Research Interests and Projects

David Sumner, Ph.D., P.Eng., Professor

Department of Mechanical Engineering, University of Saskatchewan

My research program is focused on fundamental aspects of engineering fluid mechanics related to bluff-body aerodynamics, fluid-structure interactions, and unsteady flows.  Of particular interest is the study of the fluid-structure interactions for groups of closely spaced bluff bodies immersed in uniform, non-uniform, and unsteady flows, and improving the physical understanding of complex three-dimensional separated flows.

Groups of bluff structures in these types of flows are found in many engineering applications, such as buildings, chimney stacks, offshore drilling platforms, bridge pylons, and heat exchangers.  Boundary layer separation, shear layer interaction, vortex shedding, and turbulent wakes are some of the features that characterize these flows.  For a group of chimney stacks, further complexity is introduced by the elevated jets or plumes.

The study of these fluid-structure interactions is motivated by practical industrial concerns related to flow-induced vibrations, streambed erosion, mixing and dispersal of pollutants in the atmosphere, stack downwash, and wind loading.  Prevention of flow-induced vibrations, application of flow control techniques, more effective pollution dispersion, and strategies for reducing aerodynamic drag require an understanding of the physics of the flow fields.  This understanding is needed to interpret experimental data, guide the researcher making new measurements, establish design codes and guidelines (for groups of stacks, for example), and to develop computational fluid dynamics (CFD) models. The effects of temporal and spatial non-uniformity in these flows, often encountered in engineering applications, are poorly understood and require further study.

My research group has made key contributions to the literature, or is focused on research projects, in the following areas:

• Interference effects for groups of bluff bodies
• Wakes of surface-mounted bluff bodies
• Local flow field of rooftop-mounted chimney stacks
• Unsteady flow and fluid-structure interactions involving bluff bodies
• Passive flow control devices for surface-mounted bluff-bodies
• Roughness effects in wall-bounded flows
• Behaviour of convection columns in wildfires

Bluff bodies are structures with non-streamlined shapes that significantly resist the motion of a moving fluid.  A region of separated flow occurs over a large portion of the surface of a bluff body, which results in a high pressure drag force and a large wake region.  The flow often exhibits unsteadiness in the form of periodic vortex formation and shedding.  Roshko (1954) lists three main characteristics of the flow past two-dimensional (or "infinite") bluff bodies (or cylinders):

(i)  The "bluffness" of a cylinder is related to the width of the wake, compared with the cylinder dimension.  It is almost intuitive that the bluffer body tends to diverge the flow more, to create a wider wake, and to have larger drag;

(ii)  The shedding frequency is related to the width of the wake, the relation being roughly inverse, so that the bluffer bodies have the lower Strouhal numbers;

(iii)  For a given cylinder the shedding frequency is related to the base pressure.  Generally, an increase in the base pressure is accompanied by a decrease in the shedding frequency (Roshko, 1954).

Roshko (1954) notes that a cylinder may be considered "a body whose cross-sectional shape is the same at every section along the span.  This is the so-called two-dimensional body."  By contrast, Paidoussis, Price and de Langre (2011) note:

A cylinder (κύλινδρος) is something that rolls, from κυλίω = to roll, and the Greeks have known for quite a while that square wheels do not roll.  From that perspective a "square cylinder" is rather oxymoronic.  Nevertheless, thanks to American unbridled optimism, such expressions as "square" and "rectangular cylinders" are commonplace in the technical literature nowadays, supplanting the word "prism".  (Paidoussis, Price and de Langre, 2011)

The complex nature of the flow fields of three-dimensional bluff bodies poses significant challenges for measurement, modeling, and visualization. The methods and approach I have adopted are founded primarily on experimental studies using a variety of complimentary research facilities and techniques available at the University of Saskatchewan.  These include a low-speed wind tunnel and a novel X-Y towing tank, and measurement systems such as thermal anemometry, multi-port pressure probes, laser Doppler velocimetry (LDV), particle image velocimetry (PIV), and stereoscopic particle image velocimetry (SPIV).  Recently, I have also become involved with some numerical modeling using large eddy simulation (LES).  My research team involves graduate students (M.Sc. and Ph.D. students), undergraduate research assistants, a full-time department assistant (technician), and collaboration with other faculty (in Mechanical Engineering, Civil and Geological Engineering, and other universities).

Paidoussis, M.P., Price, S.J., de Langre, E., 2011. Fluid-structure interactions: cross-flow-induced instabilities. Cambridge: Cambridge University Press.

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Last updated: August 3, 2011