Aerodynamic and Flow Characteristics of a Rough Airfoil: A Numerical Study

A computational study explores the aerodynamic and flow properties of rough-surfaced NACA-0018 airfoils using turbulent K-omega SST methodology and the control volume method. Ansys Fluent 2022 R2 was used to do the simulations, and Reynolds numbers between 5000 and 10000 were used. The analysis of how the rough surface affects the performance of the airfoil was the main goal of the study. The skin friction coefficient, drag force, lift force, pressure contours, and stream functions were among the many variables assessed. To have a thorough understanding of the flow behavior, these parameters were studied at various Reynolds numbers. The findings revealed a clear pattern: along the wall of the airfoil, the average drag force, lift force, and skin friction coefficient all increased linearly as the Reynolds number rose from 5000 to 10000. This discovery sheds important light on how the rough surface affects the overall aerodynamic performance of the airfoil. This study advances knowledge of the aerodynamic behavior of rough airfoils by illuminating the flow characteristics and their relationship to surface roughness. Significant results impact aeronautics, wind turbine design, and other airfoil applications.


Introduction
Large size, fast rotational speed, and operating at Reynolds numbers above 106 based on the mean airfoil chord length are common characteristics of commercial aircraft.In contrast, unmanned aircraft called micro air vehicles (MAVs) have smaller dimensions and travel at slower cruising speeds, giving them Reynolds numbers between 104 and 105.The low Reynolds number zone reveals distinct aerodynamic characteristics in airfoils compared to those with higher Reynolds numbers, defining the low Reynolds number zone.One significant difference in this regime is the difficulty of generating enough lift to achieve a high lift-to-drag ratio due to the likelihood of laminar separation.Furthermore, changes in friction drag have a significant effect on the overall drag coefficient [1][2].Some factors influence airfoil aerodynamic properties in the low-Reynolds number zone, including freestream turbulence level, airfoil geometries (a leading-edge shape, maximum thickness, camber strength, position of maximum camber, and so on), and the Reynolds number [3].The creation of a laminar separation bubble is one of the most important and distinguishing characteristics.The laminar separation bubble is created by laminar separation, transition, and reattachment and is known to influence airfoil aerodynamic coefficients such as lift and drag.Because 1305 (2024) 012003 IOP Publishing doi:10.1088/1757-899X/1305/1/012003 2 of such characteristics, which are distinctive of the low Reynolds number area, such as the laminar separation bubble, the low Reynolds number zone's airfoil aerodynamic properties differ dramatically from those of the high Reynolds number region.As a result, while designing airfoils for low Reynolds numbers, some general knowledge about aerodynamic properties in the high-Reynolds number area is unavailable.As a consequence, it is essential to get a fresh understanding of aerodynamic properties in the low-Reynolds number area.Several computational and experimental analyses are available to analyze airfoil aerodynamic properties.However, obtaining exact aerodynamic coefficients through experiment is challenging due to the flow.To attain a low Reynolds number environment, the speed should be decreased, and hence measured aerodynamic forces should be low.The numerical simulation, on the other hand, may create ideal circumstances and adjust for the uncertainty of experimental observations.Another advantage of numerical simulation is that it may be utilized to get detailed flow field characteristics by analyzing instantaneous flow fields.Furthermore, parametric analyses can provide a broad understanding of airfoil properties in the low-Reynolds-number zone.As demonstrated by the study of Obama et al., numerical simulation may also be employed for airfoil optimization at low Reynolds numbers [4][5][6][7].The accuracy of numerical simulation predictions is a key component of the examination of airfoil aerodynamics.Different airfoil configurations have been tested to overcome this issue.Kojima et al. found 2-D Lam insufficient for flow at high angles of attack (AOA) distant from the leading edge.They consistently saw separation, transition, and reattachment behaviors during their experiments, and the three-dimensional large-eddy simulation successfully portrayed the features of the separation bubble.The separation flow from the leading edge at high AOA was faithfully simulated in three dimensions by big eddy modelling.As a result, the three-dimensional large-eddy simulation is an effective numerical method for assessing the aerodynamic characteristics of airfoils.With the exception of high AOA, the two-dimensional laminar (2-D Lam) simulation is still applicable in the majority of situations [8][9][10].The prediction of aerodynamic properties for each numerical method is evaluated in this study using three numerical simulation approaches.Among the techniques employed are two-dimensional pressure static flow simulation, two-dimensional path line simulation, and two-dimensional steam function simulation.Each numerical result's CL and CD, or aerodynamic lift and drag forces, as well as the skin friction coefficient detachment and reattachment sites, and prediction variances between approaches, are all looked at.

Computational Method
In this study, a slightly modified NACA 0018 coordinate aero foil file was used.A NACA 0018 symmetric airfoil is often used in small-to-medium scale vertical-axis wind turbines and aerial vehicles.NACA 0018 airfoil Max thickness of 18% at 30% chord.Maximum camber is 0% at 0% chord.A good grid system is very important for quality CFD analysis.When the temperature is set, the air's density is 1.225 kg/m3 and 1.7894 kg/(m)s of viscosity.The K-omega SST model is used to calculate the turbulent viscosity [11].

Governing Equation Conservation of mass:
Mass conservation states that the net rate of change of the mass of a control volume (CV) is equal to the net rate of transport across the boundary of the CV.In vector form, (3)

Conservation of momentum:
According to the conservation of momentum, the surface forces and body forces acting on a fluid is equal to the rate of change in linear momentum of a volume flowing with a fluid [12].
Reynolds Number: A useful measure for determining whether a flow state will be laminar or turbulent is the Reynolds number, which is the ratio of inertial to viscous forces.

Generation of Domain
The geometry of the computational domain is depicted in the image below.axis.For this study, 0.1% is used as the flow speed for the speed inflow boundary condition.This is because the turbulence in the inflow is expected to be lower than in the outflow [13].

Meshing
The grid independence test is crucial to the accuracy of a CFD analysis.A 2D unstructured mesh with a total of around 375105 components is used in the current investigation.

Method of Solution:
For pressure-velocity coupling in this study, a coupled system was used.The pressure-based segregation method cannot compare to the benefits of this solver.In terms of single variables that can be in steady-state flows, the pressure-based approach is more efficient and practical.The spatial discretization gradient used a least squares cell-based method, with first-order upwind for turbulent energy and second-order upwind for momentum [14].Reynolds number, it turns upward.The lift coefficient value rises from its prior point by around 49% to 53% as the Reynolds number rises, as can also be shown in Fig. 5.These numbers pertain to the airfoil wall.In addition, we determined the skin friction coefficient in Fig. 6.Similar to the drag coefficient vs. Reynolds number graph, the skin function vs. Reynolds number graph displays a correlation.In order to raise the Reynolds number, skin function is growing by roughly 20% more than previously.Finally, we need to evaluate what we learned from the simulation and how awesome it is.Thus, the static pressure, Path lines, and steam function figures are displayed below.

Conclusions
In conclusion, the goal of this study was to examine the aerodynamic properties of the modified NACA 0018 airfoil at various Reynolds numbers.The study confirms the findings by showing that Cl and Cd values for airfoils rise with Reynolds number.This discovery supports our past simulation work and further supports the accuracy of our findings.We also looked at the stream function, path lines, and static pressure flow fields, which added more information on the behavior of the airfoil.The results improve knowledge of the aerodynamic capabilities and responsiveness to Reynolds numbers of the NACA 0018 airfoil.The Cl, Cd, and skin friction coefficients observed upward trend underline the importance of Reynolds number in affecting airfoil properties.Our understanding of the intricate flow phenomena connected to the airfoil is improved by the examination of flow field parameters.This paper evaluates the NACA 0018 airfoil redesign and offers insightful recommendations for airfoil design and optimization.Future studies should look at modifying design parameters and other aspects that affect aerodynamic performance.The examination improves our comprehension of the aerodynamic behavior of the NACA 0018 airfoil and provides a starting point for further research into the design and analysis of airfoils.

Acknowledgement
The authors acknowledge the Military Institute of Science and Technology, Bangladesh for providing support and CAD lab facilities to perform simulation.

Figure 1 :
Figure 1: Geometrical shape of modified NACA 0018While the rectangular part is 30 mm long, the quasi-section has a radius of 05 mm.The 1 mm chord length aerofoil is positioned in the domain so that its outer diameter is near the center of the diameter of the domain's semi-circular area and that its chord line corresponds with the domain's symmetry line.For an inflow moving at a velocity of 14.607 m/s, 17.528 m/s, 20.4502 m/s, 23.371 m/s, 26.293 m/s, and 29.2146 m/s for different Reynolds numbers.For the outflow, the exit boundary condition was used in the same way.Other borders were subject to the wall requirement.The flow velocity's sine component is parallel to the y-axis of the domain, whereas the cosine component is parallel to the x-

Figure 2 :
Figure 2: Grid Independence Test Higher magnification images of the meshing near the airfoil surfaces are given in Figure 3.

Figure 3 :
Figure 3: Magnified View of Meshing Solver-Setting: The calculation is carried out using ANSYS FLUENT.For estimating CL and CD fluctuation with velocity that changes with Reynolds number, the SST k turbulence model is utilized.The controlling integral formulas for the conservation of mass and momentum were resolved by this CFD tool.Boundary Conditions Inlet: The inlet velocity boundary condition is applied in the inlet for different Reynolds numbers.Outlet: It uses the pressure outlet boundary condition.The gauge pressure is kept at 0 Pa at the outflow border.Wall: In the flow geometry, stationary walls with the no-slip condition have been applied at all solid barriers.u wall =0 m/s and v wall ==0 m/s are two ways to express it mathematically.Method of Solution:For pressure-velocity coupling in this study, a coupled system was used.The pressure-based segregation method cannot compare to the benefits of this solver.In terms of single variables that can be in steady-state flows, the pressure-based approach is more efficient and practical.The spatial discretization gradient used a least squares cell-based method, with first-order upwind for turbulent energy and second-order upwind for momentum[14].

6 .
Results and DiscussionAt Reynolds Numbers, we have proportionately validated our findings.The graphical results of the validation are shown below for the drag coefficient, lift coefficient, and skin friction coefficient.

Figure 7 :
Figure 7: Steam Function flow field of Re 5000