Christoph Brehm

Experimental Investigation of Separation and Separation Control on a Laminar Airfoil

The flow field around a two-dimensional slightly modified NACA 643 618 airfoil was investigated for two chord Reynolds numbers, Re = 64, 200 and Re = 137, 000. Surfacepressure distributions and total aerodynamic forces were measured for angles of attack in the range of 12 ◦ to 20 ◦ . Experimental results were compared to numerical results obtained by Direct Numerical Simulations and calculations using the two-dimensional design code XFOIL. Additionally, acoustic measurements and flow-visualizations were performed to study the transition and the separation behavior of the flow for various angles of attack. For certain angles of attack, separation bubbles developed which had a strong influence the airfoil performance. Using the combined approach, experimental and numerical, a deeper insight into the underlying physical mechanisms and into the separation and transition behavior of the boundary layer around the airfoil was obtained. Based on these results, passive and active flow-control strategies were investigated. The flow-control strategies aimed at improving the overall airfoil performance by preventing or at least reducing regions of separated flow.

A locally stabilized immersed boundary method for the compressible Navier-Stokes equations

A higher-order immersed boundary method for solving the compressible Navier-Stokes equations is presented. The distinguishing feature of this new immersed boundary method is that the coefficients of the irregular finite-difference stencils in the vicinity of the immersed boundary are optimized to obtain improved numerical stability. This basic idea was introduced in a previous publication by the authors for the advection step in the projection method used to solve the incompressible Navier-Stokes equations. This paper extends the original approach to the compressible Navier-Stokes equations considering flux vector splitting schemes and viscous wall boundary conditions at the immersed geometry. In addition to the stencil optimization procedure for the convective terms, this paper discusses other key aspects of the method, such as imposing flux boundary conditions at the immersed boundary and the discretization of the viscous flux in the vicinity of the boundary. Extensive linear stability investigations of the immersed scheme confirm that a linearly stable method is obtained. The method of manufactured solutions is used to validate the expected higher-order accuracy and to study the error convergence properties of this new method. Steady and unsteady, 2D and 3D canonical test cases are used for validation of the immersed boundary approach. Finally, the method is employed to simulate the laminar to turbulent transition process of a hypersonic Mach 6 boundary layer flow over a porous wall and subsonic boundary layer flow over a three-dimensional spherical roughness element.

A novel concept for the design of immersed interface methods

The objective of this paper is to present a novel, robust, high-order accurate Immersed Interface Method (IIM) for advection-diffusion type equations. In contrast to other immersed methods that were designed for consistency and accuracy with a posteriori check of the numerical stability, we combine local Taylor-series expansion at irregular grid points with a local stability constraint as part of the design process. Stability investigations of the IIM are employed to demonstrate that the local stability constraint is sufficient for obtaining a globally stable method, as long as the Neumann number is less than its limiting value. One of the key aspects of this IIM is that the irregular finite-difference stencils can be isolated from the rest of the computational domain. To validate our novel immersed interface approach, two-dimensional and three-dimensional test cases for model equations are presented. In addition, this method is applied to the incompressible Navier-Stokes equations to conduct stability investigations of a boundary layer flow over a rough surface, and for investigations of pulsatile stenotic flows. Stability investigations of wall bounded flows are challenging for immersed methods, because the near wall accuracy is important for correctly capturing the characteristics of the hydrodynamic instability mechanisms, in particular regarding the wave relation between the wave velocity components close to the wall.

1/5 Scale Model of Aeromot 200S SuperXimango for Scaled Flight Research

Within the framework of a NASA STTR program, we designed and developed a 1/5 dynamically scaled model of the Aeromot (AMT) 200S motor glider (“SuperXimango”). This model is one element in a “tool box” for scaled model flight research that also includes wind/water tunnel experiments, aerodynamic design tools, and computational fluid dynamics. This 1/5 dynamically scaled model of the Aeromot 200S SuperXimango is awaiting flight testing which will allow for a validation and verification of the scaling by comparison with full scale flight test data. An important criterion in the design and development of the flight model is its ability to serve as a versatile platform for developing and testing of novel technologies. Therefore, great emphasis was placed on a modular design and construction so that certain modules can be easily exchanged.

Novel Immersed Interface Method Based on Local Stability Conditions

A novel immersed interface method is presented which is based on a local Taylor series expansion at irregular grid points whereby numerical stability is enforced through a local stability condition. In the past, various immersed interface/boundary methods were developed by solely considering the order of the local truncation error at the irregular grid points. The numerical stability of these schemes was demonstrated in a global sense by applying either a matrix stability analysis or considering a number of dierent test cases. None of these schemes used a concrete local stability condition to derive the stencils at irregular grid points for advection-diusio n type equations. This paper will show the derivation of stencil coecients at irregular grid points for the one dimensional advection-diusio n equation. The advection-diusio n equation may be viewed as a simple model equation for the incompressible Navier-Stokes equations. This paper will demonstrate that the local stability constraints can be justified in a global setting as long as the DFL-number stability limit is not reached. This paper will also present an extension of the one-dimensional immersed interface method to the two-dimensional case. In addition, the novel immersed interface method will be extended with a capability which allows for moving boundaries. Finally, several numerical sample problems as well as numerical matrix stability analysis for advection-diusio n type equations will prove the full operability of this novel immersed interface method.

Closed-loop control of low-pressure turbine laminar separation

Laminar separation from the suction side of low-pressure turbine (LPT) blades can signican tly degrade engine eciency . If laminar separation could be controlled, aerodynamic performance could be maintained over a wider operating range. Also, new and more aggressive stage designs with reduced solidity (less blades) would become possible. One example is the new L1M LPT blade, which was designed to allow for an increased aerodynamic loading by applying active o w control (AFC) in a continuous fashion. Motivated by this and other success stories where open-loop o w control was shown to result in dramatic performance improvements, the general focus has shifted towards closed-loop control which promises even greater gains and a greater robustness and capability to adjust to changing operating conditions. However, a fully satisfactory methodology for designing robust and ecien t closed-loop controllers for uids problems has not been devised yet. This paper summarizes dieren t approaches for investigating control of o w separation from the L1M blade using 2-D numerical simulations. A parameter study with open-loop control by harmonic wall normal blowing upstream of the separation was conducted to determine the optimum forcing parameters. We believe that by exploiting o w instability mechanisms the o w control can be made more ecien t. It was also demonstrated how a proportional dieren tial (PD) controller with self-adjusting parameters can be employed successfully for closed-loop control. Even better closed-loop controllers will likely become possible if a real-time prediction of the o w dynamics over a reasonably broad parameter range became available. Galerkin models perform well for one given operating point but do not generalize well. Here, neural networks were explored for making real-time predictions of the unsteady separated L1M o w eld. The resulting models are shown to be both accurate and robust and to generalize well.

Open-loop flow-control investigation for airfoils at low reynolds numbers

Within the scope of a flight research project involving dynamically scaled models, active flow control was investigated for a modified NACA 643-618 airfoil. At low-Reynolds-number conditions, the aerodynamic performance of the modified NACA 643-618 airfoil is considerably reduced by flow separation. Computational-fluid-dynamics simulations of the natural (uncontrolled) and controlled flow were carried out and provide the basis for a detailed analysis of the underlying physical mechanisms. The simulation results for the natural uncontrolled flow compare well with wind-tunnel measurements. Simulations and experiments show a large trailing-edge separation at low angles of attack and a leading-edge separation bubble at high angles of attack. To control the laminar separation and improve the overall performance of the airfoil at the low-Reynolds-number conditions, time-periodic blowing and suction through a spanwise slot is employed. A strong dependence of the effectiveness of the active flow control on the fo...

Biglobal stability analysis as an initial value problem for a stalled airfoil

A biglobal stability approach formulated as an initial value problem is used to study the occurrence of biglobal modes for the flow around a NACA0015 airfoil at a high angle of attack. For the stability analysis of the NACA0015 airfoil both steady and unsteady base flows are considered. An important focus of the paper is on elaborating the differences between the stability characteristics of steady and unsteady base flows. The connection between experimentally observed spanwise periodically occurring three-dimensional separated regions (also referred to as stall cells when occurring on aircraft wings at high angles of attack) and unstable biglobal modes is explored. The occurrence of these three-dimensional separated flow regions can have a detrimental effect on the controllability of the airplane at deep stall conditions. In this paper results from direct numerical simulations of the flow past a NACA0015 airfoil at angle of attack, α = 18◦, and Re = 1, 000 are presented. The relevance of the linearly unstable biglobal modes for the fully three-dimensional flow field will be discussed.

Novel immersed boundary/interface method for the compressible navier-stokes equations

An extension of the immersed interface method developed by Brehm and Fasel 1‐3 to the compressible Navier-Stokes equations is presented. The extension of the immersed interface/boundary method 1,2 contains some modifications to the original approach in order to incorporate characteristics of the compressible Navier-Stokes equations. The van Leer flux splitting approach utilized in the compressible Navier-Stokes solver was considered in the design of the finite dierence stencils in the vicinity of the immersed boundary. The extensive stability investigations of the immersed scheme confirm that a stable immersed boundary treatment is achieved. The simulation results for a Mach 6 boundary layer flow over a flat plate and a porous wall validate the proposed immersed boundary method for the compressible Navier-Stokes equations.

Numerical investigation of transition delay for various controlled breakdown scenarios in a Mach 6 Boundary Layer using porous walls

The influence of porous walls on the transition process of a Mach 6 Boundary Layer for different controlled breakdown scenarios is investigated using temporal direct numerical simulations. This study focusses on the non-linear regime where prior studies have mainle focussed on the linear stage. The fundamental and subharmonic resonance scenario are compared for a smooth wall case and a porous wall geometry. The porous walls showed potential to delay transition for the fundamental resonance scenario. The goal in this paper is to show that for other breakdown scenarios the porous walls still remain effective in delaying transition.