etd@IISc Collection:
http://hdl.handle.net/2005/34
2017-03-22T08:24:13ZDynamics Of Cricket Song Towards Nature-inspired MEMS Speakers
http://hdl.handle.net/2005/2571
Title: Dynamics Of Cricket Song Towards Nature-inspired MEMS Speakers
Authors: Godthi, Vamsy
Abstract: The clever designs of natural transducers are a great source of inspiration for man-made systems. At small length scales, there are many transducers in nature that we are now beginning to understand and learn from. Here, we present an example of such a transducer that is used by field crickets to produce their characteristic song. This transducer uses two distinct components—a file of discrete teeth and a plectrum that engages intermittently to produce a series of impulses forming the loading, and an approximately triangular membrane, called the harp, that acts as a resonator and vibrates in response to the impulse-train loading. The file-and-plectrum act as a frequency multiplier taking the low wing beat frequency as the input and converting it into an impulse-train of sufficiently high frequency close to the resonant frequency of the harp. The forced vibration response results in beats producing the characteristic sound of the cricket song. Based on various experimental observations reported in the literature, we model the sound production mechanism as consisting of three stages—actuator, frequency multiplier, and amplifier. We then examine how different features of the forewing govern the sound production. With careful experiments on the harp, we estimate the actual modulus of the harp cuticle and also measure the morphological features of the forewings of different field cricket species. Using this data, we construct a finite element model of the harp and carry out modal analysis to determine its natural frequency. We fine tune the model with appropriate elastic boundary conditions to match the natural frequency of the harp of a particular species—Gryllus bimaculatus. We model impulsive loading based on a loading scheme reported in the literature and predict the transient response of the harp. We show that the harp indeed produces beats and its frequency content matches closely that of the recorded song. Subsequently, we use our FEM model to show that the natural design is quite robust to structural perturbations in the file. The characteristic song frequency produced is unaffected by small variations in the spacing of file-teeth and even by larger gaps. We then attempt to predict a scaling law that crickets must use for spectrum allocation. We use our FEM model, with measurements and computations, to arrive at a predictive model that relates call frequencies of field crickets to the harp dimensions. We verify the validity of this model by using the measured dimensions of harps of nine field cricket species. We then use our model to provide possible explanations as to why the song frequency of various field crickets in our study is bounded between 3.1 kHz and 6.8 kHz. We also show that we are faced with similar challenges as crickets when designing miniature MEMS (Micro-Electro-Mechanical Systems) speakers. We present a design of MEMS speakers that is inspired by how the crickets actuate. We have been able to realize our first prototypes using simple fabrication processes. By electrostatically actuating the MEMS devices, we obtain a sound pressure of 70 dB SPL at a distance of 10 cm. We believe that with a few design and fabrication iterations, we will be able to achieve a much higher sound pressure output from the MEMS speakers.2016-09-14T18:30:00ZHigh Reynolds Number Flow Over A Backward-Facing Step
http://hdl.handle.net/2005/2542
Title: High Reynolds Number Flow Over A Backward-Facing Step
Authors: Nadge, Pankaj M
Abstract: Flow separation and reattachment happens in many fluid mechanical situations occurring in engineering applications as well as in nature. The flow over a backward-facing step represents a geometrically simple flow situation exhibiting both flow separation and reattachment. Broadly speaking there are only two important parameters in the problem, the Reynolds number(Re) based on the step height(h),and a geometrical parameter, referred to as the Expansion ratio(ER), defined as the downstream channel height to the upstream channel height. In spite of the relative simplicity of this geometry, the flow downstream is quite complex. The main focus of the present work is to elucidate the unsteady three-dimensional coherent structures present in this flow at large Re, Re>36,000,based on the step height(h). For this, we use velocity field measurements from Particle Image Velocimetry (PIV)in conjunction with hotwire anemometry measurements.
The time-averaged structure of this flow is first studied in detail, including the effect of Reynolds number(Re) and Expansion Ratio(ER), on it. These studies show that at sufficiently large Re (Re>20,000), the reattachment length becomes independent of Re. The detailed internal structure of the separation bubble is also found to be independent of Re, but for Revalues that are relatively larger(Re>36,000). At large Re, the main effect of ER ,is found to be on the reattachment length, which increases with ER and saturates for ER values greater than about 1.8. The detailed internal structure of the separation bubble has been mapped at high Re and is found to be nearly the same for all ER, when the streamwise length is normalized by the reattachment length.
In order to elucidate the unsteady coherent vortical structures, PIV measurements are done in two orthogonal planes downstream of the backward-facing step. These measurements are done for ER= 1.50 at large Re(Re=36,000) and in a large aspect ratio facility(AR= span length/step height= 24); the latter being important to avoid any effects due to span-wise confinement. In the spanwise plane parallel to the lower wall(x-z plane),instantaneous velocity fields show counter rotating vortex pairs, which is a signature of the three-dimensional vortical structures in this plane. Using conditional averaging, this counter-rotating vortex pair signature is captured right from upstream of the step, to well after reattachment. Spatial correlations are used to get the length scale of these coherent vortical structures, which varies substantially from the attached boundary layer before separation to the region after reattachment. The variation of these structures in the cross-stream (vertical) direction at reattachment and beyond gives an idea about their three dimensional shape. The circulation of these counter-rotating pairs is measured from the conditionally aver-aged fields, and is found to increase with streamwise distance reaching normalized circulation values (Γ/Uoh) of about 0.5 around reattachment.
Velocity spectra downstream of the step show peaks corresponding to both the shear layer frequency(Stsl)and a relatively lower frequency that corresponds to large-scale shedding from the separation bubble (Stb); the latter in particular being quasi-periodic. Small amplitude sinusoidal forcing at the shedding frequency(Stb) is applied close to the step, by blowing and suction, to make the quasi-periodic shedding more regular. Measurements show that this has a very small effect on both the mean separation bubble and on the counter-rotating structures in the x-z plane. This mild forcing however enables phase locked PIV measurements to be made which shows the bubble shedding phenomenon in the cross-stream plane(side view or x-y plane).
The phase-averaged velocity fields show significant variations from phase to phase. Although there is some hint of structures being shed, from these phase-averaged fields, it is not very clear. One of the primary reasons is the fact that the flow is effectively spanwise averaged, as the three-dimensional structures are not locked in the spanwise direction. To get a three dimensional view of the sheddin gphenomenon, it is necessary to lock the spanwise location with respect to the three-dimensional vortical structures before averaging across the different phases. We use the condition, u’<- urms, to locate the central plane between the counter-rotating structures, which in effect are the “legs” of the three-dimensional structure. With this condition, we effectively get a slice of the shedding cycle cutting through the “head” of the three-dimensional structure. Apart from this cut, we also get a cut between adjacent structures from the weak sweep events, with the condition u’<- urms. Using these conditions, on the phase-locked velocity fields, we effectively lock the structures in time, as well as in the spanwise direction. With this ,a clearer picture of the shedding process emerges. The flow is highly three-dimensional near reattachment and the shedding of the separation bubble is modulated in the spanwise direction owing to the three-dimensional hairpin like vortical structures in the flow. The separation bubble is seen bulged out and lifted high at locations where the head of the hairpin vortex passes, owing to the strong ejection of fluid caused by the vortical structure. On the other hand, outside the hairpin vortices, weak sweep events push the flow towards the wall and make it shallow and less prominent, with the shedding being very weak in this plane. From these observations, a three-dimensional picture of the flow is proposed.2016-06-20T18:30:00ZStudies In The Dynamics Of Two And Three Wheeled Vehicles
http://hdl.handle.net/2005/2561
Title: Studies In The Dynamics Of Two And Three Wheeled Vehicles
Authors: Karanam, Venkata Mangaraju
Abstract: Two and three-wheeled vehicles are being used in increasing numbers in many emerging countries. The dynamics of such vehicles are very different from those of cars and other means of transportation. This thesis deals with a study of the dynamics of a motorcycle and an extensively used three-wheeled vehicle, called an “auto-rickshaw” in India. The commercially available multi-body dynamics (MBD) software, ADAMS, is used to model both the vehicles and simulations are performed to obtain insight into their dynamics.
In the first part of the thesis, a study of the two wheeler dynamics is presented. A fairly detailed model of a light motorcycle with all the main sub-systems, such as the frame, front fork, shock absorbers , power train, brakes, front and rear wheel including tire slips and the rider is created in ADAMS-Motorcycle. The simulation results dealing with steering torques and angles for steady turns on a circular path are presented. From the simulation results and analytical models, it is shown that for path radius much greater than motorcycle wheel base, the steering torque and angle can be described by only two functions for each of the two variables. The first function is related to the lateral acceleration and can be determined numerically and the second function, in terms of the inverse of the path radius, is derived as an analytical approximation. Various tire and geometric parameters are varied in the ADAMS simulations and it is clearly shown that steady circular motion of a motorcycle can be reasonably approximated by only two curves–one for steering torque and one for steering angle.
In the second part of the thesis, a stability analysis of the three-wheeled “autorickshaw” is presented. The steering instability is one of the major problems of the “auto-rickshaw” and this is studied using a MBD model created in ADAMS-CAR .In an Initial model the frame ,steering column and rear-forks (trailing arms) are assumed to be rigid. A linear eigenvalue analysis, at different speeds, reveals a predominantly steering oscillation, called a “wobble” mode, with a frequency in the range of 5 to 6Hz. The analysis results show that the damping of this mode is small but positive up to the maximum speed(14m/s) of the three-wheeled vehicle. Experiments performed on the three-wheeled vehicle show that the mode is unstable at speeds below 8.33m/s and thus the experimental results do not agree with the model. Next, this wobble instability is studied with an analytical model, similar to the model proposed for wheel shimmy problem in aircrafts. The results of this model show that the wobble is stable at low speeds regardless of the magnitude of torsional stiffness of steering column. This is also not matching with the experimental result. A more refined MBD model with flexibility incorporated in the frame, steering column and the trailing arm is constructed. Simulation results with the refined model show three modes of steering oscillations. Two of these are found to be well damped and the third is found to be lightly damped with negative damping at low speeds, and the results of the model with the flexibility is shown to be matching reasonably well with the experimental results. Detailed simulations with flexibility of each body incorporated, one at a time, show that the flexibility in the steering column is the main contributor of the steering instability and the instability is similar to the wheel shimmy problem in aircrafts. Finally, studies of modal interaction on steering instabilities and parametric studies with payload and trail are presented.2016-09-08T18:30:00ZExperiments On Rolling Sphere Submerged In An Incompressible Fluid
http://hdl.handle.net/2005/2567
Title: Experiments On Rolling Sphere Submerged In An Incompressible Fluid
Authors: Verekar, Pravin Kishor
Abstract: Experiments are done using a smooth solid rigid homogeneous acrylic sphere rolling on an inclined plane which is submerged in water. The motivation for these experiments comes from a need to understand a class of solid-fluid interaction problems that include sediment transport, movement of gravel on ocean floor and river bed due to water currents. Experiments are performed in a glass water tank 15 cm wide by 14 cm deep by 61 cm long which can be tilted to desired angle. The sphere is released from rest on the inclined false bottom of the tank in quiescent water. Our experimental study has twofold aim: (1)to study the boundary layer separation, the three-dimensional eddying motion in the wake and the near-wake structure and(2) to establish hydrodynamic force coefficients by analyzing kinematical data of the sphere motion from start to till it attains terminal velocity. Experiments are carried out at moderate Reynolds number Rearound1500. Previous studies on the first problem exist in the literature for Reup to 350. Previous studies on the second problem do not clearly define the added-mass coefficient and the influence of the water tank side-walls on the drag coefficient.
In the first study, the characterization of the wake is done using flow visualization methods (fluoresce in dye visualization and particle streak visualization) and Particle Image Velocimetry (PIV). Laser light sheet obtained from an argon ion continuous laser beam is taken in different orientations to illuminate the fluoresce in dye or 14 m silver-coated hollow glass spheres. These experiments show that the wake behind the rolling sphere up to 1.6 diameters (or 1.6D) downstream is confined within height 1.2Dand width1.2D. At about 1.8Ddownstream, the wake sways alternately on either side of the equatorial plane, moving in lateral-vertical direction and moving out of the confining region; this gives zigzag appearance to the wake.
Also in these experiments, we observe that the flow separations from the surface of the rolling sphere show three separation zones. The eddies shed from the primary separation surface on the upper hemisphere are symmetrical about the equatorial plane with Strouhal number St=1.0. The primary separation is affected by the symmetrical secondary separations on the rear surface in the piggyback region — it is the region near the upper rear surface of the sphere behind the transverse equatorial plane and below the primary separation surface. The lower eddies below the primary separation zone are shed alternately on either side of the equatorial plane with shedding frequency St=0.5. Our experiments show that there is a viscous blockage of width 0.4Dat the crevice near the point of contact. On either side of the viscous blockage at the crevice, we see weak symmetric eddies. Based on our experimental observations, we proceed to build a simple physical model of the separated flow on the surface of the rolling sphere.
In the second study, the motion of the sphere is photographed and paired data of the displacement and time is obtained for the sphere motion from the start of motion till terminal velocity is reached at about 4.5 sphere diameters from the point of release of the sphere. Equation of motion of the sphere is solved numerically treating added-mass coefficient Ca and drag coefficient Cd as parameters. Experimental data is fitted on these solutions and the best fit gives the values of the force coefficients. Theoretical value of Ca equal to 0.621 is confirmed experimentally. Value of Cd is found to be 1.23 at Re=990 and it is 1.06 at Re= 1900. Side-wall effects become important for ratio of diameter of sphere to width of tank greaterthan0.20.2016-09-14T18:30:00Z