Di Peng

Measurement Techniques for Shock Movement Capture on a NACA 0012 in Unsteady Compressible Flow

The Ohio State 6” × 22” Unsteady Transonic Wind Tunnel was used to study different methods of experimentally capturing the time-varying location of a shock wave on a NACA 0012 airfoil. The unsteady wind tunnel enables a freestream Mach number oscillation of 0.51 ± 0.1 for a Reynolds number range of 17 – 43 million per meter. Fast-response pressuresensitive paint (PSP) and particle image velocimetry (PIV) were evaluated for their ability to capture time-accurate location of the oscillating shock wave. Unsteady surface pressure and temperature measurements were made using a fast-response bi-luminophore pressuresensitive paint with a single-shot intensity-based technique. This method allows for high spatial resolution surface pressure and temperature maps. Phase-locked planar particle image velocimetry data were also acquired in this experiment, demonstrating an ability to resolve a moving shock. These advanced diagnostic tools enable detailed understanding of the impact of time-varying compressibility both on and off the airfoil surface. The information obtained with each of these techniques is compared in terms of spatial resolution and accuracy with data acquired via surface pressure taps. A NACA 0012 airfoil was tested at 9, 10 and 11 degrees angle of attack and at low reduced frequencies.

A correction method of thermal radiation errors for high-temperature measurement using thermographic phosphors

In this study, a correction method based on background subtraction to remove thermal radiation from luminescent signal was proposed for intensity-based thermometry using thermographic phosphors. Mg4FGeO6:Mn was used as sensor material for a temperature range from 490 to 590xa0°C. A UV LED was used as excitation source and a spectrometer was used to collect the spectrum of both luminescence and thermal radiation for analysis. Then a CCD camera was used to conduct phosphor thermometry, including calibration and a jet impingement experiment. The calibration results showed that the measurement without correction would fail above 540xa0°C due to thermal radiation, whereas the correction method could effectively remove the effect of thermal radiation and extend the temperature range up to at least 590xa0°C. The use of band-pass filter around emission wavelength could reduce the effect of thermal radiation, but correction is still required for the case of higher temperature or lower excitation light. This correction method was then demonstrated in both steady and transient temperature measurements of a jet impingement experiment.Graphical abstract

Unsteady Pressure-Sensitive Paint Measurements on an Articulated Model Helicopter in Forward Flight

Unsteady pressure-sensitive paint (PSP) measurements are acquired on a two-bladed, articulated model helicopter rotor of 0.26-m diameter in edgewise flow to simulate forward flight conditions. The rotor operated at advance ratios of 0.15 and 0.3, while collective and cyclic pitch were set to 10o and 2.5o, respectively. A double-exposure, lifetime-based technique captured the paint luminescence after a single pulse of high-energy laser excitation, yielding a sufficient signal-to-noise ratio to avoid image averaging. In this way, transient pressure information on the blade can be measured to monitor cycle-to-cycle variations at a given azimuth position due to flow unsteadiness. The selection of porous polymer/ceramic PSP afforded high frequency response and pressure sensitivity, but the long lifetime of this paint caused blurring in the second exposure gate and the porous binder induces a large degree of temperature sensitivity. A recently-developed motion deblurring algorithm was implemented, and the temperature correction was made using temperaturesensitive paint (TSP) measurements. Surface pressure maps for the advancing and retreating blades captured a qualitatively different aerodynamic response between the two, including the signatures of tip relief, stall, and reversed flow.

Experimental Study of Vortex Dynamics during Blade-Vortex Interactions

This paper describes the first-phase work of a comprehensive study on vortex dynamics during parallel blade-vortex interaction (BVI). Study of the vortex behavior sheds light on the fundamental interaction mechanism of BVI, and provides important validation data for computational investigations. Evidence has been found in previous studies that the vortex behavior during BVI varies with Reynolds number, but the effects are not clear. In the current study, the relation between Reynolds number and vortex behavior was investigated extensively. The influence of other factors on vortex behavior, such as vortex strength, is also discussed. A pneumatic vortex generation system has been built, which is capable of producing vortices with good repeatability in location, size and strength. The experiment is performed in a low speed wind tunnel usinga planar particle image velocimetry (PIV) technique to characterize the vortex and record BVI incidence. Results indicate that the vortex behavior during BVI strongly depends on the separation distance (h/c) between the vortex and the airfoil. A decrease in vortex strength during BVI is generally observed for h/c< 0.3. The PIV results also reveal the relation between Reynolds number and the vortex strength variation during the interaction.

Quantitative stress measurement of elastic deformation using mechanoluminescent sensor: An intensity ratio model

The mechanoluminescent (ML) sensor is a newly developed non-invasive technique for stress/strain measurement. However, its application has been mostly restricted to qualitative measurement due to the lack of a well-defined relationship between ML intensity and stress. To achieve accurate stress measurement, an intensity ratio model was proposed in this study to establish a quantitative relationship between the stress condition and its ML intensity in elastic deformation. To verify the proposed model, experiments were carried out on a ML measurement system using resin samples mixed with the sensor material SrAl2O4:Eu2+, Dy3+. The ML intensity ratio was found to be dependent on the applied stress and strain rate, and the relationship acquired from the experimental results agreed well with the proposed model. The current study provided a physical explanation for the relationship between ML intensity and its stress condition. The proposed model was applicable in various SrAl2O4:Eu2+, Dy3+-based ML measurement in elastic deformation, and could provide a useful reference for quantitative stress measurement using the ML sensor in general.

Heat transfer enhancement of turbulent channel flow using tandem self-oscillating inverted flags

Heat transfer enhancement by tandem self-oscillating inverted flags was investigated for channel flow. An isolated flag was selected as the benchmark. The highly unsteady flapping motion of the inverted flag was captured using a high-speed camera and identified by the structure boundary detection algorithm. The resultant temperature fields were determined by temperature sensitive paint measurement. Three dynamic regimes were observed for the isolated inverted flag: biased, flapping, and deflected modes. The results show that the self-oscillating inverted flag in the flapping mode exhibited the most energetic motion, with a maximum amplitude of 1.75 times of the flag length and a Strouhal number 0.17. This significantly promoted heat removal from the heated wall, with a considerable local Nusselt number ratio exceeding 1.5. Tandem flags flapped synchronously with the same frequency at a low Reynolds number and a close distance G* between two inverted flags, and the phase difference depended linearly on the separation distance. Increases in the separation distance and Reynolds number led to decoupling behavior. Local Nusselt number ratio peak regions were found behind both the front and rear flags, and the heat transfer performance behind the rear flag was superior to that behind the front flag. The global Nusselt number ratio was not sensitive to the separation distance and Reynolds number for tandem flag configurations. Increasing Reynolds number reduced the mechanical loss, so operating the tandem inverted flags’ system at a high Reynolds number moderately improved the heat transfer efficiency.

Data mining of a clean signal from highly noisy data based on compressed data fusion: A fast-responding pressure-sensitive paint application

A data mining approach based on compressed data fusion is developed to extract a clean signal from highly noisy data and it has been successfully applied to flow measurement using fast-responding pressure-sensitive paint (fast PSP). In this approach, spatially resolved but noisy full-field data are fused with clean but scattered data to reconstruct full-field clean data. The fusion process is accomplished based on a compressed sensing algorithm, which has shown significantly improved performance compared with low-dimensional analysis. This is because, in low-dimensional analysis such as proper orthogonal decomposition (POD), the selection criteria of proper POD modes for reconstruction are usually based on subjective observation and the mode coefficients can be severely distorted by noise, which restricts the applications of this method to complicated flow phenomena and leads to a low-quality reconstruction. The solutions to these two problems can be expressed via mathematical optimization by determining the optimal coefficients to reconstruct clean data using the most relevant POD modes. Here, compressed sensing is used as a suitable solution to explore the sparse representation of scattered clean data based on the POD modes obtained from noisy full-field data. A high-quality reconstruction can be obtained using the optimized coefficients. The new method is first demonstrated by using fabricated patterns, demonstrating a reduction of 75% in the reconstruction error compared with POD analysis. It is thereafter successfully applied to recover the unsteady pressure field induced by a cylinder wake flow at low speed. Fast PSP measurement and microphones are used to obtain full-field but noisy pressure field data and scattered but clean data, respectively. In the cases of single and step cylinders, the reconstruction errors are approximately 5% and 25%, respectively, and the accuracy of reconstruction depends on the low dimensionality of the flow phenomena and the total number of microphone sensors. The current technique provides a reliable method to recover clean signals from strong noise, with significant potential for applications to flow measurement, control, and monitoring.A data mining approach based on compressed data fusion is developed to extract a clean signal from highly noisy data and it has been successfully applied to flow measurement using fast-responding pressure-sensitive paint (fast PSP). In this approach, spatially resolved but noisy full-field data are fused with clean but scattered data to reconstruct full-field clean data. The fusion process is accomplished based on a compressed sensing algorithm, which has shown significantly improved performance compared with low-dimensional analysis. This is because, in low-dimensional analysis such as proper orthogonal decomposition (POD), the selection criteria of proper POD modes for reconstruction are usually based on subjective observation and the mode coefficients can be severely distorted by noise, which restricts the applications of this method to complicated flow phenomena and leads to a low-quality reconstruction. The solutions to these two problems can be expressed via mathematical optimization by determining ...

A novel laminated OLED–PSP system for measurement on moving surfaces

A novel pressure-sensitive paint (PSP) system has been developed which uses an embedded organic light-emitting diode (OLED) layer as excitation, and thus eliminates the usual requirement of an external light source. This system consists of an OLED layer, an optical filter layer and a polymer-ceramic (PC) PSP layer with fast response. The OLED provides green excitation light (around 500xa0nm) for PSP, while the optical filter and (550xa0nm short-pass) removes unwanted longer wavelength light to prevent possible interference with the PSP signal. A PC-PSP layer is applied on top of the filter with well-controlled basecoat thickness (~xa010xa0μm) to allow sufficient light transmittance. It is also important that the OLED is modulated with a limited duty cycle and synchronized with the camera exposure to minimize uncertainties mostly due to the inherent heating effect of OLED. This OLED-based PSP has a pressure sensitivity comparable to the regular PC-PSP, and it is well-suited for pressure measurements on moving surfaces since its illumination field is independent of the model motion. Its advantage over a conventional PSP system was demonstrated through applications on a flapping plate under the impingement of a nitrogen jet.Graphical Abstract