LES OF PASSIVE SCALAR DISPERSION FROM AN AREA SOURCE Bharathi - - PowerPoint PPT Presentation

LES OF PASSIVE SCALAR DISPERSION FROM AN AREA SOURCE

Bharathi Boppana, Zheng-Tong Xie and Ian P. Castro

Introduction

• Surface heat flux :

1. Numerical Weather Prediction models. 2. Air quality modelling. 3. Improve the urban planning and design for human comfort.

• Field measurements (Louka et al. 1999,…, Offerle et al. 2007).
• Wind-tunnel experiments (Kovar-Panskus et al. 1999,….)
• Many 2D CFD studies (Ca et al. 1995,….,Cai et al. 2008).
• Very few 3D computations (Mathey et al. 1999,…,Yang & Shao 2008).

Objectives

• To resolve the thin thermal boundary layer.
• To estimate the surface heat flux accurately.
• To understand the heat affects on flow and dispersion.

Focus:

• 1. Effect of surface roughness type on canopy ventilation.
• 2. Horizontal advection and vertical dispersion of the scalar through

the urban canopy.

Test case

Pascheke et al., Boundary–Layer Meteorol, 2008

Uniform height staggered cubes (C10S) Non-uniform height staggered cuboids (RM10S)

Settings

Flow Flow

C10S RM10S

• p = f = 25 %
• Mean height =10 mm
• Re ~ 3000
• Flow domain:

16h x 16h x 6h (C10S) 16h x 16h x 10h (RM10S)

• Passive scalar domain:

8h x 8h

• Star-CD v4.06

Numerical details

• Unsteady + Incompressible flow: Finite volume method.
• Subgrid-scales: Smagorinsky model + Lilly damping function.
• Periodic boundary conditions in x and y directions.
• Upper boundary: Stress free conditions.
• Lower boundary + Cubes' faces: No-slip conditions.
• Driving force: Constant pressure gradient.
• Space discretization: Second-order central difference scheme.
• Constant concentration on the surface.
• Inlet: Flux=0
• Periodic in y direction.
• Upper boundary: zero gradient
• Space discretization: MARS with blending factor=0.99
• Temporal discretization: Second-order backward implicit scheme.

Grid checks

x y z

Flow

Computational domain size: 4h x 4h x 6h Xie and Castro, Flow Turbulence Combust, 2006

UG 1 1/128 1/64 1/32 1/16 z / h z ∆ h NUG1 (1/32) NUG2 (1/32) (1/16) NUG (1/16) 6

Results

Concentration field at z=0.3h above the source area

Flow

LES Experiments C10S RM10S

Flow

Good qualitative agreement with experiments (C10S)

Results

Concentration field at z=1.2h above the source area LES Experiments C10S RM10S

2 4 −2 −4 2 −2 4 x / h y / h A4 A2 A3 A1

Area-averaged concentrations above the source area Good quantitative agreement with experiments

Results (C10S)

2 4 −2 −4 2 −2 4 x / h y / h A4 A2 A3 A1

Results (RM10S)

Results (C10S)

Lateral concentration profiles downstream of source area

x / h y / h

Area source

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 6 −4 −2 2 4 6 8 10 2 4 −2 −4 −6 −8

Flux estimation

• Vertical flux is 50% of the

total surface flux and the rest is advected downstream at the height: C10S - 1.5h RM10S - 1.78h (?)

• Total surface flux from LES

and experiments differ by C10S - 3.63% RM10S - 13.28% (?) .

Results

Further work Summary

• 1. Much finer resolution is required for scalar than is needed to calculate the

flow adequately.

• 2. Good qualitative as well as quantitative agreement between simulations

and experiments.

• 3. Flux estimation is accurate, provided the grid resolution is fine enough.
• 1. Incorporation of a new scalar wall model to accurately estimate the steep

concentration gradient on a coarser resolution.

• 2. Simulation of heat transfer effects in urban canopies.