Hot-wire Probe Research Articles

Vertical concentrations gradients and transport of airborne microplastics in wind tunnel experiments

Abstract. Microplastics are an ubiquitous anthropogenic material in the environment, including the atmosphere. Little work has focused on the atmospheric transport mechanisms of microplastic nor its dispersion, despite it being a potential pollutant. We study the vertical transport of airborne microplastics in a wind tunnel, a controllable environment with neutral stability, to identify the necessary conditions for the long-range atmospheric transport of microplastics. An ultrasonic disperser generated airborne water droplets from a suspension of polystyrene microsphere microplastics (MPs) with a diameter of 0.51 µm. The water droplets were injected into the airflow, evaporating and releasing single airborne MPs. The disperser allowed for time-invariant and user-controlled concentrations of MPs in the wind tunnel. MPs were injected at 27, 57, and 255 mm above the ground. A single GRIMM R11 optical particle counter (OPC) and three Alphasense OPCs measured time-averaged MP concentration profiles (27, 57, and 157 mm above the ground). These were combined with turbulent airflow characteristics measured by a hotwire probe to estimate vertical particle fluxes using the flux-gradient similarity theory. The GRIMM R11 OPC measured vertical concentration profiles by moving its sampling tube vertically. The three Alphasense OPCs measured particle concentrations simultaneously at three distinct heights. Results show that maximum concentrations are not measured at the injection height but are rather shifted to the surface by gravitational settling. The MPs experience higher gravitational settling while they are part of the larger water droplets. For the lowest injection at 27 mm, the settling leads to smaller MP concentrations in the wind tunnel, as MPs are lost to deposition. Increasing the wind speed decreases the loss of MPs by settling, but settling is present until our maximum friction velocity of 0.14 m s−1. For the highest injection at 255 mm and laminar flow, the settling resulted in a net MP emission, challenging the expectation of a net MP deposition for high injection. Turbulent flows reverse the MP concentration profile giving a net MP deposition with deposition velocities of 3.7 ± 1.9 cm s−1. Recognizing that microplastics share deposition velocities with mineral particles bridges the gap in understanding their environmental behavior. The result supports the use of existing models to evaluate the transport of microplastics in the accumulation mode. The similar deposition velocities suggest that microplastics transported in the atmosphere can be found in the same places as mineral particles.

Hot-wire spatial resolution issues in adverse pressure gradient turbulent boundary layers

The effect of a finite length of hot-wire probe sensor length on the measured streamwise velocity fluctuations is well understood in canonical wall-bounded flow, where the small-scale energy has been found to be universal and invariant with Reynolds number. A straightforward application of that assumption to non-canonical flows such as strong adverse pressure gradient (APG) flows has, however, been hampered since the effect of Re and APG could not conclusively be studied separately due to the lack of data with a clear scale separation. The present experimental investigation at Reτ≈4000 in weak, moderate and strong APGs with different wire length shows that spatial averaging effects are not only limited to the inner layer. A note of caution is hence warranted for measurements that seemingly try to take the bias effect of spatial attenuation into account by performing measurements with albeit long but fixed viscous-scaled wire length.

Three-dimensional flow around and through a porous screen

We investigate the three-dimensional (3-D) flow around and through a porous screen for various porosities at high Reynolds number $Re = {O}(10^4)$ . Historically, the study of this problem has been focused on two-dimensional cases and for screens spanning completely or partially a channel. Since many recent problems have involved a porous object in a 3-D free flow, we present a 3-D model initially based on Koo & James (J. Fluid Mech., vol. 60, 1973, pp. 513–538) and Steiros & Hultmark (J. Fluid Mech., vol. 853, 2018 pp. 1–11) for screens of arbitrary shapes. In addition, we include an empirical viscous correction factor accounting for viscous effects in the vicinity of the screen. We characterize experimentally the aerodynamic drag coefficient for a porous square screen composed of fibres, immersed in a laminar air flow with various solidities and different angles of attack. We test various fibre diameters to explore the effect of the space between the pores on the drag force. Using PIV and hot wire probe measurements, we visualize the flow around and through the screen, and in particular measure the proportion of fluid that is deviated around the screen. The predictions from the model for drag coefficient, flow velocities and streamlines are in good agreement with our experimental results. In particular, we show that local viscous effects are important: at the same solidity and with the same air flow, the drag coefficient and the flow deviations strongly depend on the Reynolds number based on the fibre diameter. The model, taking into account 3-D effects and the shape of the porous screen, and including an empirical viscous correction factor that is valid for fibrous screens may have many applications including the prediction of water collection efficiency for fog harvesters.

The Effect of Curved Geometry on Exiting Flow of Fluidic Oscillators

Abstract Traditionally fluidic oscillators are designed to be planar. However, there are applications that may desire the exiting fluid to move in the third dimension. This could allow these oscillators to be more effective in applications such as fuel sprays, cooling flow, or flow control devices with its increase in effective spray area. This investigation designed a series of oscillators that curved the whole body and/or the exit nozzle to understand how to maximize out of plane motion. These configurations were compared to a baseline planar oscillator with no curved characteristics. Velocities were measured downstream of these oscillators within a data collection grid using a hot wire probe to determine the 3D shape of the exiting jet. Results show that configurations with only one of the two curved physical characteristics (i.e., only a curved body or a curved nozzle) produced the most curvature. Having both of the curved physical characteristics caused the nozzle width to decrease causing the axial spacing to decrease. Additionally, these curved exiting flows were only seen at mass flow rates below 40 standard liters per minute (SLPM). Higher mass flow rates caused the exiting flow to flatten, returning the flow to the baseline result of in-plane oscillations. This led to a decrease in jet spread.

Turbulence Measurements Downstream of a Combustor Simulator Designed for Studies on the Combustor–Turbine Interaction

Turbulence intensity impacts the performance of turbine stages and it is an important inlet boundary condition for CFD computations; the knowledge of its value at the turbine inlet is then of paramount importance. In combustor–turbine interaction experimental studies, combustor simulators replace real combustors and allow for the introduction of flow perturbation at the turbine inlet. Therefore, the turbulence intensity of a combustor simulator used in a wide experimental campaign at Politecnico di Milano is characterized using a hot-wire probe in a blow-down wind tunnel, and the results are compared to URANS CFD computations. This combustor simulator can generate a combination of a swirl profile with a steady/unsteady temperature disturbance. In the cold unsteady disturbance case, hot-wire measurements are phase-averaged at the frequency of the injected perturbation. The combustor simulator turbulence intensity is measured at two different axial positions to understand its evolution.

A study of the spatiotemporal structure of a turbulent boundary layer measured by the use of two hot-wire probes (Velocity time series patterns and Kolmogorov's structure function)

A study of the spatiotemporal structure of a turbulent boundary layer measured by the use of two hot-wire probes (Velocity time series patterns and Kolmogorov's structure function)

Micro-velocity fiber probe based on Fe3O4-PDMS filled microcavity for on-chip microfluidic

Real-time measurement of microflow velocities is essential for on-chip microfluidic systems and human vessels, particularly when flow velocities serve as regulators of biochemical processes and indicators of reaction outcomes. In this study, a hot-wire fiber probe for the microflow sensing was designed and fabricated by incorporating Fe3O4-polydimethylsiloxane (Fe3O4-PDMS) into the microcavity. A Fabry–Pérot (F-P) sensing cavity was formed between the single mode fiber (SMF) and Fe3O4-PDMS. The Fe3O4 enables strong absorption of 980 nm laser and generates heat as reliable heat sources. Under the influence of heat convection, the fluid removes heat to establish specific equilibrium temperatures. With the fluctuation of equilibrium temperature, Fe3O4-PDMS expands or contracts regularly, resulting in corresponding changes in lengths of F-P sensing cavities, and interference spectra also shift with changes of flow rates. In the range of 0.278 μL/s to 3.058 μL/s, the probe exhibits high sensitivities and robustness, which provides potential value detection of microfluidic chips and humans.

Reynolds number dependence of azimuthal and streamwise pipe flow structures

This paper revisits the study by Baileyet al.(J. Fluid Mech., vol. 615, 2008, pp. 121–138), adopting a higher-fidelity calibration approach to reveal subtle flow variations with Reynolds numbers that were not discernible previously. The paper aims therefore to provide insights into the characteristics of azimuthal and streamwise pipe flow structures adopting two-point joint statistics and spectral analysis for shear Reynolds numbers in the range$2\times {10^3}\le {Re_\tau }\le {16\times {10^3}}$, where${Re_\tau }$is based on the wall friction velocity$u_{\tau }$, the pipe radius$R$, and the fluid kinematic viscosity$\nu$. The streamwise velocity fluctuations were measured at four wall-normal locations,$0.1\le {x_{{2}}/R}\le {0.7}$, covering the logarithmic and core regions of fully developed turbulent pipe flow based on 35–41 azimuthal probe separations using, simultaneously, two single hot-wire probes. A uniquein situcalibration approach for both probes was adopted where a potential flow was insured, resulting in consistent and precise pipe flow data. The azimuthal velocity correlation, the cross-power spectral density and the coherence function of the streamwise velocity fluctuations are discussed, revealing a clear dependence of the azimuthal scales of the large and very large flow motions on the wall-normal location, the azimuthal separation, the streamwise wavenumber and the Reynolds number. Along the logarithmic region, a linear growth of the azimuthal scales of the large- and very-large-scale structures was observed; however, they do scale nonlinearly and reach their maximum sizes in the core region, i.e. near the centreline of the pipe. Additionally, the streamwise very-large- and large-scale motions were evaluated using the premultiplied energy spectra, showing wavelengths${\approx }{18R}$and${\approx }{3R}$for${Re_{\tau }}\approx {16\times {10^3}}$at half of the pipe radius, respectively.

Experiments on large-scale structures in fully developed turbulent pipe flow

The experimental results reported in present paper represent contributions from the Cottbus large pipe facility to DFG-priority program SPP 1881: Turbulent Superstructures. Fundamental questions regarding turbulence statistics and structures in fully developed turbulent pipe flow were addressed, utilizing thermal and optical measuring techniques integrated into the pipe facility. The facility enables unique in-situ calibration for the hot-wire probe and it has full optical access that allows performing Particle Image Velocimetry and 3D Lagrangian Particle Tracking measurements, resulting in consistent and accurate pipe flow data. A streamwise-spanwise field of view up to ≈20×2 pipe radii has been set for both optical techniques with 4–8 high resolution cameras for a friction Reynolds number range of 8⋅102≤R+≤18⋅103. The setup enabled field of measurements with an unprecedented dynamic range in turbulent pipe flow at high Reynolds numbers with spatial resolution better than 1 vector/mm. Proper Orthogonal Decomposition and Shake-The-Box algorithm were used, highlighting energy contents of the large-scale motions in fully developed turbulent pipe flow regime.

Experimental Investigation of the Structure of Disturbances from Two Pulsed Sources in the Flat-Plate Supersonic Boundary Layer

The work is devoted to the development of a new experimental method for entering controlled disturbances with a given frequency-wave structure into a supersonic boundary layer. Experimental data on the formation of disturbances from two pulsed sources (pulsed glow discharge) in the laminar flat-plate boundary layer at the Mach number equal to 2 are given. The experiments were carried out in the T-325 wind tunnel of ITAM SB RAS. The localized sources were spaced out at the same distance from the leading edge of the plate at 6 mm from each other spanwise. Flow pulsations were measured using a hot-wire probe of constant temperature anemometer, the signal was recorded synchronously with ignition of the discharges. This made it possible to distinguish the pulsations from the discharges against the background of random uncontrolled “natural” pulsations of the boundary layer. The spatial-temporal and frequency-wave structure of the generated disturbances from a single or two discharges, operating synchronously or with a time delay, are analyzed. It is found that the maximum difference in the structure of disturbances from one and two sources is observed in the central region, while at the side boundaries of the disturbances, the pulsations are close in all considered cases. In the wave spectra of disturbances from two discharges, nodes and antinodes are formed. Their position is determined by the distance between the sources and the time delay in their operation.

Experimental Investigation of the Structure of Disturbances from Two Pulsed Sources in the Flat-Plate Supersonic Boundary Layer

The work is devoted to the development of a new experimental method for entering controlled disturbances with a given frequency-wave structure into a supersonic boundary layer. Experimental data on the formation of disturbances from two pulsed sources (pulsed glow discharge) in the laminar flat-plate boundary layer at the Mach number equal to 2 are given. The experiments were carried out in the T-325 wind tunnel of ITAM SB RAS. The localized sources were spaced out at the same distance from the leading edge of the plate at 6 mm from each other spanwise. Flow pulsations were measured using a hot-wire probe of constant temperature anemometer, the signal was recorded synchronously with ignition of the discharges. This made it possible to distinguish the pulsations from the discharges against the background of random uncontrolled “natural” pulsations of the boundary layer. The spatial-temporal and frequency-wave structure of the generated disturbances from a single or two discharges, operating synchronously or with a time delay, are analyzed. It is found that the maximum difference in the structure of disturbances from one and two sources is observed in the central region, while at the side boundaries of the disturbances, the pulsations are close in all considered cases. In the wave spectra of disturbances from two discharges, nodes and antinodes are formed. Their position is determined by the distance between the sources and the time delay in their operation.

Experimental investigation of flow structure over pitching airfoil in the wake of circular cylinder

The unsteady aerodynamics play an important role in fluid-structure interaction due to its engineering applications. These types of research can be utilized to improve the fundamental analysis of bluff body-airfoil wake interaction and to achieve better design and more efficiency in engineering applications. This paper aims to conduct experimental tests over parameter space to investigate flow around a NACA0012 pitching airfoil in the wake of a circular cylinder. The one-dimensional hot-wire anemometer probe was used in an open-type wind tunnel to examine the effect of the cylinder-airfoil gap, the amplitude of pitching oscillation, and reduced frequency on the time-averaged flow characteristics in Reynolds number Rec=105. The results show that the increase of the reduced frequency at the α0=5 has a negligible effect on the flow characteristics, while its increase at the α0=10 is consequential. Pitching oscillation creates a region with a higher fluctuation in the wake center of the upstream cylinder, whose width grows with the increase in the amplitude of pitching oscillation. In addition, the distributions of the turbulent flow characteristics for GD=6and10 cases in xc=2.4 are approximately similar but different from the GD=2 case. The analytical results presented in this paper provide basic information about cylinder-pitching airfoil wake interaction.

Experimental investigation of typical surface treatment effect on velocity fluctuations in turbulent flow around an airfoil

Noise reduction of different airfoils is important because these sections are used in wind turbines, propellers, and aircraft wings. Several methods are used for passive noise reduction of sections. One of these methods is the use of surface treatment. In this research, the effect of the typical surface treatment element (finlets) on the vortex structure at different frequencies in the turbulent flow created on the NACA2412 section is investigated. For this purpose, one-dimensional hot wire probe is used. The used surface treatment in this research has a special geometry and the distance between two consecutive finlets is 6 mm (S = 6 mm). This study shows that this surface treatment element can be used for noise reduction in high frequencies. Another result of this research is the suggestion of the most suitable position to install this special surface treatment element to reduce vortex energy in all frequency ranges. This installation location is determined based on a dimensionless parameter (Xaft/h).

Design of a small pressure chamber to evaluate probe calibrations at varying Reynolds numbers

The Chair of Turbomachinery and Flight Propulsion (LTF) at the Technical University of Munich (TUM) operates a wind tunnel for probe calibration (Caljet). This paper describes the design and the validation process of a pressure chamber. The pressure chamber is used to calibrate probes for turbomachinery applications at comparable conditions to a 3.5-stages axial compressor test rig. This allows the study of Reynolds number and Mach number effects individually. The pressure chamber is designed as an additional component for the Caljet to enable probe calibration at higher mass flow densities and higher Reynolds numbers. Therefore, the original free stream jet is encased by this chamber, nevertheless the outlet of the calibration wind tunnel remains open to the environment. After a first pre-design study, numerical simulations are carried out to confirm expected flow conditions in the measurement area. Ansys Fluent is used for RANS simulations with air modeled as compressible gas. The numerical results are validated with experimental investigations that simultaneously provide a deeper understanding of the flow behavior. Therefore, a five-hole probe, a hot-wire probe, and a Fast Response Aerodynamic Probe are utilized. It is shown that the capability of the jet meets the expectations and can be used for the calibration of hot-wire probes at conditions comparable to a state-of-the-art 3.5-stages axial compressor.

CFD-supported data reduction of hot-wire anemometry signals for compressible organic vapor flows

For obtaining turbulent fluctuations in compressible organic vapor flows, computational fluid dynamics (CFD) analysis tools were used to support the data reduction process of hot-wire anemometry signals. It was found that CFD can help during the calibration process of actual hot-wire probes operated in the constant-temperature mode. Such an approach is beneficial for determining the dependency of the sensitivity coefficients on Reynolds and Mach numbers. Due to the small Kolmogorov scales, corrections are needed for probes with finite wire lengths. The experimental and numerical analysis considering the decay of turbulence downstream a grid indicated that the correction scheme proposed by Wyngaard leads to consistent results for compressible organic vapors. The new developments provide recommendations for obtaining turbulent quantities of compressible organic vapor flows utilizing hot-wire anemometry.

Aerodynamic influence of rim seal purge flow injection on the main flow in a 1.5-stage axial turbine with nonaxisymmetric end wall contouring

This paper presents an investigation of the aerodynamic influence of rim seal purge flow injection on the main flow in a 1.5-stage turbine with non-axisymmetric end walls and a bowed stator using experimental flow measurements and unsteady RANS simulations. The study focuses on the secondary vortex structures formed in the rotor passages of the 1.5-stage axial turbine rig. Through performance map measurements, it was found that the efficiency gain of the non-axisymmetric configuration is partially eliminated by the injection of purge flow. Numerical investigations, which are supported by detailed flow measurements with five-hole probes and hot-wire probes, revealed that the injection of purge air flow intensifies vortex structures near the hub, thereby generating additional losses. These resulting vortex structures are highly similar both in the axisymmetric baseline and the non-axisymmetric configuration and are the result of jet-like vortices emerging from the cavity. From these findings, it can be concluded that the non-axisymmetric contour and the bowed stator no longer provides any efficiency benefit near the hub. Only the near the casing, where the flow is not affected by the purge flow, the optimized configuration continues to improve the efficiency of the rig by homogenizing the stator outflow and thus reducing the secondary flow structures in the rotor passages.

Experimental and Numerical Considerations for the Motor-Propeller Assembly’s Air Flow Field over a Quadcopter’s Arm

The aim of the paper is to validate the analytical–numerical analysis method regarding the operating regime of the propellers of a quadcopter. The research aims to mark the flow areas whose numerical results differ from the experimental ones and to investigate the possible reasons for the discrepancies between the values. The paper presents the determination of the air velocity produced by the rotational movement of a quadcopter propeller for a stationary position of the drive motor. The velocities were determined both experimentally at various points located below the propeller plane using hot-wire anemometric probes and numerically using a time-lapse simulation with a rotating sliding table. The numerical simulations carried out consisted of the determination of the time variation of the velocity distribution developed by the propeller in the rotational movement for the different operating (power) cycles of the engine. In addition, a technique that utilizes reverse engineering to determine the propeller profile, the anemometric probe calibration, and the average velocity values measured at various points below the propeller plane for engine operating regimes that range from 60 to 90% are also presented.

Spanwise Coherence Measurements of Porous Coated Cylinders in Uniform Flow

A porous coated cylinder (PCC) generates weaker vortex shedding noise than a smooth cylinder placed in uniform flow. A key understanding of how the porous media relates to vortex shedding reduction is linked to the spanwise coherence length of the PCC at the near-wall region. To date, few studies estimate the PCC spanwise coherence to be five-to-six outer diameters in length, yet a more comprehensive study is needed. This paper presents an experimental investigation conducted in an anechoic wind tunnel using a set of polyurethane PCCs tested with varying porosity, thickness and at several Reynolds numbers. Spanwise coherence is estimated at various circumferential angles using (1) flush-mounted microphones distributed along the bare cylinder span, beneath the porous layer, to record surface pressure fluctuations, and (2) two hot-wire anemometry probes in the near-wall flow field, where one probe remains in place and the other is shifted along the span. A transfer function of the porous layer is estimated based on the surface pressure and velocity fluctuations. The results presented here give some new visions of the spatial structure of vortex shedding caused by porous materials, noise generating mechanisms and a fitting formula of spanwise coherence length of PCCs.

Cylinder aeroacoustics: experimental study of the influence of cross-section shape on spanwise coherence length

New data and review of the spanwise coherence length is provided for flows over cylinders of different cross-sections: circular of diameter d, and rectangular of sectional aspect ratios (breadth (b) to height (d) ratio AR = b/d) of 1, 2 and 3. In the present measurements, the body has both d and spanwise length of 70d fixed, and the Reynolds number (based on d) range 6000–27,000 is covered. Two-point data are obtained from two hot-wire probes, one fixed in the symmetry plane and the other moving on the corresponding spanwise axis. Their position in a cross plane are deduced from preliminary measurement of the mean flow with a single probe, allowing fair comparisons between the different geometries and the introduction of uncertainty bars on coherence length values. At all tested regimes, a very good agreement is noticed between velocity-based and pressure-based coherence experimental data. Coherence length definitions are revisited, and the aeroacoustically consistent, integral length definition is selected, allowing fair synthesis of literature data into a single chart and empirical functions. Definitions for coherence decay models (e.g. Gaussian or Laplacian) are also adapted so that coherence length and coherence integral shall be equivalent. This preliminary work on coherence data and its spanwise integration enables transparent regressions and model selection. Generally, the Gaussian model is relevant for the lift peak, while the coherence exhibits a Laplacian decay at harmonics. On average, at peak Strouhal number, the coherence length for the circular and square cylinders is of 5d while it is of the order of 15d for the rectangular sections. It is concluded that the flow over those latter geometries is still a two-dimensional dynamics at the tone frequency. These values are almost preserved over the tested Reynolds number range. Coherence length value at harmonics is extensively documented. Spanwise coherence length is also discussed as an ingredient of acoustic efficiency.

Turbulence organization in isolated roughness flow

The primary goal of this investigation was to analyze the specific evolution of the boundary layer over an isolated roughness. A specific surface consisting of rectangular elements of the same height arranged in a checkerboard pattern was used for the experiments. Throughout the experiment, the measuring cross-sections were positioned at various distances from the inlet, while maintaining a constant flow velocity of 15 m·s-1. To investigate the boundary layer topology in the streamwise directions, a 55P14 miniature hot-wire probe was employed. It is worth mentioning that the measuring position was situated at the midpoint between the roughness elements. Through the analysis of the gathered data, we identified the distribution patterns of the mean velocity profile at different measuring positions. Additionally, we estimated the integral scale of the turbulence by applying different approaches, such as the zero-crossing method (based on autocorrelation analysis), and the Roach and Karman methods (based on spectral density function). The data obtained from our experiments demonstrate that the presence of isolated roughness leads to a pronounced alteration of the velocity profile. This phenomenon is likely due to the shear layer, which typically contains the largest and most energetic vortices in the flow.