Roland Kessler

Low-Dissipation Low-Dispersion Second-Order Scheme for Unstructured Finite Volume Flow Solvers

A new low-dissipation low-dispersion second-order scheme suitable for unstructured finite volume flow solvers is presented that is designed for vortical flows and for scale-resolving simulations of turbulence. The idea is that, by optimizing its dispersion properties, a standard second-order method can be improved significantly for such flows. The key is to include gradient information for computing face values of the fluxes and to use this additional degree of freedom to improve the dispersion properties of the scheme. The scheme is motivated by a theoretical consideration of the dispersion properties for a one-dimensional scalar transport equation problem. Then, the new scheme is applied using the DLR TAU-code for compressible flows and the DLR THETA-code for incompressible flows for simulations of the moving vortex problem and the Taylor–Green vortex flow. The improved accuracy for small-scale transportation and the easy implementation make this scheme a promising candidate for efficient scale-resolvin...

Scale-Resolving Simulations with a Low-Dissipation Low-Dispersion Second-Order Scheme for Unstructured Flow Solvers

A new low-dissipation low-dispersion second-order scheme is applied to scale-resolving flow simulations using compressible and incompressible unstructured finite volume solvers. In wall-resolved and wall-modeled large-eddy simulations of the plane channel flow, the new scheme yields substantial improvements compared to the more dissipative/dispersive standard central scheme over a considerable range of Reynolds numbers. For general hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations, a numerical blending approach is derived that uses a local sensor function to switch between the new scheme in the large-eddy simulation branch and the standard scheme in inviscid flow regions. After determining a suitable sensor formulation, the hybrid numerical scheme is applied to simulate a backward-facing step flow, for which satisfactory results and a reduced grid sensitivity are obtained. To demonstrate its potential in relevant aeronautical flows, the new scheme is successfully applied to hybrid Reynolds-ave...

Scale-Resolving Simulations with a Low-Dissipation Low-Dispersion Second-Order Scheme for Unstructured Finite-Volume Flow Solvers

A new low-dissipation low-dispersion 2nd-order scheme is applied to scale-resolving flow simulations using compressible and incompressible unstructured finite-volume solvers. In wall-resolved and wall-modeled LES computations of the channel flow the new scheme yields substantial improvements compared to the more dissipative/dispersive basic central scheme over a considerable range of Reynolds numbers. For general hy- brid RANS/LES simulations a numerical blending approach is applied which uses a local sensor function to switch between the new scheme in LES regions and the basic central scheme in the outer flow field. After an a-priori determination of a consistent sensor function, the hybrid numerical scheme is used to simulate a backward-facing step flow, where satisfactory results and reduced grid sensitivity are obtained. To demon- strate its potential in relevant aeronautical flows, the new scheme is successfully applied to hybrid RANS/LES computations of a 3-element airfoil near stall and a rudimentary landing gear with massive flow separation. Note that this paper is jointly published with a companion paper by Lowe et al., in which the low-dissipation low-dispersion 2nd-order scheme is derived and theoretically analyzed, followed by a basic assessment for fundamental numerical test cases.

Overlapping Grids in the DLR THETA Code

There is an increasing interest in flow simulations in configurations where parts are moving relatively to each other. A general approach to this problem is the use of overlapping grids, often called Chimera-technique. An incompressible flow solver based on a projection scheme with a fast multi-grid method for the pressure correction equation requires an implicit coupling between the grids on all levels of the multi-grid hierarchy in order to retain the good convergence properties. The efficient implementation of the overlapping grid technique in the DLR THETA code and its validation are presented in this chapter.

Simulation of the Flow in a Human Nose

The human nose is a very complex organ. The main airways together with a multiplicity of nasal cavities and sinuses are involved in the various functions of the nose. It warms and humidifies the inspired air, filters out small particles and supports the olfaction process by transporting odor-bearing particles to the muscous membranes. The flow simulations presented in this paper are based on the geometry of a real human nose. Based on a series of CT images, a body-fitted, hybrid numerical grid was built up. The DLR THETA code is used to simulate the unsteady flow inside the nose. The transport of a marker gas is simulated to visualise the entrainment of air from the sinuses to the inspired air.

Flow Control of a Rotating Cylinder by the Use of a Structured Surface: A Visualization of Flow Structures

Within the present numerical investigation different spherical grooves are applied to a circular cylinder. By the use of the overset grid method a spinning of the cylinder is combined by an in flow from a radial direction to simulate the flow around a rotating cylinder. The cylinder based Reynolds number is set to a fixed value of Re = 1:34 x 10^5. To determine the influence of the surface structure the groove geometry is varied. As a result, governing flow structures in and around an array of spherical grooves can be observed.

Numerical Investigation of the Detaching Vortical Flow at Rotating Cylinders with Thom Discs

Due to the high cost pressure in the field of aviation, the further increase in effciency is in the focus of development. Nowadays, airfoils are highly developed and optimized. Thus, new sophisticated approaches, additional configurations and physical effects at lift generation devices have to be explored in order to further reduce drag and significantly increase lift. Focal point of these considerations is the Magnus effect, in which rotating cylinders generate a lift force. This has already been investigated in the past, in particular, different kinds of surface roughnesses, special rotor geometries, ratios of circumferential speed velocity to free stream velocity, or the effect of end plates have been considered. The Scottish engineer A. Thom achieved a significant increase of the performance by adding so called Thom discs coaxially and equidistantly mounted on the cylinder. Investigations of the Thom disc rotors (Thom rotors) have shown a maximum of the lift to drag ratio epsilon for the circumferential cylinder velocity to freestream velocity alpha = 2. Most of the conducted experimental studies have only delivered integral force values. However, detailed insights in the structure of the flow field are still missing.

Turbulence Resolving Simulations of the Flow about a Tandem Cylinder and a Rudimentary Landing Gear

Scale-resolving simulations of two separated flows using the DLR codes TAU and THETA are presented and compared to reference data. For a tandem cylinder the compressible TAU code is shown to yield equally good results as other flow solvers, and the incompressible THETA code allows for efficient and accurate mean-flow predictions of a rudimentary landing gear. Thus, the maturity of the applied models and the numerical solvers for flows with massive separation is demonstrated.

Two-phase flow modeling combining the CICSAM with the projection method

The DLR TAU code is enhanced with the CICSAM method for an algebraic reconstruction of the fluid interface. The incompressible module of the DLR TAU Code is cast in a finite-volume projection method infrastructure for variable density flows. The infrastructure is rebuilt such that very high density ratios, e.g. the ratio of gaseous helium and liquid hydrogen can be modelled. TAU is based on the dual-grid approach, i.e. the initial primary grid is broken down into an unstructured secondary or dual grid. Thus, TAU is applicable on a wide range of meshes. Structured, Cartesian grids can be handled as well as unstructured, tetrahedral ones. Several test cases are presented in which a very good agreement to experimental and analytical results can be seen. I. Introduction he present work is embedded in the German Research Cooperation Upper Stage which was initiated to coordinate and perform research on advanced cryogenic upper stage technologies 1 for the Ariane 5. Advanced technologies for upper stages are one of the primary German investigation areas. In preparation of the development of new European advanced cryogenic upper stages, the need to investigate related advanced technologies has been identified 2 . The dynamics of liquid propellant is undesired but cannot be avoided in propellant tanks of space vehicles and places high demands on both tank and feed system design. It may lead to a lowering of the feed system performance and in worst case may even lead to a total failure of the system chain. The DLR CFD code TAU delivers a numerical platform with a wide range of applications. For the present work the TAU code is enhanced to simulate the sloshing dynamics of cryogenic propellants in tanks. Until today several numerical methods have been developed to model these effects. Basically two different approaches can be distinguished amongst these methods, moving and fixed grid algorithms. Moving grid methods are not applicable for the given code infrastructure in combination with the violent sloshing to be simulated. When merging or break-up problems shall be simulated the problems occurring with moving grid methods are obvious. In fixed grid methods, however, the grid remains static throughout the computation. In respect to the flow phenomena which TAU shall be able to handle the fixed grid method delivers a suitable approach regarding its expandability to additional interface physics.

Investigation of Resolution Requirements for Wall-Modelled LES of Attached and Massively Separated Flows at High Reynolds Numbers

This work is dedicated to the resolution requirements of Large-Eddy Simulation (LES) with near-wall modelling for attached and massively separated flows at high Reynolds numbers using the DLR THETA code. Two sensors are proposed to measure the resolution quality of LES for statistically steady flows. The first sensor is based on the resolved turbulent kinetic energy and the second one considers the resolved turbulent shear stress. These sensors are applied to turbulent channel flow at Re τ = 4800 and to the flow over a backward-facing step at Re h = 37500 on successively refined meshes, and results are compared with a convergence study of the mean velocity profiles.