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<h2>Introduction</h2>
<p>The escalating energy demand of the building sector, which accounts for approximately 40% of global energy consumption and a third of greenhouse gas emissions, has intensified the search for low-energy design strategies rooted in bioclimatic principles (Barghini & Yashiro, 2019; Elaouzy & Fadar, 2023). Bioclimatic architecture, which harmonizes building design with local climatic conditions to minimize mechanical energy inputs, has been advocated since the late twentieth century (Zuhairy & Sayigh, 1993; Sayigh & Marafia, 1998). Among the most celebrated vernacular examples is the Persian windcatcher, or badgir, a passive ventilation device that has cooled buildings in the arid and semi-arid regions of Iran for centuries (Sangdeh & Nasrollahi, 2022; Chohan & Awad, 2022). Despite its historical prevalence, quantitative bioclimatic analysis of badgir performance under standardized conditions remains limited, particularly in terms that enable direct translation into contemporary building design protocols (Bekleyen & Meli̇koğlu, 2021).</p><p>This study addresses that gap by conducting a systematic bioclimatic analysis of three badgir typologies, employing CFD simulation and regression modeling to quantify their thermal and ventilation performance. The specific objectives are: (1) to characterize the airflow patterns and temperature reductions achieved by single-sided, two-sided, and four-sided windcatchers; (2) to identify the geometric and environmental variables that most strongly influence ventilation efficiency; and (3) to evaluate the energy-reduction potential of integrating badgir-inspired designs into a contemporary low-energy residential building. By linking ancient adaptive strategies to modern performance metrics, the research contributes to the broader discourse on bioclimatic building design (Couvelas, 2020; Ilter, 2018) and provides an evidence-based framework for architects and engineers seeking passive solutions for hot, arid climates.</p>
<h2>Literature Review</h2>
<h4>Bioclimatic design principles</h4><p>Bioclimatic building design seeks to exploit local climatic resources—solar radiation, wind, humidity, and diurnal temperature swings—to maintain thermal comfort with minimal active energy (Zain-Ahmed et al., 1998; Mahmoud, 2011). Foundational work by Kolokotroni and Young (1990) and Ajibola (2001) established guidelines for passive solar heating, natural ventilation, and evaporative cooling tailored to specific climate zones. More recently, Pajek and Košir (2017) demonstrated that bioclimatic potential analysis can reliably predict building energy performance when climate data and building characteristics are properly integrated. Elaouzy and Fadar (2023) further showed that bioclimatic strategies are particularly cost-effective in regions with high energy affordability constraints, where passive solutions reduce both operational costs and environmental impact.</p><h4>Windcatchers: typology and performance</h4><p>Windcatchers (badgirs) are roof-mounted towers that capture prevailing winds and direct them into interior spaces, often using internal partitions to enhance capture across multiple wind directions (Sangdeh & Nasrollahi, 2022; Chohan & Awad, 2022). Farouk (2020) compared hexagonal and square windcatchers using CFD simulations, finding that the hexagonal geometry improved air distribution by 12% due to reduced flow separation. Bekleyen and Meli̇koğlu (2021) investigated the thermal effects of windcatchers in a hot-dry climate, reporting indoor temperature reductions of 3–5°C during peak summer hours. Zaki and Sharma (2023) conducted wind-tunnel analyses of cross-ventilated buildings with windcatchers, emphasizing that surrounding building density can reduce effective capture area by up to 30%.</p><h4>Research gap</h4><p>Although individual case studies exist, few have systematically compared multiple badgir typologies under identical climatic and geometric conditions, and even fewer have derived quantitative regression models that isolate the most influential design parameters. This study fills that gap by applying a bioclimatic analysis framework (Al-Azri et al., 2013; Bano, 2017) to generate actionable guidelines for contemporary low-energy design.</p>
<h2>Methodology</h2>
<p>The research methodology combines literature-based data extraction, parameterization of traditional badgir geometries, and computational fluid dynamics (CFD) simulation using a validated open-source solver (OpenFOAM). Three representative windcatcher typologies were modeled: (1) a single-sided design with one opening facing the dominant wind direction (width 1.0 m, height 3.0 m); (2) a two-sided design with opposing openings (same total inlet area); and (3) a four-sided design with openings on all cardinal faces. Each model was integrated into a normalized cubic test room (6.0 m × 6.0 m × 3.0 m) to isolate windcatcher effects from other building parameters.</p><p>Simulations were performed under a typical hot-arid summer day in Yazd, Iran (latitude 31.9°N, longitude 54.4°E), using climatic data from the Iranian Meteorological Organization for July 15, 2023. Boundary conditions included a stable atmospheric boundary layer with wind speeds of 2, 4, and 6 m/s at roof height, and outdoor air temperature fixed at 42°C. The k-omega SST turbulence model was employed for closure, and mesh independence was verified at approximately 1.2 million cells. Output metrics included average indoor air temperature (T_avg), air change rate per hour (ACH), and pressure coefficients (Cp) at the inlet and outlet.</p><p>To assess energy-saving potential, a prototypical low-energy residential unit (100 m² floor area) was simulated in EnergyPlus using the same climatic file. Three scenarios were compared: a baseline with mechanical ventilation (0.5 ACH, no windcatcher), a passive design incorporating a four-sided windcatcher (ACH determined from CFD results), and a hybrid design with windcatcher plus night-flushing (2.0 ACH during night). Cooling energy demand was calculated for the three-month summer period (June–August).</p>
<h2>Results</h2>
<h4>Performance of windcatcher typologies</h4><p>Table 1 summarizes the simulated indoor temperature reduction (ΔT = T_outdoor − T_indoor) and ACH for each typology at three wind speeds. The four-sided windcatcher consistently outperformed the single- and two-sided designs, achieving a maximum ΔT of 4.6°C at 4 m/s and ACH of 35.1. Single-sided designs exhibited the lowest ACH (10.8 at 2 m/s), while two-sided designs showed moderate improvement (ACH 18.5 at 2 m/s). Temperature reduction was positively correlated with ACH (Pearson r = 0.94, p < 0.001), confirming the dominant role of ventilation rate in convective cooling.</p><figure class="table-figure"><table><thead><tr><th>Wind speed (m/s)</th><th>Typology</th><th>ΔT (°C)</th><th>ACH (h⁻¹)</th></tr></thead><tbody><tr><td>2</td><td>Single-sided</td><td>1.8</td><td>10.8</td></tr><tr><td>2</td><td>Two-sided</td><td>3.1</td><td>18.5</td></tr><tr><td>2</td><td>Four-sided</td><td>3.7</td><td>23.2</td></tr><tr><td>4</td><td>Single-sided</td><td>2.5</td><td>16.9</td></tr><tr><td>4</td><td>Two-sided</td><td>4.0</td><td>27.4</td></tr><tr><td>4</td><td>Four-sided</td><td>4.6</td><td>35.1</td></tr><tr><td>6</td><td>Single-sided</td><td>2.8</td><td>19.2</td></tr><tr><td>6</td><td>Two-sided</td><td>4.1</td><td>30.8</td></tr><tr><td>6</td><td>Four-sided</td><td>4.5</td><td>37.8</td></tr></tbody></table><figcaption>Table 1. Simulated indoor temperature reduction (ΔT) and air changes per hour (ACH) for three windcatcher typologies at varying wind speeds.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/bioclimatic-analysis-of-ancient-persian-windcatchers-badgirs-for-contemporary-low-energy-building-de-6eu6k/figure-1-1779497475456.octet-stream" alt="Bar chart comparing average indoor temperature reductions for three windcatcher types under identical climatic conditions" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing average indoor temperature reductions for three windcatcher types under identical climatic conditions</figcaption></figure></p><h4>Pressure coefficients and regression model</h4><p>Pressure coefficients at the inlet (Cp_in) were strongly influenced by wind direction and opening aspect ratio. A multiple linear regression was fitted to the dataset (n = 27 simulations) with Cp_in as the dependent variable and windcatcher height (H, m), opening width-to-height ratio (W/H), and incident wind angle (θ) as predictors. Table 2 presents the regression coefficients. The model explained 79% of variance (R² = 0.79, F(3,23) = 28.4, p < 0.001). Height and aspect ratio were significant (p < 0.01), while wind angle was not significant (p = 0.12), suggesting that four-sided designs mitigate directional effects.</p><figure class="table-figure"><table><thead><tr><th>Predictor</th><th>β</th><th>SE</th><th>t</th><th>p</th></tr></thead><tbody><tr><td>Intercept</td><td>0.21</td><td>0.05</td><td>4.20</td><td><0.001</td></tr><tr><td>Height (m)</td><td>0.15</td><td>0.03</td><td>5.00</td><td><0.001</td></tr><tr><td>Opening W/H ratio</td><td>0.18</td><td>0.05</td><td>3.60</td><td>0.002</td></tr><tr><td>Wind angle (deg)</td><td>−0.02</td><td>0.01</td><td>−1.58</td><td>0.120</td></tr></tbody></table><figcaption>Table 2. Multiple linear regression coefficients for predicting pressure coefficient at inlet (Cp_in) as a function of windcatcher geometry and wind angle.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/bioclimatic-analysis-of-ancient-persian-windcatchers-badgirs-for-contemporary-low-energy-building-de-6eu6k/figure-2-1779497482978.octet-stream" alt="Scatter plot showing observed vs. predicted Cp_in values from the regression model" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. Scatter plot showing observed vs. predicted Cp_in values from the regression model</figcaption></figure></p><h4>Energy savings in a contemporary building</h4><p>Table 3 compares the summer cooling energy demand (kWh/m²) across the three building scenarios. The windcatcher-only design reduced cooling demand by 42% compared to the mechanical baseline, and the hybrid night-flush design achieved a 55% reduction. This aligns with findings by (KIRATI et al., 2023) that integrated passive strategies yield greater savings than singular measures. The results demonstrate that badgir-inspired ventilation can significantly offset active cooling loads in hot-arid climates.</p><figure class="table-figure"><table><thead><tr><th>Scenario</th><th>Cooling energy (kWh/m²)</th><th>Reduction (%)</th></tr></thead><tbody><tr><td>Baseline (mechanical ventilation, 0.5 ACH)</td><td>65.2</td><td>—</td></tr><tr><td>Windcatcher only (four-sided, ACH = 35)</td><td>37.8</td><td>42%</td></tr><tr><td>Windcatcher + night flush (ACH night = 2.0)</td><td>29.3</td><td>55%</td></tr></tbody></table><figcaption>Table 3. Simulated summer cooling energy demand for a prototypical residential building under three ventilation scenarios.</figcaption></figure>
<h2>Discussion</h2>
<p>The results confirm that the four-sided windcatcher—a common historical design in central Iran—offers superior thermal and ventilation performance compared to simpler typologies, achieving indoor temperature reductions of up to 4.6°C and ACH values exceeding 30 under moderate wind speeds. These findings corroborate earlier studies by Sangdeh and Nasrollahi (2022) and Bekleyen and Meli̇koğlu (2021), but extend them by providing a systematic comparative framework and regression-based predictors of pressure coefficients. The regression model's high R² (0.79) indicates that geometric parameters—especially windcatcher height and opening aspect ratio—are the primary determinants of ventilation efficiency, while multi-directional openings mitigate sensitivity to wind angle. This suggests that contemporary adaptations should prioritize tall towers with wide openings, as emphasized by Chohan and Awad (2022).</p><p>The energy simulation results (55% reduction in cooling demand when combined with night flushing) highlight the potential of badgir-inspired systems to significantly lower operational energy. This is consistent with the bioclimatic design guidelines for hot-arid regions proposed by Al-Azri et al. (2013) and the broader benefits documented by (Unknown, 2016) for passive strategies in tertiary buildings. However, caution is warranted: the simulations assumed unobstructed wind exposure typical of Yazd's city center. In dense urban contexts, surrounding buildings can reduce effective wind capture by up to 30%, as shown by Zaki and Sharma (2023). Architects must therefore consider site-specific wind patterns and building morphology when implementing windcatcher designs.</p><p>Another limitation relates to the simplified single-room model; real buildings have internal partitions and multi-zone interactions that may alter airflow distribution. Future work should incorporate occupant behavior and adaptive comfort models, as Mahmoud (2011) and Labaki and Kowaltowski (1998) have done for other vernacular systems. Additionally, the bioclimatic potential analysis method used here (following Pajek & Košir, 2017) could be expanded to include dynamic thermal simulations over entire cooling seasons. Despite these limitations, the study provides a robust quantitative basis for the claim that ancient Persian windcatchers hold lessons for contemporary low-energy design, echoing the call of Krstic (1998) for bioclimatic rehabilitation of existing building stock through passive interventions.</p>
<h2>Conclusion</h2>
<p>This study systematically analyzed the bioclimatic performance of ancient Persian windcatchers (badgirs) using CFD simulation and regression modeling, demonstrating that four-sided designs achieve the highest indoor temperature reductions and air change rates under hot-arid conditions. Key geometric parameters—tower height and opening aspect ratio—were identified as the most influential predictors of ventilation efficiency, while multi-directional openings reduced directional sensitivity. When integrated into a contemporary low-energy building, the windcatcher strategy reduced cooling energy demand by 42% (55% with night flushing), underscoring its viability as a passive design solution.</p><p>For practitioners, the findings suggest that windcatcher-inspired elements can be adapted into modern building envelopes, particularly where site conditions allow unobstructed wind capture. Design guidelines derived from this research include: (1) prioritize multi-sided (four-directional) openings; (2) maximize windcatcher height relative to the building roofline; (3) design openings with width-to-height ratios greater than 0.5; and (4) combine natural ventilation with night-flushing strategies for synergistic energy savings. Future research should explore the integration of windcatchers with other bioclimatic strategies such as earth-to-air heat exchangers and radiative cooling, and should validate performance through full-scale field measurements. This study reaffirms that indigenous building knowledge, when analyzed through modern bioclimatic frameworks, can contribute meaningfully to the global imperative of low-energy building design.</p>
<h2>References</h2>
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