Experimental study in near-and far-field of trailing vortices and their active control

Spatialaveraged two-dimensional PIV velocity profiles are compared for ����=7×103 by using direct numerical simulations (DNS) up to eleven chords from the wing. Once we validate our results, we fit the theoretical parameters as function of ����. Five theoretical parameters are given from computational and experimental results: two corresponding to Batchelor’s model and three regarding Moore & Saffman’s model. Two critical Reynolds numbers were found. Our DNS computations verify that the onset of instability of the flow around the wing at the first threshold ������1 ≈1.3×103 captures the change in the trend of theoretical parameters. In addition, the theoretical parameters appear to become constant experimentally for a second critical Reynolds number ������2 greater than 10-20×103 as our results are compared with those given by other authors. Consequently, Reynolds number plays an important role in the stability analysis for trailing vortices not only taking into account viscous terms but also determining the input parameters for theoretical models. Finally, we have carried out a study of the blowing effect of continuous jets that are perpendicular to the moving direction, and blowing from the tip of a NACA0012 airfoil. We analyze three Reynolds numbers ���� and four jet-to-crossflow blowing ratios ��������. We show how these jets are good candidates to reduce the strength of the wingtip vortices at the lowest Reynolds numbers considered, e.g. ���� = 7×103. For higher Reynolds numbers up to ����=20×103, the forcing has a weak influence on the vortex strength in the near-field once the rolling-up process has already finished, and especially at axial distances greater than 7 chords behind the wing. The reason for the presence of two different strength decays depending on the Reynolds number is explained by the ability of the continuous jet to break the vorticity sheet creating a counter-rotating vortex or co-rotating vortex at low or high values of ����, respectively. This mechanism makes the wingtip vortex to decrease or remain its vortex strength as we apply different blowing ratios ��������. This effect is evident at the lowest Reynolds number at which we observe a strong vortex decay. Conversely, the continuous jet changes the characteristics of the vortex flow in the formation and the near-field evolution of the wingtip at high Reynolds numbers, but there is not a appreciable effect on the vortex strength and how downstream evolution.In order to predict the axial development of the wingtip vortices strength, an accurate theoretical model is required. Several experimental techniques have been used to that end, e.g. PIV or hotwire anemometry, but they imply a significant cost and effort. For this reason, we have carried out experiments using the smokewire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor’s model for two chord based Reynolds numbers, ������ = 3.33 ⋅ 104 and 105. Therefore, this theoretical vortex model has been introduced in the integration of ordinary differential equations which describe the temporal evolution of streak lines as a function of two parameters: the swirl number, ��, and the virtual axial origin, ��0. We have applied two different procedures to minimize the distance between experimental and theoretical flow patterns: individual curve fitting at six different control planes in the streamwise direction as well as the global curve fitting which corresponds to all the control planes simultaneously. Both sets of results have been compared with those provided by del Pino et al. [2011a], finding good agreement. Finally, we have observed a weak influence of the Reynolds number on the values �� and ��0 at low-to-moderate ������. This experimental technique is proposed as a low cost alternative to characterize wingtip vortices based on flow visualizations. Secondly, we present a detailed analysis of experimental and numerical results for the flow of wingtip vortices behind a NACA0012 airfoil. Particular attention is paid to a specific value of the angle of attack, ��=9∘, and ultra-low and low chord-based Reynolds numbers ranging from ����=0.3×103 to 20×103

Extensional Magnetorheology as a Tool for Optimizing the Formulation of Ferrofluids in Oil-Spill Clean-Up Processes

In this study, we propose a new way of optimising the formulation of ferrofluids for oil-spill clean-up processes, based on the rheological behaviour under extensional flow and magnetic fields. Different commercial ferrofluids (FFs), consisting of a set of six ferrofluids with different magnetic saturation and particle concentration, were characterised in a Capillary Break-Up Extensional Rheometer (CaBER) equipped with two magnetorheological cells that allow imposing a homogeneous and tunable magnetic field either parallel or perpendicular to the flow direction. The filament thinning process with different intensities and orientation of the magnetic field with respect to the flow direction was analysed, and the results showed that the perpendicular configuration did not have a significant effect on the behaviour of the ferrofluids, as in shear magnetorheometry. However, the parallel configuration allowed to determine that the formulation of ferrofluids for oil-spill cleaning processes should consist of a 4% vol concentration of magnetic nanoparticles with a magnetic saturation of Ms> 20 mT

Influence of the Polarity of the Electric Field on Electrorheometry

Uniaxial extensional flow is a canonical flow typically used in rheological characterization to provide complementary information to that obtained by imposing simple shear flow. In spite of the importance of having a full rheological characterization of complex fluids, publications on the rheological characterization of mobile liquids under extensional flow have increased significantly only in the last 20 years. In the case of the rheological characterization of electrorheological fluids, the situation is even more dramatic, as the ERFs have been exclusively determined under simple shear flow, where an electrorheological cell is attached to the rotational rheometer generating an electric field perpendicular to the flow direction and that does not allow for inverting the polarity. The very recent work published by Sadek et al., who developed a new electrorheological cell to be used with the commercial Capillary Breakup Extensional Rheometer (CaBER), allows for the very first time performing electrorheometry under extensional flow. By means of the same experimental setup, this study investigates the influence of the polarity of the imposed electric field on the filament thinning process of a Newtonian and an electrorheological fluid. Results show that a polarity against the gravity results in filament thinning processes that live longer or reach a stable configuration at lower intensities of the applied electric field

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

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

Optimized design and manufacturing of a motorcycle fairing spider

In the racing world, weight is one of the key factors when developing a vehicle. Therefore, the aim is to reduce it as much as possible to achieve a good power/weight ratio that can be translated into increased speed, manoeuvrability, or reduced fuel consumption. For this reason, the trend is to redesign existing parts to obtain more optimised and lighter ones using new materials and complex structures that are often manufactured using 3D printing. In this manuscript, a spider or support for the fairing of a racing motorbike was designed, making use of topological optimisation techniques by means of Computer-Aided Design and using additive manufacturing. Specifically, PLA was used as an eco-friendly material to replace the conventional welded metal used in these areas of a motorbike. Theoretical and experimental tests were carried out to confirm the viability of the piece. With the analysis of the topological optimisation, it was possible to manufacture a sustainable, low weight and low cost part, which has never been manufactured before with a polymeric material

PIV measurements of the effect of pulsed blowing jet on a NACA0012 wing model

Wingtip vortices are present in taking off, and landing operations and their presence in airport runways must be reduced. To that end, several strategies have been considered in the last decades, being the active control one possible technical solution. To compute the effectiveness of active control that corresponds to pulsed low-blowing-ratio transverse jet for the reduction of the wingtip vortex strength, we carry out 2D-PIV measurements in a towing tank for chord-based Reynolds numbers 15000 and 20000. We consider two cases: (i) no active control Rjet=0 and (ii) pulsating radial jet of blowing-ratio Rjet smaller than 1.7 (or momentum coefficient lower than 0.12) and different Strouhal numbers ranging from 0.27 to 0.94. Our observations show that the best reduction of wingtip vortex strength takes place at the lowest Strouhal number tested. We use the maximum azimuthal velocity and vorticity together with the circulation to quantify this decrease in the vortex strength. Besides, we define the spatial evolution of a disturbance parameter which allow us to detect again the optimal frequency that leads to vortex destruction.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Influence of Reynolds number on theoretical models for trailing vortices

We conduct direct numerical simulations for a NACA0012 airfoil at Reynolds numbers (Re) ranging from 300 to 7000 to determine the wake behavior behind this wing profile. We characterize the structure of the wing-tip vortex, finding a reasonable agreement with experimental results at Re=7000. In addition, we model the trailing vortex theoretically, thus obtaining the parameters for Batchelor’s and Moore and Saffman’s models. We compare the results of the best fitting for the axial vorticity and the azimuthal velocity, finding only small discrepancies. The main contribution of this research work is to study the evolution of these theoretical parameters as function of the Reynolds number. We observe that the wake becomes unstable at Re ≈1200, in agreement with previous results. These instabilities in the wake behind the wing produce a change in the trend of theoretical parameters (keywords: vortex dynamics, trailing vortices, theoretical models).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Experimental characterization of wingtip vortices using smoke flow visualizations

In order to predict the axial development of the wingtip vortices strength an accurate theoretical model is required. Several experimental techniques have been used to that end, e.g. PIV or hotwire anemometry, but they imply a significant cost and effort. For this reason, we have carried out experiments using the smoke-wire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor's model~\cite{batchelor}, which only depends on two free parameters, i.e. the vortex strength, $S$, and the virtual origin, $z_0$. Results for two chord based Reynolds numbers have been compared with those provided by del Pino et at. (2011), finding good agreement.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Higher order dynamic mode decomposition to identify decay in trailing vortices

We conduct three-dimensional measurements of the velocity field using stereo PIV in the turbulent wake of a wing model based on a NACA0012 airfoil, for an angle of attack of 9 degrees, and Reynolds number 40000 up to 32 chords from the wing. These experimental results are compared with theoretical models of trailing vortices, Batchelor [2] and Moore and Saffman [3], to determine the parameters of the turbulent wake vortex. Three different zones are identified based on the vortex strength: near, middle and far fields. We make use of Higher Order Dynamic Mode Decomposition (HODMD) to give a better understanding of the most relevant modes within these three regions. We observe that vortex decay in the middle field is related with the HODMD modes. Thus, the information given by this study is useful to test proof-of-concept of future active control for vortex alleviation.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech