Fast-response Pressure-sensitive Paint Research Articles

Application of pressure-sensitive paint for explosive blast measurements

This study demonstrates the application of fast response pressure-sensitive paint (PSP) to explosively driven blast wave testing. A sprayable polymer ceramic fast response PSP was applied to an aluminium disc before being coated with platinum porphyrin compound as the active luminophore. The disc was then exposed to a blast wave and the response was measured using a high-speed video camera. The PSP measured the transit of the incident shock wave clearly, albeit with a slight response delay following the instantaneous change in pressure. A time domain-based method for improving temporal response, whilst considering both spatial and temporal effects, is described. This study clearly demonstrates that the spatial distribution of a blast wave on a surface may be captured by PSP technology. Integrated parameters such as impulse can correctly be characterised using this method. This technology offers an enhanced and more efficient way of characterising blast.

Application of fast-response pressure sensitive paint to low-speed vortical flow at high angles of attack

Application of fast-response pressure sensitive paint to low-speed vortical flow at high angles of attack

Study of Unsteady Surface Flowfields on and Around Turrets with Different Protrusions

Wind-tunnel experiments were conducted to measure the unsteady surface pressure field on and around a hemisphere-on-cylinder turret of varying protrusion in subsonic flow. These measurements were obtained using fast-response pressure-sensitive paint coupled with pressure transducers. The surface pressure field data resulting from Mach 0.5 flow () over a partially submerged hemisphere, a hemisphere, and a hemisphere on a cylinder were analyzed using proper orthogonal decomposition modal analysis, as well as a variant of this approach referred to as joint proper orthogonal decomposition. The results showed that decreased turret protrusion into the freestream flow reduced the prevalence of antisymmetric surface pressure field fluctuations caused by antisymmetrical vortex shedding. The frequency associated with this fluctuation was found to be around . When a partial hemispherical turret geometry was used, it was shown that the antisymmetric mode was greatly suppressed; and the wake was dominated by a symmetric mode with a broadband spectral peak at a higher frequency of . This suggests that there is a “mode switching” as the protrusion is changed from the hemisphere to the partial hemisphere configuration. An optical flow approach was used to find the convective velocity field in the wake, from which topological flow features could be identified. The size of the wake separation region was found to grow smaller with the decreased protrusion while keeping a similar shape.

Vibration of a thin panel exposed to ramp-induced shock-boundary layer interaction at Mach 2

Flight vehicles that operate in the supersonic regime can be subject to adverse fluid–structure interactions due to their lightweight design. The presence of geometric obstructions such as control surfaces or fins can induce shocks that can interact with the boundary layer, leading to flow separation. This work investigates experimentally the interaction between a compliant panel in a Mach 2 flow under a compression ramp-induced shock-wave/boundary-layer interaction (SBLI). Thin brass panels of different thickness are investigated in a wind tunnel. Tests are performed both with and without a 20° compression ramp installed. This direct comparison allows characterization of the effect of the SBLI on the system dynamics. High-speed stereoscopic digital image correlation (DIC) and fast-response pressure sensitive paint (PSP) are used to obtain simultaneous measurements of full field deformation and surface pressure of the panels. The panel vibration is dominated by the first bending mode. Despite the forcing of the separation shock foot, the presence of the SBLI does not significantly modify the operational deflection shape, frequency, and amplitude of the dominant vibration mode, beyond what is observed for the no-SBLI case. On the other hand, analysis of the shock foot motion shows that the shock primarily oscillates at the first natural frequency of the panel. This leads to the conclusion that the shock foot oscillation is driven by the panel vibration in a one-way coupling mechanism. The SBLI does modify the higher modes, which is likely due to localized forcing by the separation shock foot. Full-field surface pressure predictions are made using first order piston theory. Results show that the fluid–structure interaction is dominated by the large region of attached flow upstream of the shock foot.

Investigations of shock–boundary layer interaction dynamics using high-bandwidth pressure field imaging

The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.

Evaluation of characteristics of fast-response pressure-sensitive paint under low-pressure conditions

In this study, fast-response polymer-ceramic pressure-sensitive paints (PC-PSP) were developed and evaluated for pressure measurement under low-pressure conditions. The PC-PSP using poly[1-trimethylsilyl)-1-propyne] (poly(TMSP)), which has high oxygen permeability under low-pressure conditions, was developed in this study. The static and dynamic characteristics of the developed poly(TMSP)-based PC-PSP were evaluated in comparison with those of conventional poly(isobutyl methacrylate) (poly(IBM)) binder and ruthenium-complex-based PC-PSPs, which have been used for pressure measurements under atmospheric pressure conditions. The particle mass content of titanium dioxide of PC-PSPs with poly(TMSP) was changed from 90 wt% to 98 wt% to increase the frequency response. The critical pigment volume concentration, so called CPVC, of the PC-PSP with poly(TMSP) and hydrophobic particles and hydrophilic particles were 95–98 wt% and 90–95 wt%, respectively. The PC-PSP using hydrophilic particles with poly(TMSP) and a particle mass content of 98 wt% could provide a cut-off frequency of approximately 4.5 kHz and a high local Stern–Volmer coefficient of 0.5 at low pressure of 2 kPa.

Pressure Sensitivity Prediction and Pressure Measurement of Fast Response Pressure-Sensitive Paint Based on Artificial Neural Network

The characterization of pressure-sensitive paint (PSP) is affected by many physical and chemical factors, making it is difficult to analyze the relationship between characterization and influencing factors. An artificial neural network (ANN)-based method for predicting pressure sensitivity using paint thickness and roughness was proposed in this paper. The mean absolute percentage error (MAPE) for predicting pressure sensitivity is 6.5088%. The difference of paint thickness and roughness between sample and model surface may be a source of experimental error in PSP pressure measurement tests. The Stern-Volmer coefficients A and B are strongly linked. Pressure sensitivity is approximately equal to coefficient B, so coefficient A is predicted using pressure sensitivity based on the same ANN, and the MAPE of predicting A is 2.1315%. Then, we try to calculate the pressure by using the thickness and roughness on a model to predict pressure sensitivity and Stern-Volmer coefficient A. The PSP pressure measurement test was carried out at the China Aerodynamic Research and Development Center. Using the Stern-Volmer coefficient calculated by the in situ method, the method in this paper, and the sample calibration experiment, the root mean square errors (RMSE) of the pressure are 47.4431 Pa, 63.4736 Pa, and 73.0223 Pa, respectively.

A platinum-based fast-response pressure-sensitive paint containing hydrophobic titanium dioxide

A platinum-based fast-response pressure-sensitive paint containing hydrophobic titanium dioxide

Uncertainty and validation of unsteady pressure-sensitive paint measurements of acoustic fields under aero engine-like conditions

Fast response pressure-sensitive paint (PSP) allows optical measurements of pressure fluctuations on a surface with high spatial and temporal resolution. This technique is evaluated for aeroacoustic measurements inside an aeroacoustic wind tunnel (AWT). The AWT is a test rig especially designed for investigating the excitation and propagation of sound under conditions typical for turbomachinery. The aim of this work is to compare the results of sound pressure measurements of tonal sound fields in a circular duct conducted with PSP and microphone arrays in order to assess the applicability of PSP in turbomachinery acoustics applications. A data analysis process is presented, which projects the camera image of the PSP data onto a given surface. To analyze the spatial pressure fluctuations, the PSP data are transformed in the frequency domain using pixel-wise fast Fourier transform. Measurements with a mean Mach number up to 0.109 and 5 kHz excitation frequency are conducted. An acoustic mode generator is used to excite the sound field with specific circumferential mode order. The pressure fluctuations obtained with the PSP measurement visualize the measured acoustic field well and allow early interpretation. The pressures of PSP and microphones are in good agreement; for example, the maximum detected deviation in pressure at 2700 Hz is 30 Pa. A preview on using radial mode analysis to decompose the acoustic field, measured by PSP, into acoustic modes is provided. The results are confirmed by a decomposition using conventional arrays of flush-mounted microphones.Graphical

Effects of hole-inlet velocity on the adiabatic film cooling effectiveness behind crossflow-fed shaped holes

In this contribution, the effect of hole-inlet velocity ratio (VRin) on adiabatic film cooling effectiveness behind crossflow-fed shaped holes is quantified using fast-response pressure-sensitive paint (fast-PSP). The measurement was performed at VRin = 0.38 and VRin = 0.5. For VRin = 0.38, the coolant and channel velocities (VRc &VRch, respectively) were varied as either VRc = 0.8 &VRch=0.3 or VRc = 1.6 &VRch=0.6, while VRin = 0.5 varies as either VRc = 0.8 &VRch=0.4 or VRc = 1.2 &VRch=0.6. The spatial–temporal features of the measured effectiveness were uncovered by the mean and unsteady data. The result affirmed the performance deterioration, which was associated with the asymmetric spreading behind the holes. The channel velocity ratio contributed to the coolant biasing, while the coolant velocity ratio contributed to the coolant spreading. The effectiveness (time-average and statistical) was correlated with identical VRin regardless of VRc &VRch; revealing the similarity and distinction from a qualitative and quantitative point of view, respectively. Based on the analysis, the obtained features indicate the coolant promotion at lower flow conditions (VRc &VRch) for both VRin = 0.38 and VRin = 0.50, although it's higher unsteadiness. Based on SOM, this could enhance the coolant coverage by the fluctuations than the local overheating by the mainstream gases. This research could assist designers to understand the unsteady features, as well as promote new cooling solutions for improved gas turbine protection in the future.

Effects of Blowing and Density Ratios on Unsteady Film Cooling Effectiveness Using Fast-Response Pressure-Sensitive Paint

Abstract The transient effect of unsteady wakes on film cooling effectiveness using a fast response pressure sensitive paint (PSP) was studied. Testing was performed in a five-passage, low-speed wind tunnel using a scaled-up high pressure, blade profile. The blowing ratios studied include 0.4, 0.8, and 1.2 with a density ratio of 1.0 and 1.5 at each blowing ratio. An upstream wake generator produced Strouhal numbers of 0.1 and 0.3 while the wind tunnel inlet mainstream Reynolds number was held at a constant value of 300,000 for every test case. The results in this paper confirm the overall time averaged reduction in film cooling effectiveness with the addition of an unsteady wake while providing more detailed information on the transient response of the film cooling effectiveness, presented in the form of film cooling effectiveness RMS values. Results show that film cooling effectiveness RMS values increase on the leading-edge region of the blade and show little change on the pressure side of the blade with an increase in the Strouhal number. On the suction side, the area-averaged effectiveness RMS values decrease for the 0.8 and 1.2 blowing ratios and remain unchanged for the 0.4 blowing ratio. A change in the density ratio of the coolant from 1.0 to 1.5 has a minimal effect on area-averaged effectiveness RMS values for all areas of the blade.

Research on global calibration technology of pressure sensitive paint based on airtight chamber

Pressure sensitive paint (PSP) technology has been widely applied in wind-tunnel test. Generally, PSP calibration data are obtained using a calibration sample in laboratory priori calibration device, which the sample is sprayed at the same time with the test model.In this study, taking the real test model as the calibration object, a global calibration technology of PSP coating on the surface of the test model is proposed, which is verified in the airtight chamber, and the effective PSP calibration data are obtained. A two-component fast response PSP coating was used in the study. In the validation test, a flying wing model is used, the flying wing model sprayed with PSP coating was placed in the airtight chamber. 11 groups of calibration tests were carried out in the environment of 0.06 MPa ~ 0.14 MPa and 30°C ~50°C, and the calibration curves obtained showed good consistency.

Swirling main flow effects on film cooling: Time resolved adiabatic effectiveness measurements in a gas turbine combustor model

Lean burn swirl stabilized combustors represent the key technology to reduce NOx emissions in modern aircraft engines. The high amount of air admitted directly trough the injection system and the higher levels of pressure and temperature requested by the engine cycle make the design of even more efficient cooling systems and the correct estimation of combustor liners heat load a crucial task for the combustor durability. One of the technologies used for the cooling of these components is the effusion cooling, which promotes the formation of a cold layer to protect the liner walls, whose cooling capabilities are described with the adiabatic effectiveness parameter. The effectiveness of these systems is severely influenced by the swirling flows generated by the swirler injectors which promote the mixing between cold and the hot gases of the combustion process. The development of the swirling flow structures inside the combustor chamber and the grade of interaction between swirling flow and the liner walls can be described by the swirl number (SN). In order to investigate the impact of the effects of the SN parameter on the adiabatic film effectiveness, a dedicated non-reactive single-sector linear combustor test rig was designed and equipped with three different axial injectors (SN=0.6−0.8−1.0) and a cylindrical holes effusion plate to simulate the liner cooling system. Film effectiveness was acquired using Fast response Pressure Sensitive Paint (FPSP) technique; the scale of the model and the acquisition frequency of 1 kHz allowed to track the effusion jets and main flow interactions. The collected results show the importance of using an unsteady analysis to perform an in-depth characterization of the mixing phenomena between the main flow and the coolant which are significantly affected by the swilring characteristics.

Unsteady adiabatic film cooling effectiveness behind shaped holes

Gas turbine as a vital component requires higher inlet temperature to improve its thermal efficiency, and thus a thin coolant film spreads over the blade surface to isolate it from hot gases. However, such coolant intensively interacts with the mainstream, leading to highly unsteady coolant coverage, which damages structural integrity, challenging its reliable and sustainable operations. Consequently, fast-response pressure-sensitive paint technique (fast-PSP) was used to measure the coolant unsteadiness behind the shaped holes, considering plenum and crossflow feeds. A steady-RANS simulation was performed to predict the large-scale flow structures and explain the mean results. The measured standard deviation (SD) and fluctuating components were used to uncover the spatial-temporal features associated with the predicted vortical structures. The mean effectiveness was dramatically influenced by the blowing ratios (M), showing attached flow at a low M and lift-off at a high M. The internal crossflow showed asymmetric spreading, resulting in deteriorated performance behind the holes. The unsteadiness was highly influenced by the energetic vortical structures, which originated from the hole entrance, forming in-hole counter-rotating vortex pair (CRVP) and vortex-tube structures. Meanwhile, the vortex-tube formed a swirl-vortex signature downstream of the hole-trailing-edge, leading to asymmetric CRVP. The swirl-vortex interacted with the CRVP windward-leg and reduced the spreading behind the holes, leading to mainstream ingestions. The combined results suggested considering the internal flow effects, which could help the designers understand the characteristics of unsteady effectiveness; promoting advanced cooling strategies for enhanced protection of future gas turbines.

Effects of crossflow-fed-shaped holes on the adiabatic film cooling effectiveness

The effects of internal crossflow-fed-shaped holes on the unsteady behavior of the adiabatic film cooling effectiveness were extensively investigated using high-resolution fast-response pressure-sensitive paint (fast-PSP). During the experiment, coolant (DR ≈ 1.0) was fed into either plenum chamber or crossflow channel at coolant to mainstream velocity ratio of VRc = 0.4-1.6, channel to mainstream velocity ratio of VRch = 0.4-0.7, and channel to coolant (hole-inlet) velocity ratio of VRin = 0.25-1.75. The mean and unsteady effectiveness were presented in the near-hole region (9D), where the significance of the internal crossflow was captured. The results affirmed the asymmetrical coolant deterioration behind the crossflow-fed-shaped holes compared with the plenum feed. The unsteadiness was originated from the hole entrance (i.e., in-hole counter-rotating vortex-pair (CRVP) and the in-hole vortex-tube structures for plenum and crossflow, respectively). The vortex-tube formed a swirl-vortex, which interacted with the CRVP at the windward side, leading to asymmetric CRVP and declined effectiveness. Meanwhile, the coolant spreading behind the crossflow-fed-shaped holes was sensitive to the hole-inlet velocity (VRin), showing symmetric spreading at VRin ≈ 1.0, biased to the windward at VRin < 1.0, and biased to the leeward when VRin > 1.0. The combined results insight our understanding into the coolant unsteadiness of the shaped-holes in-flow conditions, recommends modifying the hole-inlet shape in order to enhance the effectiveness and damp the corresponding unsteadiness.

Effect of Shock Wave Behavior on Unsteady Aerodynamic Characteristics of Oscillating Transonic Compressor Cascade

Abstract The unsteady behavior of the shock wave was studied in an oscillating transonic compressor cascade. The experimental measurement and corresponding numerical simulation were conducted on the cascade with different shock patterns based on influence coefficient method. The unsteady pressure distribution on blade surface was measured with fast-response pressure-sensitive paint (PSP) to capture the unsteady aerodynamic force as well as the shock wave movement. It was found that the movement of shock waves in the neighboring flow passages of the oscillating blade was almost antiphase between the two shock patterns, namely, the double shocks pattern and the merged shock pattern. It was also found that the amplitude of the unsteady pressure caused by the passage shock wave was very large under the merged shock pattern compared with the double shocks pattern. The stability of blade vibration was also analyzed for both shock patterns including three-dimensional flow effect. These findings were thought to shed light on the fundamental understanding of the unsteady aerodynamic characteristics of oscillating cascade caused by the shock wave behavior.

DEVELOPMENT OF A LIFETIME PRESSURE SENSITIVE PAINT PROCEDURE FOR HIGH-PRESSURE VANE TESTING

Abstract Optical measurements based on fast response Pressure Sensitive Paint (PSP) provide enhanced spatial resolution of the pressure field. This paper presents lifetime PSP at 20 kHz, with precise calibrations, and results from a demonstration in an annular vane cascade. The laser lifetime PSP methodology is first evaluated in a linear wind tunnel with a converging-diverging nozzle followed by a wavy surface. This test section is fully optically accessible with maximum modularity. A data reduction procedure is proposed for the PSP calibration, and optimal pixel binning is selected to reduce the uncertainty. In the annular test section, laser lifetime PSP was used to measure the time-averaged static pressure field on a section of the suction surface of a high-pressure turbine vane. Tests were performed at engine representative conditions in the Purdue Big Rig for Annular Stationary Turbine Analysis module at the Purdue Experimental Turbine Aerothermal Lab. The 2-D pressure results showed a gradual increase of pressure across the spanwise and flow directions, corroborated with local static pressure taps and computational results. The variation in PSP thickness was measured as a contribution to the uncertainty. The discrete Fourier transform of the unsteady pressure signal showed increased frequency content in wind-on compared to wind-off conditions at the mid-span and 30% span. Compared to the mid-span, the hub end wall region had an increase in frequencies and pressure amplitude. This result was anticipated given the expected presence of secondary flow structures at the near hub region.

On the unsteady behaviours of the adiabatic endwall film cooling effectiveness

The film cooling technique is introduced in modern gas turbines to protect the blade from the high temperature of the incoming hot gases by forming a thin coolant blank over the blade surface. However, it is known as a jet in crossflow (JICF), where coolant and mainstream interact intensively and generate complex vortices leading to highly unsteady coolant coverage over the blades surface. In this study, a fast-response pressure-sensitive paint technique (fast-PSP) was used to measure the coolant unsteadiness with a high-resolution camera. The measurements were performed in a novel single-passage transonic wind tunnel to uncover the unsteady effectiveness of the endwall surface. Such effectiveness was dramatically influenced by the blowing ratios (M), showing attached flow at a low blowing ratio and lift-off at a high blowing ratio. The effectiveness was asymmetrically distributed due to the pressure gradients, jet compounding angle, and associated complex flows. The unsteady effectiveness was highly influenced by the energetic vortical structures, which interacted with the mainstream flow immediately behind the holes. It was featured by secondary structures (horseshoe, passage, and counter vortices) beside the JICF structures. Meanwhile, the unsteadiness was originated from the middle of the passage behind the holes. It is suggested to pay close attention to the locations of the holes for further optimization. This study could help the designers to understand the characteristics of unsteady effectiveness, promoting advanced cooling strategies for enhanced protection of future gas turbines.

Time-Accurate Evaluation of Film Cooling Jet Characteristics for Plenum and Crossflow Coolant Supplies

Abstract Gas turbine film cooling creates complicated and highly unsteady flow structures. This study seeks to examine the unsteady characteristics created by different cooling hole inlet geometries using a fast-response pressure-sensitive paint (PSP) technique able to capture time-accurate measurements at 2000 frames per second, resolving frequencies up to 1000 Hz. Time-accurate and time-averaged measurements are used to evaluate the performance of a plenum-style inlet and a crossflow-style inlet in varying turbulence environments over a flat plate. The results of this study are intended to begin the process of breaking down widely accepted time-averaged film effectiveness trends into the cumulative effects of freestream turbulence and momentum flux ratio, with emphasis on the cooling jet motions they cause. Jet behaviors observed in this study include a sweeping oscillation, unsteady attachment and separation from the plate, and time-accurate and time-averaged flow bias. Cooling hole inlet geometry and momentum flux ratio affect the core of the jet, and freestream turbulence affects the periphery of the jet. Crossflow-fed cooling holes show bias to the upstream side of the cooling hole with respect to the internal crossflow direction. Plenum-fed cooling holes outperform crossflow-fed cooling holes, and the difference grows with increasing momentum flux ratio. The frequency of both plenum- and crossflow-fed cooling jet motions is influenced by the freestream turbulent velocity fluctuations. The resulting mode shapes show dominant side-to-side sweeping for higher turbulence environments and a separation and reattachment motion for lower turbulence environments. At higher momentum flux ratio, the jets were seen to increasingly favor separation and reattachment motions. The results of this study are intended to better inform existing predictive tools. With a better understanding of the time-accurate behaviors responsible for creating the commonly accepted time-averaged coolant distributions, simple predictive tools may be better equipped to accurately model film cooling flows.

Experimental Study of Oscillating Freestream Effect on the Spatiotemporal Distributions of Leading-Edge Film Cooling

Abstract An experimental study was conducted to investigate the influence of mainstream oscillations on spatio-temporal variation of leading-edge film cooling effectiveness. The investigation utilized fast-response pressure-sensitive paint (Fast-PSP) technique at high frame rate. During the experiment, coolant (i.e., CO2, DR = 1.53) was discharged into three rows of cylindrical holes. Various blowing ratios (i.e., M = 0.50, 0.75, 1.00, and 1.50) were tested under the steady (i.e., f = 0 Hz) and oscillating (i.e., f = 7 Hz and 25 Hz) conditions. The measured instantaneous effectiveness was analyzed in terms of time-averaged and phase-averaged results. The results revealed that the mainstream oscillation, consisting of simultaneous pressure and velocity oscillation, significantly influences the behavior of the film cooling effectiveness. The time-averaged effectiveness significantly decreased at high oscillating frequency (i.e., 13.0–19.8% reduction at M = 0.50, f = 25 Hz compared with f = 0 Hz), especially at low blowing ratios (i.e., M = 0.50 and 0.75). The phase-averaged results captured significant decay in the film distributions associated with backflow caused by negative pressure gradients in coolant holes at certain phases. However, the mainstream oscillation effect was relatively insignificant at high blowing ratios (i.e., M = 1.00 and 1.50), which revealed the robustness of coolant coverage at low coolant Strouhal number (i.e., high blowing ratio) under the same oscillating frequency. Furthermore, the unsteady coolant intermittency showed highly unstable film coverage at high coolant Strouhal number. The coolant decay associated with backflow at high coolant Strouhal number should be considered by the gas-turbine designers in order to improve the lifecycle of turbine blades.