#: locale=en ## Action ### PDF PopupPDFBehaviour_1FDBE065_0F1E_7147_4193_DE4BBBC820E6.url = files/0900766b8002e11f_en.pdf PopupPDFBehaviour_1FDBE065_0F1E_7147_4193_DE4BBBC820E6.url = files/0900766b8002e11f_en.pdf PopupPDFBehaviour_1A57E71B_0F0E_70C3_41A0_DEFFB23C594C.url = files/6-Axis_Force_Sensor_K6D40_500N_20Nm_MP11_20221002_en.pdf PopupPDFBehaviour_1A57E71B_0F0E_70C3_41A0_DEFFB23C594C.url = files/6-Axis_Force_Sensor_K6D40_500N_20Nm_MP11_20221002_en.pdf PopupPDFBehaviour_1B68DE5F_0F06_3144_418B_F1FFB92A1472.url = files/dax--optoNCDT-1750BL--en_en.pdf PopupPDFBehaviour_1B68DE5F_0F06_3144_418B_F1FFB92A1472.url = files/dax--optoNCDT-1750BL--en_en.pdf PopupPDFBehaviour_280CD36C_0F0E_7745_41AE_4BA4559EB529.url = files/force%20springs_rz-162u-23i_en.pdf PopupPDFBehaviour_280CD36C_0F0E_7745_41AE_4BA4559EB529.url = files/force%20springs_rz-162u-23i_en.pdf PopupPDFBehaviour_280CD36C_0F0E_7745_41AE_4BA4559EB529.url = files/force%20springs_rz-162u-23i_en.pdf PopupPDFBehaviour_280CD36C_0F0E_7745_41AE_4BA4559EB529.url = files/force%20springs_rz-162u-23i_en.pdf PopupPDFBehaviour_29DDF277_0F0E_1144_41A8_7C4EE7F4FBCA.url = files/springs_rz-162u-23i_en.pdf PopupPDFBehaviour_29DDF277_0F0E_1144_41A8_7C4EE7F4FBCA.url = files/springs_rz-162u-23i_en.pdf ## Hotspot ### Text HotspotPanoramaOverlayTextImage_73ABD443_1102_3143_416E_CED2D5069C0F.text = Back to middle HotspotPanoramaOverlayTextImage_5CFF8CD5_1106_F147_41AA_7224F151F493.text = Back to middle HotspotPanoramaOverlayTextImage_28035C93_0F02_11C3_41A9_F0FF73B4D212.text = Enter Chamber HotspotPanoramaOverlayTextImage_37853F3C_0F02_30C5_418D_3600680C67F2.text = Enter the Horizontal cylinder experiment HotspotPanoramaOverlayTextImage_D7198E68_C64A_8789_41A6_C5E5FC7CD7AE.text = Enter the building HotspotPanoramaOverlayTextImage_57707D2B_1106_10C3_419F_B556D983A746.text = Enter the grouped cylinders experiment HotspotPanoramaOverlayTextImage_EA631FF4_F9EB_D95A_41B8_E023F46C462E.text = Enter wind tunnel hall HotspotPanoramaOverlayTextImage_DBA0E061_D573_7E35_41E9_B32182F32219.text = Exit Building HotspotPanoramaOverlayTextImage_3A2384D3_0F02_1143_41AC_D5F8D426B231.text = Exit Chamber HotspotPanoramaOverlayTextImage_56A0A436_1102_10C5_41AD_BAE463A9E485.text = Exit Chamber HotspotPanoramaOverlayTextImage_EA5B0516_F9ED_6EC6_41D1_E3F3CECADD54.text = Exit Lab HotspotPanoramaOverlayTextImage_D9A8E6B0_FBE6_ABDA_41EC_63BBF07F4E01.text = Exit the Hall HotspotPanoramaOverlayTextImage_DC20E891_FBAD_67DA_41EB_4A6C397448BA.text = Exit the Hall HotspotPanoramaOverlayTextImage_C08F2E2A_FAEA_DACE_4182_13FFAE87FBD9.text = Flow direction HotspotPanoramaOverlayTextImage_2BF8918B_0BA0_AC2F_419C_F07329BBE4C2.text = Force balance HotspotPanoramaOverlayTextImage_D75AE47B_FA67_EF4E_41A9_B9D6E9A4434A.text = Inlet with Honeycomb Mesh HotspotPanoramaOverlayTextImage_DAC1852E_D575_660C_41DC_E708418BDF2C.text = Lab Entrance this way HotspotPanoramaOverlayTextImage_2A25214A_0BA0_EC29_4179_64945DAB03E2.text = Laser sensor HotspotPanoramaOverlayTextImage_EAA30554_F9EA_E95A_41DB_F342B573D4A7.text = Move closer to control devices HotspotPanoramaOverlayTextImage_2BC84C4D_0BA0_742B_4177_3F963DB2D214.text = Pressure sensor HotspotPanoramaOverlayTextImage_D3AB1BEA_FA5B_D94E_41E3_572ED96100FF.text = Step back to the Hall corner HotspotPanoramaOverlayTextImage_E415DA2D_FBDD_BACA_41E8_F35A87678364.text = Step closer to the Rotor HotspotPanoramaOverlayTextImage_D659C38B_FA7A_A9CE_41D2_51A574EF58D7.text = Step closer to the middle of the Hall HotspotPanoramaOverlayTextImage_CCDDB015_FADE_A6DA_41C5_CF720BC1F35A.text = Step closer to the motor HotspotPanoramaOverlayTextImage_E544E13C_FBDB_66CA_41E6_990C3AAE3957.text = Step forward to the middle of the hall HotspotPanoramaOverlayTextImage_3F730BE7_0F3E_1743_41A0_D367F926B052.text = To Rotor HotspotPanoramaOverlayTextImage_C3980032_FADF_66DE_41EE_C5003F05AB83.text = To the Inlet HotspotPanoramaOverlayTextImage_20272DDF_0F02_F343_41A4_F8FDBF371B83.text = 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Look around you. You will find different types of Hotspots. You can have interactions with them by clicking on them. Some take you to other scenes, AKA panorama views of the real world. Some include information, such as text, images, and videos. Some might even have question cards! So stay put, and move around with your cursor/pointer.
"Go-To Panorama" Hotspots are depicted with arrows on the ground. Follow them. Enjoy :)
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Welcome to the Chair of Wind Engineering and Fluid Mechanics (WIST) at Ruhr-Universität Bochum! This is a virtual tour of our wind tunnel, depicting two atmospheric boundary layer experiments recently conducted by WIST researchers.
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A free-vibration experiment can be done in this free-vibration test-rig, which is designed and created by Dr.-Ing. Francesca Lupi. The concept is to let the horizontal cylinder model to move freely due to wind with the associated damping and mass.
To make sure the cylinder’s movement is solely due to the wind actions, not influenced by deflection of the spring, the test rig is designed so that all eight springs (other four are on the other side) are in tension. This means that maximum oscillation in the measurement will be acceptable as long as the oscillation occur when the spring is still in tension. (Source figure: Francesca Lupi, 2019)
In the process of designing the test rig, mass of all the moving bodies must be considered, this includes the cylindrical model, traverse bar, and the accessories. Since the test-rig is designed when spring of tension, stiffness of the spring can be considered to estimate the damping of the system. Additional damper may be added, to observe ranges of frequencies.
In this Free-Vibration test rig, the measurement will focus on the oscillation of the cylinder with the given wind flow. The oscillation is measured by laser measurement. Four laser sensors are placed on the lower-middle part of the test rig, both on left and right. The process of designing test-rig is very important, where the first estimation of highest lock-in amplitude due to Vortex Induced Vibration, should be estimated properly. This is critical so that the distance coverage of the laser sensor is sufficient to observe the estimated range of amplitudes.
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Data measruements are conducted on this PC using a software called "S-Bench" (first picture). Before each experiment, the S-Bench script and sensor channels are configured. Data recordings can start by pressing the green arrow symbol on the left and will last according to the recording period set in the script. In parallel, the real-time signal through the oscilloscope is monitored to control if the applied signal is plausible.
For each experiment, there are 3 phases and for each phase measurements are recorded:
1. First reference measurement,
2. The main measurement with the wind flow,
3. Second reference measurement.
The reference measurement or "Nullmessung" refers to a short measurement where there is no wind flow in the chamber. But, can you tell, why it is important to have two reference measurements before and after the main event? (Hint: Look at the second picture)
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During the experiment, the model or test structure can be observed through the monitoring camera system. Sometimes, when the oscillations are too fast for the naked eye, plausibility check between the recorded signal and the observed oscillation can be made. This will help the researcher to better plan and perform the experiments.
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Force balance or force sensor is used to measure the force and bending moment that is resulted from the incoming wind load. In the focus of grouped cylinder experiment, only 6-Axis force/torque sensor is displayed in this virtual tour. The 6-Axis force sensor measure the force and moments in the three directions of space. This is very useful for a cantilever model such as a scaled model of steel chimney or wind turbine towers.
The force balance or force sensor that is used in the grouped cylinder experiment is K6D40 500 [N ]/ 20 [Nm] from ME-Meßsysteme. The sensor housing is made from stainless steel. Quoted from the technical specification of the sensor for conciseness, the force is applied to an annulus / 6 segments of a circle, on the end faces of the sensor. A centering hole is provided to secure the angular position. This means that customized mounting devices need to be designed according to the model. Centering and positioning of the mounting is very important to obtain an accurate experiment result. In the next slide, technical specification of the sensor is provided.
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Horizontal cylinder model is mounted on both sides of the test rig (e.g., Free-vibration or Forced-vibration test rig). Such experiment set up refers to a 2-D investigation. The circular cylinder model that is shown here has Diameter D = 150 mm and Length L = 1780 mm.
This means that the investigation will not address the three-dimensionality of aeroelastic phenomenon, and correlation length of the vortex shedding. However, 2-D experiments are still very important and they pioneered most wind tunnel campaigns.
The horizontal cylinder is not only limited to a circular cylinder cross section, but also with other cross-sections. For example, one can use rectangular or bridge deck cross section. In the use of model that is prone to flutter, the test setup should provide the measurement in three degree of freedom: horizontal (surge), vertical (heave), inclined and rotation (pitch).
Additionally, one can simultaneously measure the wind pressure by attaching pressure taps on the model’s surface.
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In the 2D experiments, one does not need the Counihan method to generate the atmospheric boundary layer, as the wind flow should be uniform along the wind tunnel height. To address the use of turbulence, one can place an additional grid, as seen in this picture, inside the tunnel chamber.
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In the inlet side of the wind tunnel chamber, a Counihan Hardware is installed. Counihan Hardware or Counihan method is a common practice and to model the atmospheric boundary layer (ABL) in the wind tunnel. The Counihan method dates back to 1969 when it was firstly introduced. It consists of three main parts: castellated barriers, counihan vortex generator, and the ground roughness. The generated ABL which considers the terrain roughness, depends on the design of the castellated barrier, the height and shape of the counihan vortex generator, and the distribution of ground roughness. Further information on each of the component is provided when you move closer to the inlet.
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In this hotspot, the short exemplary footage is shown about the free-vibration experiments with horizontal rectangular cylinder model. The model moves vertically, as the vortex resonance occurs.
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Laser sensor or laser displacement sensor is used to measure the displacement by using the concept of reflection of the radiated laser light. When there is an object placed in front of the radiated laser light, the laser ray is blocked, and the ray is reflected back to the sensor. The distance between the object and the laser sensor can be then measured. Generally, two concept of the reflection of the laser ray is confocal method and triangular method. Confocal method sensors emit and receive the laser ray in the same axis. The triangular method sensors emit the laser ray, and when the laser ray is reflected, it is reflected in an angle to a different side on the sensor (Source: Keyence.eu)
In the WISt Wind tunnel, the triangular laser sensor is used. As an example, one of the sensor is ILD1750-200 from Micro Epsilon manufacturer. This sensor has 200 mm measuring range. The minimum measuring range is 70 [mm], and the maximum measuring range is 270 [mm]. This is an important parameter as a consideration to choose which laser sensor should be used. The measuring range should be compared with the expected oscillation of the investigated model. Other important parameters to be considered are the frequency of the sensor, input and output type, and the sampling rate. (Source: Micro Epsilon)
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Obtaining the oscillation of the model is one of the main objectives of the experimental campaign of grouped cylinders. Oscillation at top can be calculated during the test with wind from the forces or bending moments measured at the base. In the case of resonance, when the vortex shedding frequency is around the natural frequency of the model, the reaction at the base is caused by the distribution of initial forces along the height. This then allows the derivation of the displacement at the top from the reaction at the base. (Source Figure: F. Lupi, 2019)
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One of the most important steps before conducting an experiment in wind tunnel, is to obtain the information of structural properties. Structural damping is the key parameter to understand how susceptible is the structure to an aeroelastic phenomenon, such as Vortex-Induced Vibration. Snap-back test can be done to obtain the structural damping. (Source: “Logarithmic decrement” by “Vietnamgeometer” through Wikipedia is licensed under CC BY-SA 4.0.)
The concept of snap-back test is to give the model a static load, and then release the given load so that the structure will freely vibrate in still air. The structure will oscillate, and the oscillation will decay depending on the structural damping of the model. Note that the structural damping includes the existence of still air.
For example, for the case of cantilever model, a weighing mass is used to give a static load to the model. This is done by connecting the model with a rope and a weighing mass. Then the mass is released by cutting the rope, so that the cantilever model will freely vibrate.
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Prandtl tube is used to measure the dynamic pressure of the undisturbed flow in the wind tunnel. This will give the information about the undisturbed wind speed. The Prandtl tube can be placed not only on the top wall, but also on the ground of the wind tunnel. For pressure measurement, the pressure of an undisturbed flow is usually used as reference pressure. This means the Prandtl tube will be connected to the box of pressure sensors and gives the reference value for all the pressure sensor channels (See information about the sensors in the control room.). Calibration of the Prandtl tube as reference pressure should be done.
The main goal of the Prandtl tube is to measure the undisturbed velocity pressure, utilizing the concept of stagnation point. In stagnation point, when the flow hits an object, all the kinetic energy is converted into pressure. The wind speed is zero at this point. The pressure at this stagnation point represent all the incoming dynamic pressure from the wind and can be measured.
Prandtl tube then utilizes this concept, which can be demonstrated with “U-tube” mechanism that connects both the stagnation point (2) and undisturbed flow with existing wind speed (3). The difference in pressure between the two points can be measure by the indication shown in height difference in the fluid inside of the U-tube.
The same concept of two different points around the Prandtl tube is used. However, instead of using U-tube, a sensitive membrane is used to measure the pressure difference between two sides. The pressure difference between the points is measured and its value is measured in Volt. (Source figure: tec-science.com)
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Pressure sensors are used to measure the wind pressure inflicted on the structure, whether positive pressure (incoming wind flow) or negative pressure (suction caused by vortex shed). Recorded pressure values are related to the wind loading values. Generally, the choice of pressure sensor depends on the measuring range that one aims for.
One of the pressure sensors that is commonly used in this wind tunnel is the Honeywell 170 PC (second picture, Source:Koss, 2001 ). The pressure sensor consists of openings on two sides, wherein between a sensitive pressure cell/membrane is placed. This sensor has a range of +/- 35 [ mbar ].
The sensor is usually placed outside of the model, due to narrow spaces in the model. This means that a tube is attached on the model in a small bore (e.g., 0.7-1 [mm]), which is then connected to one side of the sensor. The other side of the sensor is connected to reference pressure (e.g., the velocity pressure measured in Prandtl Tube). Therefore, the pressure sensor will measure the difference between the two sides, and it is able to measure the incoming wind pressure. The pressure sensor also has an electrical output which is connected to the electrical equipment and data acquisition system.
Before conducting the experiment, calibration of the pressure measurement system must be done. The calibration can be done in two phases: static phase and dynamic phase. The static calibration
was performed to establish the pressure–voltage relation for each pressure sensor, while dynamic
calibration was performed to correct the dynamic effects of tubes (Quoted from Hemida et al., 2020, adapted from Neuhaus, 2010). Only static calibration will be explained in this virtual tour.
The static calibration can be done by giving a specific value of pressure (e.g., 5 [mBar]) which can be monitored through Betz-Manometer. It is known that the pressure sensor type that is used from Honeywell has measurement range +/- 5 [V] that relates to +/- 5 [mBar]. Therefore, by giving 5 [mBar] value the voltmeter which is connected to the measurement system should show absolute value of 5V. (Source: Poufayar, 2017).
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The experiment of grouped cylinder in this virtual tour is focused on the 2-in-line arrangement with variation of the incoming wind direction. The distance-to-diameter ratio between the two cylinders is kept at 1.25. This is a small distance that is susceptible to the interference effects. In this hotspot, the previous studies investigated the 2-in-line arrangement as the basis and the reasoning of the experiment results observed in WIST wind tunnel (see videos of the oscillation of the cylinders). (Source Figure: Alam, 2013)
When the other cylinder is placed closely to the other cylinder, the effect of interference exists and based on the given distance, its effect varies on the distance. Please note that the incoming wind direction is also contributing simultaneously. However, for conciseness, only the effect of the distance between cylinder is presented in this paragraph. One of notable study that had investigated the effect of distance between 2-in-line configuration of grouped cylinders is Igarashi (1981). As shown in the figure, the distance-to-diameter ratio L/D of the two cylinders gives different effect of flow patterns around the cylinders. (Source Figure: Igarashi, 1981).
More general categorization had also been done by other works (Zdravkovich, 1987, Zhou et al. 2004, 2006, Ljungkrona and Sunden, 1993) by dividing the type of flow pattern into three categories: Extended body, reattachment, and co-shedding regime. Extended body describes the flow that the vortex shedding from first cylinder envelops the second cylinder, as the distance is very close (i.e., L/D=1-2). This means the two body can act as an extended body. Reattachment regime refers to the fact that the boundary layer of the vortex shedding of the first cylinder reattaches on the second cylinder. This gives additional forces on one of the sides of the second cylinder. This usually happens when the distance L/D is around 2-5. Co-shedding regime refers that the two cylinders are placed with enough distance, in which that vortex shedding of each of the cylinder able to completely form. Usually, the distance L/D is larger than 5. Further reading is referred to the references at the end of the slide.
As seen in the recorded videos of the observed oscillation of the two cylinders, the cylinder moves and have higher oscillation at specific nonzero wind direction. This is called a critical wind direction of interference galloping. In such way, when the two cylinders are placed close enough between each other and a certain wind direction comes, the oscillation persist and is self-sustained. Based on previous studies and wind tunnel tests, the critical wind direction on a 2-in-line configuration is predicted to be around 5-10°. (Source Figure: Ruscheweyh, 1983)
The high oscillation can be addressed with the observed value of lift coefficient of the cylinder. Schewe and Jacobs (2019) conducted a wind tunnel experiment on 2-in-line cylinder, where force measurements were performed, in the transcritical Reynolds number range. The different wind direction from -20° to 20° was observed, and nonzero lift coefficient was found around the critical wind direction.
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The first component to manipulate the wind flow, in the Counihan method, is castellated barrier. After the wind flow goes from inlet and the honeycomb mesh, castellated barrier is placed to reproduce large eddies. Reproduction of large eddies are a challenge in the wind tunnel experiment, but not impossible.
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The force-balance is installed here at the base of cantilever model, where the model is fixed. For example of the grouped cylinder experiments with cantilever model, the force balance used is K6D40 from supplier ME-Meßsysteme. The sampling rate in the scope of grouped cylinders experiment is 2000 Hz. For further details, see information about the sensors in the control room.
The force balance is connected to the data acquisition system which will be further connected to the monitoring system in the control room of wind tunnel.
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The forced vibration test is one of the testing approaches where the aeroelastic properties of the test structure can be determined. It is important as it refers to the aeroelastic stability of the test structure under phenomenon such as galloping and Vortex-Induced Vibration (Lupi et al., 2018). The concept of forced vibration test generally utilizes a propulsion unit which drives the model harmonically. Due to the greater mechanical effort in this type of test, influence of turbulence in the wind tunnel can be minimized. Therefore, pure sinusoidal oscillation can be better realized. (Source: Neuhaus, 2009)
The test rig of the forced vibration in the WISt Wind Tunnel was developed by Neuhaus (2009, 2010). The propulsion unit of the test rig will move the horizontal model, where it oscillates harmonically at given amplitude and frequency. The model can move in three degree of freedom: horizontal (surge), vertical (heave), inclined and rotation (pitch). With the given frequency, the model oscillates, and its oscillation can be measured by laser sensors. At the same time, the vertical, horizontal forces and torsional moment are measured by force balance that are placed at the two ends of horizontal model. In this way, measurement of the force and oscillation of the model can be done simultaneously. (Source: Neuhaus, 2009)
To give summary on the mechanism of the forced vibration test rig, following passages are presented. The propulsion unit (motorized equipment) drives the horizontal shaft that extends vertically to the connecting rod through the steel disk in the lower side. The connection between the vertical connecting rod and the steel disk is eccentric. The eccentricity can be manually adjusted to select the desired oscillation amplitude. The desired frequency can be adjusted through the imposed rotation speed, where the propulsion unit rotates the steel disk and mechanically will set a vertical movement of the connecting rods. Then, the connecting rod will rotate the aluminum disk limited to a small range of rotation angle.
The previously mentioned aluminum disk is then connected to a rectangular steel plate, which is connected rigidly to the external test rig frame. To achieve the pure vertical movement, the aluminum disk is connected to a crosspiece which can only move vertically and mechanically converts the rotational movement of aluminum disk to a vertical movement of the crosspiece. The crosspiece moves vertically along the guiding rods. This is important to evaluate a pure harmonic oscillation in vertical direction of the horizontal model, for the case of Vortex-Induced Vibration. (Source: Adapted from Neuhaus, 2010, taken from Lupi et al., 2018)
The maximum oscillation amplitude in this forced vibration test rig is 7.5 cm. Besides the movement in vertical direction (z), this test rig also can allow the movement to be purely rotational (α) or to be purely translational in horizontal (y) direction. The maximum frequency of oscillation is 7.25 Hz. Regarding the measurement of oscillation by laser sensors, the sampling rate is 2000 Hz. Further details about the forced vibration test setup are available in Neuhaus (2009, 2010) and Sarkic et al. (2012, 2015). (Source: Neuhaus, 2009)
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The model is mounted and fixed at the base to the force balance. The force balance sensor will measure the response at the base. Further information of the force balance can be seen in the Control Room and outside of the wind tunnel chamber.
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The specification of tensioned spring is determined in the process of designing the test rig. Most important parameters that determine the choice of springs are spring elongation range, mass of the spring and forces of the spring.
One of the spring that is selected to be used in this test-rig is tensioned spring from Gutekunst Federn, RZ-162U-23I. Please note that this is not the only spring that can be used for the test rig. A sophisticated design concept of the test rig allows the researcher to select the spring based on the given model and its natural frequency. In this virtual tour, the spring RZ-162U-23I will be used as an example.
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These pictures depict in detail the measurement equipment and respective electrical components (i.e. the amplifier, oscilloscope, measurement cards, power supply, voltmeter, multimeter, etc.).
The workstation has 8 measurement cards, each card having 16 channels. The total 128 channels for sensors are provided. This means that maximum 128 sensors can be run parallelly. Each channel refers to 1 voltage value/signal. The software "S-Bench" and the experiment script in the software allocates measurement cards to respective channels and data recording procedure.
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To able investigating different wind direction, the wood plate where the cantilever model is placed is a turntable. By turning the table, the model is rotated, in which the incoming wind flow will come in different direction from the perspective of the model. The diameter of the turntable is 1.7 m.
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Turbulence generator or precisely, a Counihan Vortex Generators, is a component and a part of Counihan technique to experimentally simulate the turbulent flow in full-scale/reality. Flow from the inlet and after passing through the castellated barrier, will develop vortices with vertical axes around the Counihan vortex generators.
The Counihan vortex generator has form of quarter-elliptic, constant-wedge-angle spires body. Height and number of vortex generator are important parameters to obtain the desired vortices which has to consider the cross-section of wind tunnel chamber.
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Two circular cylinders in-line model (2-in-line) stood as the one of the configurations the campaign of grouped cylinder experiments. Other configurations such as 2x2, 4-in-line, and 2x1 are also investigated in this wind tunnel project. The model has height H = 930 mm from the ground of wind tunnel chamber and diameter of D = 50 mm. It is constructed with a carbon tube, covered by a Styrofoam coat. On the surface of the model, a properly distributed sand-grain roughness is placed on all the surfaces of the cylinder to create a rough cylinder characteristic on Reynolds number regime. This model is a reduced scale of a real wind turbine tower. The model is fixed at the base to a force balance. The distance between two model to the diameter ratio is 1.25.
Why should the model use an additional roughness? Where the real wind turbine tower has a very smooth surface in the reality. This is because of the smaller diameter of the model, due to scaling. When a scaled model is used, and smaller dimension of the model is used, the Reynolds number change. This means the flow regime changes because the Reynolds number is different. (Source figure: ESDU 80025)
Increasing roughness of the surface of a body will make a change in the Reynolds number effect. As displayed in the figure, body with rougher surface, will have tighter curve (smaller range of drag crisis) and it reaches the later regime in smaller Reynolds numbers. Note that the diameter of the real wind turbine tower ranges around 4-6 m of the adapted case. This means, in reality, the transcritical flow occurred around the wind turbine tower. By increasing the roughness, the transcritical flow can be reproduced in the wind tunnel despite having smaller diameter and smaller Reynolds numbers. Different flow regime means that different behavior of drag, lift and Strouhal number will take place. (Source Figure: Niemann, 1990)
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Welcome to the wind tunnel experiment of grouped cylinders. The experiment aims to investigate the aeroelastic interactions and effects in several grouped configuration of cylinders. The case of grouped cylinders is based on the transportation of the wind turbine tower before they are installed in the offshore. For the scope of this virtual tour of the wind tunnel, the grouped cylinder experiment will be limited to the 2-in-line configuration, where two cylinders are placed next to each other in close distance. Different wind directions are performed to address the effect of critical wind direction. (Source: Energy Tomorrow Blog. 2015. Photo in Article “Making Offshore Operations Even Safer”. Breakingenergy.com.)
Atmospheric Boundary Layer profile of the wind flow is experimentally simulated using the castellated barriers, turbulence generators (Counihan vortex generators) and the floor/ground roughness (see the other side of the room). The ABL is simulated under the condition of neutral thermal stratification. The wind profile V(z) profile is referenced to the wind velocity measured by the velocity pressure of the Prandtl Tube. The turbulence intensity profile is determined by fitting the equation to the measured value. The wind speed and turbulence intensity profile can be seen in the figure in this slide.
In general, the grouped cylinders experiment is performed such: 1. Designing the experimental setup and parameters, such as model, structural properties (e.g., mode shape, modal mass, equivalent mass). 2. Manufacturing of the model, installation of the test setup and calibration of the sensors. 3. Validation of structural properties by snap-back or free-decay test. 4. Dynamic test with the given wind flow based on the chosen wind speed variation to be investigated. The test is performed by means of response measurement. 5. Evaluation of the measured response of test structure, for example, the oscillation at the top, lift coefficient, and Strouhal number. In this virtual tour, experiments are provided to estimate the critical wind direction between two in-line grouped cylinders.
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Why do we need ground roughness?
The ground roughness is used to act as an artificial roughness of the earth. Different terrain roughness on the earth’s surface give effect on the shape of atmospheric boundary layer (ABL). Different shape and profile of ABL gives the different wind load on the structure, as the wind speed along elevation may differs. Rougher terrain roughness gives higher turbulence near the ground in the incoming wind than the flatter roughness. (Source Figure: TU Braunschweig)
Further information on the effect of terrain roughness can be informatively described (but not limited to), by the Eurocode standard on wind action (EN-1991-1-4). Please note that the figure shown is used for informative purpose of this virtual tour. The analysis of ABL referring to Eurocode has to consider the national annex. (Source Figure: EN-1991-1-4:2005 and DIN EN-1991-1-4/NA:2010)
The actual profile that is experimentally generated by the wind tunnel depends not only on the ground roughness, but also on the former components of the Counihan method, i.e., castellated barrier and vortex/turbulence generator. Therefore, it is important to know the wind profile that occurs in the wind tunnel. Usually, in the beginning of wind tunnel experiment campaign, validation of wind profile is performed by measuring the wind speed along the height of the wind tunnel. (Source Figure: Kipsch, 2010)
In the WISt Wind Tunnel of Ruhr-Universität Bochum, one of the configurations of Counihan method can produce such wind profile, as seen in the figure. The measured wind speed can also be analyzed by its spectra where Von Karman spectrum can be observed. This is an important parameter to characterize the incoming longitudinal wind speed and to estimate the length scale of the incoming vortices. For example, of the spectrum shown in figure, it can give an information about most-dominant vortices’ frequency range referred by the peak of Von Karman spectrum. (Source Figure: Hemida et al., 2020)
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In general, the sensors that are installed on the model will have the electrical output that is connected further to data acquisition system. This depends on which sensor is used, and whether the manufacturer of the sensor requires additional accessories for data acquisition process. Each sensor that are used in the experiments will be connected to computer with measurement cards. The measurement cards provide available channel slots for each sensor. Amplifier may also be needed be needed. General sequence from sensor to the recorded data can be seen in the figure. The path from the measuring point to the recorded value in the file must be unambiguous and traceable. For example, after the installation it is strongly suggested to check the measurement circuit by checking the tightness of the connections.
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The AC 3-phase motor is governing the rotor. The AC motor is a product of Piller company, model type "KLA 1045-6", with 400 [V] voltage level, 100 [kW] power, and rotation rate of 30 up to 1500 [RPM]. The motor is checked, calibrated, and lubricated once a year.
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The WIST boundary layer wind tunnel at RUB has a length of 9.4 meters. The cross section of the chamber in the test section is 1.6 meters high and 1.8 meters wide. The maximum wind speed which can be performed is about 30 [m/s], which corresponds to 86 [m^3/s] volumetric flow rate. The wind tunnel can provide both 2D test conditions and the atmospheric boundary layer of incoming wind profile. To obtain the atmospheric boundary layer profile, the wind tunnel is equipped with castellated barrier at the beginning of the chamber, Counihan Vortex or turbulence generators, and ground roughness setups. The turn table made of wooden plate, on which a 3D scaled model may be placed, is used to adjust the incoming wind (flow) direction.
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The control board or control station is used to start and stop the operation of the wind tunnel, as well as to set the wind speed of the wind flow. The wind speed is indicated by the engine rotation, which is displayed in angular frequency (ω) [ RPM ] and frequency (f) in [ Hz ]. Once the engine rotation is stable, then the measurement/recording of the data begins.
1 ω = 2πf
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The Rotor (fan) sucks the air into the wind tunnel and creates a wind flow from inlet into the wind chamber. The fan is controlled by an AC motor on the right side. The diameter of the rotor is 2.23 meters.
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