Transient flow over a throttle body
In this Challenge I\'m setting up a transient flow of the Throttle body.
SET UP
from the steady state simulation we already know inflow velocity in the elbow shape pipe.
According to the plots red colored plot shows that average inflow velocity is approximately 190m/s. Now we can roughtly calculate how much it take for fluid to pass the whole pipe.
We see that x and y dimensions are approx 0.1m, so the length of our pipe will be maximum 0.2meters. now we can derive time which is `t=s/v`=0.001s the amount of time fluid needs to cross the entire pipe. For transient simulation I set time to be 0.1 seconds, that means that in this time fluid would be able to pass though the pipe more or less 100 times, but we have a little bit different situation here. we rotate the throttle while simulation is marching in given time.
looking at he picture above we see that the throttle body will be rotated by 25degrees from 0 to 0.002seconds then it will be stoped in it\'s new position for 0.002seconds and from 0.004 to 0.008 it will return to it\'s original position gradually, afterwards simulating additional 0.02seconds to reach steady state again.
Initial and Boundary Conditions
We are using the same initial and boundary conditions as we used in steady stete simulation.
Inlet pressure: 1.5bar
Outlet pressure: 1bar
Volume of the elbow shaped pipe: 1bar
Temperature: 300K
Mesh
Base mesh size is 0.001m and fixed embedding scale is 3 with 2 layers, meaning the smallest element in the mesh will be `0.001/2^3`= 0.000125m in other words 0.125mm
POST PROCESSING
Ilustrating rotational motion of the Throttle body
1) Mass Flow
We see that continuity has converged perfectly. we should take in account rapid decrease in mass flow before 0.002s, it\'s because of the Throttle body rotation which decreases area of flow rapidly causing reduction in mass flow while the Throttle remains it\'s position untill 0.004s, subsequently we see that mass flow is gradually increasing due to decreasing the angle of the Throttle body until 0.008s and when the simulation reaches 0.01seconds it\'s noticable that mass flow is converged.
2) Total pressure
Looking at the plots we find out that total pressure has converged but it it significantlh distinct at Inlet and Outlet, which is caused due to losses.
3) Static pressure
we notice the difference in Static pressure between Inlet and Outlet boundaries, which is the result of energy conversion, As we know that through the pipe velocity is induced we can easily conclude that static pressure (potential energy) is partially converted into dynamic pressure (kinetic enrgy). That\'s why we have higher average velocity value at the outlet, to claim this conditions look at the picture below.
4) Average Velocity at Inlet and Outlet
As we predicted, average velocity at the Outlet is higher (blue colored plot) that at the Inlet due to energy conversion.
5) Total cell count
It\'s obvious that we have significant fluctuations in total cell numbers, which is due to the throttle body rotation. if we look at the plot carefuly will notice that cell numbers are increasing untill simulation is reached 0.002s, stays constant until 0.004sec and starting decreasing before simulation reaches 0.008s and then appears to be constant again. while rotating throttle body converge cfd automatically handles to change mesh in order not to spoil quality of the fluid flow domain.
As we see in the video above during the Throttle body rotation the number of mesh elements is increased.
6) pressure and velocity contours
6.1) Front view
6.2) Back view
Watching the videos above we can easily undestand pressure and velocity relation, In the front view (6.1) we see that while the Throttle body is rotating it\'s clogging the air causing pressure increase due to increasing the surface area which collides with the coming flowing stream decreasing it\'s velocity, therefore increasing static pressure, but as we look at the back view video (6.2) we notice that at the upper surface of the Throttle body velocity is dramatically decreased but pressure is not increased accordingly, that because of circulation of flow, which creates vortexes around and the upper surface of the body due to pressure differences, creating so called \"Wake\" after air passes the Throttle.
6.3) streamlines (pressure bar shows pressure on the Throttle body surface, velocity bar represents streamline velocity)
The videos above (6.3) and (6.4) shows circulations while the airflow is passing the Throttle body generating vortexes, which at the first glance makes pressure velocity relation nonintuitive, but as we discussed above the answer is the \"Wake\" phenomenon.