[Paper Review] Large Eddy Simulation of urban boundary layer flows using a canopy stress method

Jahrul Alam, Luke P. J. Fitzpatrick|arXiv (Cornell University)|Nov 4, 2017

Wind and Air Flow Studies28 references3 citations

TL;DR

This paper proposes a large-eddy simulation (LES) method for urban boundary layer flows that combines the eddy viscosity model with a canopy stress method to accurately represent momentum loss and pressure drag from urban roughness elements without resolving their geometry. The method dynamically adapts energy dissipation by incorporating both strain and rotation tensors, achieving good agreement with body-fitted CFD simulations in predicting mean and turbulent velocity profiles, while capturing large-scale intermittency of coherent structures.

ABSTRACT

Large-eddy simulation (LES) of a turbulent flow through an array of building-like obstacles is an idealized model to study transport of contaminants in the urban atmospheric boundary layer (UABL). A reasonably accurate LES prediction of turbulence in such an UABL must resolve a significant proportion of the small but energetic eddies in the roughness sublayer, which remains prohibitive even though computational power has been increased significantly. In this article, we present a large-eddy simulation methodology to study turbulence in UABLs, where the turbulence closure is based on coupling the eddy viscosity method with the canopy stress method. Unlike the classical Smagorinsky model that considers only the strain portion of the velocity gradient tensor, we consider both the strain tensor and the rotation tensor to compute the eddy viscosity. This allows us to dynamically adapt the rate of energy dissipation to the scales of the energetic eddies in the roughness sublayer. Without employing a mesh conforming to the urban roughness elements, the effect of such solid bodies are represented in the LES model through a canopy stress method in which the loss of pressure and the sink of momentum due to the interaction between eddies and roughness elements are parameterized using the instantaneous velocity field. Simulation results of the proposed canopy stress method is compared with that of a conventional Computational Fluid Dynamics (CFD) method employing a block-structured mesh conforming around the roughness elements. For urban flow simulations, the results demonstrate that the proposed canopy stress model is accurate in predicting vertical profiles of mean and variance, as well as the temporal intermittency of coherent structures.

Motivation & Objective

• To develop a computationally efficient LES methodology for simulating turbulent flows in urban atmospheric boundary layers (UABL) without resolving individual building geometries.
• To address the challenge of high computational cost in resolving small-scale eddies in the roughness sublayer using traditional LES with adaptive mesh refinement.
• To improve turbulence prediction accuracy by incorporating both strain and rotation tensor contributions to eddy viscosity, enabling dynamic adaptation to energetic eddies in the roughness sublayer.
• To validate the canopy stress method against a conventional body-fitted CFD approach with no-slip boundary conditions on roughness elements.
• To assess the method’s ability to capture coherent structures and large-scale intermittency in urban flow dynamics.

Proposed method

• Uses large-eddy simulation (LES) with a dynamic eddy viscosity model that incorporates both the strain and rotation tensors of the velocity gradient tensor to better capture energy dissipation at small scales.
• Applies a canopy stress method to parameterize the momentum sink and pressure loss due to building-like obstacles, representing their effect via the instantaneous velocity field without resolving their surfaces.
• Implements a wall-modeling approach similar to the wall-adaptive local eddy viscosity (WALE) method to dynamically adjust turbulent stress in the surface layer.
• Avoids body-conforming meshes by modeling the roughness sublayer and canopy layer effects through a parameterized stress term added to the momentum equations.
• Performs simulations at a Reynolds number of 𝒪(2×10⁷), ensuring relevance to real-world urban flows.
• Compares results with a high-fidelity CFD simulation using block-structured, body-fitted meshes to resolve individual roughness elements.

Experimental results

Research questions

• RQ1 Can a canopy stress method in LES accurately predict mean and turbulent velocity profiles in urban boundary layers without resolving individual building geometries?
• RQ2 How does the inclusion of both strain and rotation tensors in the eddy viscosity model improve the simulation of small-scale turbulent eddies in the roughness sublayer compared to classical Smagorinsky models?
• RQ3 To what extent does the canopy stress method capture large-scale intermittency and coherent structures observed in real urban atmospheric flows?
• RQ4 How does the canopy stress method compare quantitatively with a conventional CFD approach using no-slip boundary conditions on roughness elements?
• RQ5 Does the canopy stress method remain robust and accurate when combined with adaptive mesh refinement, avoiding the grid interface errors reported in prior studies?

Key findings

• The canopy stress method produces vertical profiles of mean and turbulent velocity that are in strong agreement with those from a high-fidelity CFD simulation using body-fitted meshes.
• The method successfully captures the temporal intermittency of coherent structures, indicating that a significant fraction of unresolved energetic eddies are still resolved dynamically.
• The inclusion of both strain and rotation tensors in the eddy viscosity model enables dynamic adaptation of energy dissipation to the scales of energetic eddies in the roughness sublayer, improving turbulence modeling accuracy.
• The canopy stress approach avoids the computational cost of resolving individual buildings while maintaining accuracy, making it suitable for large-scale urban flow simulations.
• The simulation exhibits large-scale intermittency consistent with collapsing and bursting events in atmospheric turbulence, observed even in neutral stratification and urbanized regions.
• The method remains accurate at high Reynolds numbers (𝑹𝒆 ≈ 2×10⁷), demonstrating its applicability to real-world urban atmospheric boundary layer conditions.