Large perturbation flow field analysis and simulation for supersonic inlets

An analysis technique for simulation of supersonic mixed compression inlets with large flow field perturbations is presented. The approach is based upon a quasi-one-dimensional inviscid unsteady formulation which includes engineering models of unstart/restart, bleed, bypass, and geometry effects. Numerical solution of the governing time dependent equations of motion is accomplished through a shock capturing finite difference algorithm, of which five separate approaches are evaluated. Comparison with experimental supersonic wind tunnel data is presented to verify the present approach for a wide range of transient inlet flow conditions

An optimal path to transition in a duct

This paper is concerned with the transition of the laminar flow in a duct of square cross-section. Like in the similar case of the pipe flow, the motion is linearly stable for all Reynolds numbers, rendering this flow a suitable candidate for a study of the 'bypass' path to turbulence. It has already been shown \citep{Biau_JFM_2008} that the classical linear optimal perturbation problem, yielding optimal disturbances in the form of longitudinal vortices, fails to provide an 'optimal' path to turbulence, i.e. optimal perturbations do not elicit a significant nonlinear response from the flow. Previous simulations have also indicated that a pair of travelling waves generates immediately, by nonlinear quadratic interactions, an unstable mean flow distortion, responsible for rapid breakdown. By the use of functions quantifying the sensitivity of the motion to deviations in the base flow, the 'optimal' travelling wave associated to its specific defect is found by a variational approach. This optimal solution is then integrated in time and shown to display a qualitative similarity to the so-called 'minimal defect', for the same parameters. Finally, numerical simulations of a 'edge state' are conducted, to identify an unstable solution which mediates laminar-turbulent transition and relate it to results of the optimisation procedure

Run-around membrane energy exchanger performance and operational control strategies

A run-around membrane energy exchanger (RAMEE) is a novel energy exchanger that is capable of transferring both heat and moisture, which can significantly reduce the energy required to condition outdoor ventilation air. The RAMEE uses a liquid desiccant to transfer both heat and moisture between two remote air streams, making it appropriate for many applications, including building HVAC retro-fits. Both initial system start-up and changing outdoor conditions require time for the desiccant to undergo changes in both temperature and concentration, and can cause significant transient delays in system performance. Under some conditions, these transients may be beneficial by increasing the system performance. However under some conditions, the transient delays can cause a substantial decrease in performance. This thesis focuses on the development of control strategies that can be used to reduce unwanted transient delays. In order to develop these control strategies, the performance of a RAMEE is first investigated using both experimental and numerical methods. The transient numerical and experimental effectiveness results show satisfactory agreement, with a maximum root mean squared error of 10%. Both the numerical and experimental data show that a long transient time of several hours, or even several days, can occur upon initial system start-up. The numerical model is used to investigate several control strategies to reduce unwanted transient delays. The control strategies investigated are: solution and air flow control, air flow bypass, solution temperature control, and solution concentration control. The solution and air flow control are shown to reduced the start-up transient time by up to 11%, but require either a reduction in air flow or an increase in solution pumping costs. Air flow bypass proves to be a better option which provides a 16% reduction in transient time, and only requires that a bypass damper be provided for each exchanger. Solution temperature control is capable of essentially eliminating the thermal transient time (time required for the solution to reach operating temperature), but the thermal transient time is found to be a minor contributor to the overall transient time (time required for the solution to reach operating temperature and concentration) when the initial concentration of the solution is different than the steady-state concentration. When thermal and moisture transients exist, total transient times may be over 18 days. A practical temperature and concentration control strategy is developed, which can reduce transient delays by over 90% and increase performance during variable outdoor weather conditions

Flow Simulation of Supersonic Inlet with Bypass Annular Duct

A relaxed isentropic compression supersonic inlet is a new concept that produces smaller cowl drag than a conventional inlet, but incurs lower total pressure recovery and increased flow distortion in the (radially) outer flowpath. A supersonic inlet comprising a bypass annulus to the relaxed isentropic compression inlet dumps out airflow of low quality through the bypass duct. A reliable computational fluid dynamics solution can provide considerable useful information to ascertain quantitatively relative merits of the concept, and further provide a basis for optimizing the design. For a fast and reliable performance evaluation of the inlet performance, an equivalent axisymmetric model whose area changes accounts for geometric and physical (blockage) effects resulting from the original complex three-dimensional configuration is proposed. In addition, full three-dimensional calculations are conducted for studying flow phenomena and verifying the validity of the equivalent model. The inlet-engine coupling is carried out by embedding numerical propulsion system simulation engine data into the flow solver for interactive boundary conditions at the engine fan face and exhaust plane. It was found that the blockage resulting from complex three-dimensional geometries in the bypass duct causes significant degradation of inlet performance by pushing the terminal normal shock upstream

Performance of an ideal turbine in an inviscid shear flow

Although wind and tidal turbines operate in turbulent shear flow, most theoretical results concerning turbine performance, such as the well-known Betz limit, assume the upstream velocity profile is uniform. To improve on these existing results we extend the classical actuator disc model in this paper to investigate the performance of an ideal turbine in steady, inviscid shear flow. The model is developed on the assumption that there is negligible lateral interaction in the flow passing through the disc and that the actuator applies a uniform resistance across its area. With these assumptions, solution of the model leads to two key results. First, for laterally unbounded shear flow, it is shown that the normalised power extracted is the same as that for an ideal turbine in uniform flow, if the average of the cube of the upstream velocity of the fluid passing through the turbine is used in the normalisation. Second, for a laterally bounded shear flow, it is shown that the same normalisation can be applied, but allowance must also be made for the fact that non-uniform flow bypassing the turbine alters the background pressure gradient and, in turn, the turbines ‘effective blockage’ (so that it may be greater or less than the geometric blockage, defined as the ratio of turbine disc area to cross-sectional area of the flow). Predictions based on the extended model agree well with numerical simulations approximating the incompressible Euler equations. The model may be used to improve interpretation of model-scale results for wind and tidal turbines in tunnels/flumes, to investigate the variation in force across a turbine and to update existing theoretical models of arrays of tidal turbines

Modelling tidal energy extraction in a depth-averaged coastal domain

An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions. © 2010 © The Institution of Engineering and Technology

Modified Through-Flow Wave-Rotor Cycle with Combustor-Bypass Ducts

A wave-rotor cycle is described that avoids the inherent problem of combustor exhaust gas recirculation (EGR) found in four-port, through-flow (uniflow) pressure-gain wave-rotor cycles currently under consideration for topping gas-turbine engines. The recirculated hot gas is eliminated by the judicious placement of a bypass duct that transfers gas from one end of the rotor to the other. The resulting cycle, when analyzed numerically, yields a mean absolute temperature for the rotor that is 18% below the already impressive value (approximately the turbine inlet temperature) predicted for the conventional four-port cycle. The absolute temperature of the gas leading to the combustor is also reduced from the conventional design by 17%. The overall design-point pressure ratio of this new bypass cycle is approximately the same as the conventional cycle. This paper will describe the EGR problem and the bypass-cycle solution, including relevant wave diagrams. Performance estimates of design and off-design operation of a specific wave rotor will be presented. The results were obtained using a one-dimensional numerical simulation and design code

Investigation of advanced counterrotation blade configuration concepts for high speed turboprop systems. Task 4: Advanced fan section aerodynamic analysis

The purpose of this study is the development of a three-dimensional Euler/Navier-Stokes flow analysis for fan section/engine geometries containing multiple blade rows and multiple spanwise flow splitters. An existing procedure developed by Dr. J. J. Adamczyk and associates and the NASA Lewis Research Center was modified to accept multiple spanwise splitter geometries and simulate engine core conditions. The procedure was also modified to allow coarse parallelization of the solution algorithm. This document is a final report outlining the development and techniques used in the procedure. The numerical solution is based upon a finite volume technique with a four stage Runge-Kutta time marching procedure. Numerical dissipation is used to gain solution stability but is reduced in viscous dominated flow regions. Local time stepping and implicit residual smoothing are used to increase the rate of convergence. Multiple blade row solutions are based upon the average-passage system of equations. The numerical solutions are performed on an H-type grid system, with meshes being generated by the system (TIGG3D) developed earlier under this contract. The grid generation scheme meets the average-passage requirement of maintaining a common axisymmetric mesh for each blade row grid. The analysis was run on several geometry configurations ranging from one to five blade rows and from one to four radial flow splitters. Pure internal flow solutions were obtained as well as solutions with flow about the cowl/nacelle and various engine core flow conditions. The efficiency of the solution procedure was shown to be the same as the original analysis