Re-Thinking of Data Acquisition Rates in the Era of Expensive Data

It is well known that the mean of a sample converges at a rate of 1/N, where N is the number of samples, assuming that are samples are statistically independent. This paper will show the impact of non-independent samples using real data as well as investigating the efficacy of methods to determine the effective number of samples with non-independent samples are acquired.

Learning by Doing on Computational Fluid Dynamics

This work involves the methodology used in the University of Valladolid for Mechanical Engineering students to learn Computational Fluid Dynamics playing an active role. Students pretend to be engineers in a consulting or design office carrying out a fluid mechanics scale down projects. Later they act as reviewers evaluating a project from a colleague. There is a deeper understanding of the topic when they need to discuss the strategies to accomplish the project, to write a technical report and finally to justify the evaluation of other works. Furthermore, they develop their critical thought, writing skills and synthesis capacity. Multimedia material from other institutions that review the concepts learned in the course can be a suitable way to improve the understanding of concepts.

Characterization of the Custom-Designed, High Reynolds Number Water Tunnel

This paper reports on the characterization of the custom-designed high-Reynolds number recirculating water tunnel located at Oklahoma State University. The characterization includes the verification of the test section design, pump calibration and the velocity distribution within the test section. This includes an assessment of the boundary layer growth within the test section. The tunnel was designed to achieve a downstream distance based Reynolds number of 10 million, provide optical access for flow visualization and minimize inlet flow non-uniformity. The test section is 1 m long with 15.2 cm (6-inch) square cross section and acrylic walls to allow direct line of sight at the tunnel walls. The verification of the test section design was accomplished by comparing the flow quality at different location downstream of the flow inlet. The pump was calibrated with the freestream velocity with three pump frequencies and velocity profiles were measured at defined locations for three pump speeds. Boundary layer thicknesses were measured from velocity profile results and compared with analytical calculations. These measurements were also compared against the facility design calculations.

Development of an Efficient Phase Separator for Space and Ground Applications

The limited amount of liquids and gases that can be carried to space makes it imperative to recycle and reuse these fluids for extended human operations. During recycling processes gas and liquid phases are often intermixed. In the absence of gravity, separating gases from liquids is challenging due to the absence of buoyancy. This paper discusses a phase separator that is capable of efficiently and reliably separating gas-liquid mixtures of both high and low void fractions in a wide range of flow rates that is applicable to reduced and zero gravity environments. The phase separator consists of two concentric cylindrical chambers. The fluid introduced in the space between the two cylinders enters the inner cylinder through tangential slots and generates a high intensity swirling flow. The geometric configuration is selected to make the vortex swirl intense enough to lead to early cavitation which forms a cylindrical vaporous core at the axis even at low flow rates. Taking advantage of swirl and cavitation, the phase separator can force gas out of the liquid into the central core of the vortex even at low void fraction. Gas is extracted from one end of the cylinder axial region and liquid is extracted from the other end. The phase separator has successfully demonstrated its capability to reduce mixture void fractions down to 10−8 and to accommodate incoming mixture gas volume fractions as high as 35% in both earth and reduced gravity flight tests. The phase separator is on track to be tested by NASA on the International Space Station (ISS). Additionally, the phase separator design exhibits excellent scalability. Phase separators of different dimensions, with inlet liquid flow rates that range from a couple of GPMs to a few tens of GPMs, have been built and tested successfully in the presence and absence of the gravity. Extensive ground experiments have been conducted to study the effects of main design parameters on the performance of the phase separator, such as the length and diameter of the inner cylinder; the size, location, and layout of injection slots and exit orifices, etc., on the swirling flow behavior, and on the gas extraction performance. In parallel, numerical simulations, utilizing a two-phase Navier-Stokes flow solver coupled with bubble dynamics, have been conducted extensively to facilitate the development of the phase separator. These simulations have enabled us to better understand the physics behind the phase separation and provided guideline for system parts optimization. This paper describes our efforts in developing the passive phase separator for both space and ground applications.

Wingtip Devices for Tidal Turbines: Performance Improvement and Cavitation Mitigation

Wingtip devices are common in aeronautical applications and are increasingly used on wind turbines. However, their use in hydrokinetic energy conversion applications such as tidal turbines to date is minimal, due to the concern for increased bio-fouling and also the fact that there is little or no data publically available describing their cavitation characteristics. In this study, three wingtip designs were considered for hydrokinetic turbine applications: a plain foil with a rounded tip (considered the reference case), a generic wingtip device (a winglet), and a novel “split-tip” device. The tips were studied numerically and experimentally at different angles of attack. The numerical simulations were performed in OpenFOAM using the k-omega SST model to predict the lift and drag characteristics of a “base” foil with each of the three wingtip devices. Additionally the pressure and vorticity were observed. Experiments were conducted in the University of New Hampshire High-Speed Cavitation Tunnel – HiCaT. A modular experimental test bed with an elliptical foil section was developed specifically for the study. The test bed extends to the centerline of the tunnel where wingtips are attached, and has four small-diameter tube openings to accommodate pressure measurements and/or mass injection studies. Water tunnel data were obtained for lift, and cavitation inception, and compared to the numerical simulations. The numerical results show decreased vorticity with presence of the wingtip devices, however, the advantage of using wingtips for decreasing drag and increasing lift forces is not conclusively exhibited. The experimental measurements suggest that there is a significant suppression of tip vortex cavitation with the use of wingtip devices at high angles of attack (around 10 degrees), but the advantage of using the wingtip devices diminishes at lower angles of attack. It was shown by Arndt [1] that tip-vortex cavitation on hydrofoils can be related to the lift coefficient and the Reynolds number, where the cavitation index at inception is proportional to the square of the section lift coefficient and the Reynolds number based on hydrofoil chord, taken to the power m. The power exponent m has been generally accepted to be approximately 0.4. This relation is made into an equation via a coefficient of proportionality K, which depends on the wingtip and foil section geometry, and has been empirically determined to have values between 0.025 and 0.056 for previously investigated wings. While the value of the coefficient K for the reference wing tip remained comparatively constant for the range of conditions investigated (angles of attack, Reynolds numbers), it varied significantly for the foil terminated by the winglet. This may be due to the non-elliptical load distribution in the span-wise direction, but also raises the question whether the standard tip-vortex cavitation correlation for hydrofoils is applicable for general wingtip devices.

Uncertainty Quantification of Low Void Fraction Measurements Using Wire-Mesh Sensors in Horizontal Air-Water Flows

The need for a revised methodology and uncertainty quantification for wire-mesh sensor void fraction measurements in horizontal low void fraction flow conditions was identified. Two-phase flow measurements were performed at a low-pressure, adiabatic and horizontal flow loop using wire-mesh sensors over a range of water superficial velocities from 3.5 to 5.5 m/s, air superficial velocities from 0.05 to 0.9 m/s and volumetric void fractions from 1 to 16% Using this proposed analysis, a corrected trend with average percent differences of 36, 21 and 6% was obtained for the low, medium and high gas flow rate cases, respectively, when comparing the wire-mesh sensor void fractions to two-phase pipe flow models. By combining these measurements of the void fraction with those of the interfacial velocity, the gas superficial velocity was calculated based on the physical theory, and compared to the superficial velocity measured by the flowmeters for validation purposes. An estimation of the uncertainty of these parameters showed that most of the measured parameters agreed reasonably with physical theory within 20%.

Experimental Evaluation of Dual-Opposed Jet Mixer Pump Performance for Slurry Mixing

Million-gallon double-shell tanks at Hanford are used to store transuranic, high-level, and low-level radioactive wastes. These wastes consist of a large volume of salt-laden solution covering a smaller volume of settled sludge primarily containing metal hydroxides. These wastes will be retrieved and processed into immobile waste forms suitable for permanent disposal. Retrieval is an important step in implementing these disposal scenarios. The retrieval concept evaluated is to use submerged dual-nozzle jet mixer pumps with horizontally oriented nozzles located near the tank floor that produce horizontal jets of fluid to mobilize the settled solids. The mixer pumps are oscillated through 180° about a vertical axis so the high velocity fluid jets sweep across the floor of the tank. After the solids are mobilized, the pumps will continue to operate at a reduced flow rate producing lower velocity jets sufficient to maintain the particles in a uniform suspension (concentration uniformity). Several types of waste and tank configurations exist at Hanford. The jet mixer pump systems and operating conditions required to mobilize sludge and maintain slurry uniformity will be a function of the waste type and tank configuration. The focus of this work was to conduct a 1/12-scale experiment to develop an analytical model to relate slurry uniformity to tank and mixer pump configurations, operating conditions, and sludge properties. This experimental study evaluated concentration uniformity in a 1/12-scale experiment varying the Reynolds number (Re), Froude number (Fr), and gravitational settling parameter (Gs) space. Simulant physical properties were chosen to obtain the required Re and Gs where Re and Gs were varied by adjusting the kinematic viscosity and mean particle diameter, respectively. Test conditions were achieved by scaling the jet nozzle exit velocity in a 75-in. diameter tank using a mock-up of a centrally located dual-opposed jet mixer pump located just above the tank floor. Concentration measurements at sampling locations throughout the tank were used to assess the degree of uniformity achieved during each test. Concentration data was obtained using a real time in-situ ultrasonic attenuation probe and post-test analysis of discrete batch samples. The undissolved solids concentration at these locations was analyzed to determine whether the tank contents were uniform (≤ ±10% variation about mean) or nonuniform (> ±10% variation about mean) in concentration. Concentration inhomogeneity was modeled as a function of dimensionless parameters. The parameters that best describe the maximum solids volume fraction that can be suspended were found to be 1) the Fr based on nozzle average discharge velocity and tank contents level and 2) the dimensionless particle size based on nozzle diameter. The dependence on the jet Re does not appear to be statistically significant.

PIV Uncertainty: Computational and Experimental Evaluation of the Peak Ratio Method

Uncertainty quantification for planar PIV remains a challenging task. In the present study, we assess three methods that were recently described in the literature: primary peak ratio (PPR), mutual information (MI), and image matching (IM). Each method was used to calculate the uncertainty in a synthetic turbulent boundary layer flow and an experimental jet flow. In the experimental case, two PIV systems with common fields of view were used — one with a high dynamic range (which was considered as the true solution) and another with a magnification ratio of about four times less (which was considered the measurand). The high resolution PIV system was verified by comparing velocity records at a point with an LDV measurement system. PIV processing was performed with PRANA and Insight4G. In regards to the experimental flow, the PPR method performed best, followed by mutual information, and lastly image matching. This was due to better responses by PPR and MI of uncertainty to the spatially varying error distribution. Similar conclusions were made with respect to the synthetic test case.

Numerical Study of Gravity Effects on Phase Separation in a Swirl Chamber

The effects of gravity on a phase separator are studied numerically using an Eulerian/Lagrangian two-phase flow approach. The separator utilizes high intensity swirl to separate bubbles from the liquid. The two-phase flow enters tangentially a cylindrical swirl chamber and rotate around the cylinder axis. On earth, as the bubbles are captured by the vortex formed inside the swirl chamber due to the centripetal force, they also experience the buoyancy force due to gravity. In a reduced or zero gravity environment buoyancy is reduced or inexistent and capture of the bubbles by the vortex is modified. The present numerical simulations enable study of the relative importance of the acceleration of gravity on the bubble capture by the swirl flow in the separator. In absence of gravity, the bubbles get stratified depending on their sizes, with the larger bubbles entering the core region earlier than the smaller ones. However in presence of gravity, stratification is more complex as the two acceleration fields — due to gravity and to rotation — compete or combine during the bubble capture.