Optical Measurements of Wall Shear Stress with Emphasis on Flows Near Separation

53rd AIAA Aerospace Sciences Meeting

2015

This paper presents the results of numerical experiments regarding the feasibility of a micro-photonic sensor for simultaneous measurement of the wall pressure and shear stress. The proposed sensor is based on the whispering gallery mode (WGM) of two microcylindrical optical resonators. Both resonators are embedded in a flexible polymeric slab. The external pressure and the shear stress acting on the surface of the slab lead to a change in the morphology of the resonators, therefore inducing a shift in their optical resonances. Numerical experiments were carried out to investigate the effect of the geometry as well as the effect of the material properties on the induced optical mode shift. The results showed that the shear stress as well as the wall pressure can be measured simultaneously using the proposed sensor concept. Nomenclature = Pressure = Shear stress = Radius of the micro-resonators = Perimeter of the micro-resonators = Wavelength ν = Poisson's ratio 1 = Young's modulus of the slab 2 = Young's modulus of the micro-resonators = Young's modulus ratio ( = 2 1 ⁄ ) ℎ = Height of the slab ℎ 1 = Height of the center of the resonator from the bottom surface 1 = Refractive index of the slab 2 = Refractive index the micro-resonators

Section: Introductionmentioning

confidence: 99%

Investigation of a Photonic Wall Pressure and Shear Stress Sensor

Zamanian

53rd AIAA Aerospace Sciences Meeting

2015

“…Most of the currently used sensors are based on indirect measurements where the wall shear stress is determined from the measurement of another flow property. Some examples of these are hotwire/film-based anemometry [4] or heat flux gages [5], surface acoustic wave sensors [6] and laser-based velocity sensors [7][8][9][10][11]. Another frequently used method is surface oil film interferometry that provides time averaged measurement [12,13].…”

Section: Introductionmentioning

confidence: 99%

Direct measurement of wall shear stress in a reattaching flow with a photonic sensor

Ayaz¹,

Otugen

2013

Meas. Sci. Technol.

Wall shear stress measurements are carried out in a planar backward-facing step flow using a micro-optical sensor. The sensor is essentially a floating element system and measures the shear stress directly. The transduction method to measure the floating element deflection is based on the whispering gallery optical mode (WGM) shifts of a dielectric microsphere. This method is capable of measuring floating element displacements of the order of a nanometer. The floating element surface is circular with a diameter of ∼960 μm, which is part of a beam that is in contact with the dielectric microsphere. The sensor is calibrated for shear stress as well as pressure sensitivity yielding 7.3 pm Pa −1 and 0.0236 pm Pa −1 for shear stress and pressure sensitivity, respectively. Hence, the contribution by the wall pressure is less than two orders of magnitude smaller than that of shear stress. Measurements are made for a Reynolds number range of 2000-5000 extending to 18 step heights from the step face. The results are in good agreement with those of earlier reports. An analysis is also carried out to evaluate the performance of the WGM sensor including measurement sensitivity and bandwidth.

“…Most of the currently used sensors are indirect approaches where the wall shear stress is determined from the measurement of another flow property. These include hot-wire/film-based anemometry [4] or heat flux gages [5], surface acoustic wave sensors [6] and laser-based velocity sensors [7][8][9][10][11]. Another method that has been frequently used is the surface oil film interferometry [12,13], which provides time-averaged measurement.…”

Section: Introductionmentioning

confidence: 99%

Wall shear stress sensor based on the optical resonances of dielectric microspheres

Ayaz

Otugen

2011

Meas. Sci. Technol.

We report an optical wall shear stress sensor based on the whispering gallery mode (WGM) shifts of dielectric micro-resonators. The optical resonators are spheres with a typical diameter of several hundred microns and they serve as the sensing element. The wall shear force acting on a movable plate is transmitted mechanically to the microsphere. As a result of the applied force, the shape of the resonator is perturbed leading to a shift of the optical resonance (WGM). The one-dimensional wall shear stress is measured by monitoring these WGM shifts. Shape perturbations of the order of a nanometer can be detected with this optical method. The measurement resolution and range can be optimized by using dielectric sphere materials of different stiffness covering a wide range of flows. Prototype sensors using PDMS spheres have been built and validated in a laminar Poiseuille flow and in a plane wave acoustic tube.