Flow Around a Bend in a Rectangular Duct Partial Report
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Flow Around a Bend in a Rectangular Duct Partial Report Krishna Ramasubramanian pr16@illinois.edu
9/26/2022
The primary objective of this lab is to measure resultant force on air by a curved duct, determining if control volume analysis based on the linear momentum principle is valid. We’re also trying to analyze if the tangential velocity of air around the bend can be modeled with an idealization – the free-vortex model. Modeling and understanding this type of airflow is very influential in the design and shape/material optimization of fluid flow across any curved and contained chamber.
We’re first trying to measure resultant force caused by the curvature of the duct onto the air moving inside it. This requires a curved duct and a controllable air flow going through the duct. In addition, our apparatus must require a multitude of pressure transducers along the path of the travelled air to assess fluid pressure at various points of the duct. Once this setup is created, we can then use the principle of conservation of momentum and control volume to deduce the amount of force is exerted on the air because of the duct. The pressures only are exerted from inside the walls of the duct, which we can then take as 𝑭
= − ∫ 𝑝𝒏
dA as the sum of all the pressures across the inside walls’ area. Looking at control volume analysis, force can be derived as a function of linear momentum. This analysis breaks it down to 2 dimensions, making it such that there is no result of force as a result of the spanwise direction, modeled as ∑ 𝑭
ext = ∫ 𝒖
𝜌
(
𝒖
∙ 𝒏
)d
𝐴
. Lastly, using the free-vortex model, we can examine the flow of the fluid as a function of the radial distance to the center of rotation. With the theories out of the way, we can examine the results of the data that we collected with the apparatus. The forces in Appendix A tend to have Fx to be greater than Fy, and the flow rates correspond with the forces directly. The trend is consistently lower Fy than Fx for all the flow rates, and it shows that the higher the flow rate, the greater the forces are as well.
The assumption of a free vortex model (FVM) is valid until the point of deviation from the model which occurs at 75-80 mm consistently. This, however, is a very small deviation that can be explained away by slight physical discrepancies and errors in measurement. The rigid body model results in Cp creating the inverse graph of the FVM. This is the exact opposite of what is also seen at the experimental data. As a result, there is no correlation to the rigid body model, invalidating it as a means to model the physical results.
Next, the constant velocity model results in a constant pressure. The experimental pressure varies significantly as a function of the radius and does not remain constant. Therefore, this model is invalidated as well.
In conclusion, the forces inside the duct are modeled by momentum conservation and control volume analysis. Taking the integral and using the control volume both yield the same results. The flow rate also determines the amount of force that the duct exerts on the air. Lastly, we see that the FVM is the best representation of the flow pressure. The other models do not correlate to the physical data, and therefore are not a means to model the physical results.
1
2
0
5
10
15
20
25
Integration Forces
Fx = 1 Fy = 2
F (N)
Appendix A
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