Ralph Budwig

Refractive index matching methods for liquid flow investigations

A difficulty common to most optical diagnostic techniques that are applied to fluid dynamics studies is the refraction of light passing through model and/or test section walls. The method of choice to eliminate refraction problems in liquid flows is to match refractive index. This paper presents techniques for refractive index matching including, (i) arrangement of test section and model, (ii) choice of solid and liquid materials, and (iii) methods for tuning the match. In addition, a new application of refractive index matching to liquid-liquid droplet studies is presented.

The Influence of Shape on the Stresses in Model Abdominal Aortic Aneurysms

Presence of a small abdominal aortic aneurysm (AAA) often presents a difficult clinical dilemma--a reparative operation with its inherent risks versus monitoring the growth of the aneurysm, with the accompanying risk of rupture. The risk of rupture is conventionally believed to be a function of the AAA bulge diameter. In this work, we hypothesized that the risk of rupture depends on AAA shape. Because rupture is inevitably linked to stress, membrane theory was used to predict the stresses in the walls of an idealized AAA, using a model which was axisymmetric and fusiform, with the ends merged into straight opened-ended tubes. When the stresses for many different shapes of model AAAs were examined, a number of conclusions became evident: (i) maximum hoop stress typically exceeded maximum meridional stress by a factor of 2 to 3 (ii) the shape of an AAA had a small effect on the meridional stresses and a rather dramatic effect on the hoop stresses, (iii) maximum stress typically occurred near the inflection point of a curve drawn coincident with the AAA wall, and (iv) the maximum stress was a function--not of the bulge diameter---but of the curvatures (i.e. shape) of the AAA wall. This last result suggested that rupture probability should be based on wall curvatures, not on AAA bulge diameter. Because curvatures are not much harder to measure than bulge diameter, this concept may be useful in a clinical setting in order to improve prediction of the likelihood of AAA rupture.

Model studies of the flow in abdominal aortic aneurysms during resting and exercise conditions.

Pulsatile flow in abdominal aortic aneurysm (AAA) models has been examined in order to understand the hemodynamics that may contribute to growth of an AAA. The model studies were conducted by experiments (flow visualization and laser Doppler velocimetry) and by numerical simulation using physiologically realistic resting and exercise flow conditions. We characterize the flow for two AAA model shapes and sizes emulating early AAA development through moderate AAA growth (mean and peak Reynolds numbers of 362 < Re(mean) < 1053 and 3308 < Re(peak) < 5696 with Womersley parameter 16.4 < alpha < 21.2). The results of our investigation indicate that AAA flow can be divided into three flow regimes: (i) Attached flow over the entire cycle in small AAAs at resting conditions, (ii) vortex formation and translation in moderate size AAAs at resting conditions, and (iii) vortex formation, translation and turbulence in moderate size AAAs under exercise conditions. The second two regimes are classified in the medical literature as disturbed flow conditions that have been correlated with atherogenesis as well as thrombogenesis. Thus, AAA disturbed hemodynamics may be a contributing factor to AAA growth by accelerating the degeneration of the arterial wall. Our investigation also concluded that vortex development is considerably weaker in an asymmetric AAA. Furthermore, turbulence was not observed in the asymmetric model. Finally, our investigation suggests a new mode of transition to turbulence: vortex ring instability and bursting to turbulence. The transition process depends on a combination of the pulsatile flow conditions and the tube cross-sectional area change.

Two improved methods for low-speed hot-wire calibration

Two improved methods for hot-wire anemometer calibration in the low-speed range (15-95 cm/s) are described: a laminar pipe-flow method, and a shedding-frequency method. For the laminar pipe-flow method the calibration is performed in the exit plane of a fully-developed laminar pipe flow. For the shedding-frequency method the shedding frequency of a parallel mode cylinder wake is measured; the velocity is then obtained from a continuous Strouhal-Reynolds number relationship. Calibration results from the present methods were compared with each other and with results from a commercially available calibration jet. The present methods were demonstrated as a simple, but accurate means for low-speed hot-wire calibration.

Heat transfer in laminar, oscillatory flow in cylindrical and conical tubes

Abstract In order to assess the effect of frequency on transport of a passive scalar contaminant in an oscillatory flow, a piston-driven pipe flow is established. Two pipe test section geometries are used: one straight, round and uniform and the other uniformly tapering (i.e. conical). Flow is driven at frequencies characteristic of human breathing, both resting, normal and high frequency. A screen of closely spaced, parallel, thin wires is placed perpendicular to the flow, in the test section and is heated so as to dissipate a constant power into the fluid. The subsequent time-average and instantaneous temperature fields are measured, as functions of position. The results are shown to be consistent with the consequences of transport of heat by a combination of convection and diffusion. Convective transport is found to increase with frequency, at constant amplitude, but the effective diffusivity does not obey the predictions of theories which are based on an assumption of constant average axial gradient of the scalar field.

Ultrasonic particle size fractionation in a moving air stream

Identification of bio-aerosol particles may be enhanced by size sorting before applying analytical techniques. In this paper, the use of ultrasonic acoustic radiation pressure to continuously size fractionate particles in a moving air stream is described. Separate particle-laden and clean air streams are introduced into a channel and merged under laminar flow conditions. An ultrasonic transducer, mounted flush to one wall of the channel, excites a standing ultrasonic wave perpendicular to the flow of the combined air stream. Acoustic radiation forces on the particles cause them to move transverse to the flow direction. Since the radiation force is dependent upon the particle size, larger particles move a greater transverse distance as they pass through the standing wave. The outlet flow is then separated into streams, each containing a range of particle sizes. Experiments were performed with air streams containing glass microspheres with a size distribution from 2-22 microm, using a centerline air stream velocity of approximately 20 cm/s. An electrostatic transducer operating at a nominal frequency of 50 kHz was used to drive an ultrasonic standing wave of 150 dB in pressure amplitude. The microsphere size distributions measured at the outlet were compared with the predictions of a theoretical model. Experiments and theory show reasonable correspondence. The theoretical model also indicates an optimal partitioning of the particle-laden and clean air inlet streams.

Three-axis acoustic device for levitation of droplets in an open gas stream and its application to examine sulfur dioxide absorption by water droplets.

Two acoustic devices to stabilize a droplet in an open gas stream (single-axis and three-axis levitators) have been designed and tested. The gas stream was provided by a jet apparatus with a 64 mm exit diameter and a uniform velocity profile. The acoustic source used was a Langevin vibrator with a concave reflector. The single-axis levitator relied primarily on the radial force from the acoustic field and was shown to be limited because of significant droplet wandering. The three-axis levitator relied on a combination of the axial and radial forces. The three-axis levitator was applied to examine droplet deformation and circulation and to investigate the uptake of SO(2) from the gas stream to the droplet. Droplets ranging in diameters from 2 to 5 mm were levitated in gas streams with velocities up to 9 ms. Droplet wandering was on the order of a half droplet diameter for a 3 mm diameter droplet. Droplet circulation ranged from the predicted Hadamard-Rybczynski pattern to a rotating droplet pattern. Droplet pH over a central volume of the droplet was measured by planar laser induced fluorescence. The results for the decay of droplet pH versus time are in general agreement with published theory and experiments.

The Boundary Layer Over Turbine Blade Models With Realistic Rough Surfaces

Results are presented of extensive boundary layer measurements taken over a flat, smooth plate model of the front one-third of a turbine blade and over the model with an embedded strip of realistic rough surface. The turbine blade model also included elevated freestream turbulence and an accelerating freestream in order to simulate conditions on the suction side of a high-pressure turbine blade. The realistic rough surface was developed by scaling actual turbine blade surface data provided by U.S. Air Force Research Laboratory. The rough patch can be considered to be an idealized area of distributed spalls with realistic surface roughness. The results indicate that bypass transition occurred very early in the flow over the model and that the boundary layer remained unstable (transitional) throughout the entire length of the test plate. Results from the rough patch study indicate the boundary layer thickness and momentum thickness Reynolds numbers increased over the rough patch and the shape factor increased over the rough patch but then decreased downstream of the patch. It was also found that flow downstream of the patch experienced a gradual retransition to laminar-like behavior but in less time and distance than in the smooth plate case. Additionally, the rough patch caused a significant increase in streamwise turbulence intensity and normal turbulence intensity over the rough patch and downstream of the patch. In addition, the skin friction coefficient over the rough patch increased by nearly 2.5 times the smooth plate value. Finally, the rough patch caused the Reynolds shear stresses to increase in the region close the plate surface.

Scaling of Turbine Blade Roughness for Model Studies

The purpose of this note is to provide an approach to scaling turbine blade roughness so a large-scale experiment will yield useful results despite lack of detailed knowledge about the application. In the process, an apparently new approach for scaling of actual turbine blade roughness on an experimental model of a rough turbine blade is presented. Rough surface data from a first-stage high-pressure turbine rotor, estimates of engine operating conditions representative of high-performance aircraft, and assumed matches of the Reynolds number and acceleration parameter ranges are used. A scaling factor is determined by estimating and matching the nondimensional roughness (in wall coordinates) of a typical airfoil for a model.Copyright

THE ONSET AND DEVELOPMENT OF CIRCULAR-CYLINDER VORTEX WAKES IN UNIFORMLY ACCELERATING FLOWS

The influence of uniform flow acceleration on the stability and the characteristics of circular-cylinder wakes over a Reynolds-number range, 20<R<330, was investigated. Experiments were preformed to examine the temporal evolution of the wake before, during, and after the onset of the wake instability. We have demonstrated in several ways that the wake is stabilised by flow acceleration: (i) the onset of the wake instability occurs at larger Reynolds numbers than in the steady flow case, (ii) the closed wake develops to states that would be unstable in a steady flow, and (iii) once vortex shedding does occur there is a reduction in instantaneous Strouhal number. We have also examined the temporal growth rate of the wake instability and find that it is directly proportional to the applied flow acceleration. Physical mechanisms are proposed to describe the experimental observations.