**1. Introduction**

At the present time, humanity is focused on the achievement of a reliable, affordable, and decarbonized energy system. The accomplishment of this goal involves undoubtedly the use of renewable energy, with wind energy harvested through horizontal axis wind turbines (HAWTs) leading the way due to their mature development.

However, such objectives start to require rapid actions and deeper strategies to fulfill the established deadlines (such as a net-zero-emissions scenario by 2050 [1]). These may even include the consideration of the real-time demand curves and on-site production for self-consumption. In this regard, wind energy production in urban environments is gaining much attention. In such restrictive placements, the lift-type vertical axis wind turbines (VAWT) seem to be the best candidates for wind energy extraction [2,3] due to their omnidirectionality, avoiding the need for orientation mechanisms; their ability to work better in variable wind conditions; and their lower noise emission [4].

Nevertheless, in contrast with the well-established HAWTs, which present higher efficiencies and superior rated power, the VAWT turbines are machines still requiring a vast amount of research to overcome the crucial issues that prevent them from achieving a profitable and efficient development status [5]. First of all, their aerodynamics are far more complicated than conventional turbines and there is no agreement yet on the best reference rotor design [6]; additionally, the application on urban environments means

**Citation:** Santamaría, L.; Galdo Vega, M.; Pandal, A.; González Pérez, J.; Velarde-Suárez, S.; Fernández Oro, J.M. Aerodynamic Performance of VAWT Airfoils: Comparison between Wind Tunnel Testing Using a New Three-Component Strain Gauge Balance and CFD Modelling. *Energies* **2022**, *15*, 9351. https://doi.org/ 10.3390/en15249351

Academic Editors: Artur Bartosik and Dariusz Asendrych

Received: 28 October 2022 Accepted: 8 December 2022 Published: 10 December 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

facing very-poor-quality wind resources [2,7]. As a result, recent efforts in this field have been directed towards increasing performance and overall energy production through the development of both flow augmentation and control devices [8]; although, the latter are even more attractive for higher-size turbines such as those for deep-water offshore environments. Therefore, a necessary first stage comprises the study of new airfoil designs (which include these characteristics) or existing airfoils in new set-ups, both requiring a precise determination of the aerodynamic properties.

To that extent, CFD methods are a highly valuable tool to improve understanding of the airflow around the turbine blades, the interactions with the flow control devices, and the effect of power augmentation devices. Moreover, they allow the analysis of different types of geometries at a lower cost. However, the accuracy of CFD simulations depends significantly on the selection of the appropriate turbulence model, computational grid construction, and numerical characteristics (temporal and spatial schemes). For that reason, experimental validation is always required. Wind tunnel testing is widely employed, although this technique faces some intrinsic problems such as prototype–tunnel interaction (blockage) which usually forces downscaling. This in turn derives from other issues, as an intensified relevance of surface roughness [9] and the increased difficulty of measuring airfoil drag at low incidence angles [10]. Furthermore, unsteady phenomena may arise due to complex aerodynamics in cases of highly loaded airfoils. This is especially relevant for VAWT turbines where high angles of attack occur, even when flow control devices are used [11]. During the regular operation of a VAWT, i.e., during a complete rotor turn, the blade angle of attack varies continuously going from positive to negative incidences of the relative incoming flow. Thus, in the pursuit of VAWT performance enhancement, or to properly design passive flow control devices, the understanding of airfoil behavior at different angles of attack is essential. To this effect, the development of accurate, reliable and affordable equipment, useful for this purpose, is inherently interesting.

Aerodynamic performance can be estimated from the integration of the pressure distributions measured with pressure taps [12] or directly with an aerodynamic balance. The first method provides more information but limits the number of geometries that can be tested, as every prototype has to be complexly manufactured to include the pressure tabs and tubes. On the other hand, while there are a wide variety of balance designs, external balances (placed outside the test section) are the most common for airfoil testing. Within external balances, two types are distinguished, single-piece (with multi-component load cells) and multi-piece (with several load cells) [13]. Single-piece aerodynamic balances are usually expensive and not commercial, as each application usually requires a specific range distribution; thus, they are custom-manufactured [14]. Meanwhile, multi-piece balances typically need more space, although in external balances, that is not commonly problematic and, thus, they are widely used [13,14].

Recently, a new design of a three-component external multi-part strain gauge balance, intended for studying the galloping of solar trackers, has been proposed [15,16]. Due to its relevant characteristics, as different load ranges in different directions and highfrequency response, this balance has been identified as potentially attractive to test airfoils. Furthermore, its reduced size, scalability and ease of manufacturing make it even more interesting for this application.

This work presents a brand-new application of the aforementioned balance, including the testing of its capabilities, and the validation of its use for the evaluation of aerodynamic performance of VAWT airfoils. For that purpose, several prototypes have been tested in a wind tunnel using the balance, including a typical airfoil (DU06-W-200) developed for VAWT applications. Moreover, CFD simulations have been performed with recently developed turbulence models for complementary analysis and comparison.

The paper is structured as follows. Firstly, the experimental set-up used is presented in Section 2, including the description of the wind tunnel, the aerodynamic balance, and measurement procedures. Then, Section 3 describes the main characteristics of the CFD numerical modeling. Afterwards, the validation procedure is presented in Section 4. Twoand three-dimensional flat plates and the DU06-W-200 airfoil are used for comparison, taking advantage of the available data in the open literature. In Section 5, the results are provided: the validation of the balance against bibliographic results is firstly presented for both the flat plates and the airfoil. In the following section, a deeper analysis is carried out with the help of CFD modelling, including the unsteady phenomena with respect to the pitching angle. Finally, after the presentation of the results, relevant conclusions and future works are provided in Section 6.
