Solution 1: PVC Pipe
The first solution under test utilizes a 6-inch diameter PVC pipe. The pipe will be placed over the laser cavity, with a mount printed to line up the center of the pipe with the center of the HeNe laser of the Verifire.
The primary purpose of the PVC is to have the rigid structure of the tube block out any turbulence caused from fluctuations in air pressure from entering the laser cavity while having the smooth interior of the tube create a more laminar flow of air within the cavity. The low thermal conductivity of PVC of around 0.19 W/(m*K) also should help keep fluctuations of temperature from entering the laser cavity, as a low thermal conductivity indicates that the tube should not hold on to any heat from the outside environment.
The possible challenge with this solution is that, should there be any large amounts of turbulence that occur as the pipe is being placed, the turbulence may not be able to escape the inside of the tubing, therefore extra time must be taken before measurements start to allow the potential turbulence to dissipate within the tube after it is placed in the laser cavity
Solution 2: Window Screen
The second solution under test utilizes a standard aluminum window screen. This method will follow the same form as the PVC pipe, as it will consist of layers of window screen wrapped into a tube which will isolate the cavity.
Rather than fully insulating the cavity like the PVC, the window screen aims to diffuse any incoming sources of turbulence quickly into equilibrium. The aluminum has a high thermal conductivity compared to the PVC material, allowing more heat to be absorbed rather than entering the system.
A challenge of this solution is that the window screen tube is very lightweight, so it is not durable and can be easily deformed or knocked out of place.
Solution 3: Honeycomb Air Flow Straightener
Air flow straighteners are commonly employed in wind tunnel engineering applications where downstream turbulent flow is undesired. In principle, air flow straighteners are typically comprised of a length-wise thick honeycomb material that works by reducing the transverse components of velocity fluctuations (turbulence). Honeycombs are often used in conjunction with fine mesh screens that have open-area-ratios (the ratio between the area that fluid can pass through and the 2D area occupied by the screen mesh) higher than 0.57 to reduce the axial component of velocity fluctuations.
Selection of the honeycomb depends largely on two parameters: the length-to-diameter aspect ratio and the cross-sectional shape.
In wind tunnel applications, honeycomb cells with length-to-diameter aspect ratios of around 8-12 were found to be the most effective at reducing the turbulence created downstream of subsonic wind turbines. For our application, the expectation is that turbulence arises largely from HVAC systems with air flow characteristic of most other work environments. A study found that the mean wind speed measured by a static anemometer for 55 work areas within 27 factories (that encompassed a range of environments from office spaces to heavy industrial engineering) was found to be 0.3 m/s. This is much smaller than the mean speed characteristic of wind tunnels and thus a much smaller aspect ratio could be considered.
In consideration of the desired properties of the honeycomb layer as well as economic and marked availability, we have chosen aluminum hexagonal honeycomb screens from Performance MRP which are typically used for mass air flow sensors in combustion engines as the ideal candidate honeycomb structure. They are supplied in 12” x 12” rectangular sheets with ¼” cell sizes and are ½” thick lengthwise.