Author Archives: balczom

Vibration testing of tensile fabric structures

Tensile fabric structures are supporting fixtures that became popular in the second half of the 20th century. These are tensioned structures made of technical textiles, which offer a light, elegant and often extremely economical solution for covering spaces due to the used materials and solutions. In the past decades, serious research has been conducted in order to resolve the mechanical problems in relation to tensile fabric structures. Continue reading

Model Machining Tools and Workshop

Model machining tools

We are planning to acquire a 3D printer and a 3-5D milling machine to manufacture wind tunnel models. These can produce building models, model parts or terrain models from MDF or XPS foam material based on a 3D computer file. According to the plans, the machines will be operated by the Department of Freehand Drawing at the Faculty of Architecture. Their workshop has the necessary expertise and sufficient space to operate the machines, and they can be involved in other educational tasks as well (e.g. manufacturing models for the MSc theses of architecture students). This ensures the long-term utilization of the equipment.

Model preparation workshop

Along with the establishment of the wind tunnel, a model preparation workshop also has to be created inside the machine workshop of the AE building, where it will be possible to prepare the wind tunnel building or terrain models for measurements and apply the finishing touches (gluing models, assembling pressure tubes, creating EPS foam, MDF and composite parts). It is important to point out that contrary to the current workshop, this workshop can be used by students for their MSc thesis, TDK or project work. This means that the machines in the current workshop have to be relocated, the doors, windows and the bitumen floor (which is in a bad condition) have to be changed, new work tables and wood, plastic, and foam shaping small tools have to be acquired which can be used by the students.

Model storage

The minimalist renovation of the 70 m2 room in the basement of the AE building is planned so that it can be used to store wind tunnel models and satisfy the general storage needs of the laboratory, since it is in a bad condition. This includes the following: installing lighting, fixing the plaster, setting up a shelf system, creating a lifting system capable of moving models from the ground level to the existing pit and opening a door towards the stairwell of the building for the personnel.

CFD Simulation Capacity

The better quality measurement data produced by the new wind tunnel cannot only be used to directly investigate reality, but also to validate Computational Fluid Dynamics (CFD) models. In the case of atmospheric flows (e.g. examining pollutant dispersion in case of an accident), most of the field measurements are not representative since the boundary conditions for the measurement (wind velocity, direction) do not stay constant or cannot be measured in full detail for the necessary amount of time. Therefore, a wind tunnel experiment is much more suitable in these cases for validation purposes, as the boundary conditions are well known and can be measured.

Numerical simulations of atmospheric flows with great computational domain and large Reynolds numbers require great computational capacity. In other words, it needs a multi-core computer and a software license capable of handling it.

workstationHowever, a numerical simulation validated with wind tunnel experiments offers a much wider range of possibilities than solely the wind tunnel measurements. The whole 3D flow field can be investigated, thermal effects can be considered and optimization algorithms can be implemented.

Therefore, we are planning to acquire a high performance 16-24 core workstation and an ANSYS FLUENT CFD software package (capable of multi-core, parallelized simulations) for the CFD simulations of the atmospheric flows examined in the wind tunnel.

Advanced Measurement Techniques

Along with the construction of the new wind tunnel, the measurement infrastructure also has to be improved. We are planning to acquire devices with a wide range of applicability (not just for a single research project), ensuring their long-term utilization and professional payback.

1. Probe moving traverse

The most important measurement equipment of the boundary layer wind tunnel is the traverse system, which is capable of moving various probes, sensors and cameras in the three-dimensional space. The system consists of stepper motors, stepper motor controllers, linear guide rails and linear ball screw units. The project proposal OTKA K108936 (”Flow and dispersion phenomena in urban environment”) of the Department of Fluid Mechanics provided the resources to acquire several components of this system (14 m of linear guide rails and accessories which are needed to set it up in the new wind tunnel). The new traverse system follows the design of the great Göttingen-type wind tunnel’s traverse system, which was completed in 2012 and won second place at the National Instruments Hungary 2010-2011 project proposal.

Specifications

Movement range (L x W x H): approx. 14 m x 2.6 m x 1 m
Linear guide rail types: Bosch / Isel
Positioning accuracy: 0.5 x 0.2 x 0.2 mm
Movement controller: National Instruments PXI/PCI 7344/7354

2. Multi-channel simultaneous pressure measurement system

One way to determine the wind load on a building is to determine the surface integral of the pressure measurement points located on the building walls. About 200-600 measurement points are necessary for complex building geometries. The traditional measurement systems used pressure switching units, which connected the measurement points with the pressure transducer one by one, which yielded the mean and extreme pressure values at the points. This method is very time-consuming and does not provide a simultaneous, momentary pressure distribution.

The new system is based on the simultaneous measurement of miniature pressure sensors with favorable price (40-50 EUR/piece), with the help of multi-channel A/D converters. The PXI-format A/D converters will be a part of the laboratory’s existing PXI framework, and could be used to design systems with 500-600 channels. A pilot system with a smaller channel count will be used for testing before the full system is constructed.

Specifications

Planned channel count: 496-576
Sensor type: Sensortechnics HCLA, Honeywell Trustability
Sensor characteristics: amplified analog output, temperature compensation
Sensor measurement range: +/- 500 Pa … +/- 800 Pa
Combined measurement uncertainty of the sensor: <0.5% FS
Data acquisition card: National Instruments PXIe-6375, 208 Analog Inputs

3. Force and torque measurement system

The wind load on the buildings can also be determined by measuring the forces directly. This can be carried out by mounting the building on a multi-component force measurement platform. For example, the forces and moments acting at the foundation of a tall building can be determined. As the time function of the forces and moments is also important, the natural frequency of the system has to be much higher than the expected frequencies. The resources of this project proposal would be used to acquire load cells and transducers.

Specifications

Number of components: 6
Measurement uncertainty of cells: <0.2% FS
Cell measurement range: 50-100 N
Cell output voltage: max. +/- 10V
Natural frequency of cells: >1000 Hz

4. Vibration measurement using a 3D digital image correlation system (DIC)

The new laboratory will be suitable for aeroelasticity measurements, which means that it will be possible to observe the interaction between the wind forces and the vibrations of buildings/structures. An earlier measurement of an aeroelastic bridge model flutter can be seen below, which was carried out in the great Göttingen-type wind tunnel (the investigation was performed by Professor Dr. Tamás Lajos and Dr. Gergely Szabó).

 

DIC

Digital Image Correlation (DIC) system components: lighting, cameras, controller, software (Photo: Lavision GmbH)

During the investigation of aeroelastic models, if the degrees of freedom of the system well exceed the number of accelerometers in the vibration measurement system, then the exact determination of the motion is performed by the 3D digital image correlation (DIC) system. This system uses stereoscopic camera orientation to determine the spatial displacement of the reference points. If the model is painted appropriately, the number of reference points can be several million. The size of the investigated domain can be adjusted in a wide range by changing the objective, making the system viable not only for wind tunnel vibration measurements, but also for failure investigation of membrane structure materials, for example. The temporal resolution of the system is determined by the speed of the cameras. For aeroelastic wind tunnel investigations this has to be over 100 Hz.

5. Pitot-Static tubes , pressure transducers, measuring amplifiers

Multiple large-sized Pitot-Static tubes and pressure transducers are necessary to measure the reference wind speed set in the new wind tunnel.

Specifications

Number of Pitot-Static tubes: 3
Length of Pitot-Static tubes: min. 1 m
Number of pressure transducers: 3-6
Measurement range of pressure transducers: 0-5 mbar,  +/-5 mbar,  +/-1″WC
Output voltage: 0-5 V or 1-5 V
Measurement uncertainty: max 0.15% FS

6. Multi-hole probe

The most suitable way to measure the time-dependence of all three velocity components in the wind tunnel atmospheric boundary layer is by using a multi-hole (4, 5, 7-hole) probe and a corresponding measurement software. The measurement principle and a practical example can be seen on the images below.

Specifications

Min. measurement frequency: 500 Hz
Max. probe head diameter: 5 mm
Other specifications: Measurement system and software should be able to connect to the existing National Instruments analog data acquisition card

7. High performance lasers for LDA applications

lda

Solid-state laser source for LDA applications (Photo: TSI Inc.)

In the new wind tunnel, Laser-Doppler Anemometry (LDA) will be used to measure wind velocity around buildings, structures and in recirculation zones with great spacial and temporal resolution. The multi-hole probe cannot be used in these locations. The available 300 mW laser source in the laboratory is not enough for measurements with a focal length greater than 500 mm, thus there is a need for stronger laser sources. In order to limit the energy usage and avoid the complicated water cooling of the lasers, we are planning to acquire solid-state laser sources instead of gas lasers. Below  the wind tunnel measurement of a model of József Nádor Square in Budapest  can be seen with LDA, together with the results.

Specifications

Number of laser sources: 2
Wavelength: 2 different wavelengths, between 457 – 532 nm
Power of each unit: >500 mW
Other specifications:
– solid-state laser,
– the laser beam has to be shifted by 40 MHz,
– the connection has to be compatible with the optical couplers of the TSI 2D LDV system.

The New Laboratory

The most significant, and also the most challenging part of the project is the construction of the laboratory space accommodating the wind tunnel. Based on the dimensions of the wind tunnel, the necessary space for the proper recirculation of the air and the space needed to move the models, a room with a length of 30-32 m, a width of 9-10 m and a height of around 6 m is needed. The laboratory has to be:

  • heated and well insulated as it will be used throughout the year,
  • supplied with air exchange openings of appropriate size to satisfy the need for air exchange with the external environment,
  • able to shut out the light due to the occupational safety regulations regarding flow visualization and laser measurement techniques,
  • supplied with a gate and a cargo elevator to transport the models and measurement equipment.

As the wind tunnel must be located inside the AE building or its immediate vicinity (this is the only way the synergy between the wind tunnels can be exploited), and there is no sufficient space in the current laboratory, a new laboratory space has to be built. The central campus of BME is classified as a World Heritage Site buffer zone, and the AE building was built in 1936 on the filled-up former Danube riverbed. The potentially high construction costs of the laboratory space are justified by the fact that difficulties arising from the architecture, construction permission and statics have to be overcome.

8 different variants were examined for the location of the laboratory space, of which variant ‘I’ offers the widest range of applications. The detailed design of the laboratory was prepared by Norman Engineering Bureau.

The planning application documents were submitted to the building authority of District XI. (Újbuda) in December 2016, and a detailed budget plan is also available.