Investigating Boundary Layer Separation around a
Blunt Body
Sayuri Yapa1, Patrick Eddy2, Suresh Dhaniyala3
Mechanical and Aeronautical Engineering, Clarkson University
Aerosol particles are ubiquitous in the atmosphere and play an important role in local
visibility, human health, and global climate [4]. Particles are produced as a result of a variety of
natural and anthropogenic processes, such as forest fires, fossil fuel combustion, ocean waves, etc
[4-6]. Their contribution to global climate is largely through interaction with solar and terrestrial
radiation [7-9] and the magnitude of their contribution is dependent on their chemical and physical
characteristics [10-15]. While particles from natural sources are still a major fraction of
atmospheric aerosol, anthropogenic emissions are increasingly important contributors to global
climate change [16].
Aerosol and clouds contribute to the radiation budget, and hence global climate, through
two effects – direct and indirect radiative forcing. Airborne particles have a direct radiative forcing
effect because they scatter and absorb solar and infrared radiation in the atmosphere. The
complicated interaction of aerosol particles with the formation and precipitation efficiency of
liquid-water, ice, and mixed-phase clouds causes an indirect radiative forcing associated with these
changes in cloud properties. The indirect aerosol effect is estimated to be comparable to the
contribution of greenhouse gases. The large uncertainties in estimating the indirect aerosol effect is
due to a limited understanding of cloud formation processes.
Cloud formation usually requires the presence of particles that act as cloud condensation
nuclei (CCN). CCN are particles that contain water-soluble species [e.g., 19,20] and their
conversion to cloud droplets is a complicated function of atmospheric aerosol characteristics and
meteorological factors [4,21]. While the influence of aerosol physical characteristics on cloud
formation is reasonably known [21], the role of particle chemical characteristics on CCN activity
and cloud formation remains a major source of uncertainty. Accurate measurement of aerosols and
1
Mechanical and Aeronautical Engineering, Honors Student
Mechanical and Aeronautical Engineering, PhD. Student
3
Mechanical and Aeronautical Engineering, Professor
2
clouds in the atmosphere will enable accurate characterization of cloud processes and their role in
global climate.
For in-situ measurement of clouds, aircrafts are required. However, aircraft sampling
always disturbs the clouds that are being sampled, and hence potentially changing their
characteristics. In particular, cloud droplets are large in size and disintegrate on contact with the
sampling surfaces. This produces a large number of shatter particles, and contaminates any
measurement of smaller sized particles (non-activated particles or interstitial particles) that may
also be present in cloud systems. The study of interstitial particle characteristics will help
determine the role of different aerosol particles in cloud process. However, the problem of large
particle splatter has prevented any substantial study of these particles from aircraft platforms.
For interstitial particle sampling, a new blunt body sampler is being designed. The goal is to create
a region of large particle shadow behind a blunt body where activated cloud droplets do not enter or
affect, but where interstitial, or “non-activated” particles are at ambient concentrations.
When particles with large inertia (for aircraft speeds, particles greater than ~ 10m are
considered large) are forced around a blunt body at high speeds, two effects are possible. Particles
directed towards the body “splatter”, while those not directed at the blunt body will pass through
along a path of “Line of Sight”. An assumption is made that these particles lose their initial
momentum as they hit the blunt body. If the particle does not break, then the particle will probably
rejoin the large particles that have missed the blunt body and pass by the blunt body. However, if a
large particle such as a water droplet or ice particle does break up into many smaller particles,
another assumption is made that the particles stay in the boundary layer of the blunt body
(preliminary simulation results indicate that this is true).
The boundary layer is an area directly next to the surface of the body. In flows of high
Reynolds numbers, the flows can be divided into two regions: inside the boundary layer and outside
the boundary layer. Inside the boundary layer is where the forces and characteristics of friction and
viscosity are most important. The flow of the fluid will follow the shape of the surface of the body
until the energy of the flow is not sufficient to force the flow to stay attached to the body. At this
point, the flow breaks off from the surface of the body and follows the free stream. This point is
called separation. (Schlichting, 2003, and White, 1999).
The design of the instrument for cloud particle sampling is based on the theory that the
splatter particles will stay within the boundary layer of the sampler. By being able to predict and/or
control the boundary layer of the sampler, it becomes possible to control where shattered particles
go. This summer’s research is a start to the development of the cloud sampler. After placing a
cylindrical model of the cloud sampler in the wind tunnel, both quantitative and qualitative results
can be compared to previously established experimental results as well as numerical simulation
results as proof that the wind tunnel experiments are accurate. Because the cylinder has been quite
widely studied, it is most convenient to compare published results with experimental results
achieved in the wind tunnel. After a sufficiently close comparison has been established, it will then
become possible to branch out into different shapes to see if boundary layer control and prediction
can be more easily established with one shape than another.
A good portion of the research done this summer was developing a flow visualization
system for the wind tunnel. Although smoke generation (Merzkirch, 1987) was at first considered,
it became clear that this was not possible for a number of operational reasons. However, using an
ultrasonic humidifier, various widths and lengths of Tygon tubing, a uniquely designed model, a
laser system, and a digital camera, a more than satisfactory set up was achieved. After the
collection and analysis of this data in the form of digital images, experimental results achieved in
the wind tunnel can be compared to numerical and previously established experimental results.
A computational fluid dynamics software (FLUENT) was used to numerically obtain the
flow over the cylinder and other blunt bodies placed in the tunnel. The computational geometries
were refined and optimized for boundary layer calculations. Because of the large number of
resources and long computational time required, these meshes were refined to the minimal point to
result in data that made physical sense, and could be compared to existing literature (Van Dyke,
1997). In order to make sure that the results of the experiments run in the wind tunnel and the
numerical simulations could be accurately compared, it was necessary to run the simulation at
tunnel conditions of wind speed of 40 m/s and a tunnel pressure of 99 kPa. For these conditions,
the boundary layer separation point of around 104-105 degrees was obtained around the
circumference of the cylinder. This directly matched with the experimental results displayed in
Schlichting, 2003, which showed a boundary layer separation of 104.5 degrees at high Reynolds
numbers (Re > 1 x 106 ). After running the wind tunnel at the same conditions as the simulation,
digital images can be captured of the flow visualization around the cylinder using a laser system.
Once these images are analyzed, a more finite conclusion concerning boundary layer separation can
be reached.
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