A Novel Mirror-Image Technique for Wheel and Ground Simulation in Automotive Wind-Tunnels

A novel method for simulating the relative motions of the wheels and moving ground for road vehicle aerodynamics is presented. The method revisits an old concept where two identical vehicles are used and positioned so that they are mirror images, with the ground being represented by the horizontal plane of symmetry. The method involves double symmetry, where two half models (e.g. a car split down the vertical centerline) contact at the rotating wheel contact patches and the resulting (opened) vehicle halves lie on a reflection plane. This can either be the tunnel floor or the equivalent CFD plane. For some forms of physical testing this offers advantages (such as easy access to wheel cavities and requiring only one vehicle) but sealing the gap between the tunnel floor and the vehicle halves can interfere with the force balance accuracy and problems can arise with time-varying flows crossing the time averaged zero flow boundary. This paper describes the concept and CFD and model-scale EFD evaluations which were found to compare well.

Influences of Impeller Diameter and Diffuser Blades on Air-Water Two-Phase Flow Performance of Centrifugal Pump

A tandem arrangement of double rotating cascades and single diffuser cascade, proposed as a centrifugal pump with high performance in air-water two-phase flow condition, yields lower head due to the smallness of the impeller outlet in comparison with a impeller with large outlet diameter and no diffuser. Influences of impeller diameter change and installation of diffuser blades on two-phase flow performance were experimentally investigated under the case of the same volute casing. As the result, the similarity law of the diameter of impeller having the similar blade geometry and the rotational speed is satisfied even in two-phase flow condition. Comparing pump performances between a large impeller without diffuser blades and a small one with diffuser blades, higher two-phase flow performance is obtained by controlling the rotational speed of a small impeller with diffuser blades in the range of small water flow rates, while a large impeller with no diffuser gives high performance in the range of high water flow rate and small air flow rate.

Traceability of Thermal Power Measurments: Modified Venturi Tubes

This is the second of two papers describing the traceability of nuclear feedwater flow measurements. The first considered the challenges and methodology for establishing the traceability of chordal ultrasonic flow meters. This paper considers the challenges of establishing the traceability in a measurement using a flow element of the modified venturi tube type. It specifically considers the assumptions and uncertainties associated with the extrapolation, for use in the field, of tube calibration factors measured in the laboratory. To quantify these uncertainties, the in-situ performance of four modified venturi tubes is compared with the performance of four 8-path chordal ultrasonic flowmeters. The data analyzed were collected in the feeds of four steam generators in a large pressurized water reactor plant, each feed containing one meter of each type. The meters were initially calibrated in this series arrangement in a NIST traceable calibration lab and then operated in the same arrangement in the field.

Comparison of Different Methods for the Evaluation of Cavitation Damaged Surfaces

The experimental data which will be presented in this paper are the results of the comparison between different methods for evaluating damaged surfaces by cavitation erosion. The different methods are partly working in the initial stage of cavitation erosion and partly at developed cavitation erosion, where mass loss occurs. The used test rig consists basically of a rotating disc with a diameter of 500 mm on which four holes are located. Each hole generates a cavitation zone while the disc is rotating. The test objects are material specimens made of copper. Copper was used as test material in respect to reasonable durations for the tests. The specimen can be implemented in the casing of the test rig directly across the rotating disc on the diameter where the holes are located. This rotating disc test rig generates a very aggressive type of cavitation, so that mass loss, of course depending on the tested material, will appear after relatively short durations. Also the initial stage of cavitation erosion can be observed. The used test rig is very interesting regarding the possibility to apply different measuring techniques to characterize the erosive aggressiveness of cavitation. These techniques are at first the so-called Pitcount-Method, which allows investigations of cavitation erosion in the initial stage. The second one is an acoustic method, which is based on a structure-borne noise sensor and a specially developed signal processing system. The third method is the measuring of mass loss of the material specimen after several time steps. With the help of a CCD-camera and special digital image processing software, images of different cavitation conditions were recorded. The information obtained from these images should serve as support for the evaluation of the other used methods. After the evaluation with the above mentioned methods, the specimens were evaluated with a special device which works with the help of a white light interferometer. With this evaluation method three-dimensional information can be obtained in respect to the actually eroded volume of the specimens. With this information the lost mass of the specimens could be calculated directly. Especially the comparison of the results obtained from the Pitcount-Method, which is a two-dimensional evaluation method, and the three-dimensional results of the white light interferometer is an important point of the work within this paper.

Diesel Droplet Diffusion in Isotropic Turbulence With Digital Holographic Cinematography

High-speed in-line digital holographic cinematography was used to investigate the diffusion of droplets in locally isotropic turbulence. Droplets of diesel fuel (0.3–0.9mm diameter, specific gravity of 0.85) were injected into a 37×37×37mm3 sample volume located in the center of a 160-liter tank. The turbulence was generated by 4 spinning grids, located symmetrically in the corners of the tank, and was characterized prior to the experiments. The sample volume was back illuminated with two perpendicular collimated beams of coherent laser light and time series of in-line holograms were recorded with two high-speed digital cameras at 500 frames/sec. Numerical reconstruction generated a time series of high-resolution images of the droplets throughout the sample volume. We developed an algorithm for automatically detecting the droplet trajectories from each view, for matching the two views to obtain the three-dimensional tracks, and for calculating the time history of velocity. We also measured the mean fluid motion using 2-D PIV. The data enabled us to calculate the Lagrangian velocity autocorrelation function.

Computational Analysis of High Speed Flow Over a Double-Wedge for Air as Working Fluid

The work to be presented herein is a Computational Fluid Dynamics analysis of flow over a 15-degree angle double wedge for a compressible air, with Mach number of 2.95. The problem to be solved involves formation of shock waves, expansion fans and slip surfaces, so that the general characteristics of supersonic flow are explored through this problem. Shock waves and slip surfaces are discontinuities in fluid mechanics problems. It is essential to evaluate the ability of numerical technique that can solve problems in which shocks and contact surfaces occur. In particular it is necessary to understand the details of developing a mesh that will allow resolution of these discontinuities. Results including contour plots of pressure, temperature, density and Mach number will show that CFD is capable of predicting accurate results and is also able to capture the discontinuities in the flow, e.g., the oblique shock waves and the slip surfaces. The global comparison of some parameters between the numerical and the analytical values show a good agreement.

A Study of Optimization of Blade Section Shape for a Steam Turbine

In this study, an optimization design method is established for a rotor blade of a Curtis turbine. Bezier curve is generally used to define the profile of turbine blades. However, this curve is not proper to a supersonic impulse turbine. Section shape of a supersonic turbine blade is composed of straight lines and circular arcs. That is, it has several constraints to define the section shape. Thus, in this study, a blade design method is developed by using B-spline curve in which local control is possible. The turbine blade section has been changed by varying three design parameters of exit blade angle, stagger angle and maximum camber. Then flow analyses have been carried out for the sections. Lift-drag ratio of the blade section is used as the object function, and it is maximized in the optimization. Second-order response surface model is employed to express the object function as a function of design parameters. Central composite design method is used to reduce the number of design points. Then, an evolution strategy is employed to obtain the optimized section of the Curtis turbine blade.

Ambient Temperature Effects Upon the Flow in Gas Pipelines at Low Speeds

The flow field inside gas pipeline meter run is numerically simulated to determine the affects of upstream piping and temperature differences between the meter run pipe and the gas upon the flow. At bulk averaged velocities below 0.6 m/s (2 ft/s) significant changes in the velocity field are present which may alter the response of any flow meter mounted in the meter run. Examples for a bulk average velocity of 0.15 m/s (1/2 ft/s) and temperature differences with magnitudes of 27.7°C (50°F) are presented.

Effect of Tooth Bending Damage on the Leakage of Straight-Through Labyrinth Seals

Unavoidable rotordynamic impacting on labyrinth seal teeth sometimes occurs when centrifugal compressors, for example, undergo transients. Consequently, the labyrinth seal teeth are damaged or disfigured in various ways when the surface opposite to the teeth is non-abradable. Thus far, no quantitative information concerning the effect on seal leakage is available. The present work focuses on the effect of seal leakage due to such permanently bent labyrinth seal teeth. The investigation was done numerically by solving the 2-D, axisymmetric RANS equations with a finite-volume algorithm. The high-Reynolds number k-ε turbulence model was used with standard wall functions. A broad variety of tooth seal bending was studied by varying the bending curvature and the length of bending, as well as the after-bend tooth radial clearance. The results show that the bending damage drastically affects the leakage as well as the flow pattern. This is due largely to the altered clearance caused by the bending. However, other bending factors, such as the bending curvature and the percentage of tooth length that is bent, also contribute to the change of leakage and flow pattern.

Influence of Teeth Damage on Rotordynamic Instability of Impeller Eye Seals in Centrifugal Compressors

This paper investigates the influence of labyrinth seal teeth damage due to rotor impacting on the performance and the rotordynamic characteristics of impeller eye seals in centrifugal compressors. A well-established CFD-perturbation model was employed to predict the rotordynamic coefficients. The inclusion of at least an approximate shroud leakage path chamber is prefered for accurate prediction of seal-inlet swirl velocity and flow-induced rotordynamic forces. Impeller eye seals with teeth damage: (a) suffer significant leakage increases due to the increased seal clearance and (b) produce higher seal-inlet swirl velocity as well as larger rotordynamic forces, which tend to cause the system to become unstable. It was also found that distorted teeth tip geometries have an insignificant influence on both leakage and rotordynamic coefficients. The leakage path influence on seal-inlet swirl velocity W0 was also explored to thoroughly understand the rotordynamic characteristics of the eye seal subject to various degrees of teeth damage.