J. Plasma Fusion Res. SERIES, Vol.2 (1999) 153-156

Two-Dimensional Simulations of Relativistic Jet-GloudCollisions
ZHANG Hui-Min, KOIDE Shinji and SAKAI Jun-Ichi
Faculty of Engineering, Toyama University, j190 Gofuku, Toyama 930-8555, Japan(Received: 8 December 1998 / Accepted: 20 May 1999)

Abstract
We present two-dimensional simulation results of jet-cloud collision, by using a relativistichydrodynamic simulation code. It is shown that the collision processes are rather different for relativisticand nonrelativistic jet. For the relativistic jet, the collision takes the form of reflection. The deflection
angle can be greater than 90'. Thejet material is heated and suffers rapid deceleration and reaccelerationin collision region. While a nonrelativistic jet is smoothly bent to a small angle 80o, the pressure andtemperature is only a little higher than their initial values. These results are useful for understanding
some observations.

Keywords:
galaxies, jets, hydrodynamics, jet-cloud collision

1. lntrodustion

simulations, the length, density and pressure arenormalized by a, po and ps, respectively. The systemsizes are 0 S.r < 25a and -lla < y < l4a in the x and y
directions The number of mesh points are 190 x 190.
Initially, the jet, ambient medium and cloud start withequal pressure ps, and adiabatic index is l- = 5/3. The
density of ambient medium, jet and cloud are 1.0p6,0.1p6, and 1000.0p0, respectively. The jet and cloudradii a and 5a. Thejet is injected from the orifice at theleft boundary of the calculation box; xla = 0, -l 3 yla 3l. The left surface.r = 0 is bounded by the fixed
condition. We use a simplified radiative boundary
condition at x = 25a and y = -lla and y = l4a. Along

Observational studies ofjet in extragalactic radiosources show that the jets are often bent through large
apparent angles [1-5]. The bending could result from avariety of mechanisms. However, some jet apparently
changes direction at the location of the bright region andoptical knot [6]; some jets are bent by deflection angle
greater than 90o [7]. It seems that without using a jetcloud collision model it is impossible to explain them.

Some observations have also presented compelling
evidence for the relativistic flow in most of jets[-4,8,9]. Thus, relativistic simulation should also be
used to investigate jet concerned processes [10-19].

Here, we report our simulation results of jet-cloud
collision by using a special relativistic hydrodynamic(RHD) code in which a simplified total variation
diminishing (TVD) method was employed.

the z-direction for the slab geometry, we use theperiodic boundary condition. Initially, the cloud center
is localed at xla = 15, yla = -2.5. The impact parameterof collision is 2.5a for two runs. The velocities arenormalized by sound speeds u| = {fpl,o") = l. u.R =:= I for nonrelativistic and^frpKi;.nlT -1 75
relativistic cases, respectively. Time units are rl! =2. Initial Gondition and Results
We carried out two simulations for relativistic R
and nonrelativistic N case, respectively. In our@1999 by The Japan Society of Plasma

Corresponding author's e-mail(HMZ): zhanghm@ ecs.toyama-u.ac.jp
153

Science and Nuclear Fusion ResearchZhang H.-M. et al., Two-Dimensional Simulations of Relativistic JerCloud Collisions

quantities of R at the time t = 44.9t{. As seen in Figure
I the jet hits the cloud and a high pressure region isformed, where the maximum pressure is as high as
22.7ps. This region is bound on the one side by thea/v| and ?,R = a/v,R. The injection velocities are of Mach
number 4.0, and Lorentz factor 4.56 and 1.005 fot
relativistic and nonrelativistic jet, respectively.
Figure I shows the distribution of main physicallylavla'o-5*101n1q10 15xla30

xla
0.0 0.5 1.0 1.5 2.* 2 .5 3.0 3 .50/1J\5v/a'0n1.0

2.4

3 .0{b} density ilog){a} velociiy
t0.0nt2A\\\t#:='----:dYla., . .ldrss. r@r{:-a:}\*-q-1n

10 15
xla
10.0

2.9

1"5.0':qx1a

5.0 10.0 15.0 20.0 ?5.0 30.r]

20.0(d) temperature(c) pressure

Fig. 1 (al Velocity distribution of R at f = 44.92"N. The jet rushes at the cloud and forms two downstream flows, thedeflection angle of the dominant flow is about 130'. The original jet decelerates by a shock, and then the gas isreaccelerated to supersonic speed. (bl Density distribution. The cloud sharp is changed. A bow shock drives into
the cloud. (cf Pressure distribution. The high pressure region is bound by two shocks, the one is at the beamhead, the other one is a transform of bow shock dr:iven into the cloud. ldl Temperature distribution. The
temperature in the collision region is high.
r54

Zhang H.-M. et al., Two-Dimensional Simulations of Relativistic Jet-Cloud Collisionsshock driven into the cloud and on the other side by acloud gas. The maximum value of temperature T(= p/p)in the collision region is about 30 times of the initialambient temperature, or, the jet gas is heated to about 6
times of its injection temperature. The two tips of theregion are just two nozzles from which the high pressureshock at the head ofjet beam. The contact discontinuity

is located in the high pressure region. The left half ofthe high pressure region is filled with thermalized jetmaterial; the other half is high density and pressure5ylaoY/a o-5-5-30-t0

10

10 15xlax5xla
lnan

0.0

3.0(a) velocity

1.0

2.0

3.0

{bi density (log)

10IU

55yla

y/E

6-5-5-1n-1.0

1015x.la

0.5

I1nt€

2.0

3.0

4.0I

cn(d)temperalure(c) preesure

Fig. 2 lal Velocity distribution of N at r = 83.5r"N. The jet is gradually bent to about 60' by the cloud. When it touch thecloud, its velocity is decelerated but is still supersonic. (bl Density distribution. The sharp of the cloud is changed.
(cl Pressure distribution. (dl Temperature distribution. The temperature in the collision region is almost the sameas the that of injection jet.

155

Zhang H.-M. et al., Two-Dimensional Simulations of Relativistic Jet-Cloud Collisions

Acknoweldgment

gas spurts out and forms two supersonic downstream
flows. The one located on the upside is dominant, it
inherits most of the original jet material and energy. The
downstream flow is supersonic and of low density, thedeflection angle is about 130" with an opening angle.
During the collision, the jet gas suffers rapid deceleration and reacceleration. The region which takes

Sakai thanks the Cosel company for the support.
Zhang is grateful to the Japanese Education Ministry fora scholarship.

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