In this thesis, the benefits of boundary layer control (BLC) in improving aerodynamic performance and engine stability were examined in a compact, serpentine inlet exhibiting flow separation. A 1/14-scale turbofan engine simulator provided the flow through the inlet. The inlet's mass flow was measured to be 759 scfm (0.939 lbm/s) with an average throat Mach number of 0.23 when the simulator speed was 40 krpm. Boundary layer suction, blowing, and their combination were used to minimize the inlet's flow separation. The effectiveness of the suction alone and the blowing alone was shown to be approximately equivalent, and the effectiveness of the combined use of both was seen to be better than either one by itself. With blowing and suction flowrates around 1% of the simulator's core flow, the inlet's distortion was lowered by 40.5% (from 1.55% to 0.922%) while the pressure recovery was raised by 9.7% (from 87.2% to 95.6%). With its reduction in distortion, BLC was shown to allow the simulator to steadily operate in a range that would have otherwise been unstable. Minimizing the flow separation within the inlet was shown to directly relate to measurements from flush-mounted microphones along the inlet wall: as the exit distortion decreased the microphone spectrum also decreased in magnitude. The strong relationship between the aerodynamic profiles and the microphone signal suggests that microphones may be used in an active flow control scheme, where the BLC effort can be tailored for different engine operating conditions. Unfortunately, the sensing scheme used in this experiment showed the microphone signal to continue to decrease even when the separation is overly compensated; therefore refinements must be made before it would be practical in a real application. / Master of Science

The research focusses on computational analysis of vortex generation behind A-Pillar of simplified model derived from Jaguar XF that excludes air from the underside of vehicle. This vortex formation contributes in generating wall pressure fluctuations especially at speeds higher than 100km/hr. It is a collaborative work between Cranfield University and Jaguar Land Rover. Three dimensional pressure based incompressible flow using Large Eddy Simulation turbulence model is selected for computational analysis in FLUENT v14. This used high parallel computing systems available in Cranfield University. In the initial phase, three grid resolutions (coarse, medium and fine) were prepared in ICEM CFD with fine case consisting of 10 million cells. Qualitative analysis includes extraction of slices, 3-D and surface streamlines and pressure and velocity contours for capturing the unsteadiness due to the vortex formation over the front side glass surface. The iso-surface of Q captures the unsteadiness at the A-Pillar wake and side mirror wake over front side glass surface. It also reveals that the range of length scales captured were limited even at the finest grid resolution. Quantitative analysis compares the mean pressure (Cp) data with JLR results. Probes were located at 51 locations over the front side glass window that showed a good comparison; specifically for the fine grid; with maximum variation incurred at probes located in separation areas. For predicting the wall pressure fluctuations, a total of ten probes were located over the front side glass window surface. The surface pressure (static) data was recorded for 1 sec of flow-time and later imported in MATLAB for post-processing. The results obtained in 1/3rd octave band showed that the large scales were too energetic and small scales are not captured. However, comparing sound pressure levels with the Aero-acoustic Wind Tunnel (AWT); provided by JLR; it is concluded that either the grid is too coarse to resolve higher frequencies or the numerical modelling used is too dissipative to benefits the use of LES.

No description available.

A technique to infer circumferential total pressure distortion intensity found in serpentine inlet airflow was established using wall-pressure fluctuation measurements. This sensing technique was experimentally developed for aircraft with serpentine inlets in a symmetric, level flight condition. The turbulence carried by the secondary flow field that creates the non-uniform total pressure distribution at the compressor fan-face was discovered to be an excellent indicator of the distortion intensity. A basic understanding of the secondary flow field allowed for strategic sensor placement to provide a distortion estimate with a limited number of sensors. The microphone-based distortion estimator was validated through its strong correlation with experimentally determined circumferential total pressure distortion parameter intensities (DPCP). This non-intrusive DPCP estimation technique was then used as a DPCP observer in a distortion feedback control system. Lockheed Martin developed the flow control technique used in this control system, which consisted of jet-type vortex generators that injected secondary flow to counter the natural secondary flow inherent to the serpentine inlet. A proportional-integral-derivative (PID) based control system was designed that achieved a requested 66% reduction in DPCP (from a DPCP of 0.023 down to 0.007) in less than 1 second. This control system was also tested for its ability to maintain a DPCP level of 0.007 during a quick ramp-down and ramp-up engine throttling sequence, which served as a measure of system robustness. The control system allowed only a maximum peak DPCP of 0.009 during the engine ramp-up. The successful demonstrations of this automated distortion control system showed great potential for applying this distortion sensing scheme along with Lockheed Martin's flow control technique to military aircraft with serpentine inlets. A final objective of this research was to broaden the non-intrusive sensing capabilities in the serpentine inlet. It was desired to develop a sensing technique that could identify control efforts that optimized the overall inlet aerodynamic performance with regards to both circumferential distortion intensity DPCP and average pressure recovery PR. This research was conducted with a new serpentine inlet developed by Lockheed Martin having a lower length-to-diameter ratio and two flow control inputs. A cost function based on PR and DPCP was developed to predict the optimal flow control efforts at several Mach numbers. Two wall-mounted microphone signals were developed as non-intrusive inlet performance sensors in response to the two flow control inputs. These two microphone signals then replaced the PR and DPCP metrics in the original cost function, and the new non-intrusive-based cost function yielded extremely similar optimal control efforts. / Ph. D.

Measurements were made on three forward steps with step height to boundary layer ratio of approximately 3.8%, 15% and 60% and Reynolds number based on step height ranging from 6640 to 213,000. The measurements included mean wall pressure, single and 2 point wall pressure fluctuations, single and 2 point velocity fluctuations and, oil flow visualization. Pressure fluctuation measurements were made 5 boundary layer thicknesses upstream of step to 22 boundary layer thickness (or 600 step heights for smallest step size) downstream of the step. The results show that the steps remarkably enhance the wall pressure fluctuations that scale on the step height in the vicinity of the step and far downstream of the step. The decay of wall pressure fluctuations post reattachment is a slow process and elevated levels can be seen as far as 150 step heights downstream for the mid step size. The enhanced pressure fluctuations come from the unsteady reattachment region on top face of the step which was found to be a strong function of flow geometry and flow parameters such as Reynolds number. The 2 point pressure and velocity space-time correlations show a quasi-periodic structure which begins to develop close to the reattachment and grows in intensity and scale further downstream of reattachment and is responsible for the elevated pressure fluctuations downstream of the step. However, the velocity correlations lack in scale reflecting the fact that large scales reflected in pressure are masked by smaller scales that exist within them. / Master of Science

An experimental investigation was performed to ascertain the nature of turbulence in a narrow apex angle isosceles triangular duct. The study involved the design and construction of a low noise, low turbulence wind tunnel that had an isosceles triangular test section with an apex angle of 11.5[degrees]. Experiments involved the measurement of velocity fluctuations using hot wire anemometry and wall pressure fluctuations using a condenser microphone. Measurement of the velocity fluctuations reconfirms the coexistence of laminar and turbulent regions at a given cross section for a range of Reynolds numbers. The laminar region is concentrated closer to the apex while the turbulent region is found closer to the base. The point of transition is a function of the Reynolds number and moves closer to the apex as the flow rate is increased. Moreover, it was found in this investigation that traditional scaling of the turbulent statistical quantities do not hold good in this geometry. Although velocity fluctuations showed distinctive flow regimes, no such distinction could be seen in the dynamic wall pressure data. The nature of the dynamic wall pressure was uniform throughout the entire cross section suggesting that wall pressure fluctuations, unlike the velocity fluctuations, are able to travel from the base to the apex, without being damped. This implies that the relationship between the velocity and the pressure fluctuations applicable in the other systems does not hold well in a narrow apex angle isosceles triangular duct. Further, the typical scaling relationships applied to wall pressure spectra of other geometries doesn't apply in this scenario and the ratio of the RMS pressure fluctuation to the mean shear is much higher compared to a flat plate or pipe flow situation.

RESUME : Les machines axiales à rotors contrarotatifs subsoniques sont une bonne solution pour les industries où de fortes élévations de pressions et d'efficacités sont nécessaires sans augmenter le diamètre ou la vitesse de rotation des rotors. Néanmoins, le comportement des CRS et les paramètres impactant ses performances ne sont pas encore totalement compris. Cette thèse mène une investigation expérimentale sur la performance et les paramètres influents sur un étage contrarotatif. La technique de design et les méthodes de mesure sont repris sur une thèse précédente réalisée au laboratoire Dynfluid (Arts et métiers ParisTech). Trois étages contrarotatifs ont été fabriqués (JW1, JW2 et JW3) et testés sur le banc d'essai normalisé AERO2FANS. Ces machines ont été conçues pour avoir le même point de fonctionnement mais avec une répartition de charge différente. Les résultats expérimentaux se concentrent dans un premier temps sur JW1. Les grandeurs physiques regardées sont l'efficacité globale et l'élévation de pression statique pour juger de la performance globale de la machine. La fluctuation de pression pariétale et le champ de vitesse sont aussi mesurés. L'impact du changement de rapport de vitesse ou la distance entre les deux rotors sur la machine JW1 a été étudiée grâce aux grandeurs physiques décrits précédemment. Enfin dans une dernière partie, les trois machines sont comparées toujours grâce aux grandeurs physiques définies précédemment. / ABSTRACT : The counter rotating subsonic axial flow fans could be a good solution for applications where the highly improved static pressure and efficiency are required without the increase of rotational speed and fan diameter. However, the mechanisms of high performance CRS and parameters influences are not well understood nowadays. This thesis is an experimental investigation of the performance and parameter studies of two counter rotatingaxial flow ducted fans. The design and measurement methods are based on the previous research work in Laboratory Dynfluid (Arts et Métiers ParisTech). Three Counter Rotating Stages (CRS) (named JW1, JW2 and JW3) are developed and tested on a normalized test bench (AERO2FANS). These systems have same design point and differ by the distribution of loading as well as the ratio of angular velocity between the Front Rotor (FR) and Rear Rotor (RR). The first part of results focus on the JW1. The overall performance is obtained by the experimental results of the static pressure rise and static efficiency, as well as the wall pressure fluctuations recorded by a microphone on the casing wall. The parameter study is conducted to investigate the effects of the axial distance and the ratio of angular velocity between the FR and RR on the global performance and flow fields measured by Laser Doppler Velocimetry (LDV).The last part of the work is devoted to analyze the differences of the three CRS with different distribution of work, in terms of the global performance and flow features.

L'écoulement autour des véhicules produit une couche limite turbulente très proche de la paroi. Le caractère turbulent induit des fluctuations de pression pariétale qui font vibrer les panneaux du véhicule. Ces vibrations sont alors transmises à travers la structure et rayonnent du bruit dans l’habitacle. Les niveaux sonores dus à l'écoulement augmentent avec la vitesse du véhicule. Pour cette raison, cette problématique connaît un intérêt croissant dans le secteur aéronautique.Le but de cette thèse est double : comprendre les mécanismes à l’origine de ces fluctuations de pression à la paroi et prédire l’excitation de la structure avion due à l’écoulement turbulent.Pour ce faire, des calculs Large Eddy Simulation (LES) sont disponibles. Il s’agit de bases de données numériques d’écoulements de couches limites turbulentes en présence de gradients de pression favorable, adverse et nul. Ceci permet de caractériser l’écoulement sur des géométries courbes telles que la pointe avant d’un avion. L’effet du gradient de pression sur des structures cohérentes de type « hairpins » et paquets de « hairpins » a pu être identifié et quantifié à travers des méthodes de visualisation et d’analyse statistique. Une méthode d’estimation stochastique du champ de vitesse a révélé la présence de tourbillons contra-rotatifs au-dessus des paquets de hairpins. Ces tourbillons ont une vorticité opposée à celle des hairpins et un modèle de « hairpins inversé » a été proposé.En s’inspirant du travail de Ahn et al. (2010), un modèle stochastique de spectre de la pression pariétale a été développé. Il s’agit de reconstruire un champ stochastique de vitesse instantanée et d’en déduire le champ de pression à la paroi en résolvant une équation de Poisson sur la pression. Le champ de vitesse est obtenu en soumettant des structures de type paquets de hairpins à un écoulement moyen. Les caractéristiques des paquets de hairpins en fonction du gradient de pression sont basées sur l’analyse des bases LES. Les résultats de ce modèle sont comparés à ceux issues de bases de données numériques et expérimentales. Enfin, ce modèle est utilisé pour caractériser l’écoulement de couche limite turbulente dans une simulation de Statistical Energy Analysis (SEA) afin de prédire les niveaux de vibration des panneaux d’une portion de fuselage d’avion. / The flow around vehicles creates a turbulent boundary layer in the vicinity of the wall. The turbulent behavior induces pressure fluctuations that make the panels vibrate. These vibrations are then transmitted though the structure of the vehicle and radiate noise inside the cabin. The flow-induced noise levels increase with the speed of the vehicle. For this reason, aircraft manufacturers show a great interest in this topic.There are two objectives for this thesis: understand the mechanisms responsible for the wall-pressure fluctuations and predict this source of aircraft panel excitation.A study of available Large Eddy Simulation (LES) computations was performed. The database consists in simulations of turbulent boundary layer flows submitted to favorable, adverse and zero pressure gradients. This is necessary to understand the nature of the flow over curved geometries such as the aircraft flight deck. The effect of pressure gradients on coherent hairpin structures and hairpin packets could be identified and quantified based on visualization and statistical analysis methods. Linear stochastic estimation of the velocity fields revealed a pair of counter-rotating streamwise vortices above hairpin packets. These vortices have a vorticity opposite to that of the hairpins and an “inverse hairpin” model was proposed.Following the work of Ahn et al. (2010), a stochastic model for wall-pressure spectrum was developed. The idea is to build a stochastic turbulent velocity field using hairpin packets which are subjected to a mean flow. The characteristics of the packets depending on the pressure gradient are based on the analyses of the LES database. The pressure field at the wall is obtained by solving a Poisson equation. The results of the hairpin packet model are compared to numerical and experimental data. Finally, the model is used as input for a Statistical Energy Analysis (SEA) simulation in order to predict the levels of vibrations of panels submitted to a turbulent boundary layer flow over a portion of an aircraft cabin.

Les vibrations induites par l'écoulement dans le coeur du REP peuvent provoquer une usure par frottement des crayons combustibles par friction au niveau des contacts entre la cellule de grille et les crayons des assemblages combustibles. Cela peut entraîner des dommages irréversibles de la gaine du crayon combustible et compromettre la première barrière de sûreté du réacteur. Assurer l'intégrité de la gaine est une préoccupation majeure dans la sûreté du réacteur. Cependant, les spectres d'excitation des forces fluides agissant sur les crayons ne sont pas bien connus. Le but de cette thèse est d'utiliser des éléments géométriques simples pour reproduire des cellules de grilles d'un REP. SGE ont été effectuées sur une conduite annulaire avec différents maillages en utilisant le code TrioCFD. Une étude de sensibilité de maillage a été réalisée afin de proposer un maillage reproduisant correctement les résultats dans la littérature. Ces informations de résolution de maillage ont été utilisées lors de la réalisation des simulations en utilisant divers obstacles géométriques intérieurs à la conduite, i.e., des ailettes de mélange, une grille circulaire et une combinaison de grille carrée et d'ailettes de mélange. Un maillage hybride a été utilisé dans le cas des ailettes de mélange et dans le cas de cellule de grille carrée. Les caractéristiques hydrauliques ainsi que la pression pariétale ont été analysées dans chaque cas. Il apparaît que la grille carrée est une combinaison approximative du cas des ailettes de mélange et du cas de la grille circulaire. Les simulations ont été comparés avec des mesures réalisées au CEA / Flow-induced vibrations in the pressurized water reactor core can cause fretting wear in the fuel rods. Due to friction, wear occurs at the contact locations between the spacer grid and the fuel rod. This may compromise the first safety barrier of the nuclear reactor by damaging the fuel rod cladding. In order to ensure the integrity of the cladding, it is necessary to know the random fluctuating forces acting on the rods. However, the spectra for these fluid forces are not well known. The goal of this thesis is to use simple geometrical elements to check the reproducibility of realistic PWR spacer grids. As a first step, LES were performed on annular pipe for different mesh refinements using the CFD code TrioCFD. A mesh sensitivity study was performed to propose a good mesh for reproducing standard literature results. This information on mesh resolution was used when carrying out simulations using various geometric obstacles inside the pipe-mixing vanes, circular spacer grid and a combination of square spacer grid with mixing vanes. Structured mesh was generated for the annular pipe case and circular grid case. An innovative hybrid mesh was used for the two remaining cases of the mixing vanes and the square grid; keeping unstructured mesh around the obstacles and structured mesh in the rest of the domain. Both hydraulic and wall pressure characteristics were analyzed for each case. The results for the square grid case were found to be an approximate combination of the mixing vane case and circular grid case. Simulation results were compared with experiments performed at CEA Cadarache. Some preliminary comparisons were also made with classical empirical models.