Unsteady Measurements Research Articles

Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

This paper highlights steady and unsteady measurements and flow control results obtained on an Ahmed model with slant angle of 25° in wind tunnel. On this high-drag configuration characterized by a large separation bubble along with energetic streamwise vortices, time-averaged and time-dependent results without control are first presented. The influence of rear-end periodic forcing on the drag coefficient is then investigated using electrically operated magnetic valves in an open-loop control scheme. Four distinct configurations of flow control have been tested: rectangular pulsed jets aligned with the spanwise direction or in winglets configuration on the roof end and rectangular jets or a large open slot at the top of the rear slant. For each configuration, the influence of the forcing parameters (non-dimensional frequency, injected momentum) on the drag coefficient has been studied, along with their impact on the static pressure on both the rear slant and vertical base of the model. Depending on the type and location of pulsed jets actuation, the maximum drag reduction is obtained for increasing injected momentum or well-defined optimal pulsation frequencies.

Experimental study of a positive surge. Part 2: comparison with literature theories and unsteady flow field analysis

A positive surge is a unsteady open channel flow resulting from the rapid rise of the free-surface. The phenomenon may be observed in water supply canals and channels as well as in some estuaries during spring tidal conditions. The formation and development of positive surges can be predicted using the method of characteristics and shallow water equations. The paper is the second part of a study presenting the results from new experimental investigations conducted in a large rectangular channel. Detailed unsteady velocity measurements were performed with a high temporal resolution using acoustic Doppler velocimetry and non-intrusive free-surface measurement devices. Several experiments were conducted with the same initial discharge (Q = 0.060 m3/s) and six different gate openings after closure resulting in both non-breaking undular and breaking bores. A comparison between main features of the undular surges with literature theories demonstrated that the experimental data were mostly in agreement with Andersen’s theory. The analysis of unsteady flow field including Reynolds stresses confirmed and extended previous findings about positive surge hydrodynamics.

Unsteady velocity measurements in a realistic intracranial aneurysm model

The initiation, growth and rupture of intracranial aneurysms are intensively studied by computational fluid dynamics. To gain confidence in the results of numerical simulations, validation of the results is necessary. To this end the unsteady flow was measured in a silicone phantom of a realistic intracranial aneurysm. A flow circuit was built with a novel unsteady flow rate generating method, used to model the idealised shape of the heartbeat. This allowed the measurement of the complex three-dimensional velocity distribution by means of laser-optical methods such as laser doppler anemometry (LDA) and particle image velocimetry (PIV). The PIV measurements, available with high temporal and spatial distribution, were found to have good agreement with the control LDA measurements. Furthermore, excellent agreement was found with the numerical results.

Stall Warning Using Flap Hinge Moment Measurements

G ENERALLY, surface contamination of a wing leads to reductions in performance [1–3]. Premature stall due to contamination can also have devastating effects on the controllability of aircraft [4,5]. Currently, there is no established, effective method for monitoring aircraft performance and controllability in order to prevent stall under adverse conditions created by surface contamination. While angle-of-attack sensors are capable of providing an effective stall warning for an aircraft free of contaminants, the effectiveness of such systems greatly diminishes with the presence of contaminants on an aircraft wing. Gurbacki and Bragg [6] studied the steady and unsteady hinge moment behaviors of supercooled large-droplet ice-induced stall on a NACA 23012 airfoil model with a simple flap. In that study, it was found that the rms of the unsteady hinge moment increased 1 to 3 angles of attack before stall for the iced case. Thus, it was identified that the unsteady separated flow led to unsteady changes in the hinge moment, giving rise to the concept that unsteady hinge moment measurements could be used to predict ice-induced stall [7]. Such a method could be further applied to a system capable of monitoring aircraft aerodynamic performance and providing realtime predictions of the edge of a flight envelope. In addition to flight beyond the baseline-clean aircraft flight envelope, such a system could potentially detect a reduction in the envelope due to several inflight environmental contaminants resulting from icing encounters, heavy rain, surface contamination in the form of roughness, and structural damage, such as bird strikes or battle damage. To further this concept, additional development is needed beyond the initial work of Gurbacki and Bragg [6]. ThisNote reports the further development of such a technique. The specific focus of this Note is the development of a detection algorithm using the unsteady hinge moment from a simple flap on an airfoil with and without contaminants. The algorithm developed is used to predict stall several degrees before the event.

Experimental study of a positive surge. Part 1: basic flow patterns and wave attenuation

A positive surge results from a sudden change in flow that increases the depth. It is the unsteady flow analogy of the stationary hydraulic jump and a geophysical application is the tidal bore. Positive surges are commonly studied using the method of characteristics andthe Saint-Venant equations.Thearticle presents the results fromnewexperimental inves- tigations conducted in a large rectangular channel. Detailed unsteady velocity measurements were performed with a high temporal resolution using acoustic Doppler velocimetry and non-intrusive free-surface measurement devices. Several experiments were conducted with the same initial discharge (Q = 0.060m 3 /s) and six different gate openings after closure resulting in both non-breaking undular and breaking bores. The analysis of undular surges revealed wave amplitude attenuation with increasing distance of surge propagation were in agreement with Ippen and Kulin theory. Also, undular wave period and wave length data were relatively close to the values predicted by the wave dispersion theory for gravity waves in intermediate water depths.

Unsteady 2D measurement of dissolved oxygen distribution using luminescent sensor film

Abstract We have proposed a novel utilization of pressure-sensitive paint (PSP) as a luminescent dissolved oxygen (DO) sensor film, which consists of an oxygen sensing dye and polymer matrix, for time resolved DO distribution measurements. The DO measurement is based on the variation of the luminescent intensity due to oxygen quenching; thus, the unsteady DO distribution can be easily obtained from the time series emission images of the sensor film. The effect of the wettability of the sensor films on the DO sensitivity was investigated. It was clarified that the sensitivity is independent of the wettability. The feasibility test of the unsteady DO distribution measurement with the sensor film was demonstrated in the periodic oscillatory flow. The measured frequency agreed well with that measured by fluorescein seeded in the flow, showing the applicability of the DO sensor film to unsteady DO distribution measurement.

Unsteady force measurements in sphere flow from subcritical to supercritical Reynolds numbers

The flow over a smooth sphere is examined in the Reynolds number range of 5.0 × 104 < Re < 5.0 × 105 via measurements of the fluctuating forces and particle image velocimetry measurements in a planar cut of the velocity field. Comprehensive studies of the statistics and spectra of the forces are presented for a range of subcritical and supercritical Reynolds numbers. While the subcritical lateral force spectra are dominated by activity corresponding to the large-scale vortex shedding frequency at a Strouhal number of approximately 0.18, there is no such peak apparent in the supercritical spectra, although resolution effects may become important in this region. Nor does the large-scale vortex shedding appear to have a significant effect on the drag force fluctuations at either sub- or super-critical Reynolds numbers. A simple double spring model is shown to capture the main features of the lateral force spectra. The low-frequency force fluctuations observed in earlier computational studies are shown to have important implications for statistical convergence, and in particular, the apparent mean side force observed in earlier studies. At least one thousand dimensionless time units are required for reasonable estimates of the second and higher moments below the critical Reynolds number and even more for supercritical flow, stringent conditions for computational studies. Lastly, investigation of the relationship between the motion of the instantaneous wake shape, defined via the local position where the streamwise velocity is equal to half the freestream value, and the in-plane lateral force for subcritical flow reveals a significant negative correlation throughout the near wake, which is shown to be related to a structure inferred to arise from the large-scale vortex shedding convecting downstream at 61% of the freestream velocity. In addition to its utility in understanding basic sphere flow, the apparatus is also a testbed that will be used in future studies, examining the effect of both static and dynamic changes to the surface morphology.

Impact of Time-Resolved Entropy Measurement on a One-and-One-Half-Stage Axial Turbine Performance

An accurate assessment of unsteady interactions in turbines is required, so that this may be taken into account in the design of the turbine. This assessment is required since the efficiency of the turbine is directly related to the contribution of unsteady loss mechanisms. This paper presents unsteady entropy measurements in an axial turbine. The measurements are conducted at the rotor exit of a one–and-one-half-stage unshrouded turbine that is representative of a highly loaded, high-pressure stage of an aero-engine. The unsteady entropy measurements are obtained using a novel miniature fast-response probe, which has been developed at ETH Zurich. The entropy probe has two components: a one-sensor fast-response aerodynamic probe and a pair of thin-film gauges. The probe allows the simultaneous measurement of the total temperature and the total pressure from which the time-resolved entropy field can be derived. The measurements of the time-resolved entropy provide a new insight into the unsteady loss mechanisms that are associated with the unsteady interaction between rotor and stator blade rows. A particular attention is paid to the interaction effects of the stator wake interaction, the secondary flow interaction, and the potential field interaction on the unsteady loss generation at the rotor exit. Furthermore, the impact on the turbine design of quantifying the loss in terms of the entropy loss coefficient, rather than the more familiar pressure loss coefficient, is discussed in detail.

A flowmeter for unsteady liquid flow measurements

An unsteady flowmeter was developed for implementation in the Inolivent-4 total liquid ventilator prototype. The proposed design consists of a symmetrical venturi tube comprising three pressure sensors and in which flow measurement is obtained by numerically solving a slightly modified version of the unsteady Bernoulli equation. A prototype was validated experimentally by applying zero-mean sinusoidal flows. Low-frequency characterization determined the venturi discharge coefficient as a function of the Reynolds number, and higher-frequency ( ≤ 4 Hz ) measurements determined the applicable bandwidth of the device. The velocity profiles were measured in the venturi by particle image velocimetry (PIV), and the device was calibrated experimentally by comparison with an ultrasonic flowmeter and measurements from a piston pump. Results showed that quasi-steady flows could be accurately measured in the 5–60 ml/s range, while low-amplitude ( ≤ 10 ml/s ) oscillatory flows were well-measured for frequencies below 3 Hz. Finally, PIV experiments showed that the flat velocity profile assumption required for a simple solution of the flowmeter equation was valid within the operating range.

J051015 航空エンジン用低圧タービン段の高効率化に関する研究 : 回転試験機による実験及びCFD([J05101]流体機械の研究開発におけるEFD/CFD(1))

This paper presents an experimental study and numerical simulation of the unsteady flow mechanisms in a single stage axial flow turbine which is simulated the LP turbine airfoils. The target of this study is to verify the influence of the purge flow from the Rim-seal cavity on the flow field in the rotor passage, which leads to changes of the secondary flow behavior especially in the rotor hub region. To observe the unsteady flow pattern at rotor downstream, detailed 3-D unsteady measurements using a slant-type hot wire probe is performed. The result of unsteady measurement observed the secondary flow and the leakage flow in the tip region without the purge flow as well as the steady flow measurement with 5-hole probe. A three-dimensional, time-dependent numerical simulation shows the unsteady flow interactions between the main flow and purge flow.

Multi-target identification for emission parameters of building materials by unsteady concentration measurement in airtight micro-cell-type chamber

The purpose of this study is to develop a concurrent determination method that can estimate multiple emission parameters, that is, the emission rate, initial concentration and effective diffusion coefficient Dc in building materials, by a single unsteady concentration measurement. This study focused on the time history of VOC concentration in the gas phase that occurred when the target building material was covered with an airtight micro-cell. The VOC concentration in the micro-cell gradually increased and finally reached an equilibrium concentration. Under the condition of uniform distribution of initial concentration, the profile of VOC concentration in the micro-cell was determined by the order of the Dc value. A chart of the time history of VOC concentration as a function of Dc and thickness of building materials was prepared in advance by numerical analysis and then Dc was estimated by overlapping the measurement result with this chart. A chart of emission rate as a function of Dc and building material thickness was also prepared and the determination procedure of the emission rate taking into account the consistency between the 20 L small chamber method with in- and out-flow and the micro-cell method under an airtight condition was proposed. The estimation results of Dc and emission rate by this method were reasonably consistent with the results of the conventional method.

Characterisation of Interaction between Combustion Dynamics and Equivalence Ratio Oscillations in a Pressurised Combustor

In regular operation, all gas turbine combustors have a significant spontaneous noise level induced by the turbulent high power flame. This noise is characteristic for the operation as it is the result of the interaction between turbulence and combustion. Pressure fluctuations may also be generated by thermoacoustic instabilities induced by amplification by the flame of the acoustic field in the combustor. This paper focuses on the characterisation of the latter process, the combustion dynamics, in a pressurized premixed natural gas combustor. In order to predict the thermo-acoustically unstable operating ranges of modern gas-turbines with the use of an acoustic network model, it is essential to determine accurately the flame transfer function. This transfer function gives the relationship between a perturbation upstream of the flame and its combustion response, leading to acoustic forcing. In this paper, the flame transfer function is obtained by experimental means in a combustor test rig. This test rig was built in the framework of the European DESIRE project, and has the ability to perform thermo-acoustic measurements up to an absolute pressure of 5 bars. The maximum power of the setup is 500 kW. The paper presents a method to determine the flame transfer function by factorizing it in six subfunctions. Systematically these subfunctions are determined. With the method presented, acoustic measurements on the steady, unperturbed flame and on the unsteady, actively perturbed flame are performed. The effect of pressure is investigated. The steady measurements are used to provide an acousto-combustion finger print of the combustor. In the unsteady measurements, the flame transfer function is reconstructed from the measured acoustic pressures. These flame transfer functions are compared to transfer functions obtained from a numerical experiment in CFD. Good agreement is obtained.

Experimental investigation of draft tube flow instability

The draft tube of a hydraulic turbine is the component where the flow exiting the runner is decelerated, thereby converting the excess of kinetic energy into static pressure. However, in some refurbishment cases, the installation of an upgraded runner with an old draft-tube leads to an undesirable efficiency drop as the discharge is increased above the best efficiency point value. It is found to be related to a corresponding sudden variation in the draft tube pressure recovery coefficient at the same discharge. The model of a recent refurbishment which presents this instability is installed in the CREMHyG test rig. Steady and unsteady measurements of velocity and pressure fields of the complex and highly turbulent swirling flow exiting the runner have been carried out at CREMHyG (Grenoble). They included the three components of the velocity. The flow in the draft tube is rich in secondary flows and possible flow separation due to the elbow as well as to the divergent shape of this geometry. Pressure and velocity field measurements in the draft-tube have been performed. They let us identify some phenomena that may explain the origin of the unusual behavior of the draft tube. Finally, the same model, with a modified draft tube geometry that was designed to reduce the efficiency drop phenomenon has been installed. Velocity measurements were made to validate the benefit of such geometry modification.

Optimization of Anodized-Aluminum Pressure-Sensitive Paint by Controlling Luminophore Concentration

Anodized-aluminum pressure-sensitive paint (AA-PSP) has been used as a global pressure sensor for unsteady flow measurements. We use a dipping deposition method to apply a luminophore on a porous anodized-aluminum surface, controlling the luminophore concentration of the dipping method to optimize AA-PSP characteristics. The concentration is varied from 0.001 to 10 mM. Characterizations include the pressure sensitivity, the temperature dependency, and the signal level. The pressure sensitivity shows around 60 % at a lower concentration up to 0.1 mM. Above this concentration, the sensitivity reduces to a half. The temperature dependency becomes more than a half by setting the luminophore concentration from 0.001 to 10 mM. There is 3.6-fold change in the signal level by varying the concentration. To discuss an optimum concentration, a weight coefficient is introduced. We can arbitrarily change the coefficients to create an optimized AA-PSP for our sensing purposes.

Unsteady wake measurements behind an airfoil and prediction of dynamic stall from the wake

PurposeThe primary purpose of this paper is to investigate the characteristics of the unsteady flow field in the wake of Eppler‐361 airfoil undergoing harmonic pitch oscillation in both pre‐stall and post‐stall regimes.Design/methodology/approachExperimental measurements were carried out to study the characteristics of the unsteady flow field within the wake of an airfoil. All of the experiments were conducted in a low‐speed wind tunnel, and the velocity field was measured by a hot‐wire anemometry. The airfoil was given a harmonic pitching motion about its half chord axis at two reduced frequencies of 0.091 and 0.273. All experimental data were taken at the oscillation amplitude of 8°. During the experiments, the mean angle of attack was altered from 2.5 to 10° that this made it possible to study the wake in both pre‐stall and post‐stall regimes.FindingsFrom the results, it can be concluded that different velocity profiles are formed in the wake at different phase angles. In addition, the hysteresis of the velocity field in the wake is captured between increasing and decreasing incidences. It is also found that the velocity field in the wake is strongly affected by the operating conditions of the airfoil, e.g. mean angle of attack, reduced frequency and instantaneous angle of attack. Huge variations in the profiles of the wake are observed at high instantaneous angles of attack when the mean angle of attack is 10°, i.e. when the airfoil experiences significant oscillations beyond the static stall. It is concluded that this is due to dynamic stall phenomenon.Practical implicationsFindings of the present study give valuable information, which can be used to characterize wakes of micro air vehicles, helicopter's rotor blades, and wind turbine blades. In addition to this, present findings can be used to predict dynamic stall of the above applications.Originality/valueThe paper is the first to investigate the unsteady wake of Eppler‐361 airfoil and to predict the dynamic stall phenomenon of this airfoil.

Invited Review Article: Unsteady and pulsating pressure and temperature: A review of experimental techniques

Unsteady flow of liquids and gasses are important in many scientific, engineering, and biological contexts. Measurement of the characteristics of unsteady and pulsating flows is more difficult than that of static flows. Time constants of the sensors must be understood, sampling rates must satisfy basic signal processing criteria and synchronization of the measurements with the flow may be necessary. Because of development of more advanced measurement devices, there has recently been a growing interest in unsteady and pulsating flow measurements and the number of papers in this field has increased in recent years. This paper reviews the current state of the art in sensors and measurement techniques for the characterization of pressure and temperature in unsteady and pulsating flows including an analysis of the advantages and limitations of each technique.

Experimental and numerical study of the sweep effect on three-dimensional flow downstream of axial flow fans

The purpose of this work is to study the influence of the axial flow fan sweep on the downstream turbulent flow. The fans studied are three low-pressure and low-Mach-number axial flow fans, with respectively a radial, a forward and a backward sweep. Experimental and computational fluid dynamics (CFD) investigations are carried out on three fans, and the results are compared. The CFD method is a three-dimensional (3D) Reynolds average Navier–Stokes (RANS) numerical simulation with the Reynolds stress model (RSM) as the turbulence model. It allows us to compute the Reynolds stress tensor components. Unsteady velocity measurements are carried out downstream of the fans with hot-wire anemometry. The values of the three velocity components of the flow and the six components of the Reynolds stress tensor obtained from experiments and simulations are compared. Overall performances are also measured to validate the design and fan simulation. It appears that a forward sweep decreases the radial component of the velocity whereas a backward sweep increases this component. Moreover, the sweep has a significant influence on the turbulent kinetic energy downstream of the fan.

Effects of Suction and Injection Purge-Flow on the Secondary Flow Structures of a High-Work Turbine

In high-pressure turbines, a small amount of air is ejected at the hub rim seal to cool and prevent the ingestion of hot gases into the cavity between the stator and the disk. This paper presents an experimental study of the flow mechanisms that are associated with injection through the hub rim seal at the rotor inlet. Two different injection rates are investigated: nominal sucking of −0.14% of the main massflow and nominal blowing of 0.9%. This investigation is executed on a one-and-1/2-stage axial turbine. The results shown here come from unsteady and steady measurements, which have been acquired upstream and downstream of the rotor. The paper gives a detailed analysis of the changing secondary flow field, as well as unsteady interactions associated with the injection. The injection of fluid causes a very different and generally more unsteady flow field at the rotor exit near the hub. The injection causes the turbine efficiency to deteriorate by about 0.6%.

1503 水中溶存酸素濃度計測のための感圧塗料の開発(OS15-1 先端的熱流体計測法,オーガナイズドセッション)

It is important to understand the behavior of oxygen concentration distribution inside micro-channels for the improvement of micro devices or fuel cells. Most of oxygen sensors such as galvanic or polaro-graphic type cannot be used for unsteady measurements in a small system because of their long response time and large probe heads. We focused on pressure sensitive paint (PSP) for the measurement of the oxygen concentration dissolved in the water in a small system. Up to the present, the PSP techniques have been mainly applied to gas-flows. However, the PSP technique is based on the oxygen quenching of luminescent molecules and would be used in both gas and liquid-flows. In this paper, we investigated the oxygen sensitivity of three types of PSP in the water, and selected a suitable polymer for a binder of PSP.

Three Dimensional Clocking Effects in a One and a Half Stage Transonic Turbine

This paper presents an experimental investigation of the flow field in a high-pressure transonic turbine with a downstream vane row (1.5 stage machine) concerning the airfoil indexing. The objective is a detailed analysis of the three-dimensional aerodynamics of the second vane for different clocking positions. To give an overview of the time-averaged flow field, five-hole probe measurements were performed upstream and downstream of the second stator. Furthermore in these planes additional unsteady measurements were carried out with laser Doppler velocimetry in order to record rotor phase-resolved velocity, flow angle, and turbulence distributions at two different clocking positions. In the planes upstream of the second vane, the time-resolved pressure field has been measured by means of a fast response aerodynamic pressure probe. This paper shows that the secondary flows of the second vane are significantly modified by the different clocking positions, in connection with the first vane modulation of the rotor secondary flows. An analysis of the performance of the second vane is also carried out, and a 0.6% variation in the second vane loss coefficient has been recorded among the different clocking positions.