Open-Ended Curiosity-Driven Fluids Lab Project

The lab component of a fluid mechanics course permits a great opportunity for students to engage with course material. These labs can take many forms including field trips, guided inquiry exercises, formulaic lab exercises, practical/hands-on skill development, CFD and design-build-test projects to name a few. Previous literature on self-determination theory suggests that many positive results can be gained by giving students a choice in their studies. Related literature on the importance of curiosity in students suggests similar benefits. This paper describes a multi-week lab experience where students were given the opportunity to study anything remotely related to fluid mechanics with very few restrictions on implementation. The project goals were proposed by a student, or a team of two students, and then refined with the assistance of the course instructor to ensure proper scope. Pre-project surveys were used to gage the importance students place on studying material which is of personal interest and to determine how other parts of the undergraduate curriculum match up with student interest. Post-project surveys were used to gather input on the student experience of completing the curiosity project. This paper details the results from the various assessments and discusses feedback from the course instructor, lab instructors and students relating to project implementation, opportunities for improvement and some of the advantages of such a lab experience.

Investigation of Cavitation Phenomena on Noise of Underwater Propeller

Underwater propeller cavitation noise is composed of tonal blade rate noise and high frequency broadband noise. Cavitation usually increases overall sound pressure level in the various frequency ranges which depends on the type of cavitation. This research had been carry out to predict the radiated noise from a marine propeller in presence of cavitation with various cavitation types. The analysis is performed by coupling an acoustic code based on the Ffowcs Williams-Hawkings (FWH) equation to unsteady Reynolds-averaged Navier-Stokes (URANS) which able to simulate multiphase flows in rotational domains. A brief summary of numerical method used to model the cavitation around the underwater propeller are presented and the thrust and torque coefficients are validated in different flow conditions by experimental results. The radiated noise along the shaft direction and perpendicular to the shaft direction is studied on both cavitating and non-cavitating propellers. Then, to predict the radiated noise due to cavitation in marine propeller, the computed sound pressure level (SPL) for non-cavitating marine propeller is compared with the SPL for the same propeller in cavitation conditions at various cavitation number and advanced coefficients. The noise analysis helps to determine the dominant noise source of the underwater propeller in different conditions, which will provide a basis for proper noise control strategies.

Educational Results Obtained Using an Improved Two-Dimensional Panel Method Code in Undergraduate Fluid Dynamics and Aerodynamics Courses

Undergraduate required fluid dynamics and elective aerodynamics courses include substantial material on analysis techniques for forces acting on bodies in external flows. These methods include momentum integral analysis, and, for aerodynamic applications, lift computed using circulation and the Kutta-Joukowski theorem. The author presented in a previous FED meeting code development and preliminary classroom results for the implementation of a fully interactive, two-dimensional potential flow solver for flow over both rigid and flexible thin-airfoil (or sail) geometries. The intent of the development was to design a code that could be used as a virtual wind tunnel. The solver was developed in Fortran 90/95 with user interface and graphics routines developed using the high-level plotting library DISLIN for use on Windows-based computers. The analysis code solves the potential flow equations for single or multiple airfoils using a vortex panel method in which the vortex strength varies linearly along the panel and is continuous from one panel to the next. A variety of controls are available to adjust airfoil shapes and angles-of-attack. The user may also specify either rigid thin airfoil shapes, or flexible airfoils in which the final equilibrium shapes are determined by the pressure distribution. Available graphics include velocity vectors, pressure coefficient contours, and streamlines. Lift, axial and normal force coefficients are also output in the form of bar graphs. Several improvements have been implemented in the code, based on early student feedback, to improve its suitability for educational purposes in fluid dynamics and aerodynamics classes. These include pressure plot distributions over the airfoils, the inclusion of standard NACA 4-digit airfoil definitions, the output of velocity and pressure data about a closed contour for use in circulation and momentum integral analysis calculations, and improvements regarding compatibility for use on computers of widely varying screen resolutions. In this work to be presented, recent improvements to the code, and subsequent educational/student learning results based on a series of Qualtrics online student survey questions are presented. These survey questions query the students understanding of a) momentum integral analysis, b) circulation, c) lift calculations using the Kutta-Joukowski theorem, d) airfoil-to-airfoil fluid flow interactions, e) the necessity for attention to details when performing engineering analysis. The code may be downloaded for use by educators and students at other universities.

Numerical Study on the Characteristics of Natural Supercavitation by Planar Symmetric Cavitators With Streamlined Headforms

A novel supercavitation-based device named Rotational Supercavitating Evaporator (RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in RSCE for performance optimization and then design three-dimensional blades, two-dimensional numerical simulations are conducted on the supercavitating flows (with cavitation number ranging from 0.055 to 0.315) around six planar symmetric cavitators with different streamlined headforms utilizing k – ε – v′2 – f turbulence model and Schnerr-Sauer cavitation model. We obtain the characteristics of natural supercavitation for each cavitator, including the shape and resistance characteristics and the mass transfer rate from liquid phase to vapor phase. The effects of the shape of the headform on these characteristics are analyzed. The results show that the supercavity sizes for most cavitators with streamlined headforms are smaller than that for wedge-shaped cavitator, except for the one with the profile of the forebody concaving to the inside of the cavitator. Cavitation initially occurs on the surface of the forebody for the cavitators with small curvature of the front end. Even though the pressure drag of the cavitator with streamlined headform is dramatically reduced compared with that of wedge-shaped cavitator, the pressure drag still accounts for most of the total drag. Both the drag and the mass transfer rate from liquid phase to vapor phase are in positive correlation with the supercavity size, indicating that the cavitators with the elliptic and hyperbolic cosine-type forebodies could be utilized for the optimal design of three-dimensional blade shape of RSCE.

Investigation of Internal Flow Velocity Distribution and Gas Loss of High-Speed Supercavitating Flows

Internal velocity distribution is an important content of flow structure and reveals the gas loss mechanism for supercavitating flows. Considering the three-phase momentum interactions and the water-vapor mass transport, the water-gas-vapor multi-fluid model is established to simulate ventilated supercavitating flows at high speed in the frame of the nonhomogeneous multiphase flows theory. Based on the model, the gas velocity field inside supercavity is studied. In the case of supercavitating flows around disk cavitator, two vortex cores are formed in the longitudinal plane under the actions of the adverse pressure gradient in the tail and the viscous friction on cavity surface, and are symmetrically distributed about the longitudinal axis. Most inner regions in the cavity cross section are occupied by circulation flows, where the velocity is in the opposite direction of incoming flows and decreases in the radial direction. When passing the vortex center, the velocity changes direction and increases in the radial direction. Part of gas departs to wake flows from the outermost regions close to the section boundary. The results confirm Spurk’s assumption for gas entrainment in detail. It is also found that the gas velocity distribution in the cross section through vortex cores does not depend on cavitation number. Supercavitating vehicle has the similar internal velocity distribution and gas loss mechanism. Due to the added viscous effect of the enveloped body, there are multiple axisymmetrical distributed vortices inside the cavity. The relative distance between the vortex core and the cavity wall increases downstream. Computations of ventilated supercavitating flows at different Reynolds numbers show that the gas leakage is decreasing with increasing Reynolds number for a given cavitation number. This study deepens the understanding of gas loss for ventilated supercavity at high speed, and lays a foundation for further refinement of the dynamic model of the maneuvering ventilated supercavity and the control of ventilated supercavitating flows.

Numerical Study on Ventilated Cavitation Influenced by Injection of Drag-Reducing Solution

The influence of injection of drag-reducing solution on ventilated partial cavitation and supercavitation for an axisymmetric underwater vehicle is analyzed by numerical simulation. Turbulence, cavitation and multiphase models are SST k-ω, Schnerr-Sauer and Mixture models, respectively. The Cross viscosity equation is adopted to represent the fluid property of aqueous solution of drag-reducing additives. First of all, for non-cavitating conditions, the pressure distribution is obtained to determine the positions of injecting drag-reducing solution and ventilation. Then natural cavitation at different cavitation numbers is investigated for acquiring inception cavitation number. Finally, numerical simulations are conducted on the ventilated cavitating flows with and without the injection of drag-reducing solution at the cavitation number slightly smaller than the inception cavitation number (partial cavitation) and much smaller than the inception cavitation number (supercavitation). It is shown that for partial cavitation, the shape of cavity with the injection of drag-reducing solution is larger and the resistance of underwater vehicle decreases in comparison with the case without the injection of drag-reducing solution. However, for supercavitation, just viscous drag force obviously decreases, while cavity shape does not change.

Stochastic Sensitivity Analysis of Numerical Simulations of High-Pressure Injectors to Cavitation Modeling Parameters

The work focuses on numerical simulation of the cavitating flow inside high-pressure injectors. Cavitation is modeled through a transport equation for the void fraction closed by the Schnerr-Sauer relation, containing four free parameters. As for turbulence, the URANS equations are considered, together with two different closure models. The sensitivity of URANS predictions to the parameters contained in the cavitation model is investigated for a throttle geometry, for which experimental and LES data are available. In order to obtain continuous response surfaces of the quantities of interest in the parameter space starting from a limited number of simulations, a stochastic approach is adopted. First, two out of the four parameters are identified as the most important through a preliminary analysis based on 2D simulations. Then, the sensitivity of 3D simulation results to the previously identified most important parameters is investigated. The stochastic range of variability of the results contains the reference data. Finally, a parameter optimization is carried out in order to obtain the values giving the best agreement with the LES data.

Modeling of Cavitation Induced Fuel Atomization and Breakup Processes

Recent experimental and modeling studies have indicated that turbulence and cavitation behaviors within a realistic fuel injector have significant effects on the liquid atomization and spray processes. In addition to the breakup process induced by aerodynamic force at the liquid/gas interface, the effects of flow characteristics including turbulence and cavitation inside the injector nozzle on atomization have been shown to be important. The cavitation within the injector is complicated by the turbulent flow under large pressure gradient and geometry of the injector orifice. We have previously developed the “T-blob” and “T-TAB” model, for liquid fuel primary and secondary breakup predictions respectively, to account for liquid turbulence effects within the injector. The objective of this study is to further account for the cavitation effect in the atomization process of a cylindrical liquid jet. In the primary breakup model, the level of the turbulence effect on the liquid breakup depends on the characteristic scales and the initial flow conditions. These scales are further modified to include the cavitation effect. The drop size formed is estimated based on the energy distribution among wave, turbulence and cavitation modes. This paper describes theoretical development of the current model. Both non-evaporating and evaporating spray cases will be investigated to validate the newly developed cavitation-induced atomization model.

Development and Commissioning of a Supersonic Blow Down Wind Tunnel for Educational Purposes

A cost-effective test rig is presented that allows for the experimental investigation of supersonic flows for educational purposes. The individual units for the test rig were designed and built by students as part of their degrees. The test rig allows for operating times up to 10 seconds and features a nozzle test section, that can house different test objects. The divergent part of the de Laval nozzle geometry is designed using the method of characteristics for planar two-dimensional supersonic flow. State of the art 3D printing technology has been utilized to manufacture the nozzle geometry. Both optical and pneumatic measurement techniques have been adopted for the current setup. A z-type schlieren setup with two parabolic mirrors is used to perform flow visualization. The entire run can be recorded with a digital high speed camera. Stagnation pressure and temperature are measured in the pressure reservoir. Measurements are used to demonstrate basic thermodynamic effects such as the depressurization of gas-filled pressure vessels. Schlieren photography is used to graphically derive the Mach number and some aspects of Mach waves, oblique shock waves, and expansion waves are discussed. Finally, some effects of surface roughness on the flow field are addressed in this paper. Initial tests with the untreated nozzle geometry led to a fine pattern of very weak oblique shock waves in the supersonic part of the nozzle, that were caused by the finite layer thickness of the printer.