow characteristics in large square channels may be similar to those in large circular pipes. Based on existing drift-flux correlations for large circular pipes, the void fractions in the current experiment could be fairly predicted. Furthermore, the interfacial area concentrations could be predicted by existing correlations with reasonable accuracy.

Wind-tunnel studies on the characteristics of indoor/outdoor airflow and pollutant exchange in a building cluster

The diffusion of air pollutants in urban areas is essentially a multiscale problem, which has aroused increasing concerns. Wind-tunnel experiments were employed to explore the characteristics of indoor/outdoor airflow and pollutant exchange in a building cluster. Under two wind directions ( β ​= ​0°,45°), smoke visualization tests were firstly conducted to visualize the diffusion characteristics of air pollutants released from different pollution sources. Then the 3D hot-film anemometer and SF 6 quantitative detector were used to measure the turbulence parameters and pollutant distributions. The smoke visualization tests showed that under β ​= ​0°, the pollutants were mainly concentrated in the middle main street canyons (S-1 and S-2), spread downstream, and were gradually diluted. However, the influence area under β ​= ​45° was wider. The results confirmed that the indoor pollutant concentration along different floors was closely related to that around the outside target building. The window-opening mode had great influence on indoor pollutant concentration, but little influence on the other floors. Additionally, when the pollution source was located in the building leeward wake, regardless of the building height, it could reduce the indoor air quality of the leeward building. This study is helpful to understand the characteristics of flow and dispersion through different urban scales. The configurations of multiscale diffusion models. • Wind-tunnel tests are employed to study indoor/outdoor exchange in a building cluster. • Two wind directions and different window opening modes are considered. • The smoke visualization tests allow the visualization of pollutant diffusion. • Pollutants in the building wake can diffuse vertically to the top leeward rooms. • The characteristics of pollutant diffusion through different urban scales are understood.

A Continuous, Impedimetric Parylene Flow Sensor

A continuously recording, impedimetric thermal liquid flow sensor fabricated on a Parylene C thin film substrate is presented for the first time. The sensing concept, inspired by hot-film anemometry, includes an additional upstream unheated reference electrode pair which enabled attenuation of environmental drift in impedance by more than 5×. This sensor design and transduction approach was motivated by the need for an alternative to traditional hot-film anemometers for in vivo applications. An analytical model was developed to describe the axial fluid temperature surrounding the impedimetric sensor and its effect on solution conductivity. The impact of heater power was modeled and matched with experimental data; similar analysis was conducted for other parameters including channel height, ambient temperature, and electrolyte concentration. The sensor achieved a 2σ resolution of 17 μL/min over the range 43-200 μL/min. The models and fabricated device significantly expand our understanding of thermal impedimetric flow sensing. [2020-0357]

Fluid Dynamics of Condensation Process with Direct Contact in Desuperheaters

Due to the global warming and environmental implications, the focus of household heating has shifted from fossil fuels towards environmentally friendly and renewable sources. Desuperheaters have been found an attractive option as a domestic provision for the warm water; they used steam induced direct contact condensation (DCC) as the major means to warm the water. The present study has been an attempt to investigate the hydrodynamics in the Desuperheater vessel experimentally, when the pressurized pulsating steam was injected into the vessel, where, the steam jet interacted co-currently with the slow-moving water. Visual flow visualization provided an overall flow picture that showed a circulation region when the pulsating steam was injected into the slow co-currently moving water and the peaked vorticity corresponded to the steam injection duration varying from 10-60 seconds. An array of 7 Hot Film Anemometers (HFA) was traversed axially and radially to determine the velocity fluctuations at 0 – 20 cm from the steam's nozzle exit. Vertical structures were obtained that corresponded to the entrainment of the steam with the surrounding concurrently moving water. The circulation regions were thus exhibited in relation to the steam's injection durations as well as the downstream axial distance of 2 cm and 15 cm from the nozzle exit, which showed that the core local circulation at 2 cm, lost 75-79% of its circulation at 15 cm downstream of the nozzle exit.

Experimental and numerical analysis of airflow around a building model with an array of domes

Domed and curved structures are widely used in the hot arid regions of Iran and other Middle Eastern countries because of their ventilation advantages. Many experimental and numerical studies were reported for the wind flow over a single dome or single vault. However, studies of the airflow around an array of domes, which are important for designing or constructing these structures, are scarce. In this study, the wind flow around a building model with an array of domes was studied experimentally and numerically. A hot-film anemometer was used, and airflow velocity profiles around the building model in a wind tunnel were measured. The RANS approach and the large eddy simulation (LES) were used, and simulations were performed for Reynolds numbers of 43,000 and 430,000. The predicted streamwise mean velocity and root-mean-square fluctuation velocity profiles around the model were compared with the wind tunnel data, and good agreements were observed. The presented results indicated that the separation points over the domes move further downstream for the second and third domes compared with the first dome. Also, the peak suction pressure was observed over the first dome near the dome apex, and the maximum pressure was seen in the windward side of the third dome. The results were compared with the case of airflow around a single dome, and the influence of the dome's location in the array was discussed. Also, recommendations for the design or construction of buildings with an array of domes were provided.

Acoustically driven oscillatory flow fields in a cylindrical resonator at resonance.

Generation and development of acoustic waves in an air-filled cylindrical resonator driven by a conical electro-mechanical speaker are studied experimentally and simulated numerically. The driving frequencies of the speaker are chosen such that a standing wave field is produced at each chosen frequency in the resonator. The amplitude of the generated acoustic (pressure) waves is measured along the axis of the resonator by a fast response piezo-resistive pressure transducer, while the radial distribution of the oscillatory axial velocities is measured at the corresponding velocity anti-node locations by a constant temperature hot-film anemometer. For the cases studied, the acoustic Reynolds number ranged between 20.0 and 60.0 and the flow fields were always found to be in the laminar regime. The flow field in the resonator is also simulated by a high-fidelity numerical scheme with low numerical diffusion. Formation of the standing wave and quasi-steady acoustic streaming are numerically simulated by solving the fully compressible form of the Navier-Stokes equations. The effects of the sound field intensity (i.e., input power to the speaker) and driving frequency on the standing wave field and the resultant formation process of the streaming structures are also investigated.

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.

Wind tunnel study of airflow recovery on the lee side of single plants

Plants play an important role on reducing the soil erosion rate and preventing blown sand motion. The primary cause is the airflow change around the plant, especially for the lee side of plants. Although scientist have researched this topic, significant problems remain concerning airflow around plants. Therefore, we conducted a series of wind tunnel experiments to simulate average airflow speed and turbulence intensities on the lee side of eight single plants with varying characteristics under different shear velocities by utilizing a hot film anemometer. We come to the following conclusions:(1) Variation in the airflow speed along the plant downwind direction is related to the porosity and the height-to-width ratio (H/W). The weakened degree of wind speed decreases with plant porosity, and the minimum wind speeds (umin) at different heights are different for different H/W. For large H/W (H/W ≥ 2), the values of umin appear at the location of 1 H in the lee side of the plant, while the location where umin occurs for small H/W (H/W ≤ 0.5) is related to the height. The location most frequently occurs between 3 h and 5 h.(2) This paper presented a modification for relaxation equation to express airflow recovery on the lee side of plant and developed the relationships of the minimal wind speed (umin), occurring lee-side location (x0), and the characteristic length (l) in this modified relation equation, with different plant characteristic. The value of umin increases with the plant porosity (β) in a linear function of umin = 0.0183β-0.65 and the location (x0) where umin occurs and the characteristic length for wind speed recovery are proportional to the reciprocal of the ratio of plant height-to-width. Their relationships can be expressed as x0 = 1.68(H/W)−1 and l = 5.30(H/W)−1, respectively.(3) The turbulence intensity downwind direction of the plant is several times the intensity of the incoming flow, and the peak turbulence intensity can reach up to 50%. The more significantly the wind speed weakens, the more significant the increase in the turbulence intensity. The standard deviation of the wind speed varies slightly.

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions.

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Thermal dynamics modeling of a 3D wind sensor based on hot thin film anemometry

The objective of this paper is to obtain time-varying models of the thermal dynamics of a 3D hot thin film anemometer for Mars atmosphere. To this effect, a proof of concept prototype of the REMS (Rover Environmental Monitoring Station) wind sensor on board the Curiosity rover has been used. The self and cross-heating effects of the thermal structures have been characterized from open-loop measurements using Diffusive Representation. These models have been proven to be suitable in the analysis of the thermal dynamics of the sensor under constant temperature operation employing the tools of Sliding Mode Controllers. This analysis allows to understand the long term heat diffusion processes in the whole structure and how they may affect the raw output signals.

Fine-scale turbulent bursts in stable atmospheric boundary layer in complex terrain

Turbulence in the atmospheric boundary layer (ABL) is usually measured using sonic anemometers (sonics), but coarse spatial (${\sim}10$ cm) and temporal (${\sim}32$ Hz) resolutions of sonics preclude direct measurement of fine-scale parameters such as the turbulent kinetic energy (TKE) dissipation rate$\unicode[STIX]{x1D700}$. Instead,$\unicode[STIX]{x1D700}$is estimated using techniques based on Kolmogorov theory. Fine-scale measurements of ABL turbulence down to Kolmogorov scale were made with a sonic and hot-film anemometer dyad (a ‘combo’ probe) during the field campaigns of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) programme. The hot-film probe was located on a gimbal within the sonic probe volume, and was automated to rotate in the horizontal plane to align with the mean flow measured by sonic. This procedure not only helped satisfy the requirement of hot-film alignment with the mean flow, but also allowedin situcalibration of hot-film probes. This paper analyses a period of nocturnal flow that was similar to a stratified parallel shear flow. The combo-probe measurements showed an interesting phenomenon – the occurrence of strong bursts, characterized by short-term increase of velocity fluctuations and simultaneous increase of TKE dissipation rate by orders of magnitude. These bursts were indicative of unusual turbulence activity at finer (${\sim}0.1$–0.4 m) scales that are not captured by sonics since the smallest scales resolved by the latter are greater than 0.6 m. With bursting present, the spectra exhibited bumps at scales intermediate to inertial and dissipation subranges, resembling a bottleneck phenomenon. Its manifestation, although unequivocally related to bursts, may not convincingly fit into the framework of previous bottleneck-effect theories that allude to either viscous effects or buoyancy effects modifying the local energy cascade via non-local effects. The origins of burst are yet to be identified. Stratified ABL with bursts exhibits non-Kolmogorov behaviour, and hence should be modelled with caution.

Boundary layer transition determination for periodic and static flows using phase-averaged pressure data

A method of boundary layer transition measurement is presented for wind tunnel models instrumented with surface pressure taps. The measurement relies on taking a number of theoretically identical measurements at different times and then analysing the standard deviation of the pressures. Due to the slight unsteady movement of the transition position, a peak in the standard deviation of pressure σCPpeak is found at the transition position, and this is correlated with measurements of the transition position with an infrared camera and hot film anemometers. In contrast to microphone measurements, it is shown that the transition detection works for data which has been low-pass filtered with a cutoff of 1 Hz. The application to static and dynamic transition measurements on static and periodically pitching helicopter rotor blade airfoils at Mach 0.3-0.5 is demonstrated.

Influence of fan speed on airflow distribution in a batch kiln

This study reports experimental data of airflow distribution as a function of fan speed in an industrial batch kiln. Measurements were conducted with 20 hot-film anemometers distributed throughout the load at two occasions. The main result was that airflow distribution did not change significantly as the fan speed was reduced, and no positions where the air movement stopped were found. It was also found that relatively more air ran in the bolster spaces in comparison to the adjacent packages as the air ran through the load.

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Porosity Measurements

This work is focused on the nonintrusive characterization of the local and average porosity of a prototype carbon–carbon nose, representative of a reusable thermal protection system based on transpiration cooling. A study based on the x-ray computed tomography scan of the specimen has been carried out with the purpose of defining the most important guidelines for the permeability tests, which are the minimum area to be probed with a hot-film anemometer and the correct distance of the mass flux sensor from the wall. The former has been calculated from the average porosity calculation, whereas the latter has been retrieved from the statistical analysis of the dimensions, and the distribution of the void structures inside the porous network coupled to the theory of fluid flow through perforated plates. Several longitudinal and transversal sectioning planes with respect to the symmetry axis of the carbon mask have been analyzed to calculate the internal porosity from the two-dimensional images, whereas the three-dimensional reconstruction of the sample has been used to retrieve the average volumetric porosity. Both the nominal values of the two-dimensional porosity and volumetric porosity have provided the same dimension of the characteristic area to be probed with a hot-film sensor for the permeability measurements. Preliminary permeability tests, performed within the predicted dimension of the control surface, have confirmed the uniformity of the mean velocity field and allowed verifying the range of variation of the correct distance of a hot-film sensor from the wall obtained from the statistical analysis of the computed tomography images.

Helioseismology in a bottle: modal acoustic velocimetry

Measurement of the differential rotation of the Sunʼs interior is one of the great achievements of helioseismology, providing important constraints for stellar physics. The technique relies on observing and analyzing rotationally-induced splittings of p-modes in the star. Here, we demonstrate the first use of the technique in a laboratory setting. We apply it in a spherical cavity with a spinning central core (spherical-Couette flow) to determine the mean azimuthal velocity of the air filling the cavity. We excite a number of acoustic resonances (analogous to p-modes in the Sun) using a speaker and record the response with an array of small microphones on the outer sphere. Many observed acoustic modes show rotationally-induced splittings, which allow us to perform an inversion to determine the airʼs azimuthal velocity as a function of both radius and latitude. We validate the method by comparing the velocity field obtained through inversion against the velocity profile measured with a calibrated hot film anemometer. This modal acoustic velocimetry technique has great potential for laboratory setups involving rotating fluids in axisymmetric cavities. It will be useful especially in liquid metals where direct optical methods are unsuitable and ultrasonic techniques very challenging at best.

Influence of screen solidity ratio on heat transfer upon a cylinder impinged by a rectangular jet

The present work studies the enhancement of heat transfer on a cylinder impinged by a rectangular jet in presence of a metallic grid on the slot exit. The experiments are carried out to cool a smooth cylinder, electrically heated, with a rectangular jet of air, at three Reynolds numbers in turbulent flow, i.e. Re = 5100, 10,600 and 15,300, where the Reynolds number is defined with the cylinder diameter. Three metallic grids, identified by the French classification as NF06-10-20, are employed with the solidity ratio ranging from 24.3% to 67.7%, and the same diameter of the mesh filament (0.6 mm). Fluid dynamic measurements, performed with a hot film anemometer, are done to justify the thermal results. Heat transfer measurements are presented as local and mean Nusselt numbers. The comparison with the case without grid, indicated as NF00, is done at the same mass flow rate of the jet and the same power consumption of the wind tunnel to move the jet. The main goal of the paper is to evaluate how the solidity ratio of the metallic grids influences the heat transfer.

Effect of hydrophobicity on turbulent boundary layer under water

Turbulent flow field on hydrophobic surface is measured in the gravitational low-speed water tunnel using hot-film anemometer. The mean velocity profile shows that hydrophobic surface has an apparent influence on the distribution of the turbulent boundary layer in the near-wall region (y+<80). The turbulence intensity and friction on hydrophobic surface are also decreased. In this study, drag reduction rate up to 14.2%, and slip length up to 18.290μm are obtained. In view of the importance of the bursting events to the production of turbulent kinetic energy and Reynolds stress, the energy distributions of flow data at y+=20 (right in the buffer layer) are analyzed based on continuous wavelet transform (CWT), and the relation between the maximum value of energy and the bursting events is established. By proposing a new detecting method, phase averaged waveform of ejections and sweeps of the bursting events are extracted and analyzed. The results demonstrate that the intensity of the bursting events is decreased and the time scale is extended. Therefore, we concluded that hydrophobic surface decreases the turbulent fluctuations, by weakening the bursting events, and so decreases the Reynolds stress and wall friction in boundary layer, and finally results in macroscopic drag reduction in turbulence. At last, the prospect of hydrophobic drag reducing technology is discussed.