Aerodynamics Measuring Techniques and Flow Visualization Techniques - Assignment Example

AERODYNAMICS MEASURING TECHNIQUES Name: Course: Professor: Institution: Date: 1. Hot wire Anemometry (a) Types and applications There are two primary types of hot wire anemometry used. i. Constant current anemometer (CCA) ii. Constant temperature anemometer (CTA) In the CAA, a constant electric current passes through a wire exposed to the flowing liquid. Due to the balance between convective heat loss from the wire and the internally generated heat as a result of electrical resistance, the wire achieves an equilibrium temperature. CTA work by adjusting the current flowing through the wire, so that a constant temperature is maintained. This is attained by use of a feedback circuit. The current required to maintain a constant temperature is related to the heat loss (Stainback & Nagabushana, 1993). Hot wire anemometers are used to measure gas and liquid flow velocities. Areas of applications include: wind tunnels, downstream of grids, shocks and screens, boundary layers, jets, wakes, flight in atmosphere, rotating machines etc. (b) Background theory and equations for the analyses Hot wire anemometry is based on the transfer of heat from a hot wire to the colder surrounding fluid. The transfer of heat is proportional to the velocity of the fluid and the relationship between the two parameters can be established. The governing equation is shown below: Where: – Thermal energy stored by the hot wire – Heat capacity of the wire material – Power generated by heating – Heat transfer to the surrounding (c) Calibration Equations Probe is subjected to a set of velocities that have been pre-determined and the output voltage is recorded. The calibration is done in a wind tunnel and a pitot-static tube is used when measuring velocities (Lomas, 2011). King’s Law relates the power dissipated in a hot wire and the temperature difference between the air and the wire, generating the equation for calibration. Where: – bridge voltage – air velocity – Constants The constants b & c are determined by a least-squares linear regression. Taking the natural log on both sides: The relationship between U and E can be represented using a straight line, and using a linear correlation, b and c, and the calibration equation can be determined. Figure 1: Calibration graph to find the constants a&b, and the calibration equation. (d) Advantages and Limitations Advantages: High frequency response Good temporal and spatial resolution Can measure temperature Small measurement volume Easy to measure turbulent flows It is possible to measure a two-phase flow Good level of accuracy Low signal-to-noise ratio It is easier to obtain a good system of measurement for most of the measurements The analogue output gives an opportunity for analysis of frequency-domain signal and conditionally-sampled time-domain signal It is easy to make special probes It is possible to measure multi-component flows due to multi-component probes Measurements in gases, electrically conducting, transparent and opaque can be made Wire anemometers can measure instantaneous shear stresses on a wall using mounted sensors. Limitations: High turbulence flows can alter the results It is intrusive technique Liquid flows can easily contaminate the probe or the probe can break Heat loss to the support prongs and heat transfer problems. Signal noise due to circuit and radio frequency Aerodynamic problems – probes are not sensitive to the direction of flow and probe supports can interfere the flow to other sensors (Lomas, 2011). In some cases, linearization of the anemometry equation may not be accurate. The spatial resolution is limited by how long the wire is, and the size of the lowest fluctuation scales in a given flow. A problem is normally encountered when the gas is air in hypersonic flows. The temperature must be high enough for higher Mach numbers to prevent air liquefaction in the test section. 2. Laser Doppler Anemometry (LDA) (a) LDA applications LDA is a non-intrusive technique that uses a laser beam with the Dopler shift to measure velocity in fluid flows, or the vibratory or linear motion of opaque, reflecting surfaces. The measurement is linear and therefore, requires no calibration. Various applications of LDA include: turbulent and laminar flows, aerodynamic investigations, liquid flows, supersonic flows, automotive and turbines, velocity of particles, vibratory measurement and surface velocity, oceanography, hot environment velocity measurements etc. (b) Background theory and Equations for analyses LDA technique measures 1D, 2D and 3D velocity measurement and turbulence distribution in internal flows and free flows using a Doppler Effect. If a particle traveling towards the source is illuminated using a light wave, the particle receives the light with a frequency higher than that emitted. The particle scatters the light and remits it at the same frequency as received. The scattered light has a frequency shift called the Doppler frequency, which depends on the velocity of the body. The shifted light is analyzed by LDA, or spectrometric methods. Gas lasers in optical mode are ideal sources for the measurement of the Doppler shift. Laser light is focused at a small spot where all the laser energy is concentrated. As particles go through the beam, they scatter light in multiple directions (Zhang, 2010). Fractional shift in frequency/ Doppler frequency = Where: – Velocity of the particles – Velocity of light A photomultiplier converts the light received into an electrical signal, and the frequency of the photocurrent is given by: Where: – Doppler frequency – Particle velocity – Wavelength of light – Angle between the laser beams (c) LDA calibration Validity of LDA is essential in attaining reliable velocity measurements. The most common source of error is the fringe spacing, which may be uneven as a result of imperfect laser beam intersections. The fringe spacing is calibrated against a known peripheral velocity of a flywheel. The velocity of the flywheel is calculated using the wheel radius and the number of revolutions. Where: – Nominal fringe spacing W – Laser wavelength (nm) L – Foal length (mm) – Nominal beam spacing (mm) (c) Advantages and Limitations Advantages: Non-intrusive technique 1D,2D and 3D velocity measurements Wide range of velocities Small sensing volume Direct measurement, no calibration needed Does not depend on thermo physical properties of the fluid Sensitive to reverse direction Can be used in chemically reacting media and high-temperature environments Can be used in rotating machinery that cannot be fitted with physical sensors High frequency response Limitations: Require tracer particles Particles may not always accurately follow the fluid flow Single point measurements Require optical access (transparent medium) Needs seeding Do not provide continuous data 3. High response pressure transducers (a) Types and applications There are three primary types of pressure transducers. These are: i. Potentiometric pressure transducers – Use bellows, capsule, or a Bourdon tube to drive a wiper arm against a resistive element. ii. Inductive/electromagnetic pressure transducers – Have varying inductive coupling and require AC excitation. iii. Capacitive pressure transducers – This transducers use a diaphragm to deflect due to pressure, which is translated to change in capacitance. iv. Piezo-resistive strain gauge – Uses a strain gauge to detect applied pressure. v. Piezoelectric – Uses piezoelectric effect to measure strain due to pressure vi. Optical transducers – Uses physical change by optical fiber to measure strain due to pressure. High response pressure transducers are used in industrial applications to measure fluid levels, altitude and fluid flow. Automotive applications include: tire pressure, oil pressure, purge system leak detection, and manifold absolute pressure measurements. (b) Background theory and equations used in the analyses A pressure transducer is typically used to measure the pressure of liquids and gases by generating a signal output, which is a function of pressure. A transducer typically consists of two major parts, a deforming elastic material that deforms under pressure, and an electrical medium to transform the pressure into electrical signals. A common feature for the above types of transducers is that pressure introduces a mechanical movement or a strain that are measured by voltage displacement and strain gauge respectively (Bau, et al., 2008). Governing equation for voltage displacement transducers: Where: P- Measured pressure K – Nominal transducer scale factor - Output voltage – Ideal supply voltage – Actual supply voltage (c) Calibration for pressure There are many methods used to calibrate pressure transducers. Here, a focus is on the Dead-Weight Tester method. The test is done using known weights that exert pressure on a manometer fluid. The difference in column heights of the fluid in the manometer gives the pressure reading. Figure 2: Pressure manometer (d) Advantages and Limitations Advantages Capable of AC and DC response Good linearity Good stability Limitations Temperature sensitive Sensitivity to vibrations The strain-gauge type has lower sensitivity 4. Flow Visualization Techniques 

 (a) Types and applications of Flow Visualization Techniques Flow visualization techniques are used to make visible flow patterns in order to obtain quantitative or qualitative information in fluid dynamics. There are three main categories of flow visualization in fluid dynamics: i. Surface flow visualization – This method reveals the streamlines of the flow in the limit as the flow approaches a solid surface. An example is a colored oil applied on a wind tunnel. ii. Particle tracer methods – Smoke particles, dye, tufts or microspheres may be injected into a flow and their motion is traced. A flow pattern can be seen by illuminating the flow with a laser light to reveal the flow streamlines. iii. Optical methods – A flow pattern can be revealed through changes in refractive index and can be visualized by optical methods, such as the interferometry, laser sheet, shadowgraph, and schlieren photography. Flow visualization methods find a wide range of applications in both experimental and computational fluid dynamics to provide information about the fluid flow around a model (Smits & Lim, 2012). (b) Background theory Fluid visualization is a very important technique in experimental fluid dynamics, which provides an overall picture of a fluid flow field and develop new theories of flow models. (c) Advantages and Limitations Advantages: Provide important information about flows The methods do not involve complex calculations Limitations: Many flows are sensitive to small changes in boundary conditions, which means that the theoretical equations may not provide accurate results. Tracer method of using smoke particles does not work well at high velocities It may be difficult to establish actual flow pattern for a certain set of conditions due to complexity of the patterns. Optical methods require seeding 5. State of the art techniques under development i. Particle Image Velocimetry (PIV) PIV is a non-intrusive method for determining fluid flow properties by laser optical measurements that use CMOS or a single CCD camera. The technique is used to determine instantaneous velocity measurements and other fluid properties. The fluid to be tested is seeded using tracer particles that flow the fluid’s flow dynamics and then illuminated to make the particles visible. Applications of this technique are found in high lift flows, propeller flows, boundary layers, transonic flows and in-flight measurements (Adrian & Westerweel, 2011). Advantages: Non-intrusive technique The method can measure the entire 2D cross section of flow simultaneously. High speed processing of data, which can be analyzed in real time. High degree of accuracy Limitations: Particles may not follow the fluid motion perfectly due to them having a higher density. Generally, PIV methods may not be capable of measuring components on the x-axis. Safety and cost constraints brought by the use of high-resolution-speed cameras and class IV lasers. ii. Lorentz Force Velocimetry (LFV) This technique is used for fluids that can conduct electricity. The fluid is exposed a magnetic field and the drag force on the magnetic field lines is measured. The signal measured is a linear function of the velocity of flow of the liquid. A scaling law is formulated to relate the force and the velocity of the fluid. Applications include: Glass manufacturing, semi-conductor crystal growth, Metallurgy etc. Advantages Non-contact technique Can be used in aggressive fluids and high temperature environments It is possible to evaluate a number of additional parameters Limitations: Necessity to control the temperature because of the magnetic field Measurement zone is restricted by the dimensions of the magnet Rapid decay of magnetic fields. References Read More