Hot-film Measurements Research Articles

Modeling forced convection in the thermal simulation of laser cladding processes

A comprehensive methodology for the implementation of thermal convection into the finite element (FE) analysis of laser direct energy deposition (DED) cladding is developed and validated. Improved convection modeling will produce improved thermal simulations, which will in turn yield more accurate results from subsequent models seeking to predict microstructural changes, deformation, or residual stresses. Two common convection implementations, considering no convection or free convection only, are compared to three novel forced convection methods: forced convection from heat transfer literature, forced convection from lumped capacitance experiments, and forced convection from hot-film anemometry measurements. During the cladding process, the exposed surface, the surface roughness, and total surface area change due to material deposition. The necessity of accounting for the evolution of the mesh surface in the FE convection model is investigated. Quantified error analysis shows that using any of the three forced convection methodologies improves the accuracy of the numerical simulations. Using surface-dependent hot-film anemometry measured convection yields the most accurate temperature history, with L 2 norm errors of 6.25−22.1 ∘C and time-averaged percent errors of 2.80–12.4 %. Using a physically representative convection model applied to a continually evolving mesh surface is shown to be necessary for accuracy in the FE simulation of laser cladding processes.

VORTEX FORMATION IN INCOMPRESSIBLE AXISYMMETRIC FREE JETS

Free jets have been utilized extensively in many industrial applications. In general jet fluid discharging from a nozzle develops flow oscillations in the shear layer. The oscillations will roll up to eventually become toroidal vortices which increase in size with the axial distance from the nozzle. In the present work, flow visualization as well as hot-film anemometry have been employed in order to study incompressible axisymmetric free jet in moderate Reynolds numbers up to 20,000. The injection of liquid dye or micro particles associated with a laser sheet turns possible to visualize the shear layers and, consequently, the vortex formation. Hot-film measurements into the jet allow determining the flow velocity profile. Flow visualization is a qualitative tool which a broad view of the flow topology. On the other hand, hot-film anemometry is a precise quantitative tool but measurement in only one location at a time. The association of flow visualization and hot-film anemometry shows very adequate for free jet studies.

Pressure and hot-film measurements on a wind turbine blade operating in the atmosphere

In the present study the aerodynamic boundary layer at a rotor blade is investigated while the turbine is working under real operating conditions in the atmosphere. Owing to the complexity of the experimental set-up, up to now most research on transition is conducted in wind tunnels and field measurements are rare. Hence important effects such as the unsteady behavior of the inflow is not taken into account. For the current measurements the blade is equipped with a hot film at the most interesting part of the upper side midspan of the blade in order to detect non-laminar structures in the boundary layer. Furthermore, 34 pressure tubes are installed along the chord length in order to gain information about the flow field. A preliminary analysis of the hot-film measurements combined with a CFD calculation and a stability analysis based on the eN method leads to two results. Firstly it is possible to determine the state of the boundary layer (laminar or turbulent) and secondly we propose to discuss our findings in case of medium rotational speed within so called Tollmien-Schlichting scenario.

Transition on a Wing with Spanwise Varying Crossflow and Linear Stability Analysis

A new test case for challenging current transition prediction methods for three-dimensional boundary layers is designed with special focus on the effect of spanwise varying mean flow. The wind-tunnel model of this test case has a sickle-shaped planform. It is equipped with pressure taps and a special skin layup to increase the contrast of infrared transition measurements. The wing is designed to generate three-dimensional boundary layers with increasing crossflow toward the wingtip. The sickle-shaped planform with distinct kinks creates spanwise gradients, and the assumptions of linear local stability theory are therefore challenged. It is found that, at locations of high spanwise gradients, the transition location moves upstream, due to strong amplification of stationary crossflow vortices. Hot-film measurements are used to identify and characterize traveling modes, and to quantify disturbance amplifications. The wing exhibits areas of rapidly growing Tollmien–Schlichting modes, which behave in good agreement with linear stability theory, whereas crossflow-dominated regions display both stationary and traveling crossflow modes.

Analyzing guard-heating to enable accurate hot-film wall shear stress measurements for turbulent flows

In this paper we examine guard-heated hot-film sensors for the measurement of wall shear stress fluctuations in turbulent flows, using analytical and numerical techniques. The new thermal sensor designs proposed here, seek to eliminate the most severe source of error in conventional single-element hot-film sensors – which is indirect heat transfer to the fluid flow through the solid substrate. Published studies of the single-element hot-film show that a significant portion of the heat generated in the hot-film travels through the substrate before reaching the fluid, which causes spectral and phase errors in the wall shear stress signal and can drastically reduce the spatial resolution of the sensor. In addition, heat conduction parallel to the surface, within the fluid near the sensor edges, can produce significant errors when sensors are made smaller to improve spatial resolution. Here we examine the effectiveness of forcing zero temperature gradient at the edges of the hot-film in eliminating these errors. Air and water flow over such guard-heated sensors are studied numerically to investigate performance and signal strength of the guard-heated sensors. Our results show, particularly for low-conductivity fluids such as air, that edge guard-heating needs to be supplemented by a sub-surface guard-heater. With this two-plane guard-heating, a strong non-linearity in the standard single-element designs can be corrected, and substrate conduction errors contributing to spectral distortion and phase errors can be eliminated.

Laboratory observations of flow and sediment transport induced by plunging regular waves

[1] Two parallel experiments involving the evolution and runup induced by plunging regular waves near the shoreline of a sloping bed are considered: (1) a rigid-bed experiment, allowing direct (hot film) measurements of bed shear stresses and (2) a sediment-bed experiment, allowing for the measurement of pore-water pressures as well as observation of sediment suspension and bed morphological changes. Both experiments utilize the same initial bed profile and wave forcing. The experiments show that the mean bed shear stresses experienced onshore of incipient breaking are amplified by nearly a factor of 2 relative to prebreaking conditions, whereas their corresponding turbulent fluctuations are amplified even more strongly, by a factor of 5–6. The plunging processes lead to a series of vortices, whose formation may be explained as the result of shear layer instability. Measurements show that these vortices can significantly enhance peaks in the offshore-directed bed shear stresses. Moreover, near-bed pore pressure measurements indicate that these vortices cause large upward-directed pressure gradients, which in turn produce a corresponding series of suspended sediment plumes shoreward of the initial breaking event. These findings are related to the induced morphological changes over both short and long time scales. The present results are also compared and contrasted with previous experiments utilizing a similar methodology, but involving plunging solitary waves.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low Reynolds Numbers—Part I: Steady Flow Measurements

This two-part paper presents a detailed experimental investigation of the laminar separation and transition phenomena on the suction surface of a high-lift low-pressure turbine airfoil, PakB. The first part describes the influence of Reynolds number, freestream turbulence intensity and turbulence length scale on the PakB airfoil under steady inflow conditions. The present measurements are distinctive in that a closely-spaced array of hot-film sensors has allowed a very detailed examination of the suction surface boundary layer behavior. In addition, this paper presents a technique for interpreting the transition process in steady, and periodically unsteady, separated flows based on dynamic and statistical properties of the hot-film measurements. Measurements were made in a low-speed linear cascade facility at Reynolds numbers between 25,000 and 150,000 at three freestream turbulence intensity levels of 0.4%, 2%, and 4%. Two separate grids were used to generate turbulence intensity of 4% with integral length scales of about 10% and 40% of the airfoil axial chord length. While the higher levels of turbulence intensity promoted earlier transition and a shorter separation bubble, turbulence length scale did not have a noticeable effect on the transition process. The size of the suction side separation bubble increased with decreasing Reynolds number, and under low freestream turbulence levels the bubble failed to reattach at low Reynolds numbers. As expected, the losses increased with the length of the separation bubble, and increased significantly when the bubble failed to reattach.

Noise in Turbulence Measurements Using Acoustic Doppler Velocimetry

AbstractTo validate the use of acoustic Doppler velocimeters (ADVs) for the measurement of turbulent flows, experiments were conducted in an axisymmetric turbulent jet and in approximately homogenous isotropic turbulence with zero mean flow. The jet experiments show that the horizontal RMS velocities measured by the ADV were overestimated when compared to both flying hot-film anemometry measurements and the accepted values in the literature. However, the vertical component of the RMS velocity agrees well with those of other studies. This was furthermore confirmed by the ADV measurements in the isotropic turbulence with zero mean flow. Given that the overestimated RMS velocities in the horizontal directions may be caused by Doppler noise and spikes, two postprocessing filters and a Doppler noise–reduction method were applied to the jet data in an attempt to correct the data. Although the RMS velocities decreased, they remained erroneously higher than the accepted values. A noise-reduction method for axisym...

Discharge coefficient measurements of round, inclined orifices with inlet cross-flow in and against direction of inclination

In an effort to improve the validated understanding of aerodynamic losses across orifices, this publication describes discharge coefficient measurements, derived from a specially designed test rig. This rig contains a straight, square section main channel (20 × 20 mm2) with exchangeable orifice plates at one side wall with 7 mm hole diameters. The channel was supplied with compressed air at between about zero velocity and nearly the speed of sound. Static pressure ratios across orifices were varied between 1.05 and 2 in addition. Test orifices had inclinations of 0°, 30° and 60° in and against inlet cross-flow. Further, orifice length-to-diameter ratios were varied to 0.5, 1 and 2. Orifice inlet and outlet corners were sharp. The main channel velocity was determined using a pitot tube, a thermocouple and flush-mounted static pressure off-takes. Orifice downstream pressure was measured by a flush mounted static pressure off-take. Orifice through flow was determined by a calibrated hot film measurement device with constant temperature difference control, equipped with an additional thermocouple to determine fluid properties.

An Investigation of Reynolds Lapse Rate for Highly Loaded Low Pressure Turbine Airfoils With Forward and Aft Loading

This paper presents an experimental and computational study of the midspan low Reynolds number loss behavior for two highly loaded low pressure turbine airfoils, designated L2F and L2A, which are forward and aft loaded, respectively. Both airfoils were designed with incompressible Zweifel loading coefficients of 1.59. Computational predictions are provided using two codes, Fluent (with k-kl-ω model) and AFRL’s Turbine Design and Analysis System (TDAAS), each with a different eddy-viscosity RANS based turbulence model with transition capability. Experiments were conducted in a low speed wind tunnel to provide transition models for computational comparisons. The Reynolds number range based on axial chord and inlet velocity was 20,000 < Re < 100,000 with an inlet turbulence intensity of 3.1%. Predictions using TDAAS agreed well with the measured Reynolds lapse rate. Computations using Fluent however, predicted stall to occur at significantly higher Reynolds numbers as compared to experiment. Based on triple sensor hot-film measurements, Fluent’s premature stall behavior is likely the result of the eddy-viscosity hypothesis inadequately capturing anisotropic freestream turbulence effects. Furthermore, rapid distortion theory is considered as a possible analytical tool for studying freestream turbulence that influences transition near the suction surface of LPT airfoils. Comparisons with triple sensor hot-film measurements indicate that the technique is promising but more research is required to confirm its utility.

Spatially resolved skin friction velocity measurements using Irwin sensors: A calibration and accuracy analysis

This study investigates the accuracy of spatially resolved skin friction velocity uτ measurements using a large number of Irwin sensors (Irwin, 1981). A fast and simple procedure to calibrate the sensors against a two-component hot-film anemometer is introduced which leads to a universal calibration function that can then be used for all sensors. Insignificantly lower accuracies are obtained using the universal calibration function instead of individual calibration functions for each sensor. The overall accuracy of the skin friction velocity measurements averages about ±5% for uτ>0.13ms−1 and decreases strongly to ±20% at lower uτ. The main sources of errors are as follows: (i) deviations from flush mounting of the sensors with the wall surface, (ii) inhomogeneous boundary layers produced in wind tunnels, (iii) pressure transducer inaccuracies, (iv) Irwin sensor geometry variations and (v) Irwin sensor calibration errors. An excellent linear relationship between uτ measured with the Irwin sensors and the free stream velocity as well as the good agreement of Irwin sensor with independent hot-film measurements validate the reliability of the measurement setup. An exemplary measurement of the skin friction velocity distribution around a single wall-mounted rectangular block is presented, discussed and compared to literature and can serve as a test case for simulations.

Wake Formation From a Pair of Circular Cylinders Traversing Between Small- and Large-Incidence Flow Regimes

Cross flow past a pair of equal-diameter staggered circular cylinders, with either one of the pair subject to forced harmonic transverse oscillation, is investigated experimentally within Reynolds numbers Re = 525–750. The center-to-center pitch ratio and stagger angle of the cylinders at their mean position are 2.5° and 21°, respectively. Results with cylinder excitation frequencies in the range 0.07 ≤ feD/U ≤ 1.18 (D = cylinder diameter, U = mean flow velocity) at a constant oscillation amplitude (peak-to-peak) of 0.44D are reported. Flow visualization of the wake formation region and hot-film measurements of the wake velocity are reported. Emphasis is placed on the mechanisms leading to vortex shedding. Results show that the wake undergoes considerable modification with the oscillation of either of the two cylinders; this modification depends strongly on the value of feD/U. The flow patterns remain essentially the same as those of the corresponding static cases for feD/U < 0.10. However, the flow at higher oscillation frequencies than that can no longer maintain those patterns. In particular, there are distinct regions of fundamental and superharmonic synchronizations between the dominant wake periodicities and the cylinder oscillation over the whole range of feD/U. Moreover, the manner in which the wake responds to the cylinder oscillation depends strongly on whether it is the upstream or downstream cylinder which is being oscillated.

Feasibility of global skin friction diagnostics using temperature sensitive paint

This paper develops the principle of global skin friction diagnostics based on temperature sensitive paint (TSP) measurements in flows. The asymptotic form of the energy equation at a wall is derived, and its inverse solution is sought by using the variational method to extract a relative or normalized skin friction field from TSP-measured surface temperature fields. The selection of the relevant parameters (the Lagrange multiplier and the standard deviation of a Gaussian filter) and the intrinsic limitations of this method are discussed in the light of uncertainty analysis. Experimental implementation of this method is described, and the feasibility of TSP-based global skin friction diagnostics is examined in impinging jet experiments. The normalized skin friction distributions extracted by using this method are in fairly good agreement with hot-film skin friction measurements and the theoretical solutions in the normal and oblique impinging jets.

Wall shear stress in accelerating turbulent pipe flow

AbstractAn experimental study of wall shear stress in an accelerating flow of water in a pipe ramping between two steady turbulent flows has been undertaken in a large-scale experimental facility. Ensemble averaged mean and r.m.s. of the turbulent fluctuations of wall shear stresses have been derived from hot-film measurements from many repeated runs. The initial Reynolds number and the acceleration rate were varied systematically to give values of a non-dimensional acceleration parameter $k$ ranging from 0.16 to 14. The wall shear stress has been shown to follow a three-stage development. Stage 1 is associated with a period of minimal turbulence response; the measured turbulent wall shear stress remains largely unchanged except for a very slow increase which is readily associated with the stretching of existing turbulent eddies as a result of flow acceleration. In this condition of nearly ‘frozen’ turbulence, the unsteady wall shear stress is driven primarily by flow inertia, initially increasing rapidly and overshooting the pseudo-steady value, but then increasing more slowly and eventually falling below the pseudo-steady value. This variation is predicted by an analytical expression derived from a laminar flow formulation. The start of Stage 2 is marked by the generation of new turbulence causing both the mean and turbulent wall shear stress to increase rapidly, although there is a clear offset between the responses of these two quantities. The turbulent wall shear, reflecting local turbulent activities near the wall, responds first and the mean wall shear, reflecting conditions across the entire flow field, responds somewhat later. In Stage 3, the wall shear stress exhibits a quasi-steady variation. The duration of the initial period of nearly frozen turbulence response close to the wall increases with decreasing initial Reynolds number and with increasing acceleration. The latter is in contrast to the response of turbulence in the core of the flow, which previous measurements have shown to be independent of the rate of acceleration.

High-resolution hot-film measurement of surface heat flux to an impinging jet

To investigate the complex coupling between surface heat transfer and local fluid velocity in convective heat transfer, advanced techniques are required to measure the surface heat flux at high spatial and temporal resolution. Several established flow velocity techniques such as laser Doppler anemometry, particle image velocimetry and hot wire anemometry can measure fluid velocities at high spatial resolution (µm) and have a high-frequency response (up to 100 kHz) characteristic. Equivalent advanced surface heat transfer measurement techniques, however, are not available; even the latest advances in high speed thermal imaging do not offer equivalent data capture rates. The current research presents a method of measuring point surface heat flux with a hot film that is flush mounted on a heated flat surface. The film works in conjunction with a constant temperature anemometer which has a bandwidth of 100 kHz. The bandwidth of this technique therefore is likely to be in excess of more established surface heat flux measurement techniques. Although the frequency response of the sensor is not reported here, it is expected to be significantly less than 100 kHz due to its physical size and capacitance. To demonstrate the efficacy of the technique, a cooling impinging air jet is directed at the heated surface, and the power required to maintain the hot-film temperature is related to the local heat flux to the fluid air flow. The technique is validated experimentally using a more established surface heat flux measurement technique. The thermal performance of the sensor is also investigated numerically. It has been shown that, with some limitations, the measurement technique accurately measures the surface heat transfer to an impinging air jet with improved spatial resolution for a wide range of experimental parameters.

Realistic evaluation of hull performance for rowing shells, canoes, and kayaks in unsteady flow

In this study, we investigated the effect of hull dynamics in shallow water on the hydrodynamic performance of rowing shells as well as canoes and kayaks. An approach was developed to generate data in a towing tank using a test rig capable of reproducing realistic speed profiles. The impact of unsteady shallow-water effects on wave-making resistance was examined via experimental measurements on a benchmark hull. The data generated were used to explore the validity of a computational approach developed to predict unsteady shallow-water wave resistance. Comparison of measured and predicted results showed that the computational approach correctly predicted complex unsteady wave-resistance phenomena at low oscillation frequency and speed, but that total resistance was substantially under-predicted at moderate oscillation frequency and speed. It was postulated that this discrepancy arose from unsteady viscous effects. This was investigated via hot-film measurements for a full-scale single scull in unsteady flow in both towing-tank and field-trial conditions. Results suggested a strong link between acceleration and turbulence and demonstrated that the measured real-world viscous-flow behaviour could be successfully reproduced in the tank. Thus a suitable tank-test approach could provide a reliable guide to hull performance characterization in unsteady flow.

An experimental study of unsteady hydrodynamics of a single scull

The effect of hull dynamics on the hydrodynamic performance of a single scull is investigated via a combination of field trials and tank tests. The location of laminar-turbulent transition in unsteady flow is explored via several series of hot-film measurements on the bow of a full-scale single scull in unsteady flow in both towing tank and field-trial conditions. Results demonstrate that the measured real-world viscous-flow behaviour can be successfully reproduced in the tank using an oscillating sub-carriage to reproduce the surging motion measured in the field trials. It can be seen that there is a strong link between turbulence and acceleration; results show that the link is relatively insensitive to mean velocity, but that small changes in acceleration time-histories can have a marked effect, as can the presence of small waves. The impact of the location of laminar turbulent transition is investigated by way of a series of resistance tests, both with free transition and with transition forced by turbulence stimulation at two different locations. Results indicate that an aft movement of 200 mm of the location of transition can reduce resistance by almost 0.5 per cent. Unsteady tests using the oscillating sub-carriage indicate that unsteady effects add around 3 per cent to the total mean resistance with free transition.

Hybrid Control of a Turret Wake

Effects of hybrid flow control and its active and passive components on the aerodynamic characteristics of flow over a 0.254-m-diam conformal optical aperture embedded in the hemispherical cap of a cylinder turret model (D = 0.61 m) are investigated at M = 0.3-0.5 and Re D = 4.4-7.4 x 10 6 . Resulting mean flows are characterized by surface static pressure distributions and oil-flow visualizations, while the separated-flow dynamics are assessed by hot-film measurements. Active flow control is effected by arrays of piezoelectrically driven synthetic jet modules distributed in multiple arrays upstream from the aperture. Active flow control is further assisted by global flow alterations induced by a passive forward partition plate, and, when combined, constitute hybrid flow control. It is shown that the hybrid flow control combines the positive effects of its component control elements to yield superior results in any cumulative aerodynamic aspect of the separated flow. This cumulative effect of the actuation is manifested by concomitant delay of flow separation and active, dissipative suppression of turbulent motions downstream of separation. It is also demonstrated by means of direct two-dimensional wave-front measurements that the overall aerodynamic improvements correlate with substantial suppression of optical aberrations through the separated flow. Furthermore, estimated Strehl ratios for the laser beam indicate that nearly invariant Strehl ratio is established within the range of tested aperture elevation angles, yielding improvement of about 50 % for the highest elevation angle.

Experimental Study of the Plunging Motion with Unsteady Wind Tunnel Wall Interference Effects

A series of experiments were carried out to investigate unsteady behavior of the flow field as well as the boundary layer of an airfoil oscillating in plunging-type motion in a subsonic wind tunnel. The measurements involved surface-mounted hot films complimented with surface pressure. In addition, wind tunnel wall pressure distribution was acquired to furnish a baseline for the wall interference corrections. The airfoil is the section of a 660-kW wind turbine blade. The experiments were conducted at a Reynolds number of 0.42 million, and over two reduced frequencies of k = 0.06 and 0.085, at prestall, nearstall, and poststall regions. The unsteady aerodynamic loads were calculated from the surface pressure measurements, 64 ports, along the chord for both upper and lower surfaces of the model. The plunging displacements were transformed into the equivalent angle of attack. The surface hot-film measurements provided information about the boundary-layer events. The boundary-layer transition occurred via a laminar separation bubble. Variations of the surface pressure coefficients and aerodynamic loads with the equivalent angle of attack showed strong sensitivity to the reduced frequency and the mean angles of attack. The wall pressure distribution was affected by the model oscillation especially the region underneath the model.

Flow and sediment transport induced by a plunging solitary wave

[1] Two parallel experiments involving the evolution and runup of plunging solitary waves on a sloping bed were conducted: (1) a rigid-bed experiment, allowing direct (hot film) measurements of bed shear stresses and (2) a sediment-bed experiment, allowing for the measurement of pore water pressures and for observation of the morphological changes. The two experimental conditions were kept as similar as possible. The experiments showed that the complete sequence of the plunging solitary wave involves the following processes: shoaling and wave breaking; runup; rundown and hydraulic jump; and trailing wave. The bed shear stress measurements showed that the mean bed shear stress increases tremendously (with respect to that in the approaching wave boundary layer), by as much as a factor of 8, in the runup and rundown stages, and that the RMS value of the fluctuating component of the bed shear stress is also affected, by as much as a factor of 2, in the runup and hydraulic jump stages. The pore water pressure measurements showed that the sediment at (or near) the surface of the bed experiences upward directed pressure gradient forces during the down-rush phase. The magnitude of this force can reach values as much as approximately 30% of the submerged weight of the sediment. The experiments further showed that the sediment transport occurs in the sheet flow regime for a substantial portion of the beach covering the area where the entire sequence of the wave breaking takes place. The bed morphology is explained qualitatively in terms of the measured bed shear stress and the pressure gradient forces.