Unsteady Measurements Research Articles

Real-time implementation of Kalman filter for unsteady flow measurement in a pipe

A Kalman filter which estimates unsteady laminar flow in a pipe is implemented on a real-time computing system. The plant model is the optimised finite element model of pipeline dynamics considering unsteady laminar friction. A steady-state Kalman filter is built based on the model of pipeline dynamics. Pressure signals at both ends of a target section of a pipe are input to the model of pipeline dynamics, and as an output of the model an estimated pressure signal at a mid-point of the pipe is obtained. Difference between measured and estimated pressure signals at the mid-point is fed back to the model of pipeline dynamics to modify state variables of the model. According to the Kalman filter principle, the state variables of the model are adjusted so that they converge to real values. It is demonstrated that real-time implementation of the Kalman filter is possible with the sampling time of 0.1 ms.

Uniformity Study of Two-Functional Luminescent Dyes Adsorbed over an Anodized Aluminum Coating for Motion-Capturing Pressure- and Temperature-Sensitive Paint Imaging.

The pressure- and temperature-sensitive paint (PSP/TSP) technique, for steady-state and unsteady-state measurements, is becoming widespread. However, unsteady quantitative measurement is still difficult because non-uniform distribution of the probes over a test model may cause errors in the results. We focus on the dipping method that applies two luminophores into a binding material to improve sensitivity uniformity over a model surface. A bullet-shaped axisymmetric test model with motion-capturing TSP was used to evaluate the sensitivity uniformity, and three dipping methods (static, convectional, and rotational) were examined. The average peak ratios in the longitudinal direction were 1.17–1.46 for static, 1.38–1.51 for convectional, and 1.42–1.45 for rotational dipping. The standard deviations in the transverse direction were the smallest for rotational (0.022–0.033), relative to static (0.086–0.104), and convectional (0.044–0.065) dipping.

Wind turbine noise measurements in controlled conditions

To validate and reduce the uncertainty associated with noise prediction models for wind turbines, there is a need for detailed noise measurements on wind turbines in controlled conditions. However, high-quality wind tunnel campaigns on horizontal axis wind turbine models are scarce due to the large wind tunnel size needed and consequently high associated costs. To serve this purpose, an experiment using the 4.5 -m diameter Mexico turbine was set up in the large low-speed facility of the DNW wind tunnel. An overview of the experiments is given including a selection of results. Both far field microphone as well as microphone array measurements have been performed together with unsteady force measurements on five instrumented blade sections. This allows a unique insight into the relation between acoustics and the underlying aerodynamics. Overall noise characteristics of the turbine have been determined for a variety of operational conditions by varying tip speed ratio and blade pitch angles. Scaling of the noise is studied by comparing similar combinations of tip speed ratio and pitch angle for different tip speeds. The effect of blade soiling on the noise is evaluated using roughness strips, as well as the influence of yawed or misaligned inflow on the rotor noise. A comparison of calculations using a BPM model is given indicating for which operational conditions this model suffices and for which parts the noise prediction can be improved. In summary, after years of preparation, ECN and partners have performed very successful aeroacoustic experiments in the largest wind tunnel in Europe. The comprehensive high-quality database that has been obtained will be used in the international Mexnext consortium to further improve wind turbine acoustic modeling.

Dynamical aspects of a backward-facing step flow at large Reynolds numbers

A contribution to the study of separating and reattaching flows over a backward-facing step configuration is reported. Simultaneous unsteady wall-pressure and velocity field measurements are made to investigate the dynamical aspects of the separated flow field at high Reynolds numbers, up to 182,600. An array of 25 unsteady pressure sensors in the streamwise direction coupled with PIV velocity fields is used to analyze the statistical properties as well as the streamwise time–space characteristics of the separated flow. A comparison with some previous studies is done to highlight the expansion ratio influence and dependency on the main separation flow. Emphasis is placed on the dynamical aspects of turbulent flows at a wide range of Reynolds numbers, specifically in the convective motion of the vortical flow structures. Unsteady pressure spectra show, even for high Reynolds numbers, the relative dominance of low-frequency flapping motion over high-frequency modes of the large-scale vortical structure. The latter shows a ‘waked mode’ behavior, where these large-scale coherent structures grow in place and further accelerate in the downstream direction. The ability to understand the flow field unsteadiness will lead future investigations to develop active and/or passive flow control techniques in separating and reattaching flows.

Mixture-fraction imaging at 1 kHz using femtosecond laser-induced fluorescence of krypton.

Femtosecond, two-photon-absorption laser-induced-fluorescence (TALIF) imaging measurements of krypton (Kr) are demonstrated to study mixing in gaseous flows. A measurement approach is presented in which observed Kr TALIF signals are 7times stronger than the current state-of-the-art methodology. Fluorescence emission is compared for different gas pressures and excitation wavelengths, and the strongest fluorescence signals were observed when the excitation wavelength was tuned to 212.56nm. Using this optimized excitation scheme, 1-kHz, single-laser-shot visualizations of unsteady flows and two-dimensional measurements of mixture fraction and scalar dissipation rate of a Kr-seeded jet are demonstrated.

Measurement of unsteady flow structures in a low-speed wind tunnel using continuous wave laser-based TR-PIV: near wake behind a circular cylinder

The unsteady measurement of spatiotemporally varying flow structures in a low-speed wind tunnel using a continuous wave (CW) laser-based time-resolved particle image velocimetry (TR-PIV) setup was extensively evaluated in the near wake behind a circular cylinder. A CW laser with a maximum power of 25 W in combination with a high-speed camera operating at 7 kHz was used to determine the wake flows at two different free-stream flow speeds: U 0 = 5 and 10 m/s. Three different camera exposure times were selected for comparison: τ = 20, 50, and 80 μs. In the experiments, the low-repetition conventional PIV setup using the high-power pulsed laser (τ = 8 ns, 135 mJ/pulse) was used to determine the time-mean and statistical flow quantities, which served as the reference for determining the deviation in the TR-PIV measurements. At the lower flow speed of U 0 = 5 m/s, the time-mean-separated flow patterns and the streamwise velocity profiles in all of the TR-PIV systems showed satisfactory agreement with the conventional PIV measurements, along with accurate capture of the large-scale Karman vortex and its harmonic behaviors. At the higher flow speed of U 0 = 10 m/s, the measurement at τ = 50 μs gave a relatively accurate representation of the statistical flow quantities. At the longest exposure time of τ = 80 μs, considerable deviations in the time-mean streamwise fluctuation intensity and the TKE (turbulence kinetic energy) were observed due to the streaky particle image. The strong swirling motion of the large-scale vortical structures increased the deviation in the TR-PIV measurements, which increased with the increasing camera exposure time. Further POD analysis demonstrated that the leading energetic modes in the system with τ = 50 μs accurately determined the spatial features of the Karman-like vortex and its harmonic events. However, inaccurate vector representation of the second harmonic events was observed in the system with τ = 80 μs. Finally, for both flow speeds, the lower-order reconstructed phase-dependent representations of the Karman-like vortex and its harmonic behaviors were composed of the time-series velocity vector fields determined using the system with τ = 50 μs, thus providing a straightforward quantitative view of the coupled unsteady events.

Motion-Induced Unsteady Aerodynamic Loads with Development of Vortical Flow

The development of vortical flow strongly changes the properties of motion-induced aerodynamic loads. Tests were performed in the Transonic Wind Tunnel Göttingen with a half-wing model of a lambda wing. A hydraulic actuator imposed pitch oscillations with amplitudes and frequencies relevant for aeroelastic analyses. Unsteady surface-pressure and flowfield measurements were performed. With increasing angle of attack, a leading-edge vortex emerges and moves inboard. The unsteady loads show a strong dependency on the effects of the vortical flow. For the pitching cases, the effects are lagging the motion. In the rear outboard part of the wing, the unsteady pressures are small, but the amplitude of the motion is high. Hence, a significant amount of energy—force integrated over motion—is transferred from the fluid to the motion. For a free motion or deformation of the wing, this would cause a destabilization. A significant effect of the vortical flow is the discrepancy between the position where it emerges and is affected, and the position (e.g., further outboard) where the flow may generate unsteady, potentially destabilizing loads.

Unsteady flow dynamics reconstruction from mean flow and point sensors: an experimental study

This article presents a reconstruction of the unsteady behaviour of a round jet at a Reynolds number equal to 3300, from the sole knowledge of the time-averaged flow field and one pointwise unsteady measurement. The reconstruction approach is an application of the work of Beneddine et al. (J. Fluid Mech., vol. 798, 2016, pp. 485–504) and relies on the computation of the dominant resolvent modes of the flow, using a parabolised stability equations analysis. To validate the procedure, the unsteady velocity field of the jet has been characterised by time-resolved particle image velocimetry (TR-PIV), yielding an experimental reference. We first show that the dominant resolvent modes are proportional to the experimental Fourier modes, as predicted by Beneddine et al. (J. Fluid Mech., vol. 798, 2016, pp. 485–504). From these results, it is then possible to fully reconstruct the unsteady velocity and pressure fluctuation fields, yielding a flow field that displays good agreement with the experimental reference. Finally, it is found that the robustness of the reconstruction mainly depends on the location of the pointwise unsteady measurement, which should be within energetic regions of the flow, and this robustness as well as the quality of the reconstruction can be greatly improved by considering a few pointwise measurements instead of a single one. The effects of other experimental parameters on the reconstruction, such as the size of the interrogation window used for the TR-PIV processing and the accuracy of the positioning of the sensors, are also investigated in this paper.

Unsteady Flow Field and Coarse Droplet Measurements in the Last Stage of a Low-Pressure Steam Turbine With Supersonic Airfoils Near the Blade Tip

The largest share of electricity production worldwide belongs to steam turbines. However, the increase of renewable energy production has led steam turbines to operate under part load conditions and increase in size. As a consequence, long rotor blades will generate a relative supersonic flow field at the inlet of the last rotor. This paper presents a unique experiment work that focuses at the top 30% of stator exit in the last stage of an low pressure (LP) steam turbine test facility with coarse droplets and high wetness mass fraction under different operating conditions. The measurements were performed with two novel fast response probes: a fast response probe for three-dimensional flow field wet steam measurements and an optical backscatter probe for coarse water droplet measurements ranging from 30 μm up to 110 μm in diameter. This study has shown that the attached bow shock at the rotor leading edge is the main source of interblade row interactions between the stator and rotor of the last stage. In addition, the measurements showed that coarse droplets are present in the entire stator pitch with larger droplets located at the vicinity of the stator's suction side. Unsteady droplet measurements showed that the coarse water droplets are modulated with the downstream rotor blade-passing period. This set of time-resolved data will be used for in-house computational fluid dynamics (CFD) code development and validation.

Non-Ideal Compressible-Fluid Dynamics of Fast-Response Pressure Probes for Unsteady Flow Measurements in Turbomachinery

The dynamic response of pressure probes for unsteady flow measurements in turbomachinery is investigated numerically for fluids operating in non-ideal thermodynamic conditions, which are relevant for e.g. Organic Rankine Cycles (ORC) and super-critical CO2 applications. The step response of a fast-response pressure probe is investigated numerically in order to assess the expected time response when operating in the non-ideal fluid regime. Numerical simulations are carried out exploiting the Non-Ideal Compressible Fluid-Dynamics (NICFD) solver embedded in the open-source fluid dynamics code SU2. The computational framework is assessed against available experimental data for air in dilute conditions. Then, polytropic ideal gas (PIG), i.e. constant specific heats, and Peng-Robinson Stryjek-Vera (PRSV) models are applied to simulate the flow field within the probe operating with siloxane fluid octamethyltrisiloxane (MDM). The step responses are found to depend mainly on the speed of sound of the working fluid, indicating that molecular complexity plays a major role in determining the promptness of the measurement devices. According to the PRSV model, non-ideal effects can increase the step response time with respect to the acoustic theory predictions. The fundamental derivative of gas-dynamic is confirmed to be the driving parameter for evaluating non-ideal thermodynamic effects related to the dynamic calibration of fast-response aerodynamic pressure probes.

Experimental investigation of boundary layer transition in flow past a bluff body

We explore the phenomenon of drag crisis observed for the flow over bluff bodies at high Reynolds numbers. The drag coefficient reduces significantly beyond a certain Re due to the transition of the boundary layer from laminar to turbulent state. Flow past a smooth sphere and a circular cylinder is experimentally investigated for 1.0 × 105 ≤ Re ≤ 5.0 × 105 via unsteady force, surface–pressure and 2–D Particle Image Velocimetry(PIV) measurements. In case of a smooth sphere, the drag crisis is observed for Re > 3.3 × 105. The unsteady force measurements reveal that the fluctuations in the force coefficients initially increase with Re in the high subcritical regime and then experience a steep fall in the critical regime. It is found from the PIV measurements that the normal Reynolds stresses in the separated shear layer from the sphere are one order lower in magnitude for the supercritical regime in comparison to the subcritical regime. In the case of flow past a smooth circular cylinder, a two–stage drag crisis is captured using surface–pressure measurements where the boundary layer over one side of the cylinder undergoes transition around Re = 3.9 × 105 and that over the second side transitions around Re = 4.8 × 105. The transition is accompanied with increased fluctuations in the surface–pressure coefficients near the shoulders of the cylinder.

In-flight investigations of the unsteady behaviour of the boundary layer with infrared thermography

Infrared thermography (IRT) has been well established in wind tunnel and flight tests for the last decade. Former applications of IRT were focused, in nearly all cases, on steady measurements. In the last years, requirements of unsteady IRT measurements (up to 10 Hz) have been formulated, but the problem of a very slow thermal response of common materials of wind tunnel models or airplane components has to be overcome by finding a surface modification with a fast thermal response (low heat capacity, low thermal conductivity and high thermal diffusivity). Therefore, lab investigations of potential material combinations and flight tests with a ‘low cost’ aircraft, i.e. a glider with a modified wing surface, were conducted. In order to induce unsteady conditions (rapid change of laminar-turbulent boundary layer transition), special maneuvers of a glider during IRT measurements were performed.

Unsteady velocity profiling in bores and positive surges

In an open channel, steady flow conditions may be achieved when the discharge and boundary conditions remain constant for a reasonable period of time. The operation of any regulation device (e.g. gate) is associated with some unsteady surge motion. In the present study, new velocity profiling measurements were performed systematically under controlled flow conditions. Both steady and unsteady measurements were conducted in a relatively large laboratory facility. An ensemble-averaged technique was applied in unsteady flows to investigate positive surges. The experiments were repeated 25 (or 50) times for each controlled flow condition and the results were ensemble-averaged. The quality and accuracy of the Profiler data set were validated against data collected with an acoustic Doppler velocimeter, in both steady and unsteady rapidly-varied flows. A careful sensitivity analysis was conducted to test the appropriate number of runs. The results indicated that the selection of 25 runs was suitable for ensemble-averaging in rapidly-varied unsteady flows. Some instrumental error was observed however with the velocity profiler. Outside the boundary layer, the Profiler tended to produce errors in terms time-averaged velocity data and velocity fluctuations for a number of points in a profile. Overall, the study demonstrated that the propagation of positive surges is a highly unsteady turbulent process, and the performance of ADV Vectrino II Profiler in such an unsteady turbulent flow was satisfactory, provided that a careful validation was undertaken for all Profiler outputs.

Numerical and Experimental Investigation of Axial Gap Variation in High-Pressure Steam Turbine Stages

This work aims at investigating the impact of axial gap variation on aerodynamic performance of a high-pressure steam turbine stage. Numerical and experimental campaigns were conducted on a 1.5-stage of a reaction steam turbine. This low speed test rig was designed and operated in different operating conditions. Two different configurations were studied in which blades axial gap was varied in a range from 40% to 95% of the blade axial chord. Numerical analyses were carried out by means of three-dimensional, viscous, unsteady simulations, adopting measured inlet/outlet boundary conditions. Two sets of measurements were performed: steady measurements, from one hand, for global performance estimation of the whole turbine, such as efficiency, mass flow, and stage work; steady and unsteady measurements, on the other hand, were performed downstream of rotor row, in order to characterize the flow structures in this region. The fidelity of computational setup was proven by comparing numerical results to measurements. Main performance curves and spanwise distributions have shown a good agreement in terms of both shape of curves/distributions and absolute values. Moreover, the comparison of two-dimensional maps downstream of rotor row has shown similar structures of the flow field. Finally, a comprehensive study of the axial gap effect on stage aerodynamic performance was carried out for four blade spacings (10%, 25%, 40%, and 95% of S1 axial chord) and five aspect ratios (1.0, 1.6, 3, 4, and 5). The results pointed out how unsteady interaction between blade rows affects stage operation, in terms of pressure and flow angle distributions, as well as of secondary flows development. The combined effect of these aspects in determining the stage efficiency is investigated and discussed in detail.

Application of Unsteady Flow Measurement for Turbomachine Development

Application of Unsteady Flow Measurement for Turbomachine Development

Comparison of different turbulence models in simulating unsteady flow

Unsteady airflow measurement results of the experiment by Lance were rediscovered by attempting different RANS turbulence models. And the differences between the model of low Reynold models, zero equation models and standard models are compared when natural convection is dominant. The simulation results show that the robustness of standard turbulence model is superior to the low Reynold models. For unsteady airflow simulation, when the fluid flow characteristics change into natural convection, according to the characters of the required sample point and switch to the corresponding suitable turbulence model, can increase the accuracy of unsteady simulation and reduces computation time.

An improved apparatus for a wake generator using a cam mechanism and unsteady velocity measurement

An improved apparatus for a wake generator using a cam mechanism and unsteady velocity measurement

Unsteady measurements of humidity distribution in a PEFC cathode channel using thin film sensors

Unsteady measurements of humidity distribution in a PEFC cathode channel using thin film sensors

Experimental validation of CFD model of thermal fluxes through a multilayer wall

The evaluation of thermal losses through building envelope is complex due to the presence of different components such as geometrical and structural thermal bridges, multilayer walls, windows and shadow areas. In particular, in presence of material and/or shape discontinuities, the heat flux becomes two-dimensional or three-dimensional and loses its one-dimensionality. Thermal bridges are weak points of the building envelope; the measurement of heat losses through the walls is quite complicated and in particular the detection of bi-tridimensional thermal flux. The integration between the cfd analysis and the experimental study aims the improvement of the measurement technique and the assessment of the dispersion of heat fluxes through multi-layer walls both in the steady and unsteady measurement conditions. The numerical study has been directed for modelling, by means of the commercial numerical code Fluent, of the experimental test section consisting of a multi-layer wall, in which a structural thermal bridge has been inserted in order to make accurate measurements of the three-dimensional heat flux in steady state condition. The study has been developed to validate the CFD model by the experimental set-up, with the aim to investigate possible measurement methodologies of heat fluxes.

Observations of the Growth and Decay of Stall Cells during Stall and Surge in an Axial Compressor

This research investigated unsteady events such as stall inception, stall-cell development, and surge. Stall is characterized by a decrease in overall pressure rise and nonaxisymmetric throughflow. Compressor stall can lead to surge which is characterized by quasi-axisymmetric fluctuations in mass flow and pressure. Unsteady measurements of the flow field around the compressor rotor are examined. During the stall inception process, initial disturbances were found within the rotor passage near the tip region. As the stall cell develops, blade lift and pressure ratio decrease within the stall cell and increase ahead of the stall cell. The stall inception event, stall-cell development, and stall recovery event were found to be nearly identical for stable rotating stall and surge cases. As the stall cell grows, the leading edge of the cell will rotate at a higher rate than the trailing edge in the rotor frame. The opposite occurs during stall recovery. The trailing edge of the stall cell will rotate at the approximate speed as the fully developed stall cell, while the leading edge decreases in rotational speed in the rotor frame.