Harini Karthik
Age 16 | Montreal, Quebec
Online STEM Fair Regionals Ribbon (2020) | Youth Science Canada Ribbon (2020)
ABSTRACT
Drag is a true annoyance for many industrial sectors around the world. Drag force is produced by the friction of a flowing fluid flowing on the surface that lowers the energy efficiency of the system. This experiment attempts to minimize the drag force by testing theoretical biomimetic coatings. This approach was proven efficient using the digital method known as Computational Fluid Dynamics (CFD) for solving this fluid-flow problem. The morphological structures of different organisms were modeled using software tools such as Salome, OpenFOAM, and ParaView for further comparison. According to the results generated by the simulations, the drag reduced significantly for moderate Reynolds’ Number ranges. This technique can be applied to fabricate hydrophobic coatings to maximize the energy efficiency of various systems like solar panels.
INTRODUCTION
Drag is an invisible resistive force that challenges the performance of various industrial sectors. In the scientific world, drag measures the system’s energy efficiency in reference to the surface topography and the rate of the flowing fluid. Flat surfaces are observed to cause flow instabilities, thereby increasing the friction (drag). The morphological structures of biological organisms (Tetrodontophora Bielanensis, Rosaceae, and Morpho Peleides) were modeled using software tools such as Salome, OpenFOAM, and ParaView.
Computational Fluid Dynamics (CFD) was a digital method that was applied to solve this fluid-flow problem. Results indicated that streamlined surfaces are better at vortex shedding compared to planar surfaces. Vortex shedding avoids producing flow insta- bilities by creating a lower pressure region for more energy. By varying the velocity of the upcoming fluid and patterns of the morphological structures, the drag force reduced significantly (NASA Glenn Research Center, 2015). Comparatively, researchers have altered the geometric models in macroscopic applications such as automobiles, planes, and swimsuits. By modifying the surface topography on a microscopic scale, drag reduction will prevent the adhesion of fluid-like particles on the surface. This biomimetic approach is environmentally-friendly, efficient, biodegradable, and copiously available.
PURPOSE
This experimentation focuses on adopting nature’s microscopic de- formities as a surface coating to reduce drag (Fig.1.1). Theoretical biomimetic coatings were assessed based on their geometries in reference to the performance of flat surface.
Fig 1.1: The morphological structures of the biological organisms
HYPOTHESIS
If a fluid flows over a surface which exhibits a lot of vortex shedding, the drag will be reduced, as compared to a planar surface. This was assumed by nature of vortex shedding process that propagates the pressure of upcoming fluid at a direction perpendicular to the fluid flow (Fu, 2018).
METHODOLOGY
Salome, OpenFOAM and Paraview were used to analyze the identified specimen through geometric, mathematical, and visual modelling. For the purpose of experimentation, CFD’s fluid-flow package was utilized to generate external factors that influence drag, while the pressure was kept constant and numerical solutions are driven through the Finite Volume Method (Wolfram MathWorld, 2001). This method discretized the mesh to solve the fluid-flow equations. Exterior factors such as freestream velocity of fluid and morphological geometry will only affect the results, whereas the pressure of the fluid remained the same as it is incompressible. Flat surface was a constant model in this experimentation to compare the overall reduction with the methods that are used today. The temperature remained constant throughout the simulation. The governing equations of incompressible fluids are Navier-Stokes Equations in which the conservation of mass states that the mass increase is equal to the total inflow of mass (Tryggvason,G., 2011). This concept applies to the inflow of fluid coming at a higher velocity and passes by the microstructure to create a higher drag.
STAGE 1 PROCESSING
The geometries of identified morphological structures were con- structed in Salome. The crossections of morphological structures were traced on a coordinate plane and the points were copied to Salome to create a two-dimensional outline. A revolution around 360 degrees was applied for curved models such as Rosaceae and Tetrodontophora Bielanensis. The outline was extruded along the z-axis to create deep tunnels for the Morpho Peleides model. A bounding box was installed around the geometry to locate the simulation (Pluralsight, 2014). The mesh used was tetrahedrons generated using Netgen 1D-2D-3D algorithm.
STAGE 2: SOLVER
In OpenFOAM, the standard configuration of fluid properties included viscous, incompressible, laminar, and Newtonian. These characteristics were chosen for this assessment and the calculations were measured down to micrometers. The fluid flowed in a horizontal (+x) direction from inlet to outlet. The drag reduction was monitored by varying the freestream velocity between 0.001 to 1 m/s and designs of geometric bodies. The boundary conditions were defined in all regions of the solid to connect the geometric model with the environment. The velocity and pressure values were kept constant at multiple solid faces (SimScale, 2020). No slip and zero gradient conditions were applied at the walls which include the peak and bottom. After setting up the files, the simulation was executed in which iterations through timesteps generated solutions (Fig.2.3). The final solution is when the velocity profile stops changing for this steady-state fluid-flow.
Figure 2.4: Bounding box was installed around the region of the geometric model. No slip condition for velocity and zero gradient condition for pressure were applied in the walls of the entire solid body. The walls include peak and bottom represented in the image above.
STAGE 3: POST-PROCESSING
In ParaView, the results for pressure, velocity, and vorticity were viewed in a cross-sectional perspective that is perpendicular to the direction of fluid-flow. This tool enabled the usage of stream-tracers to present the velocity field aesthetically. The post-processing file contained the data for drag coefficients.
RESULTS
1 - TETRODONTOPHORA BIELANENSIS
According to the data provided in table 3.1, Tetrodontophora Biel- anensis had the lowest drag coefficient of 98.4. In comparison with the flat surface (control model) which amounted to 593, the drag coefficient was reduced by 83% for a freestream velocity of 1 m/s (Fig.3.2). The Reynolds Number remained small for all the geometries to maintain a laminar flow and predict future results according to the pattern (Fig.3.3). There was a steady fluid flow that avoided the waste of energy. The amount of vortices formed near the struc- ture lowered leading to a smaller drag coefficient.
2 - ROSACEAE
Other surface topographies such as Rosaceae have the second smallest drag coefficient of 250. The parabolic shape of Tetrodontophora Bielanensis and Rosaceae reduce drag force significantly because they lower the amount of pressure in front of the surface. This results in less energy dissipation. They possess more surface curves that eliminate flow separation and thin the boundary layer for a lower pressure drag (Aerospace engineering blog, 2018). Their rough surface topography and high freestream velocity reduced skin-friction drag. The Reynolds Number is calculated using the freestream velocity, characteristic length, fluid density, and dynamic viscosity of fluid. Rosaceae had the second smallest Reynolds Number of 155 for a freestream velocity of 1 m/s which indicates that it has thinner boundary layer compared to other models. This will result in a pressure of 5 m2/s2 closer to the curve as the fluid flows at a speed of 1 m/s.
3 - MORPHO PELEIDES
On the other hand, Morpho Peleides structure has a higher drag coefficient of 285 because of its sharp edges and flat rectangular bars that create vortices. The morphological structure of Rosaceae has a closer drag force compared to Morpho Peleides because they possess smoother surfaces that contribute to more vortices for a higher drag. Overall, the conical structures reduced drag more efficiently compared to rectangular surfaces because these patterns are better at vortex shedding. They move the vortices to another re- gion and create a region of lower pressure near the curve for more energy. Moreover, the Reynolds Numbers of Morpho Peleides is 6.45% higher than that of Rosaceae for a freestream velocity of 1 m/s. Their close proximity creates almost the same thickness of the boundary layer and pressure near the peak. Morpho Peleides has a pressure of 5.7 m2/s2 compared to Rosaceae which is 5 m2/s2.
4 - FLAT SURFACE
When the surface is not streamlined like Morpho Peleides and flat surface, the vortex shedding process does not take place effectively. Vortices are not spread out farther from the geometry, resulting in a higher drag coefficient. Flat and smoother surfaces are therefore a cause of higher drag.
Figure 3.1: The drag coefficients of all of the tested specimens are shown below according to the freestream velocity.
Figure 3.2: Drag coefficients of various surface topographies influenced by freestream velocity
Figure 3.3: The Reynolds Numbers are shown for the corresponding velocity profile of each model. The blue region represents the thickness of the boundary layer. The red portion represents the freestream velocity region.
DISCUSSION
The CFD software OpenFOAM allowed the computation of Navier-Stokes Equations. Although OpenFOAM has a high accuracy, the variation in geometry from morphology and mesh cell distribution and size can also have an influence the results. The skewed data of drag coefficients resulted from the mesh variation and the bending point was created by the curved structure difference as to a linear plane calculation. The external factors that were set before modelling resulted in internal results that showed diversity in pressure, velocity, and vorticity in the region. Standard fluid properties of incompressible water were used as a test incoming fluid to lessen the impact of adhesion on the surface. The boundary conditions of certain geometric faces set to zero velocity and pressure was justified by lessening the boundary layers, maintain- ing a laminar flow, and lowering pressure near morphology.
Higher pressure, increasing vortices, and low velocity near the curve contributed to maximize the drag. This factors are detrimental to the vortex shedding of the geometry by concentrating the vortices closer to the curve. Therefore, the prediction that lower freestream velocity will create a smaller drag was proven wrong as they tend to form boundary layers to have a negative influence on drag. Moderate freestream velocity decreased the friction applied on the morphological structure and results in a low Reynolds’ Number for minimized drag.
FUTURE IMPROVEMENTS
I would like to improve my project by testing the drag reduction for the entire topography instead of a single prototype. Nano-scale grooves and layering could be added to the micro-scale morphological structure to generate solutions mimicked from real-life structures. I would like to extend the research on various physical conditions.
REAL-WORLD APPLICATIONS
This biomimetic approach used in this computational sciences re- search supports that planar surfaces are detrimental to the drag coefficients. Since water’s kinematic viscosity was programmed in the simulation, hydrophobic surfaces could be fabricated to re- pel the flow of water molecules. For instance, the hydrophobic properties of coatings act as an automatic maintenance system for solar panels. This makes its surface resistive to weather phenome- na and fluid exposure that hinder the efficiency of the solar panels. Furthermore, this can be implemented in household materials to achieve self-cleaning properties.
POSSIBILITY OF IMPLEMENTATION IN COVID-19
Fluid mechanics plays an important role in the viral spread of SARS-CoV-2 through airborne transmission and surface contact (Cambridge Core, 2020). Reducing drag at a microscopic level will tend to repel the speech particles deposited at prevalent surfaces.
These viral particles’ adhesion to the surfaces such as steel rims in the hospital will remain for approximately 2-3 days (NIAID, 2020). To overcome the duration barrier, this biological technology will act as a surface treatment for the environment. The next steps are to incorporate more viscosity ranges for analyzing omniphobic properties.
CONCLUSION
The initial hypothesis that surfaces that exhibit a lot of vortex shedding will reduce drag significantly compared to a linear surface was supported by this experimentation. According to the results, the mor- phological structure of Tetrodontophora Bielanesis is the most efficient in reducing drag. The drag force was reduced by 83% in comparison to the flat surface for a freestream velocity of 1 m/s. It was observed that parabolic structures such as Rosaceae and Tetrodontophora Bielanesis tend to be better at drag reduction compared to sharp, rectangular structures like flat surface and Morpho Peleides by minimizing the opposing pressure acting on the geometric structure. A low Reynolds Number was maintained for a predictive analysis and laminar flow. This approach can be used for the development of hydrophobic biosurfaces to minimize the loss of energy from drag.
ACKNOWLEDGEMENTS
I would like to thank Dr. Phillip Servio (Department of Chemical Engineering; McGill University, Montreal; phillip.servio@mcgill.ca) and Jonathan Monahan (Chemical engineering undergraduate student; McGill University, Montreal; jonathan.monahan@mail.mcgill.ca) for their valuable advice and guidance. Thanks to Mrs. Magy Dimitry (LaurenHill Academy science teacher) for providing this opportunity and supporting me on this journey.
For Works Cited, see pp.4-5 in article pdf linked at top of page
ABOUT THE AUTHOR
Harini Karthik
Harini Karthik is a strong believer of biomimetics and bio-inspiration. She is an honor-roll student and an active participant in class. Her passion for engineering was sparked by nature and nurtured by experts on the significance of Computational Fluid Dynamics and surface modelling. She followed the path of 3Is: Imaginative, Insightful, and Intuitive, which allowed her to conduct research on drag reduction through surface geometry modification. She hopes to continue phase 2 of her assessment this fall. During her free time, Harini enjoys programming, volunteering, and playing badminton.