Skip to main content

Spatio-temporal measurement and plasma-based control of crossflow instabilities for drag reduction

Periodic Reporting for period 2 - GLOWING (Spatio-temporal measurement and plasma-based control of crossflow instabilities for drag reduction)

Reporting period: 2020-08-01 to 2022-01-31

Delay of laminar-turbulent flow transition on aircraft wings can potentially reduce aerodynamic drag by up to 15%, reducing emissions and fuel consumption considerably. The main cause of laminar-turbulent transition on commonly used swept wings is the development of crossflow (CF) instabilities. Despite their importance, our fundamental understanding of CF instabilities is limited due to inability of current measurement techniques to capture their complex and multi-scale spatio-temporal features. This severely limits our ability to delay CF transition, which is further impeded by the lack of simple, robust and efficient control concepts.
In this project we will achieve unprecedented spatio-temporal measurements of CF instabilities and develop a novel active flow control system that can successfully delay transition on swept wings. To achieve these goals, we bring forth a unique combination of cutting-edge technologies, such as tomographic particle image velocimetry, advanced plasma-based actuators and linear/non-linear stability and control theory.
Spatio-temporal volumetric velocity measurements of CF instabilities will be achieved at three important stages of their life, namely inception, growth and breakdown, providing breakthrough insights into the underlying physics of swept wing transition and turbulence production. The results will be used to postulate and validate linear and non-linear stability and control theory models and provide top benchmarks for high-fidelity CFD. The unprecedented wealth of information, enabled through these advances, will be used to design and demonstrate the first synergetic plasma-based laminar flow control system. This system will feature minimum-thickness plasma actuators, able to suppress the growth of CF instabilities and achieve and sustain considerable transition delay at high Reynolds numbers. These advances will finally enable robust and efficient laminar flow on future air transport.
The work in the first half of the project has been progressing according to schedule. The main achievements so-far are the following:

- design, fabrication and commissioning of several experimental facilities for the necessary experiments
As part of the experimental activities in Task 1 and Task 4, design and fabrication of several experimental items is completed. The first includes a complete instrumentation of the M3J/LTT facility towards enabling accurate spatio-temporal measurments of crossflow instabilities using PIV. New sCMOS type of digital cameras are integrated in conjunction with two OPTRIS PI 640 infrared cameras for transition detection. For the dedicated receptivity studies in Task 1 a second "leading edge" setup is designed and fabricated providing good optical and physical access to the receptivity region. This setup is destined for the anechoic tunnel facility.

- development and validation of several numerical codes necessary
As part of Task 2 and 4 several numerical and theoretical tools have been developed. The first is a non-linear extension of the Parabolized Stability Equations (NPSE) able to model the development of primary crossflow instabilities and their harmonics.As a further extension a fully elliptic Harmonic Navier Stokes solver has been developed and tested able to model effects of roughness. The two tools are complemented with a new inverse boundary layer solver, able to model viscous-inviscid interactions

- receptivity scaling to roughness amplitude and location (journal publication under review, abstract provided here)

The effect of discrete roughness elements on the development and breakdown of stationary crossflow instability on a swept wing is explored. Receptivity to various element heights and chordwise locations is explored using a combination of experimental and theoretical tools. Forcing configurations, determined based on linear stability predictions, are manufactured and applied on the wing in a low turbulence facility. Measurements are performed using infrared thermography, quantifying the transition front location and spectral characteristics, and planar particle image velocimetry, providing a reconstruction of stationary crossflow instabilities and their associated growth. Measurements are corroborated with simulations based on non-linear parabolised stability equations. Results confirm the efficacy of discrete roughness elements in introducing and conditioning stationary crossflow instabilities. Pri- mary instability amplitudes and resulting laminar-turbulent transition location are found to strongly depend on both roughness amplitude and chordwise location. The Reynolds number based on element height is found to satisfactorily approximate the initial forcing amplitude, revealing the importance of local velocity effects in non-zero-pressure gradient flows. Direct estimation of initial perturbation amplitudes from non-linear simulations suggests the existence of pertinent flow mechanisms in the element vicinity, active in conditioning the onset of modal instabilities. Dedicated velocimetry planes, elucidate the development of a momentum deficit wake which rapidly decays downstream of the element followed by mild growth, representing the first experimental evidence of transient behavior in swept wing boundary layers. The outcome of this work identifies a strong scalability of the transition dynamics to roughness amplitude and location, warranting the upscaling of roughness elements to more accessible, measurable and spatially resolved configurations in future experiments.

- receptivity to unsteady plasma forcing(journal publication under review, abstract provided here)

This paper investigates the effects of the spanwise-uniform Dielectric Barrier Discharge (DBD) plasma actuator (PA) on crossflow instabilities (CFIs) and the eventual laminar- turbulent transition on a 45◦ swept wing at chord Re = 2.17 × 106. Two major parameters of the PA operation are scrutinized, namely the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements (DRE) are installed near the leading edge (LE) to promote and condition a critical stationary crossflow vortex (CFV) mode. Numerical solutions of the boundary layer equations and Linear Stability Theory (LST) are used in combination with the experimental pressure distribution to provide predictions on the most critical stationary and travelling CFIs. By means of infrared thermography, the laminar-turbulent transition front is visualized and quantified. Further quantitative measurements of the boundary layer velocity are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the ability of PAs to induce unsteady perturbations in a broad range of frequencies which can potentially promote transition to turbulence, through triggering of travelling CFIs. When the PA is located closer to the LE, low frequency CFIs are found to be significantly amplified. Conversely, when the PA is located further downstream, it is found to promote strongly amplified high frequency CFIs. Finally, the spanwise- wavenumber spectra extracted from velocity measurements demonstrate that induced higher order CFIs of type I & III essentially originate from the imbalance between the positive and negative travelling perturbations induced by the forcing. Specifically, positive travelling perturbations play a dominant role in type III structures while the negative travelling perturbations dominate the formation of type I instabilities. The different roles of positive and negative travelling perturbations are attributed to the velocity gradients caused by the modulation of the base flow due to the persistent stationary CFVs.

- full DNS of crossflow instabilities and interaction with spanwise rougness (journal publication under review, abstract provided here)

The interaction between forward-facing steps of several heights and a pre-existing critical stationary crossflow instability of a swept-wing boundary layer is analysed. Significant degrada- tion of laminar flow on wings of subsonic aircraft due to such spanwise-distributed geometric imperfections has stimulated a series of numerical and experimental studies in recent years. These have given rise to contradictory conclusions with regard to the effect of step-induced flow features on the major stability characteristics of a pre-existing stationary crossflow perturbation. In this work, Direct Numerical Simulations (DNS) are performed of the incompressible three- dimensional laminar base flow and the stationary distorted flow that arises from the interaction between an imposed primary stationary crossflow perturbation and the steps. These DNS are complemented with solutions of the linear and the non-linear Parabolised Stability Equations (PSE), used towards identifying the influence of linearity and non-parallelism near the step. A fully stationary solution to the Navier-Stokes equations is numerically enforced, in order to isolate the mechanisms pertaining to the interaction of the stationary disturbance with the step.
Results provide insight into the salient modifications of the base laminar boundary layer due to the step and the response of the incoming crossflow instability to these changes. The fundamental spanwise Fourier mode of the disturbance field gradually lifts up as it approaches the step and passes over it. The flow environment around the step is characterised by a sudden spanwise modulation of the base-flow streamlines. Additional stationary perturbation structures are induced at the step, which manifest in the form of spanwise-aligned velocity streaks near the wall. Shortly downstream of the step, the fundamental component of the crossflow perturbation maintains a rather constant amplification for the smallest steps studied. For the largest step, however, the fundamental crossflow perturbation is significantly stabilised shortly downstream of the largest step. This surprising result is ascribed to a modulation of the kinetic energy transfer between the base flow and the fundamental perturbation field, which is brought forward as a new step interaction mechanism. Possible non-modal growth effects at the step are discussed. Furthermore, the results from DNS indicate significant amplification of the high-order harmonic crossflow components downstream of the step.
- Isolation of possible breakdown mechanism due to elevated roughness

New insights are gained here, particularly involving a newly discovered mechanisms governing the breakdown of crossflow vortices under supercritical roughness. By the end of the project this mechanisms will be generalised and scaled as well as conceptualised into a predictive model.

- Discovery of transient growth mechanisms in vicinity of roughness elements

This was an unexpected development emerging from the spatio-temporal measurements. The implications are profound, as this explains many discrepancies in past predictions. By the end of the project, we want to confirm this discovery by using the newly developed numerical methodologies.

- Demonstration of BFM-plasma based control of crossflow instabilities

The basic machanism behind BFM has been experimentally measured and will be conceptualised. Steps are taken to upscale it to higher Reynolds numbers.