Aerodynamics of ground-mounted solar panels: Test model scale effects Aly Mousaad Aly a,b,n , Girma Bitsuamlak b a Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, USA b WindEEE Research Institute, Department of Civil and Environmental Engineering, Western University, The Boundary Layer Wind Tunnel Laboratory, London, ON, Canada N6A 5B9 article info Keywords: Wind loading Wind tunnel testing Geometric scale Low-frequency turbulence High-frequency turbulence Wind spectra Solar panel Computational fluid dynamics abstract Most boundary-layer wind tunnels (BLWTs) were built for testing models of large civil engineering structures that have geometric scales ranging from 1:500 to 1:100. However, producing aerodynamic models of the solar panels at such scales makes the modules too small, resulting in at least two technical problems. First, the resolution of pressure data on such small models becomes low. Second, the test model may be placed in the lower portion of the boundary-layer that is not a true representative of a real world scenario, due to high uncertainty in wind velocity. To alleviate these problems, development of a standardized testing protocol is very important. Such protocol should account for different time and geometric scales to design appropriate wind tunnel experiments that can allow accurate assessment of wind loads on the solar panels. The current paper systematically investigates the sensitivity of wind loads to testing ground-mounted solar panels, both experimentally (in a BLWT) and numerically (by computational fluid dynamics (CFD)), at different geometric scales. While mean loads are not significantly affected by the model size, peak loads are sensitive to both the geometric scale and the spectral content of the test flow. However, when the objective is to predict 3-s (three seconds) peak loads, large models can be tested in a flow that has reduced high-frequency turbulence. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Background Boundary-layer wind tunnel (BLWT) testing of structures is an industry wide accepted procedure and is considered the main source of information for wind load calculations and codification. The majority of BLWTs are built for testing large civil structure models with geometric scales ranging between 1:500 and 1:100. Accordingly, producing representative aerodynamic models of ground-mounted solar panels at such scales makes the models too small, resulting in at least two technical problems. First, resolution of pressure data on such small models becomes low, and second, the model will be located in a portion of the boundary-layer that might not be a representative of case in the real world. For instance, small ground-mounted solar panels (or even low-rise buildings), when scaled down, may have a height that is less than the smallest floor roughness element in upstream wind. This may cause high uncertainty in measured wind speed, in addition to significant interference effects from instruments (e.g., pressure tubes (see Fig. 1)). To alleviate these problems, devel- opment of a standardized and practical modeling procedure that accounts for different time and spatial scales is important. This will help to design appropriate wind tunnel experiments and hence to determine the wind loads accurately on the solar panels. Because of the lack of full-scale data, aerodynamic loads on civil structures in general, and on the solar panels in particular, are not often calibrated or validated with the prototype data. Therefore, the scale effects are not well quantified yet. It is clear that a wind tunnel boundary-layer should exhibit similar velocity and turbu- lence intensity profiles as those of the atmospheric boundary- layer. What is not well understood is how to select model scale ratios to minimize scale effects, under the constraints of the tunnel′s size which limits the production of large scale turbulence (limited integral length scale). 1.2. Literature review Lee (1975) studied the effects of turbulence scale on mean forces on square prisms. The results indicated that care must be taken when modeling a building in natural wind to ensure that ratios of turbulence scale size to building dimensions remain constant. In addition, at high values of turbulence scale, the drag tends to have a large constant value compared to low turbulence scales. Cook (1977) presented a graphical/analytical procedure for the determination of a model scale factor in boundary-layer wind tunnels. The approach is based dominantly on the determination of the integral scale of turbulence, L ux . The Importance of the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jweia Journal of Wind Engineering and Industrial Aerodynamics 0167-6105/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jweia.2013.07.007 n Corresponding author. Tel.: +1 225 578 6654; fax: +1 225 578 4945. E-mail addresses: aly@LSU.edu, aly.mousaad@polimi.it (A.M. Aly), gbitsuam@UWO.ca (G. Bitsuamlak). Please cite this article as: Aly, A.M., Bitsuamlak, G., Aerodynamics of ground-mounted solar panels: Test model scale effects. Journal of Wind Engineering & Industrial Aerodynamics (2013), http://dx.doi.org/10.1016/j.jweia.2013.07.007i J. Wind Eng. Ind. Aerodyn. ∎ (∎∎∎∎) ∎∎∎–∎∎∎