Volkan Otugen

Experimental study of a round jet impinging on a convex cylinder

The flow physics of a free round air jet prior to impinging on a convex cylindrical surface is discussed. Two components of mean velocity, normal stress and shear stress profiles were obtained using a novel fibre-optic, two-simultaneous velocity component laser-Doppler velocimetry probe. Velocity profiles obtained at seven axial locations in the impinging jet case are compared to profiles obtained at eight axial locations away from the jet exit to determine the surface effects on the free jet. The Reynolds number of the flow based on the jet diameter was Re = 25 000 and the convex cylinder was located at x/d = 4.0 (jet exit velocity, Uj = 24 m s−1; jet diameter, d = 15.24 mm; circular cylinder diameter, D = 60.5 mm). Flow visualization results show that the initially axisymmetric jet becomes a three-dimensional flow, wraps itself on the cylinder across the cylinder and also behaves similar to a wall jet along the cylinder axis. The jet axial mean velocity starts reducing sharply one diameter (1d) away from the cylinder. The jet axial, radial, tangential normal stresses and the shear stress are not affected by the presence of the surface in the vicinity of the jet axis until 0.05d away from the surface, and are affected near the jet edge about 0.75d away from the surface.

Optical Force Sensor Based on Whispering Gallery Mode Resonators

This paper discusses a novel micro-optical force sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes (WGM). A small change in the size, shape or optical constants of the microsphere causes a shift in the resonant frequency (or the WGM). For example, a compression force applied to the microsphere will lead to a change in both its shape and its index of refraction distribution. These changes will result in a shift of the WGM. By monitoring this shift, the magnitude of the applied force can be determined. The WGM shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photo diode on the other end. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips (due to destructive interference) in the intensity of the light transmitted through the fiber. Current results demonstrate the WGM shifts due to compression force applied to micro-spheres along the polar direction. The measurements also indicate a force measurement resolution of ~ 10 N with the current sensor design.

High-speed transient sensing using dielectric micro-resonators

In this paper, we demonstrate the use of whispering gallery mode (WGM) resonators for high-speed transient sensing. In the typical WGM sensor, the micro-resonator modes are interrogated by coupling light from a tunable laser through a single mode optical fiber. The laser is tuned over a narrow range by thermo-optic effect, and mode shifts in the transmission spectrum through the fiber are observed. For high-speed applications, thermal inertia of the optical system impedes the proper tuning of the laser, limiting the WGM sensor applications to slow varying phenomena. In order to use the sensors for high-speed transient applications, we tune the DFB laser using a harmonic rather than a ramp waveform and calibrate the laser response at various input frequencies and amplitudes using a Fabry-Perot interferometer. WGM shifts are tracked using a fast cross-correlation algorithm on the transmission spectra. We demonstrate dynamic force measurements up to 10 kHz using this approach. We also present a simple lumped-heat capacity thermal model to predict the laser response.

High Resolution Micro-Optical Wall Shear Stress Sensor

We report an optical wall shear stress sensor based on the whispering gallery modes (WGM) of a dielectric sphere. Radial deformations of such spheres, for example caused by shear stress, results in a shift in the WGMs, thereby allowing one to monitor the effect (shear stress) causing such shifts. The sensor is composed of a sensing element, which is a movable plate flush to the wall. The sensing element is attached to a lever on one end, and the other end is in contact with the sphere. Thus, the shear force felt by the sensing element is transmitted to the sphere mechanically through the rotation of the lever. Previous experimental results with these spheres showed force resolutions as good as ~10 -10 N, which for a sensing element of ~650 m

High-Resolution Whispering Gallery Mode Force Micro- Sensor Based on Polymeric Spheres

Previous experimental studies of whispering gallery mode (WGM) sensors have indicated that microspheres with diameters ranging between 300-950 μm may have force resolutions reaching 10N [1]. In the present, we expand on the previous investigatons. Here, we carry out a systematic analysis and experiments to investigate the sensitivity, resolution and bandwidth limits of WGM-based force sensors. Expressions for WGM shifts due to applied force in the polar direction are obtained for microspheres of various dielectric materials, in the diameter range of 300-950 μm. The analyses are compared with experimental results for Polymethylmethacrylate (PMMA) Polydimethylsyloxane (PDMS) microsphere sensors. The present analysis shows that the strain effect on WGM shifts dominate over that of mechanical stress. It also indicates that force sensitivities of the order of a 1pN are possible using hollow PDMS spheres. The sensor bandwidths (based on the mechanical properties of the sensor material alone) range between 1 kHz and 1 MHz. These results have significance also from the point of view wall shear stress since the same force sensing concept can be used for the development of high-sensitivity skin friction sensors.

The effect of thermal barriers on sound wave propagation

Sound propagation through regions of non-uniform temperature distribution in a gas is studied numerically. The main objective of this study is to determine the impact of temperature gradients on the sound wave parameters and to evaluate the effectiveness of using regions of hot gas as sound barrier. Such regions of hot gas (low acoustic impedance) can be generated by remotely depositing energy into a selected volume of gas, for example, by means of electrical discharge. Sound attenuation through the hot gas region is studied systematically for a range of sound wave and thermal field parameters by solving the two-dimensional unsteady compressible Eulers equations along with the ideal gas state equation using a finite volume scheme. Particular attention is given to the practical case when sound wavelength is comparable to the thickness of the thermal barrier. The present two-dimensional model indicates that considerable sound attenuation can be achieved at large incidence angles and a critical angle for total internal reflection is possible.

Multi-layered resonators for optimized electric field detection

In this study, we carry out an analytical investigation to determine the efficacy of multi-layer dielectric micro-sphere resonators as high-resolution electric field sensors. The use of a large number of layers with different electrical and mechanical properties allows for optimum dielectric constant and elastic modulus gradients within the sphere to optimize sphere resonator’s sensitivity to external electric field. The external electric field applied to a dielectric sphere induces an elastic deformation of the resonator (electrostriction effect), leading to shifts in its whispering gallery optical modes (WGM). The non-uniform distribution of the dielectric constant leads to a gradient in the electric field within the sphere. This in turn induces non-uniform body force on the sphere when subjected to an external electric field. By also appropriately varying the elastic modulus of the layers, the electrostictive deformation of the sphere can be optimized. In this paper we present a mathematical model describing the effect of the electric field on the WGM shift of layered micro-spherical resonators.

Demonstration of a Novel Micro-Photonic Wall Pressure Sensor

In this paper we propose a novel micro-photonic sensor concept based on the morphology dependent resonances (MDR) of dielectric microresonators. The microresonator, spherical in shape and a few hundred microns in diameter, serves as a sensing element. A membrane is used to mechanically transmit pressure to the microsphere. As a result of the applied pressure, the morphology of the microsphere is perturbed, leading to a shift in the optical resonances (MDR). The applied pressure is thus monitored by recording the optical shift. Preliminary experiments using a prototype sensor design show promise for future applications.

Thermal gradient effects on the propagation of nonlinear pressure waves in a gas

A numerical study of wave propagation through gases with nonuniform temperature distributions will be presented. The main objective of this study is to determine the impact of temperature gradients on high‐intensity, initially sinusoidal pressure waves. Particular emphasis is paid to wave reflection, transmission, and any influence a high‐temperature region may have on nonlinear behavior. Ultimately, the performance of thermal barriers in attenuating nonlinear waves is evaluated. The concept of using regions of hot gas inside an ambient environment has potential in aeroacoustic applications, such as jet screech mitigation. This analysis considers one‐dimensional compressible unsteady Euler equations with an ideal gas state equation, applied over a uniform domain. The domain is composed of two regions with uniform and equal gas properties (ambient conditions) separated by a third region with higher gas temperature and, accordingly, lower density. Pressure is uniform throughout the domain. We introduce a high‐intensity sinusoidal wave into this medium. The shape and extent of the high‐temperature zone is varied to study the effect of this region on wave propagation. Further, wave reflection and transmission are studied for a range of wave and thermal field parameters. Results for nonlinear pressure waves are compared to linear acoustic waves.