Laser doppler velocimetry pdf

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Explore Documents. Uploaded by imsandy Did you find this document useful? Is this content inappropriate? Report this Document. Flag for inappropriate content. Download now. Laser Doppler Velocimetry. Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. Authors Authors and affiliations R. The Laser-Doppler-Velocimetry LDV is an optical method to measure local flow parameters of transparent gaseous or liquid flows.

Without explaining in more detail it should be noted that in a modified version the same technique can be applied to velocity measurements of surfaces of solids. In comparison to traditional flow measurement techniques, e.

The LDV technique makes a very high spatial and temporal resolution possible. This process is experimental and the keywords may be updated as the learning algorithm improves. The demodulated image data is further processed as presented in Meier et al. Those image pairs can be either two consecutive images of one SPDA or the images of two SPDAs where one detector array runs with a fixed delay with respect to the other.

Measurements Two experiments are presented in this section. The second experiment presents a more complex setup using two SPDAs. To demonstrate the benefit of such a system the velocity distribution on a rotating disk is measured. For the ILDV measurement the co-planar light sheet configuration was chosen to measure one of the in-plane velocity components, specifically the component in the main flow direction of the jet.

The telescope is needed to control the thickness of the light sheet. After the cylindrical lens the light sheet is split up using a beam splitter BS.

The deflected part is then redirected into the same direction as the non deflected part using a mirror M. The distance between the beam splitter and the mirror can be used to adjust the crossing angle of the two light sheets.

For this experiment it was set to 2. The flow was seeded using a Laskin nozzle generating particles with a size of approx. The seeding density had to be adjusted such that both measurement techniques work. The Doppler measurement usually requires a higher seeding density than the PIV measurements as in the Doppler measurement the single particles do not have to be resolved, and more particles per pixel lead to a higher signal intensity.

Therefore a simultaneous measurement was possible. To correct for the different observation angles and fields of view of the two cameras the same checkerboard target was recorded with each camera and remapped onto the same coordinate system using a 2-dimensional projective transformation.

Figure 4 shows the measured vertical velocity distribution using ILDV. The shape of the jet is clearly visible. Due to the low velocity the jet flow is laminar and shows only little spreading.

On both sides of the jet where the flow velocity drops to zero the signal becomes noisy. This is due to the fact that zero velocity means zero Doppler shift, and the camera noise becomes the dominating part of the signal. Figure 4: Velocity map of the measured velocity distribution of the jet. This time delay between two images in a single detector system is determined by the frame rate of the camera. A system using two cameras eliminates the problem as the two cameras can acquire the images with much smaller time delays between the two images.

With smaller time delays the crossing angle between the laser sheets can also be reduced since the time the signal needs to be persistent can be reduced, too. The development of such a camera system is currently ongoing and first measurements of the velocity distribution on a rotating disk were performed. At the same time, the issue of the noisy signal at low velocities can be avoided by pre-modulating the frequency of one of the light sheets as it is done in classical point measurement LDV systems to eliminate the directional ambiguity.

Experimental conditions and desired particle size are the biggest factors in the selection of particle generation method. Many practical generators and particle materials to be used with these generators are commercially available.

More information about particle generation techniques can be found in Albrecht et al. The preliminary design was more of a conceptual idea rather than an actual design. This design is shown in Figure 5. As mentioned in section 1. This original design used 1-mm. The light then passes through a beam splitter that allows the beams to pass through unimpeded to the focusing lens.

The design is more conceptual due to the way the transmitting part of the probe and receiving part are connected. Also, a beam splitter plate that acts in the way described, allowing light to pass through from one direction but reflect when coming from the other direction, was not commercially available at the desired size. The final issue came with the 1-mm.

These lenses proved to be impossible to glue into place without getting glue on the front or back side of the lenses. Also, due to their small size, they were easily lost and extremely hard to handle.

This was accomplished by machining a block to hold the transmitting, receiving, and probe head tubes seen in Figure 5. This block is shown in detail in Figure 5. There is a third unseen hole in the figure machined from the back side of the block with a diameter of 0.

The probe head long-neck extension tube was glued into the 0. The 0. Most of the remaining parts were purchased and are listed in Table 5. Table 5. A right angle mirrored prism was purchased and two 0. Figure 5. The third problem was solved by finding lenses with the same focal length of the 0. The lenses chosen are listed as collimating lenses in Table 5. These lenses were approximately four times bigger in area than the smaller lenses and proved to be much easier to work with.

The lenses were glued onto the tubes that the fiber terminator tubes slid into. First, these fiber-terminator holder tubes were centered and glued onto a machined separation plate with a thickness of 0. The next step was to apply a thin film of slow drying model glue around the rim of each of the fiber-terminator holder tubes and place each lens on the tubes.

Next, the fiber-terminators were inserted into the fiber-terminator holder tubes. Each lens was then glued permanently into place using cyanoacrylate jewelry glue. Each fiber-terminator holder tube was then inserted into a larger tube to complete the construction of the transmitting fiber-terminators holder assembly. The assembly was then glued into a larger tube which was then glued into the block in Figure 5.

The receiving optics assembly was constructed next. A fiber optic cable was terminated and inserted into a steel structural tube similar to the transmitting fiber-terminators. This tube was then inserted into an octagonal tube machined by the machine shop and held in place with a screw. A block to allow set screws to be used was machined and placed over the tube that joins the receiving optic assembly to the main probe assembly which was then glued into place.

The octagonal tube was inserted into a larger tube and held in place with eight set screws to allow movement in three directions. Finally, the receiving lens from Table 5. The receiving optic assembly is shown in Figure 5. The assembly was inserted and glued into the top of the block in Figure 5. The final piece of the probe is the probe head. This part consists of a 0. This is the most important part of the probe as it was the component kept as small as possible which was the main purpose of this probe.

This tube was glued into the main probe structure to complete the construction of the probe. The final probe design is shown in Figure 5. This was due to the small parts needing to be extremely precise in their placement and the nature of glue contracting while drying. The process, however, was fairly straightforward. First, the two collimated transmitting beams had to be focused by pulling the terminator tubes in and out until the beams came to a point at a far distant wall.

Next, a microscope objective was placed at the focal length of the focusing lens. Now the two beams passing through the microscope objective had to be made to overlap onto each other at the focal point of the transmitting lens.

This was done by twisting the terminator tubes and adjusting their holders as needed. The beams emerging from the microscope objective were observed at a distant wall, which made it easy to observe the crossing of the beams. Once the beams overlapped, the tubes were glued into place.

The next step was to send a beam through the receiving optics and make it overlap the other two beams as well. This was much easier as the set screws gave very precise control over the fiber. Once all three beams were crossed, the probe was ready for use. A diagram for the system is shown in Figure 6. The on-table optics were used to create the two laser beams required for velocity measurements and to couple them with fiber-optic cables that transferred the beams to the borescopic probe.

The probe was used to generate the measurement probe volume and to collect the light scattered by the seeding particles as they passed through the probe volume and couple the light into the receiving fiber-optic cable.

This cable transferred the light to the data acquisition and reduction units. Figure 6. First, the components of an LDV system are described in detail followed by a discussion regarding the on-table optics. Finally, the data acquisition and reduction units are discussed. The laser was discussed in section 3.

This section will focus on the transmitting and receiving optical components. The transmitting optics first separate the laser beam emerging from the laser into the desired number of beams.

They then apply the appropriate frequency shift to one of the beams to eliminate directional ambiguity. The optics then redirect the beams to fiber-optic cables, collimate the laser beams emerging from the fibers, and focus them to form the measurement probe volume.

Each component will now be described in its own sub-section. They are used for redirecting the laser beams in such a way that other components, such as lenses and beam splitters, can be easily located within the confines of the table used.

Flatness, reflection loss, and damage threshold are the important properties to consider when choosing mirrors with the latter being the most important. The damage threshold of the mirrors must be higher than the power density of the laser beam.

Lenses have multiple purposes in an LDV system. Some lenses are used to focus the laser beams to a desired focal point, while others are used to reduce a laser beam to a desired diameter.

Lenses are also used to couple laser beams with fiber-optic cables and to collimate laser beams. The selection of lenses depends on the purpose they will serve, their size, and their focal length. LTFC are mechanical devices with up to six degrees of freedom adjustment capability. They house the fiber-optic cables and help to align the cables to receive the laser beams. Single-mode and multi-mode fiber optic cables are commercially available and some are able to maintain the polarization of the light that forms the desired, undistorted fringe pattern in the measurement probe volume.

Creating the desired measurement probe volume or achieving a specific beam-waist to launch a beam to a fiber-optic cable is accomplished using beam collimation.

Highly disturbed measurement probe volumes result from uncollimated beams. Lenses and prisms are both commonly used to collimate laser beams. The adjustment comes from the ability to change the distance between the two lenses.

This type of collimator allows the expansion of the beam and the beam-waist to be adjusted. Equations 6. The desired measurement probe volume is achieved by expanding the laser beams before they reach the front focal lens Albrecht et al.

Beam expansion and collimation are shown in Figure 6. The divergence angle of the laser beam as it exits the fiber-optic cable is given by Equation 6. The lenses used for collimation must be placed at their focal length away from the fiber-optic cables.

The focal length, f, is chosen based on the desired collimated beam radius as seen in Equation 6. Bragg cells are the most common beamsplitters. Bragg cells divide a laser beam into two or more beams that are frequency shifted by the modulator frequency and its harmonics. The design usually uses the zeroth order, non-shifted, beam and the first order, first harmonic, beam.

This fact makes polarization of the laser light very important in an LDV system. Polarizers are used to reorient the polarization direction of laser beams. Polarizers consist of materials with anisotropic properties. Polarizers exhibit different refractive indexes in two orthogonal directions Albrecht et al. One index is for the ordinary wave while the other is for the extraordinary wave. The assorted polarization states are shown in Figure 6.

Substituting into Equation 6. Substituting this into Equation 6. Half-wave and quarter-wave plates are usually prior to the laser beams entering a beam splitter in a conventional LDV system. This allows the desired polarization to be achieved before the beam is split. This allows the transmitting optics and beam splitter to be rotated without having to rotate the laser.

A schematic of the on-table optics used for the work done in this thesis is shown in Figure 6. The laser used as the light source for the work done in this thesis was a Spectra-Physics Ar-Ion laser Innova C with a maximum output of 8W-all-lines. The laser beam emerging from the laser has a diameter of 0. The Bragg cell splits the laser into two beams of equal strength.

The zeroth order beam passes through without a frequency shift while the first order beam experiences a Bragg shift of 50MHz. The LTFCs are equipped with five-axis adjustment capabilities. The LDV probe is described in detail in Chapter 4 and will not be discussed again here. The fiber-optic cable carries the collected light to a photomultiplier tube Electron Tubes SB which converts the light information into an electrical signal. A schematic showing this process is shown in Figure 6.

A single channel was used on the frequency domain processor. The processor was attached to the data-acquisition and reduction computer via a firewire port. The first setup produced the pipe jet flow while the second setup produced the convergent nozzle jet flow.

A particle seeding generator was used to provide the seeding for both flows. A TSI six-jet atomizer was used for providing the seeding to each of the flows. Di-Octyl-Phtalate was used as the seeding particle material. The atomizer generated particles with a mean diameter of 0. The atomizer is shown in Figure 6. The smoke then travels through a long rubber hose to the flow generator.

The long rubber tube minimizes the amount of large oil particles that reach the flow because most of the large particles either collect on the walls of the tube or settle to the bottom. This makes the seeding particles delivered to the flow more uniform.

The tube transfers the smoke to the settling chamber of the flow generators to assure adequate mixing of seeding particles with the flow before emerging from the jet exit. Some effects of entrainment of air into the jet were not able to be measured with this setup due to the fact that only the emerging flow is seeded. This is known as in-homogenous seeding. The entire room encasing the flow would need to be seeded to acquire accurate measurements of the flow velocity at the edges of the jet.

Fuchs et al.