Anemometry Lab Protocol (Mini Project) Vortex flowmeter A Vortex flowmeter is a flowmeter for measuring fluid flow rates in an enclosed conduit. Composition of vortex flowmeter A vortex flowmeter comprising: a flow sensor operable to sense velocity variations due to vortex- shedding of a fluid in a passage and to convert the flow fluctuations to a flow sensor signal in the form of an electrical signal; and a signal processor operable to receive the flow sensor signal and to generate an output signal corresponding to the velocity variations due to vortex-shedding of the fluid in the passage. The schematic and the typical flow meter is shown in Fig 1. 5. Electronics 4. - 3. Sensor 2. Sensor Post Shedder Bar 1. Flexure Meter Body Figure 1: Schematic and typical vortex flowmeter Industrial applications Vortex flowmeter is a broad-spectrum flow meter which can be used for metering, measurement and control of most steam, gas and liquid flow for a very unique medium versatility. It has a high stability and high reliability with no moving parts, has a simple structure and a low failure rate. The flowmeter is relative economical because of its simple flow measurement system and easy to maintain. It is widely used in petroleum, chemical industry, metallurgy, food, paper, electricity, industry boiler, urban pipeline heating, water supply, coal gas and other industries. Von-Karman Vortices When a bluff body (e.g. cylinder, square or triangle) is facing a fluid flow around it, unsteady separated vortices are generated in an alternating and periodic pattern. These swirling vortices are known as Von- 1 Karman vortices (shown in Fig. 2) and they are related with a frequency known as vortex shedding frequency or Strouhal frequency. Flow Direction Bluff body Von-Karman vortex street Figure 2: Unsteady flow over cylinder Strouhal Number (St) In dimensional analysis, the Strouhal number (St) is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincence Strouhal, a Czech physicist who experimented in 1878 with wires experiencing vortex shedding and singing in the wind. Figure 3 shows the plot of the Strouhal number of a cylinder over a range of Reynolds number. Note that over a Reynolds number value of 300 (Re>300) the Strouhal number is almost constant and has a value of 0.2 for circular cylinder. The Strouhal number is different for various shapes. Strouhal Number (S) 0.47 0.4- 0.3 0.2 St 0.2 0.1 Smooth Surface- → Rough Surface 0 40 102 103 104 105 106 107 Reynolds Number (UD/v) Figure 3: Strouhal number versus Reynolds number for a circular cylinder Reynolds number is defined as Re = pUd and Strouhal number as St where, μ 'p' is the density of the flowing fluid in kg/m³ 'U' is the velocity of the flowing fluid in m/s 'd' is the thickness of the bluff body in m 'μ' is the dynamic viscosity of the flowing fluid in kg/ms 'f' is the Strouhal frequency in Hz. 2 = fd U Parts of the Experimental Set-up 1 Suction Pump Suction pump is a device which draws air from the atmosphere to induce flow inside the flow tube. As the impeller rotates it creates suction pressure inside the pump, hence air from the atmosphere rushes into the pump. 2 Flow Control Valve Flow control valve regulates the amount of flow which enters the flow tube. This valve is responsible for the velocity of the air inside the flow tube when pump is switched on. 3 Flow Tube It is a constant area pipe where, ideally there is no change in bulk air velocity along its length. It is similar to test section in wind tunnels. Triangular model is mounted in this tube. 4 Hot Wire Sensor Hot wire sensor is a very fine wire (on the order of several micrometres) electrically heated to some temperature above the ambient. Air flowing past the wire cools the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed. (For more details read wikipedia article) 5 Anemometer An anemometer is a device used for measuring wind speed. In this experiment it is a hot wire anemometer. The principal of a hot wire anemometer is based on a heated element (Hot wire sensor) from which heat is extracted by the colder impact airflow. The temperature of the hot wire is kept constant via a regulating switch, and the voltage is directly proportional to the air velocity. 6 NI-DAQ Data acquisition commonly known as DAQ is the process of sampling signals that measure real-world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. DAQ system typically convert analog waveforms into digital values for processing. In this experiment, the value of voltage from the anemometer is stored as digital signals in a computer using National Instruments DAQ. 7 LabVIEW It is a software from National Instruments to view the data and analysis. The signal and its spectrum is visualised in this experiment. 3 4 །།། Inlet from Atmosphere Triangular Model Hot Wire Sensor U Strouhal Frequency f Von Karman Vortices Lab-View Screen NI - DAQ Anemometer Figure4:Schematic Diagram Hot-wire holder Flow Control Valve 8- To Suction Pump Flow Tube Configuration 1 U Configuration 2 U d St=0.196 St = 0.154 Reference: Two-dimensional wake dynamics behind cylinders with triangular cross-section under incidence angle variation Journal of Fluids and Structures 63 (2016) 302-324 Figure 5: Configurations Formula and Terms St = fd = fd St Strouhal number f - Vortex shedding frequency in Hz d-Width of the trianglular cylinder U - Free Stream Velocity Main Observations from the Experiment 1) As we open the throttle the flow rate increases. Hence the Velocity increases inside the flow tube. 2) When Velocity increases the Strouhal frequency 'f' increases. 3) At a given control valve position, the vortex shedding frequency of configuration 1 is greater than configuration 2. Figure 6: Formula and Observations 5 сл