Mach Number Dependence of Flow Instability around a Spiked Body

TL;DR

This study investigates how flow speed, characterized by Mach number, influences the unsteady flow behaviors around a spiked body, revealing pulsation at low Mach and oscillation at high Mach numbers through numerical simulations.

Contribution

It is the first to analyze the effect of Mach number on flow unsteadiness around a spiked body using axisymmetric Navier-Stokes simulations across a wide Mach range.

Findings

Flow pulsates at Mach 2 and 3, transitions to oscillation at Mach 5-7.

Flow remains stable at Mach 4.

Strouhal number limit of 0.2 characterizes pulsation and oscillation modes.

Abstract

A forward-facing aerospike have been identified as a passive flow control device for enhancing the aerodynamic efficiency and reducing the heat transfer in high-speed flows. In addition, it has been reported that the presence of a spike brings in unsteadiness in the form of oscillation and pulsation to the structure. Previous researchers have investigated the aerothermodynamic coefficients, together with offering a detailed explanation of the flow physics and associated unsteadiness, and their dependence on the spike's geometric characteristics (spike nose, and length-to-fore-body diameter ratio, L/D). This work focuses on ascertaining the role of flow speeds (free-stream Mach number), and their energy content, in governing the physics around a spiked body, which is yet to be established. Numerical investigation has been carried out using axisymmetric Navier-Stokes laminar flow solver for Mach number range of 2.0 to 7.0. A round-tip spike with flat-face cylindrical after-body have been simulated for spike length ratio of L/D = 2.0, with spike diameter to fore-body diameter of 0.1. The flow unsteadiness has been analyzed with drag and pressure coefficients variation at different Mach numbers. It was found that the flow field around the spiked blunt nose behaves in pulsation mode at lower Mach numbers 2, 3 and transition to oscillatory mode at higher Mach numbers 5, 6 and 7, while remain almost stable at Mach 4. The limit of Strouhal Number for characterizing the pulsation and oscillation modes at various Mach numbers for spike length of L/D = 2 with flat after-body is observed as 0.2, however it may very well depend on other geometric parameters of spike and after-body.