Hot-wire Anemometry Measurements Research Articles

Pulsatile Turbulent Flow in Straight and Curved Pipes – Interpretation and Decomposition of Hot-Wire Signals

Pulsatile turbulent flows in curved pipes at high Womersley and Reynolds numbers are prevalent in various components of internal combustion engines, in particular in the intake of the exhaust manifold. Despite their technological importance, there appears to be a lack of experimental data both with regard to straight and bent pipes. The present paper addresses this gap through phase-locked hot-wire anemometry measurements in a highly pulsatile turbulent flow through straight and bent pipes and compares the results with those obtained under steady flow conditions. The aim is to understand to some extent the effect of pulsations on the turbulent flow itself and for that purpose different decomposition methods are applied to the data in order to reveal the underlying turbulence from the pulsatile signal. Besides classical phase-averaging, also temporal filtering and singular value decomposition have been employed to investigate the decomposed turbulence statistics in terms of their pulsatile and turbulence contributions. Results show that—due to the large scale separation between the turbulence and pulsations—both decomposition techniques provide similar results and highlight, that the statistics from the turbulent part of the pulsatile flow resemble those of the steady one.

Aeroacoustic source analysis in a corrugated flow pipe using low-frequency mitigation

Our study is focused on a phenomenon often encountered in flow carrying pipes, since flow instabilities caused by geometric features may generate acoustic signals and, thereafter, interact with these signals in such a way that powerful pure tones are produced. A modern example is found in the so-called ‘singing risers’, or the gas pipes connecting gas production platforms to the transport network. But the flow generated resonance in a fully corrugated circular pipe may be silenced by the addition of relatively low frequency flow oscillations induced by an acoustic generator. Experiments reported here, aimed at investigating in more detail the coupling between the flow in the pipe, the acoustically generated flow oscillations and the emitted resulting noise, are performed in a specifically designed facility. A rectangular transparent channel using glass walls enables us to use optical techniques to describe in detail the flow field in the corrugation vicinity, in addition to more standard hot-wire anemometry and acoustic pressure measurements with microphones, with and without the acoustically generated low-frequency oscillations.

The CoLaPipe--the new Cottbus large pipe test facility at Brandenburg University of Technology Cottbus-Senftenberg.

The CoLaPipe is a novel test facility at the Department of Aerodynamics and Fluid Mechanics, Brandenburg University of Technology Cottbus-Senftenberg (BTU Cottbus-Senftenberg), set up to investigate fully developed pipe flow at high Reynolds numbers (Re(m) ⩽ 1.5 × 10(6)). The design of the CoLaPipe is closed-return with two available test sections providing a length-to-diameter ratio of L/D = 148 and L/D = 79. Within this work, we introduce the CoLaPipe and describe the various components in detail, i.e., the settling chamber, the inlet contraction, the blower, bends, and diffusers as well as the cooling system. A special feature is the numerically optimized contraction design. The applications of different measuring techniques such as hot-wire anemometry and static pressure measurements to quantitatively evaluate the mean flow characteristics and turbulence statistics are discussed as well. In addition, capabilities and limitations of available and new pipe flow facilities are presented and reconsidered based on their length-to-diameter ratio, the achieved Reynolds numbers, and the resulting spatial resolution. Here, the focus is on the facility design, the presentation of some basic characteristics, and its contribution to a reviewed list of specific questions still arising, e.g., scaling and structural behavior of turbulent pipe flow as well as the influence of the development length on turbulence investigations.

Experimental Investigation of a Free Round Jet with Dean Vortices

Results of the experimental investigations of free round jet with Dean vortices formed in curved channel are presented. Hot-wire anemometry measurements of three-dimensional mean velocity profile were performed, smoke visualization pictures cross and longitudinal sections at nozzle exit and downstream were obtained. The features of jet development at acoustic excitation of 40 Hz are shown

High spatial range velocity measurements in a high Reynolds number turbulent boundary layer

Here, we detail and analyse a multi-resolution particle image velocity measurement that resolves the wide range of scales prevalent in a zero pressure gradient turbulent boundary layer at high Reynolds numbers (up to Reτ ≈ 20 000). A unique configuration is utilised, where an array of eight high resolution cameras at two magnification levels are used simultaneously to obtain a large field of view, while still resolving the smaller scales prevalent in the flow. Additionally, a highly magnified field of view targeted at the near wall region is employed to capture the viscous sublayer and logarithmic region, with a spatial resolution of a few viscous length scales. Flow statistics from these measurements show good agreement with prior, well resolved hot-wire anemometry measurements. Analysis shows that the instantaneous wall shear stress can be reliably computed, which is historically known to be challenging in boundary layers. A statistical assessment of the wall shear stress shows good agreement with existing correlations, prior experimental and direct numerical simulation data, extending this view to much higher Reynolds numbers. Furthermore, conditional analysis using multiple magnification levels is detailed, to study near-wall events associated with high skin friction fluctuations and their associated overlaying structures in the log region. Results definitively show that the passage of very large-scale positive (or negative) velocity fluctuations are associated with increased (or reduced) small-scale variance in wall shear stress fluctuations.

On the appropriate filtering of PIV measurements of turbulent shear flows

The three-dimensional spatial filtering and measurement noise associated with experimental planar and three-dimensional (3D) particle image velocimetry (PIV) measurements is investigated using a combination of direct numerical simulations (DNS) and experimental databases. Spatial filtering velocity fields from a DNS of a zero-pressure-gradient turbulent boundary layer (TBL) at resolutions typical of PIV experiments are shown to underestimate Reynolds stresses by as much as 50 %. Comparison of experimental PIV measurement of a turbulent channel flow and 3D tomographic PIV measurements of a TBL with higher-resolution simulations and hot-wire anemometry measurements show that in real experiments, measurement noise acts to offset this effect. This is shown to produce measurements that appear to provide a good estimate of the turbulent fluctuations in the flow, when in reality the flow is spatially under-resolved and partially contaminated by noise. Means of identifying this noise are demonstrated using the one-dimensional (1D) velocity power spectra and the 1D transfer function between the power spectra of the unfiltered velocity field and the power spectra calculated from the filtered experimental measurement. This 1D transfer function differs from the commonly used sinc transfer function of PIV owing to the integrated effect of filtering in multiple directions. Failure to incorporate this difference is shown to overestimate the maximum resolved wave number in the 3D spectra of the planar PIV by close to 10 %, while conversely underestimating the maximum resolved wave number in the 3D PIV by 50 %. Appropriate spatial filtering of the experimental data is shown to remove the noise-dominated small-scale fluctuations and bring the data inline with that which should be obtained for a noiseless PIV measurement at the corresponding spatial resolution.

Evaluation of RANS Models in Predicting Low Reynolds, Free, Turbulent Round Jet

In order to study the flow behavior of multiple jets, numerical prediction of the three-dimensional domain of round jets from the nozzle edge up to the turbulent region is essential. The previous numerical studies on the round jet are limited to either two-dimensional investigation with Reynolds-averaged Navier–Stokes (RANS) models or three-dimensional prediction with higher turbulence models such as large eddy simulation (LES) or direct numerical simulation (DNS). The present study tries to evaluate different RANS turbulence models in the three-dimensional simulation of the whole domain of an isothermal, low Re (Re = 2125, 3461, and 4555), free, turbulent round jet. For this evaluation the simulation results from two two-equation (low Re k-ɛ and low Re shear stress transport (SST) k-ω), a transition three-equation (k-kl-ω), and a transition four-equation (SST) eddy-viscosity turbulence models are compared with hot-wire anemometry measurements. Due to the importance of providing correct inlet boundary conditions, the inlet velocity profile, the turbulent kinetic energy (k), and its specific dissipation rate (ω) at the nozzle exit have been employed from an earlier verified numerical simulation. Two-equation RANS models with low Reynolds correction can predict the whole domain (initial, transition, and fully developed regions) of the round jet with prescribed inlet boundary conditions. The transition models could only reach to a good agreement with the measured mean axial velocities and its rms in the initial region. It worth mentioning that the round jet anomaly is still present in the turbulent region of the round jet predicted by the low Re k-ɛ. By comparing the k and the ω predicted by different turbulence models, the blending functions in the cross-diffusion term is found one of the reasons behind the more consistent prediction by the low Re SST k-ω.

Effect of free-stream turbulence on flow characteristics over a transversely-grooved surface

An experimental investigation has been performed to study the free-stream turbulence influence on the flow over a transversely-grooved surface. Free-stream flow at 8.1m/s with turbulence intensities of 0.5% and 4.4% have been provided over smooth and grooved surfaces. Mean velocity profiles, fluctuating velocity moments and turbulence length scales are studied using data obtained from hot-wire anemometry measurements. Introducing quasi-isotropic free-stream turbulence of 4.4% to the flow resulted in fuller velocity profiles, increased boundary layer thickness and significantly augmented streamwise turbulence intensity throughout the boundary layer in flows over both smooth and grooved surfaces. Probability density functions of the velocity fluctuation show that interaction of quasi-isotropic free-stream turbulence with wall turbulence leads to nearly-isotropic turbulence in a small portion of the boundary layer in the near-wall region; while the remaining boundary layer experiences an anisotropic turbulence. In other words, with the introduction of additional turbulence generated by the grooves, the momentum of the fluid particles is increased, resulting in better flow separation resistance. Accordingly, the flow over a grooved surface showed an increase in turbulence intensity, and hence in energy dissipation rate. This is amplified under free-stream turbulence conditions as a result of the interaction of groove-generated turbulence with free-stream turbulence in the boundary layer. The energy cascade span as indicated by the integral to Kolmogorov ratio is also widened by the grooves.

Aerodynamic characterization of the wake of an isolated rolling wheel

The flow around the wheels of road vehicles exhibits complex unsteady 3D phenomena, including massive boundary layer separations, recirculation areas and 3D vortical structures. Although the mechanisms generating the wake main vortices have been identified, their exact topology is still indeterminate and a description of their unsteady evolution has not yet been proposed.The aim of this paper is to characterize the structure and unsteady motion of an isolated wheel wake. For this purpose, wind tunnel investigations have been carried out with a smooth rotating wheel in contact with a moving belt and compared to URANS simulations based on the geometry used for experiments.CFD calculations appeared to have successfully predicted the main features of the flow. Particle Image Velocimetry and hot-wire anemometry measurements coupled with numerical simulation results enabled the origin and the nature of the main vortical structures in the near-wake to be confirmed and their unsteady behaviour to be investigated. The results show good agreement with previous knowledge and provide new insight into the unsteady features of the wake.

1042 AN EXPERIMENTAL STUDY ON THE WAKE OF A RECTANGULAR FOREBODY WITH SUCTION APPLIED OVER THE SURFACES

Previous studies have shown that a modification of the boundary layer of a bluff body can change the wake properties like the vortex shedding frequency and the base drag. In the present study the boundary layer has been modified by applying suction over the top and bottom surfaces of a bluff body. A level of suction above a certain threshold has been shown to give an asymptotic suction boundary layer. For the asymptotic suction boundary layer the boundary layer properties become independent of the free stream velocity and hence the boundary layer Reynolds number. The bluff body studied can be described as a rectangular forebody with a smooth leading edge and a blunt trailing edge. Hot-wire anemometry measurements have been carried out in the boundary layer of the forebody. For four different Reynolds numbers, all within the laminar regime, the suction coefficient has been varied in the range from the neutral case to the case for which the asymptotic suction boundary layer is reached. It is observed that as the suction coefficient approaches the asymptotic suction boundary layer threshold, the Strouhal number based on the shedding frequency approaches a constant value. The base pressure decreases as the boundary layer thickness decreases. Furthermore it is shown that the Strouhal number does not always decrease for an increase of the displacement thickness.

Control of a Transitioning Boundary Layer using Surface-Mounted Electro Active Polymers

Micro air vehicles (MAV) are a major focus of aerodynamics today with many military as well as civilian applications. MAV flight is dominated by the unsteady characteristics of low Reynolds number flows. Three-dimensional separations may occur at random on different parts of the wing, causing highly unsteady forces and moments, as well as full wing stall. In the present work an Electro-Active Polymer (EAP) actuator was examined as a feasible flow control actuator with application to low Reynolds number flows. The performance of the EAP was evaluated using a laser displacement device, where the displacements generated under selected driving frequencies and input voltages were recorded. It was shown that the deflections could reach up to 0.35 mm at frequencies below 40 Hz. The surface deflection was found to be parabolic across the span and width of the actuator. Hot-wire anemometry measurements were conducted on a flat plate equipped with a trailing edge flap in a low Reynolds number wind tunnel to study t...

Modeling of Electrodynamic Zero-Net Mass-Flux Actuators

electrodynamic zero-net mass-flux (ZNMF) actuator for flow control applications. The electrodynamic actuation schemeemploysamagneticassemblytoestablisharadialmagnetic fieldpatternwithanimbeddedcurrent-carrying coil winding. The coil is attached to a composite diaphragm having a rigid center boss and compliant outer annular region. The magnetic field acting on the coil assembly is modeled using a one-dimensional magnetic circuit. The composite diaphragm is modeled as a transversely isotropic, annular clamped plate. An overall lumped-element electromechanical system model is obtained by combining previously developed lumped models of the cavity/neck structures with lumped models for the composite diaphragm and electrodynamic actuation. The complete lumpedelement model is used to provide insight regarding performance characteristics and design limitations. The model is validated over a frequency range of 10–1000 Hz with hotwire anemometry measurements for both slot and orifice configurations for the jet. The model is useful as a design tool for electrodynamic ZNMF actuators. Nomenclature Amagnet = cross-sectional area of the permanent magnet, m 2 a = outer radius of the diaphragm, m aorif = axisymmetric orifice radius, m Bavg = average magnetic flux density acting on the coil placed in the magnetic assembly, T Bgap = magnetic flux density in the magnet gap predicted by magnetic circuit, T Br = remnant magnetic flux density of the permanent magnet, T b = inner radius of the compliant part of the diaphragm or the radius of the center boss, m CaC = acoustic compliance of the cavity, m 5 =N CaD = acoustic compliance of the diaphragm, m 5 =N CmD = mechanical compliance of the diaphragm, m=N c0 = speed of sound in air, m=s dC = diameter of the cavity, m dslot = diameter of the rectangular slot, m dT = overall diameter of the ZNMF actuator, m Ememb = Young’s modulus of the material of the compliant region of the diaphragm, Pa fN = undamped natural frequency of the diaphragm, Hz fr = steady-state resonant frequency of the diaphragm, Hz f1 = lower 3 dB cutoff frequency, Hz

Scaling of statistics in wall-bounded turbulent flows

High-resolution laser Doppler anemometry (Lda) and hot-wire anemometry (Hwa) measurements are utilized to study a zero-pressure-gradient turbulent boundary layer over the range of momentum thickness Reynolds number of 1170–3720. The primary objective is to investigate the near-wall behavior of this type of flow. We are particularly interested in possible Reynolds- and Kármán-number dependencies. The experimental results are in excellent agreement with most recent direct numerical simulations (Dns), which allow direct comparison of detailed results such as peak value and position of streamwise Reynolds stress, wall values of skewness and flatness factors, and turbulence dissipation rate. Systematic changes of some of these parameters with Kármán number are found when scaled with the inner parameters. A remedy seems to be the alternative mixed scaling that is based on uτ3/2U01/2, instead of uτ2, which admits direct influence of the outer velocity scale on the wall parameters.

Flow and heat transfer inside a PV/T collector for building application

In certain building applications, the PV installation is extended to cover also south- or west-facing walls, taking care to circulate cooling air to the back of the panels. The cooling effect maintains a high conversion efficiency of the PV panels and the heated air may be exploited by the HVAC or service water heating system. Sizing and design of the double façade system is critical to its energetic performance. In this paper, the results from flow visualization and hot wire anemometry measurements performed on the basic structural module of a double-skin photovoltaic (PV/T) façade are discussed. The concept and its feasibility have been presented in a previous paper. The results of transient outdoor measurements with the testing device have been reported elsewhere. The air flow and turbulence field inside the cavity is analyzed in the present paper by means of indoor measurements with the testing device. The results are combined with CFD computations to calculate heat transfer coefficients and improve our understanding and modeling of the specific PV/T concept.

Band-pass filtered velocity statistics in decaying turbulent box

For homogeneous isotropic turbulence study, the acquisition of band-pass filtered velocity increments (FVI) in a non-forced turbulent box is still a challenge both experimentally and numerically. Turbulence and associated physical processes, at a given instant, are permanently contaminated by a forcing process which can seldom be universal. The situation tends to be the origin of intermittency and the non-Gaussian probability density distribution for acceleration and velocity gradients. To reveal implied mechanism, grid turbulence is adapted to observe non-perturbed homogeneous isotropic turbulence. The velocity increments (VI) can be obtained following Comte-Bellot and Corrsin (GCBC) by means of two point-two time shifted velocity measurements. It is difficult to obtain decaying turbulence (DT) at large turbulent Reynolds number without pollution coming from walls. Nevertheless it is also significant to investigate DT in low Reynolds number regimes to determine non-polluted tendencies. The similarity of DT between particle image velocimetry (PIV) and hot wire anemometry measurements by GCBC are presented. Here we focus our tendency on VI and FVI probability density function (PDF) shapes in this letter. In conclusion, the tendency to Gaussian shape in inertial zone wavenumbers, demonstrates that there will be no intermittency if turbulent cascade is not perturbed.

Spatial resolution correction for hot-wire anemometry in wall turbulence

We investigate spatial resolution issues in hot-wire anemometry measurements of turbulence intensity and energy spectra. Single normal hot-wire measurements are simulated by means of filtering direct numerical simulation (DNS) of turbulent channel flow at \(Re_\tau = 934\). Through analysis of the two-dimensional energy spectra from the DNS, the attenuation of the small-scale energy levels is documented, especially in the near-wall region. The missing energy displays anisotropic characteristics, and an attempt is made to model this using an empirical equation, thus providing a correction scheme for all wall normal locations. The empirical model is assessed using experimental boundary layer data and shown to effectively correct both the streamwise one-dimensional energy spectra and turbulence intensity at a Reynolds number significantly above that of the DNS.

Investigation into the Strouhal numbers associated with vortex shedding from parallel-plate thermoacoustic stacks in oscillatory flow conditions

This paper investigates vortex shedding processes occurring at the end of a stack of parallel plates, due to an oscillating flow induced by an acoustic standing wave. Here the hot-wire anemometry measurement technique is applied to detect the velocity fluctuations due to vortex shedding near the end of the stack. The hot-wire fast time response enables detailed frequency spectra of the velocity signal to be obtained, which can be used for identifying the dominant frequencies associated with vortex shedding, and thus allow calculation of the corresponding Strouhal numbers. By varying the stack configuration (the plate thickness and spacing) and the acoustic excitation level (the so-called drive ratio), the impact of the stack blockage ratio and the Reynolds number on the Strouhal number has been studied in detail. Furthermore, in the range of Reynolds numbers between 200 and 5000 a correlation between the Strouhal number and Reynolds number has been obtained and compared with analogous relationships in the steady flow. Particle Image Velocimetry (PIV) is also used to visualize the vortex shedding processes within an acoustic cycle, phase-by-phase, in particular during the part of the cycle when the fluid flows out of the stack—selected cases are shown for comparisons with hot-wire measurements.

Wavelet analysis of unsteady flows: Application on the determination of the Strouhal number of the transient wake behind a single cylinder

This paper presents experimental results of the accelerating and decelerating flow in the wake of a cylinder obtained by means of hot wire anemometry measurements in a wind tunnel with high blockage ratio. The analysis was done in Fourier and wavelet spaces. The Strouhal number for Reynolds numbers up to 3×104 was studied in a transient flow and compared with the results obtained from steady flows at several velocities uniformly distributed from Re=1.7×103 to 3×104. Results show that the wavelet analysis is a valuable tool to deal with both transient and stationary random phenomena and that is able to capture the characteristics of the transient flow as well as the Fourier analysis can do with the steady state acquisitions.

Experimental study of a tip leakage flow: wavelet analysis of pressure fluctuations

A wavelet-based conditional analysis of unsteady flow and sound signals highlights the role of intermittent perturbations both in the sound generation and the unsteady field of an aerofoil tip leakage flow experiment. It is shown how the most probable flow perturbations generated at the pressure side tip edge are convected through the gap and swept downstream along the suction side past the trailing edge tip corner, where they radiate sound. The nascent sound sources are identified and localized in the clearance between 40% and 60% of the chord. It is also found that the time dependence of the averaged intermittent structures scales with the inverse of the square root of the mean velocity and a physical interpretation based on a simple potential vortex model is proposed. The data are retrieved from an experiment that has been carried out at low Mach number (Ma < 0.3) in an anechoic test facility. A single motionless instrumented NACA 5510 aerofoil was mounted into the potential core of an open rectangular jet between two plates with an adjustable clearance. The tip leakage flow was ensured by the 5% camber and a 15° angle of attack. A large database obtained by a variety of measurement techniques is thus available for the present analysis. More specifically, the conditional approach is applied to joint far field, wall pressure and particle image velocimetry (PIV) measurements. The wall pressure probes are located along the suction side tip edge and on the tip inside the gap, whereas the PIV plane is parallel to the mid-gap plane. Additional joint wall pressure and single hot-wire anemometry (HWA) measurements are also analysed with a hot-wire probe located near the trailing edge tip corner. The conditional averaging is triggered by high-energy wavelet events selected in a reference signal by setting a threshold to the so-called local intermittency measure.

Experimental Investigations of an Internal Flow Generated by Porous Injection

Abstract An anisothermal channel flow generated by a porous injection is investigated in details for different Reynolds numbers of the injection in order to highlight the impact of the microstructure of porous material on the flow development. Two types of porous materials, being characterized by different matrices and pore sizes are studied: a coarse bronze porous plate (30% porosity and 100 μm average pore diameter) and a stainless steel porous plate (30% porosity and 30 μm average pore diameter). Particle image velocimetry, hot-wire anemometry, and cold wire thermometer measurements lead to the comparison of mean profiles, rms profiles, and energy spectra for the velocity and temperature fields. Two-point spatial correlations for the fluctuating velocity are also calculated. In the case of the coarse bronze plate, the flow is slightly fluctuating with big space coherence. In the opposite, the results obtained with the fine pore plate show a flow close to a fully developed turbulent channel flow. The comparison of the aerodynamic field with computational simulations based on a Reynolds-averaged Navier-Stokes (RANS) model underlines the difficulties to reproduce exactly the evolution of the mean and fluctuating velocities in all the explored part of the channel.