Session ML: General Fluid Mechanics I

PIV Analysis of Prandtl's Flow Visualization Movies

Probably some of the oldest time-resolved PIV image sequences still available today were recorded on film during the late 1920's and early 1930's by Prandtl and his colleagues Tietjens and M\"{u}ller using free surface water flumes at the Kaiser-Wilhelm-Institute for Fluid Research (G\"{o}ttingen), now the Max Planck Institute for Self Organisation. Recorded at 20 frames per second the films visualize the process of unsteady flow separation and vortex generation on surface piercing objects such as airfoils or cylinders. The visualization was achieved by means of small particles (aluminum powder, ferrous mica or lycopodium powder) scattered on the water surface. Illumination from above resulted in high contrast images of the random particle distribution that are very well suited for PIV analysis. Modern PIV software is used to process digitized versions of the films, made available by the ``Institut f\"{u}r wissenschaftlichen Film'' (www.iwf.de) in G\"{o}ttingen. In addition to the surface flow field the time evolving vorticity field and other quantities can now be visualized which by itself shows the importance of carefully documenting and archiving valuable data.   

POD Analysis of Jet-Plume/Afterbody-Wake Interaction

The understanding of the flow physics in the base region of a powered rocket is one of the keys to designing the next generation of reusable launchers. The base flow features affect the aerodynamics and the heat loading at the base of the vehicle. Recent efforts at the National Center for Physical Acoustics at the University of Mississippi have refurbished two models for studying jet-plume/afterbody-wake interactions in the NCPA's 1-foot Tri-Sonic Wind Tunnel Facility. Both models have a 2.5 inch outer diameter with a nominally 0.5 inch diameter centered exhaust nozzle. One of the models is capable of being powered with gaseous $H_2$ and $O_2$ to study the base flow in a fully combusting senario. The second model uses hi-pressure air to drive the exhaust providing an unheated representative flow field. This unheated model was used to acquire PIV data of the base flow. Subsequently, a POD analysis was performed to provide a first look at the large-scale structures present for the interaction between an axisymmetric jet and an axisymmetric afterbody wake. PIV and Schlieren data are presented for a single jet-exhaust to free-stream flow velocity along with the POD analysis of the base flow field.   

Interaction of a wall jet with wake behind a bluff body in an open channel

Junction flow around a bluff body mounted in bed has been the subject of investigation for decades because of the unique phenomenon associated with the generation and dynamics of the horseshoe vortices. However, a more interesting flow could be the case where a small bleed flow in the form of a wall jet is allowed to pass beneath the bluff body and let interact with the horseshoe vortex. If this flow is strong, it can completely detoriates the effect of the horseshoe vortex and may result into a complex three-dimensional flow. To explore the characteristics of this kind of flow a sharp-edged bluff body was mounted in an open channel flow of nominal flow depth of 100 mm. Three flow configurations were examined: a) the body was mounted firmly on the floor, b) the body was lifted 5 mm from the bed and c) the body was lifted 10 mm from the bed. Particle image velocimetry measurement was performed at three horizontal planes, at $y$/$H$ = 0.10, 0.50 and 0.80, respectively, where $H$ is the nominal depth of flow. Time-averaged flow parameters were examined at selected streamwise locations at different vertical elevations to examine the effect of wall jet to change the characteristics of the wake.   

Passive Scalar Transport in Pipe Flows

The problem of passive scalar transport in pipe flows has a long standing history. Recent work has been re-examining the concentration evolution during the initial transient timescale before G.I. Taylor's 1953 theory becomes applicable. Using high resolution digital photography we experimentally investigate this transient concentration evolution in laminar pipe flow. Gravitational effects associated with non-homogeneous densities induced by the passive scalar are shown to play a role, especially at short timescales, and need to be carefully mitigated through density matching. In density matched experiments, we observe anomalous behavior in the form of the development of non-zero skewness and identify the relevant timescales of these anomalies. Comparisons with theoretical predictions, including recent advancements based on a mathematically rigorous stochastic differential equations approach, will be presented.   

Experimental and Numerical Investigation of Supercritical Carbon dioxide compact heat exchanger

The use of super-critical carbon dioxide is gaining importance because of its use in Brayton cycles, to increase the cycle efficiency and reduce the initial capital investment, for high temperature energy conversion system. In order to reduce the capital cost, one improvement which was thought, is the use of compact, highly efficient, diffusion bonded heat exchangers for the regenerators. In this presentation we will focus on the experimental measurements of heat transfer and pressure drop characteristics within mini-channels. Two test section channel geometries were studied: a straight channel and a zigzag channel. Both configurations are 0.5m in length and constructed out of 316 stainless steel with a series of nine parallel 1.9mm semi-circular channels. The zigzag configuration has an angle of 115 degrees with an effective length of $\sim $0.58m. Heat transfer measurements were conducted for varying ranges of inlet temperatures, pressures, and mass flow rates. Numerical simulations have been performed using Fluent 12.0 to complement our experimental program. This is an ongoing program and we will be showing our recent progress we have made in last six months.   

Generation of Vertical Density Layering by Moving Boundaries

A vertically moving boundary in a stratified fluid can create and maintain vertical layering of density (i.e., a horizontal density gradient). We theoretically and experimentally investigate the scenario in which the gradient of density is sharp and localized, as well as non-diffusive. A careful experimental study of a vertically towed fiber through a sharp, stably stratified, highly viscous fluid will be presented. We observe a vertical column of dense fluid entrained by the moving fiber with a well-defined diameter. We investigate various theoretical models for predicting the diameter of this column, including an approach from an unbounded fluid domain limit, a lubrication model, and finally linear stability analysis of a family of exact solutions for bounded and unbounded domains. We compare these theoretical predictions with the experimental observations.   

Sky dancer: an intermittent system

Sky dancers attract people sight to make advertising. What is the origin of those large vertical tubes fluctuations above an air blower? This study complements the previous one [1] about the system analysis from a dynamical system point of view. As a difference from the ``garden hose-instability'' [2], the tube shape has got ``break points''. Those ``break points'' separate the air-filled bottom tube portion from its deflated top portion. We record the tube dynamics with a high-speed videocamera simultaneously that we measure the pressure at the air blower exit. The intermittent pressure evolution displays picks when the tube fluctuates. We compare those overpressure values with the ones that appears in a rigid tube whose exit is partially obstructed. [1] F. Castillo Flores {\&} A. Cros ``Transition to chaos of a vertical collapsible tube conveying air flow'' \textit{J. Phys.: Conf. Ser.} \textbf{166}, 012017 (2009). [2] A. S. Greenwald {\&} J. Dungundji ``Static and dynamic instabilities of a propellant line'' \textit{MIT Aeroelastic and Structures Research Lab, AFOSR Sci.} Report: AFOSR 67-1395 (1967).   

Simplified models of high-aspect-ratio ellipsoids under shear

Inspired by rheoscopic flow visualization, we extend the classic study of the motion of small ellipsoidal particles under shear, focusing on simplifications obtained by consideration of the extreme aspect ratios typical of rheoscopic particles (e.g., Kalliroscope). In particular, the long-time behavior of scalene (triaxial) elllipsoids can in some cases be well-approximated by a low-order model in the appropriate aspect ratios. We enumerate and describe the generic long-time motions of such particles in the lowest-order model. We then investigate changes induced by inclusion of the physically-appropriate first-order correction, with particular attention to a periodic wobbling motion special to scalene (cf. axisymmetric) ellipsoids.   

Innovative Method for Greatly Reducing Flow Resistance and Obtaining Well-Ordered Continuous Flow

In this paper, firstly, the experiment on the flow resistance of the aerated pipe flow is introduced. And some experimental research on comparison between different volumes of air entrained is presented. Secondly, the characteristics of Gravity Pipe Flow under the action of Torricelli's Vacuum, shortly called as GPFUTV are dissertated, including creative and functional design, fundamental principle, etc. Under GPFUTV condition the water flow in the tube is full-pipe and continuous, colorless and non-aerated, high-speed and non-rotational as distinguished from laminar flow. Thirdly, an appeal in relation to the experimental research, the applied studies and basic theory research is given. For instance, the well-known Reynolds' experiment under GPFUTV condition, the potential for GPFUTV to be developed for deep seawater suction technology, seawater intake pipe of OTEC and lifting technology for deep ocean mining in Fe-Mn concretions, flow stability and flow resistance under GPFUTV condition, etc.   

Onsager reciprocity relations derived from Maxwell reciprocity relations

Based upon recent work by the author [H. Brenner, ``Bi-velocity hydrodynamics,'' Physica A 388 (2009) 3391-3398] devoted to the foundations of fluid mechanics and transport processes, a simple model of linear irreversible thermodynamics (LIT) is developed. On the basis of this the Onsager Reciprocal Relations (ORR) applicable to LIT are shown by purely macroscopic arguments to be a direct consequence of Maxwell's Reciprocal Relations (MRR) governing equilibrium thermodynamics. Among other things, this work answers a fundamental question raised by B. D. Coleman and C. Truesdell [``On the reciprocal relations of Onsager,'' J. Chem. Phys. 33 (1960) 28-31] regarding the need for a rational basis for choosing a compatible set of forces and fluxes for which the ORR are a priori valid.