## 2 Design Process Outline
### Preliminary Design  
The preliminary design consisted of the selection of the IWT typology, its basic characteristics and sizing, together with the required first approximation calculations. For this preliminary design, recommendations for the design of general-purpose wind tunnels have been followed [40; 41; 42; 43; 44; 45; 46; 47], and the required calculations have been carried out for the use of wind tunnels in icing tests. First of all, the existing space requirements and the objectives of the tunnel (the maximum speed to be reached) were defined, followed by the definition of the most basic characteristics of the IWT: type of circuit, type of test section, main dimensions of the test section, etc. Financial criteria have also been taken into account throughout the whole process. Secondly, each of its sections has been defined in detail using the formulas obtained from the various studies on wind tunnel design [40]. These formulas are based on analytical one-dimensional models, experimentally corrected. These corrections and experimental adjustments are based on the abundant existing literature, and the model has been widely used in tunnel design for decades [48]. Figure 2: Phases of the design process. Given the existing restrictions on the overall dimensions, the design has consisted of an iterative process of successive refinements of the dimensions of the different sections (their areas and their lengths and conicities), so that the general tunnel design criteria are met, as well as the specific requirements of this case. This iterative process has allowed the geometry of the IWT to be optimised, while meeting the constraints. In the case of any wind tunnel, the power required for its operation and its energy consumption are crucial. But this is especially so in the case of an IWT, due to the need to keep the air temperature low, for which any energy loss in the form of heat (from the operation of the fan and from the friction of the air passing through the circuit) will have to be compensated for by a cooling system. For this reason, the design has been optimised for maximum efficiency (or minimum pressure drop to be compensated for by the fan), which means minimum aerodynamic heat loss. In an IWT, the minimum temperature that can be reached or the maximum test time is limited by the cooling system, which has to compensate not only for the heat losses of the refrigerated chamber itself, but also for the heat generated by the fan and air friction. The pressure drop has been calculated for each of the sections and added together to give the total pressure drop in the tunnel. The dynamic pressure is determined by the law of continuity, so energy losses by components reduce the total pressure (and therefore the static pressure). The total pressure drop must be compensated for by the fan and is therefore related to the required fan power. In addition, it is needed, together with the flow rate, to define the fan. The loss coefficient of each section can be defined as the ratio of the pressure loss in section \(\Delta H\) to the dynamic pressure \(q\) at the entrance to the section:  
\[K=\frac{\Delta H}{q}=\frac{\Delta H}{\frac{1}{2}\rho V^{2}}\leftrightarrow \Delta H=K\cdot q \tag{1}\]  
The loss coefficient of each section can be calculated using semi-empirical formulas, as explained above, according to [PERSON] et al. [40], where the different methods for each type of section are summarised. An energy ratio was also calculated, which is the ratio of the jet power (the power in a flowing jet) in test section \(P_{t}\) to the rate of \"flow losses\" around the circuit, and is a measure of the efficiency of the wind tunnel. This ratio is usually reduced by 10% due to leaks and seals, and in case of a flow separation affecting the diffuser, it can be as low as 60%, so the power requirement could be doubled.