Flow Physics of Fluidically Controlled Attachment in Separation Cells

Internal flows subjected to adverse pressure gradients are susceptible to three-dimensional separation on flow boundaries that can result in flow instabilities and significant losses. Active, surface-integrated flow control offers an attractive approach for mitigating these adverse effects by delaying separation or bypassing it altogether. The present investigations focus on the interactions between a separation cell that forms over a diffuser surface and a spanwise array of fluidically oscillating jets that lead to flow attachment with specific emphasis on understanding the actuation-induced changes in the structure and dynamics of the base flow. These effects are investigated in two diffuser configurations having significant differences in their inlet conditions namely, an open-end diffuser duct branching from a channel, and a curved surface insert that forms a diffuser within a channel using planar and stereo particle image velocimetry. Actuation is effected by spanwise arrays of surface-integrated fluidically oscillating jets that issue tangentially to the diffuser’s surface. It is shown that separation cells formed in the adverse pressure gradient are receptive to fluidic actuation and that increasing actuation strength incrementally delays separation by the manipulation of the flow dynamics in the vicinity of separation and creation of spanwise concentrations of streamwise vorticity that subdivide the separation cell of the base flow into smaller spanwise-periodic reattachment cells that mitigate the adverse effects of reversed flow along the surface. The demonstrated control of separation indicates that these active flow control technologies have the potential for improving system performance in multiple internal flow applications including diffusers, flow diverters, and engine inlets.Ph.D

Characterizing the Unsteady Dynamics of Cylinder-Induced Shock Wave/Transitional Boundary Layer Interactions Using Non-Intrusive Diagnostics

The objectives of this study were to provide time-resolved (1) characterizations of shock wave/transitional boundary layer interactions using schlieren flow visualization, and (2) correlations of unsteady shock motion to boundary layer features. The characteristics of cylinder-induced shock wave/transitional boundary layer interactions in a Mach 2 freestream flowfield were studied experimentally. The Reynolds number in the Mach 2 facility was 30,000,000 m-1. Incoming boundary layers were in transitional and fully turbulent states. Characterizing the shock wave motion was based on tracking the position of the shock wave on the model surface in schlieren images. The motion of the shock waves revealed an high-intensity resonance. When analysis of high-speed schlieren images were combined with unsteady pressure-sensitive paint studies, it was concluded that upstream scaling exhibited characteristics of laminar flow interactions, whereas the downstream separation mirrored turbulent interactions. This high-intensity resonance was duplicated using a blunt fin shock generator and an axisymmetric model. Furthermore, the unsteady dynamics of a boundary layer separation precursor upstream of the separation shock was highly correlated to the motion of the upstream influence (UI) shock and separation shock. The motion of the UI shock, separation shock and boundary layer separation precursor suggest that the unsteadiness in transitional interactions was driven by instabilities in the boundary layer. An initial characterization with changing Reynolds number and edge Mach number was made in the appendix

Minnowbrook V: 2006 Workshop on Unsteady Flows in Turbomachinery

This volume contains materials presented at the Minnowbrook V 2006 Workshop on Unsteady Flows in Turbomachinery, held at the Syracuse University Minnowbrook Conference Center, New York, on August 20-23, 2006. The workshop organizers were John E. LaGraff (Syracuse University), Martin L.G. Oldfield (Oxford University), and J. Paul Gostelow (University of Leicester). The workshop followed the theme, venue, and informal format of four earlier workshops: Minnowbrook I (1993), Minnowbrook II (1997), Minnowbrook III (2000), and Minnowbrook IV (2003). The workshop was focused on physical understanding of unsteady flows in turbomachinery, with the specific goal of contributing to engineering application of improving design codes for turbomachinery. The workshop participants included academic researchers from the United States and abroad and representatives from the gas-turbine industry and U.S. Government laboratories. The physical mechanisms discussed were related to unsteady wakes, active flow control, turbulence, bypass and natural transition, separation bubbles and turbulent spots, modeling of turbulence and transition, heat transfer and cooling, surface roughness, unsteady CFD, and DNS. The workshop summary and the plenary discussion transcripts clearly highlight the need for continued vigorous research in the technologically important area of unsteady flows in turbomachines. This volume contains abstracts and copies of select viewgraphs organized according to the workshop sessions. Full-color viewgraphs and animations are included in the CD-ROM version only (Doc.ID 20070024781)

An experimental investigation of cavity flow

Of particular interest are the flow structure and dynamics associated with open shallow rectangular cavities at low Mach numbers for various length-to-depth ratios. At the Reynolds number investigated, it is the presence of convective instabilities through the process of feedback disturbance that gives rise to a globally unstable flowfield. Using an instrumental wing model with a cut-out an experimental investigation of a cavity flowfield exhibiting ‘fluid-dynamic’ phenomenon has been completed. A post-processing module for the PIV image data was constructed which optimised the data fidelity and accuracy while improving upon velocity spatial resolution. These improvements were necessary to capture the flow scales of interest and minimise the measurement error for the presentation of velocity, velocity-derivative and turbulent statistics. It is shown that the hydrodynamics that is intrinsic to the cavity flowfield at these inflow conditions organises the oscillation of small- and large-scale vortical structures. The impingent scenario at the downstream edge is seen to be crucially important to the cavity oscillation and during the mass addition phase a jet-edge is seen to form over the rear bulkhead and floor. In some instances this jet-like flow is observed to traverse the total internal perimeter of the cavity and interact with the shear layer at the leading edge of the cavity, this disturbs the normal growth of the shear layer and instigates an increase in fluctuation. The coexistence and interplay between a lower frequency mode dominant within the cavity zone and the shear layer mode is seen to shed large-scale eddies from the cavity. This modulation imposes a modification to the feedback signal strength such that two distinct states of the shear layer are noted. Concepts for the passive reduction of internal cavity fluctuation are successful although modifications to the shear layer unsteadiness are encountered; an increase in drag is implied

An investigation of the flow characteristics in the blade endwall corner region

Studies were undertaken to determine the structure of the flow in the blade end wall corner region simulated by attaching two uncambered airfoils on either side of a flat plate with a semicircular leading edge. Detailed measurements of the corner flow were obtained with conventional pressure probes, hot wire anemometry, and flow visualization. The mean velocity profiles and six components of the Reynolds stress tensor were obtained with an inclined single sensor hot wire probe whereas power spectra were obtained with a single sensor oriented normal to the flow. Three streamwise vortices were identified based on the surface streamlines, distortion of total pressure profiles, and variation of mean velocity components in the corner. A horseshoe vortex formed near the leading edge of the airfoil. Within a short distance downstream, a corner vortex was detected between the horseshoe vortex and the surfaces forming the corner. A third vortex was formed at the rear portion of the corner between the corner vortex and the surface of the flat plate. Turbulent shear stress and production of turbulence are negligibly small. A region of negative turbulent shear stress was also observed near the region of low turbulence intensity from the vicinity of the flat plate

On the aerodynamic performance of automotive vehicle platoons featuring pre and post-critical leading forms

A considerable body of work exists concerning the aerodynamic optimisation of the vehicle form in isolation. Some valid generalised conclusions have been reached concerning optimal and sub-optimal key vehicle geometries and their relevant flow mechanics; generalised test forms representing various characteristic vehicle geometries - “squareback”, “notchback” and “fastback” - have been developed and extensively studied, with critical geometries highlighted. The study of organised vehicle convoys has similarly been researched since the early 1970’s primarily as a means to increase traffic throughput on existing road arterials, with ultimate “future-generation intelligent transport systems” envisioning scenarios where vehicles on major arterials may travel under fully automatic control, allowing possibilities in vehicle organisation not previously envisioned. Initial research simply considered reducing the spacing between vehicles travelling in localised groups to similar destinations - “platoons” - with traffic throughput scaling positively with platoon length and reduced spacing. The significant majority of research in this field is dedicated to developing concepts that increase traffic throughput; aerodynamic concerns are only recently being explored, however it is clear from relevant research concerning tandem bluff bodies that aerodynamic interaction is heightened with closer proximity. A variety of recent studies examining aerodynamic force effects in platooning confirm advantages for all vehicles in homogenous platoons of squareback and notchback geometries. The case for fastback geometries is unclear, with preliminary studies suggesting that there can be an increase in the drag force of trailing vehicles in the wake of a fastback geometry. The work presented explores the fundamental phenomena underscoring the performance of two fastback vehicles travelling in close proximity. Vehicles are simulated using a well-known reference automotive form. A primary extension to existing works concerns effect of changing the leading vehicles geometry to one of two different (yet practically characteristic) fastback configurations, constituting an important variable known to offer (in isolation) two markedly unique flow structures and drag force coefficients. A series of wind-tunnel experiments were performed where rear slantback angles were varied and measurements of pressures, forces and flow visualisations were made on upstream and downstream models in addition to interrogation of the intervening gap flow field. It is demonstrated that irrespective of the upstream models form (and thus irrespective of dominant flow phenomena for such a model considered in isolation), force characteristics remain broadly similar for leading and trailing models in the platoon, primarily owing to the development of streamwise vortices originating from the C-pillar of the leading model which are shown to entrain a high-momentum flow between them, impinging on the trailing model forebody. A variety of methods - from qualitative flow visualisation to spectral methods applied to dynamic data - are employed to demonstrate that even at the closest spacing examined, salient flow phenomena of the leading and trailing models are broadly retained. A detailed investigation of gap flows and trailing model spectra effects as a function of leading model geometry and model spacing is also presented

Advanced Fluid Dynamics

This book provides a broad range of topics on fluid dynamics for advanced scientists and professional researchers. The text helps readers develop their own skills to analyze fluid dynamics phenomena encountered in professional engineering by reviewing diverse informative chapters herein

Shear layer dynamics of a reacting jet in a vitiated crossflow

The jet in crossflow (JICF) is a canonical shear flow that is present in a number of practical configurations including industrial gas turbines. Its complex flow topology, heavily influenced by underlying hydrodynamic instabilities, makes it an attractive configuration to implement when the mixing performance is critical. Past studies analyzing the behavior of non-reacting jets have noted that the overall performance of JICF configurations can be tied to the behavior of the shear layer, which influences both near-field and far-field jet dynamics. As a result, techniques used to manipulate jet mixing and penetration, such as active jet modulation, require an understanding of the dominant instability characteristics of the shear layer. Although this configuration finds extensive use in reacting applications, the hydrodynamics of reacting flows are often fundamentally different from non-reacting flows, and few studies have analyzed the influence of heat release and reactions on JICF dynamics. In addition to varying the momentum flux ratio (J) and the density ratio (S) this study presents a novel method of systematically moving the flame position with respect to the shear layer to gauge its impact on shear layer stability. High speed optical diagnostics including Stereoscopic PIV, OH-PLIF and OH* chemiluminescence were used to quantify the flowfield and infer the behavior of the reaction zone. Moving the flame inside the shear layer was observed to significantly change the jet topology as the shear layer vortices (SLV) were completely suppressed. This was further quantified through a growth rate defined based on tracking the swirling strength of SLV structures. Other structural characteristics including the location of mixing transition were shown to be highly correlated with this extracted growth rate. Time-resolved velocity data was further used to quantify the shear layer spectrum by extracting the dominant instability frequencies and classify the instability behavior as convectively and globally unstable. In order to explain the observed instability behavior, the counter current shear layer (CCSL) model was used to extract an analogous stratification parameter (S’), which along with the counter current velocity (Λ) ratio was shown to capture the stability behavior of both non-reacting as well as reacting configurations.Ph.D