CFD ANALYSIS AND SIMULATION OF SHOCKWAVE GENERATION
Dr. S. P. Vendan #1, K. A. Mohamed Isqhak*2, R. Prabakaran *3, N.Sugajen*4 and M.Pravinkumar*5
#1

Associate Professor, Department of Mechanical Engineering, PSG College of Technology, Coimbatore-641004. 1 spvendann@yahoo.com

*

IV Year BE Mechanical Engineering, Department of Mechanical Engineering, PSG College of Technology, Coimbatore-641004. 2 mailmepravin@gmail.com 3 prbkrn1991@gmail.com

Abstract. This paper involves computational fluid dynamics analysis and simulation of a shock wave generating equipment. Shock waves are produced by suddenly exposing a high pressure region to a low pressure region. This involves in the design of an arrangement that acts as a valve which separates high pressure region and low pressure region such that the valve opens suddenly (i.e., in the order of milliseconds) and thus producing shock waves. The instantaneous rise in pressure and temperature of a medium can be used in a variety of industrial applications Key words: Shock waves, Shock tubes, CFD, Pneumatic Valve

1 Introduction
The ability of shock waves to instantaneously increase the pressure and temperature in a medium of propagation enables their use for many novel industrial applications[1]. In some sense the presence of a shockwave propagating in an enclosed medium can be similar to a furnace where, in addition to temperature, even pressure can go up instantaneously and remain at elevated levels for a short time and then come back to ambient conditions. There is no other method by which one can achieve high pressure and temperature in a medium so quickly. Shockwaves are essentially non-linear waves that propagate at supersonic speeds. Such disturbances occur in steady transonic or supersonic flows, during explosions, earthquakes, hydraulic jumps and lightning strokes. The ability of shock waves to instantaneously increase the pressure and temperature in a medium of propagation enables their use for many novel industrial applications There is no other method by which one can achieve high pressure and temperature in a medium so quickly. As a result shock wave loading of materials can be very fascinating leading to new industrial applications based on them. Some of the applications include shockwave assisted gene transfer, preservative injection into wood slats, oil extraction, drug delivery, metal forming, reduction of bioburden in natural products, oil extraction

from sandalwood and paper manufacturing. Shock wave tubes are also used in research laboratories as wave reactors for kinetic studies and material systems.

2.1 Principle Shockwaves are produced when a high pressure region is suddenly exposed to low pressure region. The shock tube in its simplest form a uniform cross-section tube divided into a driver and driven sections by a diaphragm. Both sections are filled with gases with the driver section at higher pressure. When the diaphragm is suddenly allowed to burst, a plane normal shock travels down the driven section heating the gas there instantaneously. This is replaced by using a pneumatically actuated sliding piston that produces the effect of the diaphragm of the conventional shock tubes. This diaphragm less method has the advantage that it does not pollute the low pressure region by minute fragments produced by the rupture of the diaphragm. A largeamplitude wave formed by the sudden compression of the medium through which the wave moves. Shock waves can be caused by explosions or by objects moving through a fluid at a speed greater than the speed of sound. 2.2 Shock Tube The shock tubes are one of the most common and easiest way to produce shock waves in laboratories. They are used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale. Shock tubes (and related impulse facilities such as shock tunnels, expansion tubes, and expansion tunnels) can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities[4]. Shock tubes are also used to investigate compressible flow phenomena and gas phase combustion reactions. More recently, shock tubes have been used in biomedical research to study how biological specimens are affected by blast waves. But the use of rupturing diaphragms results in (1) an uncontrollable amount of impurities inside the shock tube, (2) a relatively low reproducibility of shock conditions, and (3) very costly and time-consuming tasks of replacing the ruptured diaphragm after each and every shot. Shock tube using diaphragm less method has the advantage that it does not pollute the low pressure region by minute fragments produced by the rupture of the diaphragm. In order to clarify the piston sliding process for the piston-actuated structure, a simplified model based on the motion equation of its main sliding piston has been developed. Such a model allows the piston sliding time to be estimated. The piston sliding time is of importance mainly because it may influence the distance where the shock wave will be formed downstream and the characteristics of the shock wave itself.

Fig. 1 Shock tube

2.3

Diaphragm less Structure

The diaphragm less structure of the design is shown in the figure. The piston at the initial state acting as a diaphragm separates the low pressure region from the high pressure region. When actuated by two pneumatic driver pressure sources of different magnitude, the piston is slides over the chamber holding air at higher pressure. Initially the actuation is done by a low pressure source to overcome static frictional force. Once the piston starts sliding a certain distance it is accelerated by the higher driver pressure. It is suitably designed such a way that in the beginning of the process only one driver pressure is in effect to the sliding process as shown in the figure. The piston is accelerated such that it opens the high pressure region to low pressure region in very small interval of time. Depending on the interval of time, the shockwave is strong in case of smaller time (≈µs) or weak in case of longer interval. If the opening time is higher, the shock formation distance, which is the measurement of the distance at which the shock Mach number reaches maximum, increases. This sliding of the piston produces a single pulse of shock waves. The green port on the top right side is the low pressure port and the port near it is the high pressure port. The following figure 3 and figure 4 show the two extreme positions of the piston. Initially the piston acts as a closed valve and at the end of a half-cycle it is fully open and it needs to be brought back to the original position to get ready for the next pulse. This is actuated by using a three-way pneumatic valve in the other side of the arrangement to provide exhaust during backward stroke and driver pressure during forward stroke. All the pressure ports are connected to three-way non return valves.

Fig. 2 Diaphragm less structure

2.3

Operating Conditions

The body center and the main assembly is made of steel whereas the piston is made of Aluminium with Teflon coating. The fluid that is used for the production of shock waves is air. The actuation is pneumatic i.e., the driver gas is air. The magnitudes of the driver gas pressures influence the sliding time of the piston and calculating them is an objective. A differential pressure of 50 bar is to be maintained between the pressure chamber and the exhaust in order to achieve shock waves. The whole setup is maintained at 300 K.

Fig.3 Sectional View(Fully Closed)

Fig.4 Sectional View(Fully opened)

2.4 Operation The scheme for generating the shock waves through this diaphragm less structure is as follows: (i)The piston is at fully closed position (ii)By the opening of the three-way valve in the low pressure driver port the piston is pushed forward (iii) As it crosses the higher pressure port it is accelerated to higher speeds by the driver gas (iv)Once it crosses the critical position shockwave pulse is produced (v)Now it is brought to rest by the dampers at the other end (vi)By the pneumatic valve at this end the piston is brought back to its original position 2.5 Optimal pressure for pneumatic source The time taken by the piston to slide across the critical displacement i.e., from the instant the differential pressure begins to open to the instant it is completely open is the opening time. It is determined for different actuating pressures ranging from 10 to 20 bar are determined using a dynamic simulation software. The data obtained is plotted as a graph between the actuating pressure and opening time. The pressure corresponding to optimum opening time one millisecond opening time was interpolated using linear interpolation method.
TABLE I OBSERVATIONS S.No. 1 2 3 4 5 6 Actuating Pressure (bar) 15 16 17 18 19 20 Opening Time (ms) 1.13 1.03 0.87 0.66 0.39 0.25

Opening Time (s )

Fig.5 Opening time vs Pressure

3 ANALYSIS

3.1 Dynamic analysis using the optimised pressure After obtaining the appropriate pressure corresponding to the required opening time the various kinematic parameters of the motion is simulated using the model of the experiment in a dynamic motion evaluation software. The figure below shows the displacement of the piston across the time along the x-axis.

Fig.6 Time vs Piston displacement

Fig. 7 Time vs Piston velocity

Fig. 8 Time vs Piston Acceleration

3.2

CFD Analysis

The net force acting on the piston can be formulated as follows Fn(t) = AfPd(t)-fAfPd(t) where Af is the area of the flange of the piston, Pd is the pressure exerted by the driver gas on the flange and f is the friction co-efficient[6]. Dynamic model cannot be analysed using CFD software so piston at five different position is analysed/This pressure exerted on the flange is determined by modeling the driver gas flow at various positions of the piston. Thus after finding the force acting the acceleration can be determined even more accurately than previous analysis. Thus acceleration, an(t) = Fn(t)/mp where mp is the mass of the piston. This is of significance as it is required for the design of the dampers. The model should not have any leakage so we close all the open ends as shown below

Fig 9 Adding details to model make flow analysis

Fig 10 Meshing model creating fluid model The figure below shows the force acting on the flange initially when the low pressure driver gas is alone acting at different iterations while solving using a CFD software[4].

Fig. 11 Iterations vs Force The figure below shows the force acting on the flange when the low pressure driver gas is alone acting on it.

Fig.12 Pressure distribution

Fig. 13 Iterations vs Force The total force acting on the flange at the end of the piston displacement is 8593.77 N. The vector diagram of the pressure distribution is shown in following figure.

Fig.14 Pressure distribution

4 CONCLUSION A piston actuated structure has been developed to replace diaphragms in conventional shock tubes. From the analysis of piston sliding process for a piston actuated shock tube the various parameters contributing to the objective was examined. The differential pressure, speed at which the pressure barrier is opened and the gap provided for the shock wave generation contribute maximum to the creation. The acceleration and the force acting are indirectly determined using numerical techniques.

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