Download - Lecture Objectives
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Lecture Objectives
• Review wall functions• Discuss: Project 1, HW2, and HW3• Project topics
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Surface boundary conditions and log-wall functions
)log(1*
2/1
EyyV
E is the integration constant and y* is a length scale
dydV
t
The assumption of ‘constant shear stress’ is used here. Constants k = 0.41 and E = 8.43 fit well to a range of boundary layer flows.
y*=(/Vt)
- von Karman's constant
2/1
tV
Friction velocityCorrection
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K- turbulence model in boundary layer
dydV
t
y
1
2/3
Wall function for
Wall function for k
2kCt
Eddy viscosity
Wall shear stress
2/1
1C
k
V
) )/V(
log(1t
2/1
EyV
)
)/(
log(12/1
2/1
EyV
)log(1*
2/1
EyyV
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HW2
• Problem 1
• Problem 2
• K- RNG vs. LES vs. DNS
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DNS • Example from the previous project class
Boundary conditions !
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LES vs. RANS• Examples form the ongoing project
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Cough simulation
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What do you see with RANS
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What do you see with LESVelocity Vorticity
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Calculation time cost Modeling of unsteady cough and particle dispersion - period of 1 minute-mesh size: 500,000; particle #: 600,000; time step: 0.001s-Computers: 4 core 3.4 MHz
10
1) RANSSteady state
calculation of background flow
3-6 hours
Conversion to unsteady
Calculation Time (2-5 day)
Cough +particle injection
(1s)
Particle dispersion
(60 s)
24-48 hours 6-12 hours
Calculation Time (2-4 weeks)
RANS to getSteady state
background flow
LES+part. inject (1s)
LES+part. dispersion
(60 s)
LES calculation to get fully developed turbulent flow
(calculation of 1500 seconds)
1~2 weeks 2-3 weeks
10
3-6 hours
2) LES
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Comparison: Velocity
LES
RANS
Measurements
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Comparison: Peak Velocity and Turbulence
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13
Comparison: Cumulative Exposure
7 micron particles
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HW3
• Questions?
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Project 1
Airpak; How to : •Define occupancy zone•Zero diffusion •Refine mesh•Define isosurface•….
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Final project topics
• You will define the project topic• Examples from previous years
– Single side natural ventilation– Atrium ventilation - design problem – Rain water collection – Ventilation effectiveness – parametric study– Surface convection– Surface mass transfer