本研究利用單相機粒子影像測速法（Particle Image Velocimetry, PIV）與雷射誘導螢光法（Laser-Induced Fluorescence, LIF），建立一套適用於小尺度熱驅動流場之速度與溫度場量測及後處理流程。實驗以小型方形水槽作為測試區域，並由水槽底部施加局部加熱，以形成非等溫熱驅動流場。PIV 影像用於取得速度場，LIF 螢光影像則透過灰階值–溫度校正關係進行溫度場重建。由於本研究使用同一台相機與相同量測平面進行影像擷取，因此速度場與溫度場可在對應的空間位置上進行配對與耦合分析。
在實驗設備方面，本研究使用 532 nm 奈秒雷射作為 PIV 粒子照明與 LIF 螢光激發光源，並以 hollow ceramic microspheres 作為 PIV 示蹤粒子，Uranine 作為 LIF 螢光染料。由於 LIF 影像中會同時包含 Uranine 螢光訊號與示蹤粒子造成的局部亮點，因此本研究在溫度重建前先進行粒子雜訊去除，以降低粒子亮點對灰階值判讀的影響。完成粒子去除後，各 grid window 之平均灰階值被用於建立灰階值–溫度校正曲線。校正結果顯示，在本研究的光學與影像處理條件下，灰階值與溫度之間呈現近似線性關係，因此本研究採用線性擬合作為溫度場重建方法。
本研究使用兩種分析視窗設定進行速度與溫度場分析。第一組設定為 interrogation window（IW，分析視窗）= 96 × 96 pixels、moving window（MW，移動視窗）= 48 × 48 pixels，對應 67 × 67 grid arrangement；第二組設定為 IW = 128 × 128 pixels、MW = 64 × 64 pixels，對應 50 × 50 grid arrangement。透過將 LIF 溫度資料整理至與 PIV 速度場相對應的 grid points，本研究可進一步分析時間平均速度場、時間平均溫度場、速度與溫度標準差、流向速度擾動與垂直速度擾動相關性所代表的動量通量，以及垂直速度擾動與溫度擾動相關性所代表的紊流垂直熱通量。
結果顯示，局部加熱會使水槽內產生明顯的浮力驅動循環流。時間平均垂直速度分布顯示，加熱區域附近出現向上運動，而水槽其他區域則形成補償性的向下流動。時間平均流向速度分布則顯示，此垂直運動同時伴隨水平回流，形成循環流結構。瞬時溫度場結果顯示，隨著加熱時間增加，靠近加熱側的局部高溫區域逐漸變得明顯。標準差分析進一步顯示，速度與溫度變化在空間中並非均勻分布，其中較明顯的速度變化主要出現在與上升流、下降流及回流路徑相關的區域。
動量通量分析顯示，流向速度擾動與垂直速度擾動在不同區域呈現不同的正負相關性，表示流向動量會受到垂直方向速度擾動影響而產生傳輸。紊流垂直熱通量分析則顯示，垂直速度擾動與溫度擾動之間亦存在空間上非均勻的相關性。當其值為正時，表示較熱流體傾向向上運動，或較冷流體傾向向下運動，代表向上的熱傳輸；當其值為負時，則表示相反的擾動相關性。這些結果說明，局部加熱不僅形成平均循環流，也會造成由速度與溫度擾動所引起的動量與熱量傳輸。
整體而言，本研究證明單相機 PIV/LIF 系統結合適當影像處理方法，可用於小尺度非等溫流場中速度與溫度耦合特性之分析。本研究所建立之流程可作為未來在有限設備條件下進行小尺度熱流場量測、溫度場重建與熱傳輸分析之參考方法。

This study established a single-camera Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF) measurement and post-processing procedure for analyzing the velocity and temperature characteristics of a small-scale thermally driven flow. A small square water tank was used as the test section, and localized heating was applied from the bottom of the tank to generate a non-isothermal thermally driven flow field. PIV images were used to obtain the velocity field, while LIF fluorescence images were processed to reconstruct the temperature field through a grayscale–temperature calibration relationship. Since the same camera and measurement plane were used for image acquisition, the velocity and temperature fields could be matched at corresponding spatial locations for coupled analysis.
In the experimental setup, a 532 nm nanosecond laser was used as the illumination source for PIV particles and the excitation source for LIF fluorescence. Hollow ceramic microspheres were used as PIV seeding particles, while Uranine was used as the fluorescent dye for LIF temperature-field reconstruction. Since the LIF images contained both Uranine fluorescence signals and local bright spots caused by seeding particles, a particle-noise reduction procedure was applied before temperature reconstruction to reduce the influence of particle-induced bright spots on grayscale analysis. After particle removal, the grid mean grayscale values were used to establish the grayscale–temperature calibration curve. The calibration results showed that, under the optical and image-processing conditions used in this study, the grayscale value and temperature had an approximately linear relationship. Therefore, linear fitting was selected as the temperature reconstruction method.
Two analysis-window settings were used for the velocity and temperature field analyses. The first setting used an interrogation window (IW) of 96 × 96 pixels and a moving window (MW) of 48 × 48 pixels, corresponding to a 67 × 67 grid arrangement. The second setting used an IW of 128 × 128 pixels and an MW of 64 × 64 pixels, corresponding to a 50 × 50 grid arrangement. By arranging the LIF temperature data on the corresponding PIV grid points, the time-averaged velocity field, time-averaged temperature field, standard deviations of velocity and temperature, momentum flux represented by the correlation between streamwise and vertical velocity fluctuations, and turbulent vertical heat flux represented by the correlation between vertical velocity and temperature fluctuations were further analyzed.
The results show that localized heating generated a clear buoyancy-driven circulation inside the tank. The time-averaged vertical velocity distributions indicated upward motion near the heated region and compensating downward motion in other parts of the tank. The time-averaged streamwise velocity distributions further showed that the vertical motion was accompanied by a horizontal return flow, forming a recirculating flow structure. The instantaneous temperature fields showed that, as the heating time increased, local elevated-temperature regions became more apparent near the heated side of the tank. The standard deviation results further indicated that the temporal variations of velocity and temperature were spatially non-uniform, with stronger velocity variations mainly occurring in regions associated with rising flow, descending motion, and return-flow paths.
The momentum flux results showed that the streamwise and vertical velocity fluctuations exhibited different positive and negative correlations in different regions, indicating that streamwise momentum was transported through vertical velocity fluctuations. The turbulent vertical heat flux also showed spatially non-uniform correlations between vertical velocity and temperature fluctuations. A positive value indicates that warmer fluid tends to move upward or colder fluid tends to move downward, representing upward heat transport. A negative value indicates the opposite fluctuation correlation. These results suggest that localized heating not only forms a mean recirculating flow but also induces fluctuation-driven transport of momentum and heat.
Overall, this study demonstrates that a single-camera PIV/LIF system combined with appropriate image-processing methods can be used to analyze the coupled velocity and temperature characteristics of a small-scale non-isothermal flow field. The proposed procedure provides a practical reference method for future small-scale thermal-flow measurements, temperature-field reconstruction, and heat-transport analysis under limited equipment conditions.

[1] Shuo-Jun Mei, Chao Yuan, “Urban buoyancy-driven air flow and modelling method: A critical review,” Building and Environment, vol. 210, 108708, 2022.
[2] Kiyoshi Uehara, Shuzo Murakami, Susumu Oikawa, Shinji Wakamatsu, “Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons,” Atmospheric Environment, vol. 34, p. 1553–1562, 2000.
[3] Xiaomin Xie, Chun-Ho Liu, Dennis Y.C. Leung, Michael K.H. Leung, “Characteristics of air exchange in a street canyon with ground heating,” Atmospheric Environment, vol. 40, p. 6396–6409, 2006.
[4] W.C. Cheng, Chun-Ho Liu, Dennis Y.C. Leung, “On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow,” Atmospheric Environment, vol. 43, p. 3682–3690, 2009.
[5] Davide Marucci, Matteo Carpentieri, “Effect of local and upwind stratification on flow and dispersion inside and above a bi-dimensional street canyon,” Building and Environment, vol. 156, p. 74–88, 2019.
[6] Jiggar Shah, Claudio Mucignat, Ivan Lunati, Thomas Rösgen, “Simultaneous PIV–LIF measurements using RuPhen and a color camera,” Experiments in Fluids, vol. 65, no. 3, 2024.
[7] Ronald J. Adrian, “Particle-imaging techniques for experimental fluid mechanics,” Annual Review of Fluid Mechanics, vol. 23, p. 261–304, 1991.
[8] J. Westerweel, “Fundamentals of digital particle image velocimetry,” Measurement Science and Technology, vol. 8, p. 1379–1392, 1997.
[9] Richard D. Keane, Ronald J. Adrian, “Optimization of particle image velocimeters. Part I: Double pulsed systems,” Measurement Science and Technology, vol. 1, p. 1202–1215, 1990.
[10] Richard D. Keane, Ronald J. Adrian, “Theory of cross-correlation analysis of PIV images,” Applied Scientific Research, vol. 49, p. 191–215, 1992.
[11] J. Sakakibara, R. J. Adrian, “Whole field measurement of temperature in water using two-color laser induced fluorescence,” Experiments in Fluids, vol. 26, p. 7–15, 1999.
[12] J. P. Crimaldi, “Planar laser induced fluorescence in aqueous flows,” Experiments in Fluids, vol. 44, p. 851–863, 2008.
[13] V. K. Natrajan, K. T. Christensen, “Two-color laser-induced fluorescent thermometry for microfluidic systems,” Measurement Science and Technology, vol. 20, 015401, 2009.
[14] S. Funatani, N. Fujisawa, H. Ikeda, “Simultaneous measurement of temperature and velocity using two-colour LIF combined with PIV with a colour CCD camera and its application to the turbulent buoyant plume,” Measurement Science and Technology, vol. 15, p. 983–990, 2004.
[15] Yongling Zhao, Yunpeng Xue, Shuojun Mei, Yuan Chao, Jan Carmeliet, “Enhancement of heat removal from street canyons due to buoyant approaching flow: Water tunnel PIV-LIF measurements,” Building and Environment, vol. 226, 109757, 2022.
[16] Yunpeng Xue, Yongling Zhao, Shuo-Jun Mei, Chao Yuan, Jan Carmeliet, “The significant impacts of urban canyon configuration on non-isothermal flow, ventilation, and heat removal: Insights from PIV-LIF measurements,” International Journal of Heat and Fluid Flow, vol. 110, 109594, 2024.
[17] R. S. Volkov, P. A. Strizhak, “Research of temperature fields and convection velocities in evaporating water droplets using Planar Laser-Induced Fluorescence and Particle Image Velocimetry,” Experimental Thermal and Fluid Science, vol. 97, p. 392–407, 2018.
[18] Ye Lin, Toshiaki Ichinose, Yukio Yamao, Hideaki Mouri, “Wind velocity and temperature fields under different surface heating conditions in a street canyon in wind tunnel experiments,” Building and Environment, vol. 168, 106500, 2020.
[19] Markus Raffel, Christian E. Willert, Steve T. Wereley, Jürgen Kompenhans, “Particle Image Velocimetry: A Practical Guide,” Springer, 2007.
[20] Sven Scharnowski, Christian J. Kähler, “Particle image velocimetry - Classical operating rules from today’s perspective,” Optics and Lasers in Engineering, vol. 135, 106185, 2020.