Sub-Optimal and Optimal Control of Unsteady Boundary-Layer Separation
Funded by: Engineering and Physical Sciences Research Council (EPSRC), UK
Professor Kevin Cassel and Chetan Sardesai

A two-dimensional channel with localized suction from the upper surface is considered as a framework within which to consider sub-optimal and optimal control of an unsteady separating boundary layer. A quasi-steady (sub-optimal) approach is adopted in which the control input is optimized at each time as the unsteady flow evolves. Two cost functionals are implemented and compared; they both use a boundary-based performance measure that minimizes the difference between the wall shear stress and a target distribution corresponding to the unseparated boundary layer. The two control mechanisms considered, with corresponding penalty functions in the respective cost functionals, are a domain-based control, involving a body force throughout the boundary layer, and a boundary-based control, involving the normal wall velocity. The sub-optimal control results will be compared with a fully nonlinear optimal control approach using the unsteady boundary-layer equations as the governing state equations.

A Computational Model of Cephalic Arch Hemodynamics in Arterial Venous Fistulas
Funded by: University of Chicago
Professor Kevin Cassel, Michael Boghosian, and Mary Hammes (UoC)

A computational fluid dynamics (CFD) model is being developed of the hemodynamics within the cephalic arch after insertion of an arterial venous fistula (AVF). This model will allow for determination of the representative geometric and flow features within the cephalic arch that may contribute to development of cephalic arch stenosis (CAS) in dialysis patients after fistula creation, which dramatically alters the hemodynamics in the cephalic arch. An accurate in vivo model of hemodynamics will provide a data platform on which to improve the overall design of fistula implants based on a fundamental understanding of the factors that influence their patency.

Level-Set Method for Simulation of Two-Phase Flow in Micro- and Macro-Scale Heat Transport Devices
Funded by: NASA
Professor Kevin Cassel, Professor Jamal Yagoobi, and Michael Dominik

A fundamental understanding of the electrically driven dielectric liquid film flow in two-phase micro- and macro-scale heat transport devices is sought. The level-set method is incorporated with computational fluid dynamics (CFD) to investigate various electrode geometries and optimize their performance for two-phase EHD conduction pumping phenomena. The numerical data will be compared with experimental that are being obtained in parallel. This study will improve our understanding of the interaction between electric fields with heat transfer and mass transport. For example, in microdevices, the interaction between the double layer and heterocharge layers will be elucidated allowing for design of devices with improved efficiency.

High Reynolds Number Wall-Bounded Turbulence
Professor Hassan Nagib

Nearly all currently used commercial codes for computation of flow in applications including aeronautics, energy generating machines, and weather prediction rely on the von Karman constant. We examine the overlap parameters of the logarithmic law for available experimental and computational date from turbulent boundary layer, pipe, and channel flows, over wide ranges of Reynolds numbers, using composite profiles fitted to the mean velocity. This reveals that boundary layers with streamwise pressure gradients, and pipe and channel flows display von Karman coefficients that are not universal. Therefore, we conclude that the von Karman constant exhibits dependence on not only the pressure gradient but also the wall-bounded flow geometry, thereby raising fundamental questions regarding turbulence flow theory and modeling for all wall-bounded flows. Along the way, we also examine various alternatives to the log law, including various forms of power laws, and conclude that there is no reason to abandon a perfectly coherent and successful leading order model (the von Karman-Millikan-Rotta-Clauser log law) for descriptions which have problems modeling available data, and in some cases promise a final asymptotic state we will probably never be able to verify.

Flow Control of Separation and Circulation and their Impact on Improved Aerodynamic Performance
Professor Hassan Nagib

Zero-net mass flux oscillatory jets introduced from span wise slots at various locations on the upper surface of steady and oscillating airfoil models are shown to be effective in controlling lift, moment and drag coefficients over the range of Mach numbers over 0.4. This control is demonstrated over a wide range of mean angles of attack from light to deep stall conditions on several airfoil cross sections with and without flaps. With non-dimensional frequency and amplitude of the forcing unchanged, we find comparable modifications of the aerodynamic coefficients throughout this Mach number range. Near the higher end of this Mach number range, local supercritical conditions are experienced near the leading edge and shocks are present. Even in these cases the flow control was found to be effective with slot positions near the location of the shock. Therefore, it appears that this active flow control technique is only limited by the ability to generate the adequate forcing conditions at the higher Mach numbers required for applications such as rotorcraft, and aircraft requiring high lift for short takeoff and landing or controllable drag for rapid maneuverability.

Jet and Cavity Noise Suppression Using High Frequency Excitation & Ultrasonic Actuators
Funded by: AFOSR and Lindbergh Foundation
Professor Ganesh Raman, Shekhar Sarpotdar, Kedar Chaudhari, and Rakesh Ramachandran

This project explores new methods to reduce the aeroacoustics caused by aircraft, including noise from jets and cavities using high frequency actuators. Instead of the current passive flow control techniques, like lobed nozzles and chevrons, which are unable to adapt to changing working loads of the aircraft, there are plans to develop an active flow control actuator that can be turned on during take-off and turned off during cruising time, to retain maximum fuel economy. In addition, the high frequency fluidic flow control actuators have no moving parts, making the operation simple and highly reliable. A high bandwidth powered resonance tube actuator that is potentially useful in noise and flow control applications has also been developed and characterized, under funding from the United States Air Force.

Integration of Wind Turbines with Tall Buildings
Funded by: US Department of Energy and IIT ERIF
Professor Dietmar Rempfer

This research is developing design concepts for energy-sustainable in order to develop practical methods for the integration of wind power plants into the design of tall buildings ("sky scrapers"). The intention is to size the power plants such that they can provide at least enough power to make the building energy autonomous on average, possibly even providing some excess energy that can be sold and fed into the public power grid, while at the same time optimizing the design subject to a number of disparate requirements.

Optimization of Vertical and Shrouded Wind Turbines
Funded by: US Department of Energy
Professor Dietmar Rempfer

This project is interested in optimizing the power output of wind turbines, focusing on two particularly important issues:

• Finding optimal ducts that can be used in conjunction with either horizontal- or vertical-axis machines (HAWTS or VAWTS), which will allow the wind turbines to produce energy in excess of the Betz limit. Computational aerodynamics in conjunction with optimization algorithms are being used for this work.
  
  
• While HAWTs are well developed at the current technological state of the art, and can produce power output close to the theoretical limit, VAWTs are much less understood from a fundamental aerodynamics point of view. In particular, typical power coefficients of VAWTs are lower by a factor of about two than the ones for HAWTs. Detailed investigations of the aerodynamics of such turbines are therefore being performed, followed by an optimization of their power output with the goal of achieving performance similar to the one of HAWTs.

Development of Spectral Element Code for Unsteady Geometries
Professor Dietmar Rempfer

This project has as its goal the development of a highly accurate computational fluid mechanics code that will be based on a spectral element approach. In contrast to existing codes of this type, we will allow grids that vary in time, and that include portions of the grid that are in relative motion with respect to other grid regions. This will allow us to perform accurate simulations of flow through rotating machinery as well as, ultimately, flow through or around deformable geometries. For high Reynolds numbers we will also include the option of sub-grid scale modeling via a scale-independent LES (Large Eddy Simulation) approach.

Remote Flow Sensing of Complex Systems: Steps towards Contaminant Dispersion Modeling at the Urban Scale
Funded by: Illinois Consortium of the NASA Space Grant
Professor Candace Wark, Bruno Monnier, and Paritosh Mokhasi

Contaminant dispersion at the urban level has become a major concern in recent decades. Pollutant and toxic chemical releases, intentional or not, need to be monitored and detected quickly. Prediction of the flow field in an urban area based on a few measurements is essential in order to provide the best possible response. To improve on existing solutions, a combination of various measurement tools (Hot Wire Anemometry, Stereoscopic Particle Imagery, Static Pressure measurements via an array of microphones & Laser Doppler Velocimetry) is being used to study the airflow around wall mounted obstacles in a turbulent boundary layer. To make the problem more tractable, Proper Orthogonal Decomposition is used to lower the dimensionality of the problem in projecting the velocity field onto a set of optimal basis functions and associated coefficients. The challenge lies then in finding the best way to correlate the various information to construct accurate dynamical models, hybrid measurement models (direct or indirect) and state space models that will provide robust remote flow sensing capability.