Mory Gharib

Miniature and MOEMS Flow Sensors

Recent progress in the development of a miniature Laser Doppler Anemometer (LDA) and a micro optical shear stress sensor is described. Miniaturization of these sensors has been achieved with the use of integrated optics and micro fabrication techniques. This paper describes the fabrication of the two sensors and presents an experiment for the evaluation of the sensors. The results show perfect agreement between the boundary layer velocity gradient performed with the LDA, and the measurements obtained with the shear stress sensor. The range of experimental conditions suitable for the wall shear sensor is reported. Finally, we describe the application of the sensors in a series of tests performed at the William B. Morgan Large Cavitation Channel of the Navy’s Carderock Division, in Memphis, Tennessee.

An optical MEMS-based shear stress sensor for high reynolds number applications

In an effort to extend wall shear stress measurements to high Reynolds number flows, a new MEMSbased optical shear stress sensor was fabricated and tested in the 2 feet wind tunnel at the California Institute of Technology for Reynolds numbers of up to 5.6 x 106. The description of this sensor and the test results are reported in this paper. The sensor, the Dual Velocity sensor, designed using recent developments in diffractive and integrated optics, was small enough to be embeddable in test models. The sensor measured the average flow velocity at two probe volumes located within the first 110 micrometers above the flush-mounted sensor surface. The velocity gradient at the wall was estimated by fitting the Spalding formula to the average velocity measurements, once mapped using the inner-law variables u+ and y+. The results obtained with the Dual Velocity sensor were in excellent agreement with measurements obtained in the same tunnel using other techniques such as the oil film interferometry technique and with another MEMS-based optical shear stress sensor, the Diverging Fringe Doppler sensor. All wall shear stress measurements were also in agreement with those calculated from boundary layer surveys obtained with a miniature LDV.

Quantitative Flow Visualization - Toward a Comprehensive Flow Diagnostic Tool

Abstract Quantitative flow visualization has many roots and has taken several approaches. The advent of digital image processing has made it possible to practically extract useful information from every kind of flow image. In a direct approach, the image intensity or color (wavelength or frequency) can be used as an indication of concentration, density and temperature fields or gradients of these scalar fields in the flow (Merzkirch, 1987). For whole-field velocity measurement, the method of choice by experimental fluid mechanicians has been the technique of Particle Image Velocimetry (DPIV). This paper presents a novel approach to extend the DPIV technique from a planar method to a full three-dimensional volume mapping technique useful in both engineering and biological applications.

Recent Progress in Flow Visualization Techniques toward the Generation of Fluid Art

This paper describes recent progress in flow visualization techniques from the viewpoint of visual art incorporating fluid motion. The images of fluid art introduced here are categorized into four groups: the reflected or refracted patterns of free surface motion in nature and in a controlled environment, the coherent turbulent phenomena of fluid flow, and the fluid motion induced by the physical properties of fluids. It is shown that flow visualization techniques, which were originally developed in the field of engineering, have been successfully applied to the creation of artistic images.

The effects of forced boundary conditions on flow within a cubic cavity using digital particle image thermometry and velocimetry (DPITV)

Buoyancy-driven convection within a cavity, whose sidewalls are heated and cooled, is a problem of great interest, because it has applications in heat transfer and mixing. Most studies to date have studied one of two cases: the steady-state case or the development of the transient flow as it approaches steady state. Our main concern was to study the response of the cavity to time-varying thermal boundary conditions. We therefore decided to observe the flow phenomena within a convection cavity under sinusoidal thermal forcing of the sidewalls. To map the flow properly, it is necessary to have simultaneous kinematic and thermal information. Therefore, the digital particle image thermometry and velocimetry (DPITV) is used to acquire data. Implementing this technique requires seeding the flow with encapsulated liquid crystal particles and illuminating a cross section of the flow with a sheet of white light. Extraction of the thermal and kinematic content is in two parts. For the first, the liquid crystals will reflect different colors of the visible spectrum, depending on the temperatures to which they are subjected. Therefore, calibrating their color reflection with temperature allows for the extraction of the thermal content. For the second part, the kinematic information is obtained through the use of a digital cross-correlation particle image velocimetry technique. With the use of DPITV, the flow within a convection cavity is mapped and studied under steady forcing and sinusoidally forced boundary conditions at the Brunt-Vaisala frequency. For the sinusoidally forced case, three cases are studied. In the first, the heating between the two walls is in phase. In the second, the heating between the two walls is 180° out of phase. In the third, the heating between the two walls is 90° out of phase. For steady forcing, the thermal plots show that the flow develops a linearly stratified profile within the center of the cell. At the sidewalls, however, owing to forcing, hot/cold thermal boundary layers develop at the left/right walls. These hot/cold thermal boundary layers then turn around the upper-left/lower-right corners and develop into intrusion layers that extend across the top and bottom walls. The vorticity and streamlines show that the bulk of the fluid motion is concentrated around the walls, whereas the fluid within the center of the cell remains stationary. For the sinusoidally forced cases, the thermal plots show the existence of many thermal “islands,” or pockets of fluid where the temperature is different with respect to its surroundings. The vorticity plots show that the center of the cell is mostly devoid of vorticity and that the vorticity is mainly confined to the sidewalls, with some vorticity at the top and bottom walls. For the 0° forcing, the streamlines show the development of two counterrotating rollers. For the 180° forcing, the streamlines show the development of only one roller. Finally, for the 90° forcing, the streamlines show the development of both a two-roller and a one-roller system, depending on the position within the forcing cycle.

Analytical and experimental investigations of dual‐plane particle image velocimetry

In its classical form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double‐pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross‐correlation methods, are used to determine the projections of the velocity vectors onto the plane between the two light sheets. The amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light‐sheet planes. The mathematical background of this method is described. Numerical simulations show the influence of the main parameters (e.g., light‐sheet thickness, light‐sheet displacement and out‐of‐plane flow component) on the resolution and reliability of the new method. A new recording procedure and its results will be shown to demonstrate the ease of operation when applying this technique to liquid flows.

Quantitative Flow Visualization

Abstract: Quantitative flow visualization has many roots and has taken several approaches. The advent of digital image processing has made it practical to extract useful information from every kind of flow image. In a direct approach, the image intensity or color (wavelength or frequency) can be used as an indication of concentration, density and temperature field, or gradients of these scalar fields in the flow. 1 For whole‐field velocity measurement, the method of choice for experimental fluid mechanicians has been digital particle image velocimetry (DPIV). This paper presents a novel approach to extend the DPIV technique from a planar method to a full three‐dimensional volume mapping technique.

A REYNOLDS NUMBER INDEPENDENT MODEL FOR TURBULENCE IN COUETTE FLOW

In this paper we study theoretically and computationally the dynamics of linearized stream-wise constant Navier-Stokes equations, under external time varying deterministic disturbances.

Mechanisms of Enhanced Heat Transfer for Oscillating Circular Cylinders

Experiments were conducted to determine the relationship between wake structure and heat transfer for an oscillating circular cylinder in cross-flow. An internally heated cylinder was suspended in a water tunnel and oscillated transverse to the freestream. The cylinder’s heat transfer coefficient was measured over a wide range of oscillation amplitudes and frequencies. By comparing these results to the known wake mode regions in the amplitude-frequency plane, relationships between wake mode and heat transfer were identified. Representative cases were investigated further by using digital particle image thermometry/velocimetry (DPIT/V) to simultaneously measure the temperature and velocity fields in the near-wake. This revealed more detail about the mechanisms of heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, are the primary cause of the heat transfer enhancement.Copyright

Applications of DDPIV to Studies Associated with Road Vehicles

The quantification of experimental flows is a problem that poses several challenges, the most obvious of which is how to extract motion from an “invisible” phenomenon. In general, flows can be analyzed through a sequence of still images (Singh 1991). For example, the motion of patterns generated by dye, clouds or particles can be used to obtain such a time sequence of still images. The main problem with using a continuous-intensity pattern, generated by scalar fields (e.g., dye patterns), is that it must be somehow discretized and contain variations of intensity at all scales before mean and turbulent velocity information can be obtained (Pearlstein and Carpenter 1995). In this respect, the discrete nature of images generated by seeding particles has made particle tracking the method of choice for whole field velocimetry. Displacement and, thus, velocity information can be extracted through statistical methods and other methods such as particle tracking. The spatial resolution of this method depends on the number density of the particles.