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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - TRANSEP (Flow physics and interaction of laminar-turbulent transition and flow separation studied by direct numerical simulations)

Teaser

The vision of this work is to overcome the failure of Computational Fluid Dynamics (CFD) to tackle one of the central unsolved fluid physics problems, namely predicting the sensitive flow physics associated with laminar-turbulent transition and flow separation. A recent...

Summary

The vision of this work is to overcome the failure of Computational Fluid Dynamics (CFD) to tackle one of the central unsolved fluid physics problems, namely predicting the sensitive flow physics associated with laminar-turbulent transition and flow separation. A recent, highly influential report by NASA clearly states that the major shortcoming of CFD is its “… inability to accurately and reliably predict turbulent flows with significant regions of separation”, most often associated with laminar-turbulent transition. The research address this shortcoming and develop and utilize computational methods that are able to predict, understand and control the sensitive interplay between laminar-turbulent transition and flow separation in boundary layers on wings and other aerodynamic bodies.

We will be able to understand enigmas such as the recent results from experiments where the laminar area of a wing grows after a smooth surface have been painted (increased roughness), or the drastic changes of laminar-turbulent transition and separation locations on unsteady wings, or the notoriously difficult interaction of multiple separation and transition regions on high-lift wing configurations. For such flows there have been little understanding of flow physics and few computational prediction capabilities. Here we will perform simulations that give completely new possibilities to visualize, understand and control the flow around such wings and aerodynamic bodies, including the possibility to compute and harness the flow sensitivities. We tackle these outstanding flow and turbulence problem using the new possibilities enabled by multi-peta scale computing.

These research questions are highly important in the design of new fuel efficient aircraft, where the high drag of the turbulent and separated flow needs to be avoided as much as possible. If laminar flow control is utilised in a modern transport aircraft a typical fuel savings of 15% is envisaged.

Work performed

1. First turbulence simulation of flows over wing section at Re=1,000,000

We have conducted well-resolved large-eddy simulations (LESs) of the turbulent flow around a NACA4412 wing section up to a numerically high Reynolds number of 1 million (based on inflow velocity and wing chord length). We have considered a small angle of attack of 5 degrees to ensure that the turbulent boundary layers around the airfoil remain attached, and we have used the high-order spectral-element code Nek5000. We carefully designed the numerical setup, including resolution and accuracy requirements, and implemented a relaxation-term-based filter for the LES in Nek5000. The LES results were validated against fully-resolved direct numerical simulations (DNSs) of the same flow case. We also developed a complete framework to compute turbulence statistics and power-spectral densities in the moderately complex geometries under study.

2. Data-driven methods for studying turbulence

We have developed a data-driven method for predictions of turbulence quantities in spatially-developing boundary layers. In particular, we employed a system identification approach where high-fidelity numerical data are used to build single and multiple-input linear and non-linear transfer functions. The developed methodology has great potential for implementation in experiments and realistic flow control applications.

3. First high fidelity simulations of pitching airflows at moderately high Reynolds numbers

We have conducted high-fidelity simulations of unsteady wings at two widely different Reynolds numbers. We studied the unsteady boundary layer transition of a laminar wing undergoing forced pitch oscillations at a moderately high Reynolds number of 750,000. The study lead to the development of a low-dimensional model for the approximation of unsteady aerodynamic loads and a much improved understanding of the origin of non-linear aerodynamic response of unsteady laminar airfoils.

Another study involved the study of unsteady aerodynamics of pitching airfoils at low Reynolds numbers (Re=100,000) which brought to light the complex dynamical behavior of unsteady separation bubles. The study found that unsteady separation bubles can undergo state changes from convective to absolute instability and that such state changes cause abrupt changes in the boundary-layer characteristics of the airfoil, leading to large variations in aerodynamic loading.

4. Large scale simulations of transition under free-stream turbulence in boundary layers and low-pressure turbine flows

We also study effects of the free-stream turbulence characteristic length scales and intensity on the transition in an incompressible flat-plate boundary layer by means of DNS. Computations are performed using the spectral element code Nek5000. Numerically-generated homogeneous isotropic turbulence upstream of the leading edge is designed to imitate the characteristics of the grid-generated turbulence in the wind tunnel experiments. Various combination of levels of the free-stream turbulence intensity and integral length scales are simulated. To ensure the quality of the data, classical turbulence statistics and integral quantities are carefully evaluated showing close agreement with the corresponding experimental data. The aim is to study more closely the nucleation process, validate and extend the existing model for spot nucleation.

We also study both the effect of the level of free-stream turbulence and the effect of the wake on the low-pressure turbine blades. The homogeneous and isotropic freestream turbulence is prescribed at the inlet as superposition of Fourier modes with a random phase shift. In a second stage of the study, cylinders moving in front of the leading edge of the turbine are included to model the effect of the wake coming from upstream blade. That is done using the tool NekNek which allows to run at the same time two different simulations which communicates with each other at each time step

Final results

The progress beyond the state of the art is contained in three main results of the project:

Incorporation of the possibility to perform unsteady aerodynamics in the Nek5000 open access software and the use of this software to understand the fluid physics of the transition and separation processes associated with the pitching airfoil study.

Understading of the roughness induced instability in the swept wing RECEPT experiment as a localised wave packet growth, giving an earlier instability compared to fully turbulent transition at the roughness location.

The first fully wall resolved accurate LES simulations of a 1.000.000 Reynolds number turbulent wing ever performed.

During the second phase of the project we expect to perform more ground breaking simulations of the same type, for example we are on the way to, for the first time, determine the bifurcation to an unsteady fluid interaction through a global instability calculation for a realistic wing.