Hot-wire Anemometry Techniques Research Articles

Experimental investigation of the variation of the local gas velocities in a cold flow pebble bed reactor (PBR) using a hot wire anemometry technique

Obtaining accurate results and new benchmark data for the local velocity of the gas flowing within the pebble bed reactor (PBR) is a key step for understanding and benchmarking simulations of the thermal hydraulics in the PBR core as it significantly affects the design and safety operation of these reactors. Therefore, this work focused on studying the local gas velocities inside a pebble bed using a sophisticated hot wire anemometry (HWA) technique, which was supported with a novel probe-protector case that protected the probe, allowing the measurements to be conducted at various locations in a pebble bed with pebble diameter of 5cm and an aspect ratio of 6. The measurements were conducted at various superficial inlet gas velocities (0.3≤Ug≤2.4m/s), covering both transitional and turbulent flow conditions (993.78≤Re≤7950.24) at three axial levels and four radial locations (r/R=0, 0.33, 0.67, 0.9). The results highlighted the effect of the wall on the variation of the local gas velocities inside the pebble bed, as higher gas velocities were recorded at the near-wall region, where the void fractions are higher, providing a path of the least resistance to the flow of the gas, compared to the center of the bed. Furthermore, a second order polynomial correlation with an R2=93.96% and an AARE equal to 1.47% was developed for the prediction of the local gas velocity inside the bed, within the experimental range of the study. The accurate gas velocity measurements obtained in this study can serve as benchmark data for the validation of CFD simulations coupled with heat transfer calculations.

On the application of statistical turbulence models to the simulation of airflow inside a car cabin

Computational fluid dynamics simulations of airflow inside a full-scale passenger car cabin are performed using the Reynolds averaged Navier–Stokes equations. The performance of a range of turbulence models is examined by reference to experimental results of the streamwise mean velocity and turbulence intensity profiles, obtained using the hot-wire anemometry technique at different locations inside the car cabin. The models include three linear eddy-viscosity-based variants, namely, the realizable k–ε, the renormalization group k–ε, and the shear-stress transport k–ω models. The baseline Reynolds stress model (BSL-RSM), a second-moment-closure variant, and an Explicit Algebraic Reynolds Stress Model (BSL-EARSM) are also investigated. Visualization of velocity vectors and streamlines in different longitudinal planes shows a similar airflow pattern. The flow topology is mainly characterized by jet flows developing from the dashboard air vents and extending to the back-seats compartment resulting in a large vortex structure. Additionally, a comparison between numerical and experimental results shows a relatively good agreement of the mean velocity profiles. However, all models exhibit some limitations in predicting the correct level of turbulence intensity. Moreover, the realizability of the modeled Reynolds stresses and the structure of turbulence are analyzed based on the anisotropy invariant mapping approach. All models reveal a few amounts of non-realizable solutions. The linear eddy-viscosity-based models return a prevailing isotropic turbulence state, while the BSL-RSM and the BSL-EARSM models display pronounced anisotropic turbulence states.

Application of synthetic jet arrays for flow separation control on a circular “hump” model

This research investigates the effectiveness of a spanwise array of synthetic jet actuator (SJA) for the control of boundary layer separation over a circular “hump” model. The influence of geometrical and operational parameters – including actuator position, velocity ratio (i.e., the ratio of the peak exit jet velocity of actuators to the freestream velocity of cross flow, VR) and the actuation waveform – on the flow separation control are investigated using hot-wire anemometry (HWA) and particle image velocimetry (PIV) techniques. The effect of the position of SJA array on flow separation control has been studied experimentally over a considerable length of the hump chord (from the “apex” of the model to near the “trailing edge”) for the first time. The investigation looks in more detail into the mechanisms behind the alleviation of adverse pressure gradient as a key factor controlling the flow separation. The investigation of the effect of VR on the performance of SJAs shows the importance of the momentum injection in the mitigation of the momentum deficiency as another important factor in turbulent boundary layer flow separation. A holistic overview of the control parameters allowed to reveal a considerable change in the separation flow patterns. The results show that the best performance of SJA array from the viewpoint of separation control occurs at the velocity ratio of 1.85 with a reduction of the length of recirculation region of around 42.6 and 44.2% by using sine and square waves excitation of SJAs, respectively.

Experimental investigation of the airflow generated by the human foot tapping using the hot-wire anemometry

Human-walking-induced particle resuspension in indoor environments is believed to be an important source of particulate matter. Aerodynamic disturbance generated by the human foot during a gait cycle are the main driver for particle detachment and dispersion in the room. In this work, the hot-wire anemometry technique was employed to investigate the airflow generated by one phase of the human gait cycle: the foot tapping. This phase was simulated by a mechanical simulator that consists of a wooden rectangular 25 × 8 × 1.2 cm plate, and a servomotor that allows downward and upward rotations of the plate with a constant velocity. A correction procedure based on the hot-wire velocity measurements and the analytical solution of Falkner–Skan has been derived to correct the hot-wire readings in the near-wall region. Results show a sharp increase of airflow velocity in front of the simulator after the simulator rotation. Transverse hot-wire measurements downstream of the simulator show that the profile of the maximal velocities reaches a peak at a distance y = 8 × 10−3 m from the wall. The expulsed air from the volume under the simulator propagates downstream from the foot to reach near zero velocity values at 0.15 m away from the top of the simulator.

EXPERIMENTAL STUDY OF THE EFFECT OF THE WAKE GENERATED BY OSCILLATING GURNEY FLAP

The study of the turbulent near wake of an airfoil provided with flow control devices shows the flow configuration in such conditions and the starting vortices mechanism, while, the far wake offers information about the general performance of the model. The objective of the present work is to study the develop of the fluid-dynamic structures found in the NACA 4412 airfoil wake, as well as the development of the same structures when flow con-trol techniques are applied by means of a oscillating Gurney Flap place in the lower surface of the wing model, close to the trailing edge. Tests were per-formed at a given Reynolds numbers and the flow control system was set at different frequencies. In order to study the effect of the control mechanism on the wake, hot wire anemometry techniques were used. Two components of the velocity vector were measured - longitudinal and vertical - by means of a vertical array of three sensors acquiring simulta-neously. The intention is to quantify the flow general configuration at the airfoil wake for different test conditions. The velocity vectors will be analyzed, together with the turbulence intensity and integral scales.

An investigation of airfoil dual acoustic feedback mechanisms at low-to-moderate Reynolds number

An experimental investigation was performed in an anechoic wind tunnel involving acoustic measurements, hot-wire anemometry and surface flow visualisation techniques to investigate airfoil tonal noise generating mechanisms. Tests were conducted using a NACA 0012 airfoil at corrected angles of attack of 0° and 1.58° and Reynolds numbers of 50,000 to 150,000. A dual acoustic feedback model is presented, where feedback processes act independently on the airfoil pressure and suction surfaces between the point of boundary layer separation and the trailing edge. It is proposed that the tones generated on both airfoil surfaces, with the same or similar frequencies on each surface, interfere constructively. The primary tone possesses near exact frequencies on both surfaces, whereas the secondary tones have larger differences in frequencies between both surfaces, thus explaining their relative magnitudes based on acoustic superposition. This model provides a better comparison with the experimentally obtained tonal frequencies than the existing feedback models. Despite this agreement, the feedback model cannot perfectly predict the acoustic tones as the tones are not perfectly equispaced. An empirical feedback length is calculated by reverse engineering an acoustic feedback length scale by using the recorded primary tone as an input that also minimises the secondary tone prediction errors. This empirical length closely matches the dual acoustic feedback model presented in this paper.

Bypass Boundary Layer Transition on Flat Plate by Adverse Pressure Gradient

Bypass boundary layer transition in flows on flat plate by adverse pressure gradient was investigated experimentally. It was measuered cases with combination of adverse pressure gradient by different free stream turbulence intenzity. Hot wire anemometry technique was used. Measuerement were made on flat plate in closed wind tunnel. Adverse pressure gradient was set by diffuser in tested section of wind tunnel. Grid turbulence of free stream was controlled by screen. Hot wire anemometry technique was used, intermitency factor was evaluated. Results were compared wih cases with simpliest conditions.

Wake flow modification behind a square cylinder using control rods

In the present study, flow behind a square cylinder is investigated using control rods. The control rods are placed in the shear layer and studied using particle image velocimetry, hotwire anemometry and flow visualization techniques. Experiments are conducted in a low-speed wind tunnel for three different Reynolds numbers 485, 805 and 1290. However, detailed analysis of flow is only presented at a Reynolds number of 485. The aim is to modify the wake behind a square cylinder for possible suppression of vortex Street and drag reduction. The control rod diameter is kept constant as 0.2 times of the cylinder diameter. Depending upon the gap between control rods and the cylinder, the flow structure, vortex dynamics, the distance of the shear layer roll up behind the cylinder is modified. An attempt is made to correlate the different flow structures with corresponding drag forces. Several parameters such as time-averaged drag coefficient, Strouhal number, recirculation length, fluctuating velocity, turbulence intensity, and Reynolds stress are measured. To understand the flow phenomena and its intricate structure, flow visualization images are also captured in the same setup with smoke visualization. Maximum up to 22% drag reduction is observed at a particular control rod position.

Characteristics of the wake formation and force distribution of the bistable flow on two cylinders side-by-side

This numerical study with experimental support examines the bistable phenomenon along two parallel smooth cylinders submitted to a uniform turbulent cross-flow. The spacing ratio P/D of the cylinders is 1.26, being D the cylinder diameter and P the distance between centers. The computational domain and the experimental tests section are equivalents. The Reynolds number based on the free mean velocity and cylinder diameter was Re = 2.2 × 104. For the numerical analysis, the Navier–Stokes equations were solved using finite volumes method for a URANS simulation (unsteady Reynolds averaged Navier–Stokes) with the kω-SST-SAS (shear stress tensor-scale adaptive simulation) turbulence model. Measurements were performed by means of hot wire anemometry technique, firstly to determine actual boundary conditions of flow velocity and turbulence intensity at the test section entrance, secondly to obtain velocity and velocity fluctuation at the wake of the cylinders in different positions along the cylinder height to corroborate the numerical results. By means of the numerical results of velocity, the process of the wake formation is described, showing that the bistability does not occur simultaneously along the cylinder, corroborated by a delay on the switching process of the simultaneously measured velocities and the cross-correlation function. This effect appears also in the results of cylinder wall pressure distribution as well as in lift and drag coefficients.

Investigation of the impulse action of a membrane on the straight wing boundary layer

The hot-wire anemometry technique is used to study the development of controlled disturbances in a straight wing boundary layer. Three-dimensional surface oscillations with large amplitude generate two types of disturbances: the localized longitudinal structures and wave packets. In downstream direction, the intensity of localized longitudinal structures decreases. The wave packets manifest themselves near the fronts of longitudinal localized structure in the flow region with adverse pressure gradient. In the separation flow area, an intensive growth of the wave packets amplitude is observed. The spatial development of the wave packets coincides with the development of three-dimen-sional Tollmien–Schlichting wave under similar conditions.

On the chaotic nature of bistable flows

Evidences from experimental measurements in tube banks submitted to turbulent cross flow in quadrangular and triangular arrangements show the presence of instabilities, being associated to the phenomenon of bistability, known in the simplified case of the flow on two cylinders side-by-side. Although not completely understood the study of the dynamic process of bistability can reveal new features about the chaotic behavior of these time series. As chaotic time series are observed routinely in experiments on physical systems, a study of the characteristics of bistability can be performed with the aid of techniques already established. This work presents a study about the determination of the Lyapunov exponents from experimental time series of bistable flows on the simplified geometry of two parallel circular cylinders and on a set of rows of a triangular tube bank. Time series of flow velocity and velocity fluctuations were obtained by means of the constant hot-wire anemometry technique in an aerodynamic channel. Discrete wavelet transform was used to make a multilevel decomposition of the series in several bandwidth values, accordingly with a selected decomposition level. Thus, the turbulence can be dissociated from the original signals, allowing a more accurate study to be conducted on the obtained state spaces. With these filtered time series, a state space reconstruction was performed by the method of time delays or Takens’ method, while the percentage of false neighbors together with the embedding dimension were useful for the choice of some of the analysis parameters. The Rosenstein’s method was applied to calculate the largest Lyapunov exponent of the time series. Easy to implement, this method is robust with respect to changes in most immersion parameters. Results show that the flow after two circular cylinders placed side-by-side and after two rows tube bank in triangular arrangement present positive largest Lyapunov exponents. This means that bistability has a clear chaotic behavior. In contrast, the flow after a five row tube bank is random.

Application of hot-wire anemometry for experimental investigation of flow distribution in micro-packed bed reactors for synthesis gas conversion

The knowledge of the flow distribution inside microstructured reactors is valuable e.g. for improving the reactor design, developing flow distributors, and optimizing the catalyst loading method. The applicability of the hot-wire anemometry (HWA) technique for experimental determination of the flow distribution inside a multi-stack micro-packed bed reactor is demonstrated for the first time. The anemometry data is then evaluated in relation to the reactor performance for methanol synthesis under relevant industrial operating conditions. A 400 µm long hot-wire connected to a constant temperature anemometer was applied for scanning the flow out of a specially designed reactor model, clamped on a motorized table equipped with a precise positioning system. The anemometry measurements revealed a nonuniformity in the catalyst packing, partially resulting from the reactor design. The flow distribution is poorer for smaller particles and for wider particle size distributions, and worsens as the superficial flow velocity (and the pressure drop) decreases. The effects of the packing nonuniformity on the reactor performance appear minor under methanol synthesis conditions. They could, however, turn out significant upon pushing the overall conversion in the whole reactor towards equilibrium as the synthesis reaction is exothermic, and temperature increase does alleviate the problem. The reactor design should be modified instead.

Flow Characterization and Modeling of Strong Round Synthetic Jets in Crossflow

This study focuses on the flow field of a round synthetic jet issuing into a crossflow. Six cases are investigated in which the jet-to-freestream velocity ratios range between 2.8 and 8.3. This corresponds to a medium-to-strong transverse jet, which is characterized using both two-dimensional hot-wire anemometry and particle image velocimetry techniques. A self-similar model is developed for the centerline trajectory and velocity of a transverse synthetic jet. The resulting scaling laws are validated with experimental data. The scaling coefficients are found to be independent of the synthetic jet strength (characterized by the stroke ratio) for large-velocity-ratio cases, provided that the jet velocity is calculated based on the momentum flux of the leading vortex ring of the synthetic jet. Furthermore, the velocity component parallel to the crossflow is enhanced over the crossflow velocity in the near field and fundamentally different from continuous jets. This can be explained by the induced effect of the asymmetric vortex rings in the near field, which are a result of the interaction between the crossflow and the synthetic jet.

Experimental investigation of flow field behind triangular prisms at intermediate Reynolds number with different apex angles

An experimental study of flow around triangular prism with effect of apex angle is reported. The focus of the study is towards understanding the effect of apex angle on flow structure and related forces occurring. Four triangular prisms with apex angle 30°, 45°, 60° and 90° respectively are reported at Reynolds number of 520 (based on cylinder diameter). The Particle image velocimetry (PIV), hotwire anemometry and flow visualization techniques are used to characterize the complete flow field. Results obtained in the present work are Strouhal number, drag coefficient, time averaged as well as instantaneous velocity field, statistical quantities like rms velocity, turbulence intensity, and shear stress. A linear relationship between vortex shedding frequency and apex angle is being observed. A minimum in drag coefficient and maximum in Strouhal number is seen for apex angle 30°. Corresponding small wake width is seen from flow visualization images. The power spectra shows additional peak other than dominant frequency peak due to non linear interaction between the Karman vortices.

Experimental investigation of flow field behind two tandem square cylinders with oscillating upstream cylinder

In this paper, the fluid flow around two identical square cylinders arranged in tandem has been discussed. The upstream cylinder is oscillated in the transverse direction while the downstream cylinder is held stationary. The experiments are conducted at a Reynolds number of 295. Both the influences of spacing between two cylinders, as well as the effect of oscillation, of the upstream cylinder are investigated using Particle image velocimetry, hotwire anemometry and flow visualization techniques. The spacing ratios are varied from 1.5 to 5 while the upstream cylinder is oscillated in harmonics of the vortex shedding frequency of a stationary cylinder at the fixed amplitude. It is observed that there is a strong effect of spacing between the cylinders on vortex shedding mechanism and flow structures. When two cylinders are in tandem there is a critical distance below which vortex shedding of the upstream cylinder is suppressed. Three distinct flow regimes are captured and the detailed investigation of the intricate flow field is done both spatial and temporal domain. The vortex shedding frequency of a single stationary square cylinder is measured with hotwire anemometer and this frequency is being used to oscillate the upstream cylinder with its sub-harmonic, harmonic and super-harmonic frequency ratios. The effect of forcing frequency on flow interference, wake oscillation frequencies, aerodynamic forces and turbulence statistics have been studied for these spacing ratios. Flow fields are investigated in terms of the time-averaged drag coefficient, stream traces, vorticity contours, turbulence statistics and the time-dependent flow field is captured using flow visualization, vorticity contours, and Strouhal number.

Comparison of PIV and Hot-Wire statistics of turbulent boundary layer

The paper shows a cross checking of turbulent boundary layer measurements using large field of view PIV and hot-wire anemometry techniques. The time-resolved PIV method was used for the experiments. The measuring plane was oriented perpendicularly to the wall and parallel to the mean flow. Hot wire measurement has been performed using the special probe with perpendicular hot wire. The HW point measurements were performed in the same place as PIV experiments. The hot-wire probe has the wire length of l+ < 20 in considered range of Reynolds numbers. Various evaluation methods were applied on PIV data. The profiles of statistical characteristics of streamwise velocity components were evaluated from the data. Mean values, standard deviations as well as skewness and kurtosis coefficients were compared for a few values of Reθ. Reynolds number ranges from 1000 to 5500. The result shows that with the increasing Reynolds number the attenuation of fluctuations maximum in PIV measurements occurs with respect to Hot-Wire measurements, however representation of velocity fluctuations using the PIV method is satisfactory. The influence of wall-normal fluctuation component on Hot-Wire near wall peak was also investigated.

Flow disturbance due to presence of the vane anemometer

This paper presents the results of the preliminary experimental investigations of the disturbance of velocity field resulting from placing a vane anemometer in the analyzed air flow. Experiments were conducted in a wind tunnel with a closed loop. For the measurement process, Particle Image Velocimetry (PIV) method was used to visualize the flow structure and evaluate the instantaneous, two-dimensional velocity vector fields. Regions of inflow on the vane anemometer as well as flow behind it were examined. Ensemble averaged velocity distribution and root-mean-square (RMS) velocity fluctuations were determined. The results below are presented in the form of contour-velocity maps and profile plots. In order to investigate velocity fluctuations in the wake of vane anemometer with high temporal resolution hot-wire anemometry (HWA) technique was used. Frequency analysis by means of Fast Fourier Transform was carried out. The obtained results give evidence to a significant spatially and temporally complex flow disturbance in the vicinity of analyzed instrument.

Experimental study of bypass flow in near wall gaps of a pebble bed reactor using hot wire anemometry technique

Coolant flow behavior through the core of an annular pebble bed reactor is investigated in this experimental study. A high frequency hot wire anemometry system coupled with an X-probe is used for measurement of axial and radial velocity components at different points within two near wall gaps at five different modified Reynolds numbers (Rem=2043–6857). The velocity profiles within the gaps verify the presence of an area of increased velocity close to the pebble bed outer reflector wall, which is known as the bypass flow. Moreover, the characteristics of the coolant flow profile are seen to be highly dependent on the gap geometry. The effect of Reynolds number on the velocity profiles varies as the geometry of the gap changes. The time histories of the local velocities measured with considerably high frequency are further analyzed using power spectral density technique. Power spectral plots illustrate substantial spatial variation of the energy content, spectral shape, and the slope of the energy cascade region. A significant correlation between Reynolds number and characteristics of the velocity power spectra is observed.

Static and dynamic characteristics of turbulent flow in a closed compound channel

The hot-wire anemometry technique was employed to study the velocity distribution and dynamic characteristics of the flow in two types of closed compound channels. The compound channels were built using two rectangular tubes attached to the walls of a wind channel separated by a distance d. In this configuration, a narrow gap is connected to a main subchannel. The results revealed an interesting flow that was quite similar to a mixing layer. All axial velocity profiles presented turning points at the main subchannel/ narrow gap interface. By using quantities from the mean flow velocity distribution, the flow could be described by a hyperbolic tangent function, and a Strouhal number was suggested as well. Regardless of the channel type, the dimensionless frequency was almost constant, resulting in a Strouhal number of approximately 0.10.

Experimental study of the bistability in the wake behind three cylinders in triangular arrangement

By means of hot wire anemometry technique and flow visualizations, the presence of the phenomenon of the bistability around three cylinders in two triangular arrangements is investigated in an aerodynamic channel and in a water channel. Bistability occurs in flows over sets of bluff bodies forming a flip-flopping wake characterized by a biased flow switching at irregular intervals, which can represent an additional source of dynamic instabilities. Results of flow around three cylinders in triangular arrangement are presented for two configurations: one cylinder upstream and two downstream, and vice versa. The experimental data are analyzed by means of statistical, spectral and wavelet tools. The joint analysis in time and frequency domains through wavelets allows the detection of non-permanent flow structures. The results show the presence of bistable flow in the configuration with one cylinder upstream and two downstream, but no bistable effect was detected in the second configuration, where the flow shows a distinct shedding frequency.