Dynamic Mode Decomposition Analysis of Flow Separation in a Diffuser to Inform Flow Control Strategies

In this work, a large eddy simulation (LES) of a typical subsonic diffuser provides data used to analyze coherent structure in a separated flow with dynamic mode decomposition (DMD). From this, a low–dimensional approximation, which retains the main dynamic characteristics of the original flow fields, is obtained. In particular, specific dynamic structures associated with a unique frequency are isolated. The spatial structures of the real and imaginary parts of the DMD mode are similar but with a phase difference. The contribution of the conjugate modes to the evolution of the DMD modes over time is discussed. The dominant frequency is found to be related to the wake mode. The scale of wake will saturate, and the shear layer will become weaker and merges into the wake structure as it develops downstream. This allows direction for effective flow control strategies using this information.

Predicting Unsteady Flow Parameters in a Subsonic Air Diffuser Using MacCormack’s Explicit Method

A numerical procedure is presented to predict the flow characteristics inside a subsonic diffuser by solving Navier-Stokes' equations, using MacCormack’s explicit method. The flow is assumed to be viscous, compressible, unsteady and two-dimensional. The grid model suggested for the diffuser has 20 points in the horizontal direction and 30 points in the vertical direction. The numerical solution has shown reasonable results with a 2D variation of flow properties inside the diffuser and the steady state solution can be satisfied by 600-900 loops only. The obtained results of the present study are compared with those obtained by using a numerical code of National Project for Application-oriented Research in CFD (NPARC) as well as those obtained from a previous experimental study and give an acceptable range of errors (about ± 15%).

Actuator selection and placement for localized feedback flow control

The selection and placement of actuators and sensors to control compressible viscous flows is addressed by developing a novel methodology based upon the eigensystem structural sensitivity of the linearized evolution operator appropriate for linear feedback control. Forward and adjoint global modes are used to construct a space of possible perturbations to the linearized operator, which yields a small optimization problem for selecting the parameters that best achieve the control objective, including where they should be placed. The method is demonstrated by informing actuation to suppress amplification of the instabilities in boundary layer separation in a high-subsonic diffuser. Complete stabilization is observed in the separated shear layer for short downstream distances at modest Reynolds number. Higher Reynolds numbers and longer distances are expected to be more challenging to stabilize; here the control informed by the procedure still substantively suppresses amplification of instabilities. It is also demonstrated that more complex actuator–sensor selections may not yield superior controllers.

Redesign of the B-1B Bomber Inlets for Improved Supersonic Performance

This paper presents a conceptual study of two alternative inlet concepts for the United States Air Force B-1B bomber to provide for improved supersonic performance with expansion of capabilities to high-altitude, high-speed flight at Mach 2.0. The two inlet concepts are two-dimensional, variable-ramp inlet systems designed to replace the current fixed-geometry, pitot inlets of the B-1B. One inlet incorporates a two-ramp system, while a second inlet incorporates a two-ramp system containing an isentropic contour. The entire inlet system including the supersonic diffuser, throat, cowl lip, and subsonic diffuser sections was designed to maximize the total pressure recovery at the engine fan face to achieve maximum thrust by the engine at Mach 2.0 conditions. Analytic methods implemented into the MATLAB and the NASA SUPIN codes are used to design and analyze the two-dimensional inlet concepts. In addition, high-fidelity WIND-US computational fluid dynamics (CFD) simulations were used to verify the results of the analytic design methods. The results suggest that at Mach 2.0, the total pressure recovery of the inlets could increase from 0.70 to 0.94. The inlet capture area and cowl drag increased; however, the overall improvements resulted in a 98% thrust increase over the existing inlet at the design point.

A Numerical Study for Flow Excitation and Performance of Rampressor Inlet considering Rotor Motion

A unique supersonic compressor rotor with high pressure ratio, termed the Rampressor, is presented by Ramgen Power Systems, Inc. (RPS). In order to obtain the excitation characteristic and performance of Rampressor inlet flow field under external excitation, compression inlet flow of Rampressor is studied with considering Rampressor rotor whirling. Flow excitation characteristics and performance of Rampressor inlet are analyzed under different frequency and amplitude of Rampressor rotor whirling. The results indicate that the rotor whirling has a significant effect for flow excitation characteristics and performance of Rampressor inlet. The effect of rotor whirling on the different inlet location excitation has a definite phase difference. Inlet excitation becomes more complex along with the inlet flow path. More frequency components appear in the excitation spectrum of Rampressor inlet with considering Rampressor rotor whirling. The main frequency component is the fundamental frequency, which is caused by the rotor whirling. Besides the fundamental frequency, the double frequency components are generated due to the coupling between inlet compression flow of Rampressor rotor and rotor whirling, especially in the subsonic diffuser of Rampressor rotor inlet. With the increment of rotor whirling frequency and whirling amplitude, the complexity of Rampressor inlet excitation increases, and the stability of Rampressor inlet performance deteriorates.

Numerical Research on the Flow Field and Performance of a Ram-Rotor and a Scrampressor

The design methods of typical supersonic aircraft intakes and shock wave compression technology have been applied to ram-rotor, a new attractive compression system. A ram-rotor is a typical structure including the compression ramp, the throat and the subsonic diffuser; a scrampressor is similar to ram-rotor, the only different is that scrampressor has no subsonic diffuser. Base on the preparatory work, it has been found that these two structures have different advantages respectively. So, in this paper, the three dimensional Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulent model are used to simulate numerically the flow field of the ram-rotor and the scrampressor at the design and at the off-design conditions. The back pressure and rotational speed are mainly considered which may affect the flow field and the total performance. It has been found that back pressure can not have influence on the flow field before the throat outlet obviously. With increasing of the back pressure, the position of the flow separation zone and shock train move forwards to the inlet. The rotational speed changes the shock wave structure of the ram-rotor and scrampressor evidently. With the rotational speed increasing, each shock wave moves to the outlet and the shock wave number decreases. The ram-rotor and scrampressor structure is similar, except the ram-rotor flow structure has a large flow separation zone after the throat outlet. The compression capability of the ram-rotor is higher than that of the scrampressor. The total performance of the scrampressor is better than the ram-rotor.