influence of micro jets on the flow development in the...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:01 70

I J E N S IJENS February 2019IJENS © -IJMME-2828-191301

Abstract— In this paper, Computational fluid dynamics

method is used to simulate the supersonic flow. Convergent-

divergent (C-D) nozzle have been used with sudden expansion.

The base pressure controlled by using the microjets of 1 mm of

orifice diameter is arranged at ninety degrees at PCD 13 mm. The

Mach number is 1.87, and the area ratio of 3.24 was considered

for the present study. The L/D of the duct was used 10, and the

nozzle pressure ratio (NPR) considered for simulation was from

3, 5, 7, 9 and 11. The two-dimensional planar model has been

used using ANSYS commercial software. The total wall pressure

distribution and Mach number variation from the inlet to the

outlet was observed. From the results, it is found that the

microjets are capable of controlling the base pressure, the loss of

pressure and decreases in the drag. In the present study, the C-D

nozzle designed and modeled: K-ε standard wall function

turbulence model has been used and validated with the

commercial computational fluid dynamics (CFD).

Index Term— C-D nozzle, Wall Pressure; Flow Control; NPR;

CFD; Microjets.

I. INTRODUCTION

In engineering application, the sudden expansion of the flow

in supersonic regimes is significant and has applications in

many fluid flow problems. In rocket and jet engine test-cells

systems have been used to simulate the upper atmosphere flow

field; a jet discharging yields a low pressure which is very low

as compared to the atmospheric pressure. However, CFD

analysis has its advantage to simulate the flow field’s plays an

important role in current technologies of design optimization

by providing very accurate solutions for a given problem with

different commercially available tools. Recently, Khan et al.

identified the velocity and pressure effect in a suddenly

expanded converging-diverging (C-D) nozzle flow for area

ratio 6.25 [1], [2] and area ratio 3.24 [3] with and without the

presence of the micro-jets at the base using the CFD.

From literature, it is found that CFD simulation by different

available design and analysis tools have been used from the

last two decades. Some of them are highlighted here; the CD

nozzle used for convert pressure energy to kinetic energy in

order to yield thrust using CFD method in various performance

parameters [4]. CFD was used to optimize fluid flow in a

supersonic rocket nozzle at a various divergent angle of the

nozzle by using 2D axis-symmetric model [5]. The De-Laval

nozzles were used to convert the thermal and pressure energy

into kinetic energy using the CFD software ANSYS Fluent and

compared with theoretical results [6]. High-speed CD nozzle

has been modeled and optimized the flow after the throat at a

certain point using ANSYS FLUENT [7]. Numerical

simulation has been used to optimize the flow in a convergent-

divergent nozzle for Mach number M = 2.6 at nozzle exit using

the RANS equations with k-ω SST turbulent model [8]. CFD

simulation has been carried out to study the pressure and

velocity affects for different designed Mach numbers, area

ratios and length to diameter ratio of the CD nozzle [9]–[13].

Evaluating the optimum fineness ratio (ratio of length to

maximum diameter) of the human-powered submarine of

different shapes to reduce the drag force on the body using

CFD simulation [14]. Numerical simulation was carried out for

Box-wing configuration and simulated non-planar

configuration using ICEM CFD and ANSYS CFX solver [15].

Investigated the flow-field by a numerical approach using CFD

simulation to investigate the efficacy of the supersonic Mach

numbers due to the flow from the supersonic nozzle exhausted

in a larger circular duct [13].

From numerical simulation, the laminar flow in a sudden

augmented pipe subjected to a uniform suction speed has been

analyzed [16]. Different numerical methods have been used to

solve and account the viscous effects of the flow and compared

with the available results from the literature. It is concluded

that along the length of the duct with the progressive increase

in the values of blowing speed applied at the walls the vortices

generated near the step wall dwindles. Furthermore, separation

control using microjets has been examined in a canonical flow

such as a modified backward facing ramp [17] and for

aerospace applications for two-dimensional airfoils [18], [19].

Khan et al. [20]–[29] experimentally investigated the

mechanism to assess the performance of the control

mechanism by the small jets at various level of expansion to

regulate the base pressure in a suddenly expanded circular

ducts at moderate and high supersonic Mach numbers. The

result thus produced showed that the highest gain in the base

pressure by more than 100 percent for Mach number 2.58.

For C-D nozzle with sudden expansion, the variation in

mean velocity profiles along x-axis where the four microjets

are placed at the designed Mach number has been studied to

identify the effects of the control mechanism [30]. An

experimental investigation has been carried out to investigate

Sher Afghan Khan1, Abdul Aabid1, and C Ahamed Saleel2

1Department of Mechanical Engineering, Faculty of Engineering, International Islamic University Malaysia,

Kuala Lumpur, Malaysia 2Mechanical Engineering Dept., College of Engineering, King Khalid University, Kingdom of Saudi Arabia

Influence of Micro Jets on the Flow

Development in the Enlarged Duct at

Supersonic Mach number



the control of base pressure with and without the control in the

form of the microjets with axi-symmetric suddenly expanded

duct in the supersonic regime [31]. Experiments work has been

conducted in order to investigate the base pressure variation

from an axisymmetric nozzle having an exit diameter of 10

mm and Area ratio of 4.84 [32], [33]. This investigation

focused attention on the outcome of the wind-tunnel tests

conducted to control the base pressure in the recirculation

region and also investigated the efficiency of the control

mechanism to manipulate the base pressure in a suddenly

expanded duct [34], [35]. The effects of microjets were

investigated in a sudden expansion of air for different area

ratio and L/D of the circular pipes, at subsonic and sonic flow

regimes by [36]. Investigated the airflow from CD axi-

symmetric nozzles expanded suddenly into the circular duct of

larger cross-sectional area than that of nozzle exit area,

focusing attention on the base pressure and the flow

development in the duct [37]–[47]. A technique for estimating

the efficiency of microjets in a flow which is exhausted into

the large duct from a CD nozzle has been investigated. In the

evaluation for low, medium and high Mach numbers, the

pressure distribution of the flow behind the nozzle exit is

computed, and the results indicate the presence of oblique

shock, Mach waves, and expansion fan when the shear layer is

exiting from the nozzle exit which depends upon the level of

expansion [48]–[50].

To control base pressure at the sudden expansion static as

well as the dynamic control cylinder has been used to regulate

the base flows. The effect of the control mechanism has been

identified when rotated clockwise inside the recirculation zone

at different locations at the base region to reduce the drag. The

study considered to control the base pressure for different

Mach number, area ratio, L/D, and NPR. The study shows the

effectiveness of the control mechanism to control the pressure

with sudden expansion for CD nozzle, and it also discussed the

effect of the low-cost base drag reduction technique [40], [51]–

[61].

The objective of this paper is to investigate the flow field in

a C-D nozzle. The parameters considered, in this investigation

are the L/D ratio of 10, and the NPR considered are 3, 5, 7, 9,

and 11 for an area ratio of 3.24. The flow-field is pressure and

Mach number. The 1 mm orifice diameter microjets have been

employed at the base of the nozzle to optimize the

effectiveness of the control and the influence of the microjets

on the flow development in the duct. The results have been

shown using contours and base pressure as well as pressure

plots for different NPR.

II. PROBLEM DEFINITION

Figure 1 illustrates the designed CD nozzle for Mach

number 1.87. Four microjets are used of 1mm of diameter and

located at the pitch circle diameter of 1.3 mm. This study aims

to analyze the pressure and Mach number flow for L/D 10 and

different NPR using 2D CFD simulation with and without

microjets. These microjets are operated at sonic Mach numbers

even though the stagnation pressure in the control chamber

was very high. To generate supersonic flow instead of circular

orifice we should have C-D shapes of the microjets.

Fig. 1. CD nozzle with enlarged duct

The procedure for the data analysis was followed as in Ref.

[17]. In this paper, the step height is 3 mm having area ratio as

2.56, and the level of expansion in the control tank is similar to

that of the storage tank. This research aims to study and

quantify the possibility of the flow regulation where small jets

are used as the blowing in the base area as the flow regulation

mechanism for restraining the base pressure.

The dimensions of the CD nozzle with suddenly expanded

duct are mentioned in table I. Table I

Dimension of CD nozzle

Mach Number 1.87

Inlet diameter (Di) 28.72

mm

Throat diameter (Dt) 8.648

mm

Exit diameter (De) 10 mm

Extended diameter (D) 18 mm

Convergent length

(Lc)

35 mm

Divergent length (Ld) 12.926

mm

Extended length (Le) 180 mm

Micro-jets diameter

(Dm)

1 mm

III. FINITE ELEMENT METHOD AND ANALYSIS

Figure 2(a) illustrates the finite element two-dimensional

planar model of CD nozzle and figure 2(b) illustrate closed of

finite element meshing. ANSYS Workbench has been used

and created a structural mesh, the number of elements has been

used high to create the fine mesh in a closed area at the edge of

the planar body. Total, 36,388 binary nodes, were generated

for the 2D planar model.

(a)



(b)

Fig. 2: 2D planar fluid body (a) Finite Element Model (b)

Meshing.

A. Setup for Solution Initialization

Computations of the flow field inside the control volume were

done using RANS (Reynolds-Averaged Navier-Stokes)

equations with the k-ε standard turbulent model [62].

The most important settings that have been applied are;

• Solver: steady, absolute, 2D planar pressure-based

• Model: viscous, k-ε standard wall function, the

Energy equation

• Fluid: air, ideal gas, viscosity by Sutherland law,

three coefficient methods

• Boundary conditions: inlet, pressure inlet (pa); outlet,

pressure outlet; wall, wall

• Solution method: Pressure (standard); density,

momentum, turbulence kinetic energy, turbulence

dissipation rate, energy (second-order upwind)

• Solution initialization: standard, from inlet •

Reference value: inlet (solid surface body)

IV. RESULTS AND DISCUSSION

A. Validation of Finite Element Model

In order to validate the finite element (FE) results, the

configuration considered as shown in figure 1 of Khan et al.,

(2003) is selected. Khan et al. (2003) conducted experiments at

Mach 1.87 for area ratio 3.24 for NPR from 3 to 11. The

transducer used for the data acquisition take three hundred

fifty samples per second and takes the average of all the data

and then display on the monitor and at the same times, it writes

on the hard disk. By comparing the experimental results

obtained by Khan et al., (2003) and the present FE results, the

agreement is presented in figure 3 for the different cases. Fig.

3 shows the matching of the wall pressure and the flow field in

the duct for various NPR for different L/D ratios of the present

study. The results obtained by the simulated are within the

acceptable limit.

(a)

(b)

(c)



(d)

(e)

(f)

Fig. 3. Wall pressure distribution for different NPR and L/D

B. Pressure Flow

Fig. 3 to 7 illustrate the pressure flow from the inlet to the

outlet without control. In this case, the optimized pressure is in

Pascal and wall is considered from the inlet to the outlet of the

C-D nozzle. Fig. 4 to 8 show the wall pressure plot and

contour which indicate that the pressure variation has a sudden

change when the level of expansion NPR increases. Also, it is

seen that the length of the recirculation zone at the corner of

the C-D nozzle is high which results in low pressure at the

base area and gives more drag. Therefore, to control base flow

and hence reduce the base drag in sudden expansion region we

have placed the microjets which will break the vortex at the

base, and that will result in manipulation of the recirculation

zone. This process will increase the base pressure and will also

result in forwarding movement of the reattachment point of the

flow which as shown in fig. 9 to 13. When micro jets

employed at the base region of the sudden expansion the wall

pressure with and without the control remains the same.

Moreover, for NPR 3 the microjets behave differently due to

the low-pressure inlet and the jets are highly over expanded.

With further increase in the NPR, the level of overexpansion

will go down, and the control effectiveness will improve.

(a)

(b)

Fig. 4. NPR 3 without microjets control (a) Contours (b) Plot



(a)

(b)


(a)

(b)


(a)

(b)




(a)

(b)


(a)

(b)

Fig. 9. NPR 3 with microjets control (a) Contours (b) Plot

(a)

(b)




(a)

(b)


(a)

(b)


(a)

(b)

Fig. 13. NPR 11 with micro jets control (a) Contours (b) Plot

C. Mach Number

The effect of Mach number also considered ascertaining the

losses incurred given the wavy nature of the flow field. In this

case, the Mach number is increasing at the exit of the CD

nozzle whenever jets are under-expanded, and for

overexpanded there will be an expansion fan or oblique shock

wave which will result in increase or decrease of Mach



number. Fig. 14 to 18 shows the Mach number variation

without micro jets control. The contours and plots have been

considered to optimize the flow variables. From the Figs. 13

to 17, show the recirculation is very high at the base corner.

Moreover, to control this enhanced recirculation zone the

microjets becomes active, and suction becomes very low this

means that the control has resulted in a decrease of the drag

and the downward direction in the downstream the flow at the

exit of the nozzle which may result in increases the Mach

Number which is shown in fig. 19 to 23. For low-pressure

variation, the Mach number varies differently, and at the exit

of the nozzle, the value of the Mach number is low for other

cases the value will become high.

(a)

(b)


(a)

(b)


(a)



(b)


(a)

(b)


(a)

(b)

Fig. 18. NPR 11 without microjets control (a) Contours (b)

Plot

(a)



(b)


(a)

(b)


(a)

(b)


(a)



(b)


(a)

(b)


D. Flow Circulation at Base Region

Figs. 24 to 25 indicate the flow field in the re-circulation zone

with and without control. In the absence of control, there is a

re-circulation zone seen creating a low-pressure region.

However, when the control is employed at the base, it can

break the dominant vortex present in the base region and able

to increase the base pressure.

Fig. 24. Without Microjet Control

Fig. 25. With Microjet Control

V. CONCLUSIONS:

Based on the above discussions we may draw the following

conclusions:

The base pressure flow field and the Mach number

have been considered to optimize the solution using

CFD simulation.

After designing the nozzle for a specific Mach

number, the nozzles were fabricated. After fabrication

of the nozzle, the nozzle was calibrated. Finally, the

calibrated Mach number is used for simulations for

different NPR and L/D ratio of 10 for the area ratio of

3.24.

The effect of micro-jets was found to be useful for a

given level of expansion and a fixed level of inertia.

From the above results, it is concluded that total

pressure varying from the inlet to the outlet and value

of total pressure is high for NPR 11.

As pressure decreases due to the formation of the

expansion fan at the nozzle exit, the velocity will also

increase, and the simulation results proved that the



pressure is low by considering the total wall pressure

flow and velocity is high by monitoring the Mach

number at the exit.

Results indicate the flow field in the re-circulation

zone with and without control. In the absence of

control, there is a recirculation zone seen creating a

low-pressure region. However, when the control is

employed at the base, it can break the dominant

vortex present in the base region and able to increase

the base pressure.

The validated results obtained through simulation as

well as experiments will be handy during the initial

design stage of the aerospace vehicle.

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