Showing papers by "Defence Research and Development Laboratory published in 2004"

Comparison of fracture models to assess the notched strength of composite/solid propellant tensile specimens

Abstract: Modifications are made in the well-known inherent flaw model as well as in the stress fracture models for accurate prediction of notched tensile strength of composite laminates containing holes or slits. To examine the adequacy of these modifications, fracture data on different composite materials reported in the literature are considered. The notched strength estimates from the present fracture models are found to be close to the existing test results. An attempt is made to correlate the fracture data of center crack tension specimens made of nitramine and HTPB-based propellant materials. The present analysis results from the above simple models are found to be in good agreement with test results. This study confirms the applicability of the above three fracture models to solid propellant materials having relatively low stiffness and strength. Since the notched strength estimates of composite/solid propellant tensile specimens are close to the test results, any one of them can be utilized while evaluating the notched tensile strength of specimens.

Experimental and numerical investigation of confined unsteady supersonic flow over cavities

Abstract: Experimental investigations were carried out to study the acoustic radiation from a rectangular wall mounted cavity in a confined supersonic flow. The free-stream Mach number was maintained at 1·5 and the cavity length-to-depth ratio was varied from 0·43 to 5·0. Acoustic measurements made on the top wall show jumps in the dominant frequency as the cavity behaviour changes from shallow-to-square-to-deep cavity. Numerical simulations of this unsteady two-dimensional flow using the commercially available software FLUENT have also been carried out. Unsteady pressure data at the same location in the flow field as the pressure transducers in the experiments was collected. FFT analysis of the unsteady pressure data was performed to obtain the dominant acoustic frequencies. The values for these dominant frequencies predicted by the numerical calculations agree well with experimental data. The numerical study also predicts the frequency jump observed in experiments.

Numerical Investigation of the Intake Flow Characteristics for a Ramjet Engine with and without Heat Addition in the Combustion Chamber

Abstract: The flow field in the supersonic mixed compression axisymmetric as well as two-dimensional intake have been investigated numerically. The flow characteristics, such as pressure, temperature, and velocity have been obtained. An enlarged chamber with a supersonic convergent-divergent nozzle has been integrated at the air intake exit to simulate the ramjet engine. Suitable heat addition at the inner surface of the wall has enabled the simulation of combustion chamber condition. The well-known modes of operation of the ramjet engine air intake have been predicted by the mathematical simulations. A comparative study for supercritical, critical, and subcritical -; conditions has been presented. The predicted flow features including shock locations and the static pressure distributions along the cowl, as well as the centre body are in agreement with the available experimental results in the published literature. The effect of thermal input in the combustion chamber on the flow characteristics of the air intake has been studied in detail. The characteristic curve of generic supersonic air intake can be obtained through these numerical studies.

Numerical Investigation on Effect of Inlet Dump Angle on Ramjet Combustor Performance

Abstract: Ramjet combustion is a highly complex phenomenon involving simultaneous momentum, heat and mass transport processes. It involves a variety of complexities such as high temperature variation, intricate flame chemistry and liquid – gas phase coupling, which are formidably challenging for a complete theoretical analysis. The governing equations with appropriate boundary conditions for flow in a ramjet dump combustor with two side inlets 90 apart are solved using the flow simulation software FLUENT. Standard k H equations based on generalized Boussinesq eddy viscosity concept are employed for the prediction of turbulent flow features. The gas phase heat and mass transfer have been modelled by Eulerian approach, while Lagrangian formulation is used for tracking the fuel particles. In liquid phase modelling, the momentum, energy and species balances are carried out by treating the droplet as a lumped system. Probability Density Function (PDF) approach is employed to describe the turbulent fluctuations in the mixture fraction. Predictions of temperature inside a ramjet combustor were compared with the limited experimental data available and these were found to match reasonably well. A detailed study has been carried out on the baseline combustor geometry under consideration for air inlet dump angles of 30,45 and 60 and the results are discussed in this paper.

Kinetic Scheme for Computational Aeroelastic Analysis of 2-D Airfoils in Transonic Flows

Abstract: A new formulation based on a Kinetic scheme namely the Kinetic Flux Vector Splitting scheme on Moving Grids(KFMG) for solving Euler equations has been developed for unsteady flow computations around oscillating bodies. The necessary grid movement has been achieved using spring analogy method on structured grids. An integrated Aeroelastic code has been developed by coupling the KFMG Euler solver with a 2-DOF structural dynamics model of an airfoil. Results obtained for steady and unsteady flows problems are compared with literature data and are discussed in detail. The aeroelastic response characteristics for NACA 64A006 airfoil undergoing pitch and plunge motions have been obtained using the present aeroelastic code. The well known ‘Transonic Dip’ phenomenon has been captured for NACA 64A006 airfoil using the present Computational Aeroelastic code.

An Experimental Investigation of Transverse Liquid Injection in Confined Supersonic Flow over Cavities

Abstract: An experimental investigation was carried out to study the acoustic radiation of a rectangular wall mounted cavity in confined supersonic flow under the influence of transverse injection of distilled water (surrogate fuel). This work is a part of an ongoing research program to identify cavities, which will enhance mixing using acoustic radiation in supersonic combustors with liquid fuel. The free-stream Mach number was 1.5 and the cavity length to depth ratio was varied from 0.69 to 5.0. The effect of liquid injection location and injection pressure (5-20 bar) on the cavity dynamics was studied. Acoustic measurements carried out on the top wall of the test model showed shifts in oscillating frequencies and changes in amplitude due to liquid injection. Instantaneous Schlieren images were obtained to visualize the shock structures generated by liquid injection and their interaction with the shock structures generated by the cavity. The shock induced from the liquid injection changes the flow conditions at the cavity leading edge leading to changes in cavity behavior. The fluid dynamic coupling of flow conditions at cavity leading edge and cavity depth lead to increased amplitudes for few cavity depths compared to the case of absence of injection. This was experienced for cavities with L/D around 2 and the amplitude increased by 50%. Schlieren pictures indicate that a normal shock stands upstream of the cavity leading edge for the cases where increase in amplitudes was experienced.