Hot-wire Velocity Measurements Research Articles

Turbulent boundary layer control with DBD plasma actuators

The flat-plate turbulent boundary layer at Re τ = 1140 is manipulated using a spanwise array of bidirectional dielectric barrier discharge (DBD) plasma actuators. Based on the features of no moving mechanical parts in the DBD plasma control technology and hot-wire anemometer velocity measurements, a novel convenient method of local drag reduction (DR) measurement is proposed by measuring the single-point velocity within the linear region of the viscous sublayer. We analyze the premise of using the method, and the maximum effective measurement range of −73.1% < DR < 42.2% is obtained according to the experimental environment in this work. The local drag decreases downstream of the center of two adjacent upper electrodes and increases downstream of the upper electrodes. The magnitude of the local DR increases with increasing voltage and decreases as it moves away from the actuators. For the spanwise position in between, the streamwise distribution of the local DR is very dependent on the voltage. The variable-interval time-average detection results reveal that all bursting intensities are reduced compared to the baseline, and the amount of reduction is comparable to the absolute values of the local DR. Compared with previous results, we infer that the control mechanism is that many meandering streaks are combined together into single stabilized streaks.

Small-width wall-attached Coandǎ jets for flow control

The flow dynamics of small-width wall-attached jets generated by a Coand-effect nozzle is investigated by unsteady Reynolds-averaged Navier–Stokes simulations. The data are validated by comparison with hot-wire velocity measurements performed on the same flow configurations. The jets exhibit a complex topology strongly influenced not only by the spanwise vorticity (as usually observed in wall jets) but also by a vorticity component normal to the wall and induced by the shear layer developing on the jet sides. This results in an original U-shaped jet whose characteristics are studied in detail for three different mass flow rates. The robustness of the flow topology on a larger range of injected mass flow rates is finally presented and discussed in terms of the injected momentum near the wall. The resulting flow profiles point out that our injector is expected to be a promising candidate for active flow control in gas-turbine compressors for aeronautical and energy applications.

Pulsed jet phase-averaged flow field estimation based on neural network approach

Single hot-wire velocity measurements have been conducted along a three-dimensional measurement grid to capture the flow-field induced by a 45 $$^\circ$$ inclined slotted pulsed jet. Based on the periodic behavior of the flow, two different estimation methods have been implemented. The first one, considered as the reference baseline, is the conditional approach which consists in the redistribution of the experimental data into space- and time-resolved three-dimensional velocity fields. The second one uses a neural network to estimate 3D velocity fields given spatial coordinates and time. This paper compares the two methods for a complete flow-field estimation based on hot-wire measurements. Results suggest that the neural network is tailored to capture the phase-averaged dynamic response of the jet induced by the actuator, and identify the coherent structures in the flow field. Interesting performances are also observed when degrading the learning database, meaning that neural networks can be used to drastically improve the temporal or spatial resolution of a flow field estimation compared to the experimental data resolution.

Simulation of spectators’ aerodynamic drag using porous models approximation

Evaluation of the thermal comfort is essential for complex ventilation systems design. Assessment of thermal indices requires representative velocity and pressure fields' values. When simulating the air flow in large facilities such as stadium, the effect of crowds' geometrical features needs to be captured. Using porous models approximations to simulate the aerodynamic effect of detailed spectators' geometry reduces the required mesh size and associated processing time. This paper investigates the use of different porous media models approximations for capturing the effect of large crowds inside complex building systems, such as stadiums. Their efficiency of capturing the effect of spectators on the air flow were compared to the simulation of the exact spectators' geometry. The exact spectators' geometrical model was of a stadium tiers section with 28 spectators. Using a wind tunnel, the exact spectator's model results were validated against a 1:10 scaled physical model. The experiments included PIV and hot-wire velocity measurements. The results of the pressure drop were used to obtain the coefficients needed to utilize the porous models. Compared to the exact spectators' case, the three-dimensional porous volume model approximation yielded an average absolute error of 24.5% in velocity, while the two-dimensional porous jump model yielded results with an average error of 1.5%. In comparison to the exact model cooling load, the results yielded a difference of 6% for the 2D porous jump and 6.5% for the 3D porous volume. Nevertheless, both models yielded more representative results than the case of simulation of empty bleachers.

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.

Conditions for formation of the blue whirl

This paper presents a laboratory study of the relation between blue whirls and fire whirls in terms of circulation (swirl) and energy-release rate. The blue whirl is a small, completely blue, soot-free flame that was originally seen when it evolved from more traditional fire whirls burning liquid hydrocarbons on water. The experimental apparatus consists of two offset quartz half-cylinders suspended over a water surface, with fuel injected onto the water surface from below. The flow circulation is calculated using the diameter of the enclosure and hot-wire velocity measurements made at the inlet gap between the half-cylinders. The heat-release rate was varied by adjusting the volumetric supply rate of liquid n-heptane, and is calculated assuming complete combustion. Results show that stable blue whirls form in a narrow range of circulation and energy-release rate close to a previously cited extinction limit. A scaling law derived from the data, based on the length scale of the enclosure, shows that the transition to a blue whirl depends on the gap size between the half-cylinders of the enclosure.

Flow Statistics and Visualization of Multirow Film Cooling Boundary Layers Emanating From Cylindrical and Diffuser Shaped Holes

The research presented in this paper strives to exploit the benefits of near-wall measurement capabilities using hotwire anemometry and flowfield measurement capabilities using particle image velocimetry (PIV) for analysis of the injection of a staggered array of film cooling jets into a turbulent cross-flow. It also serves to give insight into the turbulence generation, jet structure, and flow physics pertaining to film cooling for various flow conditions. Such information and analysis will be applied to both cylindrical and diffuser shaped holes, to further understand the impacts manifesting from hole geometry. Spatially resolved PIV measurements were taken at the array centerline of the holes and detailed temporally resolved hotwire velocity and turbulence measurements were taken at the trailing edge of each row of jets in the array centerline corresponding to the PIV measurement plane. Flowfields of jets emanating from eight staggered rows, of both cylindrical and diffuser shaped holes inclined at 20 deg to the main-flow, are studied over blowing ratios in the range of 0.3–1.5. To allow for deeper interpretation, companion local adiabatic film cooling effectiveness results will also be presented for the geometric test specimen from related in-house work. Results show “rising” shear layers for lower blowing ratios, inferring boundary layer growth for low blowing ratio cases. Detachment of film cooling jets is seen from a concavity shift in the u′rms line plots at the trailing edge of film cooling holes. Former rows of jets are observed to disrupt the approaching boundary layer and enhance the spreading and propagation of subsequent downstream jets. Behavior of the film boundary layer in the near-field region directly following the first row of injection, as compared to the near-field behavior after the final row of injection (recovery region), is also measured and discussed. The impact of the hole geometry on the resulting film boundary layer, as in this case of cylindrical verses diffuser shaped holes, is ascertained in the form of mean axial velocity, turbulence level (u′rms), and length scales profiles.

Effect of fan and shroud configurations on underhood flow characteristics of an agricultural tractor

ABSTRACTIn the current study, effects of two different underhood geometry modifications namely; fan position relative to shroud and fan tip clearance, on airflow through an agricultural tractor engine cooling system are investigated by utilizing CFD modeling. For the characterization of each modification, the underhood components are reduced to a domain of cooling systems including only fan, shroud, and radiator that allows saving in computational time and cost. The models are validated using a custom designed underhood flow setup, where hotwire velocity measurements at proximity to the radiator are conducted. The mass flow rate and the uniformity of the airflow through the radiator are quantified as performance parameters. For the former modification, position of fan relative to shroud, the computations are performed for the percentages of fan projection into shroud (FPiS) varying from 78% to 0% at fan rotational speeds of 2060 and 2800 rpm. The optimum fan location is found to be around 56–60% for both rotational speeds that leads 8% increase in mass flow rate compared with the pre-design location of 74%. In addition, a noticeable reduction in the relative magnitude of the RMS velocity through the radiator is obtained for the moderate FPiS values. For the latter modification, fan tip clearance, the computations are performed for the tip clearance values varying from 5.25to 12 mm at a fan rotational speed of 2060 rpm. As the tip clearance decreases up to 6 mm, air mass flow rate can be improved by 7%. However, further reduction in tip clearance does not improve the air mass flow rate instead it decreases, suggesting an optimum tip clearance value. The results suggest that the proper geometrical modifications in engine cooling systems might induce significant improvement in the aforementioned performance indicators.

Turbulence Structure around an Asymmetric High-Lift Airfoil for Different Incidence Angles

An exhaustive investigation of the structure of the turbulence around an asymmetric FX 63-137 wind turbine airfoil is carried out in this paper. Reliable hot-wire velocity measurements, made at the Xixon Aeroacoustic Wind Tunnel, are presented with the aim of analyzing the turbulent flow features. The probe was placed at two different positions along the streamwise direction, one over the airfoil and the other at the wake, both on the suction and pressure side. These measurements were performed in order to capture the evolution of the flow and its behavior at the wake. The experimental data were collected at a Reynolds number of 350000 for several incidence angles to explore their influence in the turbulence characteristics. The data processing from the dual hot-wire, capable of measuring two velocity components, allowed to achieve half set of the Reynolds stresses, the turbulence intensity and the degree of anisotropy. The boundary layer and wake size were estimated from the Reynolds stress components. In addition, the production term of the turbulence kinetic energy budget is calculated to visualize the unsteadiness energy inside the boundary layer. As a result of these analyses, it was observed that the transversal fluctuations were higher than the longitudinal ones. Besides, an alternative description of the turbulence structure is obtained when a frequency analysis of the motion is provided, disclosing a clear change in the spectra tendencies in the wake and boundary layer regions. This analysis, combined with the degree of anisotropy analysis, was helpful to define a transition zone between the clearly distinguishable instability zone and the free-stream zone. Finally, the integral length scale of turbulence was estimated from the area under the autocorrelation function of the velocity fluctuations. The combination of the results of this work have provided a wide description of the turbulent behavior of the flow around the airfoil and present a clearer physical picture of the phenomena.

Oscillations of a planar impinging jet induced by a potential-core confined airfoil

The flow features of a planar jet modified by an airfoil located within the potential-core region have been investigated experimentally by hot-wire velocity measurements and the flow control mechanism is then analyzed using linear stability theory. The jet–airfoil impinging configurations are varied by changing the angle of attack (α) of the airfoil and adjusting the impingement distance (L). The test results show that, at small angles of attack, the impinging jet displays two independent instability regions, i.e. the jet shear layer and the airfoil wake, in which the jet fundamental instability wave and the airfoil wake instability wave evolve downstream respectively. However, at large angles of attack, the impinging jet exhibits self-sustained oscillations where the jet shear layer is characterized by the wavelength and frequency of the airfoil wake. As a result, both the momentum thickness and the volume flow rate increase significantly. Linear stability analysis using a local concept of absolute/convective instability suggests that such self-sustained oscillations can only be observed when there is a sufficiently large absolutely unstable flow region behind the airfoil. The measured resonant frequency matches well with the absolute instability frequency at the streamwise location where the flow transition from absolute instability to convective instability occurs.

Flow regimes in a trapped vortex cell

This paper presents results of an experimental investigation on the flow in a trapped vortex cell, embedded into a flat plate, and interacting with a zero-pressure-gradient boundary layer. The objective of the work is to describe the flow features and elucidate some of the governing physical mechanisms, in the light of recent investigations on flow separation control using vortex cells. Hot-wire velocity measurements of the shear layer bounding the cell and of the boundary layers upstream and downstream are reported, together with spectral and correlation analyses of wall-pressure fluctuation measurements. Smoke flow visualisations provide qualitative insight into some relevant features of the internal flow, namely a large-scale flow unsteadiness and possible mechanisms driving the rotation of the vortex core. Results are presented for two very different regimes: a low-Reynolds-number case where the incoming boundary layer is laminar and its momentum thickness is small compared to the cell opening, and a moderately high-Reynolds-number case, where the incoming boundary layer is turbulent and the ratio between the momentum thickness and the opening length is significantly larger than in the first case. Implications of the present findings to flow control applications of trapped vortex cells are also discussed.

Sub-harmonic broadband humps and tip noise in low-speed ring fans.

A joint experimental and numerical study has been achieved on a low-speed axial ring fan in clean inflow. Experimental evidence shows large periodic broadband humps at lower frequencies than the blade passing frequencies and harmonics even at design conditions. These sub-harmonic humps are also found to be sensitive to the fan process and consequently to its tip geometry. Softer fans yield more intense humps more shifted to lower frequencies with respect to the fan harmonics. Unsteady turbulent flow simulations of this ring fan mounted on a test plenum have been achieved by four different methods that have been validated by comparing with overall performances and detailed hot-wire velocity measurements in the wake. Noise predictions are either obtained directly or are obtained through Ffowcs Williams and Hawkings' analogy, and compared with narrowband and third-octave power spectra. All unsteady simulations correctly capture the low flow rates, the coherent vortex dynamics in the tip clearance and consequently the noise radiation dominated by the tip noise in the low- to mid-frequency range. Yet, only the scale-adaptive simulation and the lattice Boltzmann method simulations which can describe most of the turbulent structures accurately provide the proper spectral shape and levels, and consequently the overall sound power level.

Scale-by-scale energy budget in a turbulent boundary layer over a rough wall

Hot-wire velocity measurements are carried out in a turbulent boundary layer over a rough wall consisting of transverse circular rods, with a ratio of 8 between the spacing (w) of two consecutive rods and the rod height (k). The pressure distribution around the roughness element is used to accurately measure the mean friction velocity (Uτ) and the error in the origin. It is found that Uτ remained practically constant in the streamwise direction suggesting that the boundary layer over this surface is evolving in a self-similar manner. This is further corroborated by the similarity observed at all scales of motion, in the region 0.2⩽y/δ⩽0.6, as reflected in the constancy of Reynolds number (Rλ) based on Taylor’s microscale and the collapse of Kolmogorov normalized velocity spectra at all wavenumbers.A scale-by-scale budget for the second-order structure function 〈(δu)2〉 (δu=u(x+r)-u(x), where u is the fluctuating streamwise velocity component and r is the longitudinal separation) is carried out to investigate the energy distribution amongst different scales in the boundary layer. It is found that while the small scales are controlled by the viscosity, intermediate scales over which the transfer of energy (or 〈(δu)3〉) is important are affected by mechanisms induced by the large-scale inhomogeneities in the flow, such as production, advection and turbulent diffusion. For example, there are non-negligible contributions from the large-scale inhomogeneity to the budget at scales of the order of λ, the Taylor microscale, in the region of the boundary layer extending from y/δ=0.2 to 0.6 (δ is the boundary layer thickness).

An aerodynamic study on flexed blades for VAWT applications

There is renewed interest in aerodynamics research of VAWT rotors. Lift type, Darrieus designs sometimes use flexed blades to have an 'egg-beater shape' with an optimum Troposkien geometry to minimize the structural stress on the blades. While straight bladed VAWTs have been investigated in depth through both measurements and numerical modelling, the aerodynamics of flexed blades has not been researched with the same level of detail.Two major effects may have a substantial impact on blade performance. First, flexing at the equator causes relatively strong trailing vorticity to be released. Secondly, the blade performance at each station along the blade is influenced by self-induced velocities due to bound vorticity. The latter is not present in a straight bladed configuration. The aim of this research is to investigate these effects in relation to an innovative 4kW wind turbine concept being developed in collaboration with industry known as a self-adjusting VAWT (or SATVAWT). The approach used in this study is based on experimental and numerical work. A lifting line free-wake vortex model was developed. Wind tunnel power and hot-wire velocity measurements were performed on a scaled down, 60cm high, three bladed model in a closed wind tunnel.Results show a substantial axial wake induction at the equator resulting in a lower power generation at this position. This induction increases with increasing degree of flexure. The self-induced velocities caused by blade bound vorticity at a particular station was found to be relatively small.

Swept-Wing Boundary-Layer Transition and Turbulent Flow Physics from Multipoint Measurements

Multipoint surface shear stress and hot-wire velocity measurements were obtained within a three-dimensional, swept-wing boundary layer. Proper orthogonal decomposition, spectra, and spatial correlations were used to identify the presence of flow structures and their spatial evolution. The first proper orthogonal decomposition crossflow spatial eigenmode from the shear stress obtained in the laminar region has a wavelength of 12 mm, which is consistent with the crossflow vortex spanwise periodicity. The first proper orthogonal decomposition crossflow spatial eigenmode obtained in the turbulent region is not nearly as distinct, but a large spatial structure approximately 24 mm in span is observed, suggesting that, through the transition process, two neighboring crossflow vortices are interacting with a stronger one, canceling a close weaker one. The proper orthogonal decomposition was also applied in span to hot-wire velocity measurements at 3 mm above the wing surface in both the laminar and turbulent regimes. Flow structure in the turbulent flow regime has been identified that is very similar to that obtained in the laminar transition zone. This suggests that the initial instability and transition physics continue to play a key role even after the flow has transitioned to turbulence.

The Aerodynamic behavioral Study of Two Wing's Wake Flow in Tandem Arrangement

This paper is aimed to study the aerodynamic properties and flow structures of flow field between two wings in tandem arrangement, including the experiments of force and velocity measurement in the wind tunnel. From the hot-wire velocity measurements, the spectral analysis is also conducted to observe the flow development of the trailing vortices. It is found that there is a sudden drop of shedding frequency distribution at the distance of 4 to 5 times the chord length from the front wing when AOA=10°, which determines the distance between the front wing and following wing, and understand the dynamic response of following wing under the effect of trailing vortices from the front wing at constant angle of attacks. In addition, the aerodynamic performance of the following wing is drastically affected due to the impingement of the tip vortices or wake from the front wing as well.

Concave Surface Boundary Layer Flows in the Presence of Streamwise Vortices

Concave surface boundary-layer flows are subjected to centrifugal instability which results in the formation of streamwise counter-rotating vortices. Such boundary layer flows have been experimentally investigated on concave surfaces of 1 m and 2 m radius of curvature. In the experiments, to obtain uniform vortex wavelengths, thin perturbation wires placed upstream and perpendicular to the concave surface leading edge, were used to pre-set the wavelengths. Velocity contours were obtained from hot-wire anemometer velocity measurements. The most amplified vortex wavelengths can be pre-set by the spanwise spacing of the thin wires and the free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different vortex growth regions can be identified. The occurrence of a secondary instability mode is also shown as mushroom-like structures as a consequence of the non-linear growth of the streamwise vortices. Wall shear stress measurements on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By pre-setting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of the streamwise vortices will respectively occur.

On the use of hot-wire anemometry in pulsating flows. A comment on ‘A critical review on advanced velocity measurement techniques in pulsating flows’

In their recent topical review, Nabavi and Siddiqui (Meas. Sci. Technol. 2010 21 042002) recommended the use of hot-wire anemometry for velocity measurements in pulsating flows, especially at high frequency. This recommendation is misleading. The procedures invoked by these authors are valid only for small-amplitude fluctuations, which are of little interest for pulsating flows. When large-amplitude velocity changes occur without flow reversal, new procedures for the correction of the nonlinearities of the hot wire are required. This case was thoroughly investigated for a constant-voltage anemometer by Berson et al (Rev. Sci. Instrum. 2009 80 045102). In addition, we show that when flow reversal occurs—a case most relevant to pulsating flows—accurate hot-wire velocity measurements are not possible because heat transfer is not well defined when velocity passes through zero and changes direction.

On the onset of turbulence in natural convection on inclined plates

The problem of determination of the turbulence onset in natural convection on heated inclined plates in an air environment has been experimentally revisited. The transition has been detected by using hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity fluctuations (measured through turbulence intensity) start to grow. Experiments have shown that the onset depends not only on the Grashof number defined in terms of the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting quite well the experimental data.

Comparison between Measurements and Predictions of Fan Broadband Noise on a Low Speed Fan Rig

This paper presents a comparison between measurements and predictions of the fan broadband noise in a low speed fan rig. It is assumed here that the broadband noise is due to the interaction between turbulence generated in the wake of the rotor and the downstream stator vanes. The contribution due to the background turbulence level is also included in the analysis. The stator vanes are assumed to be unloaded flat plates and the wakes are assumed to satisfy two-dimensional plane wake theory. The wake turbulence velocity correlation function is assumed to be periodic in the blade pitch direction (i.e., cyclo-stationary). Aerodynamic input data to the model, both steady and unsteady, is taken directly from hot wire velocity measurements. Comparisons between predicted and measured sound power spectra are in reasonable agreement with spectral shape being particularly well captured by the model. We first review the details of the broadband noise prediction model. Hot wire velocity data will then presented by way of validating the model simplifying assumptions, and to provide the input data to the model. Finally, comparisons between the measured and predicted sound power spectrum will be presented. The work reported here was undertaken as part of the European project, PROBAND.