Experimental Fluid Dynamics

A comprehensive computational and experimental study has been performed at the NASA Langley Research Center as part of the Quiet Aircraft Technology (QAT) Program to investigate the unsteady flow near a leading-edge slat of a two-dimensional, high-lift system. This paper focuses on the experimental effort conducted in the NASA Langley Basic Aerodynamics Research Tunnel (BART) where Particle Image Velocimetry (PIV) data was acquired in the slat cove and at the slat trailing edge of a three-element, high-lift model at 4, 6, and 8 degrees angle of attack and a freestream Mach Number of 0.17. Instantaneous velocities obtained from PIV images are used to obtain mean and fluctuating components of velocity and vorticity. The data show the recirculation in the cove, reattachment of the shear layer on the slat lower surface, and discrete vortical structures within the shear layer emanating from the slat cusp and slat trailing edge. Detailed measurements are used to examine the shear layer formation at the slat cusp, vortex shedding at the slat trailing edge, and convection of vortical structures through the slat gap. Selected results are discussed and compared with unsteady, Reynolds-Averaged Navier-Stokes (URANS) computations for the same configuration in a companion paper by Khorrami, Choudhari, and Jenkins (2004). The experimental dataset provides essential flow-field information for the validation of near-field inputs to noise prediction tools.

The conventional suction geometry is not efficient for higher capacity of pump and thus reduced discharge on the delivery side. Intake manifold is being designed for this work. The previous configuration would be studied using CFD techniques while pursuing the objective of arriving at the most efficient geometry for the given application. The marginal increment in the discharge for the Centrifugal Pump tends to depreciate with each marginal rise in capacity the pump; especially for the higher order pumps (25HP and above). The prominence of vortices along with turbulent flow at the regions in the suction pipe affects the flow of water and consequently the discharge. The discharge the `sump' is not favorably designed for aiding the intake through the suction pipe. This work would focus on Design alternatives for minimizing the vortices within the suction pipe and enhancing the discharge through possible use manifold at the suction end. Alternatively, efforts would be pursued for addressing the Design of the Sump (Tank) for facilitating the flow of water at the suction end while smoothing out the in pipe.

Simulations have been performed with spatial uniformity assumed, in order to assess the effect of tentative wall reactions on autoignition delay times and on a pressure -ambient temperature diagram of oxidation phenomena in the case of n-butane. Reactions which depend on the type of reactor wall coating have been considered for HO and RO radicals with estimated rate constants. The inhibiting effect of these walls has been simulated for closed vessel autoignition delay times as well as for ignition boundaries at pressures below 1 atm. The minimum temperature at which reaction occurs in jet-stirred reactor simulations is similarly affected 2 2 .

Simulations have been performed with spatial uniformity assumed, in order to assess the effect of tentative wall reactions on autoignition delay times and on a pressure -ambient temperature diagram of oxidation phenomena in the case of n-butane. Reactions which depend on the type of reactor wall coating have been considered for HO and RO radicals with estimated rate constants. The inhibiting effect of these walls has been simulated for closed vessel autoignition delay times as well as for ignition boundaries at pressures below 1 atm. The minimum temperature at which reaction occurs in jet-stirred reactor simulations is similarly affected 2 2 .

Accuracy of velocity-vorticity ( V, ω)-formulations over other formulations in solving Navier-Stokes equation has been established in recent times. However, the issue of non-satisfaction of solenoidality conditions on vorticity is not addressed in the literature which can possibly lead to non-physical solution. In this respect, here, we have developed and reported conservative rotational form of the ( V, ω)-formulation which preserves the solenoidality condition on vorticity in a much simpler way compared to other formulations. Superiority of rotational form over the conventional Laplacian form of ( V, ω)-formulation is also shown [by comparing the results for flows inside cubical lid driven cavity (LDC)]. For solving the 3D Navier-Stokes equation using a staggered grid, we use optimized compact schemes for (a) interpolation and (b) evaluation of first and second derivatives. As illustrations, we have solved problems of (i) flow inside a 3D lid driven cavity (LDC), whose solutions are compared with experimental results reported by Koseff and Street [J. Fluids Engg. 106, 390-398 (1984)] and (ii) 3D

Microbubble dynamics in distilled water were experimentally investigated during three stages: formation, expansion, and detachment,
emphasizing the combined effects of nozzle size and surface hydrophobicity. To achieve superhydrophobicity, nozzles with radii of 5, 15, 25,
and 50 lm were fabricated using laser precision machining technology and treated with trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane.
High-speed, high-spatial-resolution imaging was used to precisely measure the contact angle, instantaneous volume, radii of curvatures and
detachment time of the microbubbles. The experimentally measured data were used to calculate the forces acting on bubbles, including the
Young–Laplace pressure, surface tension, buoyancy, momentum, and added mass force. The results show that superhydrophobic nozzles generate larger, more elongated bubbles with extended detachment time, whereas non-hydrophobic nozzles form smaller, spherical bubbles that
detach faster. Furthermore, it was observed that increasing the nozzle size decreases the size of bubbles generated by superhydrophobic nozzles, while it increases the bubble size for non-hydrophobic nozzles. A force balance analysis confirmed that in the expansion stage, the pressure force on bubbles from non-hydrophobic nozzles drops, while the surface tension and momentum force remain constant, leading to
faster bubble growth and detachment. In contrast, for bubbles from superhydrophobic nozzles, the pressure force decreases gradually, while
the surface tension changes continuously, resulting in a prolonged attachment and the formation of larger bubbles.

Laser-based in-cylinder diagnostics are well established in engine research. The requirement of large-scale optical accesses, however, makes the application expensive and time consuming. It furthermore limits the engine operation range to low loads and speeds. We introduce laser excitation and imaging optics with a minimal outer diameter of 10 mm (imaging optic) respectively 9 mm (excitation optics). The imaging optics allows the observation of a 30×30 mm 2 field with a working distance of 35-42 mm and provides a light intensity, which is higher than standard large-scale UV optics at the same image magnification. The particular features of the endoscopes are demonstrated in experiments at a fired IC engine, for example OH chemiluminescence and UV-LIF measurements.

This paper is mainly about the areas of non-Newtonian fluid mechanics which are not investigated or not appropriately and sufficiently investigated. In fact, this should also include emerging areas of research in the field of non-Newtonian fluid mechanics due to new scientific and technological developments and advancements. The purpose of the paper is to highlight and draw the attention to these areas so that researchers (especially the young researchers and newcomers to research such as PhD students) invest their resources and efforts in these areas instead of investing in other areas which are previously investigated and hence they are of less priority from this aspect. Apart from the obvious benefit of "leveling up" in research, the attention to these rather neglected and nonexplored areas of research can be beneficial at the scientific and individual levels since it can lead to breakthroughs and new discoveries in these areas of research by inspecting and assessing their potentials and impacts at the theoretical and practical levels and probing their beneficial applications. We will also provide a brief discussion about the possibility of introducing novel tools and methods in these areas of research (and in non-Newtonian fluid mechanics research in general) as well as highlighting some of the existing limitations of the past and current research in the field of non-Newtonian fluid mechanics (noting that this discussion should help in achieving the ultimate objective of this investigation).

This experimental study focuses on exploiting laser Doppler velocimetry (LDV), a non-intrusive technique, for rheological characterization based on analyzing flows of Carbopol solutions at two distinct concentrations and two different temperatures within a pipe. The velocity profiles obtained using LDV and the pressure drops associated with each flow rate were exploited to establish the behavioral law of aqueous Carbopol solutions. Two approaches were used: the first was an analytical velocity model to fit the experimental profile, and the second used the first derivative of the experimental velocity profile and the pressure drops to reconstruct the flow curve. In addition, a third reference characterization was carried out using a rotary rheometer equipped with a vane geometry. This study's three rheological characterization methods showed excellent agreement concerning the Herschel-Bulkley model. Finally, all the laws resulting from these three methods were validated using an empirical law relating to Darcy's coefficient of friction.

Local field correction particle image velocimetry (LFCPIV), which was first presented in 1997, is the only correlation PIV method able to resolve flow structures smaller than the interrogation window. It presents advantages over conventional systems and thus offers an alternative in the field of super-resolution methods. Improvements of the initial version are likely to promote its application even further. The issues defining some of these possible improvements were already indicated in the paper that originally introduced LFCPIV, but not developed. This work presents refinements and also simplifications of the technique, so that it can be applied using current algorithms of advanced correlation PIV systems. Furthermore, these refinements reduce the measurement error and enlarge the range of application of LFCPIV. In particular, the application of the system is no longer constrained to images with mean distances between particles larger than 4 pixels. Besides that, the use of interrogation windows smaller than in its previous version is evaluated. This allows multigrid LFCPIV implementations. The results show how multigrid LFCPIV can obtain better measurements than can the usual multigrid PIV, but still the refined version of the LFCPIV technique performs even better, at the expense of a larger computing time. The performance of these methods is evaluated for synthetic and real images. This includes examples in which the ability to cope with gradients in velocity, gradients in seeding density and the presence of boundaries is highlighted.

Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance is evaluated carrying out the following two different tasks: -Comparison of robustness and accuracy for large and small scale flow structures. This is carried out using three techniques: the conventional Stereo PIV, the Stereo-LFC PIV and the Stereo-Multigrid PIV enhanced with image distortion. -Insight on the limit of resolvable scales for the Stereo-LFC. This task is relevant because the resolution attainable by this combination is higher than what has been obtained by the rest of the herein used algorithms. The first task has been achieved using synthetic images. Afterwards the coherence of the results has been checked with real images. The results show improvement of Stereo-LFC PIV in respect to Stereo-Multigrid PIV enhanced with image distortion. The performance of Stereo-LFC when only large scales are involved shows an increase of the dynamic range of measurable vorticity. When small scales are analysed, the magnitude of the error resulting when using Stereo-LFC is about half of the one obtained for the Stereo-Multigrid measurements. Results with errors below 20% have been achieved for some of the cases with peak vorticities as large as 1.8 Dt -1 (in the absence of out-ofplane displacements), out-of-plane loss of particle pairs of 65% (with a low peak vorticity of 0.06 Dt -1 ) and peak vorticities as large as 1.5 Dt -1 with 50% particle pair loss. For the second task most of the information has been obtained using real images. It has been found that the resolution limit is very dependent on the robustness of the algorithms against image defects and variability. The results show a remarkable improvement when using the Stereo-LFC PIV processing, although a full quantification and characterization would need further study because of the variety of noise sources possible in a real image.

In the past decade, advances in electronics technology have made larger imaging sensors available to the experimental fluid mechanics community. These advancements have enabled the measurement of 2-component 2-dimensional (2C-2D) velocity fields using particle image velocimetry (PIV) with much higher spatial resolution than previously possible using a single camera. Although previously reported experiments have incorporated multiple-camera arrays to acquire high spatial resolution PIV, using a single large camera can greatly reduce the complexity of the experimental setup as well as the error introduced by the calibration between the cameras. In this paper, the ability of a single large sensor for high spatial resolution PIV is demonstrated by performing the measurement of a zero-pressure-gradient turbulent boundary layer (ZPG-TBL). In post-processing the PIV images, the lens distortion error is of particular importance, as the lens distortion error increases with the size of the im...

The analysis of the internal flow structure and performance of a specific fluidic diverter actuator, previously studied by time-dependent numerical computations for subsonic flow, is extended to include operation with supersonic actuator exit velocities. The understanding will aid in the development of fluidic diverters with minimum pressure losses and advanced designs of flow control actuators. The selfinduced oscillatory behavior of the flow is successfully predicted and the calculated oscillation frequencies with respect to flow rate have excellent agreement with our experimental measurements. The oscillation frequency increases with Mach number, but its dependence on flow rate changes from subsonic to transonic to supersonic regimes. The delay time for the initiation of oscillations depends on the flow rate and the acoustic speed in the gaseous medium for subsonic flow, but is unaffected by the flow rate for supersonic conditions.

Experimental Study of the Power Profile Airfoil Equipped with Plasma Flow Control LIBIN DANIEL, JAMEY JACOB, Oklahoma State University -This presentation discusses results from an experimental study of the power profile airfoil at low Reynolds number. The power profile airfoil was developed by AMO Smith and consists of a blunt trailing edge shape with two wall jets near the trailing edge. The replacement of streamlining with properly designed blowing is used to prevent flow separation and additionally offers potential applications as a powered high-lift system, propulsive system, or low inertia control device. The 2D wind-tunnel model consists of the 22.5% thick power profile airfoil equipped with a movable trailing edge plug to direct flow along the trailing edge streamline. Compressed air was passed into the model via a plenum with flow conditioning devices to create pressure backdrop to allow uniform blowing at the trailing edge. The effects of varying jet momentum coefficient and trailing edge positioning on the aerodynamic characteristics are observed with both wake surveys and PIV. The impact of plasma synthetic jet actuators (PSJA) placed along the trailing edge of the power profile airfoil is also discussed. PSJA operation is compared to the baseline power profile airfoil both alone and working with the blowing to provide additional control authority.

Thrust Vectoring Flow Control Using Plasma Actuators JAMEY JACOB, MICHAEL BOLITHO, Oklahoma State University -Thrust vectoring flow control is demonstrated using plasma synthetic jet actuators (PSJA). The PSJA is a geometric variant of a plasma actuator, consisting of a symmetric electrode array that results in a circular region of dielectric barrier discharge plasma. Quiescent flow PIV measurements of the PSJA reveal that the flowfield on actuation resembles that of a zero-mass flux or synthetic jet that is useful for flow control, particularly separation reduction. Like synthetic jets, unsteady pulsed actuator operation results in formation of multiple vortex rings. The output jet momentum is found to be affected by the power input, actuator dimension and pulsing frequency. While increasing the input power increases the maximum jet velocity, an optimum range of pulsing frequencies and actuator dimensions are observed to exist in order to maximize jet momentum. By asymmetrically varying the plasma input parameters, such as frequency, amplitude and duty cycle, it is possible to control the jet angle. Vectoring using high frequency pusling akin to synthetic jets is more effective than vectoring by modifying steady control inputs and differences in control effectiveness are due primarily to the time scales associated with the vortex formation.

Smart structures sense external stimuli, process the sensed information, and respond with active control to the stimuli in real or near-real time. The response to external stimuli can be through deforming or deflecting the structure, or through communicating the sensed information to another control center. Smart materials enact a deformation or deflection of the structure by changing their physical properties when subject to either an electric, magnetic or thermal load. Multifunctional structures (MFS), in addition to supporting loads, incorporate sensors to detect and evaluate loads or failure, and to interact with the surrounding electromagnetic environment. MFS represent a new manufacturing and integration technology by which communications and electronics equipment are integrated into conformal load-bearing structures. The technology is enabled by advances in large-scale integrated electronics packaging, lightweight composite structures and high-conductivity materials. In multifunctional structures, electronic assemblies (multichip modules), miniature sensors and actuators are embedded into load-carrying structures, along with associated cabling for power and data transmission. This level of integration effectively eliminates traditional boards, boxes, large connectors, bulky cables, and thermal base plates, thereby yielding major weight, volume and cost savings, over traditional structural and electronic packaging techniques. The Aeronautics Enterprise works closely with it customers and partners to ensure that its technology, products and services are developed to levels where customers can confidently make decisions regarding the application of those technologies. This strategy allows the enterprise to support the national aeronautics goals: Maintain the superiority of U.S. aircraft and engines; Improve safety, efficiency and affordability of the global air transportation system; Ensure the long-term environmental compatibility of the aviation system; and Achieve aircraft-like operations and costs for launch systems. Research programs are in place to develop technology that provides benefits in the following areas: Performance a Efficiency and affordability Survivability Safety and security Capacity and efficiency Environment, and Cost per pound to place payload into low-earth orbit. System Benefit Framework for the National Goals National Goals Maintain the S ' ' of U.S. Aircraft an NASA R&T BASE LEAD ROLES To take advantage of the unique capabilities that exist at each of the research centers, the Aeronautics Research and ~e c h n o l o ~~ Base consists of six programs: Airframe Systems Aviation Operations Systems Flight Research Information Technology Propulsion Systems, and Rotorcraft. Each element is led where the center of gravity of relevant expertise exists. The Airframe Systems Program is led at Langley Research Center. Airframe systems are those systems that determine or characterize the performance of an aircraft, including airframe-propulsion system integration. Key elements of airframe systems include the following: Conceptual design Aerodynamic design, development, and testing Structural design, development, and testing (including airframe materials) Flight crew station design, integration, and testing, and Airborne systems (including flight mechanics, controls, electromagnetics, and flight-crucial systems) design, integration, and testing. The Agency Center of Excellence for Structures and Materials, one of the key elements of airframe systems, is located at Langley Research Center. ASP0 LEVEL-HI PROGUMS Airframe systems technology development for some specific vehicle classes are addressed in other Aeronautics Enterprise programs. The Advanced Subsonic Technology (AST) Program has focused technology efforts for general aviation, civil tiltrotor, and subsonic transport aircraft. In addition, Ames Research Center leads the Research and Technology Base portion of the Rotorcraft Research Program. The High-Speed Research (HSR) Program is developing technologies that will allow the decision on the feasibility of building an economically viable supersonic civil transport aircraft. Reusable launch vehicle (WV) technology is being developed in the X-33 Program. While technology developments in the focused programs are aimed at relatively nearterm applications, the Airframe Systems Program has the following characteristics: Breakthrough technologies High-rismigh-payoff * Revolutionary Expanding Frontiers * Pioneering research for the twenty-first century Requires paradigm shifts A sound investment in America's aeronautics future, and Complemented by focused programs. The Airframe Systems Program Office (ASPO) has planned a set of innovative Level-I1 research programs that are focused along advanced conceptual (systems) studies, transport aircraft, high-performance aircraft, and fundamental aeronautical concepts and methods. These programs are shown in the figure. In order to assure that the research planning and implementation is responsive to customers' needs, the management of the Airframe Systems Program centers around vehicle classes and long-term aeronautics research needs. A Systems Studies and Analysis element provides independent assessments, high-payoff research and concepts, and data for deciding investment strategies. The Level-I1 programs are managed within the Aeronautics Systgms Analysis Division, the Civil Transportation Office, the High-Performance Aircraft Office, and the Fundamental Concepts and Methods Office. The Rotorcraft, Airspace Operations, and Information Systems managers serve as the Langley points of contact for those programs. The figure shows how the Airframe Systems Level-I1 programs align with the national goals. This slide presents a brief description of the following Level-I1 programs: Megaliner Error-Proof Flight and Aircraft Electronic Systems * Advanced Life Extension (ALEX) Integral Airframe Structures (L4S) Alternate Civil Capacity, Environment, Efficiency, and Safety Solutions (ACCESS), and Advanced Subsonic Transport Aircraft Research (ASTAR). This presentation contains details about the following programs: Megaliner ALEX * IAS. These programs involve a significant amount of structures research. Airframe Systems Programs v Revolutionary subsonic long-haul transports Error-Proof Error-Proof flight deck and mission critical systems Systems (Error-Proof) Advanced Life Airworthiness of thick, metallic structural Extension (ALEX) components typical of wing structure, in the aging commercial transport fleet Integral Airframe Low cost large, integral, metallic structures Structures (IAS) Alternate Civil Capacity, Cooperative activities with industry or other Environment, Efficiency, government agencies 81 Safety Solutions (ACCESS)

In this paper we continue our previous investigation about energy minimization in the flow of fluids through tubes and networks of interconnected tubes of various geometries. We will show that the principle of energy minimization holds independent of the geometry of the tubes and networks of such interconnected tubes and independent of the type of fluid in such geometries where in this regard we consider generalized Newtonian fluids. We consider in this investigation the flow of Newtonian fluids through tubes and networks of interconnected tubes of elliptical, rectangular, equilateral triangular and concentric circular annular cross sectional geometries. We also consider a combination of geometric factor with a fluid type factor by showing that the principle of energy minimization holds in the flow of some non-Newtonian fluids (namely power law, Ellis and Ree-Eyring fluids) through tubes and networks of interconnected tubes of elliptical cross sections. The relevance of this study extends beyond tubes and networks of fluid flow to include for instance porous media and electrical networks.

In this paper we continue our previous investigation about the use of stress function in the flow of generalized Newtonian fluids through conduits of circular and noncircular (or/and multiply connected) cross sections where we visualize the stages of yield in the process of flow of viscoplastic fluids through tubes of elliptical, rectangular, triangular and annular cross sections. The purpose of this qualitative investigation is to provide an initial idea about the expected yield development in the process of flow of yield-stress fluids through tubes of some of the most common non-circular (and non-simply-connected) cross sectional geometries.

The consequences of poor hull surface conditions on fuel consumption and emissions are well-known. However, their rationales are yet to be thoroughly understood. The present study investigates the hydrodynamics of fouling control coatings and mimicked biofouling. Novel experimental roughness function data were developed from the “young” fully turbulent flow channel facility of the University of Strathclyde. Different surfaces, including a novel hard foul-release coating, were tested. Finally, the performance of a benchmark full-scale containership was predicted using Granville’s similarity law scaling calculations. Interestingly, the numerical predictions showed that the novel hard foul-release coating tested had better hydrodynamic performance than the smooth case. A maximum 3.79% decrease in the effective power requirements was observed. Eventually, the results confirmed the practicality of flow channel experiments in combination with numerical-based methods to investigate hull roughness effects on ship resistance and powering. The present study can also serve as a valuable guide for future experimental campaigns using the fully turbulent flow channel facility of the University of Strathclyde.

The necessity to improve the hydrodynamic performance of marine vessels has confirmed the urgency of conducting further investigations on biomimetic tubercles. In this work, towing tank tests were conducted at a range of speeds on conveniently prepared flat plates coated with 3D printed humpback whales mimicked tubercles.
The results showed that the artificial tubercles induced a drag reduction of up to 1.3% compared to the bare flat plate. Following the experimental campaign presented, some relevant considerations for ship owners and naval architects will be proposed.

In this paper we continue our previous investigation about the use of stress function in the flow of generalized Newtonian fluids through conduits of circular and noncircular (or/and multiply connected) cross sections where we inspect the flow of Ree-Eyring fluids in tubes of elliptical cross sections. We derive analytical expressions for the flow velocity profile and for the volumetric flow rate. The obtained analytical expressions were tested against the available analytical expressions for the special cases of Newtonian flow in circular tubes, Newtonian flow in elliptical tubes and Ree-Eyring flow in circular tubes and the results were identical. The obtained analytical expressions were also tested for sensible trends, tendencies and correlations and they passed all these tests.

The Gradient Scheme framework provides a unified analysis setting for many different families of numerical methods for diffusion equations. We show in this paper that the Gradient Scheme framework can be adapted to elasticity equations, and provides error estimates for linear elasticity and convergence results for non-linear elasticity. We also establish that several classical and modern numerical methods for elasticity are embedded in the Gradient Scheme framework, which allows us to obtain convergence results for these methods in cases where the solution does not satisfy the full H 2 -regularity or for non-linear models.

Recent technological advances in airplane construction techniques and materials employing bonded and milled aluminum skins and composite materials allow for the production of aerodynamic surfaces without significant waviness and roughness, ...

The methods employed by Martin [1971] and Chandna et al. [1982] to steady plane flows have been applied to the plane viscous flows in a porous medium using the Darcy - Brinkman - Lapwood equation. Various flows and corresponding geometries have been investigated.

Significant advancements have been made in the experimental investigation of drag reduction techniques for bluff bodies. This study explores various methods, including active and passive flow control strategies, surface modifications, and innovative aerodynamic shapes, aimed at minimizing aerodynamic drag. Extensive wind tunnel testing and computational fluid dynamics (CFD) simulations were conducted to analyze the effects of these techniques on drag reduction. The results demonstrated that specific surface modifications, such as vortex generators and riblets, effectively delayed flow separation and reduced drag coefficients. Additionally, active flow control methods, such as synthetic jets and plasma actuators, showed promise in dynamically altering flow characteristics to achieve drag reduction. The findings of this research provide valuable insights for the design and optimization of bluff bodies in automotive, aerospace, and civil engineering applications, ultimately contributing to improved energy efficiency and performance.

In the state-of-the-art PIV, image sensors are growing in size and PIV algorithms are increasing in spatial resolution capabilities. These technological advances allow for the simultaneous measurement of increasingly larger spatial scales ranges in a single PIV snapshot. To take fully advantage of the available spatial scales range, PIV setup constraints establish a coupling between the laser sheet thickness and the time between laser pulses. This condition did not apply with older technologies. The resulting value for the optimum laser sheet thickness is in a range where the superposed flow information in the direction of the imaging sensor introduces special characteristics errors at the small scale limit. This may compromise the measurement of flow quantities like dissipation and turbulent kinetic energy (tke), for which the contribution of the small and medium scales may be relevant. Within this scenario the assessment of the smallest scale resolved in the PIV measurement is nec...

Multigrid particle image velocimetry (PIV) is an open path in the search for high-resolution PIV methods. It is based on an iterative scheme that uses the information of initial processing to adapt the method parameters in order to improve the measurements. This is mainly performed by reducing the size of the interrogation windows and shifting them. In multigrid PIV, two sources of error can significantly affect the final measurement quality: (1) the error coming from the amplitude response of the initial large interrogation windows to spatial frequencies; (2) the error originating from the truncation of particles at the borders of the final small interrogation windows. By applying weighting functions and using symmetric direct correlation both errors can be reduced, respectively. These techniques have been separately tested in the past, but a joint implementation has not yet been analysed. This task is fulfilled and both sources of error are further clarified. For this purpose, a one-dimensional single wavelength displacement field is used. This gives us the opportunity to analyse the non-linear behaviour of PIV, together with the influence of basic parameters on it. In addition to this, the multigrid method, so far described, is enhanced by compensation of the particle pattern deformation. The metrological performance of this advanced method is tested using synthetic images and the results are compared with those delivered by established PIV methods. Coherence between these results and those obtained in a real image is also detailed.

Conventional particle image velocimetry (PIV) processing techniques based on correlation show a limited range of application when they are used to describe a concentrated vorticity field. This work focuses on the evaluation of these range limits. In this frame, the application of an advanced PIV technique, namely Local Field Correction Particle Image Velocimetry (LFCPIV) is also evaluated, showing a significant performance enhancement. The magnitude of the measurement error is obtained for conventional and advanced PIV by means of synthetic images of an analytical vortex flow. This gives information on the behaviour to be expected when operating beyond the classical limits of applicability of conventional PIV algorithms. The study of relevant effects, like group-locking, allows for further understanding the sources of the measurement error. The knowledge of these characteristics helps in taking decisions regarding the experimental setup as well as the kind of PIV method to use for a certain application. Real PIV images obtained by partners within Europiv 2 are also analysed and the results commented.

Particle image velocimetry (PIV) is now applied with confidence in industrial facilities such as large wind tunnels, yielding data not possible before. Despite the difficulties that arise in such environments, the conventional PIV methods can provide high quality data, especially when dealing with spatial and temporal slowly varying values of the flow magnitudes. Obtaining highly spatially resolved velocity fields is still challenging due to the inherent difficulties in these industrial facilities, such as large velocity gradients, background light and reflections. This work has focused on: (i) the behaviour of conventional PIV when the conventional limits of the processing algorithm are approached or surpassed and (ii) which kind of advanced methods can reduce the main sources of error that are characterized in this paper. The focus is on the description of vortex flows. Group locking, a major source of error, is introduced, modelled and metrologically characterized. The part of the study devoted to advanced methods deals with one that has already shown to be of profit in images from industrial facilities, local field correction PIV (LFC-PIV). This is a robust and an accurate method, able to obtain a high yield of measurements in these environments, without the need of external adjustment to each particular situation. To illustrate this point, some examples of processing of real images are given. The conclusions of this work suggest guidelines about the error figures when measuring flows with embedded vortex using conventional and advanced methods in industrial facilities.

Local field correction particle image velocimetry (LFC-PIV) has become an established alternative among high-resolution PIV techniques. Previous works by the authors introduced its implementation by means of simple algorithms. In these works the initial limitation of the method, which was related to the mean distance between particles, was removed. Comparison with other contemporary high-resolution techniques indicates that it offers advantages in robustness and accuracy. The trade-off for this better performance is a heavier computing load. Until now, the computing time that this load requires has not been characterized in detail, since this computing time could be substantially reduced by accepting a reduction in accuracy. This paper focuses on the characterization of the trade-off between time and accuracy, thus offering a new perspective to PIV. In this field, LFC-PIV offers a wide range of possibilities that are described in the paper. Several alternative schemes for LFC-PIV are tested, together with an analysis of the influence of the number of iterations. Performance figures for both accuracy and expended time are given. Metrological evaluation is carried out over synthetic images. A test of coherence between these results and the performance on real images is also presented. The paper shows that even for a limited number of iterations this technique offers advantages.

In the past, the authors have addressed the tasks of assessing read-out and peak-locking errors separately. In this paper an improved approach is tested for assessing both errors simultaneously. The rationale is that, generally these errors are coupled in PIV images and the procedures described in previous works are not suitable to determine both errors together, especially when CCD cameras read-out error is relevant. The sources for each error are different enabling for any combination of relative magnitude between errors. The magnitude of the CCD read-out error seems to be related to camera operating conditions (temperature, humidity and sensor illumination) besides its own design, while peak-locking is related to the size of the tracer particle images, the truncation of particle images by window borders and the subpixel peak fitting or image deformation algorithms. The application of PIV in challenging conditions can generate situations where both errors combine with similar magnitude. For the error assessment in these conditions, the work proposes a methodology of general application to PIV. Once it is described, the application to selected synthetic and real cases is offered. The results indicate the possibility to asses bias errors in the order of 0.1 pixels for both sources (peak-locking and CCD read out) simultaneously. In addition, the determination of zones where these errors are not the dominant ones is presented.

Particle Image Velocimetry with Local Field Correction (LFC-PIV) has been tested in the past to obtain two components of the velocity in a two dimensional domain (2D2C). When compared with conventional correlation based algorithms, this advanced technique has showed improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the Stereo technique, able to obtain three components of the velocity (2D3C PIV). To successfully apply stereoscopy, the laser sheet width is usually larger than in 2C velocimetry, in order to acquire the out of plane displacement of the particles. This peculiarity can make questionable the need for improved spatial resolution. In particular, if some factors concur the measurement of small scale features of the velocity along the laser sheet can be spoiled by the velocity variations across it, caused by the flow structures. Being the spatial resolution capabilities of LFC its main advantage, this paper focuses in establishing if there are advantages left for LFC-PIV in situations where only large scale features are of interest, thus making it difficult for this technique to show its attractiveness. The remaining benefits still to be analyzed are the robustness and the ability to cope with large displacement gradients. For both aspects, the performances of 2D3C-LFC-PIV are explored. These performances are characterized and compared with conventional and other advanced algorithms, when applied to synthetic PIV images. In addition, the coherence between these results and those coming from experimental PIV images is presented and discussed.

In this study, the heat transfer and friction characteristics of four different rib geometries-45 angled, Vshaped, W-shaped and M-shaped ribs in a two-pass stationary channel have been numerically investigated. The aspect ratio (Height to Width) of the cooling channel was 1:1 (square). The rib pitch-to-rib height ratio (p/e) and the rib-height-to-channel hydraulic diameter ratio (e/D h) were 16 and 0.125 respectively. The Reynolds number was varied from 20,000 to 70,000. For the computations, the Reynolds averaged NaviereStokes (RANS) equations were solved with the commercial software ANSYS Fluent using the realizable version of k-ε (RKE) model. The heat transfer results were benchmarked with experiments on a test rig with similar geometries and flow conditions. Detailed analysis of the flow characteristics in the two-pass channel was carried out so as to understand the interaction of the ribinduced secondary flows and the bend-induced secondary flows and their contribution to heat transfer enhancement. The heat transfer enhancement provided by V-shaped ribs was 7% higher than 45 ribs, 28% higher than W-shaped ribs and 35% higher than M-shaped ribs. However, the pressure penalty for Vshaped ribs was 19% higher than 45 ribs, 24% higher than W-shaped ribs and 28% higher than M-shaped ribs. On comparing the overall thermal hydraulic performance, V-shaped and 45 ribs were observed to perform significantly better than W-shaped and M-shaped ribs.

In this study, the heat transfer and friction characteristics of four different rib geometries-45 angled, Vshaped, W-shaped and M-shaped ribs in a two-pass stationary channel have been numerically investigated. The aspect ratio (Height to Width) of the cooling channel was 1:1 (square). The rib pitch-to-rib height ratio (p/e) and the rib-height-to-channel hydraulic diameter ratio (e/D h) were 16 and 0.125 respectively. The Reynolds number was varied from 20,000 to 70,000. For the computations, the Reynolds averaged NaviereStokes (RANS) equations were solved with the commercial software ANSYS Fluent using the realizable version of k-ε (RKE) model. The heat transfer results were benchmarked with experiments on a test rig with similar geometries and flow conditions. Detailed analysis of the flow characteristics in the two-pass channel was carried out so as to understand the interaction of the ribinduced secondary flows and the bend-induced secondary flows and their contribution to heat transfer enhancement. The heat transfer enhancement provided by V-shaped ribs was 7% higher than 45 ribs, 28% higher than W-shaped ribs and 35% higher than M-shaped ribs. However, the pressure penalty for Vshaped ribs was 19% higher than 45 ribs, 24% higher than W-shaped ribs and 28% higher than M-shaped ribs. On comparing the overall thermal hydraulic performance, V-shaped and 45 ribs were observed to perform significantly better than W-shaped and M-shaped ribs.

The inviscid, internal and external axial corner flows generated by two intersecting wedges traveling supersonically are obtained by use of a secondorder shock-capturing, finite-difference approach. The governing equations are solved iteratively iri conical coordinates to yield the complicated wave structure of the internal corner and the simple peripheral shock of the external corner. The numerical results for the internal flows compare favorably with existing experimental data.

This study focuses on the effect of a sweep angle between the upstream mean flow direction and the shock plane normal in a canonical oblique shock wave/boundary layer interaction. We study a configuration based on the experiments of : an incident shock impinges on a turbulent boundary layer developing at free stream Mach number M = 2.7 and a Reynolds number Re θ = 3200. The sweep angle is implemented by adding a spanwise velocity component (crossflow) at the inflow and enforcing periodicity conditions on the side boundaries. Several sweep angles are investigated and compared to a canonical unswept case. A viscosity is selected for each case in an effort to keep a constant Re θ across the whole range of sweep angles. The mean flow structure is analyzed, showing a strongly skewed flow around the separation zone that increases in size compared to a reference unswept case. Pressure loads on the wall surface are also higher for swept cases; and the characteristic low-frequencies of the flow separation also slightly increase.

Motivated by growing demands for aircraft noise reduction and for revolutionary new aerovehicle concepts, the late twentieth century witnessed the beginning of a shift from singlediscipline research, toward an increased emphasis on harnessing the potential of flow and noise control as implemented in a more fully integrated, multidisciplinary framework. At the same time, technologies for developing radically new aerovehicles, which promise quantum leap benefits in cost, safety and performance benefits with environmental friendliness, have appeared on the horizon. Transitioning new technologies to commercial applications will also require coupling further advances in traditional areas of aeronautics with intelligent exploitation of nontraditional and interdisciplinary technologies. Physics-based modeling and simulation are crucial enabling capabilities for synergistic linkage of flow and noise control. In these very fundamental ways, flow and noise control are being driven to be more closely linked during the early design phases of a vehicle concept for optimal and mutual noise and performance benefits.

Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction at supersonic speeds. Zero-pressure gradient boundary layers with an adiabatic wall, a Mach number of M∞ = 2.5, and a Reynolds number based on momentum thickness of Re θ = 1720 are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric Vgroove riblets with nominal spacings of 20 and 40 wall units. The DNS results confirm the few existing experimental observations and show that a drag reduction of approximately 7% is achieved for riblets with proper spacing. The influence of riblets on turbulence statistics is analyzed in detail with an emphasis on identifying the differences, if any, between the drag reduction mechanisms for incompressible and high-speed boundary layers.

The Lattice-Boltzmann-based solver PowerFLOW is used to perform direct numerical simulations of the transitional flow over an airfoil at Reynolds number equal to 0.657 million. The leading edge of the airfoil is covered with sand particles, represented by polyhedra, to mimic the grit used in experiments. The sensitivity of the laminar to turbulent transition to the size of these particles, grid resolution, spanwise length is evaluated and rectangular trips are also tested.

In this paper, the transition between steady and unsteady regimes of the two-dimensional gas flow past a moving square cylinder confined between two parallel plates with equal temperatures was investigated for different subsonic and transonic speeds and blockage ratios B = 3, 5, 10 and 20 (B is the ratio between microchannel height and square size). The influence of B on the drag coefficient of the square, pressure at stagnation point and transition between steady and unsteady regime were studied. A set of transition curves were obtained for different blockage ratios. The results showed a similar shape of the neutral curves, which shifts to lower Knudsen and Mach numbers when B decrease. A continuum approach based on the Navier-Stokes-Fourier equations was basically applied to all flow regimes. Navier-Stokes-Fourier equations was calculated numerically by using pressure based finite volume algorithm SIMPLE-TS. For the initial transition velocity slip regime, the results were compared to data obtained by using the particle Direct Simulation Monte Carlo (DSMC) method.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The performance of lifting bodies such as hydrofoils is determined by the flow state and particular attention should be paid to flow separation, as this greatly affects the generated lift and drag. Sailors are used to look at telltales (woolies) to trim their sails or steer their yacht. A French company developed an electronic telltale for sails based on a strain gauge activated by a silicon strand, with the appropriate signal processing to deliver the same information as a classical wool-made telltale, basically attached or separated flow. This new sensor proved useful when woolies are not visible or to deliver a signal to feed a control system, such as the autopilot for example. It was also applied to wind turbines to control the blade pitch. Mer Agitée is now developing an equivalent hydrodynamic e-Telltale to be used on hydrofoils and rudders to help trimming and controlling. The present work presents the investigation of a foil section fitted with this new sensor in a water tunnel, combining force and PIV measurements with the sensor signal, on a wide range of angle of attack. Results show that the hydro e-Telltale enables detecting the flow separation and anticipate stall, and possibly allows for detecting the boundary layer transition to turbulence. The feedback from this new sensor could be used for example in a control loop to make an "Anti-Stall System" on a rudder or a foil.

We propose to combine the active vortex generators with the particle image velocimetry (PIV) measurements and post-processing streamwise vortex characterization algorithms into a feedback based closed-loop control system for wind turbine applications. We develop two vortex identification and characterization methods that use PIV realizations for the purpose of a real-time (online or on-the-fly) feedback-based control. Both methods can extract centers and strengths of streamwise vortices generated behind active vortex generators in a turbulent boundary layer flow, and we show how to integrate those in a closed-loop control strategy. For demonstration purposes we use stereoscopic PIV measurements at the wind tunnel facility obtained in the transverse-wall-normal plane behind active vortex generators. A robust algorithm is using the Q-criteria and the integration of vorticity of each extracted vortex. Results show that a moving window average of a small number of instantaneous fields i...