1 Introduction to the Wind Tunnel - Part I(1).pdf

Aerospace Engineering Department AERO 335 Aerodynamics – I Laboratory Introduction to the Wind Tunnel Part I – Overview & Components of Wind Tunnel Objective: To get an overview of the Wind Tunnel avai...

Aerospace Engineering Department AERO 335 Aerodynamics – I Laboratory Introduction to the Wind Tunnel Part I – Overview & Components of Wind Tunnel Objective: To get an overview of the Wind Tunnel available in the laboratory and learn about its components Equipment Required: Wind Tunnel with all accessories Prepared by: Mr. Mohammad Abdul Majid Siddiqi  Introduction to the Wind Tunnel – Part I 1. General Overview The Wind Tunnel is designed for bench top operation, with a square, transparent working section and a variable-speed fan for wind speed control. The operating range is nominally 0 – 32 m/s with no model installed in the working section. The maximum velocity achievable will vary with the type of model installed and depends on the blockage created by the model (most of the models available for use in the tunnel are designed for use at lower velocity). The tunnel is designed with an inlet flow straightener and a contraction ratio (of 9.4:1) to give well developed air flow through the working section. Experiments of flow around bodies can be performed using the wind tunnel, including a visual indication of flow path as well as measurement of static and total pressures, lift and drag. The tunnel incorporates an interface (electrical console), which provides connection to a PC. The Wind Tunnel software provides sensor output logging and fan control as well as performing any required calculations for each experiment. 2. Equipment Diagram Descriptions of a few items in the above equipment diagram: Label No. Description Label No. Description 2 Flow Straightener 10 Fan 3 Contraction section 11 Electrical Console 5 Static Pressure Sensor (electronic) 12 Inclined Manometer 6 Roof Tappings 13 Displacer 7 Circular Opening Hatch 15 Working Section 8 Smaller Hatch AERO 335 Page 2 of 10  Introduction to the Wind Tunnel – Part I 3. Description Overview The Wind Tunnel has a square working section (15) that is designed for bench top operation. Air is drawn in through the working section by a variable speed fan (10) located at the discharge end of the tunnel. A honeycomb type flow straightener (2) and a 9.4:1 contraction ratio (3) ensure well developed air flow through the working section. Accessories include an electronic manometer bank, which, together with the Electrical Console and Wind Tunnel software, allows full electronic monitoring and recording of the measured pressures on a PC. Important note on pressure measurement using the tunnel To minimise turbulence inside the working section the fan is mounted at the discharge end of the tunnel so that air is sucked through the working section. When the fan is operating the pressure inside the working section is therefore sub-atmospheric and any static pressure measurement will be slightly below atmospheric pressure. When using the Inclined manometer, the bottom of each tube is connected to a common water reservoir and the top of appropriate tubes are connected to the tunnel or a model inside the tunnel. At atmospheric pressure (no air flow) each manometer tube will indicate the same level at the bottom of the tube that is coincident with the level in the water reservoir. As the air velocity increases the static pressure falls inside the tunnel and water is drawn up the relevant tubes i.e. lower pressure results in larger readings on the manometer. The left hand tube on Inclined Manometer is connected to the static pressure tapping at the rear of the working section to provide a datum for measurements inside the tunnel. This measurement can also be used for calculating the air velocity at the entrance to the working section. Any manometer tube left disconnected is open to atmosphere and therefore shows the atmospheric datum. Absolute pressures in the tunnel may be determined by relating the tunnel datum to the atmospheric datum then adding the measured barometric pressure. Total pressure, as the sum of the static and dynamic pressures, will be higher than the static pressure and will therefore give a smaller differential between the (sub-atmospheric) reading and the outside air pressure, and thus a lower reading on the manometer than that for static pressure. For example, when using the Pitot Static tube, the static tapping will register higher on the manometer than the total pressure tapping. This is the opposite of normal convention when a Pitot Static tube is used in free air (where the total head reading would be greater than the static head reading). An illustration is provided below. AERO 335 Page 3 of 10  Introduction to the Wind Tunnel – Part I Ignoring frictional losses, the Total pressure (stagnation pressure) in the free stream will be equal to the atmospheric pressure. Hence, Total pressure measured using the Pitot Static tube will be very close to the atmospheric pressure indicated in unused tubes of the manometer. Note that when the absolute local total pressure is greater than the absolute local static pressure, the manometer reading for total pressure will be lower than the reading for static pressure. Also note that, usually, local static pressure = tunnel static pressure. Exceptions occur when the cross-sectional area at the point of measurement is modified, for example when using the Bernoulli Apparatus (Venturi). Pressures in the tunnel are sub-atmospheric due to the increased velocity and reduced cross- sectional area. Wind Tunnel Components This section describes in detail a few important components of the wind tunnel. 1. Working Section The working section (15) is 150 mm (6”) square and constructed from clear acrylic to give good visibility of the models in operation. The overall length of the working section is 455mm. Appropriate model / instrumentation mounting points are included in the side wall and roof of the working section. The entire base of the working section is also removable to allow the insertion of large or complex models such as the Bernoulli Apparatus, Boundary Layer Plate or alternative models. 2. Electrical Console The Wind Tunnel contains the Electrical Console (11), which allows the equipment to be controlled from a suitable PC via a USB port. 3. Static Pressure Sensor An electronic pressure sensor (5) mounted in a tapping through the side wall at the rear of the working section measures the static pressure inside the working section, allowing the AERO 335 Page 4 of 10  Introduction to the Wind Tunnel – Part I instantaneous air velocity to be calculated and displayed on the computer. The support plug incorporating the pressure sensor can be interchanged with the upstream blanking plug in the roof to allow measurement of the static pressure when using the optional Bernoulli Apparatus. 4. Circular Hatch Many of the optional models are mounted through a circular opening (7), 120 mm diameter, in the front wall of the working section. These models are permanently mounted on a hatch cover to seal the opening (flush with the inside wall of the working section to avoid disturbing the flow). The hatch cover is secured by quick release clamps on the side wall of the working section allowing rapid change from one model to another. Where necessary, the hatches incorporate an angular scale allowing the model to be manually rotated to known angles. The standard hatch cover supplied with the Wind Tunnel incorporates a central boss with a hole, locating slot and clamping screw. This feature allows models to be mounted securely in the working section when performing flow visualisation studies. 5. Small Hatch A second, smaller hatch (8) behind the model mounting position allows models to be installed downstream of the main model. A plain hatch cover is installed until this option is fitted. 6. Roof Tappings Three tappings (6) in the roof of the working section allow the flow visualisation system or the Pitot Static tube to be inserted. These tappings are located at the start of the working section, upstream and downstream of the model mounting position. Each tapping incorporates AERO 335 Page 5 of 10  Introduction to the Wind Tunnel – Part I a blanking plug, flush with the inside wall of the working section, that can be fitted when the tapping is not used to avoid disturbances in the working section. 7. Fan Air flow through the working section is generated by a fan (10) located at the outlet end of the wind tunnel. The fan is fitted with a protective grill on the outside to prevent personnel from coming into contact with the rotating blades. Care must be taken when installing a model to ensure that the model is secure before starting the fan. A model that is not secure could be sucked into the rotating fan blades causing damage to the model and damage to the fan. 8. Flow Visualisation The working section incorporates a simple technique for flow visualisation around any of the optional models. A lightweight twine follows the flow contour around the model and shows if and where boundary layer separation (breakaway) occurs and where the flow becomes turbulent or reverses. The twine passes through a stainless steel ‘L’ shaped tube that is mounted in a support plug that can be located in the roof of the working section at three alternative positions, i.e. the start of the working section (the usual position) and upstream and downstream of the model mounting position. The support plug incorporates an ‘O’ ring to retain the tube where it is positioned. A simple adjustment arrangement allows the length and position of the twine to be varied. The vertical position of the twine can be varied by sliding the ‘L’ shaped tube up or down in the support plug. The horizontal position of the twine can be varied by rotating the ‘L’ shaped tube in the support plug. The length of the twine can be varied by allowing more or less twine to pass through the tube then securing the twine to the tube by sliding the ‘O’ ring over the end of the tube. Adjustment of the length is best carried out when the Wind Tunnel is operating. The end of the twine should be tied to the ’O’ ring before operating the fan so that the twine cannot accidentally enter the working section and become entangled with the fan. 9. Manometers A manometer bank is required for use with some of the models. Two options are available: a 13 tube inclined water manometer or a 16 channel electronic manometer. AERO 335 Page 6 of 10  Introduction to the Wind Tunnel – Part I With the electronic manometer the readings and data logging are integrated with the wind tunnel operational software. With the water manometer the readings can still be integrated and recorded, but need to be entered into the computer manually. Both manometers incorporate quick release connectors that allow appropriate models or instruments to be connected. a. Inclined Manometer Bank A bank of 13 transparent tubes inclined at 30° to measure small pressure differences (0 – 160 mm H2O) using water as the working fluid for safe operation and convenience in use. When installed on the wind tunnel, the manometer is located inside the frame below the test section to the left hand side of the Electrical Console. The manometer (12) incorporates a water reservoir with a screw operated displacer (13) to allow rapid adjustment of the datum level in the manometer. Any change in the level in one tube affects the level in all of the other tubes because they are connected to the common reservoir. After each adjustment to the model, the wind speed etc. the displacer should be screwed up or down as required to restore the tube(s) at atmospheric pressure to the original datum. All readings can then be recorded relative to a common datum. The manometer incorporates quick release connectors on the side for rapid connection to appropriate models and instruments. The 10 way connector is connected to tubes 1 to 10 and the two separate connectors are connected to tubes 11 and 12. AERO 335 Page 7 of 10  Introduction to the Wind Tunnel – Part I A sliding cursor is fitted to each manometer tube. These can be slid along the tubes to record the different water levels. The reading is then preserved when a change is made allowing comparison of results. Alternatively, a set of readings can be preserved when the fan is switched off. The cursors also make the calculation of differential readings easier and help to reduce parallax error. All of the cursors can be slid to the bottom or top of the tubes when not required. Each bold engraved line on the backboard corresponds to 10 mm H2O, each fine line corresponds to 2 mm H2O (the reading is magnified by a factor of x2 because the tube is inclined at 30°). For conversion to alternative engineering units: 1 mm H2O = 9.80665 Pascals (N/m2) 1 mm H2O = 0.001 422 334 PSI 1 mm H2O = 0.039 37 Inches H2O As described already (‘Important note on pressure measurement using the tunnel’ on page 3), the static pressure in the working section will be sub-atmospheric when the fan is operating. Reducing pressure will be displayed as increasing head on the inclined manometer because the tappings in the working section are connected to the top of each manometer tube and reduced pressure will suck water up the tube. Stagnation pressure in the working section will be very close to atmospheric pressure, allowing for frictional losses, i.e. a low reading on the manometer when the fan is in operation. The relative values can be converted to absolute values if an illustration of typical pressure behaviour is required. b. Electronic Manometer An electronic console incorporating 16 differential pressure sensors, each with a range of 0- 178 mm H2O. When installed on the wind tunnel, the electronic manometer is located inside the frame below the test section to the left hand side of the Electrical Console. The electronic manometer can be secured to the frame by transferring one of the straps from the console to the manometer. The electrical supply for the manometer is obtained from the outlet socket on the front of the console. AERO 335 Page 8 of 10  Introduction to the Wind Tunnel – Part I A common tapping ensures that all of the differential pressure sensors are referenced to atmospheric pressure. Quick release connectors (7x single and 1x 10-way) allow for rapid connection to models and instruments. The electronic manometer connects to the control PC using a second USB port on the PC, and the readings are fully integrated with the wind tunnel control software for ease of use (the software has to be used). As described already (‘Important note on pressure measurement using the tunnel’ on page 3), the stagnation pressure in the working section will be very close to atmospheric pressure, allowing for frictional losses, when the fan is in operation. To match the results from the inclined manometer, static pressure readings below atmospheric pressure are displayed as positive values so static pressure will be greater than the corresponding total pressure readings. The relative values can be converted to absolute values if an illustration of typical pressure behaviour is required. 10. Pitot Static Tube A miniature Pitot Static tube mounted in a support plug that can be located in the roof of the working section at three alternative positions, i.e. the start of the working section and upstream and downstream of the model mounting position. The support plug incorporates an ‘O’ ring to retain the Pitot Tube where it is positioned and allows the tube to traverse over the full height of the working section to measure the velocity profile inside the working section of the tunnel. The Pitot Static tube is constructed from two concentric stainless steel tubes. The inner tube is open at the tip and measures the Total head. The outer tube incorporates a ring of small holes in the side that measure the static head. The overall diameter of the Pitot Static tube is 4 mm to give a stiff assembly without unduly disturbing the airflow downstream and the ‘L’ shaped arrangement, with the tip pointing into the flow, gives minimal disturbance at the point of measurement. The two flexible tubes from the Pitot Static tube incorporate a quick release connector that allows it to be connected to one of the optional manometers. AERO 335 Page 9 of 10  Introduction to the Wind Tunnel – Part I The Pitot Static is of Prandtl design and may be used with a negligible correction up to angles of yaw of at least 5 degrees. 11. Bernoulli Apparatus A Venturi profile that is installed in the working section of the tunnel via the removable floor. The Venturi incorporates 11 pressure tappings in the floor, connected via flexible tubing to quick release connectors to suit the inclined or electronic manometers. The Venturi occupies the full height of the working section and the width varies from full width at the inlet and outlet to 100 mm at the throat. It is manufactured from clear acrylic for full visualisation. By itself, the Bernoulli Apparatus may be used to show the variation in static pressure with change in cross section, but when used in conjunction with the Pitot Static tube the Bernoulli equation can be fully demonstrated. When using Bernoulli Apparatus, the static pressure sensor should be moved from the tapping in the rear wall to the upstream tapping in the roof of the working section to avoid errors in the static pressure measurement caused by the wall of the Venturi downstream of the rear tapping. AERO 335 Page 10 of 10