An experimental investigation of cavity flow

Of particular interest are the flow structure and dynamics associated with open shallow rectangular cavities at low Mach numbers for various length-to-depth ratios. At the Reynolds number investigated, it is the presence of convective instabilities through the process of feedback disturbance that gives rise to a globally unstable flowfield. Using an instrumental wing model with a cut-out an experimental investigation of a cavity flowfield exhibiting ‘fluid-dynamic’ phenomenon has been completed. A post-processing module for the PIV image data was constructed which optimised the data fidelity and accuracy while improving upon velocity spatial resolution. These improvements were necessary to capture the flow scales of interest and minimise the measurement error for the presentation of velocity, velocity-derivative and turbulent statistics. It is shown that the hydrodynamics that is intrinsic to the cavity flowfield at these inflow conditions organises the oscillation of small- and large-scale vortical structures. The impingent scenario at the downstream edge is seen to be crucially important to the cavity oscillation and during the mass addition phase a jet-edge is seen to form over the rear bulkhead and floor. In some instances this jet-like flow is observed to traverse the total internal perimeter of the cavity and interact with the shear layer at the leading edge of the cavity, this disturbs the normal growth of the shear layer and instigates an increase in fluctuation. The coexistence and interplay between a lower frequency mode dominant within the cavity zone and the shear layer mode is seen to shed large-scale eddies from the cavity. This modulation imposes a modification to the feedback signal strength such that two distinct states of the shear layer are noted. Concepts for the passive reduction of internal cavity fluctuation are successful although modifications to the shear layer unsteadiness are encountered; an increase in drag is implied

Experimental investigation of oscillatory heat release mechanisms and stability margin analysis in lean -premixed combustion

Lean-premixed combustion has become an acceptable means of achieving ultra-low NOx emissions from land-based gas turbines. Further reduction may be possible through the use of hydrogen augmented or syngas fuels. However, advanced combustor designs developed to utilize these technologies often encounter thermoacoustic instabilities that may significantly hamper engine performance and shorten component life-cycles. These dynamics, although not fully understood, occur through a complex interaction between variations in heat release rate and acoustic properties of the system, and can be exacerbated by variable fuel properties in natural gas and syngas applications.;Theoretical models of thermoacoustic instabilities have attempted to describe the coupling process through reduced-order models that represent mechanisms suspected of contributing to variations in the heat release rate such as variations in fuel/air mixing, fluctuations of heat release through vortex shedding and periodic changes in the flame structure. These reduced-order models have demonstrated only a modest ability at predicting instabilities even in relatively simple systems. This may be due to the inherent complexity from interacting processes, the use of over-simplifying assumptions and the lack of experimental verification.;In this study a simple conical flame, used to reduce the number of contributing mechanisms, is utilized to experimentally evaluate the relationship between the heat release rate and variations in the flame surface area. Results indicated that while area perturbations can adequately describe the magnitude of heat release fluctuations, the area perturbations are not a direct indicator of the phase of heat release needed for closed-loop stability analysis.;Time-resolved particle image velocimetry was used to quantify the near-field acoustics and the dilatation rate field in the pre- and post-flame regions of the flow. Measurements indicated that multi-dimensional acoustics dominate the pre-combustion flow field with radial and axial acoustic velocities of similar magnitudes. Variations in the flame structure potentially due to alternating regions of positive and negative flame stretch were also observed and may result in variations in the flame speed. As it is common to assume constant flame speed and one-dimensional acoustics, the experimental identification of these altered mechanisms may help to resolve discrepancies compared to a number of published reduced-order models

Vortices Characterization In A Cavity Flow At Low Speed

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2003Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2003Kavite içi daimi ve daimi olmayan akışlar çeşitli mühendislik uygulamalarında karşımıza çıkmaktadır.Bu tez kavite akışı içerisindeki döngülerin davranışlarını istatistiksel olarak incelemek amacıyla yapılmıştır. Bu yüzden geniş ve güvenilir bir veri tabanı oluşturulmaya çalışılmıştır. Deneysel içerikli bu çalışma düşük hızlı bir hava tünelinde parçacık izleme yöntemi (PIV) kullanılarak gerçekleştirilmiştir. Deneyler kavite uzunluğunun derinliğine oranı dört olan dikdörtgen kavite geometrisi (açık ve sığ) için yapılmıştır. Kavitenin derinliği ve gelen akımın hızı esas alınarak hesaplanmış üç farklı Reynolds sayısı ile çalışılmıştır. Akışın giriş koşullarının laminer rejime uyduğu görülmüştür. Döngülerin PIV ölçümleri ile elde edilen anlık hız alanlarından çıkartılması wavelet analiz yöntemi ile sağlanmıştır. İncelenen kavite akışında akım yapışması gözlenmediğinden akışın “açık tip” kavite akışı olduğuna karar verildi. Ayrıca akışın birbirine ters yönde dönen iki döngü ile temsil edildiği tespit edildi. Reynolds sayısı arttırıldığında bu her iki döngünün de akışın gelmekte olduğu yöne doğru hareket ettikleri ve aynı zamanda kavitenin arka basamağında konumlanan döngünün boyutunun büyüdüğü görüldü. Wavelet analizi ile belirlenmiş döngülerin genel dağılımına bakıldığında bunların kavitenin ön basamağından çıkan bir jet akışına benzediği görüldü. Döngülerin iki ana üretim kaynağı olduğu tespit edildi. Saat yönünde dönen döngülerin ayrılmanın meydana geldiği kayma tabakası içerisinde oluştuğu ve yollarına kavitenin arka basamağında oluşan ana döngü aracılığıyla devam ettikleri belirlendi. Saatin ters yönünde dönen döngülerin ise kavitenin arka duvarında gelişen sınır tabaka etkisiyle oluştuğu ve genellikle kavitenin ön tarafında konumlandığı görüldü. Döngülerin boyutlarının ise yaklaşmakta olan sınır tabaka kalınlığı ile ilişkili olduğu ve Reynolds sayısı arttırıldığında azaldığı tespit edildiFlows over cavities exhibit various steady and unsteady phenomena, and one of concern in several engineering disciplines. The present thesis is aimed to understand the nature of the vortex behavior in a cavity flow through the analysis of statistical data and to create a wide and reliable database. This study which is of experimental nature was performed in a subsonic wind tunnel using particle image velocimetry (PIV). The tests were made on a rectangular cavity with length to the depth ratio of 4. Three different Reynolds numbers, were examined (Re=4000,9000,13000). The inlet conditions was investigated using hot-wire anemometry and was found to be laminar. The vortices have been identified by using a wavelet analysis technique applied to instantaneous velocity fields obtained by PIV. The results showed that the investigated flow was belonging to an open type and was mainly characterized by two large circulating bubbles turning in opposite directions. As the Reynolds number increased, the downstream recirculation bubble became larger and its center moved through the leading edge. An over view of the positions occupied by the vortices was found similar to a jet configuration attached to the leading edge. Two main areas of vortex production were identified. Clockwise vortices are created inside the shear layer, convected by the trailing recirculation bubble. Anti-clockwise vortices were mainly created by the trailing wall-layer. Their concentration at lower Reynolds number seems to be higher in the leading edge recirculation bubble. It is observed that the size of the vortices is linked to the size of the incoming boundary layer and this size decreases as the Reynolds number increases.Yüksek LisansM.Sc

Jet Mixing Enhancement by High Amplitude Pulse Fluidic Actuation

Turbulent mixing enhancement has received a great deal of attention in the fluid mechanics community in the last few decades. Generally speaking, mixing enhancement involves the increased dispersion of the fluid that makes up a flow. The current work focuses on mixing enhancement of an axisymmetric jet via high amplitude fluidic pulses applied at the nozzle exit with high aspect ratio actuator nozzles. The work consists of small scale clean jet experiments, small scale micro-turbine engine experiments, and full scale laboratory simulated core exhaust experiments using actuators designed to fit within the engine nacelle of a full scale aircraft. The small scale clean jet experiments show that mixing enhancement compared to the unforced case is likely due to a combination of mechanisms. The first mechanism is the growth of shear layer instabilities, similar to that which occurs with an acoustically excited jet except that, in this case, the forcing is highly nonlinear. The result of the instability is a frequency bucket with an optimal forcing frequency. The second mechanism is the generation of counter rotating vortex pairs similar to those generated by mechanical tabs. The penetration depth determines the extent to which this mechanism acts. The importance of this mechanism is therefore a function of the pulsing amplitude. The key mixing parameters were found to be the actuator to jet momentum ratio (amplitude) and the pulsing frequency, where the optimal frequency depends on the amplitude. The importance of phase, offset, duty cycle, and geometric configuration were also explored. The experiments on the jet engine and full scale simulated core nozzle demonstrated that pulse fluidic mixing enhancement was effective on realistic flows. The same parameters that were important for the cleaner small scale experiments were found to be important for the more realistic cases as well. This suggests that the same mixing mechanisms are at work. Additional work was done to optimize, in real time, mixing on the small jet engine using an evolution strategy.Ph.D.Committee Chair: David Parekh; Committee Member: Ari Glezer; Committee Member: Jeff Jagoda; Committee Member: Richard Gaeta; Committee Member: Samuel Shelto

The Vortex Dynamics of Laminar Separation Bubbles

Laminar separation bubbles (LSBs) are common features in low Reynolds number flows, and can have considerable performance impacts in applications such as hydrofoils, small-to-medium scale wind turbines, micro and unmanned aerial vehicles, glider planes, and aircrafts operating at low speed or high altitude. In particular, the very presence of an LSB can cause loss in lift, increase in drag, and/or unwanted noise emissions, while also leaving the flow in an unstable configuration, as only slight changes in the environment or operating conditions can lead to further detrimental effects, such as the sudden onset of stall. All of these performance impacts are rooted in the laminar--turbulent transition process of an LSB, where disturbance growth in the unstable shear layer leads to its roll-up and the formation of coherent shear layer vortices that govern the reattachment process and are the source of unsteady loads and noise emissions. Therefore, a comprehensive understanding of LSB transition and vortex dynamics is a prerequisite to the development of effective control strategies. The work completed as part of this thesis is at the forefront of this effort, as flow development in laminar separation bubbles is studied and a new forcing technique is developed and tested. The supporting data is experimental and is collected primarily by means of particle image velocimetry. First, flow development in a nominally two-dimensional LSB formed over an airfoil is studied. Forcing at the LSB fundamental frequency and the first subharmonic of this frequency are found to inhibit and promote the prevalence of vortex merging in the LSB, respectively. When left to develop naturally, the flow development is characterized by the periodic roll-up of the separated shear layer upstream of the mean maximum height location. The vortices are strongly two-dimensional at formation, but quickly develop spanwise deformations with downstream convection, leading to their breakdown to smaller scales near the mean reattachment point. The deformations take the form of spanwise undulations in the vortex filaments, which tend to develop at wavelengths ranging between one and seven times the streamwise wavelength of the structures. These undulations continually intensify, ultimately leading to the breakdown of the vortices, while re-orienting vorticity from the spanwise direction into the streamwise and wall-normal directions, creating hairpin-like structures. An instability mechanism is hypothesized to be responsible for the development of these spanwise undulations, and a new forcing technique is developed to target it. The technique is capable of producing deterministic, three-dimensional disturbances modulated to a desired spanwise wavelength, while holding all other parameters (amplitude, frequency, and streamwise wavelength) constant. This is achieved using two alternating current, dielectric barrier discharge (AC-DBD) plasma actuators arranged in streamwise succession, which are operated simultaneously. The upstream actuator produces a spanwise uniform disturbance, which is then spanwise modulated by the output of the downstream actuator, with a relative phase delay introduced in order to spatially superimpose the two outputs. The technique is verified to produce the desired disturbance characteristics through a detailed experimental characterization that considers both quiescent and in-flow conditions. The effects of this forcing technique and the subsequent growth in spanwise modes is studied in an LSB formed over a flat plate subject to an adverse pressure gradient. Disturbance growth is tracked throughout the LSB, identifying small amplitude disturbances of a frequency matching the primary Kelvin-Helmholtz instability that undergo convective amplification downstream of the mean separation point. In comparing results from forcing the flow with two and three-dimensional disturbances, with the latter modulated to a spanwise-to-streamwise wavelength ratio of 2:1, disturbance amplitudes in this region are higher for the three-dimensional case, indicating a preferential amplification of spanwise modes in the upstream boundary layer. While this growth may result from the underlying stability of the upstream boundary layer, it could also stem from a low frequency modulation of the base flow, as significant spanwise non-uniformities of the same wavelength are found in the unforced natural flow. Nevertheless, stability predictions in the LSB find that, regardless of the forcing scenario, the normal (two-dimensional) modes are subject to the highest amplification rates throughout the length of the LSB, while disturbance modes of an oblique wave angle of less than 30 deg. experience comparable, yet reduced, growth rates. Thus, disturbance growth in the LSB is confirmed to be spanwise wavelength dependent. The effectiveness of the spanwise modulated forcing, in terms of effecting change in disturbance and flow development, is justified, as its wavelength ratio (2:1) corresponds to a wave angle of 26.5 deg., while the other three-dimensional forcing configurations considered are less effective on account of their larger wave angles (33.7 and 45 deg.). The effect of unstable spanwise modal growth on the development of the LSB shear layer vortices and the ensuing vortex dynamics is assessed. The small amplitude perturbations tracked through the fore portion of the LSB manifest in the shear layer vortices, imparting a spanwise wavelength, if present, in the vortex filaments. Thus, in the case of two-dimensional forcing, the shear layer vortices remain largely two-dimensional until their breakdown, while for the three-dimensional forcing case, significant spanwise undulations develop at the 2:1 ratio prescribed by the forcing. The filaments surge forward in the streamwise direction downstream of the three-dimensional actuator's active regions, while lagging behind at spanwise locations downstream of the actuator gaps. A continual intensification of vortex stretching ensues, leading to rapid filament deformations. Through supporting observations from a simplified vortex filament model, the undulatory shape of the vortex filament is shown to self-induce a net rotational motion, causing the streamwise forward and rearward sections of the filament to tilt away and toward the wall, respectively. This, coupled with the wall-normal velocity gradient, causes the continual stretching of the filament. These vortex motions are observed consistently for all LSBs studied throughout this thesis, and apply more broadly to all LSBs, since regardless of how a spanwise undulations is initially produced, if present, a vortex filament will tend to develop in the way shown in any near-wall shear flow. Thus, these dynamics are found to be intrinsic to the breakup process of shear layer vortices in laminar separation bubbles

Shock-cell noise investigation on a subsonic/supersonic coaxial jet

The work is aimed at the experimental investigation of shock-cell noise on a coaxial jet with subsonic primary stream and supersonic secondary stream. This kind of noise is nowadays an important component of the total noise emitted by aeronautic engines, particularly affecting cabin noise in cruise conditions. In this thesis, the design and commissioning of a new supersonic coaxial jet rig, at the von Karman Institute for Fluid Dynamics (BE), are discussed, with a specific focus on the design choices that have been made to obtain good flow quality, low background noise and the possibility to perform a variety of flow and acoustic measurements. The maximum achievable Mach number at the outlet of the primary (central) and secondary (annular) nozzles is equal to 2.2, with a baseline operating point being M_p=0.89, M_s=1.21. To commission the facility, several test campaigns on a supersonic single stream jet were conducted using PIV in synchronous with microphones mounted on a polar antenna. Multiple screech harmonics and subharmonics tones have been documented, showing a directivity pattern similarly to the supersonic broadband noise (BBSAN). Turbulence integral length scales have been computed using correlation functions. The average of Reynolds shear stress fields shows the presence of lobes in the jet near field which are the trace of a standing wave caused by the screech. Following the commissioning, the coaxial jet has been investigated. Multiple combinations of pressure conditions for the primary and secondary flows have been tested. Acoustic measurements have been performed in synchronous with the PIV, which has been applied for the first time in the literature on a supersonic coaxial flow. The presence of both screech and broadband noise was recorded in the majority of the tests, and a directivity pattern was recognized for the latter. For a certain pressure conditions, the screech tone naturally disappeared. Experimental evidences suggest this may be related to a complex shock interaction occurring at the end of the primary nozzle. A simple method to infer the screeching dynamics from the spatial correlation functions was proposed. The correlation suggests the presence of a pulsation (or breathing) motion of the internal jet, which is cause/effect of the screech. A second screech mode was also retrieved from acoustic data, for which, the correlation functions suggest the presence of a sinusoidal motion of the internal jet

Jet noise : aeroacoustic distribution of a subsonic co-axial jet

The noise generated by aircraft can be easily heard by those living under the flight path of passenger or cargo carriers. It is considered an environmental pollutant and is treated as such by the International Civil Aviation Organization (ICAO) who monitor and review noise levels. The ICAO imposes substantial fines on those carriers who do not adhere to the decibel limitations. With the new limit or `stage' enforced in 2006, aircraft manufacturers (including jet engine manufacturers) are seeking ways to reduce the noise created by an aircraft. A 1/150th scale model, based on the exit geometry typically found on commercial jet engines, was designed and manufactured at Warwick. The laboratory jet flow conditions operated at 0.7 Mach. The work presented in this thesis looks at the noise generated in a subsonic, co- owing jet, with particular focus given to the distribution sound sources from 5 kHz to 80 kHz (0.375 St to 6.0 St). An acoustic mirror mounted on a motorized 3-way traverse measured radiated sound in the co-flowing jet to produce 2D sound source maps. This is done using combinations of smooth cowl and chevrons for the core and bypass nozzles. For frequencies less than 30 kHz, a reduction of noise was observed using the bypass chevron nozzle compared with the bypass smooth cowl nozzle. Laser Doppler Anemometry (LDA) was used to reveal the 2D flow dynamics of the jet, supporting the acoustic distribution results with velocity profiles of the flow. The change in the flow dynamics with different nozzle combinations is discussed and different regions of the flow were identified

Fluorescent particle tracers for surface hydrology

Surface water processes control downstream runoff phenomena, waste and pollutant diffusion, erosion mechanics, and sediment transport. However, current observational methodologies do not allow for the identification and kinematic characterization of the physical processes contributing to catchment dynamics. Traditional methodologies are not capable to cope with extreme in-situ conditions, including practical logistic challenges as well as inherent flow complexity. In addition, available observational techniques are non-exhaustive for describing multiscale hydrological processes. This research addresses the need for novel observations of the hydrological community by developing pioneer flow characterization approaches that rely on the mutual integration of traditional tracing techniques and state-of-the-art image-based sensing procedures. These novel methodologies enable the in-situ direct observation of surface water processes through remote and unsupervised procedures, thus paving the way to the development of distributed networks of sensing platforms for catchment-scale environmental sensing. More specifically, the proposed flow characterization methodology is a low-cost measurement system that can be applied to a variety of real-world settings spanning from few centimeters rills in natural catchments to riverine ecosystems. The technique is based on the use of in-house synthesized environmentally-friendly fluorescent particle tracers through digital cameras for direct flow measurement and travel time estimations. Automated image analysis-based procedures are developed for real-time flow characterization based on image manipulation, template-based correlation, particle image velocimetry, and dimensionality reduction methodologies. The feasibility of the approach is assessed through laboratory-designed experiments, where the accuracy of the methodology is investigated with respect to well-established flow visualization techniques. Further, the transition of the proposed flow characterization approach to natural settings is studied through paradigmatic observations of natural stream flows in small scale channel and riverine settings and overland flows in hillslope environments. The integration of the proposed flow sensing system in a stand-alone, remote, and mobile platform is explored through the design, development, and testing of a miniature aerial vehicle for environmental monitoring through video acquisition and processing

Experimental studies on shock wave interactions with flexible surfaces and development of flow diagnostic tools

Nowadays, light-weight composite materials have increasingly used for high-speed flight vehicles to improve their performance and efficiency. At supersonic speed, sonic fatigue, panel flutter, severe instabilities, and even catastrophic structural failure would occur due to the shock wave impingement on several flexible components of a given structural system either internally or externally. Therefore, investigation on shock wave interaction with flexible surfaces is crucial for the safety and performance of high-speed flight vehicles. This work aims to investigate the mechanism of shock wave interaction with flexible surfaces with and without the presence of the boundary layer. The first part involves the shock wave generated by supersonic starting jets interaction with flexible surfaces and the other one focuses on shock wave and boundary layer interaction (SBLI) over flexible surfaces. A novel miniature and cost-effective shock tube driven by detonation transmission tubing was designed and manufactured to simulate the supersonic starting jet and investigate the interaction of a supersonic starting jet with flexible surfaces. To investigate the characterization of this novel type shock tube, the pressure-time measurement in the driven section and the time-resolved shadowgraph were performed. The result shows that the flow structure from the open end of the shock tube driven by detonation transmission tubing agrees with that of conventional compressed-gas driven shock tubes. Moreover, this novel type of shock tube has good repeatability of less than 3% with a Mach number range of 1.29-1.58 when the weight of the NONEL explosive mixture varies from 3.6mg to 12.6mg. An unsteady background oriented schlieren (BOS) measurement system and a sprayable Polymer-Ceramic unsteady pressure sensitive paint (PC-PSP) system were developed. The preliminary BOS result in a supersonic wind tunnel shows that the sensitivity of the BOS system is good enough to visualize weak density variations caused by expansion waves, boundary layer, and weak oblique shocks. Additionally, compared with the commercial PC-PSP from Innovative Scientific Solutions Incorporated (ISSI), the in-house developed unsteady PSP system has higher pressure sensitivity, lower temperature sensitivity, and photo-degradation rate. To identify the shock movement, distortion and unsteadiness during the processes of the supersonic starting jet impingement and shock wave boundary layer interaction (SBLI) over flexible surfaces, an image processing scheme involving background subtraction in the frequency domain, filtering, resampling, edge detection, adaptive threshold, contour detection, feature extraction, and fitting was proposed and applied to process shadowgraph and schlieren sequences automatically. A large shadowgraph data set characterized by low signal to noise ratio (SNR) and small spatial resolution (312×260-pixel), was used to validate the proposed scheme. The result proves that the aforementioned image processing scheme can detect, track, localize, and fit shock waves in a subpixel accuracy. The mechanism of the interaction between the initial shock wave from a supersonic starting jet and flexible surfaces was investigated based on a square shock tube driven by detonation transmitting tube. Compared with that of the solid plate case, flexible surfaces can delay the shock reflection process because of the flexible panel deformation generated by the pressure difference between the top and the bottom. The delay time is around 8µs in the case of 0.1mm thick flexible surface, whereas it declines to around 4µs in the case of 0.3mm thick flexible surface because of the lower flexibility and deformation magnitude. However, interestingly, the propagation velocity of the reflected shock wave is basically the same for the solid plate and flexible panels, which means the flexible surface doesn’t reduce the strength of the reflection wave, although it delays its propagation. Also, there is not an apparent difference in the velocity of the reflected shock wave in the case of different incident shock Mach numbers when Ms varying from 1.22 to 1.54. These experimental results from this study are useful for validating numerical codes that are used for understanding fluid-structure interaction processes