The aim of our study is to determine the formation of the recirculation zone in the snorkel. Therefore, we have to take into acount several parameters : flow rate of air in the engine, speed of the car, boundary conditions, viscosity, ...
1st case : U = 80 m/s
First, we decided to study the flow
with a car velocity of 80 m/s. This value corresponds to
a nominal standard velocity used by many design offices.
We do not notice any recirculation zone.
We decided to validate our simulation
by checking the Bernoulli equation. Therefore, we
measured the values of pressure and velocity at the
intlet and the outlet of the snorkel.
We also decided to check the cross
section of the flow by taking into account physical
Cross section of pressure
We can notice that the cross section of
velocity appears like parabolic, which corresponds to
what we expected, regarding to Poisseuille flow results.
In those first simulations, we did not see any recirculation zone.
2nd case : U = 200 m/s
Willing to determine the formation of a recirculation
zone, we decided to increase the speed of the car.
We wondered if the problem could come from the number of iterations in time. To check wether the time was correct or not, we studied the residual graph.
3rd case : influence of physical parameters
In order to obtain the formation of a
recirculation zone, we decided to change the boundary
conditions and to substitute rugosity by a sliding wall.
By deviding by 10 the value of artificial viscosity, we could finally see the formation of a recirculation zone. The equation of Bernoulli could not be checked anymore since the loss of load modified the cross section of velocity.
We can see this zone on the picture below :
The resprentation of the flow by velocity vectors allow us to see the reciculation zone :
If you want to see a simulation film click here.
We can wonder why such recirculation
zone did appear. From a physical point of view, the
viscosity increases the friction loss within the pipe so
that this losses are rather progressive. Minor losses can
be neglected. By decreasing the viscosity of the fluid,
we let reciculation zone the possibility to appear
through minor losses.
Study of the recirculation zone
We decided to calculate the value of the losses due to the recirculation zone and to study the influence of the number od iterations on that value.
number of iterations = 30 000
We can take a look at the cross sections of velocity at the inlet of the snorkel :
inlet : P = 102 900 Pa ; U = 56.4 m/s
Regarding to these values, the losses amount to 1000 mCE.
number of iterations = 430 000
inlet : P = 102 248 Pa ; U = 50.6 m/s
Regarding to these values, the losses amount to 3971 mCE, which is quite a lot.