Current Research Projects
Applications of Active Flow Control to Unsteady Flow
Funded by: AFOSR
Professor David Williams, Wesley Kerstens, Vien Quach, and Seth Buntain
This research focuses on the application of modern active flow control techniques to unsteady aerodynamics and
fluid flows. In particular, with support from the Air Force Office of Scientific Research, the group is exploring
methods to enhance the maneuverability of small unmanned and micro-air vehicles. The ability to reduce the
unsteady forces in gusting flows and the ability to execute bird-like perch landings maneuvers is also of interest.
Fundamental studies into the energy exchange mechanisms between gusting winds and micro air vehicles is also being
examined as a way of increasing the range and endurance. In addition to unsteady aerodynamics, new flow meters
with very low pressure losses and large range of application are being developed. Research into improving the
performance of axial flow compressors used on aircraft engines is also underway.
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
- 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.