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Greg Wright
Honors Proposal
Can Juncture Flow Drag be Reduced with Dillets
Problem:
When two surfaces which are not parallel to each other intersect, there is additional drag
due to this juncture. This drag is inevitable, but minimizing this drag will optimize the
two surfaces. A demonstration that shows this juncture between two surfaces is the effect
of water across a bridge pier and the riverbed. When the fluid reaches the pier, it can
either go around the object or up and down. The fluid does all of the above, but when the
fluid goes downward, it erodes the riverbed. This erosion effect is called scour.
This problem is seen in many objects such as the juncture between a wing and fuselage
and keel and haul of a sailboat. In these examples, this juncture produces an increase in
drag compared to that if the non-lifting surface was not present. This drag decreases the
aero or hydrodynamic efficiency, thus decreasing the performance of the object. The drag
is caused by many factors, the skin friction, the vortexes formed at the juncture (induced
drag), and pressure drag. All of these components form the total drag at the juncture. In
reducing the drags, the fluid bypassing the object will be smoother, thus allowing the
object to be able to move faster using the same amount of power used before. The
juncture between the keel and hull will be analyzed more in depth and this research will
be able to be carried over to other such junctions. In minimizing the drag at this juncture,
the boat will become more efficient in terms of speed as well as maneuverability.
Background of Related Research:
Studies have been done between the juncture of a bridge pier to the river bed in which it
sits on. At this juncture, scour occurs. When this water comes in contact with the bridge
pier, it can no longer continue on its path so one of the ways it diverts is to go down, but
with the riverbed there, the water starts to erode this. This erosion continues until
equilibrium exists. As seen in Figure 11, the scour effect is seen while the water is passing
by the pier. Also, the direction of the water is shown and its tendencies while going
around the pier. The arrows in the figure help show why scour takes place along a bridge
pier. The horseshoe vortices are a major part of scour in that they rotate the particles
away from the object. They are then swept away by the moving fluid.
Figure 1: Scour around a pier
When the water encounters the pier, it erodes part of the riverbed around the entire pier
and deposits the materials beyond the pier. This is seen in Figure 21, which illustrates an
experiment in a water flume. The flow direction is from left to right. In Figure 2, the pier
disrupts flow around the pier, as well as the surrounding area after the pier. This elevated
sand level is the result of the scour around the pier.
Flow
Direction
Figure 2: Flume experiment
Proposed Investigation:
Since nature usually keeps everything in equilibrium, the eroded shape around the bridge
pier should be in keeping with the minimum energy outlay of the flow. Optimum in this
sense is the least amount of drag around the object. Based on this idea and that of
previous work by Visser 3 on the AmericaOne team and Xiao 4, the concept of a reverse
fillet or a dillet as illustrated in Figure 3, was suggested as a means for drag reduction.
Figure 3: Dillet concept
The idea behind the experiments is that the optimum shape between two objects could be
captured by placing a hydrofoil inside a water flume with sand being the bed of the
flume. This optimum shape is would be the dillet. The water flume will provide the
moving water around the foil. The sand will erode around the foil creating the dillet. If
nature does provide the equilibrium position, a carved out shape will form around the
hydrofoil. This shape will have to be documented for its specific shape, as well as the
fluid conditions such as density, flow velocity and Reynolds number. Also, time lapse
photos will be taken in order to document what shapes it formed at various times during
the erosion. The erosion won’t happen instantly, which is why the time lapse will be very
informative as to what happens around the foil, as well as where the sand is deposited.
Once the shape around the foil is documented, extensive tests will have to be taken in
order to document the shape electronically so that tests can be done through Computer
Fluid Dynamics, similar to Xiao. Also, the hydrofoil shape will be recorded to be able to
compare the dillet shape to the size of the foil.
Timeline:

March-May: complete the water flume experiment and record the shape (not
electronically)

August-October: Record the shape in the computer

November: Run CFD with shape about a haul-foil shape (foil shape is same as
one used in flume)

December-January: Analyze data from CFD

February: Complete thesis

March: Defend thesis
References:
1) Jensen, Larsen, Frigaard, Vos, Christensen, Hansen, Solberg, Hjertager, Bove;
“Offshore Wind Turbines situated in Areas with Strong Currents”; Morten Sand
Jensen; January 2, 2006.
2) R. Balachandar, J.A. Kells; “Local channel scour in uniformly graded sediments: the
time-scale problem”; Water Sciences Group, Department of Civil Engineering,
University of Saskatchewan; April 4, 1997.
3) http://www.americaone.org/video/teamvideos-nd.html (1998).
4) Weng, Xiaoliang, “A Numerical Study of Juncture Flow with a Dillet” MS thesis,
Clarkson University, July 2006.