Session HT: Experimental Techniques V

Demonstration of a Novel Micro-Optical Wall Pressure Sensor Concept Based on Whispering Gallery Mode Resonators

We present a novel micro-optical wall pressure sensor concept based on the optical resonance (whispering gallery mode or WGM) shifts of polymeric spheres. The spherical resonators, which are typically a few hundred microns in diameter, serve as the sensing element. The pressure acting on a polymeric membrane flush with the wall is transmitted by simple contact with the microsphere. The force acting on the microsphere in the contact region perturbs its morphology (both its shape and refractive index) leading to a shift in its WGM. By measuring these changes in the resonance frequency, measurements of wall pressure is accomplished. Due to the unusually large optical quality factors associated with sphere WGMs, pressure measurements with extremely high sensitivity are possible.   

Volumetric measurements in bubbly wakes using V3V

The new V3V volumetric PIV system from TSI is used to investigate cavitating and bubbly flows in wakes. Data obtained using the V3V system are compared to more conventional measurements and the efficacy of the system in measuring void fraction and velocity fields in multiphase flow is addressed.   

Experimental Measurements of Skin Friction in Air and Water Micro-Channels

Surface Stress Sensitive Film (S3F), is a relatively new experimental sensor that provides continuous measurements of skin friction and pressure on aerodynamic and hydrodynamic surfaces. This sensor is based on the distortions of an elastic polymer film which deforms under the action of the applied normal and tangential loads. Skin friction and pressure gradients are determined by monitoring these distortions and applying a finite element model to the elastic film. This technique has been demonstrated by performing quantitative measurements of pressure and skin friction in several wind tunnels, water tunnel, and channel flows. This paper will focus on experimental measurements in fully developed micro-channels that have been used to validate the S3F measurements. Comparisons between S3F, theoretical relations based on Reynolds number and Poiseuille Flow, and experimental measurements of skin friction based on monitoring the pressure gradient in the channel indicate agreement of better than 5{\%}.   

Dynamic Calibration Technique for the Micro-Pillar Shear-Stress Sensor MPS$^{3}$

Based on magnetic excitation a dynamic calibration technique for the micro-pillar shear-stress sensor MPS$^{3}$, which allows to determine the local wall-shear stress in turbulent flows by optically measuring the velocity gradient within the viscous sublayer of turbulent flows, is described. The proposed dynamic calibration technique allows to assess the micro-pillar dynamic response for different flow media up to approximately 10kHz. The results do convincingly agree with the findings of a second-order analytical approximation based on experimentally determined damped eigenfrequencies and damping coefficients. Measurements for different sensor geometries and in various fluids show the sensor to possess transfer functions ranging from a flat low-pass filtered response to a strong resonant behavior. The results further indicate the pillar to possess a very constant transfer function amplitude at frequencies reasonably below the resonance making it ideal for the measurement of fluctuating wall-shear stress.   

Magnetic suspension system for measuring drag on a sphere in superfluid helium

An apparatus to investigate oscillation of a 3 mm niobium sphere in superfluid helium has been built. A Nb-Ti superconducting solenoid is used to suspend a sphere made of niobium; meanwhile a similar superconducting quadrupole magnet centers and helps to stabilize the ball at one location in the flow channel. The niobium sphere is levitated by the superconducting magnetic suspension system; then the oscillation is obtained by dropping the ball from one equilibrium point to a lower equilibrium point via reducing the magnetic field. The sphere's oscillation is then recorded with a high-speed video camera. Drag force is calculated through its relation to the maximum velocity decay rate.   

Generation of Homogeneous and Isotropic Turbulence with Small Mean Flow

We designed an apparatus that generates nearly homogeneous and isotropic turbulence with small mean flow. The apparatus is shaped as an icosahedron containing 140 liters of water. The flow is driven by 12 independently controlled propellers, each at one of the vertices of the icosahedron. We carried out Lagrangian particle tracking measurements experiments with measurement volumes up to size of $10\times10\times10 [cm^3] $, comparable with the integral length scale. The results show that the apparatus is able to provide nearly homogeneous and isotropic turbulence with a Taylor microscale Reynolds number $R_\lambda \approx 600$. The measured mean flow is less than 20\% of the fluctuating velocity throughout the observed volume. We also measured the Lagrangian structure function of position and velocity in this flow and compared with theoretical predictions by Zybin et al [PRL. 100, 174504 (2008)].   

Schlieren and shadowgraph techniques for fluid physics experiments -- a brief tutorial

Schlieren and shadowgraph techniques are often and broadly applied to the types of flows reported at APS DFD meetings. The simplest optics usually suffice to reveal the strong refractions generated by gas-liquid interfaces, drops, bubbles, and the like. Only rarely is the exquisite sensitivity potential of the schlieren method invoked, while the low-sensitivity background-distortion schlieren method is used more often. Nonetheless there are pitfalls in applying these optical techniques: loss of information from over-ranging due to the tradeoff between sensitivity and measuring range, the inadvertent appearance of schlieren cutoff in an otherwise-pure shadowgram, etc. In some cases the fact that the visualization of a phenomenon is grounded in either schlieren or shadowgraphy is not even recognized. These issues are surveyed and example images are shown.   

Predicting the visibility of a chemical vapor plume using schlieren optics

Chemicals plumes from a freely-evaporating liquid surface and from the exit of a circular pipe are considered. For the freely-evaporating case, the visibility of fourteen chemicals was tested in two schlieren optical systems. One system was a modest bench-top system and the other was a lard system of extraordinary sensitivity. Plume visibility was found to be a function of the vapor pressure and vapor refractive index. An empirical fit to the plume-visibility data, compared with the sensitivities of these systems (measured using a standard-lens method), suggests guidelines for predicting the visibility of plumes of other chemicals using other schlieren equipment. For the circular opening case, plume visibility of the same chemicals was found to be a function of plume geometry and refractive index. The peak light-ray deflections (also measured with a standard lens) caused by plumes of two different sizes were found to scale based on plume geometry. This scaling information and plume refractive index can be used to predict plume visibility for arbitrary chemicals in arbitrary systems, if the system sensitivity is known. One application of this work lies in the optical detection of plumes emitted by contraband material.   

Development of Fast Responding Pressure-Sensitive Paint based on Luminescent Polymer

The development of fast responding pressure-sensitive paint (PSP) based on a luminescent polymer (PTMST) is discussed. The luminescence of PTMST is sensitive to temperature, which can be used as a global temperature sensor. This polymer is based on poly (1-trimethylsilyl-1-propyne), which is known as one of the highest gas permeable polymer. We combined with a pressure-sensitive luminophore of platinum porpholactone (PtTFPL) to create a fast responding, two-color PSP. The luminescent peak of PtTFPL lies 750 nm, which can be separated from the luminescent spectra of PTMST to give an ideal two-color PSP. Because of its high gas permeability, the present PSP gives the pressure response on the order of milliseconds. The luminescence from PTMST can be used to compensate the temperature dependency of PtTFPL. Steady-state calibrations show that the temperature probe of PTMST provides the temperature sensitivity over the calibrated range (273 K to 333 K) without the pressure sensitivity. The pressure sensitivity of the pressure probe of PtTFPL is 0.6. In the final version, we will include unsteady pressure field measurements with temperature-compensation.   

Novel Volumetric Size and Velocity Measurement of Particles Using Interferometric Laser Imaging

Global Sizing Velocimetry (GSV) is a recently developed technique for characterizing the particle size distribution and flow velocity in a plane and in this research we extend this measurement to a volume through a laser scanning system. In GSV, a LASER sheet is used to illuminate translucent particles in a spray or flow field and the camera image is de-focused a known distance to create interference patterns. The diameters of the particles in the flow field are calculated by measuring the inter-fringe spacing in the resulting interferogram. Particle Imaging Velocimetry (PIV) techniques are used to compute velocity by measuring the particle displacement over a known short time interval. Researchers have recently begun applying GSV techniques to characterize sprays in a plane as it offers a larger area of investigation than other well known techniques such as Phase Doppler Anemometry (PDA). In this paper we extend GSA techniques from the current planar measurements to a volumetric measurement. The approach uses a high speed camera to acquire GSA images by scanning multiple planes in a volume of the flow field within a short period of time and obtain particle size distribution and velocity measurements in the entire volume.