WEBVTT 00:00.110 --> 00:00.800 Hello everyone. 00:00.800 --> 00:04.250 Welcome back to the CFD using Openfoam Beginner to Intermediate course. 00:04.340 --> 00:10.430 In this class we will be seeing the types of boundary conditions, practical implementation, examples 00:10.430 --> 00:16.460 of those boundary condition turbulence modeling, introduction and setting up and running some Rans 00:16.460 --> 00:17.240 simulation. 00:18.020 --> 00:20.120 So what are the types of boundary condition? 00:20.120 --> 00:22.070 As we already saw we have two. 00:22.070 --> 00:25.790 One is Dirichlet boundary condition and the other is Neumann boundary condition. 00:25.790 --> 00:29.720 To do a Dirichlet condition we will be defining a fixed value. 00:30.530 --> 00:34.040 So it specifies a fixed value for a variable at the boundary. 00:34.040 --> 00:36.860 The example is like inlet velocity in a pipe. 00:36.860 --> 00:42.050 The application will be used when the exact value of the variable is known at the boundary. 00:42.050 --> 00:44.990 The open form keyword to do this is fixed value. 00:44.990 --> 00:47.690 Notice that the v is capital because it's the second word. 00:48.590 --> 00:49.850 I took a velocity file. 00:49.850 --> 00:50.720 For example. 00:51.350 --> 00:54.750 In this we will be specifying the inlet as a velocity. 00:54.750 --> 00:57.420 So here I have done inlet as a function. 00:57.420 --> 01:00.000 This will be defining in the u file. 01:00.000 --> 01:01.800 So the type will be fixed value. 01:01.860 --> 01:07.440 The value will be uniform 100 which means it is one meters per second in x direction. 01:07.440 --> 01:13.440 If you want it in the negative x direction, you can just mention it minus one and it will be, uh, 01:13.440 --> 01:20.360 directed in the opposite direction if you see Neumann boundary condition, it is a fixed gradient. 01:20.360 --> 01:24.740 So it specifies a fixed gradient or rate of change for a variable at the boundary. 01:25.100 --> 01:29.330 The example is heat flux at the surface of the wall or a heated wall. 01:29.360 --> 01:30.470 The application is. 01:30.470 --> 01:33.770 It is used when the derivative of the variable is known at the boundary. 01:33.890 --> 01:36.950 So the open form keyword is zero gradient. 01:37.040 --> 01:41.860 It is not constrained to using zero gradient, but it is the most commonly used. 01:42.490 --> 01:48.160 You can also define other types of gradients with specific values if you want, so that is also possible. 01:48.160 --> 01:53.200 Here I have say taken the same velocity file, but here we will be defining the outlet. 01:53.200 --> 01:59.440 So this outlet is set to type zero gradient because we do not know what is the value at the outlet because 01:59.440 --> 02:01.000 we define at the inlet. 02:01.000 --> 02:02.620 So we won't give at the outlet. 02:02.620 --> 02:04.300 The solver will calculate it for us. 02:05.100 --> 02:08.220 Instead, we will be defining maybe zero pressure at the outlet. 02:08.640 --> 02:10.770 Then we have symmetric boundary condition. 02:10.770 --> 02:14.100 It assumes no flux across the boundary. 02:14.190 --> 02:19.080 The center line of a symmetric flow problem is one such example. 02:19.080 --> 02:24.090 The application is used to reduce computational effort by exploiting the symmetry in the problem. 02:24.090 --> 02:32.300 The openfoam keyword is using just symmetry plane, so it is mostly useful in cases like a nozzle instead 02:32.300 --> 02:33.500 of doing entire 3D. 02:33.500 --> 02:36.590 Even if you take a 2D section, you are still going to see the flow. 02:36.590 --> 02:41.780 But even in a 2D, you don't have to simulate both the top half and the bottom half. 02:42.410 --> 02:47.510 Just simulating the top half is enough because it is going to be symmetric along the center axis. 02:47.510 --> 02:52.160 So you can define the center axis face as symmetry plane and it would still work. 02:52.730 --> 02:56.530 Then we have the periodic boundary condition, which is also known as cyclic. 02:56.620 --> 03:02.920 It assumes the solution repeats itself in a periodic manner, which means the flow in a periodic array 03:02.920 --> 03:10.570 of obstacles can be modeled in such a way that the outlet of the pipe or outlet of the flow domain can 03:10.570 --> 03:12.610 be given as the input back. 03:12.610 --> 03:18.390 So when we are getting a value at the outlet, we can give it as the inlet as well. 03:18.390 --> 03:20.430 So that is what the cyclic says. 03:21.150 --> 03:25.590 So the application is it is used in simulations of a repetitive structures or flow. 03:25.590 --> 03:30.600 And if you have only a small section of pipe, but if you want to consider it as an infinite length, 03:30.600 --> 03:35.640 then you can do the outlet are given to the inlet thing. 03:35.640 --> 03:40.320 So it is mostly periodic or cyclic, and it can be considered as an infinite pipe. 03:41.600 --> 03:46.400 The openfoam keyword is just cyclic, so we have two things. 03:46.400 --> 03:48.470 So one is periodic one and periodic two. 03:48.470 --> 03:51.740 So if you see both has the type cyclic. 03:51.740 --> 03:55.520 But the neighborhood patch corresponds to the opposite of the one. 03:55.520 --> 04:00.380 If you see the periodic one we have neighbor patches periodic two and periodic two has the neighbor 04:00.380 --> 04:01.490 patches periodic one. 04:01.520 --> 04:05.300 This is how we are defining the coupling of both those patches. 04:05.320 --> 04:12.370 and it will be coupled so that one of the output value is given to the input of other value. 04:13.150 --> 04:16.540 The practical implementation example is what we are going to see now. 04:17.290 --> 04:19.840 We have a backward facing step of a 2D. 04:19.840 --> 04:24.940 So if you haven't watched the meshing tutorial already, please check out this YouTube link of my YouTube 04:24.940 --> 04:29.320 channel where I have step by step explained how to prepare the mesh for this. 04:29.490 --> 04:32.010 We will set up both laminar and turbulent case. 04:32.010 --> 04:36.930 For this problem we will use incompressible state solver which is simple form. 04:37.500 --> 04:40.200 So now we will see how to do the laminar case. 04:41.220 --> 04:43.590 This is the geometry which we are using. 04:43.590 --> 04:46.770 It has inlet outlet fixed walls and front and back. 04:46.800 --> 04:50.790 If you see the front end back is set to empty because we are treating it as 2D case. 04:51.450 --> 04:52.830 Then we have the velocity. 04:53.600 --> 04:54.320 The velocity. 04:54.320 --> 05:00.350 We have used Dirichlet boundary condition, where we are defining the uniform velocity as 1D minus three, 05:00.350 --> 05:02.810 which is ten power minus three in the x direction. 05:02.840 --> 05:04.250 Of course, meters per second. 05:04.490 --> 05:06.110 And the outlet is zero gradient. 05:06.110 --> 05:10.610 If you see correspondingly for the pressure, we are not defining the inlet because we already defined 05:10.610 --> 05:11.660 it in the velocity. 05:12.320 --> 05:15.050 So we are defining it as zero gradient in the inlet. 05:15.050 --> 05:17.920 And for the outlet we are setting zero pressure. 05:17.920 --> 05:19.990 So it is also in a unit. 05:19.990 --> 05:23.650 So we will be defining fixed value and the value is uniform zero. 05:23.650 --> 05:26.470 And for fixed walls for velocity it is no slip. 05:26.470 --> 05:29.410 It is just nice way of saying that the fixed value is zero. 05:29.410 --> 05:34.330 Notice that the s is capital and for pressure we are saying zero gradient. 05:34.330 --> 05:38.980 So zero gradient is the way to tell the solver that you have to calculate it for me, and I do not know 05:38.980 --> 05:43.320 it already on front and back is obviously empty because we are doing a 2D case. 05:43.710 --> 05:48.360 If you see the transport properties, we are using a Newtonian model with the kinematic viscosity of 05:48.360 --> 05:55.440 1.56 into ten power minus five, and the simulation type is laminar for this tutorial and in the control 05:55.440 --> 06:01.770 dict, the application is simple form start time is zero and we will be stopping at 2000. 06:01.770 --> 06:03.690 That's the maximum number of iterations. 06:04.350 --> 06:09.470 But if the solution converges faster, we will not go till 2000. 06:09.470 --> 06:11.120 That's how steady state cases work. 06:11.360 --> 06:15.290 So these are the results which we are expecting out of the tutorial. 06:15.290 --> 06:18.290 So now I will jump to the terminal. 06:18.710 --> 06:23.660 Okay so this is the case where we will be working on. 06:23.660 --> 06:28.190 So I will open that location by doing the command explorer dot x space dot. 06:28.190 --> 06:31.090 So this is exactly the case which I showed you. 06:31.090 --> 06:32.800 All the case files belong to this. 06:33.040 --> 06:35.200 Now we are going to run the block mesh. 06:36.790 --> 06:38.380 So I ran the block mesh. 06:38.530 --> 06:41.260 And then the next command is simple form. 06:43.120 --> 06:46.750 Now I will let the simulation run and let it converge. 06:47.680 --> 06:49.030 And we will wait for it. 06:54.660 --> 06:58.380 Okay, now the simulation has converged at 483 iterations. 06:58.410 --> 07:04.620 As you can see, since it converged, it did not bother to go till, uh, 2000 iterations. 07:04.650 --> 07:06.690 Now we can visualize the results. 07:06.810 --> 07:08.490 I'll open the paraview file. 07:11.340 --> 07:12.510 I'll click on apply. 07:13.110 --> 07:15.000 Okay, now we have got the pressure. 07:15.000 --> 07:19.780 But since we are interested only in the last time step where it converged, we will press this and go 07:19.780 --> 07:22.360 to the last time step which is 483. 07:23.260 --> 07:29.950 I'll press on this to set the value to the, uh, range, which is visible here, and we can visualize 07:29.950 --> 07:30.790 the pressure. 07:31.120 --> 07:36.010 As you can see, uh, we are getting a good, steady state over here. 07:36.850 --> 07:37.300 Okay. 07:37.300 --> 07:43.620 Now, the most important, uh, result, which we are seeing in our backward facing step is the reattachment 07:43.620 --> 07:46.380 length and the recirculation region. 07:46.380 --> 07:51.180 So we will see how to visualize the recirculation region by plotting streamlines. 07:51.180 --> 07:54.750 So I will press on this which will plot the streamlines. 07:54.750 --> 07:55.920 Click on apply. 07:55.920 --> 07:58.170 Initially it will show only the pressure colors. 07:58.170 --> 08:01.410 You can change it to velocity colors. 08:01.410 --> 08:03.810 Now if you want to hide the line you can press on this. 08:03.810 --> 08:04.710 It will hide the line. 08:04.710 --> 08:09.530 You can see that there is a recirculation region under Reattachment point for this. 08:09.530 --> 08:14.390 But this tutorial is not about calculating the calculating the reattachment point, so I am not going 08:14.390 --> 08:15.320 to touch upon that. 08:15.650 --> 08:23.300 If you think like this are too many number of stream lines, then you can probably reduce it like this 08:23.300 --> 08:24.710 is a good number. 08:24.710 --> 08:27.440 We are able to see distinct lines here. 08:27.560 --> 08:30.280 So this is how we will be plotting stream lines. 08:30.730 --> 08:37.300 And uh, if you just want to see how it looks without the stream lines, then you can go here, hide 08:37.300 --> 08:37.600 this. 08:37.600 --> 08:38.800 And this is your result. 08:39.010 --> 08:41.140 So that's it about the laminar case. 08:41.140 --> 08:43.330 Now we will see the turbulent case. 08:44.080 --> 08:45.400 So what is turbulence. 08:45.550 --> 08:50.260 The characteristics of a turbulent flow are irregularity diffusivity and high Reynolds number. 08:50.260 --> 08:55.410 The importance of turbulence in engineering is that it is an important aspect to consider, as it plays 08:55.410 --> 09:00.540 a crucial role in fields like fluid mechanical system, aeronautical engineering, and environmental 09:00.540 --> 09:01.080 engineering. 09:01.110 --> 09:03.270 Also weather and HVAC. 09:04.440 --> 09:07.890 The turbulence model overview is mostly these. 09:07.920 --> 09:11.370 We are mostly interested in only three types of turbulence modeling. 09:11.370 --> 09:16.020 One is direct numerical simulation, where we will be solving the entire Navier-Stokes equation without 09:16.020 --> 09:16.800 any modeling. 09:17.780 --> 09:21.830 It is computationally expensive and not feasible for higher Reynolds number. 09:21.830 --> 09:27.560 Then we have large eddy simulations which will resolve large scale turbulence structures while modeling 09:27.560 --> 09:32.480 smaller scales, and it is suitable for unsteady flows with moderate computational cost. 09:32.480 --> 09:38.150 Then we have Reynolds averaged Navier-Stokes, popularly known as Rans, or in Openfoam terms, it's 09:38.150 --> 09:44.750 RA ra as it solves the time averaged equation of motion with turbulence effects modeled widely used 09:44.750 --> 09:47.540 due to its balance between accuracy and computational cost. 09:47.540 --> 09:50.690 So this is the introduction to turbulence modeling. 09:50.990 --> 09:53.180 We will be using Rans throughout this course. 09:53.180 --> 09:57.890 In trance there are mostly three main uh models. 09:57.890 --> 10:04.640 One is known as k epsilon where it is a two equation model where it is, uh modeled based on turbulence 10:04.640 --> 10:08.220 kinetic energy, which is k, and the dissipation rate epsilon. 10:08.610 --> 10:10.920 It is suitable for general purpose applications. 10:10.920 --> 10:15.900 Then we have the most popular one, which is the k omega SST model. 10:16.500 --> 10:21.630 It is also a two equation model based on turbulence, kinetic energy and specific dissipation rate, 10:21.630 --> 10:22.650 which is omega. 10:22.800 --> 10:28.740 Commonly used in external aerodynamics and turbomachinery where the walls need to be modeled so SST 10:28.740 --> 10:31.110 is shear stress transport model. 10:31.560 --> 10:34.410 So we will be modeling shear stress better in this. 10:34.410 --> 10:40.140 Then we have the Spalart-allmaras model, which is a one equation model designed specifically for aerospace 10:40.140 --> 10:43.770 application to predict boundary layer behavior and aerodynamic performance. 10:43.950 --> 10:48.180 It is mostly used in external aerodynamics for airfoil wings and aircraft. 10:48.420 --> 10:53.010 So now we will be seeing backward facing step 2D turbulent case tutorial. 10:55.420 --> 10:57.280 So I'll go here. 10:57.610 --> 11:00.760 I will, uh, enter the second tutorial. 11:02.170 --> 11:03.040 I'll clear this. 11:03.550 --> 11:04.000 Okay. 11:04.000 --> 11:06.850 Now we are going to see the boundary files again. 11:06.850 --> 11:08.800 The boundary is going to remain in the same. 11:08.800 --> 11:09.970 The mesh is the same. 11:10.210 --> 11:15.070 And for velocity we have given 0.05m/s. 11:15.070 --> 11:17.440 It's quite high from what we used already. 11:17.440 --> 11:23.350 Then we have the K, which is, uh, turbulent kinetic energy for inlet. 11:23.380 --> 11:25.360 We are giving a fixed value and value. 11:25.360 --> 11:32.260 We are giving a reference value which is uniform 1.09 into ten power minus three. 11:32.260 --> 11:34.450 It is a 0.061 percentage. 11:34.450 --> 11:41.440 If you want to know how to calculate k omega and epsilon, there is a beautiful documentation from Openfoam. 11:41.440 --> 11:42.590 You can check that out. 11:43.070 --> 11:47.330 So they have given all the standard formula to calculate it, which you will be using as the initial 11:47.330 --> 11:48.080 condition. 11:48.290 --> 11:54.500 And for fixed walls we are using k q r wall function and again the value is referenced to internal field. 11:54.650 --> 11:56.690 Front and back is again empty. 11:56.990 --> 12:06.080 Then for new T we are giving the inlet as zero outlet, also as zero fixed walls as new two blended 12:06.080 --> 12:07.780 wall function, which is a function. 12:07.960 --> 12:13.570 And if you want to know more about these functions, you really have to check the documentation because 12:13.570 --> 12:14.710 there are a lot of function. 12:14.710 --> 12:18.250 We can't just explain all the function in the videos. 12:18.910 --> 12:22.960 So uh, we have the front and back HMD. 12:22.960 --> 12:26.980 So new T is just turbulent kinematic viscosity. 12:27.280 --> 12:31.860 Then uh, the next thing is we are having the omega here. 12:31.860 --> 12:34.710 So sorry the PPT has went backside. 12:34.710 --> 12:40.080 So it is omega and we have given it a pretty big value. 12:40.080 --> 12:46.440 So there is a reason since we want it to converge to a right value, we are giving it a very big value 12:46.440 --> 12:50.220 and it will correct it to the correct value. 12:50.220 --> 12:55.100 So we are trying to give a very big value and asking the solver to correct it for us. 12:56.090 --> 12:59.720 And uh, if you notice in new T we are giving it calculated. 12:59.720 --> 13:00.800 So that is also same. 13:00.800 --> 13:05.750 We are setting it as zero, a very low value and asking the solver to calculate it for us. 13:05.750 --> 13:07.340 So that's what happening. 13:08.210 --> 13:11.000 And in the control dict it is going to remind the same. 13:11.000 --> 13:15.410 And this is what we are uh expecting out of the simulation. 13:15.590 --> 13:16.010 Okay. 13:16.010 --> 13:18.020 Now I will run block mesh. 13:18.670 --> 13:19.540 It is done. 13:19.540 --> 13:20.860 Now I will run simple form. 13:23.350 --> 13:26.320 Let's wait for the simulation to end. 13:26.680 --> 13:31.240 Now the simulation has converged at 1378 iterations. 13:31.270 --> 13:33.550 Now we can visualize it using Paraview. 13:38.680 --> 13:40.570 Okay, we have got the pressure. 13:41.200 --> 13:44.790 And we have also got the velocity. 13:46.440 --> 13:50.730 Now, if you want to plot the streamlines, of course you can do it here. 13:51.540 --> 13:53.310 I'll change it to 250. 13:53.400 --> 13:54.450 Click on apply. 13:57.120 --> 14:01.470 Now you see we have got many recirculation regions. 14:01.470 --> 14:05.430 So this is what turbulence does to the flow domain. 14:06.320 --> 14:07.010 Okay. 14:07.040 --> 14:09.680 Now we have done the simulation. 14:10.130 --> 14:13.910 If you have any questions please feel free to contact me. 14:14.480 --> 14:21.230 And if you want to check the boundary conditions or types of boundary for each of the turbulence values 14:21.230 --> 14:26.060 like k, epsilon and omega, you can check the documentation and set it up. 14:26.060 --> 14:31.580 There are many tutorial cases which you can refer to and try it on your own.