Unsteady Computational Fluid Dynamics in Aeronautics
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Many of the newest aircraft designs require the sort of innovative research that is routine for the AE AFM group. The cylindrical fuselage and moderately swept wings of older airplanes is quickly being replaced by a wide array of blended wing-body, joined- wing, vertical take-off and land, hypersonic scamjets, and even flapping wing vehicle designs. The growth and success of unmanned aerial vehicles UAVs and the critical need for highly fuel-efficient and environmentally responsible systems are leading to substantial innovation in aerospace configurations.
The AE AFM program advances prediction and control of fluid mechanics as a means of developing highly capable, efficient, and safe aircraft, launch and reentry vehicles, rotorcraft, novel UAV configurations, and wind energy systems.
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This work is of critical importance to the commercial aerospace industry, government agencies e. The AFM group has access to state of the art facilities for both experimental and computational research. The experimental facilities include the John J. Harper Wind Tunnel , located in the basement of the Guggenheim building. The tunnel has a 7 x 9-foot test section and a speed range of 10 to feet-per-second. If a majority or all of the turbulent scales are not modeled, the computational cost is very low, but the tradeoff comes in the form of decreased accuracy.
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In addition to the wide range of length and time scales and the associated computational cost, the governing equations of fluid dynamics contain a non-linear convection term and a non-linear and non-local pressure gradient term. These nonlinear equations must be solved numerically with the appropriate boundary and initial conditions. An ensemble version of the governing equations is solved, which introduces new apparent stresses known as Reynolds stresses. This adds a second order tensor of unknowns for which various models can provide different levels of closure.
It is a common misconception that the RANS equations do not apply to flows with a time-varying mean flow because these equations are 'time-averaged'. In fact, statistically unsteady or non-stationary flows can equally be treated. There is nothing inherent in Reynolds averaging to preclude this, but the turbulence models used to close the equations are valid only as long as the time over which these changes in the mean occur is large compared to the time scales of the turbulent motion containing most of the energy. Large eddy simulation LES is a technique in which the smallest scales of the flow are removed through a filtering operation, and their effect modeled using subgrid scale models.
This allows the largest and most important scales of the turbulence to be resolved, while greatly reducing the computational cost incurred by the smallest scales. Regions near solid boundaries and where the turbulent length scale is less than the maximum grid dimension are assigned the RANS mode of solution.
As the turbulent length scale exceeds the grid dimension, the regions are solved using the LES mode. Therefore, the grid resolution for DES is not as demanding as pure LES, thereby considerably cutting down the cost of the computation.
Direct numerical simulation DNS resolves the entire range of turbulent length scales. This marginalizes the effect of models, but is extremely expensive. The coherent vortex simulation approach decomposes the turbulent flow field into a coherent part, consisting of organized vortical motion, and the incoherent part, which is the random background flow. The approach has much in common with LES, since it uses decomposition and resolves only the filtered portion, but different in that it does not use a linear, low-pass filter.
Instead, the filtering operation is based on wavelets, and the filter can be adapted as the flow field evolves. Goldstein and Vasilyev  applied the FDV model to large eddy simulation, but did not assume that the wavelet filter completely eliminated all coherent motions from the subfilter scales. This approach is analogous to the kinetic theory of gases, in which the macroscopic properties of a gas are described by a large number of particles.
PDF methods are unique in that they can be applied in the framework of a number of different turbulence models; the main differences occur in the form of the PDF transport equation. The PDF is commonly tracked by using Lagrangian particle methods; when combined with large eddy simulation, this leads to a Langevin equation for subfilter particle evolution. The vortex method is a grid-free technique for the simulation of turbulent flows. It uses vortices as the computational elements, mimicking the physical structures in turbulence.
Vortex methods were developed as a grid-free methodology that would not be limited by the fundamental smoothing effects associated with grid-based methods. To be practical, however, vortex methods require means for rapidly computing velocities from the vortex elements — in other words they require the solution to a particular form of the N-body problem in which the motion of N objects is tied to their mutual influences. A breakthrough came in the late s with the development of the fast multipole method FMM , an algorithm by V.
Rokhlin Yale and L. Greengard Courant Institute. This breakthrough paved the way to practical computation of the velocities from the vortex elements and is the basis of successful algorithms. They are especially well-suited to simulating filamentary motion, such as wisps of smoke, in real-time simulations such as video games, because of the fine detail achieved using minimal computation. Software based on the vortex method offer a new means for solving tough fluid dynamics problems with minimal user intervention.
Among the significant advantages of this modern technology;. The vorticity confinement VC method is an Eulerian technique used in the simulation of turbulent wakes. It uses a solitary-wave like approach to produce a stable solution with no numerical spreading. VC can capture the small-scale features to within as few as 2 grid cells. Within these features, a nonlinear difference equation is solved as opposed to the finite difference equation.
VC is similar to shock capturing methods , where conservation laws are satisfied, so that the essential integral quantities are accurately computed. The Linear eddy model is a technique used to simulate the convective mixing that takes place in turbulent flow. It is primarily used in one-dimensional representations of turbulent flow, since it can be applied across a wide range of length scales and Reynolds numbers.
This model is generally used as a building block for more complicated flow representations, as it provides high resolution predictions that hold across a large range of flow conditions. The modeling of two-phase flow is still under development.source url
Unsteady Computational Fluid Dynamics In Aeronautics
Different methods have been proposed, including the Volume of fluid method , the level-set method and front tracking. This is crucial since the evaluation of the density, viscosity and surface tension is based on the values averaged over the interface. Discretization in the space produces a system of ordinary differential equations for unsteady problems and algebraic equations for steady problems. Implicit or semi-implicit methods are generally used to integrate the ordinary differential equations, producing a system of usually nonlinear algebraic equations. Applying a Newton or Picard iteration produces a system of linear equations which is nonsymmetric in the presence of advection and indefinite in the presence of incompressibility.
Such systems, particularly in 3D, are frequently too large for direct solvers, so iterative methods are used, either stationary methods such as successive overrelaxation or Krylov subspace methods.
Coupling of an unsteady aerodynamics model with a computational fluid dynamics solver
Krylov methods such as GMRES , typically used with preconditioning , operate by minimizing the residual over successive subspaces generated by the preconditioned operator. Multigrid has the advantage of asymptotically optimal performance on many problems. Traditional [ according to whom? By operating on multiple scales, multigrid reduces all components of the residual by similar factors, leading to a mesh-independent number of iterations.
For indefinite systems, preconditioners such as incomplete LU factorization , additive Schwarz , and multigrid perform poorly or fail entirely, so the problem structure must be used for effective preconditioning. CFD made a major break through in late 70s with the introduction of LTRAN2, a 2-D code to model oscillating airfoils based on transonic small perturbation theory by Ballhaus and associates. CFD investigations are used to clarify the characteristics of aortic flow in detail that are otherwise invisible to experimental measurements. To analyze these conditions, CAD models of the human vascular system are extracted employing modern imaging techniques.
A 3D model is reconstructed from this data and the fluid flow can be computed. Blood properties like Non-Newtonian behavior and realistic boundary conditions e. Therefore, making it possible to analyze and optimize the flow in the cardiovascular system for different applications. These typically contain slower but more processors.
For CFD algorithms that feature good parallellisation performance i.