Darius Modarress

The Application of Advanced Methods in Analyzing the Performance of the Air Curtain in a Refrigerated Display Case

Computational Fluid Dynamics (CFD) modeling is effectively coupled with the experimental technique of Digital Particle Image Velocimetry (DPIV), to study the flowfield characteristics and performance of the air curtain of a medium-temperature open vertical refrigerated display case used in supermarkets. A global comparison of the flowfield and quantification of the entrained air into the case indicate that there is a considerable amount of cold air spillage from a typical display case that is replaced by the ambient warm entrained air across the air curtain, lowering the energy efficiency of the case. The computational model that is developed from the marriage of CFD and DPIV techniques provides a reliable simulation tool that can be used for the design optimization of air curtains. A correct estimate of the infiltration rate by changing different parameters in a validated computational simulation model will provide a feasible tool for minimizing the spillage of the cold air, and thereby designing more energy efficient open display cases.

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.

Micro-Optical Sensors for Boundary Layer Flow Studies

This manuscript describes optical MEMS (or MOEMS)-based microsensors for near wall boundary layer flow and particle field analysis. The sensors have been developed to measure a variety of parameters including flow velocity, surface speed, skin friction, and particle sizing. The surface mounted sensors measure flow velocity and/or flow velocity gradients as close as 70 microns from the wall. The sensors have been successfully used in a number of filed tests and flow facilities at different Reynolds numbers. They have also been used on-board full-scale vehicles. These compact and embeddable sensors incorporate specially designed diffractive optical elements and use single-mode optical fiber or integrated diode lasers for illumination.Copyright

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.

Micro-Optical Sensors for Underwater Velocity Measurement

This manuscript describes recent progress in the development and fielding of optical MEMS-based Mini-and Micro-sensors for surface and underwater applications. The non-intrusive sensors have been developed to measure vehicle speed, boundary layer velocity profile, and skin friction. The MiniLDVTM and Micro velocimeters have been used as a speed sensor in torpedoes, scaled submarines, and surface vessels at sea and in tow tanks. The same instruments have also been used for boundary layer profilometry at variety of Reynolds numbers. MicroSTM shear stress sensors have been used for drag reduction studies in full-scale sea trials. The small size and integrated electronics of the micro-sensors make them especially appropriate for applications in UUV and AUV. In all cases, particles present in the ocean water were found to be adequate both in concentration and size as scattering sources for the measurement. The building blocks for MEMS-based optical sensors are described and examples of sea trials are presented.

Diffractive optics for particle velocimetry and sizing

Beam-shaping diffractive optical elements are used to create structured light patterns in fluid flows. Particle scattering results in detected signals that can be used to determine the particle velocity and size.

Instantaneous whole field measurement of velocity and size of air microbubbles in two-phase flows using DDPIV

Defocusing digital particle image velocimetry (DDPIV) is the natural extension of digital particle image velocimetry (DPIV), planar or quasi three-dimensional, to a true and unique three-dimensional PIV technique. This work presents the defocusing optical concept by which the depth information can be retrieved, thus overriding the limitation to in-plane measurements of actual PIV techniques, either standard or stereo-based. The concept is implemented into a three-dimensional imaging system specifically designed for the purpose of mapping two-phase bubbly flows. Digital images of the bubble field are recorded and analysed to provide information both on the physical location of every single particle/bubble and on its respective size, which is estimated from the scattered light intensity. The calculation of the true three-component velocity field is done by local spatial cross-correlation between two consecutive sets of particle/bubble locations. The spatial resolution and uncertainty limits are established based on a simplified model of the defocusing optical system. Accuracy measurements show that the average error on the displacements is about 0.02 pixels. The methodology used to measure the size is laid out by application of the Mie scattering theory. A DDPIV prototype instrument was fabricated on specific requirements. The instrument records high resolution images of the bubble field and is capable of providing bubble size and bubble location within a cubic foot volume. The technique is applied to the study of the dynamics of sub-millimeter air bubbles in a three-dimensional vertical flow generated by a propeller. Velocity, bubble size distribution and void fraction for these flows are discussed.

Miniature illuminator for laser Doppler velocimeter assembled on micromachined silicon optical bench

We have built a miniature illuminator for Laser Doppler velocimeter on micromachined silicon optical bench utilizing a novel optical scheme. We used two intersecting coherent beams from the two opposing facets of semiconductor laser die to form a standing interference pattern needed for the particle detection and velocity measurement. Such devices are of interest to NASA for investigating wind patterns and dust loading on planets with atmosphere. They have been applied to problems where the liquid or gas flux must be characterized without disturbing the flow. In addition, the small probe volume makes possible local flow characterization and profiling. The device fabrication, and the results of the fringe characterization and velocity measurements are presented and discussed.