Stability analysis of flow structures in hovering using robotic experiments and flow visualizations

We investigate the kinematics and stability of hovering flight making use of a robotic experimental device that simulate the movement of insects or birds. We carried out this analysis based on the characterization of this movement with flow visualizations and particle image velocimetry -PIV-. First, we characterized the kinematics of the robotic device inside water. Therefore, it has been verified that the robotic experiments follow the desired input signal precisely. Second, we give qualitative and quantitative information from the experimental tests as a function of frequency and angular amplitude. The kinematics of the hovering flight produces vortices that are stable or unstable around the rigid flat plate, as well as one transient regime that has been also found between these two latter states. Finally, dimensional velocity field plus associated vorticity have been characterized in these flow regimes through PIV measurements, and we also validated the reproducibility of experiment finding excellent agreement between different set of experiments.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Nonlinear evolution of harmonically forced perturbations on a wingtip vortex

Wingtip vortices are created by flying airplanes due to lift generation. The vortex interaction with the trailing aircraft has sparked researchers’ interest to develop an efficient technique to destroy these vortices. Different models have been used to describe the vortex dynamics and they all show that, under real flight conditions, the most unstable modes produce a very weak amplification. Another linear instability mechanism that can produce high energy gains in short times is due to the non-normality of the system. Recently, it has been shown that these non-normal perturbations also produce this energy growth when they are excited with harmonic forcing functions. In this study, we analyze numerically the nonlinear evolution of a spatially, pointwise and temporally forced perturbation, generated by a synthetic jet at a given radial distance from the vortex core. This type of perturbation is able to produce high energy gains in the perturbed base flow (10^3), and is also a suitable candidate for use in engineering applications. The flow field is solved for using fully nonlinear three-dimensional direct numerical simulation with a spectral multidomain penalty method model. Our novel results show that the nonlinear effects are able to produce locally small bursts of instability that reduce the intensity of the primary vortex.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Theoretical parameters of trailing vortices versus aspect ratio of wing models

We perform 2D-PIV measurements to characterize trailing vortices in NACA0012 wing models for aspect ratios ranging between 1 and 2.5, and for chord-based Reynolds numbers from 7000 to 40000. Firstly, and regarding the influence of the Reynolds number, the increase of this dimensionless parameter generates a more concentrated and intense vortex, presenting, therefore, an increase in all its characteristic magnitudes: maximum azimuthal velocity and vorticity. Secondly, the greater the aspect ratio, the greater the vortex strength is observed. Thirdly, the radial location of the peak of the azimuthal velocity has a strong decay as the aspect ratio increases for Re=7000, but it changes its trend for Re=40000.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.. Ministerio de Economía y Competitividad (Spain) Grant No. DPI2013-40479-P and DPI2016-76151-C2-1-R and Junta de Andalucía Grant No. P11-TEP-7776

DPIVSoft-OpenCL: a multicore CPU-GPU accelerated open source code for 2D Particle Image Velocimetry

We present a translation of the original Matlab DPIVSoft code to a complete open source code implemented in Python, to perform Particle Image Velocimetry (PIV) in two-dimensions, in parallel, and with interrogation window shifting along with the double-pass window deformation approach using multiple iterations for each pass. The added value of the code is the use of the Open Computing Language (OpenCL) library to parallelize the original code on multiple Intel Central Processing Units (CPUs) and/or Graphics Processing Units (GPUs), so it can be run on all commercially available GPUs. Examples of flow application are included in the text using synthetic images generated from DNS data from John Hopkins Turbulence Database (JHTD) (Perlman, 2007), showing about 90x speedup over the previous Matlab implementation for a given test case.This research has been supported by one grant from the Ministerio de Economía y Competitividad of Spain (Grant No. DPI2016-76151-C2-1-R) and partially by the project B4-2019-11, 0837002010 from the Universidad de Málaga and the project PID2021-124692OA-I00 from the Ministerio de Ciencia e Innovación // Partial funding for open access charge: Universidad de Málaga / CBU

Effect of the axial jet on the optimal response in Batchelor vortex

En este póster se estudia la respuesta óptima del torbellino de Batchelor para distintos números de onda. Se demuestra que incluso teniendo la velocidad axial, un torbellino es capaz de tener grandes amplificaciones de energía.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Wingtip vortex in a NACA0012 airfoil and its active control

Contribución mediante sesión pósterWe conduct experiments in a towing-tank to analyse the flow patterns of wingtip vortices in a NACA 0012 airfoil. In this experimental research, we provide PIV measurements and flow visualisations. Without active control, several parameters are given experimentally as function of the Reynolds number, so we compare these data with the theoretical models of Batchelor, and Moore and Saffman together with DNS. Secondly, we analyse the effect of a continuous injection in the spanwise direction. The continuous jet has a strong influence on the wing-tip vortex formation. We explore this effect at low chord based Reynolds number ranging from 7000 up to 20000. We change the aspect ratio of the injection, R, defined as the ratio of the velocities between the jet (Uj) and free-stream (U). For R=1, we find that the jet strongly affects the wingtip vortex formation with a sudden decrement of the axial vorticity and the azimuthal velocity. This technique is a challenge and a promising tool to reduce the intensity of the vortex core.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

On the effect of low blowing ratio continuous jets on wingtip vortex characteristics

Vortices are an unavoidable effect of flight, which appear behind the wing with a bounded length. The strength of these vortices, which are extremely stable, is due to the lift force [1]. That is the reason why this phenomenon is especially relevant during take-off and landing operations. In these situations, when aircraft are departing from or arriving to the airport runways, the following aircraft might feel two counter-rotating vortices which remain long time under normal environmental conditions. Unfortunately, this huge rotation of airflow patterns always destabilizes the following aircraft. Consequently, trailing vortices have a mighty influence on the air traffic control of airport runways, and they have justified the research interest in this topic since the 1960's [2]. However, aeronautical engineers are still searching for different technological strategies to breakdown these wingtip vortices.Short-Term Postdoctoral Stay of Talent Attraction Plan of Andalucía TECH ICE. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

On the optimaL response of q-vortex

Wingtip vortices represent a hazard for the stability of the following airplane in airport highways. These flows have been usually modeled as swirling jets/wakes, which are known to be highly unstable and susceptible to breakdown at high Reynolds numbers for certain flow conditions, but different to the ones present in real flying airplanes. A very recent study based on Direct Numerical Simulations (DNS) shows that a large variety of helical responses can be excited and amplified when a harmonic inlet forcing is imposed. In this work, the optimal response of q-vortex (both axial vorticity and axial velocity can be modeled by a Gaussian profile) is studied by considering the time-harmonically forced problem with a certain frequency ω. We first reproduce Guo and Sun’s results for the Lamb-Oseen vortex (no axial flow) to validate our numerical code. In the axisymmetric case m = 0, the system response is the largest when the input frequency is null. The axial flow has a weak influence in the response for any axial velocity intensity. We also consider helical perturbations |m| = 1. These perturbations are excited through a resonance mechanism at moderate and large wavelengths as it is shown in Figure 1. In addition, Figure 2 shows that the frequency at which the optimal gain is obtained is not a continuous function of the axial wavenumber k. At smaller wavelengths, large response is excited by steady forcing. Regarding the axial flow, the unstable response is the largest when the axial velocity intensity, 1/q, is near to zero. For perturbations with higher azimuthal wavenumbers |m| > 1, the magnitudes of the response are smaller than those for helical modes. In order to establish an alternative validation, DNS has been carried out by using a pseudospectral Fourier formulation finding a very good agreement.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Frequency response of Batchelor vortex.

We carried out a frequency response analysis of Batchelor vortex model in two different spatial configurations: punctual and annular jet. The theoretical base flow corresponds to the experimental setting of a wing model with airfoil NACA0012 for a chord-based Reynolds number Rec=40000 and angle of attack of α=9◦. We found that Batchelor model presents a gain in the annular jet configuration higher than the punctual jet for a pair of parameters k and ωf . The results of this research work will be used to propose future candidates of active control.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Ministerio de Ciencia e Innvocación, Proyecto Nacional PID2021-124692OA-I0

Numerical Investigation on the Effect of Active Injection Location on the Frequency Response of a Batchelor Vortex.

This study investigates the effect of the variation of the active control application distance on the frequency response of a Batchelor vortex. The theoretical base flow pertains to the experimental configuration of a wing model with a NACA0012 airfoil at an angle of attack of α=9◦ and a chordbased Reynolds number of Rec=40000. The injection was implemented using punctual and annular jet configurations. The findings of this research will serve as a basis for the optimization of the experimental parameters that characterize potential candidates for active control.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech