**Abstract**

Numerical simulations are carried out to study the effect of divergence angle and adverse pressure gradient on the movement of shock wave train in a scramjet isolator. The commercial software tool ANSYS Fluent 16 was used to simplify two dimensional Reynolds averaged Navier Stokes equation with compressible fluid flow by considering the density-based solver with standard K-ε turbulence model. The species transport model with single step volumetric reaction mechanism is employed. Initially, the simulated results are validated with experimental results available in open literature. The obtained results show that the variation of the divergence angle and back pressure on the scramjet isolator has greater significance on the flow field. Also, with an increase in the back pressure, due to the intense turbulent combustion, the shock wave train developed should expand along the length and also moves towards the leading edge of the isolator leading to rapid rise in the pressure so that the pressure at the entrance of the isolator can match the enhanced back pressures.

**Keywords:** CFD, scramjet, isolator, divergent angles, back pressure and combustion

## **1. Introduction**

The hypersonic air-breathing jet engines are designed to operate in the supersonic-combustion ramjet engine when the Mach number is more than 6. A scramjet engine incorporates with isolator, combustor, and nozzle. Isolator in the scramjet engine is widely used since its inception. Isolator in the scramjet engines is a typical component since it has a significant effect on the dual-mode engine transition. The hypersonic vehicle operates at a particular time during the ascent phase [1]. These effects happen due to the absence of mechanical compressor, free stream air as well as compression ratio. The propulsion vehicle maintained the engine by using inlet and isolator. The main task of the isolator is to differentiate the combustion pressure that occurs in the combustion chamber and should not reach the inlet [2].

Boundary layer interaction observed in the isolator when the dual-mode engine runs with ramjet engine. The air, pressure movement in an isolator, as well as shock train, are maintained the position of the Mach wave train. The compressed air is adjusted in isolator to match with the condition that can enter into the combustor. When the pressure introduced in the reverse direction of the combustor zone obtained the variation in pressure from the isolator and combustor. The difference between isolator and combustor zone is adjusted by changing the shock wavelength [3]. While designing the isolator, need serious concern about the unstart phenomenon. The isolator may lead to severe effect due to high speed in the flight. The length of the isolator part in the scramjet engine maintains at a certain weight. The required shear and shock waves provided to avoid the communication of the instabilities that will arise and affect the inlet [4]. The system developed using the hypersonic inlet isolator under Mach 4 and Mach 5 flight conditions [5]. In [6], reported that the decrease in the pressure at the inlet of the domain is observed with an increase in the isolator length. The shock train in a fixed 2-D scramjet inlet with isolator showed some results by increasing wall and decreasing the total temperature [7]. In Mach 5 inlet-isolator model, the shock train jumping moments captured by separating flow at the head of the shock train and the contraction ratio of the local throat-like shape [8]. The scramjet isolator decreases the static pressure, and it becomes sharper. The experiment conducted on a constant-area scramjet isolator and observed that was relatively stable with time-resolved and low-frequency pressure [9]. In [10], the numerical simulation influences the movement of free stream characteristics leading to separation with an increase in adverse pressure. The dynamic model of the shock train is predicted on the shock wave layer. The dynamic model cannot suppress the pressure gradient as high as the other sustains [11]. In [12], the complex compression and expansion waves exist in the isolator, causing large stream-wise and transverse gradients upstream of the shock train. The adverse gradient pressure in stream-wise decreases with the duct curvature [13]. In [14], the experiments compared with the conventional approaches using boundary layer interaction large-eddy simulation of a hypersonic of Mach 8 flight vehicle. In [15, 16], they have conducted experiments on the multiple shock wave/turbulent boundary layer interactions in a rectangular duct using Mach numbers 2.45 and 1.6. Carroll et al. observed that the length of the communications and the tendency towards a repeated oblique was scaled directly with the level of confinement. The study of unstart and unstarted flows in an inlet/isolator model strongly associated with boundary-layer separation [17]. The numerical solutions of the Naiver-stokes equations for the interactions of a shock wave and turbulent boundary layer varying from 7.93 to 12.17, at a free-stream Mach number of 2.96 and Reynolds number 1.2 107. The free-stream predicts accurate results. When shock strength and overall rise pressure for the low viscosity pressure asymptotes. The large-eddy 3-D analysis in the area of uniform cross section with low aspect ratio rectangular duct geometry is studied [18–22].

**2. Physical model, simulation methodology and validation**

*Numerical Investigation of the Shock Train in a Scramjet with the Effects of Back-Pressure…*

In this work, analysis has been carried out on the scramjet isolator of uniform cross section to analyze the movement of shock wave train to study the relation between shock train and the interaction of the boundary layer formation. As shown in **Figure 1** to study the impact of adverse pressure and the significance of divergent angles on the behavior of shock wave train are studied using ANSYS Fluent 16 [23]. The atmospheric air is injected at the entrance of the isolator with 220 mm length and 32 mm height. The hydrogen fuel is injected transversely from the either sides of the wall at a distance of 232.8 mm of the inlet of the computa-

The commercial software ANSYS Fluent 16 [23] was used to simplify twodimensional compressible fluid flow by considering the density-based solver with standard K-ε turbulence model, Reynolds-averaged Navier Stokes equation with finite volume method was considered. The species transport model with single step volumetric reaction mechanism is considered to simplify the combustion model (finite rate/eddy dissipation model) [24–27]. To maintain the proper mixing and optimizing the combustion phenomena in supersonic flow RANS approach is the most effective and faster method. The standard K-ε turbulence model is chosen due to its ability of simplifying the negative pressure gradient in the case of transverse

The appropriate governing Eqs. (1)–(5) describing the continuity equation, Navier Stokes equation and combustion model for fluid flow is written as [21, 22, 28, 29].

> *∂ ρu <sup>j</sup> ∂xi*

> > *μeff*

*∂ui ∂x <sup>j</sup>* þ *∂u <sup>j</sup> ∂xi*

¼ 0 (1)

þ *Sui* (2)

*∂ρ ∂t* þ

¼ � *<sup>∂</sup><sup>ρ</sup> ∂x <sup>j</sup>* þ *∂ ∂x <sup>j</sup>*

Conservation of momentum (Navier–Stokes equation)

*∂ ρuiu <sup>j</sup> ∂x <sup>j</sup>*

**of computational fluid dynamics code**

*DOI: http://dx.doi.org/10.5772/intechopen.92555*

**2.1 Physical model**

tional domain.

injection flow field.

**Figure 1.**

**5**

Continuity equation:

*<sup>∂</sup>*ð Þ *<sup>ρ</sup>ui ∂t* þ

*Schematic diagram of the scramjet combustor.*

**2.2 Simulation methodology**

In the open literature, by varying the adverse pressure gradient at the exit of the isolator the motion path and characteristics of the shock wave train are obtained. In this work, we intend to study the impact of combustion phenomena on shock wave train. Therefore, the significance of the angle of attack on the wall surface of the domain and the effect of the adverse pressure gradient on the movement of the shock wave train are analyzed. The present analysis focused on different divergent angles, i.e., 0, 0.5, 1, and 1.5° with constant pressure gradient of 90 kPa, and also with different negative pressures of 80 and 100 kPa with constant cross-sectional area of the isolator is discussed. All these effects are studied on similar computational domain with similar solver type parameters. The rest of the paper is as organized as follows, Physical model and simulation methodology is discussed in Section 2, the effects of back pressure and angle of attack are discussed in Section 3. *Numerical Investigation of the Shock Train in a Scramjet with the Effects of Back-Pressure… DOI: http://dx.doi.org/10.5772/intechopen.92555*
