DESIGN PROCEDURE AND HONEYCOMB SCREEN IMPLEMENTATION TO THE AIR

Wind tunnels are the most widely used tools when comes to validation of Reynold number. Most of wind tunnels use various air straighteners or flow conditioners to achieve as most unified air flow as it’s possible. This article deals with design and creation procedure of air flow straightener – honeycomb screen. Based on mathematical relations and empirical experience defines dimensions and characteristics for subsoni c wind tunnel of the Air Transport Department of University of Žilina. Wind tunnel equipped with a suitable screen provides more relevant and accurate data, which are crucial for final validation of results of test objects.


Introduction
The wind tunnel is a tool to study fluid flows around a body and the forces generated by the fluid-structure interaction (Panda & Samanta, 2016). The main concerns of aircraft aerodynamics are reducing drag, reducing noise, and improving lift forces on aerodynamic objects (Łusiak, 2020). Wind tunnels are used as a key tool in various engineering applications such as building aerodynamics, study of boundary layers, simulation of atmospheric layers, small-scale testing of aircraft/vehicle models and subsequent validation of numerical predictions They offer a rapid, economical, and accurate means for aerodynamic research (Kulkarni et. al., 2011).
The Reynolds (Re) number is a quantity which engineers use to estimate if a fluid flow is laminar or turbulent. This is important, because increased mixing and shearing occur in turbulent flow. The Reynolds number is calculated using mean velocity, pipe diameter, density, and viscosity, and is valid for any fluid. The Reynolds number is also dependent upon the geometry of the pipe, as well as the roughness of the walls. Yet, no successful analytical methods for determining the Reynolds number have been developed due largely to the difficulty associated with predicting turbulent flow, and so Reynolds numbers for flow through pipes or around immersed objects must be determined experimentally (Yang, 2012).
In view of this necessity, a subsonic wind tunnel has been installed at Žilina Airport in Dolný Hričov in the year 2020. The layout of this tunnel with the dimensions of various components is shown in Fig. 1. It is an open-circuit type wind tunnel with a square test section of 0.35 x 0.35 m 2 and 0.650 m length. Air flow is established in the test section by using an axial fan powered by 400 W electric motor. This subsonic wind tunnel consists of four segments or sections. The inlet section and diffuser are made of laminated wood. The process of lamination was done with the usage of five levels of plywood, which were glued together in special forms in order to establish curved profiles.
In terms of simplicity of construction and economy, the bearing structure of the test section, which is shown in Fig. 2., is composed of aluminium strips, which were welded to the desired profile. To ensure thorough visibility of the air flow inside, the walls of the test section are fitted with LED light elements and acrylic glass, which reinforces the structure and dampens possible vibration.
Generally, in order to achieve better performance in terms of laminar flow in the test section various methods can be applied. The air straighteners or screens are often used mainly to minimize the turbulence level in the test section.
According to (Škultéty, 2018), the lower the Reynolds number the more laminar airflow can be expected. The screens cascade large-scale turbulent fluctuations into smaller scales. These decay in the settling duct must be sufficiently long to allow sufficient decay, while minimizing boundary-layer growth (Cattafesta, 2010).
According to (Kulkarni et. al., 2011) honeycomb is a passage of ducts, laid along the axis of the main air stream to suppress the cross velocity components, which are initiated during the swirling motion in the air flow during entry, The cross-sectional shapes of honeycombs may be square, circular and regular hexagonal cells, as shown in Fig. 3.

Honeycomb design criteria
To improve the relevance of output data, it is necessary to design and create an optimal screen suited for the conditions of this wind tunnel.
According to (Metha & Bradshaw, 1979), as far as the design method is concerned, the honeycomb length, the cell hydraulic and the porosity are design key factors.
The honeycomb porosity is the ratio between the actual flow cross-section area and the total cross-section area, and it is expressed by Eq. (1).
Two main conditions must be verified in the honeycomb design (Metha & Bradshaw, 1979). The honeycomb length and cell hydraulic diameter ratio must be in the range between 6 and 8 and the honeycomb porosity should be greater than 0.8 (see Eq. (2) and Eq. (3)) As shown in Fig. 4, the honeycomb screen is defined by its geometric dimensions such as: inside honeycomb cell length lhoney; outside honeycomb cell length lg honey; honeycomb cell diameter dhoney; honeycomb cell thickness Shoney; honeycomb solid sheet divisions z (Mauro, 2017).
The internal and external honeycomb cell lengths may be determined thorough Eq. (4) and Eq. (5) The solid sheet divisions can be evaluated using the same method (see Eq. (6)).
In order to calculate the solid area of the honeycomb, considering a single division, the solid area is the sum of two rectangular areas and two trapezoidal areas (see Fig. 3.). These areas can be calculated using Eq. (7) and Eq. (8).
= ℎ ℎ (7) To determine the honeycomb porosity, it is necessary to determine the number of divisions height-wise and width-wise. The number of divisions height-wise is defined as the ratio between the settling chamber cross-section height and the honeycomb divisions, as showed in Eq. (9).
The number of divisions width-wise is the ratio between the settling chamber cross-section width and the sum of honeycomb cell thickness and the half honeycomb cell diameter (see Eq. (10)).
It is evident that for square cross-sections Eq. (11) comes out.

ℎ =
Therefore, the cross-section area of the honeycomb solid sheet is calculated using Eq. (12).
In a dual way, it is possible to define the honeycomb solidity as the ratio between the cross-section area of the solid sheet and the settling chamber cross-section area. According to the solidity definition, Eq. (13) comes out.
Comparing Eq. (13) and Eq. (19), it is easy to achieve the conclusion that the honeycomb and porosity are complementary factors. Thus, Eq. (14) comes out.
Therefore, knowing the honeycomb solidity, the porosity can be calculated using Eq. (14). Thus, the criterion expressed in Eq.
The design criterion in Eq.
(2) should be verified. Thus, the cell hydraulic diameter should be calculated. Firstly, the cell area is calculated using Eq. (15).
It should be noted, that both criteria expressed by Eq.
(3) must be verified at the end of the honeycomb design procedure.

Installation criteria
Final screen was placed into straight section which is part of the inlet section. The straight section provides almost 0.14 m long space right in front of the test section. The diameter of the straight section is the same as the test section, 0.35 x 0.35 m.
Based on the mentioned in the previous chapter, the final dimensions of honeycomb were set as is shown in Tab. 1. and Fig. 5.  Due to financial costs and simplicity the honeycomb screen itself was 3D printed using FDM method, which is the most frequent method for printing 3D objects such as functional prototypes and concept models (Pecho, 2019). It involves melting the working material and applying it with thin threads one on top of the other. When it freezes, the material binds together to form a hard and durable coating (Iljaszewicz, 2020). Because of the large size of the honeycomb screen, the whole printing process was divided into four individual parts (see fig.  6.). All four parts were glued together and create the final form of honeycomb screen. In order to remove possible roughness after the print, all surfaces were subjected to mechanical treatment. Sides of the honeycomb were designed to have tapered profile so it can fit right into a straight section of the inlet section (see Fig. 7.). This option reduced the possibility of vibration or incorrect placement of the screen. In addition, the smoke nozzles are located right behind the air straightener, thus the air mixed with a smoke can easily access to the its cells.

Results
Verification of velocity and laminar flow was performed after the installation of honeycomb screen. Verifications were carried out by usage of anemometer, which is a device used for measuring wind speed (air velocity) and in form of visual check. The Fig. 8. shows how the air flow looked before the installation of air straightener. The air flow is separated. Individual streamlines are separated from each other in the test section all around the tested wing profile, which presents unwanted conditions for relevant outputs of experimental measurement and visual analyses. Installation of honeycomb screen made a significant improvement in the terms of laminar flow inside the test section as we can see in Fig. 9., individual streamlines create one uniform stream or air flow which over floats tested wing profile.
However, the drawback of this improvement is the air velocity loss. The anemometer showed huge reduction of air velocity inside the test section. Before the installation, the wind tunnel could reach speed of almost 11.1 m/s (40 km/h). After the installation, the speed was reduced almost twice. Anemometer showed only 6.9 m/s (25 km/h).

Conclusion
In conclusion, it is possible to claim that the main goal was successfully fulfilled. On the other hand, the size of the honeycomb screen itself is responsible for huge air velocity drop inside the test section, mainly because it creates unwanted drag.
In order to achieve higher air velocity inside the test section, it is suggested to replace an axial fan which was powered by 400 W electric motor for a stronger one. The wind tunnel meets the specified criteria for running the experiments. The aim of the paper was to describe the design and structure of the honeycomb screen construction for specific requirements and conditions, specifically for the subsonic wind tunnel of Air Transport Department of The University of Žilina. During the design phase, the authors followed mathematical methods for calculating the dimensions of open-type subsonic wind tunnels.

Future work
A new stronger electric motor is currently being installed along with the frequency converter. The old 400 W electric engine will be replaced for a new 4 kW electric engine. The 10 times stronger fan can provide enough power a thus air velocity for experimental analysis in subsonic wind tunnel.