Thierry Jardin

On the non-intrusive evaluation of fluid forces with the momentum equation approach

The aim of this paper is to discuss the advantages and difficulties linked with the experimental application of the momentum equation approach as a non-intrusive way to predict the unsteady loads experienced by an airfoil in motion. First, in order to evaluate the influence of the varying parameters relative to the calculation of the corresponding drag and lift coefficients, numerical flow fields obtained by means of DNS are used. The comprehension of the impact of the spatial and temporal resolutions, velocity accuracy or third velocity component on the estimation of forces allows us to quantify the accuracy of the approach and helps in specifying the parameters setting which could lead to a consistent experimental application. In a second step, the approach is applied to experimental flow fields measured through the use of time resolved particle image velocimetry (TR-PIV). A low Reynolds number flow around an impulsively started airfoil is considered. The loads and vorticity flow fields are correlated and compared with those obtained by DNS.

Wake Instabilities behind an Axisymmetric Bluff Body at Low Reynolds Numbers

This paper aims at understanding the mechanisms that lead to the onset of chaos in the wake of blunt based axisymmetric bluff body. On the basis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 800, we show that the flow undergoes multiple transitions, successively giving rise to the Steady State SS and to the Reflectional Symmetry Preserving RSP a , RSP b and RSP c wake states. In particular, the RSP c state is characterized by intermittent vortex stretching denoting the onset of chaos and the potential occurence of a third instability that superimposes to the first and second instability associated with state RSP a and RSP b respectively. Interestingly, the reflectional symmetry plane that characterizes the RSP states is still retained. Hence, chaos is triggered before the symmetry breaking and the occurence of the Reflectional Symmetry Breaking RSB state observed at higher Reynolds numbers.

Analysis of the Turbulent Wake Generated by a Micro Air Vehicle Hovering near the Ground with a Lattice Boltzmann Method

This paper presents numerical investigations undertaken to analyze the turbulent flow produced in the wake of a micro air vehicle rotor interacting with the ground. Two configurations are investigated: a free rotor and a shrouded rotor. The Reynolds number based on the chord and tip speed is Retip =0.86 × 105, which corresponds to a challenging flow where leading-edge separations are commonly observed. The numerical simulations are performed with a Reynolds-averaged Navier–Stokes approach and a large-eddy simulation (by means of a lattice-Boltzmannmethod), combinedwith an immersed boundary approach. The comparison of numerical data with measurements shows that the mean flow and the turbulent shear stresses are accurately predicted close to the ground and in the rotor wake. However, some discrepancies remain in the prediction of the rotor torque and thrust, mainly due to the difficulty to reproduce the flow near the rotor walls. An analysis is conducted to identify and understand the different sources of turbulent production. The numerical simulations show also that the presence of a shroud contributes, at a given thrust, to reduce the velocity and the turbulent intensity at the ground.

Aerodynamic Performance of a Hovering Microrotor in Confined Environment

This paper aims at understanding how the aerodynamic performance of a hovering microrotor is affected by horizontal and vertical wall proximity. Toward that end, experiments are performed to extract aerodynamic loads and velocity flow fields from strain gauges and high-definition stereoscopic particle image velocimetry measurements, respectively. The results show that horizontal wall boundary conditions contribute to enhancing aerodynamic performance, whereas vertical boundary conditions have a negligible impact. Enhancement of aerodynamic performance arises from distinct flow physics, such as rotor wake expansion or Venturi effects, that depend on the configuration considered. These results open the path toward the development of micro air vehicles dedicated to the exploration of highly confined environments.

Root Cutout Effects on the Aerodynamics of a Low-Aspect-Ratio Revolving Wing

Direct numerical simulations of the flow past a low-aspect-ratio revolving wing are performed. The wing undergoes an impulsively started 180 deg revolution about a vertical axis at angles of attack 15, 30, and 45 deg and chord-based Reynolds number 1000. The root cutout is varied at a fixed wing radius, R=4 chords, and the effects on the flow structure and aerodynamic performance of the wing are evaluated. It is shown that an optimum in aerodynamic efficiency exists at low root cutout. Results suggest that this optimum is due to the competition between low Reynolds number effects at the wing root and root vortex effects. In addition, it is shown that a large root cutout can inhibit leading-edge vortex burst that occurs at high angles of attack. However, despite the associated recovery in pressure forces near the wing tip, this inhibition has no significant impact on aerodynamic performance.

Revisiting Froude’s Theory for Hovering Shrouded Rotor

This paper extends Froude’s momentum theory for free propellers to the analysis of shrouded rotors. A one-dimensional analytical approach is provided, and a homokinetic normal inlet surface model is proposed. Formulations of thrusts and power for each system component are derived, leading to the definition of optimum design criteria and providing insight into the global aerodynamics of shrouded rotors. In the context of micro-air vehicles applications, assessment of the model is conducted with respect to numerical data. Overall, comparison between numerical and analytical results shows good agreement and highlights the sensitivity of the model to viscous effects.

Application of a lattice Boltzmann method to some challenges related to micro-air vehicles

The demand for micro-air vehicles is increasing as well as their potential missions. Whether for discretion in military operations or noise pollution in civilian use, the improvement of aerodynamic and acoustic performance of micro-air vehicles propeller is a goal to achieve. Micro- and nano-air vehicles operate at Reynolds numbers ranging from 103 to 105. In these conditions, the aerodynamic performance of conventional fixed and rotary wings concepts drastically decreases due to the increased importance of flow viscous forces that tend to increase drag and promote flow separation, which leads to reduced efficiency and reduced maximum achievable lift. Reduced efficiency and lift result in low endurance and limited payloads. The numerical simulation is a potential solution to better understand such low Reynolds number flows and to increase the micro-air vehicles’ performance. In this paper, it is proposed to review some challenges related to micro-air vehicles by using a Lattice-Boltzmann method. The method is first briefly presented, to point out its strengths and weaknesses. Lattice-Boltzmann method is then applied to three different applications: a DNS of a single blade rotor, a large eddy simulation of a rotor operating in-ground effect and a large eddy simulation of a rotor optimised for acoustic performance. A comparison with reference data (Reynolds Averaged Navier-Stokes, DNS or experimental data) is systematically done to assess the accuracy of lattice-Boltzmann method-based predictions. The analysis of results demonstrates that lattice-Boltzmann method has a good potential to predict the mean aerodynamic performance (torque and thrust) if the grid resolution is chosen adequately (which is not always possible due to limited computational resources). A study of the turbulent flow is conducted for each application in order to highlight some of the physical flow phenomena that take place in such rotors. Different designs are also investigated, showing that potential improvements are still possible in terms of aerodynamic and aero-acoustic performance of low-Reynolds rotors.

Immersed Boundary Lattice Green Function methods for External Aerodynamics

In this paper, we document the capabilities of a novel numerical approach - the immersed boundary lattice Greens function (IBLGF) method - to simulate external incompressible flows over complex geometries. This new approach is built upon the immersed boundary method and lattice Greens functions to solve the incompressible Navier-Stokes equations. We show that the combination of these two concepts allows the construction of an efficient and robust numerical framework for the direct numerical and large-eddy simulation of external aerodynamic problems at moderate to high-Reynolds numbers.