Dynamical systems analysis of a zero-equation transition model for sensitivity to initial conditions
Adynamical system analysis is performed on the zero equation: W transition model (Sandhu, J. P. S., and Ghosh, S., “A local correlation-based zero-equation transition model,” Computers & Fluids, Vol. 214, 2021, p 10475) to study the sensitivity to initial conditions. The analysis is performed for homogeneous and non-homogeneous flow, focusing on the approximated intermittency function used in the model. The analysis showed that the use of turbulent to molecular viscosity ratio (turbulent Reynolds number) in the approximate intermittency functionwas the reason for the: W transition model’s sensitivity to initial conditions, particularly for cases with low freestream turbulence intensity. A comparison with other models in the literature showed that the use of wall distance in such functions aids in avoiding the sensitivity issue.
A simplified local correlation-based zero-equation transition model
In this paper we present a simplified form of the local correlation-based zero-equation kγ transition model (Sandhu, Jatinder Pal Singh, "Local-Correlation Based Zero-Equation Transition Model for Turbomachinery," Proceedings of the ASME 2019 Gas Turbine India Conference, Volume 1: Compressors, Fans, and Pumps; Turbines; Heat Transfer; Structures and Dynamics) derived from the one-equation γ transition model (Menter, F. R., Smirnov, P. E., Liu, T., and Avancha, R., “A One-Equation Local Correlation-Based Transition Model,” Flow, Turbulence and Combustion, vol. 95, 2015, pp. 583–619). The new model is easy to implement and saves computational time and memory. The proposed model is validated against the standard T3 series flat plate test cases (with and without pressure gradient), Hultgren and Volino series, Aerospatial A-Airfoil and the Eppler-387 airfoil. Results indicate that the transition predicted with the new model is similar to the one-equation γ transition model in most cases and compares reasonably well with experimental data. The predicted transition is also gradual in some cases and the method provides a savings in computational memory and time (in most of the flat-plate test cases) over the γ model.
A mathematical method for modelling compliant camber morphing airfoil geometries
This paper describes the development of two mathematical methods to analytically model variable camber morphing wing airfoils. The variation in camber is achieved through a combination of droop nose and trailing edge deflections, at the desired hinge locations, from the base airfoil configuration. The surface deflections are achieved in a compliant manner with allowance for variations in the order of morphing. The first method focuses on the preservation of the physical accuracy of morphing by preserving the airfoil shape and camber length of the base airfoil while using camber deflection as the basis for morphing. The second method directly morphs the surface coordinates and allows for the specification of rigid portions at the extreme ends of the airfoil. The results of both codes are directly compared with the results of several published prototypes of morphing wings to cement the accuracy and validity of the two methods arrived at.
Adaptation of 2D unstructured mesh based on solution gradients
The work presented in this paper attempts to improve the grid (and consequently solution) for compressible-flow simulations performed with a 2D finite-volume Euler solver for unstructured grids, using mesh adaptation based on gradients of flow parameters, (pressure and pseudo-entropy, and grid geometry (cell areas, nodal distances etc.). The procedure requires solution (primitive variables) reconstruction at grid nodes, which are interpolated using linear polynomials in 2 dimensions or inverse distance based methods, and cell-averaged gradients, which are computed using the Green-Gauss method. The adaption is terminated if the global maximum displacement of any node is less than ε, where ε is a small user defined length scale. Results indicate that the method is capable of clustering the grid near an oblique shock and a contact wave, which results in sharper resolution of the discontinuities.
Numerical investigation of passive flow control using permeable and wavy walls in oblique shock-wave/boundary-layer interaction
This work investigates the effect of a porous medium/permeable wall in the region of flow separation induced by an impinging oblique shock/boundary-layer interaction (SBLI) at Mach 2.0 using 2D numerical simulations. The study presented here includes an investigation of cases with no control, with permeable walls, and wavy non-permeable walls with varying waviness. The effect of the position and extent of the permeable wall is also investigated. The porous region is modeled as a cavity filled with a square array of circular cylinders (rendered as circles in 2D). The computations are performed using a parallel, finite-volume solver for compressible flows on structured grids. An immersed-boundary method is used to represent the porous region. Menter’s k − ω SST model is used to model turbulence. Results are presented using pressure contours (to reveal shock structure), streamline patterns, and near-surface velocity and pressure. It is observed from the plots that the limiting case of a non-permeable wall produces better results compared to the permeable wall, indicating that passive blowing (from) and suction (into) the permeable wall does not produce the desired effect of energizing the boundary layer and mitigating flow separation in this case.
2D Immersed Boundary Solver for Compressible Laminar Flows
In this paper, an immersed-boundary method developed for compressible viscous flows is further improved to account for better mass conservation and address numerical instabilities associated with the velocity reconstruction. The 2D embedded object is represented as a set of line segments along with their outward unit normal vectors. A forcing method that leverages the finite-volume approach is used, wherein the solution at cell interface that lie near the boundaries of the embedded solid is reconstructed. A cut-cell type approach is proposed to the determine the effective flow through area of a cell face that is intersected by the immersed boundary to improve mass conservation; however, the boundary of the embedded object is not reconstructed in its entirety. This method shall is validated for supersonic inviscid flow past a bump in a channel and a circular cylinder, transonic viscous flow past a NACA0012 airfoil and supersonic viscous flow past a circular cylinder. The results are compared with simulations from literature using contours of flow properties, surface pressure and show fair to good agreement.
Effectiveness of micro-vortex generators in tandem in high-speed flows
Micro-vortex generators offer an alternative to boundary-layer bleed and suction to mitigate flow separation due to shock/boundary-layer interaction. In the last two decades, a number of devices have been investigated either in isolation, wherein the focus has been on studying the flow physics, or in tandem, for studies in flow-separation control. While studies of vortex generators in a supersonic free-stream (without a separate shock/boundary-layer interaction) have generally focused on the understanding of the flow downstream of single devices, their effect on the flow while being used in tandem have not been looked into in as much detail. This work investigates the effect of inter-device spacing in a systematic manner to optimize the configuration of two micro-vortex generators placed side-by-side. A set of objective functions is designed using boundary-layer integral properties and are determined for various inter-device spacing. Simple, slotted, and ramped-vane devices are investigated in this work. Results show that increased device spacing reduces device drag but also worsens boundary-layer health. RANS computations are performed using an immersed-boundary method that renders the vortex generator as a point cloud. The effect of the inter-device spacing of the vortex generators on the mitigation of flow separation is finally tested using simulations of a Mach 2.5 impinging oblique-shock/boundary-layer interaction. Flow-separation profiles indicate that the ramped-vane device provides better mitigation of separation compared to the slotted device and its performance improves with reduction in inter-device spacing.
Numerical study of camber morphing in naca0012 airfoil
Camber morphing is an effective way to control the lift generated in any airfoil and potentially improve airfoil efficiency (lift-drag ratio). This can be especially useful for fixed wing UAVs undergoing different flying manoeuvres and flight phases. This work investigates the aerodynamic characteristics of NACA0012 airfoil morphed by the Single Corrugated Variable Camber (SCVC) morphing and Double Corrugated Variable Camber (DCVC) morphing approach. The airfoil is reconstructed from the camber line using a Radial Basis Function (RBF) based interpolation method (J. H. S. Fincham and M. I. Friswell, “Aerodynamic optimisation of a camber morphing aerofoil,” Aerosp. Sci. Technol., 2015). The aerodynamic analysis is done by employing two different finite volume solvers: OpenFOAM and ANSYS-Fluent, and a panel method code (XFoil). Results reveal that the aerodynamic coefficients predicted by the two finite-volume solvers using a fully turbulent flow assumption are similar but differ from those predicted by XFoil. The aerodynamic performance of morphed airfoils are nearly equal or lower than that of the baseline airfoil at lower values of coefficient of lift whilst at large values of the morphed airfoils display superior aerodynamic performance. At identical morphing angles, the aerodynamic characteristics of SCVC and DCVC airfoils are almost identical. Finally, it is observed for a fixed angle of attack, that an optimum morphing angle exists for which the aerodynamic efficiency becomes maximum.
Optimization of the thrust to lift ratio using camber morphing in flapping airfoil
The objective of the work presented in this paper is to maximize the ratio of thrust to lift for a flapping NACA0012 airfoil using camber morphing. The Fish Bone Active Camber (FishBAC) is chosen as the camber morphing mechanism for this study. This mechanism can be used to improve the aerodynamic performance of an airfoil to meet the requirements of multiple flight conditions. The numerical method used for optimization is based on the method of steepest ascent. The optimization parameters include the amplitude of trailing edge deflection in morphing and its phase difference with flapping kinematics. An immersed-boundary method is used to solve the two-dimensional, unsteady, laminar flow past a flapping and morphing airfoil.