Overcoming wind shadow

patrick on 12|11|2010
Filled under: dynamic mappings

Introduction

Building in arid conditions requires to be aware of the surroundings, and having good knowledges about the few natural resources that are available. Using the right resources to serve the right purpose is one of the main stake in those conditions. In this environment wind is one of the more precious resources available as it allows not only to produce energy but also to create natural ventilation for the buildings.
The aim of this study is to understand the main wind patterns on the Ras Al-Kaimah EPFL middle-east site, and to understand the role played by wind shadows in the usage of wind as a energy resource.

Convections

Regarding the fact that we are in a seaside situation, we can admit that wind will be ruled by convections and generating this way a land breeze and a sea breeze.


(source:http://www.bzhecume.com/surflog/read.php?6,346110,346810)

Land breeze will occur mainly during night when ground temperature is colder than sea temperature. During daytime the situation will be inverted creating a seabreeze.

Data

Regarding the variations per hour of windspeed over one year (2009), two main wind patterns can be established.

On this example green color represents the hours between 7am and 6pm, which is the supposed working time of a campus

Regarding the results on twelve months we can clearly observe that the wind has a dominant NW direction during day and SE during night.

Applied to the site those directions confirm the convection theory.

Wind shadow

Wind shadow is a phenomenon occurring when the wind air flow encounters an obstacle. After impact the wind flow is perturbed over a certain distance creating depression zones.

(source: Lee, Jeffrey A. and Zobeck, Ted M., 2002, Proceedings of ICAR5/GCTE-SEN Joint Conference, International Center for Arid and Semiarid Lands Studies, Texas Tech University, Lubbock, Texas, USA Publication 02-2 p. 458

This phenomenon can be applied to dunes, if those one present a steep slope on one of their side. Regarding the height (h) of the dune the wind flow will be perturbed on a distance of four to ten times “h”.


(source: Victor Olgyay, Design whith climate)

Same thing for building. After impact the air flow will be perturbed on a distance of seven times the height of the building.

Analysis

Wind shadow is an important factor that has to be take in count if we want to use wind to produce energy or for natural ventilation. Regarding this an analysis of the site mophology was necessary to establish the presence wind shadow producing dunes.

Once this map established the mapping process for the analysis had to be defined

First step of the mapping process is to define the dune’s ridge, once this done we have to established several points on this ridge that will “project” a wind vector and allow to establish the length of the wind shadow (regarding the relationship between the height of the obstacle and the projected length).
Last step is to rescale the vector if it encounter an obstacle such as terrain deformation or other dune.

Through this process a map was established regarding the changing wind direction during the day.

The different projections create different “patterns” that have to be taken in consideration for the upcoming buildings design

Design stratgies

After the mapping analysis, three design strategies were defined to overcome the wind shadow effect.







Those three strategies will help to figure out, how to establish a master plan for the upcoming EPFL middle east campus.

Link to presentation in pdf

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